Phototriggered structures: Latest advances in biomedical applications

Abstract

Non-invasive control of the drug molecules accessibility is a key issue in improving diagnostic and therapeutic procedures. Some studies have explored the spatiotemporal control by light as a peripheral stimulus. Phototriggered drug delivery systems (PTDDSs) have received interest in the past decade among biological researchers due to their capability the control drug release. To this end, a wide range of phototrigger molecular structures participated in the DDSs to serve additional efficiency and a high-conversion release of active fragments under light irradiation. Up to now, several categories of PTDDSs have been extended to upgrade the performance of controlled delivery of therapeutic agents based on well-known phototrigger molecular structures like o-nitrobenzyl, coumarinyl, anthracenyl, quinolinyl, o-hydroxycinnamate and hydroxyphenacyl, where either of one endows an exclusive feature and distinct mechanistic approach. This review conveys the design, photochemical properties and essential mechanism of the most important phototriggered structures for the release of single and dual (similar or different) active molecules that have the ability to quickly reason of the large variety of dynamic biological phenomena for biomedical applications like photo-regulated drug release, synergistic outcomes, real-time monitoring, and biocompatibility potential.

Graphical abstract

Here, the design, photochemical properties and essential mechanism of the most important phototriggered structures for the release of single and dual (similar or different) active molecules have been discussed.

1. Introduction

Over the past decade, special and great regard has been paid to the combination of the stimuli-triggered molecules with active biomaterial owing to multiple advantages in human healthcare such as controlled delivery of diagnostic, regulated drug delivery, therapeutic and pharmaceutical factors1, 2, 3, 4, 5, 6. The triggered molecules are defined as the units, which can change their structure due to isomerization, dimerization or bond cleavage in response to an external stimulus for the release of active molecules7, 8, 9, 10, 11, 12, 13. Living organisms in nature are rich in examples that can reversibly regulate their configuration and properties in response to environmental stimuli. Heat-shock transformation in bacteria¹⁴, camouflage in chameleons¹⁵ and color changes in echinoderms in response to light¹⁶ are wide ranging instances for this. Such triggered materials with the ability of responding to stimulus are considered to be an important class of advanced materials that can be utilized in biomedical applications and nanomedicine, particularly in the development of stimuli-triggered drug delivery systems (DDSs)17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28.

So far, several external stimuli have been developed, such as light29, 30, 31, 32, 33, 34, 35, temperature36, 37, 38, magnetism39, 40, 41, 42, ultrasound^43,44 and electricity⁴⁵. Among the mentioned external stimuli, light stimulation has attracted exceptional attention due to its ease of application without chemical contaminants, high spatial resolution, noninvasive nature and exact temporal and spatial control^29,46, 47, 48, 49, 50, 51. Wider ranges of the light wavelengths from ultraviolet (UV, λ_max = 200–400 nm) to visible (Vis, λ_max = 400–750 nm) or near-infrared (NIR, λ_max = 750–2000 nm) can be employed to trigger photo-sensitivity. Compared to UV and visible light, NIR light has less photo-toxicity, better tissues penetration depth and reduced background signal for biological applications. However, the application of NIR light is restricted due to its long wavelength, which has not enough energy to disrupt of chemical structures through bond-breaking or conformation-switching to triggered DDSs52, 53, 54, 55. Recently, this issue solved by two-photon actuation (Section 3.1.1) in the range of 650–900 nm or upconversion NPs (UCNPs) technologies⁵⁶. In comparison, UV light is a somewhat inferior nominee due to its toxicity under prolonged treatment and poor tissue penetration capacities (around 10 mm) due to light scattering and absorbance by intrinsic biological chromophores^57,58. However, attempts have been provided to address these restrictions by a micro-light (MLight) source that can be implanted locally inside the human body^59,60. In contrast, visible light can lead minor damage than UV light in vivid systems that has been recommended as an alternative to phototriggered DDSs (PTDDSs)⁶¹.

Phototriggers, also known as photo-removable protecting groups (PRPG), undergo an irreversible dissociation by selectively breaking a chemical bond can release leaving groups (LG) as bioactive molecules at specific time under light irradiation (Fig. 1)⁶². Therefore, a PTDDS can distribute a bioactive agent upon a specific wavelength light instantly at the preferred place in instant to attain a focused high value of drug while reducing generally injected dosage level and total poisonous effects as a result of their non-invasiveness and spatiotemporal accuracy⁶³. This proves greatly assure for drugs with adverse toxicity and side-effects or for targeted therapy efficiency of them. With increasing applications of light-responsive DDSs particularly in biomedical applications, new improvements of phototriggers are needed to fulfill the requirements for better sensitivity, low toxicity, structural simplicity, desirable solubility in the targeted media, faster kinetics, effective fluorescence embodiment and an adjustable and strong absorption spectrum above 300 nm. So far, several phototriggers such as o-nitrobenzyl (ONB)^64,65, coumarin^66,67, p-hydroxylphenacyl (pHP)^68,69, hydroxycinnmate^70,71, benzoin (Bnz)72, 73, 74, nitroindoline (NI)75, 76, 77 and 8-bromo-7-hydroxylquinoline (BHQ)^78,79 have been applied in DDS during the past decade.

Figure 1.

The general representative of phototriggered molecules.

To date, to further improvement the therapeutic results and reduce side effects, integration of nanotechnology with phototriggered molecules has opened new horizons for synergistic therapies²⁵. Nano-platforms such as polymeric nanoparticles (NPs), liposomes, metal–organic cages and metallic NPs have provided a complementary means for delivery of therapeutic agents into diseased cells and tissues using safe and effective directions80, 81, 82, 83, 84, 85. In this review, we will focus on latest developments of conventional phototriggers to provide more effective therapies against serious illnesses with reduced the damage to healthy cells and open new perspective toward favorable bioavailability and drug delivery efficiency.

2. Phototriggered drug delivery systems (PTDDSs)

PTDDSs facilitate the release of LGs such as therapeutic agents or drug cargos through different mechanisms including bond cleavage86, 87, 88, 89, isomerization90, 91, 92, photo-oxidation^93,94, photo-reduction^95,96, cross-linking^97,98 and photocaging/uncaging^99,100, by light irradiation. These transformations can be caused disruption and dissociation of extant structures or even altering the lower critical solution temperature (LCST) transition^8,101,102. Here, we discuss important latest advances on phototriggered molecules together with their essential mechanisms and their conditions for stimulation based on two types of light-induced drug release. One is the direct release of the drug as a LG by photochemical bond cleavage of p-hydroxyphenacyl (pHP), o-hydroxycinnamate, tetraphenylethylene (TPE), and coumarin substituted and the other is the photoinduced disruption of nanoscale structures such as micelles and MOCs having nitroaryl derivatives, thioketal, maleimide-anthracene, spiropyranes (SP) and azobenzenes (Azo) moiety to release the drug cargo. In addition, the dual-releasing phototriggers as a novel combination therapy with the ability to release two anticancer drugs are discussed at the end of review (Fig. 2).

Figure 2.

Schematic illustration of PTDDSs.

