Strategies for transportation of peptides across the skin for treatment of multiple diseases

ABSTRACT

An established view in genetic engineering dictates an increase in the discovery of therapeutic peptides to enable the treatment of multiple diseases. The use of hypodermic needle for delivery of proteins and peptides occurs due to the hydrophilic nature, sensitivity toward proteolytic enzymes and high molecular weight. The non-invasive nature of the transdermal delivery technique offers multiple advantages over the invasive route to release drugs directly into the systemic circulation to enhance bioavailability, better patient compliance, reduced toxicity and local irritability. The transdermal route seems highly desirable from the pharmaco-therapeutic and patient compliance point of view, however, the lipophilic barrier of skin restricts the application. The use of several techniques like electrical methods (iontophoresis, sonophoresis etc.), chemical penetration enhancers (e.g. protease inhibitors, penetration enhancers, etc.) and nanocarriers (dendrimers, lipid nanocapsules, etc.) are utilized to improve the passage of drug molecules across the biomembranes. Additionally, such clinical interventions facilitate the physicochemical characteristics of peptides, to enable effective preservation, conveyance and release of therapeutic agents. Moreover, strategies ensure the attainment of the intended targets and enhance treatment outcomes for multiple diseases. This review article focuses on the techniques of peptide transportation across the skin to advance the delivery approaches and therapeutic efficiency.

Plain language summary

Executive summary.

• Peptides show multiple therapeutic advantages for the management of various diseases.

• However, poor oral bioavailability, interference of digestive enzymes, and patient non-compliance for parenteral administration limits its use.

• Transdermal delivery shows advantages like higher efficacy, absence of first-pass metabolism compared to oral route, and direct absorption into the bloodstream.

• Despite the promising potential, the lipophilic barrier of skin, physicochemical properties of peptides restrict the transdermal application.

• Various strategies are developed for skin penetration of peptides.

• Utilization of nanocarriers offers enhancement in chemical drug stability and reduction in adverse effects by modulating drug the release.

• Overall therapeutically effective formulations and optimized carrier systems, will enable successful transdermal delivery systems systems for theranostic applications, and personalized medicine in near future.

Plain language summary

Article highlights.

Potential of peptides

• Peptides demonstrate multiple therapeutic advantages for the management of multiple diseases.

• However, limited therapeutic efficiency results due to poor oral bioavailability, interference of digestive enzymes, and patient non-compliance for parenteral administration.

Challenges for transdermal delivery of peptides

• Various challenges are associated with transdermal delivery of peptides such as skin, physicochemical properties of peptides, etc.

Strategies/Approach of skin penetration of peptides

• The use of external stimulants, permeation aids, chemical modulators, prodrugs aid to overcome the limitations for transdermal delivery of peptides.

• However, these approaches may not be suitable for cosmetics or sensitive eye areas.

• Therefore, a gap is identified to develop systems that optimize the delivery of actives into the skin and minimize adverse effects.

Nanocarrier systems for peptide delivery

• Nanocarriers offer various advantages like enhancement in chemical drug stability and reduction in adverse effects by modulating the drug release for longer periods.

• Nanotechnology-based formulations facilitate enhanced topical drug delivery, and maintain the integrity of skin barrier, thus offering superior advantages compared to other approaches.

• Multiple nanocarrier systems are utilized such as polymeric nanoparticles, liposomes, niosomes, solid lipid nanoparticles, dendrimers, nanostructured lipid carriers, metal-based peptide complexes, microneedles for successful transdermal delivery of peptides.

1. Introduction

Peptides offer numerous therapeutic advantages for the management of multiple diseases, but their potential is constrained by several issues, like poor oral bioavailability, interference of digestive enzymes and patient non-compliance for parenteral administration. Therefore, ongoing research in peptide therapy is necessitated to address the challenges and enhance the therapeutic benefits [1]. Poor permeability caused by physicochemical obstacles, such as hydrophilicity, high molecular weight and low stability in the presence of extremely highly acidic environments along with proteolytic enzymes, limits the oral delivery of peptides. Earlier research studies demonstrated the enhancement in the half-life of proteins by employing strategies such as conjugation with PEG polymer, serum albumin, glycosylation and substitution of D-amino acids with L-amino acids to enable a reduction in the dose of peptides [2]. In vivo studies aid in development of clinical programs and the biotransformation data collectively drives the fabrication of novel peptides, clinical trial designs and monitor data studies to enhance peptide delivery for effective patient therapy as well as compliance [3]. Precocious puberty must be treated with biomolecules such as leuprolide, a synthetic peptide that targets the luteinizing hormone-releasing hormone receptor (LH-RH) and is used for the reduction of risk associated with certain cancers and endometriosis [4]. For the population affected by cancer, leuprolide is available as implant or depot formulations but faces various drawbacks related to patient compliance and safety [5]. Recent research employed iontophoresis to effectively enhance the transdermal absorption of leuprolide and the pharmacokinetics parameters for transdermal delivery of leuprolide using patches. The patches were supplied with electrical current and compared with those of subcutaneous administration and the results were mostly similar, demonstrating the utility of this method for administration [6]. Transdermal delivery shows higher efficacy against the oral route under lower exposure to enzymes in the skin, absence of first-pass metabolism and direct absorption into the bloodstream. The therapeutic peptides are locally administered within the target area of the skin, for example, antimicrobial peptides possess the potential to manage dermatitis, skin infections, etc. whereas, peptides with anti-aging properties are incorporated in cosmeceutical products. This review article features the existing techniques for transdermal delivery of peptides by overcoming the skin barriers with modification in physical and chemical characteristics of the proteins and peptides to achieve desirable therapeutic effects.

2. Challenges for transdermal delivery of peptides

2.1. Skin as a barrier to the absorption of drugs

Ideally, drugs with a molecular weight under 500 Da i.e., Nicotine (M.W.162 Da), hydrophilicity below 1 mg/ml, lipophilicity in the range of 1–3 on log P scale i.e., scopolamine (log p = 0.98) and melting point below 200°C i.e., fentanyl melts at 87.5°C; are successfully delivered across the skin. Drug permeation across the stratum corneum (SC) via sweat duct as well as hair follicles are the two potential pathways for transportation through the skin. The flux from the steady state provides negligible contribution throughout the skin. However, the limitations associated with the transdermal delivery of peptides are shown in Figure 1. Moreover, the SC acts as an essential route for the transportation of large-sized molecules which are polar, colloidal particles and polymers however, these are unable to diffuse readily across the skin. Ten to fifteen layers of keratinocytes are present in the SC resembling a brick-and-mortar structure in an intracellular matrix acts as an effective barrier. The keratinocytes serve as the bricks and the matrix, made up of sterol/wax esters, cholesterol, free fatty acids, cholesterol sulfate, long-chain ceramides and triglycerides, serves as the mortar. Due to such high lipophilicity of skin, passive permeation of hydrophilic peptides like beta-defensins, insulin, vasopressin etc., are limited and the presence of several protease enzymes (e.g. matrix metalloproteins, serine, cysteine etc.) destabilizes the peptide structure to render it inefficient. Hence, a transdermal delivery method significantly improves the stability and transportation of proteins across the skin layers with high efficacy [7]. The enzymes such as endopeptidases including collagenase, elastase, caseinolytic enzymes, fibrinolysis and kallikrein-kinin system as well as exopeptidases consisting of carboxypeptidase and amino-peptidase are present in skin layers with the potential to break N- or C-terminal peptide bonds. Peptides show excellent therapeutic efficacy, however, due to their high hydrophilicity, penetrability across the skin remains poor. To boost skin penetration and address the related permeation issues, a variety of approaches are employed [8,9].

Figure 1.

Limitations in transdermal drug delivery systems of peptides.

2.2. Physicochemical properties of peptides as a barrier for transdermal delivery

Peptides, with their remarkable potential for targeted therapy due to their high specificity and diverse functionalities, are attracting significant interest in transdermal delivery. However, their successful transdermal delivery remains a significant challenge due to several inherent physicochemical properties. The size of therapeutic peptides typically falls within the range of 500–5000 Da, exceeding the passive diffusion limit in the outermost layer of the skin, where SC acts as the primary barrier for transdermal delivery. Additionally, the net charge of a peptide can influence their interaction with the SC as it encompasses a predominantly negative charge because of carboxylic acid groups in corneocytes (dead skin cells). This leads to repulsive interactions for negatively charged peptides, further hindering their permeation [10]. Hydrophilicity is another key physicochemical property that restricts peptide delivery across the skin. Most therapeutic peptides exhibit high hydrophilicity, limiting the partitioning in a lipid-rich environment of the SC as it primarily consists of fatty acids, cholesterol and ceramides in the form of hydrophobic barrier to impede hydrophilic molecules passage. Consequently, peptides show passive diffusion on difficulty through the SC due to their unfavorable partitioning behavior [11,12].

Furthermore, peptides are predisposed to enzymatic degradation in the skin for example proteases, are the enzymes that are abundant in the dermis and epidermis, the two inner skin layers. These enzymes can rapidly degrade peptides and significantly reduce the therapeutic agent availability for targeted action. Additionally, proteins and peptides are sensitive toward the proteolytic enzymes like aminopeptidases present inside the different layers of skin. However, the high elimination rate in human plasma is reflected as the lesser duration of half-life and further lower rate of intradermal elimination [13].

