Microneedle-mediated treatment for superficial tumors by combining multiple strategies

Abstract

Superficial tumors are still challenging to overcome due to the high risk and toxicity of surgery and conventional chemotherapy. Microneedles (MNs) are widely used in the treatment of superficial skin tumors (SST) due to the high penetration rate of the stratum corneum (SC), excellent biocompatibility, simple preparation process, high patient compliance, and minimal invasion. Most importantly, MNs can provide not only efficient and rarely painful delivery carriers, but also combine multi-model strategies with photothermal therapy (PTT), immunotherapy, and gene therapy for synergistic efficacy. To promote an in-depth understanding of their superiorities, this paper systematically summarized the latest application progress of MNs in the treatment of SST by delivering various types of photosensitizers, immune signal molecules, genes, and chemotherapy drugs. Just as important, the advantages, limitations, and drug release mechanisms of MNs based on different materials are introduced in the paper. In addition, the application of MN technology to clinical practice is the ultimate goal of all the work. The obstacles and possible difficulties in expanding the production of MNs and achieving clinical transformation are briefly discussed in this paper. To be anticipated, our work will provide new insights into the precise and rarely painful treatment of SST in the future.

Graphical Abstract

Introduction

Superficial skin tumors (SST), such as breast cancer, basal or squamous cell carcinoma (BCC), head and neck cancer, human oral epidermoid carcinoma, and melanoma, impose a great burden on patients both physically and psychologically due to the specificity of tumors color, shape, and location [1, 2]. Surgical excision is a traditional treatment method. However, in most cases, surgical treatment is often affected by changes in the patient’s internal indicators, so it cannot be the first choice for treatment after tumor diagnosis [3–5]. At the same time, surgical treatment is required to accurately diagnose the tumor sites before surgery and to ensure that the tumor tissue can be completely removed after surgery, which undoubtedly increases the difficulty of the treatment process. In the same way, not all patients with SST are suitable for surgical resection. Therefore, there is an urgent need to develop effective, safe, and easy-to-use methods to increase the cure rate of SST.

At present, the research on the treatment of SST is tending to adopt percutaneous drug delivery and locally targeted therapy. The prototype and preparation techniques of MNs were first proposed around 1950 [6]. As an emerging drug delivery carrier, MNs are small in shape, convenient to use, low in cost, safe and non-toxic. Compared with oral administration, MNs can avoid the metabolic effect of the digestive system and improve drug bioavailability. For another thing, compared with injection administration, MNs can effectively reduce patients’ pain sensation, improve patients’ compliance, and avoid the occurrence of needle sickness. At present, MNs have been applied to medical beauty [7], tumor treatment [8], diabetes treatment [9–11], disease screening and diagnosis [12], antibacterial [13], and other fields, which have achieved good feedback. With the improvement of industrial technology, MNs have also been successfully applied in the vaccination of influenza vaccines in clinical practice [14, 15]. The mechanism of action between MNs and skin based on different preparation methods is shown in Fig. 1 [16–20].

Fig. 1.

Schematic of the mechanism of action between MNs and skin. The preparation process of drug-loaded solid MNs with a two-step casting method [24], Copyright 2021 American Chemical Society; b droplet-born air-blowing method [25], Copyright 2021 Elsevier; and c free radical polymerization method [26], Copyright 2022 Royal Society of Chemistry

In the treatment of SST, the combined application of chemotherapy, immunotherapy, gene therapy, photothermal therapy (PTT), photodynamic therapy (PDT), and other multiple treatment methods provides an important prerequisite for the cure of tumors [21, 22]. Taking chemotherapy as an example, paclitaxel (PTX), 5-fluorouracil (5-FU), doxorubicin (DOX), and cisplatin (DDP) are commonly used in clinical cancer treatment. However, due to the difference in drug solubility and the limitation of responsive release, it is often difficult to successfully deliver all drugs to tumor targets. The skin is a barrier between the body and the external environment. In this case, MNs can interact with fibrin tissue at the superficial surface of the skin and successfully deliver the loaded drugs to the SST targets to achieve intelligent drug release [23]. Therefore, the combination of MNs and multiple tumor diagnosis and treatment strategies has special significance for the treatment of SST.

Based on the above points, we reviewed the MN-mediated different diagnostic and therapeutic approaches for SST, looking forward to laying a solid knowledge foundation for increasing the cure rate of clinical SST.

Superficial skin tumors

Breast cancer, BCC or squamous cell carcinoma, head and neck carcinoma, human oral epidermoid carcinoma, and melanoma are currently high incidences of SST. Taking melanoma as an example, surgical resection, chemotherapy, gene intervention, and locally targeted therapy are all effective treatments [27]. In the nineteenth century, surgical resection and lymphadenectomy were almost the only treatment for primary melanoma. However, once melanoma had metastasized or spread throughout the body, surgery would not be able to meet its therapeutic effect. In the middle of the twentieth century, chemotherapy drugs became the next breakthrough in the treatment of melanoma. However, the toxicity of chemotherapy drugs made the survival rate of melanoma patients extremely low. With the accumulation of treatment and survival data of melanoma patients and the improvement of treatment methods, BRAF mutation and MAPK mutation are considered to be the causes of melanoma. BRAF mutation occurs in about half of melanoma patients. It is easily targeted and inactive in healthy cells. Therefore, BRAF intervention has become a breakthrough in melanoma treatment [28].

Different from the treatment of melanoma, non-melanoma, mainly represented by cell carcinoma, adopt targeted therapy, which is mainly because BCC and squamous cell carcinoma rarely show metastasis. Most of the clinical manifestations were local tumors, which were relieved after radical surgery or radiotherapy [29]. At present, immunosuppression and radiotherapy can be used to treat rare cell carcinoma. Targeted delivery of coded gene molecules, enzyme inhibitors, chemotherapy drugs, monoclonal antibodies, and other strategies have shown better and better therapeutic prospects in the treatment of non-melanoma skin cancers [29–31].