3. PTDDSs based on photochemical bond cleavage

The photochemical bond cleavage strategy assists intramolecular self-immolation to the release of the bioactive molecules such as enzymes, neurotransmitters, cell-signaling molecules, fluorophores, fragrances and drugs on a particular site with exact control of their dosages using light irradiation length^86,103. The pHP, o-hydroxycinnamate, nitroaryl, TPE, and coumarin derivatives are good examples for these purposes, which noticed as a critical step for biomedical applications of phototriggers.

3.1. p-Hydroxyphenacyl

The pHP groups are good phototrigger example for the study of very fast biological procedures with high photochemical quantum yield104, 105, 106, however, due to non-fluorescent behavior and excitation wavelength below 400 nm, pHP group receives less attention as a delivery agent¹⁰⁷. To overcome these issues, Barman et al.¹⁰⁸ have incorporated 2-(2′-hydroxyphenyl)benzothiazole (HBT) moiety to pHP derivative toward the design a photo-induced DDS, entitled p-hydroxyphenacyl-benzothiazole-chlorambucil (pHP-Benz-Cbl), see Scheme 1.

Scheme 1.

Synthesis of the pHP-Benz-Cbl and its potential photorelease mechanism.

First, the salicylaldehyde 1 transformed to derivative 3 using Friedel–Crafts acylation and treated with the anticancer drug chlorambucil (Cbl) to afford 4. Afterward, the treatment of 4 with 2-aminothiophenol 5 yielded the pHP-Benz-Cbl as excited-state intramolecular proton transfer (ESIPT)-assisted phototrigger for the very fast photorelease of Cbl inside the cell (15 min) using the visible wavelength (≥410 nm). Herein, the pHP group integration on the HBT caused a distinct fluorescence discolor from green to blue after photorelease and assisted in the deprotonation of pHP segment to accelerate the release process via ESIPT. Upon visible light irradiation, pHP-Benz-Cbl (with an intense green-emission band) excites to the singlet state undertakes a fast ESIPT, where a proton translocation occurs from the pHP to the benzo-thiazole segment, producing 7 and subsequently zwitterionic 8. The intermediate 8 exceeds during proficient intersystem crossing (ISC) to triplet excited state that converts to a supposed spirodiketone 10 with the parallel release of the Cbl accompanied by a photo-Favorskii rearrangement. Finally, the hydrolytic ring opening of spirodiketone 10 yielded pHP-Benz-COOH 11 with an intense blue-emission band owing to conjugation disconnection from a phenolic hydroxy functional group to a carbonyl moiety. This phenomenon is obvious in emission spectrum (Fig. 3a) during 15 min. Fig. 3b‒d, show the real-time monitoring of Cbl release by pHP-Benz-Cbl within malignant neoplastic disease tissue (MDA-MB 231). At first, the tissues have green fluorescence because of pHP-Benz-Cbl sorption (Fig. 3b, 0 min). After 10 min visible light irradiation with λ_max = 410 nm, two different green and blue fluorescence detected, signifying the Cbl incomplete release (Fig. 3c). In conclusion, after 15 min discolor from green to blue, suggesting a complete photorelease of Cbl drug and highest toxicity level (above 90%) toward cancer tissues (Fig. 3d).

Figure 3.

(a) Increase of emission spectra of pHP-Benz-Cbl during 0–15 min, and real-time monitoring of the Cbl liberation from pHP-Benz-Cbl during visible-light irradiation (b) 0 min, (c) 10 min and (d) 15 min by confocal microscopy. Reprinted with permission from Ref. 108. Copyright@2016 John Wiley & Sons, Inc.

Singh et al.¹⁰⁹ recently developed pHP based DDS with an excellent uncaging capacity in the area of 700 nm. Aggregation-induced-emission (AIE) chromophores have gained immense interest in biomedical purposes due to their unique possessions including exceptional photostability, excellent luminescence and biocompatibility110, 111, 112. In this way, they incorporated the naphthalene group into the pHP moiety so that a strong internal charge transfer occurred, causing in a red-shift in absorption bond and improved two-photon uncaging cross-section (Scheme 2). The prepared photo-induced DDS, pHP-Naph-Cbl exhibits exceptional properties like two-photon absorption in the phototherapeutic window (700 nm), high real-time monitoring ability due to a specific fluorescence color change from greenish-yellow to blue through the Cbl release and demonstrates AIE behavior, therefore remarkably releases the Cbl in the aggregated state with distinct fluorescence discolor.

Scheme 2.

Two-photon uncaging of pHP-Naph-Cbl.

3.1.1. Two-photon excitation uncaging

The two-photon excitation (2 PE) uncaging is a growing alternative method to evade the photo-toxicity of UV light and enhance spatial resolution via three-dimensional (3D) imaging in various fields, especially in cell biology113, 114, 115, 116, 117, 118. This non-linear absorption occurrence can excite a molecule by simultaneous absorption of two photons with about half-energy instead of one, which doubles the corresponding irradiation wavelength compared with conventional one-photon excitation (1 PE). The phototrigger subsequently becomes red or NIR light, which their wavelengths can penetrate deeper into tumor cells and decrease the photodamage under treatment. Therefore, designing structures with two-photon photosensitivity is one of the most prevalent challenges in biomedical treatment. Recently, Klausen et al.¹¹⁹ provide a systematically review to the explanation and design of two-photon phototriggered structures in uncaging of bioactive molecules.

3.2. Tetraphenylethylene

Among the various AIE chromophores, tetraphenyletheylene (TPE) and its analogs have achieved considerable importance in the field of cancer therapy mostly through their potential to perform as photosensitizers for photodynamic therapy (PDT) and cellular imaging^120,121. The TPE derivatives are one of the ideal units for assembling macrocycles and cages due to its simple C₂ symmetry and as minimum tetratopic reaction situations^122,123. To use the benefits of AIE and ESIPT combination, the Parthiban et al.¹²⁴ group developed a PTDDS using connection of TPE with pHP-Cbl (TPE-pHP-Cbl) that, released the Cbl only in their aggregated state under visible light (λ ≥ 410 nm) irradiation with high real-time monitoring ability. This organized PTDDS due to its AIE phenomenon showed distinct fluorescence. Although, the visible light activated based on DDS by Barman et al.¹⁰⁸ and Parthiban et al.¹²⁴ did not display hopeful uncaging capacity in the area of 650–950 nm, which obstructs their sensible usage in the biomedical applications.

In the following, Parthiban et al.¹²⁵ synthesized for the first time a photo-induced nano-DDS with strong fluorescence by a TPE 13 functionalized with 4 equivalent Cbl as anticancer agent (TPE(Cbl)₄ NPs) (Scheme 3). The four-armed phototriggers TPE(Cbl)₄ was initially synthesized by Friedel–Crafts acylation and then reacted with Cbl. Subsequently, the TPE(Cbl)₄ NPs were synthesized by re-precipitation methods. They proved that 4 equiv of Cbl is librated in an ordinal procedure when the TPE(Cbl)₄ NPs induced by visible light through a C–O bond cleavage. Furthermore, TPE(Cbl)₄ and the released photoproducts exhibited a PDT property during drug release.