3. Strategies/Approach of skin penetration of peptides

3.1. External stimulants

3.1.1. Iontophoresis

Iontophoresis is originated from the Greek word “Ionto” to mean “ions” and “phoresis” means “to bear”. The procedure involves the application of electric impulses of low intensity to increase the penetration of molecules (like charged/ionized) across the skin layers and based on two principles: electro-osmosis and electro-repulsion [14–16]. The ions are injected from the electrode applied to the skin due to electro-repulsion. By the electro-osmosis theory, the solvent consisting of neutral or cationic molecules exhibit a bulk flow when the voltage is applied across the charged skin layers [17]. The three main paths via which iontophoresis primarily occurs are the paracellular route via corneocytes, the appendageal route (shunt pathway) via secretory gland, hair follicles, sweat ducts and the transcellular pathway across the cells. Due to the presence of protein and peptides in modern therapy, subcutaneous administration as intramuscular or intravenous injection possesses a challenge due to its physicochemical properties and stability. Iontophoretic delivery delivers negatively charged corticosteroids for antiviral delivery used to treat herpes oro labialis lesions (also known as “fever blisters”), oral ulcers (also known as “cancer sores”) and dermatological conditions like hyperhidrosis of the feet, axillae, palms, etc. [18]. In the diagnostic applications, the iontophoretic transport induces sweating and the collection for further analysis is a primary diagnostic test for cystic fibrosis [19]. With the advantages of ease of handling, home therapy and patient compliance (10 patients), the novel iontophoretic device “Drionic” was utilized to treat hyperhidrosis axillaris. This unilateral therapy was administered daily for a duration three weeks, followed by hygrometry and colorimetry. Sweating recovered to normal levels in three patients who experienced palmar hyperhidrosis along with two patients with plantar hyperhidrosis, indicating successful treatment of hyperhidrosis [20]. Four patients demonstrated sweat rate reduction on their palms, while three patients showed sweat rate reduction moderately on their soles. Sweat inhibition was only moderate in the remaining group whereas the side effects were low during the palmoplantar hyperhidrosis treatment. In other cases, axillary hyperhidrosis was only reduced mildly [21].

Improvements are also evident in the administration of short peptides, especially growth hormone-releasing factor (amino acids 1–44), LH-RH model tripeptides (vasopressin) and insulin [22]. Iontophoresis along with different formulations and chemical enhancers by Boinpally et al. found that the variety of use of chemical penetration enhancers and solvents in a systematic manner. Cyclosporin from lecithin vesicles transported peptides across the skin effectively by anodal iontophoresis in in vitro evaluation in comparison to microemulsions used for insulin delivery. These peptides include calcitonin, octreotide, LHRH, arginine vasopressin (AVP), Thyrotropin-releasing hormone (TRH), cyclosporin, insulin and nafarelin [23]. The use of pulsed current demonstrates enhancement for transdermal permeability of LHRH peptides, a lipophilic and negatively charged peptide [24]. Iontophoresis serves as a preferred method for peptide delivery, mainly for hydrophilic peptides, however the transport impedes if lipophilic moieties are present due to their bulky structure and positively charged residues. This can reduce iontophoretic transport and prevent skin accumulation, for more precise drug administration in a non-invasive iontophoresis system (Figure 2) [25].

Figure 2.

Iontophoresis-aided transdermal permeation of drug.

Michiue et., al. determined the use of iontophoresis, a non-invasive low electric current technique, to deliver myostatin inhibitory peptides (MIPs) to skeletal muscle. MIPs hold the promise for treating sarcopenia, an age-related muscle loss condition. The research is the first to demonstrate delivery of biomolecules like peptides to muscles from skin to muscles through iontophoresis. A significant increase in skeletal muscle weight (1.25-fold) by iontophoresis for a specific MIP (MID-35) compared with control system was observed. Additionally, such trends suggest an increase in both new and mature muscle fibers, along with potential changes in gene expression related to muscle growth. These findings warrant further exploration to understand the mechanisms behind iontophoresis-mediated MIP delivery and its impact on muscle gene regulation. Iontophoresis offers a potentially transformative approach for non-invasive intramuscular drug delivery, particularly for muscle wasting disorders. This patient-friendly alternative could improve treatment adherence compared with traditional invasive injection formulations [26].

These studies demonstrate the use of iontophoresis for effectively delivering the negatively charged corticosteroids, antiviral drugs and peptides, with the hypothesis that iontophoresis enhances the transdermal drug delivery by leveraging electric currents. Additionally, when compared with conventional delivery methods, such as injections, iontophoresis offers several advantages, including ease of handling, patient compliance and the potential for home therapy. The novel iontophoretic device “Drionic” demonstrated significant efficacy in treating hyperhidrosis, with some patients of normal sweating levels. These findings highlight iontophoresis as a promising alternative to more invasive methods, particularly for conditions like hyperhidrosis and muscle wasting disorders.

Furthermore, iontophoresis showed enhanced delivery of peptides like growth hormone-releasing factor, LHRH and insulin, especially when used with formulation changes and chemical enhancers, as demonstrated by Boinpally et al. In comparison to microemulsions used for insulin delivery, anodal iontophoresis significantly improved the transport of cyclosporin from lecithin vesicles across the skin. These comparisons underscore the versatility of iontophoresis in enhancing transdermal permeability for a variety of therapeutic agents [25].

Despite the promising results, the study showed certain limitations like the iontophoretic delivery of lipophilic peptides with bulky structures near positively charged residues was less efficient which could impede drug transport and accumulation in the skin. Future research studies will explore new strategies to overcome such limitations, such as optimizing the electrical parameters for development of novel formulation enhancers, or integrating advanced nanocarrier systems to improve the delivery of lipophilic therapeutic peptides. Additionally, the study demonstrated significant muscle growth following the delivery of myostatin inhibitory peptides (MIPs) via iontophoresis, the underlying mechanisms of iontophoresis-mediated MIP delivery and its impact on muscle gene regulation for further investigation. Future studies required for elucidating these mechanisms, as well as exploring the long-term effects and potential clinical applications of iontophoresis in treating muscle wasting disorders [20–23].

3.1.2. Sonophoresis/phonophoresis

This technique is an ultrasound-based technique to painlessly transmit the ionized molecules through the skin. To produce acoustic waves, a piezoelectric crystal generates mechanical energy from electrical energy [27]. The resultant waves disturb the lipid bilayer by the formation and disruption of microbubbles for the generation of shock waves and assist the permeation through the skin [28]. The high acoustic intensity builds the channels in skin enabling the transdermal distribution of macromolecules [29]. Additionally, sonophoresis pre-treatment substantially enhances the dermal penetration as well as the accumulation of the therapeutic agents in the lower epidermal layer (Figure 3) [30]. Sonophoretic transdermal delivery devices are employed for the enhancement in the penetration of peptides to manage arthritic pain, whereas cosmetic applications like skin rejuvenation are also fulfilled. To efficiently transmit heparin along with heparin of low molecular weight in biologically therapeutic doses, in vitro experiments on porcine and in vivo studies on murine skin demonstrated low-frequency ultrasound increased the skin permeability [31]. To improve skin permeability in vitro along with in vivo, ultrasound-mediated transdermal administration of low molecular weight heparin (LMWH) caused aXa blood levels to remain persistent. Before ultrasonography, the skin conductivity of 0.01 kohm-cm² was observed. A 50-fold increase in the conductivity was reported after ultrasound exposure. High skin permeability was maintained for a minimum of 8 h after ultrasonic treatment. Due to skin repair mechanisms, permeability gradually returns to the baseline value [32]. Ultrasound assisted deliveries demonstrate advantage like effective transdermal distribution, to deliver proteins and peptides such as erythropoietin, insulin, IFN-g and heparin etc., across the skin. The commercial availability of various ultrasonic transducers, as well as ultrasonic baths show disadvantages like clinical setting restrictions, complex and large-scale handling [33]. In a research study, insulin was administered in six pigs with and without ultrasound followed by measurement of the blood glucose levels for 90 min. Compared with the 3121 mg/dl at 30 min, the blood glucose levels declined to the level of 725 mg/dl after the time of 60 min (p = 0.05) and 9123 mg/dl at the time of 90 min mg/dl increase seen in animals in control group (p > 0.05). The results demonstrated the weight for non-invasive transdermal insulin administration for diabetes management is both feasible and potentially effective when ultrasonography provides transdermal insulin to animals resembling to human size and weight [34].

Figure 3.

Sonophoresis for effective transdermal drug delivery.

During in vitro sonophoresis of rat skin, Cyclosporin A (CsA) topical transport aided by low-frequency ultrasound was studied. Investigators found the impact of exposure time and intensity on CsA deposited deep within the skin. In comparison to low-frequency ultrasound, seven-times higher CsA was confined in the skin owing to passive diffusion mechanism. However, the interaction between low-frequency ultrasound and additional techniques, like electroporation chemical enhancers (laurocapram and sodium lauryl sulfate) etc., dramatically increased the efficiency of low-frequency ultrasound to boost the external dispersion of CsA [33]. The findings support the hypothesis that high-intensity acoustic waves create channels in the skin, thereby enabling the transdermal distribution of macromolecules. Sonophoresis pre-treatment was shown to significantly increase the dermal penetration and accumulation of therapeutic agents in the lower epidermal layers, which aligns with the research goals. Sonophoresis offers distinct advantages over traditional transdermal delivery methods such as passive diffusion or iontophoresis.

Despite the promising results, the study shows limitations that require further investigation. For example, while sonophoresis was effective in increasing skin permeability and enhancing drug delivery, prolonged exposure to ultrasound led to a decrease in the biological activity of heparin in porcine skin. This highlights the need for optimizing exposure times and intensities to balance enhanced permeability with the preservation of drug efficacy. Additionally, the bulky and cumbersome nature of commercially available ultrasonic transducers limits the practical application of sonophoresis to clinical set-up. Future research should focus on developing more compact and portable devices that can be used in non-clinical environments, such as home therapy.