The different types of MNs in the treatment of tumors

Solid MNs for tumors treatment

The MNs are shaped like an upside-down pushpin with sharp tips, which is the key point of penetration during interaction with the skin. The idea of MNs was first put forward in the 1970s. It was not established research till with the rise of microfabrication technology in the 1990s [32]. In recent years, various materials including polymers, ceramics, stainless steel, glass, metals, and non-metals have been used for the preparation of solid MNs [33]. After solid MNs form drug delivery channels on the skin surface, the drug is slowly released under the action of ISF and pH gradient and then absorbed by the body [32]. Currently, although solid MNs are widely used in the treatment of tumors, the release efficiency of loaded drugs is often affected by the MN carrier matrix. Last but not least, when metal solid MNs are inserted into the skin, there is a chance of causing irritation, swelling, erythema, discoloration, or further side effects, which will be fatal for those patients who are sensitive to certain matrix materials. Repeated insertion of MNs at the same location also may cause the above problems, even at the tumors targets [32, 34–36].

Hollow MNs for tumors treatment

To solve the problems of low drug loading, slow drug release, and material degradability of solid MNs, experts designed and developed hollow MNs for the delivery of drugs and small molecules in the 1990s. The design of the hollow MNs is to retain the head of the MNs, hollow out the center matrix of the sharp MN tails, and then replace them with the delivered small molecule substances and drugs [37, 38]. At present, hollow MN-mediated vaccine intradermal delivery and drug intradermal delivery have been proved to be promising methods to improve the effectiveness of drug preparations [39–42]. Compared with intravenous administration, the drug delivered by hollow MNs could achieve 10 h of sustainable release while maintaining the same dosage [42]. At the same time, hollow MNs have been proved to be able to completely deliver up to 2 mL of liquid and deposit 75% of the administered drug in the skin [39, 41]. However, the preparation of hollow MNs requires a series of complex means, such as the use of vacuum pumping. It is also important for researchers to effectively solve the problem of mutual penetration between matrix and drugs in the preparation process. With the end of the central drug release process, the mechanical strength of the MNs is greatly weakened based on the original strength, which will undoubtedly lead to the toxic side effects of MNs residues in the tumor sites [37, 38].

Dissolving MNs for the treatment of tumors

At the beginning of the twenty-first century, dissolving MNs were developed as new green and environmentally friendly drug carriers. The dissolving MNs are made of some biocompatible, water-soluble matrix materials, regardless of whether they will fracture in the skin and biocompatibility issues [25]. Currently, materials such as maltose, polyvinylpyrrolidone (PVP), trehalose, dextran, hyaluronic acid (HA), and carboxymethyl cellulose (CMC) can be used to prepare the dissolving MNs [43–46]. Natural polymers and natural polysaccharides can be dissolved or degraded when they come into contact with body fluids due to their special structure and existing state in nature. The main advantage of polysaccharides is that there are a large number of hydrophilic groups in their structure, such as hydroxyl, carboxyl, sulfonic group, or other branched chain groups, which provide conditions for them to become excellent soluble MN matrix [47]. Many studies on the safety of dissolving MN patches have shown that animal organs and cells will not be affected by the matrix materials after repeated application of MNs, which indicates that dissolving MNs are extremely safe and widely used with relatively lower limitations [25, 48, 49].

Hydrogel MNs for tumors treatment

Hydrogel healing has long been regarded as a good means of repairing damaged tissues. At the beginning of the twenty-first century, hydrogel MNs were first developed as an ideal drug delivery vehicle. In the preparation of hydrogel MNs, polysaccharides, polypeptides, and polymers are high-quality raw materials. These materials are loose, porous, and hydrophilic to a certain extent. When MNs are partially inserted into the skin surface, they will absorb water and swell. Due to the above characteristics, based on the osmotic pressure difference or temperature difference between ISF and the MN matrix, after fully absorbing the ISF, the pores between the polymer materials are opened larger, leading to drug release [50–53]. It is also not negligible that the porous characteristics of hydrogel MNs make it possible to adjust the release rate of drugs through different degrees of cross-linking, which has more advantages than soluble MNs in drug delivery [54–56]. However, most of the existing hydrogel MNs are used for the extraction and detection of ISF. Hydrogel MNs play a more prominent role in terms of tumor diagnosis and prognosis instead of tumor treatment, which may become a major obstacle to hydrogel MNs in tumor intervention [51, 57, 58].

Coated MNs for the treatment of tumors

To make the drug-loaded MNs penetrate the skin more quickly and improve the stability of the MNs, experts have developed a type of coated MNs by directly coating the drug around the tips of the MNs at the beginning of the twenty-first century. The coated drug can be a molecular prototype, polymer film-modified, or nano-modified drug so that the drug coated on the surface of the MNs can be released quickly and accurately when it contacts the ISF [59–61].

The boundary of different types of MNs gradually becomes indistinct in the application. The combination of coated MNs, solid MNs, hollow MNs, and hydrogel MNs can further enhance the mechanical properties of the MNs while maintaining good drug release properties [62, 63]. At the same time, MNs for treatment by changing the structure have also been developed, such as the use of double-layer MNs. In general, to reduce the dissolution of the backing layer as much as possible, the double-layer MNs prepared by the two-step pouring method can make it possible that the needle tips of the lower layer has a stronger drug release capacity [63, 64]. A schematic diagram of different types of MNs is shown in Fig. 2. The years, advantages, disadvantages, and preparation methods of different types of MNs have been summarized in Table 1.

Fig. 2.

Schematic diagram of different types of MNs. a Flow chart of drug delivery by solid MNs, coated MNs, dissolving MNs, and hollow MNs [65].

Copyright 2018 Elsevier. Representative images of b dissolving MNs [66], Copyright 2021 Wiley; c coated MNs [67], Copyright 2017 Elsevier; and d hydrogel MNs [52], Copyright 2020 Wiley

Table 1.