Scheme 3.

Synthetic route and potential mechanism for produced singlet oxygen and sequential Cbl release.

Upon photolysis, TPE(Cbl)₄ NPs get excited to their singlet state and afterward to their triplet states through ISC. Then, TPE(Cbl)₄ NPs as a photosensitizer produce singlet oxygen and undergo heterolytic fission of the carbonate ester C–O bond to cause anion-pair intermediate 16. After that, reaction of anion pair intermediate 16 with H₂O generated the photoproducts PP-1, PP-2, PP-3 and PP-4 sequentially, together with the ordinal Cbl release in their aggregated state. During cellular uptake, the cells exhibited a deep-green color due to existence of TPE(Cbl)₄ which has the AIE process capability (Fig. 4a). Upon 10 and 25 min visible irradiation, respectively (Fig. 4b and c), the amount of the fluorescence initially reduced and a weak green fluorescence observed that obviously signifies the breakup of TPE(Cbl)₄ NPs and disseminated Cbl inside the tumor cell. Incubation of tumor tissue (HeLa cell) with non-fluorescent dichlorodihydrofluorescein diacetate (DCFDA) and TPE(Cbl)₄ NPs displays weak green fluorescent (Fig. 4d) that after irradiation due to generation of a singlet oxygen through TPE(Cbl)₄ NP a strong green fluorescence observed (Fig. 4e and f). In fact, non-fluorescent DCFDA convert to green fluorescent dichlorofluorescein owing to oxidation by produced singlet oxygen species. Prominently, they confirmed that TPE functionalized with 4 equivalents of Cbl are very efficient (lower 16% viability) compared to 1 equivalent of Cbl against cancer cells thanks to the synergistic effects of 4 equivalent of released Cbl and PDT activities.

Figure 4.

(a–c) Confocal microscopy representations of TPE(Cbl)₄ NPs in HeLa cell; (a) 0, (b) 10 and (c) 25 min irradiation, and (d–f) generation of a singlet oxygen via TPE(Cbl)₄ by DCFDA in HeLa cell; (d) 0, (e) 15 and (f) 25 min irradiation. Reprinted with permission from Ref. 125. Copyright@2019 American Chemical Society.

In the other research work by the same group, they synthesized tetraphenylethylene conjugated pHP NPs (TPE-pHP-H₂S) for the controlled liberation of hydrogen sulfide (H₂S) upon exposure to visible light without the assistance of any peripheral reagent¹²⁶. This H₂S donor triggered by light displays both AIE and ESIPT properties by TPE and pHP moieties, respectively. Furthermore, a real-time monitoring at the cellular level is possible with a simple fluorescence color change from yellow to green after photorelease.

3.3. o-Hydroxycinnamate

According to preceding considerations^127,128, Paul et al.¹²⁹ studied the application HBT as an ESIPT moiety in o-hydroxycinnamate fragment (HBT@o-hydroxycinnamate). As shown in Scheme 4, the authors designed the fluorescent phototrigger HBT@o-hydroxycinnamate by the attachment of HBT moiety to the o-hydroxycinnamate group for rapid and shortest release (60 min) of methyl salicylate 21 with distinct fluorescence color change from orange (because of the ESIPT occurrence) to blue (due to formed benzothiazole-coumarin 20) following photorelease. The light-induced conversion of the (E)-photoisomer to the (Z)-photoisomer causes the release of alcohol in company with a coumarin as by-product (with intense fluorescent property and emission wavelength). The potent fluorescent of coumarin byproduct can be useful in the release of alcohol derivatives from these systems. Although, the limitation of this work is the phototrigger activates in the UV region (≥365 nm) as well as the strong chromophore benzothiazole-coumarin byproduct 20 operates as an optical filter.

Scheme 4.

Photoinduced uncaging of methyl salicylate 21 from compound HBT@o-hydroxycinnamate.

3.4. Coumarin substituted

Among the synthetic phototriggers, coumarin-based derivatives as another important class of phototriggered molecules exhibit unique fluorescence visualization and liberate free drug molecules through a C–O bond breaking upon exposure light irradiation^102,130, 131, 132, 133, 134. Accordingly, some researchers nominate a coumarin incorporated with heterocycle derivatives as the PTDD system for making the photo-induced release of antitumor agents^56,135,136. In general, coumarins may be utilized both as a cross-linker and as a divisible moiety especially owing to their faster release rate compared to other phototriggered molecules such as ONB derivatives¹³⁷. Coumarin functionalized at the 7-position with an electron-donating group (EDG) and at the 3-position with an electron-withdrawing group (EWG) display red-shifted absorption and emission in the blue-green light area (Scheme 5). The design of derivative 22 was a critical point burgeoning in the development of cages for the biological and biomedical studies138, 139, 140, 141, 142, 143. Wang et al.¹⁴⁴ examined the synthesis of two carbazole-coumarin derivatives 23 (with –COOH group) and 24 (with triphenylphosphonium (TPP) group) for the photocontrolled release of Cbl, in an in vitro model of cancer cells. The amine group of the carbazole moiety supplies as an EDG and the lactone segment of the coumarin serves as an EWG and permits the visible light of 405 nm to trigger the photodecomposition of the carbazole-coumarin-Cbl connection. Carbazole-coumarins 23 and 24 showed emission bands at 360 and 450 nm, correspondingly.

Scheme 5.

Structures of (a) designed coumarin 22 and (b) a visible-light-triggered drug release of 23, 24.

The photochemical release of Cbl drug is similar to what will be expressed later in Scheme 24³⁵². Carbazole-coumarin 24 with the lower IC₅₀ due to synergistic effect of chemo-drug strength and photosensitization proposed the high efficiency of the TPP structure in raising the bioavailability carbazole-coumarin-drug derivatives.

Scheme 24.

The Acr-Cbl-Vpa structure and subsequent release of Vpa and Cbl.

Klausen et al.¹⁴⁵ in other attempt attached an EWG and a π-conjugated linker through a vinyl-acceptor function at the 3-position of the coumarin, the respective results conveyed Fmoc-protected glycine release both at 700 and 900 nm. As already mentioned (Section 3.1.1), contrary to UV–visible light, the 2 PE by NIR wavelengths affords different benefits including profounder penetration in tumor cells, abridged photo-damage and inherent 3D resolution. A tiny collection of advanced dipolar coumarinylmethyl structure 26–31 that demonstrate great two-photon sensibility at two supplementary wavelengths in the NIR spectral area is illustrated in Scheme 6.

Scheme 6.

Structures of π-extended coumarins 26–31.

Further investigation revealed that the existence of the fused-ring on the EWG and most powerful EWG in the categories can intensify the photorelease efficiency. Therefore, compounds 26–28 are not proper for uncaging applications during C–O bond cleavage upon excitation. Meanwhile, a distinct performance examined for developed analogs 30 and 31 with a fluorenyl component in the conjugated π-connector.