Moreover, the combination of low-frequency ultrasound with other techniques, such as electroporation and chemical enhancers, significantly improved the transdermal delivery of CsA. Further studies are needed to explore the synergistic effects of these combinations on a wider range of therapeutic agents. Understanding these interactions will be crucial in optimizing sonophoresis for various clinical applications [32,33].

3.1.3. Photomechanical waves

Photomechanical waves or laser-induced stress waves occur when a drug reservoir is supported by a laser target (such as polystyrene) placed on the skin. When a laser was deployed on a target, the energy emitted was collected by the tissue via the generation of compression waves or pressure pulses to govern the barrier function of SC and generate transient channels [35,36]. Drug diffusion occurs passively via the momentary channels and the use of pressure waves restores the SC barrier function gradually and without any pain or discomfort [37,38]. For example, involvement of clinical applications such as insulin delivery in rats as in vivo experiments reduced the blood glucose levels. However, the transdermal distribution of peptides that employ such a specified technique is unlikely to be feasible. Erbium: YAG (Er: YAG) lasers used in an in vivo study on naked mice for demonstration of improved and optimized skin absorption for gene-based medications. The permeation research study on Er: YAG in vitro laser therapy boosted antisense oligonucleotides (ASO) permeability up to 30-fold and was related to the ASO molecular mass and laser fluence engagement. The photomechanical waves of Er: YAG lasers and SC ablation effect caused DNA to be expressed all over the skin, from the epidermis to the subcutaneous layer. The study demonstrates the potential of the Er: YAG laser used to transport ASOs and DNA via SC in a well-controlled way and non-invasive technique [39]. The studies confirmed the hypothesis for the ability of laser-induced stress waves to create temporary channels in the SC to allow for passive drug diffusion. However, the findings also indicated that the feasibility of this technique for peptide delivery is limited, as the transdermal distribution of peptides may not be effective with this method.

3.1.4. Jet injections

Jet injectors synthesize the high-velocity jets (>100 m/s) with an orifice of about 150 μm in diameter and utilizes compressed gas/air to puncture the skin, for drug administration into the epidermis. High velocity transdermal powdered technique accelerates the powdered therapeutic compounds (fine drug particles of about 20 μm–100 μm) into the mucosal sites with energy of transient helium gas. Additionally, this technique is pain-free and causes no damage to the skin [40]. The PowderJect^® device uses high-velocity powder injections to deliver multiple drugs and biotechnological actives, including salmon calcitonin (s-CT). The injections are performed at 60 bar pressure and illustrate a deterioration of more than 10% in serum calcium levels [41]. The frequent use of needle injections for vaccinations is the main reason for blood-borne pathogen infections linked to injections (human immunodeficiency viruses, hepatitis C and hepatitis B). Liquid jet injectors are invented to overcome such complications. This technology allows to target the genetic materials and proteins for the epidermis-antigen-presenting cells, permits the probability of needle-free immunization and proves to be beneficial in children as well as needle-phobic patients [42,43]. These findings support the hypothesis that use of jet injectors of high-velocity can effectively deliver powdered therapeutic compounds and biotechnological actives without causing pain or skin damage. The success of the PowderJect^® device to deliver multiple drugs, including s-CT and achievement of a significant reduction in serum calcium levels further validate the effectiveness of this technology [43]. While jet injectors show significant promise, there are limitations that need to be addressed in upcoming research. One limitation is the potential variability in drug absorption and distribution, which may be influenced by factors such as skin thickness and composition. Additionally, while the technique is generally pain-free, individual variations in pain perception affects patient acceptance and compliance. Future research should focus on optimizing the parameters of jet injectors, such as pressure settings and particle size, to ensure consistent and effective drug delivery across the patient populations. Moreover, long-term studies are needed to evaluate the safety and efficacy of jet injectors, particularly in comparison to other needle-free delivery technologies such as microneedles or electroporation. Exploring the potential for combining jet injectors with other transdermal enhancement techniques may be valuable to expand their applicability to a wider range of therapeutic agents, such as larger molecules and biologics.

3.1.5. Laser ablation

Laser ablation technique employs laser beam to aid transport of drug molecules via the SC with the use of topical patches or gels. Application of laser radiation, involves vibrational energy absorbtion by the skin components to cause vibrational heating. As the water molecules within the irradiated cells reach the boiling point, a micro-explosion occurs to result in a controlled SC ablation [44]. The intensity of energy incident on to the skin allows to remove SC in a structured way and ablation can be accomplished at two optimal wavelengths. The tissue proteins absorb at a wavelength of 2940 nm, while tissue water absorbs at 2790 nm [45]. A laser ablation technique is created to deliver both drugs lipophilic and hydrophilic drugs Figure 4. However, the structural changes in the therapeutic molecules need to be accessed for safety. Er: YAG laser (2940 nm) ablates the skin in a regulated fashion to increase the penetration of 5-fluorouracil with lesser thermal damage and higher efficacy. Antithymocyte globulin (ATG) and basiliximab, two commercially available antibodies used to treat immunosuppression, were delivered transdermally via fractional Er: YAG laser to enhance antibody permeability. In comparison to hypodermic injection, transdermal administration of vaccines are susceptible to epidermal Langerhans and dermal dendritic cell metabolism that result in a negligible immune response at therapeutic doses [46,47]. Amplification of pDNA expression in the epidermis and subcutis after laser ablation with Er: YAG exposure was observed in in vitro permeation of ASOs [48]. Therefore, it remains necessary to alter the exposure time, laser influence and depth of the microporation for successful transdermal delivery by utilising the laser ablation approach to manage the safety and effectiveness of the process [44].

Figure 4.

The mechanism of peptide permeation via skin treated with full-surface and ablative lasers.

Since the laser ablation technique shows promise, there are limitations that must be addressed. One significant concern is the potential for structural alterations in therapeutic molecules during laser exposure, which may affect their stability and effectiveness. Additionally, the transdermal administration of vaccines using this method may lead to metabolism by dermal dendritic cells and epidermal Langerhans cells, to result in a negligible immune response at therapeutic doses. This is a critical limitation when considering the use of laser ablation for vaccine delivery. Studies are required to investigate the long-term effects of repeated laser exposure on skin integrity and the potential for cumulative damage [49,50]. Furthermore, the development of novel laser systems that can selectively target specific layers of the skin or therapeutic molecules may provide a solution to the current limitations, to make laser ablation a more viable option for a broader range of transdermal drug delivery applications.

3.1.6. Radio-frequency thermal ablation

The specified technique is effective in case of the electrosurgery and treatment for malignant tumors as the principle of Radio-frequency (RF) thermal ablation involves utilization of heat produced by high-frequency alternating electric current with an oscillation frequency of 200–1200 kHz. RF waves travel through the electrodes cause tissue ions to move around, heat up and lead to cell ablation with coagulative necrosis. Microelectrodes are inserted directly into the tumour to achieve this purpose. A rise in temperature from 60 to 100°C causes coagulation, permanent damage the tumour tissue. However, a temperature increase above 100°C causes tissue to vaporise and act as an insulator to stop the propagation of heat and results in reduction of the RF thermal ablation effectiveness [51,52].

RF thermal ablation offers several advantages over other tumor ablation techniques, such as cryoablation and microwave ablation. Unlike cryoablation, that uses extreme cold to freeze and destroy tissue, RF ablation uses heat, to allow for greater precision in targeting tumor cells while minimizing damage to surrounding healthy tissue. Additionally, while microwave ablation also uses heat, RF ablation remains generally more controllable because it allows for precise adjustment of the temperature range. However, both RF and microwave ablation face similar limitations when tissue vaporization occurs, potentially reducing treatment efficacy.

In vivo percutaneous penetration HGH by ViaDerm™ displayed a relative bioavailability of ∼80% against the subcutaneous injection, where the microchannel generator device on the application of pressure, gets activated to result into thermal ablation. 50 m electrode arrays show increased gene expression in ViaDerm™-treated viable epidermis and these levels were superior to those shown after application of DNA formulation to the skin surface. It developed RF-microchannels for the transport of nanoparticles and genetic material [53]. Treatment of benign thyroid nodules causes symptoms like dysphasia, neck pain, dysphasia, discomfort, a foreign body sensation in the throat and cough, as well as cosmetic issues or thyrotoxicosis in cases of thyroid nodules that are capable of own operation (AFTNs) [53].

Research involves developing more advanced microelectrode systems that can precisely control the temperature and depth of RF energy application. Additionally, research could explore the combination of RF ablation with other therapeutic modalities, such as chemotherapy or immunotherapy, to enhance treatment outcomes for malignant tumors. Moreover, further studies are needed to explore the potential of RF ablation in drug delivery, particularly in optimization of the parameters for transdermal delivery systems like ViaDerm™ to improve bioavailability and patient outcomes.

3.1.7. Suction blister ablation

The suction blister ablation method causes the formation of small blisters under the influence of a vacuum into the upper surface and the release of the drug in the dermal circulation. Compared with other transdermal delivery methods, suction blister ablation offers the advantage of direct drug entry into the dermal circulation, that particularly acts beneficial for achievement in rapid and complete bioavailability. Unlike traditional transdermal patches, that rely on passive diffusion and often suffer from incomplete drug absorption, suction blister ablation ensures that the drug bypasses the SC barrier entirely. However, while this method is effective for short-term, high-precision drug delivery, it presents significant drawbacks that limit its broader application.

100% bioavailability was reported during testing of the technique to utilize the antidiuretic peptide 1-deamino-8-D-arginine vasopressin (DDAVP). This technique utilized Cellpatch^® (a commercial product) and the transdermal delivery of morphine tested in postoperative patients revealed an absence of eurythmic pain as well as scar formation. However, the drawback of pronounced hyperemia in the de-epithelialized dermis indicates the failure of the method for chronic disease treatment. Owing to, the drawback of the process it is unlikely to be utilized as the future choice of transdermal delivery [54].