The summary of the years, advantages, disadvantages, and preparation methods of different types of MNs

Types of MNs	Years	Advantages	Disadvantages	Preparation methods	Refs
Solid MNs	1970s put forward 1990s rise	Excellent mechanical performance Strong puncture ability	Low security High toxicity Complex preparation process Serious matrix residue	Two-step etching method Free radical polymerization method Laser technology	[26, 36, 68–70]
Hollow MNs	1990s	High drug loading High drug delivery efficiency	Poor mechanical performance Complex preparation process	Solvent-casting method Microelectromechanical technology Laser technology	[38, 39, 41, 70, 71]
Dissolving MNs	At the beginning of the twenty-first century	Good biodegradability High security Low toxicity Wide range of use	Low drug loading	Solvent-casting method Three-step centrifugation method Droplet-born air-blowing method Two-layer centrifugation method	[72–79]
Hydrogel MNs	At the beginning of the twenty-first century	ISF extraction test Excellent swelling performance Adjustable mechanical strength Adjustable drug release rate	Narrow scope of application Complex matrix composition Insoluble matrix materials	Freeze–thaw cycle method Solvent-casting method	[51, 79–81]
Coated MNs	At the beginning of the twenty-first century	High drug delivery efficiency Excellent mechanical performance	Low drug loading Complex preparation process Low security	Wet etch process Dip coating method	[22, 59, 82–84]

MN-mediated treatment and diagnosis of superficial tumors

PDT and PTT

In recent years, to deal with the occurrence of various skin cancers, synergy strategy [85, 86], drug function modification strategy [87], spatiotemporal controllable drug release strategy [50, 73, 88], local targeted drug delivery strategy, [5, 89], and other methods have been developed. Among them, the local targeted therapy strategy has become a potential treatment mode due to its remarkable drug concentration and few side effects. In the treatment of skin tumors, the targeted delivery of photosensitizers into tumor cells will be an effective therapeutic strategy [90]. Under the irradiation of a light source with a specific wavelength, cytotoxic substances are produced, which act on the target tissue to kill pathological cells, induce microvascular injury, and stimulate the immune response, which is PDT [91–93]. It is worth noting that after photosensitizers take effect in the body, the target position is often accompanied by an increase in local temperature, which can destroy the tumor cell membrane and denature proteins and then induce tumor cell apoptosis. In general, this above process is called PTT, which is inseparable from the PDT [94]. In addition, heating can also increase blood flow at the tumor sites, and the rapid flow of blood drives the drug deep into the tumor to play a therapeutic role.

Conjugated phthalocyanine

Phthalocyanine is a near-infrared dye containing a phthalocyanine structure, which belongs to the second generation of photosensitizers. It is widely used due to its excellent photodynamic efficiency while reducing the photosensitivity of the skin. Tham et al. [95] combined phthalocyanine with a self-developed mesoporous nanocarrier. To maximize the therapeutic efficacy, MNs are combined to facilitate the penetration of the substance into the deeper tissues of the skin. The results showed that the penetration was 27.2% and 63.1% respectively for the pretreatment without MNs and the treatment with MNs.

Indocyanine green (ICG) is a special phthalocyanine dye that can generate reactive oxygen species after absorbing near-infrared energy, which can produce high cytotoxicity and destroy the cellular structure of tumors. On the other hand, the absorbed energy is further converted into heat energy, which increases the local temperature of the tumor to achieve the purpose of killing tumor cells [96]. To improve the stability of ICG, Pei et al. [97] developed a safe and effective delivery system to ensure the synergistic effect of simultaneous delivery of chemotherapy drugs and photothermal agents to the tumor sites. The dissolving MN patches based on PVP showed excellent skin penetration and the tips of the MN patch were dissolved by ISF to release DOX and ICG at the tumor sites, which reached a local high temperature of 50 °C within 2 min. At the same time, it was observed that the MNs were completely dissolved without residue.

Metal compound

The operation of irradiation often causes side effects such as itching, blisters, and dermatitis on the skin. The means of PTT for the treatment of tumors avoid the symptoms of extracellular leakage and dermatitis. Metals have good optical properties and strong cellular uptake. In terms of practical ability, metal complexes have more excellent structural plasticity. In these studies, the doping of metal compounds can enhance the photothermal conversion effect of polymer photosensitizers [98–102].

CuS is an ideal photosensitizer. Zhao et al. [94] used MNs as the delivery carrier of the photosensitizer CuS and the anticancer drug camptothecin (CPT), achieving the goal of intelligent time–space drug release. As shown in Fig. 3, the MNs coated with CuS showed excellent photothermal stability after repeated irradiation. Under the irradiation of the laser, the structure of MNs begins to be destroyed, thus releasing anticancer drugs. Apoptosis experiment showed that near-infrared (NIR) radiation was the main cause of cell apoptosis, which indicated that the delivery of MNs would not produce substances harmful to the body (Fig. 3d). The composition of dissolving MN matrix PVP and PVA also provided an argument for the safety of MNs. According to the in vivo photothermal effect evaluation experiment of B16 tumor-bearing mice, the existence of MNs and photosensitizers made the body temperature reached 50 °C within 1 min after irradiation, which indicated that the drug delivery channels produced by MNs were effective (Fig. 3e).

Fig. 3.

Schematic diagram of temperature change of MN PTT loaded with metal photosensitizers. a Temperature change curves of CPT-CuS-ZIF-8@HA, CPT-CuS-ZIF-8, CuS-ZIF-8, CuS, and ZIF-8 under laser irradiation (1.0 W/cm²) at different time points. b Temperature change curves of CPT-CuS-ZIF-8@HA at different concentrations under NIR irradiation. c Photothermal conversion stability curves of CuS and CPT-CuS-ZIF-8@HA for three cycles. d Schematic diagram of apoptosis analysis. e The thermographic images of blank DMN, CuS-ZIF-8@MN, and CPT-CuS-ZIF-8@HA@MN under NIR irradiation at different time points [94].

Copyright 2021 Elsevier

Moreira et al. [103] co-encapsulated DOX and AuSS nanorods in PVP MNs system for PTT of tumor-bearing mice. The research showed that the existence of AuSS nanorods can make the system produce higher temperatures within 5 min, which can enhance the sensitivity of cancer cells to chemotherapy drugs, and accelerate protein denaturation and cell membrane destruction. Among numerous photothermal materials, AuNCs have good chemical stability, a simple preparation process, and an excellent photothermal effect. On the one hand, the AuNCs can be taken up by cells within 10 min, and the cell vitality decreased by 50% after being irradiated by near-infrared. On the other hand, the existence of AuNCs can also significantly improve the mechanical strength of the MNs, which is mainly due to the enhanced aggregation of polymer materials by the photosensitizer [104].