Bojtar et al.¹⁴⁶ synthesized a click-and-release system based on coumarin with a supplementary degree of spatiotemporal manage for the liberation of the caged kinds (Scheme 7). They synthesized vinylene-linked coumarinyl-tetrazine 32 as a photocage that jointed with several amino acids including, Boc-phenylalanine, Fmoc-lysine and Boc-tyrosine-tBu-ester as the model caged substrates. The presence of vinylene tetrazine moiety in the photocages 33–35 quenches the fluorescence of the coumarin moiety and they were fairly photostable and no liberation of the amino acids was perceived following blue laser lighting (λ_max = 488 nm).

Scheme 7.

Structure and conditional uncaging of the photocages.

However, fluorescence capability was changed upon altering the tetrazine 33–35 in a bioorthogonal reaction with bicyclo[6.1.0]non-4-yn-9-ylmethanol (BCN) 36. Recently, bioorthogonal reactions allow researchers to label or manipulate biological systems in a single experiment147, 148, 149, 150, 151. The BCN-conjugated derivatives 37–39 demonstrated the most fluorescence (with a bright green-emission band) as well as the light-induced bond breaking to fast liberation of all three amino acids 44–46. Fig. 5a and b clearly demonstrate that the live cells remediated alone with tetrazine derivative 47 demonstrated a tiny fluorescence growth while cells treated with TPP-BCN and then with 47 displayed bright yellow emission of coumarin 51 after irradiation (Fig. 5c and d). This can be due to the formation of BCN-conjugated derivative 48 caused by the bioorthogonal reaction of 47 with TPP-BCN according to Fig. 5e. The tetrazine derivative 47 have an excellent ability for uncaging monitoring process. Further, BCN with triphenylphosphonium moiety (TPP-BCN) used as a tiny organelle marker, to focus the bioorthogonal reaction into the mitochondria (Fig. 5e)¹⁵².

Figure 5.

(a, b) Confocal images of tissues remediated alone with tetrazine derivative 47 (a) before and (b) after irradiation and (c, d) cells treated with TPP-BCN and then with 47 (c) before and (d) after irradiation and (e) reaction of tetrazine derivative 47 with TPP-BCN. Reprinted with permission from Ref. 146. Copyright@2020 American Chemical Society.

4. PTDDSs based on photoinduced disruption of nanoscale structures

4.1. Nitroaryl derivatives

Recently, nitroaryl derivatives including the ONB, o-nitro-2-phenethyl and o-nitro-anilide (ONA) groups used phototriggers to assemble photocages for the regulated liberation of important metallic ions, drugs or bioactive molecules153, 154, 155, 156, 157. In the ONB derivatives, an aci-nitro intermediate can form via an intramolecular hydrogen transfer to the nitro group (tautomerization) by irradiation. Despite their extensive applications, ONB photocages often show low quantum yield values (<0.05) owing to resonance stabilization of aci-nitro intermediates and their application is limited due to the highly absorbing and reactive side products^72,158. A one photon blue visible light responsive polymeric carrier (up to 500 nm) based on ONB derivatives as a subcutaneously implanted depot developed by Carling et al.¹⁵⁹ and it has been used for medical cargo release in the hydrophobic microenvironments including the internal space of a polymeric particle. The improvement of one-photon visible photo-responsive structures represents an attractive alternative for in vivo applications due to very shorter lighting time period with lower powers consumption and is significantly less dangerous to the tissues than UV and NIR laser lights. In this work, to facilitate polymerization and enhance the kinetics of photo-degradation, the butanediol derivative 2-(4′-N-dimethylamino-4-nitro-[1,1′-biphenyl]-3-yl)butane-1,4-diyl dicarbonyl (ANBB) was firstly synthesized^160,161 and then basic tertiary amine groups introduced in the polymer backbone for promotion photo-cleavage in hydrophobic environments (Scheme 8).

Scheme 8.

(a) Schematic representative of Dex@PP52 and its swelling and Dex release under visible light irradiation and (b) in vivo photorelease of Dex@PP52 depot upon exposure to blue visible light. Reprinted with permission from Ref. 159. Copyright@2014 The Royal Society of Chemistry.

The tertiary amine functional group connected to photo-responsive polymer spine 46 (PP52) facilitate deprotonation of the aci-nitro intermediate directing to β-elimination and photo-cleavage according to Scheme 8. Therefore, the visible light irradiation of PP52 induces swelling and release of dexamethasone (Dex) as an anti-inflammatory agent in vivo. The therapeutic effects of Dex@PP52 were more effective than free Dex in local inflammation because of the release of Dex with greater efficiency in the target tissue, rapid diffusion and remaining as local depot on-demand at the injection site with minimizing adverse side effects. Dex@PP52 together with the NIR fluorescent probe IR780 permitted in vivo real-time monitoring of the depot release. Photograph Scheme 8b, demonstrates the in vivo photorelease of Dex@PP52 depot upon exposure to blue visible light. Advantages of this photo-responsive carrier were the strong visible light absorption above 500 nm and photo-reactivity in hydrophobic surroundings.

4.2. Thioketal linkers

4.2.1. Reactive oxygen species-responsive thioketal carriers

The irregular biochemical change of reactive oxygen species (ROS) level (lack/excess) in the disease sites can promote different diseases like autoimmune, cardiovascular, neurodegenerative and etc.162, 163, 164, 165, 166, 167, 168, 169. This issue has motivated investigators to utilize imparity ROS amounts for creating ROS-responsive drug carriers. Thioketal, thioacetal, thioether, vinyldithioether, aryloxalate, selenium, tellurium and arylboronic esters linkers are effective ROS-cleavable chemical groups undergoing bond breaking upon treatment with ROS, therefore, have already been utilized to synthesis ROS-responsive DDSs170, 171, 172, 173, 174, 175, 176. Chen et al.¹⁷⁷ synthesized an amphiphilic and cationic ROS-responsive poly(β-amino ester) (PBAE_ROS) via insertion of a thioketal segment in its monomer unit to attain synergistic antitumor outcomes integrated with photothermal/photodynamic therapy (PTT/PDT) and chemotherapy treatment according to Scheme 9. Hereupon, PBAE_ROS was used for load of photosensitizer IR780 (a near NIR dye, which has both intense PTT and PDT efficacies under laser illumination) and chemotherapeutic drug doxorubicin (DOX). Because the poly(β-amino ester) chains of PBAE_ROS as hydrophobic moiety and side-chain hydroxyl groups as hydrophilic were conjugated, it was predictable that the poly(β-amino ester) chains and hydroxyl groups form a core/shell architecture in an aqueous medium, respectively. Nanomicelles of PBAE_ROS@IR780-DOX were prepared under stirring with dropwise addition of DOX and IR780 in DMSO and surface modification of polymer PBAE_ROS@IR780-DOX (PPID) was accomplished through propylene glycol alginate sodium sulfate (PSS) using an easy nanoprecipitation procedure (Scheme 9). The positive charges on the surface of the PPID nanosystem facilitate the rapid penetration into the tumor tissues because of interplay with the negatively charged cell membranes. Upon exposure 808 nm laser light, PPID NPs cause a quick rise temperature and generated a wide range of cytotoxic ROS. The ROS generated by the IR780 advanced further breakage of the thioketal bonds and release of DOX, thereby synergistic PTT/PDT and chemotherapy efficiencies was observed in vitro photorelease. Further examinations unraveled the formation of by-products acetone 53 and dithiol 54 upon the cleavage of the thioketal bond of PBAEROS.