Given these limitations, suction blister ablation might not be a widely adopted method for transdermal drug delivery, especially for chronic conditions. Future research should focus on modifying the technique to minimize dermal irritation and improve patient comfort. The achievement involves development of new materials or methods to create blisters in a more controlled and less traumatic manner. Additionally, exploring the combination of suction blister ablation with other transdermal delivery technologies may overcome the current limitations and expand its applicability in clinical settings.

3.1.8. Thermal portion/Thermal ablation

Thermal ablation technique results due to heat provided to the skin surface for tissue vaporization. The localized heating may remove the SC at the site(s) of heat application to create microchannels in the skin, and permit drug delivery non-invasively with formation of small microchannels to avoid undesirable effects such as pain, bleeding, irritation and infection [55]. Thermal ablation brings about the formation of micropores ablating the SC by utilizing high temperatures. Altea Therapeutics synthesized PassPort^®, a patented technology patch of metallic filaments for diabetes management. Electric energy transformed into thermal energy causes an increase in temperature at the localized skin area and vaporizes to create microchannels and multiple diverse drug molecules like insulin to penetrate the deep layers of skin after electric pulse application on the filaments. During post ablation process, the skin starts to heal and the epidermis layer starts to regenerate [56]. Clinical use of PassPort^® for administration of insulin encompasses positive phase 1 trial data and sustained constant plasma levels of insulin for 12 h. Recombinant H5 hemagglutinin PassPort^® patch delivery reported, that the immunomodulators augment the antibody response in mice and boost the immune defenses against the extremely pathogenic avian H5N1 influenza virus [57,58].

3.2. Permeation aids

3.2.1. Penetration enhancer

Penetration enhancers promote the drug absorption into the bloodstream through the skin barriers via disruption of tight junctions or membrane fluidization. Penetration enhancers are classified as chelators that include EDTA, salicylates, p-diketones, citric acid, enamines and N-acyl collagen derivates. Penetration enhancers function in two ways via: transcellular pathway and paracellular pathway. On exposure to surfactants, the intestinal mucosa of goblet cells in rats displayed a modification in the viscosity of bronchial mucus. The paracellular transport pathway for the absorption of drug molecules was found to be more efficient due to the lack of proteolytic enzymes. However, the barrier functionalities for paracellular diffusion are the integrity of tight junction, or zonula occludens, dependent on extracellular calcium ions. Exposure to the chelating agent lowers the calcium ions concentration to alter the integrity of the tight junction and increases the bioavailability of the therapeutic agent [59]. According to research by Nagase et al., polymeric enhancers with cationic end-groups in polyethylene glycol (PEG) or polydimethylsiloxane block copolymers enhanced the characteristics of the polymer. The study also revealed a significant enhancement in the hydrophilic and hydrophobic therapeutic activity. The PEG chain length and polydimethylsiloxane showed an impact on the efficacy of the block copolymer and increased the partition coefficient at the skin surface instead of skin diffusion coefficient [60]. Administration of high surfactant concentrations and/or other chemicals to the epidermis is not ideal and may even be hazardous as it leads to dermatitis. Unless significant quantities of surfactants or other additives are applied, low absorption of actives such as IFN-α through the nasal mucosa or the skin is observed [61]. The skin integrity or physicochemical characteristics are momentarily harmed when penetration enhancers like N-alkyl-aza-cycloheptanones (Azone) for desglycinamide arginine vasopressin or DEE are used [62]. Ex vivo penetration of Leu-enkephalin via the hairless mouse skin was demonstrated and improved by addition of non-ionic surfactant, N-decylmethyl sulphoxide [63,64].

The research demonstrated that penetration enhancers, particularly those with cationic end-groups in polymeric structures like PEG or polydimethylsiloxane, can significantly improve both hydrophilic and hydrophobic drug absorption. Compared with other transdermal drug delivery methods, penetration enhancers offer a targeted approach to improve the drug absorption by directly modification of the skin properties. Future research need to focus on development of safer and more effective penetration enhancers to minimize the skin damage to provide better drug absorption. Exploration of the combination of enhancers with other transdermal delivery technologies, such as microneedles or iontophoresis, could also provide synergistic effects, improving both efficacy and safety. Additionally, long-term studies are needed to assess the cumulative effects of repeated use of penetration enhancers on skin health and to optimize their formulations for various therapeutic applications.

3.2.2. Protease inhibitors

Proteolytic degradation of therapeutic proteins is inhibited by protease inhibitors that aid in delivery via the transdermal route. By chelation of metal ions or by covalent bonding with the active site, the enzyme inhibition can be achieved. Reduction in protein degradation during iontophoresis for dermatological uses or local delivery of protein i.e. the recovery of IGF-1 was 99.98% in human skin for protease inhibitor combination and an increase in stability of IGF-1 was observed in the presence of protease inhibitor like phenylmethanesulfonyl fluoride [65].

3.2.3. Cell-penetrating peptides (CPPs)

Protein transduction domains (PTDs) move across plasma membranes due to their neutral or cationic nature and are also known as CPPs used to deliver cargo molecules into cells, including solid lipid nanoparticles, liposomes proteins and oligonucleotides. Pep-1, YARA, polyarginine, penetration, transport and the CPP from the transactivator of transcription (TAT) are some of the most widely used CPPs [66]. The structure of CPPs resembles that of the barrier membranes to result for better absorption and used to distribute different forms of conjugated proteins or peptides, amino acids, either chemically or genetically. Application to the skin includes an analysis of the fusion proteins or peptides due to the ability to be absorbed by or move through the skin. TAT peptide, also referred to as HIV-1 TAT, in conjugation with tripeptide glycine. GKH (Lysine-Histidine) is a molecule involved in lipolysis (fat breakdown), generated from parathyroid hormone and used in cosmetic procedures for injecting into the skin to facilitate the lipolysis process. Gene fusions of CPPs with therapeutic proteins result in the formation of fusion proteins with the potential to penetrate deeply into the skin. The fusion proteins contain multiple CPPs such as TAT, R9 and K9, and large therapeutic proteins like catalase and CuZn SOD1 [67]. According to Schutze-Redelmeier, the antennapedia transduction sequence peptide (ANTP) triggered the release of an antigenic peptide made up of eight amino acids generated from the ovalbumin protein (OVA 257–264). After topical application, it elicits a potent immune response to generate 264 peptides. Conjugated CPPs provide efficient skin penetration, nevertheless, skin degradation and rapid clearance of drugs are significant concerns of this route. Therefore, fabrication of nanoformulation assists in overcoming such obstacles in the delivery of drugs. As reported earlier, the TAT peptide was detected by fluorescent dyes (DID oil, 1,1′-dioctadecyl-3,3,3′,3′-tetramethylindodicarbocyanine) contained in nano lipid crystals whereas Perchlorate was conjugated to the surface. Nanoparticles were encapsulated with celecoxib (CXb) and localized to skin appendages for efficient penetration and to achieve a sustained drug release profile [68].

3.3. Prodrugs & chemical modulators

To decrease the hydrophilic character and transit period over the skin, peptides can be conjugated with lipophilic moieties to create lipophilic prodrugs, for exhibition of greater SC retention. INF-α delivered as an acyl derivative, displayed an increased penetration by 2.5-to fivefold. Improved penetration of insulin and TRH were achieved and no alteration in parent peptide due to derivatization with fatty acids was observed [69]. Compared with traditional transdermal delivery methods, lipophilic derivatization offers a distinct advantage by enhancement in the SC penetration of peptides. This is a critical improvement over standard methods that often struggle with the poor permeability of hydrophilic molecules.

Derivatization of TRH involves lauric acid to form a chemical bond with N-terminal pyroglutamyl group for the development of a more lipophilic product. Compared with the parent TRH, the activity of the lauryl derivative in the central nervous system and endocrine activity was only minimally reduced. Peptides are generally transported across biomembranes by lipophilic derivatization [70]. The enhancement in the lipophilicity of the peptides, the ester of palmitic acid (N-hydroxysuccinimide) was utilized to bond with terminal amino groups and p-methoxybenzoxycarbonyl azide was employed to protect the glycine-amino terminus. Two pure insulin derivatives-Bl-mono palmitoyl and Bl, B29-dipalmitoyl were discovered to be more lipophilic than the original insulin in HPLC. The hypoglycaemic effects of both products were evaluated in vivo in rat models following intravenous and intramuscular injections. Following intravenous administration, the findings showed mono-derivative was more therapeutically effective than the di-derivative and demonstrated a longer duration of therapeutic effectiveness against the original insulin. Additionally, an enzyme immunoassay illustrated the reduction in capacity of derivatives for immunoreactivity [71].