Precursor photosensitizers

The direct delivery of photosensitizers required for PTT treatment of skin tumors by MNs is a very effective strategy. However, it is also affected by the stability and drug loading of the photosensitizers. Delivery of precursor photosensitizers would be a good solution. For example, 5-aminolevulinic acid is gradually converted to its metabolite protoporphyrin IX (PPIX) after delivery into the human body. This is a highly efficient photosensitizer, which can combine with oxygen under the irradiation of excitation light, resulting in a certain photochemical reaction, which eventually leads to cell death [105]. While promoting the activation of the pro-drug at the subcutaneous tumor site, it does not hinder the restoration of the anticancer properties of its parent drug [106, 107].

Gene therapy

At present, experts believe that gene therapy has great potential in the treatment of tumors or other inflammatory diseases, especially after the outbreak of COVID-19, which can be seen in the general acceptance of nucleic acid [108]. Gene transfection and gene silencing are the main means of gene intervention. Silencing the expression of mutant genes may become the key to curing SST.

However, the stability of nucleic acids cannot be ignored. The storage date, storage method, usage, dosage, etc., will become a thought-provoking problem in the large-scale production of nucleic acid-loaded pharmaceutical dosage forms. Now, the problems listed above will be solved by the advent of MNs. Nucleic acid stability is maintained by the dryness of the MN encapsulation process. MNs can coat various low molecular and macromolecular substances on the surface during the preparation process, which will facilitate self-administration in terms of clinical. In addition, in the case of loading unstable biomolecules or vaccines, the improvement of the stability of dry materials can reduce the requirements for cold chain storage, which will facilitate the use of genetic means and MNs for tumor treatment [83, 108].

Gene transfection

Gene transfection is defined as the process of delivering nucleic acid-containing biological functions into cells of living organisms and further maintaining their biological functions. P53DNA, siRNA, and other synthetic DNAs with tumor suppressor effects are commonly used nucleic acids that can intervene in the cell cycle of tumor cells and inhibit the growth, metastasis, and spread of cancer cells. Physical injection, electroporation, and ballistic delivery are common gene delivery methods. However, the gene transfection efficiency is limited due to factors such as DNA molecular weight, charge, and enzymatic degradation. At the same time, the above methods are usually accompanied by limitations such as pain, high price, and professional operation. The emergence of MNs successfully solves the safety problems of other viral vectors and effectively reduces the side effects caused by the carrier materials. Studies have shown that MNs for gene transfection can simultaneously improve DNA stability within solid matrices and increase DNA delivery. Compared with the PVP matrix, the PVA matrix can enhance the in vitro transfection ability of genes better [109, 110].

In the process of gene therapy, how to enhance the uptake of genetic material is a major problem for experts. Ionic polymers with opposite charges are a perfect solution [81]. Li et al. [23] and his team took advantage of the acidic skin environment to coat the pH-responsive polyelectrolyte multilayer membrane (PEM) on the surface of polycaprolactone (PCL) MNs for the delivery of p53 plasmids, thereby achieving rapid gene delivery freed. In vivo experiments showed that the tumor-inhibiting effect of the MN experimental group was about twice that of the injection group. Grace Cole’s research showed that the use of the freeze-drying method increased the loading of DNA nanoparticles in soluble MNs, and maintained long-term stability and ideal tumor inhibition effect while ensuring mechanical properties [111].

Gene silencing

In addition to delivering tumor suppressor genes into cells to inhibit tumor growth, RNA interference is also considered one of the effective ways to inhibit the growth cycle of tumor cells. To examine the efficiency of gene silencing exhibited by siRNA when it is delivered into the body employing MN carriers, the delivered siRNA is modified with cholesterol to better penetrate the stratum corneum of the skin and into the cytoplasm. The study of Deng et al. showed that GAPDH gene expression was reduced by 66% after MN injection [68]. Tang and his team members used MNs for siRNA delivery to achieve targeted enrichment of anticancer genes at xenograft sites. Combining with MNs allowed siRNA to have higher stability and better permeability. The team successfully achieved the silencing of the HPV-16-E6 oncogene, and at the same time, it did not show major adverse reactions and biological toxicity [112].

Because the main cause of melanoma is BRAF gene mutation, silencing the expression of this gene may become the key to curing the disease. At the same time, the gene silencing efficiency can be further evaluated by calculating the protein expression of the target gene and the relevant control group. Peptide nano-complexes can promote the transfection of target genes and accelerate the silencing of target genes, which makes MNs express more obvious advantages in the treatment of skin melanoma [84]. Due to the relatively simple preparation process of siRNA, it is expected to achieve large-scale industrial production in the future.

Chemotherapy

Chemotherapy combined with different medical technologies to treat tumors has become an important means of clinical cancer treatment. Though chemotherapy methods have great toxicity and adverse reactions, for now, the status of chemotherapy methods will not be replaced by other methods in the next few years or even decades [113].

Chemotherapy combined with nanotechnology

Nanocarriers including liposomes are widely used due to their excellent drug delivery properties and low adverse drug reactions in recent years [114]. A growing number of studies have found that single-agent chemotherapy shows more limitations for intractable tumor diseases. For this problem, Ahmed et al. [115] used MNs for the delivery of the dual anticancer drug DOX and celecoxib, which showed that the drug delivery method of the MN carrier could make the anticancer drug penetrate the skin more effectively and have stronger cytotoxicity than the common type of drug delivery method.

The incorporation of nanotechnology and novel drug formulations has led to further advances in MNs in the treatment of oncological diseases. This conclusion is supported by Abdelghany and members of his team. They gave a systematic review of the results comparing a single chemotherapeutic drug with curcumin (Cur) MNs with nanosuspension technology. In vitro studies showed that the use of MNs greatly enhances the intradermal penetration of Cur. The delivered chemotherapeutics penetrated the skin up to 5 times the depth of MN insertion. Meanwhile, as shown in Fig. 4a, Cur-loaded soluble HA MNs combined with micelles were also successfully prepared. Experiments showed that the MNs released more than 70% of the drug within 6 h, and could be completely dissolved in about 2 min after insertion (Fig. 4b, c). Cur-loaded micelles enabled targeted aggregation at tumor sites, resulting in drug accumulation [75, 87].