Scheme 9.

Preparation of PPID NPs and their mechanisms via integrated with PTT/PDT and chemotherapy.

The fluorescence microscopic photographs of Hep1-6 tissues after various remediations with free DOX, PPID without laser and PPID with laser lighting are revealed in Fig. 6. Few tumor cells died after 24 h remediation with free DOX (IC₅₀ = 1.3 μg/mL), a greater number of the cells significantly died with PPID NPs (IC₅₀ = 0.72 μg/mL) and these cells were approximately completely died after laser illumination (IC₅₀ = 0.22 μg/mL) meaning that PPID NPs possibly will merge PTT/PDT and chemotherapy to apply synergistic effects in cancer tissues.

Figure 6.

Fluorescence microscopic photographs of live and dead Hep1-6 tissues after various treatments. Reprinted with permission from Ref. 177. Copyright@2019 The Royal Society of Chemistry. Scale bar: 200 μm.

4.2.2. o-Nitroaryl thioketal carriers

In an interesting study, Men et al.¹⁷⁸ have presented a new cleavable polymer containing o-nitroaryl thioacetal structure with more stability than reported thioketal and thioacetal-based polymers to ROS. They first used o-nitrobenzaldehyde (o-NBA) and a dithiol derivative for the preparation of thioacetal polymer containing thiol terminated polyTNB. It can be easily conjugated to methoxypolyethylene glycol maleimide (mPEG-maleimide) through mPEG-maleimide click chemistry. The prepared amphiphilic polymers PEG-polyTNB capable of forming NPs in a combination of THF/dioxane with Nile Red as a hydrophobic drug pattern have been utilized as a light-cleavable nano-drug assembly (Scheme 10). The treatment of PEG-polyTNB with some ROS types specified a significant shift in the oxidation potential of these thioacetal compounds due to the presence of EWD group (NO₂) of NBA, demonstrating improved stability of the polymer against ROS condition compared to the often-reported thioacetal polymers. In addition, irradiation of PEG-polyTNB with UV-A (365 nm) led to ONB product 59 that is subsequently reduced to benzisoxazole derivative 62 and lastly the thioester amine derivative 64 produced, see Scheme 10. Indeed, light leads to degradation of PEG-PolyTNB NPs surface and induced liberation of the Nile red as a hydrophobic drug pattern.

Scheme 10.

Synthesis of PEG-polyTNB NPs and its phototriggered release.

4.2.3. Metal–organic cage thioketal carriers

Recently, in a very attractive piece of work, the metal–organic cages (MOCs) were functionalized with ROS-cleavable thioketal linker by Shen et al.¹⁷⁹ (Scheme 11). In this attempt, a ROS-responsive thioketal platform was developed to decrease some dilemmas restrictive biomedical application of MOCs. MOCs are a class of coordination-driven assembly of metallic ions and organic compounds with special hollows that are attractive for DDS180, 181, 182, 183, 184, 185, 186, 187, 188. The central cavity of MOCs provides new opportunities for controlled microenvironments with distinct shape and size that the cavity properties can be modified simply with infinite structural possibilities¹⁸⁹. This allows that MOCs will be further extended for synergistic result in biomedical fields190, 191, 192, 193. The ultra-small size of MOCs makes possible tumor penetration; however, the fast release and negligible agglomeration at the cancer cells restrict their biomedical uses194, 195, 196, 197, 198, 199. As well as, the hydrophobicity of the MOC surfaces progresses internalization on the cancer sites leading to reduced blood circulation period and decreased biocompatibility.

Scheme 11.

Synthesis of ZnPC@polySCage and related photoinduced ROS-cleavable pathway.

To this end, a ROS-responsive thioketal platform is developed to decrease some dilemmas restrictive biomedical application of MOCs. As described, in this study 3,5-bis(pyridin-4-ylethynyl)aniline (BPA) and PEGylation dithioketal BPA (mPEG-SS-BPA) coordinate with Pd²⁺ to form MOCs and then it filled with photosensitizer Zinc Phthalocyanine (ZnPC) in the cavity called ZnPC@polySCage. Heteroleptic strategies were utilized to synthesis of ZnPC@polySCage and construct a cantellated tetrahedral cage using Pd(II) ions and two bent dipyridyl ligands200, 201, 202. The prepared ZnPC@polySCage can easily penetrate in the depth of cancer cell due to its nanoscale particle size (less than 10 nm). Then, the cavity-loaded ZnPC provided ROS for the degradation of PEG thioketal bond and PDT to destroy the cancer tissues. Interestingly, the PEG dissociation caused surface switching from hydrophilic to hydrophobic and automatically and simultaneous accumulation of MOCs (sizes as big as 671 nm). Therefore, the cellular uptake, tumor accumulation and the PDT efficacy were significantly enhanced under subsequent laser irradiation in the tumor cell. As illustrated in Fig. 7a. The mice mediated with ZnPC@polySCage upon exposure to light compared to only irradiation, ZnPC@polySCage without irradiation, ZnPC with irradiation, and ZnPC@polyCage with irradiation exhibited the significant anti-tumor effect that showed the phototriggered cellular sorption, drug aggregation and the PDT efficacy. Also by comparison, the in vivo safety evaluation results revealed that ZnPC@polySCage can be a prospective structure for tumor treatment with attractive biocompatibility and security. The main difference between ZnPC@polySCage and ZnPC@polyCage is that ZnPC@polyCage does not have thioketal linker in its structure that caused by the reaction of 1,3-bis(pyridin-4-ylethynyl)benzene (BPB), PEGylation BPA (mPEG-BPA) as well as Pd²⁺ and then filled with ZnPC according to Fig. 7b. The ZnPC@polyCage showed weaker tumor activity compared with ZnPC@polySCage due to diminished aggregation of ZnPC loaded in MOCs.

Figure 7.

(a) The photographs of the cancer tissues after the mice were sacrificed and (b) synthesis of ZnPC@polyCage. Reprinted with permission from Ref. 179. Copyright@2021 John Wiley & Sons, Inc.

4.3. Maleimide-anthracene linker

Maleimide-anthracene linkers with excellent thermal stability (up to 200 °C) have been utilized as mechanophores to explore the effects of macromolecular architecture or micellar aggregation on the duration of cycloreversion203, 204, 205, 206, 207, 208. Mechanophores, a class of stimuli-responsive compounds, have recently fascinated the interest of engineers due to their prospective applications as stress sensors209, 210, 211, 212, 213. These mechanically responsive molecules undergo fluorescent/color changes by the imposition of a mechanical stimulus such as stretch or rotation214, 215, 216, 217. More recently, Kabb et al.²⁰³ designed a crosslinker rely on the maleimide-anthracene linkage and employed in preparing exclusive networks that exhibit a fluorescence response when damaged by compressive forces. From a mechanistic point of view, the cycloreversion of adduct occurs through pressure and release of the fluorescent anthracene groups as shown in the top inset of Scheme 12. Accordingly, Cheng et al.²¹⁸ recently synthesized ultra-stimuli responsive systems based on multiarmed poly(ethylene glycol)-block-poly(ε-caprolactone) (PEG-b-PCL) copolymer-linked by maleimide-anthracene as a photoresponsive segment to organize a globular phototriggered micellar NPs (maleimide-anthracene@PEG-b-PCL) in aqueous and phosphate buffered saline solution for the controlled drug release (Scheme 12).