Improvement in encapsulation efficiency of liposomes and enhancement in the distribution through the skin in combination with the chemical modification into lipophilic derivatives of the peptides improves the permeability across the mucosal barrier. IFN-α with replacements of 10, 11 and 12 palmitoyl to lysine residues improved the cutaneous absorption by six-times [72]. Tetra gastrin modified with fatty acids leads to increased lipophilicity and reduced degradation of the tetra gastrin. Additionally, the SC remains more permeable due to the greater stability [73]. Nimodipine plasma levels were maintained with minimal variation owing to the use of transdermal patches containing 4% w/w limonene as a penetration enhancer, an ethanolic gel and a pressure-sensitive adhesive. Nimodipine inherits better skin tolerability due to its increased bioavailability compared with the tablet formulation and lack of any local limonene-related skin irritation. The intestinal barrier appears to be more easily penetrated by hydrophobic peptides like cyclosporin than by hydrophilic peptides. As a result of this finding, acylated derivatives of insulin and TRH contained palmitic and lauric acid, respectively to create an increased transmural permeability in the intestine and urinary tract. The results reported 60–80% retention of parent peptide activity, a significant rise against the control group [74]. Accumulation of the drug in the spleen and liver was observed after intravenous administration of lecithin-linked human IgG. Fatty acylation was used to enhance the bioavailability and drug circulation time. This method involves modification of drug molecule to form vesicle-like structures, as demonstrated in the development of the prosome-azidothymidine. In vitro testing was performed using human lymphocyte line MM46 and mouse sarcoma cells [75]. Fab fragments were used for trifluoperazine targeted delivery to brain glial cells across the blood-brain barrier [76]. Human tumor necrosis factor (recombinant) was fatty acylated (C2-C18) to improve the encapsulation in liposomes and with higher in vitro cytotoxicity. The neurotransmitter, vasoactive intestinal peptide, was modified by stearoyl residues and decreased in patients with sexual dysfunction to topically deliver in a rat model to show improved delivery at the desired site [77]. The novel chemical modification approach for transdermal delivery of proteins and palmitoylation of IFN-α increased the delivery to cutaneous and percutaneous regions in the treatment of condylomata acuminate [78].

The future of using prodrugs for transdermal delivery of peptides holds significant promise, with a potential advancements in both efficacy and patient compliance. Prodrugs, which are chemically modified versions of active peptides designed to enhance their permeability and stability, can address some of the major challenges associated with transdermal delivery, such as limited skin penetration and rapid degradation. By conjugating peptides with lipophilic moieties or employing innovative chemical modifications, prodrugs can improve the retention and absorption of these therapeutic agents through the SC. Future research may focus on optimizing prodrug formulations to achieve better bioavailability and targeted release profiles. This approach could enable the non-invasive delivery of peptides that are currently administered via injections, to enhance patient comfort and adherence to treatment regimens. The lipophilic derivatization of peptides represents a promising strategy for enhancement in transdermal delivery. By increasing the retention and permeability of peptides in the SC, this approach shows the potential to improve the bioavailability and therapeutic effectiveness of various peptide-based drugs. However, further research is needed to address the challenges associated with this technique, particularly concerning the preservation of peptide bioactivity and optimization of delivery systems for clinical applications.

3.4. Nanocarrier systems

The SC acts as a primary obstacle to transdermal peptide delivery and can be overcome via various methods like iontophoresis, electroporation, chemical enhancers, microneedles and ablation techniques. However, these techniques may not be suitable for cosmetics or sensitive eye areas. Therefore, a gap is identified to develop systems that optimize the delivery of actives into the skin at different layers and minimize systemic effects.

Utilizing nanocarriers offer the advantages such as enhancement in chemical drug stability and reduction in adverse effects by controlling the release for longer periods. Despite limited skin permeability to nanocarriers, nanoparticulate systems are beneficial in treatment conditions like atopic dermatitis and psoriasis, where the skin barrier is compromised. Nanocarrier-based formulations are particularly advantageous in acne treatment with targeted delivery to hair follicles. Nanotechnology-based formulations facilitate enhanced topical drug delivery and maintain the integrity of skin barrier, thus offering superior protection compared with other approaches. A variety of nanocarriers commonly employed for transdermal delivery are listed in Table 1.

Table 1.

Commonly employed nano/microparticles for transdermal delivery of peptides.

Drug	Nanoparticle type	Preparation	Efficacy	Ref.
Human Growth Hormone	Transferosome	Film hydration method	In vitro: 76.58% of drug release at 10 h	[37]
Desmopressin acetate	Microemulsion	Adding emulsifier	In vitro: 6% of the drug reached the acceptor compartment	[38]
Ovalbumin	Hyaluronic acid nanocapsules	Solvent replacement technique	22-times in vitro penetration rate higher than aqueous solution	[39]
Insulin	Biphasic lipid	Lipid layer hydration method	In vivo: The level of insulin 35.1 ± 9.3 mIU mg-1 after 48 h	[42]
Insulin	Niosome	Film hydration method	Prolong the release and increasing the stability of insulin	[46]
Insulin	Niosome	Lipid evaporation method	In vivo: Blood glucose level decreases 47.49%	[44]
Insulin	Transfersome	Rotary evaporation sonication	In vitro: drug release at 24 h: 83.11 ± 3.782%	[45]
Insulin	Biphasic lipid	Lipid layer hydration method	In vivo: Blood glucose level decreases 43.76 ± 3.8%	[49]
Ovalbumin	Nanogel	Semibatch procedure	Temperature dependent release	[53]

3.4.1. Polymeric nanoparticles

Nanoparticulate delivery emerges as an effective strategies for targeted drug therapies by improving the stability of bioactive compounds and reducing toxicity to healthy cells. These delivery systems offer a convenient means of tuning physical and chemical properties, to reduce drug degradation, enhance permeation of the drug and enable controlled delivery. Moreover, polymeric nanoparticles tend to aggregate predominantly via the follicular pathway, for suitable hair follicle-targeted delivery. Polymeric nanoparticles are the versatile and innovative approach to transdermal drug delivery, with significant enhancements over traditional methods. These nanoparticles are crafted from biocompatible and biodegradable polymers, with nanodimensional size that can encapsulate a wide range of therapeutic agents, including both hydrophilic and lipophilic compounds. The use of polymeric nanoparticles in transdermal delivery systems shows numerous benefits, due to their unique properties and capabilities [79].

One of the primary advantages of polymeric nanoparticles is their ability to improve the penetration of drugs through the skin barrier. The nano size of these particles allows them to interact with the SC and traverse the lipid matrix more effectively than larger particles. The enhanced penetration facilitates the delivery of drugs to deeper layers of the skin or even systemic circulation, depending on the formulation.

Polymeric nanoparticles also offer controlled or sustained release profile of encapsulated drugs. The controlled release pattern ensures that therapeutic levels of the drug are maintained over an extended period, reducing the need for frequent dosing and demonstrates patient compliance. Additionally, the ability to fine-tune the polymer composition and nanoparticle size allows for the customization of release kinetics to meet specific therapeutic requirements. The use of polymeric nanoparticles also enhances the stability and solubility of encapsulated drugs. Polymers can protect drugs from degradation and improve their solubility in the formulation for better bioavailability and efficacy. Moreover, polymeric nanoparticles can be engineered to target specific skin layers or receptors, by enhancing the precision and effectiveness of drug delivery [79].

Kokcu et al. [80], reported the encapsulation of GHK peptide within poly(lactic-co-glycolic acid) (PLGA) nanoparticles using the water-in-oil-in-water (W/O/W) technique combined with ultrasound. The GHK peptide garnered attention due to its protective effects against UVB radiation and reactive carbonyl species (RCS), and the ability to facilitate stem cell regeneration and proliferation of basal epidermal cells for the facilitation of new blood vessel formation. PLGA was selected as the nanoparticle matrix due to its non-toxic nature and the generation of metabolizable lactic acid upon decomposition, further metabolized into water and carbon dioxide. The GHK peptide-loaded particles exhibited 223 nm of size, with 94% entrapment efficiency. In vitro release assay revealed 50% release of the peptide within 7 h, and 90% release was achieved up to 48 h. Additionally, the GHK tripeptide exhibited skin permeability of -9.697 cm/h.

Biopolymers and biopolymeric combinations encompassed multiple advantages for long-term formulation stability. For instance, the transforming growth factor (TGF-β1) was successfully encapsulated within PLGA and polymeric microparticles to facilitate the regeneration of bone. Additionally, insulin-loaded poly(ϵ-caprolactone)-polyethylene glycol-poly(ϵ-caprolactone) (PCL-PEG-PCL) nanoparticles were fabricated using the double emulsion (w/o/w) solvent evaporation method, to exhibit effective delivery and therapeutic efficacy of insulin in diabetic rats. The NPs possess specific characteristics suitable for protein-drug delivery, with controlled degradation rates [81,82], and to tailor hydrophobic or hydrophilic drugs, depending on the encapsulation requirements and the desired stability of the drug. Furthermore, NPs offer low immunogenicity with higher safety profile to minimize the risk of rejection and ensure suitability for clinical use [83,84].

3.4.2. Liposomes

Liposomes serve as ideal carriers for peptide delivery, due to their ability to encapsulate non-polar, polar and amphiphilic amino acids derived from protein hydrolysates. The liposomes can be fabricated with ease using food-grade materials. Encapsulation with a targeted release of both hydrophobic and hydrophilic compounds enables targeted therapy [85]. The advantages of liposomes include non-toxicity, biodegradability and the potential to encompass various lipophilic and hydrophilic substances. However, its comparison with other encapsulation methods such as nanocapsules, emulsions, SLNs and nanosphere, the design of liposome is more prone to the fusion and leakage of entrapped molecules while preparing and preserving them, and digesting within the gastrointestinal tract (GIT) [86]. A notable advantage of liposomes is the ability of skin penetration by merging with lipids within the SC or dissociation on the SC surface, with the penetration of lipid molecules. Furthermore, sebaceous glands also serve as a good option for the entry of liposomes which acted as a reservoir for controlled delivery [87,88].