Fig. 4.

Preparation process and in vitro release studies. a Principle of preparation of Cur-loaded soluble HA MNs. b Dissolution after 45 s, 90 s, and 125 s of insertion into pigskin. c In vitro drug release profiles of Cur-loaded micellar MNs [87].

Copyright 2020 Springer

Mojeiko et al. [116] also tried to use a combination of MNs and microemulsions to explore innovative ways to treat breast cancer, and to use the MN carriers to assist the penetration of the drug-loaded microemulsions. The higher frequency of use of the MN roller and the shorter MN length innovatively reduced moisture loss and damage to the skin barrier. Lan et al. [117] reported the MN-mediated targeted delivery of the first-line anticancer drug DDP. Under the delivery effect of the soluble MNs, the inhibition degree of DDP on head and neck cancer tumor cells reached 58.6%. In addition, no hepatotoxicity, pulmonary toxicity, and nephrotoxicity were detected except in tumor tissue.

Chemotherapy combined with osmotic solubilized technology

The researchers evaluated the potential of hydrogel MNs to deliver anticancer drugs to prove the feasibility of MNs combined with osmotic enhancement technology for the treatment of SST. Drug accumulation in skin, plasma, and visceral tissues was observed within 1 week of anticancer drugs delivered using MNs [79]. Huang et al. [118] encapsulated DOX and trametinib into self-prepared dextran methacrylate hydrogel MNs, which could penetrate 600 μm into the skin for drug release. Compared with the injection group and the single-administration group, the dual-delivery MN group showed a more outstanding melanoma inhibitory effect. The presence of MNs and some chemical penetration enhancers can enhance the delivery of targeted drugs. Under the action of MNs and oleic acid, a chemical permeation enhancer, anticancer drugs, and honokiol could increase drug delivery by 3 times and 27 times respectively [119]. One of the main goals of curing BCC is to enhance the permeability of specific drugs for the treatment of BCC, such as imiquimod. Sabri et al. [120] developed imiquimod-loaded polyvinylpyrrolidone-vinyl acetate (PVPVA) MNs. The lesion site of the disease is mainly located at about 400 μm below the skin surface, which makes it possible to realize the delivery of BCC-targeted drugs with MNs. Sabri et al. used MN device to evaluate the permeability of imiquimod cream, which showed that only a small amount of effective penetration occurred when a simple cream was applied to pig skin. Most of the drug was recovered during subsequent skin cleansing. After pretreatment with MNs, the amount of imiquimod delivered through the stratum corneum barrier was about 3 times that of the ointment, and the skin permeability was significantly improved [69, 121, 122]. In addition, Naguib et al. [123] demonstrated a 4.5-fold increase in 5-Fu passing through the skin when the skin was pretreated with MNs.

Immunotherapy

From a physiological point of view, making good use of the immune ability of patients when they are sick is a breakthrough point of many experts’ research. At present, the combination of MNs and immunotherapy has successfully delivered some therapeutic agents required for tumor treatment into human cancer cells, such as monoclonal antibodies, vaccines, and immune checkpoint inhibitors [79, 124, 125].

The delivery of immune signaling molecules

It is well known that the growth rate of tumors has a great relationship with the environment and location. Generally speaking, the environment in which the tumor is located will have a great impact on the growth of breast cancer tumor cells, and even the environment in which the tumor is located can be reshaped. As a specific immune signaling molecule, PD-L1, when combined with PD-1 on lymphocytes, allows tumor cells to escape immune in vivo [126, 127]. Thus, experts have developed a new idea to deliver a large amount of anti-PD-1 (aPD-1) antibodies in vitro into the tumor tissue microenvironment to competitively bind to PD-1, thereby reducing the chance of tumor cell immune escape. Lan et al. [76] successfully developed MN arrays loaded with pH-responsive tumor-targeting lipid NPs. It can precisely deliver aPD-1 and DDP to cancer foci. To improve the efficiency of aPD-1 delivery, a physiologically responsive MN delivery device was simultaneously developed for the treatment of cutaneous melanoma. Wang et al. [78] reported an innovative self-degradable MN patch that continuously transported aPD-1 in a physiologically controlled way. In this system, MNs encapsulated aPD-1 and glucose oxidase (GOx), which converts blood sugar to gluconic acid. The production of the acidic environment promoted the self-dissociation of NPs, leading to the release of aPD-1. It was found that a single administration of the MN patch induced a strong immune response in a B₁₆F₁₀ mouse melanoma model compared to MNs without GOx or intratumoral injection of free aPD-1 with the same dose, which demonstrated that the MN patch-assisted system improved cancer immunogenicity significantly.

MN-assisted delivery of DNA vaccines [89, 128, 129], tumor cell lysates [130, 131], immune adjuvants required for immunotherapy, or immune checkpoint inhibitors [71, 132] has also been developed for the treatment of superficial tumors. Subunit vaccines are an ideal choice for immunotherapy due to their good biosafety. However, there is still a common problem of low immunogenicity. Auxiliary delivery of immune adjuvants may be a logical solution to the problem. Many researchers used soluble MNs as the delivery carriers of ovalbumin (OVA). Duong et al. [133] proposed a dissolvable MN mixture for triggering vaccine and adjuvant release in skin tissue, which elicited stronger antigen-specific antibody responses than subcutaneous administration. Dissolving MN cocktail enhances antibody recall memory after challenge compared to traditional vaccination. Compared with intramuscular injection, immune signaling molecules delivered through the hollow MN array elicited significantly higher antibody responses and higher numbers of interferon-secreting lymphocytes [39, 74, 134, 135]. As shown in Fig. 5, the experimental group of receptor agonists (R848) delivered by MNs showed better immune effects than the control group and the subcutaneous injection group, specifically in the increase of the number of antibodies, immune factors, and immune cells and the decrease of tumors size. Most importantly, the insertion and dissolution of MNs were highly safe and low toxic, which could be observed from the safety evaluation of the experiment [135].