Scheme 12.

Structure of maleimide-anthracene@PEG-b-PCL micelles, controlled drug loading and phototriggered drug release.

A part from various advantages like biodegradability and biocompatibility for clinical usages, the application of PEG-b-PCL polymeric micelles as DDS still is limited by the poor hydrolytic dissociation of the PCL segment in the aqueous surroundings. To overcome this obstacle a maleimide-anthracene linker as an ultra light-sensitive group was attached to the PEG-b-PCL polymers to generate micelles with unique responsiveness and amphiphilic characteristics. The including of the three PEG fragments in the copolymer structures significantly enhanced hydrophilicity and cause to the development of self-assembled hierarchical arrangements in water with DOX as a hydrophobic drug. As shown in Scheme 12, when DOX-loaded micelles irradiated with UV light (254 nm) for 10 s, the maleimide-anthracene linker disrupted, segments of 65, 66 produced and the drug rapidly and completely released from micellar NPs. These micellar NPs exhibited a small critical micellar concentration (below 10⁻⁵ mg/mL), potential micellar stability, improved-controlled photoresponsive property, appropriate drug-loading and ultra-sensitive light-responsive drug-release performance (for only 10 s) and could develop the protection and efficiency of chemotherapy.

4.4. Photochromic derivatives

In recent years, photochromic compounds integrated into the supramolecular systems have been extensively applied in phototriggers-based DDSs^137,219, 220, 221, 222, 223, 224, 225, 226, 227. To date, a variety of photochromic derivatives such as stilbene228, 229, 230, spiropyranes231, 232, 233, 234, 235, spirooxazine^236,237, dithienylethene238, 239, 240, 241, azobenzenes242, 243, 244, 245, 246 and 1,3-diazabicyclo[3.1.0]hex-3-ens, as a rather less renowned photochromic derivative247, 248, 249, 250, 251 have been developed that undergo structural changes through rotation or inversion on the original double bond, ring-opening process or the mixture of both processes in response to light irradiation. PTDDSs can be either reversible or irreversible depending on the type of the substrates that absorb the light. PTDDSs containing non-photochromic moiety such as ONB or coumarins mostly undergo an irreversible reaction, whereas PTDDSs based on photochromic molecules show reversible reactions and disordered substrate can be recovered again after removing the applied light. In addition, different light wavelengths can be used to induce light responsiveness. These items show the importance and benefits of utilizing photochromic derivatives in PTDDSs. Lately, almost all PTDDSs based on photochromic molecules are functionalized with photochromic spiropyranes (SP) or azobenzenes (Azo) that can isomerize under UV light irradiation and change their hydrophobic-hydrophilic balance.

4.4.1. Spiropyranes (SP)

SP, as a family of photochromic molecular switches has so far gained particular attention in different DDSs due to inherent biocompatibility and their multi responsiveness to endo-/exogenous stimuli^60,252, 253, 254, 255, 256, 257. SP undergoes a trigger-induced reversible conversion between the non-planer, hydrophobic and bleached spiro (SP) structure, likewise, the planer, hydrophilic, zwitterionic and colored merocyanine (MC) structure by the breaking of the C_spiro‒O bond^258,259. These conversions are highly sensitive to environmental conditions including surface or solvent polarity, pH, ionic strength dependence, hydrogen bonding and polar–polar interfaces. Therefore, when this molecule was incorporated into the polymers after switching they are able to stimulate a considerable alteration in the polymer properties like polymer hydrophobicity^219,260,261. The majority of phototriggered DDSs expressed here are founded on induced disruption and disorder of the drug carriers that experience an untimely release before arrival at the desired site. In addition, due to the non-reversibility and probable toxicity of the residual carrier pieces it can be appropriate to enable reversible photo-induced DDSs. They work through the intermolecular interplay (work of adhesion) among polymer, drug and solvent without the disruption of the carrier and according to desorption or release of the drugs262, 263, 264. In an exciting work, Ghani et al.²⁶⁵ established a photo-responsive interpenetrating polymer networks (IPN) as a new PTDDS, for the first time by the notion of work of adhesion. The work of adhesion declares when, the hydrophobic SP unit in the polymer converts to the hydrophilic MC, the phototriggered drug release can be done without decay and disconnection of the PTDDS. Therefore, the release (desorption) and adsorption of the drug can be frequently changed on and off, only by changing the light on and off. In this study, supercritical carbon dioxide (scCO₂) tools were employed to produce IPNs using saturating silicone elastomers (as the host polymer) and copolymers of the photochromic spiropyran methacrylate (SPMA) as the guest polymer with varying hydrophilicity. Overall, the guest polymer mixture, particularly the hydrophilicity of the guest polymer, was the main point in the work of adhesion, untimely and triggered releases. The triggered-release of five different drugs doxycycline hyclate, dopamine, levodopa, prednisone and curcumin with varying hydrophobicity was evaluated, as illustrated in Scheme 13. After illumination of IPNs upon 365 nm UV light, conversion of the monoecious hydrophobic spiropyran to the hydrophilic zwitterionic merocyanine resulted due to a C–O bond division. As a consequence, the release of the drug was done as a result of the intermolecular interplays among the drug, the guest polymer and the solvent without the disruption of the system. Further, for the first time the release of drug can be increased or halted upon illumination in the form of reversible.

Scheme 13.

Desorption and liberation of bioactive components from the photoactive guest polymer in an IPN.

4.4.2. Azobenzenes (Azo)

Azo derivatives are excellent molecular switches that show great potential application in DDSs because they can be spontaneously, efficiently and reversibly switched between linear trans and bent cis forms under UV–visible light illumination266, 267, 268, 269, 270, 271, 272, 273. This transformation generates the supramolecular azo components reversible self-assembly and dissociation during light irradiation (UV and visible)274, 275, 276, 277, 278, 279, 280, 281. Also, using hydrophobic interplays and intermolecular van der Waals interactions, trans-Azo with an almost flat structure can spontaneously penetrate to the host molecules hollow and undertake complexation, while cis-Azo is unable to perform this due to the size difference between the host and guest structure282, 283, 284, 285, 286, 287, 288, 289, 290, 291, 292, 293, 294, 295. So, cis-Azo as a dynamic component and trans-Azo as a passive component are candidates for DDS and controlled arrangements^268,296, 297, 298, 299. Wang et al.³⁰⁰ have prepared and studied a new in vivo DDS with hypoxic-responsive Azo bridge via efficient reduction of Azo segment to aniline groups using reductases and their surroundings oxygen deficiency, causing in the hypoxia-triggered drug release and giving a synergistic chemotherapy drug DOX with PDT on the inhibition of cancer progress. In other research, Bian et al.³⁰¹ designed a photo-responsive silicone in accordance with host-guest interplays between thiolated Azo and β-cyclodextrin (β-CD) intended for controlled cell adhesion of particular cells (MCF-7). The selected tumor cell capture, particular aptamer (Section 2.2, a 25 mer DNA aptamer as GCA GTT GAT CCT TTG GAT ACC CTG G), was attached to the thiol-terminated-azobenzene by chemical coupling reactions for smart capture of MCF-7 when incubating a mixture of cells. Upon UV illumination, the Azo isomerized from trans to cis photoisomer and the cis-Azo cannot be known by β-CD thus releasing the captured MCF-7 cells. These innovative results give a novel avenue for the separation and assessment of tumor cell, mainly for regulated drug release. Therefore, Azo derivatives can be suitable for pharmaceutical and biomedical science due to its responsibility to light, hypoxia and enzymes, so showing gaining increasing attention in site-specific smart cargo delivery and prodrug302, 303, 304, 305, 306. The employment of Azo derivatives for triggered prodrugs and DDSs, and application of photoswitchable azo-based prodrugs, has been previously reviewed³⁰⁷.