Advanced skin transport of peptides necessitates the creation of specialized liposomal structures such as ethosomes, niosomes, transferosomes, etc. For example, insulin delivered via liposomes and micelles showed low penetration via the SC. However, transdermal delivery of liposome formulations successfully achieved the desired therapeutic effectiveness and similarly, the encapsulation technology contributes to peptide delivery. Liposomes of human epidermal growth factor and wound-healing properties were 200 percentage greater than those of the active in a saline solution. Liposomal encapsulation of interferon-(IFN) enhanced skin deposition, but not macromolecule penetration. To improve wound healing with liposomal IFN delivery in a healthy SC, it was however not necessary to disrupt the SC [89]. Liposomes improved the insulin delivery whereas delivery of insulin and CsA through lecithin-based flexible vesicles showed interesting results for controlled release. Conventional and ultra flexible vesicles were generated by CsA as a model medication and placed non-conclusively on the skin of mice whose abdomen skin was surgically removed. In vitro studies involved vesicles tested for their ability to improve cyclosporin, vesicles were applied to mice skin by permeation technique, and drug serum concentrations were assessed in vivo. Cyclosporin, flexible vesicles were proven to distribute into the bloodstream during transdermal distribution better than conventional vesicles. The two functional mechanisms of penetration and fusion were considered to be responsible for the enhanced delivery [90]. Liposomes are pliable vesicles with enhanced drug delivery abilities found to work well for the topical and transdermal administration of active compounds to increase the transport of numerous active substances in animals and humans, for challenging therapeutics like plasmids and insulin, cannabinoids, peptides, DNA and organic cations [91].

3.4.3. Niosomes

Niosomes are closed bilayer structures formed by cholesterol and a single non-ionic surfactant with an alkyl chain, such as sorbitan esters and polysorbates, in aqueous medium. These are non-ionic surfactant-based vesicles, a cutting-edge advancement in transdermal drug delivery systems, offering several advantages over traditional delivery methods. Composed of non-ionic surfactants, cholesterol and sometimes other additives, niosomes form bilayer vesicles that encapsulate both hydrophilic and lipophilic drugs. This unique structure enhances the stability, solubility and bioavailability of the encapsulated drugs. Compared with liposomes, niosomes offer improvement in stability, higher skin penetration at lower surfactant concentrations, with enhanced bioavailability. Due to the surfactant characteristics, niosomes can modify the SC structure, to make it more permeable [92]. In transdermal delivery, niosomes are valued for their ability to penetrate the skin barrier more effectively than conventional formulations. The vesicular nature of niosomes allows them to interact with the lipid matrix of the SC, potentially disrupting it and facilitating the transport of the drug through the skin. This enhanced penetration can lead to improved drug absorption and bioavailability, making niosomes particularly useful for delivering drugs that have poor skin permeability.

Niosomes also offer controlled and sustained release of drugs, which helps in maintenance of therapeutic drug levels over extended periods. These characteristics not only enhance the efficacy of the treatment but also improves patient compliance by reducing the frequency of dosing. Additionally, niosomes are less likely to cause irritation or sensitivity in comparison to other surfactants or solubilizers, making them suitable for sensitive skin applications. The flexibility in fabrication of niosomes-by varying the surfactant composition and lipid content allows for the customization of delivery systems to meet specific therapeutic needs. The adaptability, coupled with their ability to improve drug penetration and provide controlled release, using niosomes acts as a promising technology in the field of transdermal drug delivery. Their applications explored in various therapeutic areas, including dermatology, pain management and hormone replacement therapy, highlighting their potential to enhance treatment outcomes and patient adherence [93].

In one study, niosomes were fabricated using Span 60, cholesterol and diacetyl phosphate (DP) as the surfactant at specific molar ratios (10:6:0.5). The charge on the niosomes was modified by altering the amount of DP. GHK-Cu-loaded niosomes (copper-bound glycyl-L-histidyl-L-lysine) were fabricated via thin-film hydration method. The stability of the noisome was assessed at 60°C for 4 weeks, followed by storage at 75% relative humidity (RH) and 40°C for an additional 4 weeks before analysis via high-performance liquid chromatography (HPLC) analysis. The GHK complexed niosomes demonstrated a t_1/2 of 89.95 days. In addition, degradation products were observed in samples with DP due to the presence of the phosphoric acid moiety in DP. The moiety can act as a donor of hydrogen atoms and become negatively charged in more basic solutions to form cleavage of nucleophiles in the peptide and the phenomenon was observed in samples subjected to stronger acid hydrolysis conditions [94–100].

3.4.4. Solid lipid nanoparticles

SLNs are biocompatible nanosystems of 50 to 1000 nm in size, composed of a solid lipid core encircled by surfactants and [95] the solid lipid core shows a critical role in drug release pattern, protects the drug from degradation and prevents a rapid release phenomenon [96,97]. Various methods, such as double emulsion and solvent diffusion, can be employed for the preparation of SLNs, allowing for the entrapment of hydrophilic or hydrophobic actives. SLNs are composed of solid lipids, which are solid at ambient and body temperatures, combined with surfactants and stabilizers to form nanoparticles typically ranging from 50 to 1000 nm in size. SLNs unique structure allows to encapsulate both hydrophilic and lipophilic drugs, to enhance their stability and bioavailability [98].

The application of SLNs in transdermal delivery capitalizes on their ability to penetrate the skin barrier and deliver therapeutic agents directly to the targeted site. The small size of SLNs facilitates their passage through the SC, the outermost layer of the skin, while their solid lipid core can protect sensitive drugs from degradation and control their release over time. The controlled release mechanism ensures a prolonged therapeutic effect by reducing the need for frequent dosing and improving patient compliance.

Additionally, SLNs pertain to an ability to enhance the penetration of drugs into deeper skin layers by utilizing various mechanisms, such as disruption of the lipid structure of the SC or interaction with the skin's natural lipid matrix. This capability makes SLNs particularly useful for delivering drugs that are otherwise challenging to administer transdermally, including peptides, proteins and other macromolecules. The versatility and efficacy of SLNs in transdermal drug delivery offer promising prospects for advancing treatment options across various therapeutic areas. By improving drug stability, enhancing skin penetration and providing controlled release, SLNs represent a significant step forward in optimizing the delivery of therapeutic agents through the skin.

The efficacy of SLNs as nanocarriers in peptide delivery was demonstrated using CsA as a model peptide. CsA shows potential for effective anti-inflammatory treatments like psoriasis, dermatitis, hive, eczema, etc. Softisan 649, a solid lipid commonly used as a lanolin substitute in the cosmetic industry, was chosen for the SLN formulation due to its skin adhesion properties and non-occlusive effect. The resulting SLNs exhibited an average size below 250 nm, entrapment efficiency of 88% and -22 mV zeta potential. Storage stability tests conducted at 4°C and then room temperature for 8 weeks demonstrated minimal SLN aggregation and increment in zeta potential because of the reconfiguration of Softisan 649 during the synthesis of SLNs. Biocompatibility studies illustrated the SLN's compatibility at CsA concentrations up to 0.6 mg/ml, as well as enhanced in vitro permeation in comparison to free CsA in a pig ear skin model [99,100].

Similarly DEETGEF, a heptapeptide was encapsulated within cocoa butter SLN using hot high-pressure homogenization. The nanoparticles exhibited the average size of 173 nm, zeta potential of -54 mV, encapsulation efficacy of 90.8% and a core lipid melting temperature of 27°C as determined by differential scanning calorimetry (DSC). The formulation activated protective enzyme genes such as NQO1, HMOX1 and PRDX1 in ex vivo skin models, to provide skin protection. It also increased 8-OHdG levels and reduced DNA damage caused by UV exposure [101,102].

3.4.5. Dendrimers

Dendrimers are nanoscale three-dimensional structures resembling macromolecules, known for their radial symmetry. The central atom covalently linked to other atoms and forms an inner core structure of dendrimers whereas the external composition is highly branched, resulting in a dendritic architecture. The branches create cavities that can encapsulate bioactive molecules, like proteins and peptides used in various pathological treatments, as well as for storage, transport and controlled delivery [103].

Multiple types of dendrimers are synthesized using methods such as click chemistry, divergent and convergent strategies, for the incorporation of chemical elements, and different designs. Particularly for drug delivery, dendrimers have garnered attention due to their biocompatible characteristics, including non-toxicity, non-immunogenicity and the ability to efficiently cross biological barriers such as cell membranes, blood vessels, blood-brain barrier and the intestines. Moreover, dendrimers exhibit high stability in blood, prolonged half-life and targeting ability. Additionally, dendrimers manufactured with high purity however may exhibit some level of toxicity in vitro when surface functionalized with anionic and cationic groups. Nevertheless, the toxicity might be mitigated by surface functionalization using PEGs or fatty acids [104,105].

Dendrimers in transdermal drug delivery systems, explore for the delivery of nonsteroidal anti-inflammatory drugs, such as indomethacin, aiding in the pain-related treatment like arthritis and osteoarthritis. The TAT/PAMAM modification of the dendrimer system was reported earlier, where the HIV TAT was surface functionalized as a CPP to increase the uptake of constructed plasmid DNA by cells, and serve as a non-viral vector to deliver DNA plasmid release via transdermal route [106]. Dendrimers serve as excellent enhancers for drug penetration and represent highly versatile nanostructures with significant potential in medicine for peptide delivery.

3.4.6. Nanostructured lipid carriers

Nanostructured lipid carriers (NLCs) are considered as a promising advancement in drug delivery technology. Unlike traditional SLNs, NLCs are composed of a mixture of liquid lipids and solids, to create a more complex and adaptable matrix. The unique structure of NLCs offers several advantages like encapsulation of a wider range of drugs compared with SLNs, including both hydrophilic and lipophilic molecules. The ability to manipulate the ratio of solid and liquid lipids allows the controlled drug release with optimization of therapeutic effect. Additionally, NLCs exhibit superior stability compared with SLNs, with improved resistance to physical and chemical degradation during storage. Such studies demonstrated successful transdermal delivery of various peptides using NLCs, including those with analgesic and anti-inflammatory properties. This opens exciting possibilities for developing novel peptide-based therapeutics with significant bioavailability and targeted action [107,108].