Fig. 5.

Effectiveness and safety evaluation after treatment with OVA/R848 MNs. a Effectiveness evaluation: in vivo immune response after treatment with OVA/R848 MNs. The schematic diagrams are respectively the cumulative release of OVA, the expression of IgG specifically generated by OVA after MNs treatment, and the percentage of CD8⁺ T cells specifically generated by OVA in the spleen. n = 5 (**P < 0.01, ****P < 0.0001). b Effectiveness evaluation: cumulative release of R848, local distribution of photothermal effect in vivo, and the change of tumor size after implantation of MNs. c Safety evaluation: dissolution of MNs, H&E staining of tumors, and important organs of mice treated with OVA and R848. Scale bar = 100 μm [135].

Copyright 2018 American Chemical Society

Immune monitoring and diagnosis

In addition to using immunotherapy to treat tumors, pro-drug strategy and nano-drug delivery strategy have also been proved to be ideal treatment methods in the early diagnosis and treatment of tumors [136–139]. However, it is undoubtedly more beneficial for patients to be able to accurately diagnose corresponding diseases through specific indicators at the early stage of tumor onset. Traditional tumor detection and diagnosis mainly rely on the identification of tumor biomarkers, which usually come from patients’ blood, urine, and other biological samples [140]. Taking melanoma as an example, the biomarkers of melanoma include P16, BRAF V600E, S100B, NRAS, and ALK. The detection of these markers can effectively diagnose melanoma [70]. However, the routine means of extracting biological samples not only brings serious physical trauma but also requires corresponding histological treatment and nursing of patients [141, 142]. Compared with traditional biomarker detection methods, the MN monitoring and diagnosis devices have the advantages of simplicity, rapidity, and strong patient compliance [143], which is meaningful for patients.

Currently, Yang et al. [144] introduced a hydrogel MN patch for rapid and easy capture of Epstein-Barr virus cell-free DNA from ISF in situ around 15 min, with a maximum capture efficiency of 93.6%. Dervisevic et al. [143] fabricated an electrochemical transdermal immunesensor using high-density silicon MN arrays, which achieved the simultaneous extraction and detection of breast cancer biomarker ErbB₂ successfully. The Si-MN-based immunosensor with an Au coating indicated excellent skin model penetration and good analytical performance in the extraction and electrochemical quantification of ErbB₂ biomarkers. Chen et al. [145] achieved a blood-free sample process by inserting MNs into the animal detection area. The MN-based assay could detect the occurrence of breast cancer within 7 days. For comparison, the classical blood test needed 14 days to validate the occurrence of breast cancer.

In addition to monitoring and diagnosis, prevention is also an indispensable step in the process of tumor discovery and treatment. How to deliver the vaccine to the tumor site becomes the key to solving the problem. The degradable MNs can deliver the composition into the tumor tissue site, to carry out transdermal vaccine inoculation, achieve significant inhibition of the growth and proliferation of mouse melanoma, and further prevent the occurrence of tumors [146].

Other methods used to treat tumors

The growth, metastasis, and spread of tumors require more energy and glucose than normal tissues. Depleting the glucose and energy near the tumor tissue will become one of the very potential tumor treatment methods. Therefore, a method called “starvation therapy” come into being. Glucose oxidase can specifically oxidize glucose and reduce the energy and oxygen near the tumor tissues. Therefore, it has been widely used in the “starvation therapy” of various tumor diseases. Glucose oxidase has poor stability and is easily enzymatically hydrolyzed. When it was encapsulated in dissolving MNs, drug release could be achieved through the dissolution of MN carriers in vivo, which could effectively enhance the stability of the enzyme. Therefore, the leakage and safety risks associated with local intratumoral injections are avoided [72]. The application of MNs ensured the continuous delivery of glucose oxidase to achieve accumulation at the tumor site [147].

However, despite the innovativeness of this treatment method, due to the complexity and intractability of tumor diseases, it is difficult to cure tumors only by a single starvation therapy. Moreover, in the long-term application of this method for tumor treatment, adverse reactions such as hypoglycemia will inevitably occur. Therefore, today’s research trend is increasingly favoring starvation therapy as a topical adjuvant therapy to assist other treatments such as PTT and immunotherapy.

Synergistic approach to tumors therapy

A single skin tumor treatment approach exhibits various limitations under the test of clinical translation. Combining two or more treatments can have a positive effect, improving treatment efficiency while greatly reducing side effects.

Chemotherapy is the most basic and common cancer treatment. At the same time, PTT is relatively in-depth research in recent years, and it is also the most potential tumor treatment method. When the chemotherapeutic drugs are delivered to the skin surface by various types of MNs, the infrared photothermal agents required for PTT are delivered simultaneously. After being irradiated by infrared lasers, these materials will produce a certain photothermal effect, which synergizes with the chemotherapeutic effects of the abovementioned chemotherapeutic drugs to destroy tumors [73, 99, 104]. Sun et al. [77] co-encapsulated PTX and excellent photosensitizer IR780 in self-developed self-assembled nanomicelle soluble MNs for photothermal-chemotherapy synergistic treatment of melanoma. After the MNs are dissolved in the skin, the self-assembled nanomicelle can deliver the first-line anticancer drug PTX to the deep part of the tumor to the greatest extent. Compared with the injection method, the delivery of MNs enabled the drug to accumulate at the tumor site without reservation and effectively achieve a cure rate of more than 80% while ensuring certain biocompatibility. Chen et al. [148] used MNs for the co-delivery of lanthanum hexaboride and DOX. When the local tumor was exposed to near-infrared light, lanthanum hexaboride could quickly induce a thermal ablation area, the local temperature could reach 50 °C, and DOX could be released in a large range while heating, thereby further inducing tumor cells to ablate die.

The purpose of the combination of gene therapy and PTT is to deliver some target genes that can produce anti-cancer effects together with photothermal agents into human tumor cells. Xu et al. [149] chose MNs to co-deliver p53 DNA and ICG derivative IR820 into skin cells for the treatment of human oral epidermoid carcinoma. The study found that the local temperature of the part irradiated by near-infrared light increased by about 15 °C.