4.4.3. Dual-stimuli-responsive phototriggers

4.4.3.1. Dual-stimuli-responsive SP-triggers

In recent years, the development of dual or multiple stimuli-responsive supramolecular DDSs has attracted increasing attention to suggest a safe and more efficient controlled drug release in cancer microenvironments and with better stability in normal cells using the synergistic retort to diverse triggers and thus reduce the hurt to normal tissues308, 309, 310, 311, 312, 313. Razavi et al.³¹⁴ developed an interesting amphiphilic light/temperature sensitive block copolymers 67 using a hydrophobic poly(methyl methacrylate) (PMMA) segment with spiropyran unit as a light responsive group (UV and visible light irradiation) and poly (N-isopropylacrylamide) (PNIPAM) segment as a temperature-responsive moiety for controlled DOX release (SP-PNIPAM@DOX, Scheme 14). The amphiphilic copolymers can be self-assembled to nanomicelles in acetone and water solution with a hydrophobic core PMMA and the SP units and a hydrophilic PNIPAM shell. Subsequently, the anticancer drug DOX was loaded into the polymer assemblies 68. When SP→MC isomerization was performed upon exposure to UV light (365 nm), a movement of the polar MC units to the micelles surface, together with shrinkage of the micellar assemblies induced and DOX release triggered from micelles. This process was completely reversible under visible light irradiation. Besides, the shrinkage of the micellar assemblies occurred in response to temperature increase. PNIPAM has a LCST range of 31–33 °C and the PNIPAM's LCST demonstrated light-dependence. Under UV light irradiation, the LCST of PNIPAM enhanced to 37 °C due to the formation of the polar and water-soluble MC form after isomerization. Therefore, a considerable enhance in release of DOX was done under UV light irradiation and at temperatures over the PNIPAM's LCST (T = 40 °C) in acidic media. These polymer assemblies can be more efficient and applicable for multi-responsive DDSs by light, temperature and pH^315,316.

Scheme 14.

Self-assembly of SP-PNIPAM of and controlled release of DOX in different conditions.

Further study by the same group, considered a multi-responsive polymer assembly based on SP and poly(dimethylaminoethyl methacrylate) (PDMAEMA) as a multi-responsive and hydrophobic polymer³¹⁷. The PDMAEMA are generally used as a multi-responsive block toward different stimulants like temperature, pH and CO₂ gas318, 319, 320, 321. In this study, dynamic light scattering (DLS) consequences indicated that the dimension of polymeric assemblies altered in response to light irradiation, temperature rising (above the LCST of PDMAEMA) and also pH variations (from 5 to 9) accordingly, DOX release was controlled by light irradiation, temperature changes and pH.

4.4.3.2. Dual-stimuli-responsive Azo-triggers

Cheng et al.³²² developed pH/temperature responsive adenine poly(propylene) glycol-functionalized boron nitride nanospheres (BN-APPG) for regulated drug delivery and release in answer to internal pH and temperature stimuli in tumor tissues. While the cancer microenvironment has a somewhat higher temperature and a less pH than the surrounding healthy cells, these nanospheres showed excellent anticancer effect in vitro.

To further the development of multiple stimuli responsive DDSs with photo-response performance, Zhang et al.³²³ designed and synthesized an interesting dual light/pH-responsive supramolecular polymer via the host-guest interaction between β-CD-graft-poly(2-(dimethylamino)ethyl methacrylate) (β-CD-g-PDMAEMA) and Azo substituted poly(ε-caprolactone) (Azo-PCL) (Scheme 15). The supramolecular polymer β-CD-g-PDMAEMA@Azo-PCL can encapsulate DOX as spherical drug-loaded micelles and exhibits dual stimuli response to changes in light and pH. The hydrophobic PCL linkage as supramolecular micelles core is sensitive to light irradiation and the hydrophilic PDMAEMA moiety utilized as the micelle shell for response to pH changes. According to Scheme 15, Azo with trans-configuration can penetrate to the β-CD hallow during the host-guest complexation upon visible light irradiation. After UV illumination, the Azo configuration changes from the trans to cis, which cannot be recognized by the β-CD molecules. Therefore, the supramolecular micelles interrupt, causing the release of the DOX drug. Alternatively, in the acidic pH, the completely protonated tertiary amine groups in PDMAEMA moieties develop intense electrostatic repulsion interactions, so accelerating the drug release from the supramolecular micelles³²⁴.

Scheme 15.

Preparation, self-assembly and drug loaded of β-CD-g-PDMAEMA@Azo-PCL.

In addition, the electrostatic repulsion, accompanied by low protonation of Azo moieties triggers better inconstancy on the hydrophobic hallow and Azo-PCL core, therefore the host-guest complexation based on β-CD/Azo was weakened and DOX release from micelles was accelerated³²⁴. Also, under acidic medium, the re-protonation of the DOX amino functional groups enhances the DOX solubility in aqueous media and the micelle cores decay sooner, ensuing in the fast DOX release³²⁵. UV light irradiation in acidic surroundings more enhances the DOX release than a single stimulus due to synergistic effects of pH and UV light irradiation. These behaviors offer the possibility for the dual DOX-targeted and DOX-controlled release based on diverse physiological situations.

Gao et al.³²⁶ synthesized a dual light/temperature responsive supramolecular polymer brush through the host-guest complexation between β-CD structure on star-like side sequences and Azo moiety as a flexible spine, which supplies a novel platform for the preparation of self-assemblies with special sizes from unimolecular micelles, multi-molecular micelles and vesicles in water via alternating UV light irradiation and increasing temperature (Scheme 16). Polymer brushes are a unique kind of joined copolymers that are consists of a pliable unit and interlocked side chains327, 328, 329, 330, 331, 332, 333, 334, 335, 336. Upon UV light irradiation, the micelles or vesicles are disconnected due to the β-CD is divorced from the Azo backbone during photo-triggered isomerization from trans to cis conformer. Subsequently, the convened micelles or vesicles are able to accumulate into strawberry superframes via rising temperature. This report could be eventuality applied in DDSs.

Scheme 16.