A study by Jiang et al. explored the use of functionalized NLCs to improve the delivery of pain relief using lidocaine hydrochloride (LID), a topical anesthetic. The SC presents a significant barrier to drug penetration and to overcome this challenge, researchers decorated NLCs with both chemical penetration enhancers (CPEs) and CPPs. A novel compound, pyrenebutyrate-PEG-DSPE (PB-PEG-DSPE), was synthesized to combine hydrophobic, hydrophilic and lipid functionalities. The lipid group integrates into the NLC, while the other groups enhance drug delivery. Additionally, TAT peptide, a well-known CPP, was incorporated to further facilitate cellular uptake of the anesthetic. The resulting TAT/PB LID NLCs demonstrated a larger size (153.6 nm) compared with undecorated NLCs (115.3 nm) due to the additional functional moieties. However, all the NLCs were stable and exhibited a narrow size distribution with negative zeta potential. In vitro studies showed significantly enhanced percutaneous penetration of LID using TAT/PB LID NLCs compared with other formulations, including single-ligand decorated and undecorated NLCs. The study suggests a synergistic effect of the combined CPE and CPP functionalization. Furthermore, in vivo studies confirmed that TAT/PB LID NLCs delivered superior anesthetic compared with other formulations. These findings highlight the promise of functionalized NLCs as a novel and effective topical anesthetic system [109].

Another study by Yuan et. al. combined ropivacaine (anesthetic) and meloxicam (anti-inflammatory) in TAT-modified NLCs (TAT-NLCs-RVC/MLX) to offer a promising strategy for post-surgical pain management. These nanocarriers with particle size of 133.4 ± 4.6 nm, zeta potential of -20.6 ± 1.8 mV enhanced skin absorption due to TAT modification. In animal studies, TAT-NLCs-RVC/MLX demonstrated superior pain relief compared with control group. This co-delivery system may provide long-lasting pain control with reduced side effects by simultaneously targeting both pain and inflammation [110].

3.5. Metal-based peptide complexes

Metal complexes demonstrate a critical role in drug delivery because of their unique properties and capabilities. These complexes offer multiple advantages, such as targeted delivery, increased stability, prominent controlled release, enhanced solubility and improved bioavailability of drugs [111,112]. The coordination of metal ions with drug molecules allows for the formation of stable complexes and protecting the drug from degradation with enhancement in pharmacokinetic profile. Additionally, metal complexes may be modulated to release the drug in a controlled manner, for prolonged therapeutic effects. The ability of metal complexes to selectively bind to specific receptors or target sites enables targeted drug delivery with lesser off-target effects and reduction in systemic toxicity [113].

The versatility of metal complexes stems from the wide variety of metal ions and ligands available, for the fine-tuning of their properties to meet specific therapeutic needs. For example, metal complexes can be designed to exploit the unique biological environments of target tissues, such as the acidic pH of tumors or the presence of specific enzymes, to release drugs in a controlled manner. This targeted approach not only enhances the therapeutic efficacy of the drug but also minimizes side effects by reducing off-targeted interactions and systemic toxicity. In addition to their role in drug delivery, metal complexes explored for their intrinsic therapeutic properties, particularly in the treatment of cancer, where metal-based drugs like cisplatin showed standard care. The exploration of metal complexes in drug delivery holds significant promise for the development of more effective, targeted and safer therapeutic strategies across a range of medical conditions.

A study involved carnosine, a dipeptide, exerts its effects by reducing oxidative stress through various mechanisms, including scavenging ROS, chelating metal ions and neutralization of secondary oxidized metabolites like peroxyl radicals. Additionally, carnosine was recognized for its anti-aging properties, involvement in cellular lifespan regulation and contribution to a rejuvenating effect. To enhance skin penetration, carnosine was complexed with magnesium ions and the presence of magnesium ions primarily in the SC suggests the involvement in functions of the epidermis like maintaining homeostasis barriers and differentiation. The complexation with magnesium ions potentially enhances the skin penetration of carnosine. The carnosine-magnesium complex showed to increase the concentration of carnosine by 1.5-times under the reconstructed human epidermis (RHE) model, to indicate enhanced peptide permeation through the skin [114]. The complex formation occurred through the coordination of the anionic form of the peptide (negatively charged) with magnesium ions. The positively charged complex interacted with amino groups of skin proteins to facilitate peptide permeation.

The formation of metal-drug complexes must be carefully controlled to avoid potential toxicity or unintended interactions with biological tissues. Additionally, while the carnosine-magnesium complex demonstrated significant permeation, further studies are needed to explore the long-term safety and efficacy of such complexes in clinical set-up. Moreover, the specific interactions between the metal ions and skin proteins facilitate the permeation into deeper investigation to optimize the delivery system. Future research should focus on exploring other metal ions and peptides to determine the ability of this approach and to refine the design of metal complexes for transdermal delivery. The complexation of drugs with metal ions represents a promising strategy for enhancing transdermal delivery, particularly for peptides like carnosine. By improving the stability, permeation and bioavailability, metal complexes can potentially revolutionize the transdermal drug delivery systems. However, careful consideration of the safety, efficacy and specific interactions involved in metal complexation is necessary for the successful translation of this approach into clinical practice.

3.6. Microneedles

Microneedles (MNs) are small needle-like structures, usually made of biodegradable polymers and used to improve transdermal drug administration. Typically MNs are lower microns in size, fabrication of different shapes and sizes to provide trans-epidermal delivery. MNs create micron-sized pores in the skin for drug delivery and avoid the stimulation of pain nerves upon insertion (Figure 5).

Figure 5.

Peptide delivery via microneedles across skin.

Microneedles, an innovative technology, attained considerable attention for their potential to revolutionize transdermal drug delivery. The needle-like structures offer a minimally-invasive approach to bypass the skin's formidable barrier, the SC and enable the efficient delivery of various therapeutic molecules, including peptides. The creation of microscopic pores in the skin by microneedles enhance drug permeation while minimizing the pain and discomfort associated with conventional needles. These tiny needles, ranging from solid, dissolving to coated, hydrogel and electrospun types, each offer distinct advantages for peptide administration. Solid microneedles create precise micro-channels for peptide absorption, while dissolving microneedles release their peptide payload directly into the dermis and then dissolve, by reducing the discomfort and injury risks. Coated microneedles combine solid needle structures with a peptide coating for targeted delivery and hydrogel microneedles, made from hydrophilic polymers, provide a controlled/sustained release of peptides. Electrospun microneedles, with their porous, fibrous structure, enhance the stability and distribution of peptides. The key benefits of microneedles include enhanced penetration through the skin barrier, minimized pain, improved bioavailability, reduced systemic side effects and the ability to offer controlled/sustained action. These advantages make microneedles a transformative approach in peptide delivery, promising more effective and patient-friendly therapeutic options [113–118].

For skin pre-treatment and drug-loaded transdermal patches or semisolid topical formulations for drug administration different types of MNs are used. The drug is administered to the skin through passive diffusion via the formed pores [115,116]. Investigations into the administration of insulin using solid MNs resulted in 29% reduction in blood glucose levels at 5 h and established the potential of coated MNs to deliver insulin to the skin. Drugs are transported and deposited within the skin by coating MNs with drug solution and releasing it through breakdown. Tween 20, Poloxamer 188 and other surfactants are employed to assist MN coating, while stabilizers such as trehalose, glucose, sucrose, dextrans and inulin protect the medicine during drying. Desmopressin is delivered through coated MNs to treat hemophilia A, diabetes insipidus and bed wetting in young children [117]. To increase patient compliance and to provide control release, soluble MNs with encapsulated drugs are made with biodegradable polymer and dissolved after applying on the skin. Various skin illnesses are treated with soluble MNs for the administration of CsA [118,119].

The design of these bio-responsive MN patches leverages common pathological and physiological triggers, including blood glucose levels, oxygen concentration, enzyme activity and pH. This approach holds significant promise for treating various diseases [120]. In another study for diabetes (type 1 and advanced type 2), traditionally, open-loop management relies on blood glucose monitoring and subsequent insulin injections. However, imprecise dosing can lead to complications or even fatal insulin shock due to severe hypoglycemia. Recent advancements focus on developing closed-loop glucose-responsive medications that mimic the function of a healthy pancreas in an autonomous manner. A common strategy involves utilization of glucose oxidase (GOx) to initiate an enzymatic reaction. In the presence of oxygen, GOx converts glucose into gluconic acid, resulting in a decrease of local oxygen and pH levels while simultaneously increasing hydrogen peroxide (H₂O₂) concentration [121].

Yu et al. exemplified this approach by designing an insulin delivery system using MNs loaded GOx and insulin-containing vesicles as a glucose-responsive system. These vesicles were self-assembled from hypoxia-sensitive hyaluronic acid conjugated with 2-nitroimidazole. In a hyperglycemic state, the GOx-mediated enzymatic oxidation creates localized hypoxia. Additionally, triggers the vesicle dissociation due to the conversion of hydrophobic 2-nitroimidazole component to hydrophilic 2-aminoimidazoles, resulting in the controlled release of insulin with rapid responsiveness behaviour. Beyond hypoxia, researchers have also explored exploiting enzymatically generated H₂O₂ for insulin delivery self-regulated in nature. Hu et al. integrated Gox-insulin-loaded H₂O₂-responsive polymeric vesicles into MN array for controlled insulin release [122].