The combination of PTT and immunotherapy can inhibit the growth and spread of tumors and has no significant effect on the safety of experimental animals. The MN system loaded with immunologic agents and photosensitizers is expected to show great therapeutic potential in clinical translation [85, 86, 150]. He et al. [24] delivered semiconducting polymer nanoparticles co-modified with poly and immune adjuvant PIC HA to the tissue site of mouse melanoma tumors through the soluble MNs. Semiconductor polymer NPs have good photothermal conversion efficiency. The existence of dissolving MNs enables the NPs to have higher efficacy of PTT while reducing the toxic and side effects of delivery. As shown in Fig. 6, the experimental results showed that the immune signal molecules and photosensitizers delivered by MNs could significantly increase the local temperature of tumors and the number of immune cells. The synergistic therapy based on the MN delivery platform has a significant tumor-suppressive effect, providing a potential tumor treatment modality in the treatment of SST.

Fig. 6.

The immunity study of tumors in vivo. a ICD induced by ICG with irradiation in vitro. Percentage of CD86CD80 DCs after MN treatments (means ± SD, n = 3). b Infrared thermographic maps and temperature elevation profile of MN-treated mice under laser irradiation (808 nm, 0.35 W/cm²). c Schematic illustration of a bilateral B16 tumor model and the administration schedule. Tumor volume after being treated with MNs [86].

Copyright 2020 American Chemical Society

In conclusion, we summarized the study of MNs combined with multiple therapeutic strategies against SST in this paper, which was expected to provide insights into the development of more potential therapeutic platforms for targeting superficial tumors. Different types of MNs, characteristics of MNs, matrixes, the loaded drugs, the diseases treated with MNs, and the treatment methods in cancer therapy are summarized in Table 2.

Table 2.

Different types of MNs, characteristics of MNs, matrixes, the loaded drugs, the diseases treated with MNs, and the treatment methods in cancer therapy

MN types	Characteristics	Matrixes	The loaded drugs	Diseases	Treatment methods	Refs
Solid MNs	High mechanical strength Low biosafety Low biocompatibility	Stainless steel	Dabrafenib, trametinib	Melanoma	PTT	[95]
		Silicon	Cholesterol-modified housekeeping gene (Gapdh) siRNA	Genetic disease	Gene therapy	[68]
		Oscillating MN device dermapen	Imiquimod	BCC	Chemotherapy	[69]
		HA, stainless steel	Itraconazole	BCC	Chemotherapy	[122]
Dissolving MNs	Excellent biocompatibility Excellent biosafety Release the loaded drug rapidly Wide range of applications Simple preparation process	HA	5-Fu; ICG	Human epidermoid cancer, Melanoma	PTT Chemotherapy	[96]
		PVP K30: PVA 103 = 6: 1, PVP K90	CuS NPs, CPT	Melanoma	PTT; Chemotherapy	[94]
		HA	5-ALA	Skin lesions	PTT	[105]
		PVP	Cationic delivery peptide (RALA), DNA	Immunogenic disease	Gene therapy	[109]
		PVA	RALA/pDNA NPs	Cervical Cancer	Gene therapy	[111]
		PVA	CU-NS	Skin cancer	Chemotherapy	[75]
		HA	Cur	Melanoma	Chemotherapy	[87]
		Sodium carboxymethylcellulose (SCMC)	LCC-NPs	Head and neck cancer	Chemotherapy	[117]
		Maltose	Honokiol, Oleic acid	Breast cancer	Chemotherapy	[119]
		PVP	Imiquimod	BCC	Chemotherapy	[120]
		PVP	aPD-1, DDP	Oral squamous cell carcinoma	Chemotherapy; Immunotherapy	[76]
		PVA	RALA/pDNA NPs	CRPC	Immunotherapy	[128]
		CS: NMP = 2 g/80 ml	Polymeric nanocomplex of paclitaxel-encapsulated sulfobutylether-β-cyclodextrin (SBE)/mannosylated N,N,N-trimethylchitosan (mTMC)/DNA	Melanoma	Immunotherapy	[89]
		P127: PEG = 7: 3	Toll-like receptor 7/8 agonist (R848), ovalbumin protein	EG-7 lymphoma	Immunotherapy	[135]
		HA	Ovalbumin protein	EG-7 lymphoma	Immunotherapy	[74]
		HA: PVP K30 = 15%: 40%	GOx	Melanoma	Starvation therapy	[72]
		PVP K30: PVA 103 = 1: 4, PVP K90	GOx, CAT, ICG	Melanoma	Starvation therapy	[147]
		PVP K90	PTX, IR780	Melanoma	PTT Chemotherapy	[73]
		HA	AuNC, DOX	Melanoma	PTT Chemotherapy	[104]
		HA	Chloroquine, CQ, IR780	Melanoma	PTT Immunotherapy	[150]
		PVA: PVP = 2: 1	Lanthanum hexaboride, DOX	Breast Cancer	PTT Chemotherapy	[148]
		HA	p53 DNA, IR820	Human oral epidermoid carcinoma	PTT Gene therapy	[149]
		PVP K90	Poly(cyclopentadithiophene-alt-benzothiadiazole), immune adjuvant polyinosinic–polycytidylic acid	Melanoma	PTT Immunotherapy	[24]
Coated MNs	High transdermal efficiency Prepare the coating solution separately Complex preparation process and mold The coating tends to become uneven	PCL	p53 DNA, PEI	Human oral epidermoid carcinoma	Gene therapy	[82]
		PCL	Dimethylmaleic anhydride-modified polylysine (PLL-DMA), p53 DNA	Human oral epidermoid carcinoma	Gene therapy	[23]
		Stainless steel	BRAF siRNA	Melanoma	Gene therapy	[84]
		Polyethylene glycol (PEG)	Gold nanorod, DOC	Human oral epidermoid carcinoma	PTT Chemotherapy	[99]
Hydrogel MNs	Controlled drug release Excellent biocompatibility Excellent biosafety High risk of drug leakage	GelMA	pDNA, poly(β-aminoester) (PBAE) NPs	Tissue regeneration and cancer therapy	Gene therapy	[81]
		PVA: PEG = 20%: 7.5%	Bevacizumab	Lymphomas Secondary metastatic tumors	Chemotherapy	[79]
		DexMA	DOX, Tra	Melanoma	Chemotherapy	[118]
Biodegradable MNs	Excellent biodegradability Low toxicity	HA	aPD1, GOx	Melanoma	Immunotherapy	[78]
		HA	Cytotoxic T-cell epitope peptide	Melanoma	Immunotherapy	[146]
Core–shell structure MNs	Complex preparation process High drug loading relatively	Oligomeric sodium hyaluronate, PVP: HA = 31.25%: 33%; 40%; 50%	ICG, aPD-L1	Melanoma	PTT Immunotherapy	[85]
		PVP: PVA = 25%: 15%	1-Methyl-tryptophan, ICG, indoleamine 2,3-dioxygenase (IDO)	Melanoma	PTT Immunotherapy	[86]
Injectable MNs	Complex use process	Stainless steel	Cholesterol-modified housekeeping gene (Gapdh) siRNA	Cervical Cancer	Gene therapy	[112]
Derma roller® MNs	High market transformation efficiency	HA	DOX, CEL	Melanoma	Chemotherapy	[115]

Current status of clinical research mediated by minimally invasive MNs

In recent years, several clinical trials on the efficacy and safety of MNs have been published. MNs have been used to treat skin diseases [151, 152], vaccines, or insulin delivery [153, 154], which all showed similar or more effective results than controls. In other specific studies, MN-mediated drug delivery was more effective than drug application alone or placebo MNs. In the treatment of some non-tumor diseases, MNs are relatively safe, causing only mild adverse reactions. MNs have advantages over traditional therapies, such as requiring no specialized training to implement, reducing costs through the use of fewer vaccines, and avoiding needle phobia [155]. However, we are infrequent in the treatment of any tumors or superficial tumors. We suspect that it is related to the severity and complexity of the tumor diseases. For now, many indicators have not been adequately compared between groups, and further research is needed to be concluded.

Conclusion and outlook

In short, tumors that develop on the body^’s surface can try to use MNs as an ideal drug delivery system. MNs can form unobstructed drug release channels while interacting with the skin so that the loaded drugs or other adjuvant therapy components can penetrate deep into tumor cells. At present, experts’ researches on MNs provide a clear idea for the clinical treatment of skin cancer. We reviewed the current research progress of many minimally invasive MN-mediated tumor therapies, including PTT, gene therapy, immunotherapy, and starvation therapy. Their developments hold promise for the treatment of superficial tumors and have shown high potential for clinical conversion. The synergistic use of multiple methods also holds great promise for the treatment of patients. However, despite the latest exciting outcomes from the minimally invasive MNs for superficial tumor diseases therapy, several challenges and limitations remained that need to be developed before further clinical translations. First of all, the drug loading of MNs still has a small killing effect on tumor cells, especially for patients with advanced cancer. In addition, even if the MNs achieve the ideal drug loading, due to the limitations of the use of the MNs, it will be difficult to use the MNs to treat some internal tumors after the occurrence of the tumors. Although there have been reports of biodegradable MNs applied between teeth and gingival tissue to treat periodontitis, and reports of MN patches applied to cardiac stromal cells [80, 156], there are also many difficulties in realizing the real clinical translation of these theoretically ideal methods.

To solve these problems and meet the special needs of tumor therapy, more methods and strategies should be explored to improve the drug load of MNs, including the combination of new nanomaterials, chemical modification of drug molecules, and so on. In addition, more MN-mediated treatments for superficial tumors should be widely developed and their corresponding therapeutic mechanisms should be elucidated to realize multi-modal treatments against these diseases. In addition, to maintain the inherent properties of the MNs and pay attention to the biosafety of the material itself, more materials should be developed. Last but not least, there is also an urgent need to develop some minimally invasive in vivo transplantation treatments and apply this system to the treatments of more diseases, which will be the direction of our lives to strive for.

Abbreviations

MNs

Microneedles

SST

Superficial skin tumors

SC

Stratum corneum

PTT

Photodynamic therapy

BCC

Basal cell carcinoma

LHRHa

Luteinizing hormone-releasing hormone analogs

NPs

Nanoparticles

DLP

Digital light processing

DOX

Doxorubicin

DOC

Docetaxel

PDT

Photodynamic therapy

MAPK

MAP kinase

ICG

Indocyanine green

PTX

Paclitaxel

MSN

Mesoporous silica nanoparticles

5-Fu

5-Fluorouracil

CaO₂@Mn-PDA NFs

Calcium peroxide-polydopamine MN nanoformulation

PPIX

Protoporphyrin IX

PEM

Polyelectrolyte multilayer membrane

PCL

Polycaprolactone

NIR

Near-infrared

PVPVA

Polyvinylpyrrolidone-vinyl acetate

aPD-1

Anti-PD-1

OVA

Ovalbumin

NK cells

Natural killer cells

HA

Polyglycolic acid

CPT

Camptothecin

5-ALA

5-Aminolevulinic acid

CU-NS

Curcumin nanosuspension

Cur

Curcumin

LCC-NPs

Lipid-coated cisplatin nanoparticles

CS

Chitosan

NMP

N-Methylpyrrolidone

GOx

Glucose oxidase

PVP

Polyvinylpyrrolidone

CAT

Catalase

AuNC

Gold nanocage

CRPC

Castrate resistant prostate cancer

p53 DNA

p53 expression plasmid

PEI

Polyethylenimine

pDNA

Plasmid DNA

GelMA

Gelatin methacryloyl

DexMA

Dextran methacrylate

Tra

Trametinib

CEL

Celecoxib

Author contribution

Meng Wang: investigation, conceptualization, writing—original draft, writing—review and editing. Xiaodan Li: writing—review and editing. Wenzhen Du: writing—review and editing. Minge Sun: writing—review and editing. Guixia Ling: conceptualization, supervision. Peng Zhang: supervision.

Availability of data and materials

Not applicable.

Declarations

Ethics approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Conflict of interest

The authors declare no competing interests.