Polyboronic acid (PBA) offers a unique approach for on-demand insulin release within MN arrays. PBA exhibits a well-defined equilibrium among anionically charged and uncharged forms, with the ability to bind cis-diols (1,2- or 1,3-) and shift this equilibrium toward the negative charge. In addition, this property leveraged to develop PBA-based systems for controlled insulin release, as demonstrated by Yu et al. [123,124]. Their work presents a “smart insulin patch” for blood glucose regulation in pre-clinical models. The system incorporates PBA units in the polymeric matrix of the MNs array. These PBA units reversibly form complexes with glucose, inducing matrix swelling for insulin release. Consequently, the electrostatic interactions between insulin and the polymers weaken by facilitating a glucose-responsive release mechanism. In vivo studies using diabetic mice and minipigs demonstrated the effectiveness of this PBA-based MN patch. Consecutive applications successfully maintained normal blood glucose levels. Notably, a single, coin-sized patch regulated blood sugar levels for more than 20 h in diabetic minipigs.

Extension of this concept, Wang et al. developed a novel PBA-based hybrid patch for the co-delivery of glucagon and insulin in a glucose-dependent manner. The innovative patch design incorporates two distinct modules co-polymerized from the same set of monomers but with varying ratios. These monomers include 3-(acrylamide) PBA, 2-aminoethyl methacrylate hydrochloride and 1-vinyl-2-pyrrolidinone. Employing a mask-mediated sequential photopolymerization technique, the researchers strategically incorporated insulin and glucagon into the co-polymerized matrix at a ratio of 3:1, mimicking the physiological composition of pancreatic β-cells and α-cells. The hybrid patch demonstrated promising results in a diabetic mouse (type 1) model, for control of hyperglycemia while minimizing the hypoglycemia occurrence. These studies collectively highlight the significant potential of PBA for developing bioresponsive MN arrays capable of controlled and personalized insulin delivery, offering a promising avenue for improving diabetic management [125].

Hollow MNs employ dispersion/drug-loaded hollow-spaced structures wherein macromolecules like proteins, vaccines and oligonucleotides are injected into the skin and adsorbed into the dermal layer. Hydrogel forming MNs, fabricated from swellable polymers undergo structural enlargement upon inserting into the skin. Microchannels are created between the drug patch and the capillary circulation as a result of contact with the interstitial fluid, permitting controlled drug release in a regulated pattern. Delivery of macromolecular dextran employs hydrogel MNs to improve ocular delivery [126]. Desmopressin, a hormone made up of synthetic peptides, is typically prescribed for the treatment of enuresis and administered via intranasal and oral routes due to the limited, variable absorption and results in an unsuitable routine for young children. On exposure to guinea pigs to solid desmopressin-coated microneedle for 15 min, significant levels of desmopressin were reported during the skin analysis. Desmopressin was also administered through a transdermal route with the assistance of Macroflux^® technology: a microneedle array bypasses the epidermal barrier and a high bioavailability of 85% w/w was achieved [52]. Table 2 mentions the different examples and their outcomes in transdermal peptide delivery.

Table 2.

Examples of enhanced peptide delivery across the skin.

Peptide	Delivery Method	Outcome	Ref.
Heparin	Chemical enhancer combination	100-fold increased skin permeability	[2]
Insulin	Electroporation, Iontophoresis and high voltage pulsing	Increased penetration by enhanced flux	[3]
Insulin	Photochemical waves, MNs, peptide transporters, iontophoresis	Increased penetration and decreased blood glucose level in diabetic rats	[3,4]
Desmopressin	Microneedles	Increased blood concentration in 5-15mins in hairless guinea pigs	[17]
Recombinant H5 hemagglutinin vaccine	Thermal ablation	Stimulation of immune system against avian H1N1 influenza virus in mice	[19]
IFNα	Chemical modification	5-6 folds increase in skin penetration	[27]
IFNα	Liposomes	Increased skin penetration	[28]
LHRH	Chemical enhancer combination	100-fold increased skin permeability	[29]
LHRH	Iontophoresis	Tenfold increased skin permeability	[30]
Nafarelin	Iontophoresis	Increased flux in cadaver skin	[36]
Calcitonin	Iontophoresis	Increased blood concentration	[38]

Despite the promising advancements in MN technology for drug delivery, several limitations and challenges persist. Scalability and cost-effectiveness in manufacturing complex microneedle structures remain significant hurdles, necessitating optimization of production techniques. Drug stability during coating and compatibility with microneedle materials also pose concerns, requiring further research into advanced stabilizers and formulation strategies. Achieving consistent and adequate penetration depth across varying skin types is crucial, highlighting the need for more precise control mechanisms. Patient safety and comfort must be continuously assessed to minimize potential discomfort or adverse reactions. Navigating regulatory requirements and conducting extensive clinical trials for new microneedle products can be complex and time consuming. High-dose drug delivery presents another challenge, calling for microneedle systems with greater drug-loading capacities. To address these limitations, future research should focus on developing enhanced materials and manufacturing techniques, optimizing microneedle design and functionality, integrating biosensors for real-time monitoring and conducting extensive real-world testing. Cross-disciplinary collaboration will be essential to overcome these challenges and advancing microneedle technology for more effective and patient-friendly drug delivery solutions.

4. Clinical relevance & future perspective

The field of medicine is transforming personalized approaches that cater to individual needs. Transdermal peptide delivery emerges as a powerful tool in this revolution, offering a multitude of benefits for patients across various therapeutic areas like pain management, dermatological treatments, cancer therapy, neurological disorders etc. The most significant advantage of transdermal peptide delivery lies in the ability to facilitate personalized medicine. Bypassing the GIT and first-pass metabolism, this approach allows for precise dosing and minimizes systemic side effects. This empowers healthcare professionals to tailor treatment regimens based on a patient's unique needs and sensitivities. For instance, children often struggle with oral medications due to taste or swallowing difficulties. Transdermal peptide patches could revolutionize paediatric care by providing a painless, non-invasive alternative [127,128]. Similarly, the elderly population may experience reduced gastric motility or impaired liver function. Transdermal peptides offer a solution to ensure consistent drug levels without burdening the clearance function of liver.

Beyond personalized dosing, transdermal delivery holds immense potential for chronic disease management. Patients with diabetes could benefit from insulin analogues delivered via patches, simplifying treatment and reducing the risk of hypoglycemia associated with injections. Chronic pain management can also be revolutionized with transdermal peptide-based analgesics, such as enkephalin derivatives, offering sustained relief and improved patient comfort and compliance. The targeted nature of transdermal delivery makes it ideal for dermatological applications. Topical peptides can modulate immune responses or promote skin barrier repair, offering a novel approach to atopic dermatitis and psoriasis. The localized treatment minimizes systemic exposure and the risk of unwanted side effects. In cases of wound healing, peptides like human beta-defensins can be delivered transdermally, directly targeting the wound site and promoting tissue regeneration [129,130].

The burgeoning field of aesthetic medicine is also embracing transdermal peptides. Anti-aging formulations containing collagen-stimulating peptides delivered through patches can enhance skin elasticity, reduce wrinkles and improve overall texture. Similarly, peptides targeting hair follicles offer a discreet and convenient option for patients seeking hair regrowth solutions through transdermal patches. Compliance and quality of life for patients are significantly enhanced by transdermal patches. This approach can revolutionize treatment for those with psychiatric disorders. Peptides targeting neurotransmitter receptors, like neuropeptide Y, could be delivered transdermally, potentially improving adherence and benefiting patients with anxiety, depression, or schizophrenia. Additionally, transdermal patches simplify the medication regimens for patients with multiple chronic conditions by reducing pill burden and enhancing overall treatment adherence [131,132].

However, realizing the full potential of transdermal peptide delivery requires concerted efforts from researchers, clinicians and pharmaceutical companies. Collaborative research and development are crucial to create effective and safe formulations for various therapeutic needs. Furthermore, healthcare professionals need to be equipped with the knowledge and tools to integrate this innovative approach into their practice.

5. Conclusion

Transdermal penetration of peptides is superior to other delivery routes due to enhancement in bioavailability, low rate of metabolism for peptides, lower elimination rate and short half-life of the peptide (in intra(epi-)dermal degradation terms) making transdermal distribution feasible. As a result, the skin becomes the perfect location for optimal stratum corneum penetration. Several approaches, like iontophoresis, chemical penetration enhancers and sonophoresis explore to bypass the skin barrier (stratum corneum) and assist in topical and transdermal delivery. Mechanical techniques like iontophoresis show potential for delivering peptides to the surface of skin. Utilizing chemical modifiers like protease inhibitors helps to overcome the enzymatic obstacles for proteins and peptides. Microporation is an increasingly recognized and effective technique for enhancing the transdermal delivery of large molecules, like macromolecules. Additionally, other technologies like microneedles, metal complexation, cell-penetrating peptides and chemical modifications demonstrate their utility in facilitating the transdermal delivery of peptides, with multiple options for this purpose. Furthermore, the discovery of new molecules or the sustainable production of bioactive peptides remains crucial for the pharmaceutical industry to offer a wider range of products to consumers with ease of accessibility. Considerable efforts are directed toward developing processes that derive bioactive peptides from agro-industrial waste, to address environmental concerns and promote alignment with global initiatives for a circular economy. Through the collaboration of well-designed formulations and optimized carrier systems, successful transdermal delivery is expected to become a reality in the near future, supporting personalized medicine, diagnostics and theranostics.

Author contributions

J Bhavsar: visualization, writing (original draft). K Kasture: visualization, writing (original draft). BV Salvi: visualization, writing (original draft). P Shende: conceptualization, writing (review and editing), supervision.

Financial disclosure

This paper was not funded.

Competing interests disclosure

The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.