Self-assemblies of unimolecular, multi-molecular micelles and vesicles through the host-guest interaction.

In the interesting work, Xiao et al.³³⁷ developed a dual stimuli-responsive azo-trigger to independent controlled release of multiple drugs. Since matrix metalloproteinases (MMPs) concentration is overexpressed in cancer tissues, they utilized it as interior stimulus for the controlled release of macromolecular drugs. UV light also used as secondary external stimulus for release of small molecule drugs. According to Scheme 17, the nanospheres were assembled by five bilayers of two particular polymer chains containing poly(acrylic acid-graft-azo-graft-proline-leucine-glycine-valine arginine-adamantane) (PAA-g-azo-g-PLGVR-AD) and poly(asparticacid-graft-β-CD) (PASP-g-β-CD) via layer-by-layer (LbL) method. The PASP-g-β-CD prepared by the ring-opening reaction of poly(l-succinimide) 81 with mono(6-(2-aminoethyl) amino-6-deoxy)-β-cyclodextrin 80 that functionalized firstly by p-toluenesulfonyl chloride and then ethylenediamine. The hydrophobic cavities of PASP-g-β-CD supplies possibilities to load guest molecules via non-covalent interactions. Also, the click reaction of alkynyl-terminated 76 with an adamantly azide derivative of 77 afford the PAA-g-azo-g-PLGVR-AD polymer. The azo-alkynyl-terminated 76 was prepared by cautiously controlling the reaction between conditions from both 69 to 71 and 73 to 76. This system can load two different drugs, macromolecular drugs were adsorbed in the hollow central cavity of nanospheres while, small molecule drugs attached by α-CD that aggregated on layers during the host–guest interaction between α-CD and Azo moiety. In this system, dextran₅₀₀₀-fluorescein isothiocyanate (Dex₅₀₀₀-FITC, with green fluorescence color) as a macromolecular model drug, α-CD-rhodamine B (α-CD-RhB, with red fluorescence color) as a small molecule model drug, and squamous cell carcinoma (SCC-7 cells) as high MMP activity were chosen. In the tumor tissues connection between AD and β-CD can be interrupt because PLGVR peptide hydrolyzed by MMPs and the macromolecular drugs regularly release from cavity. In addition, under UV irradiation, α-CD modified small molecule drugs that still bonded to PAA-g-azo polymers released from layers during photo-triggered Azo isomerization from trans to cis that this moment, the liberation of the small drugs occurs. The confocal microscopy images of SCC-7 cells confirm that the release of dual drugs can restricted by various stimulation of MMP or UV light irradiation, allowing cocktail treatment for tumor tissues.

Scheme 17.

Synthesis route of five bilayers nanospheres and schematic illustration of drugs release.

5. Dual-releasing phototriggers

Recently, dual-releasing PTDDSs have become a promising approach mainly in biomedical applications over single-releasing phototriggers due of their exceptional properties of quick and clean opening with spatio-temporal manage in the releasing two anticancer drugs and its application in combination treatment338, 339, 340, 341, 342, 343. Dual-releasing PTDDSs can be released two equivalent similar or different active molecules upon exposure to light as sequential (in order) or subsequent (irregular) according to Scheme 18. In the sequential release, the second active molecule is usually in the locked state when the first active molecule release simultaneously during light irradiation. Hence, this approach can exploit for the controlled release of two different active molecules selectively via stepwise pathways.

Scheme 18.

The schematic illustration of sequential and subsequent dual-releasing phototriggers.

5.1. Sequential dual-releasing phototriggers

5.1.1. Acetyl-nitrobenzyl (ANB) moiety

Firstly, Bochet et al.³⁴⁴ established a phototrigger for the sequential release of different LGs by orthogonal photolysis, although, this method requires two- or more-chromophoric structures. Since, the 4-acetyl-2-nitrobenzyl (ANB) segment is able to orthogonal photochemical release of distinct LGs, Kammari et al.³⁴⁵ synthesized a mono-chromophoric structure 76 during several steps³⁴⁵. The mono-chromophoric structure 82 design based on two well-known phototriggered molecules including ANB and phenacyl for the sequential release of two different LG in the presence of a chemical activator and UV light irradiation (<350 nm) (Scheme 19). Upon irradiation, initially, carboxylic acid or alcohol (R–OH) released essentially from ANB segment 82 which its photochemistry is firstly based on a conventional intramolecular 1,5-H shift by the nitro group 82 and formation of aci-nitro intermediate 83. Subsequently, formation of benzoxazolidines 84 and ring opening occur to release the first LG along with 2-nitrosobenzaldehyde derivative 86^156,346. The second LG (R′CO₂H) in the phenacyl position of 86 is photochemically released in the presence of a hydrogen atom donor like 2,2-propanol and light if required through photochemical electron transfer³⁴⁷. The most important restriction of this study is the necessity of using a chemical activator together with light for the second liberation phase and the sequential manage blocked if the 2,2-propanol (chemical activator) appended at first.

Scheme 19.

The structure and photochemistry of phototrigger substituted 82 in the benzylic and the phenacyl positions with LGs.

5.1.2. o-Hydroxycinnamate

In a valuable study, Paul et al.³⁴⁸ proposed a new approach for the sequential and in situ release of second active molecule from mono-chromophoric phototrigger 88 no need for an activator (Scheme 20). The first release (alcohol derivatives) occurs from o-hydroxycinnamate 88 as phototrigger I, due to the photoisomerisation of the double bond afterward lactonization during UV light irradiation. The second release (carboxylic acid derivatives) take places in situ simply from the generated coumarin 89 as second phototrigger, this step is located in an inactivated state throughout the first deprotecting step. The intended system also has real time monitoring capabilities owing to generation of fluorescence coumarin derivatives.

Scheme 20.

The structure and in situ liberation of phototrigger substituted 88 with different LGs.

5.1.3. Metallopolymer

He et al.³⁴⁹ developed a sequential dual-releasing phototriggered metallopolymer (Poly@Ru/PTX) for release of photosensitizer ruthenium complex (Ru) and paclitaxel (PTX) as an anticancer drug for the first time (Scheme 21). The polymer filament includes methoxy polyethylene glycol (MPEG) and piperidine-functionalized polycarbonate (PTMCP) that Ru complexes and PTX covalently appended to the polymer backbone using a superficial amino-alkynoate click polymerization³⁵⁰. This type of innovative connection could let Ru and PTX to attain cancer spots concurrently via enhanced permeability and retention (EPR) effects and eschew unwanted drug release in the bloodstream. The red-light irradiation of the cancer cells released the anti-tumor Ru complexes straightforwardly and produced reactive singlet oxygen (¹O₂). The released ¹O₂ can cleave the ROS-sensitive β-aminoacrylate bond during oxidative degradation resulting to the liberation of PTX and other products 91, 92. The in vitro and in vivo investigations of poly(Ru/PTX) confirm a synergistic effects of chemotherapy PTX with PDT with exceptional cancer accumulation, cytotoxic activity (lower 32.4% viability) and high biosafety under red light irradiation.

Scheme 21.

The synthetic route of poly(Ru/PTX), cleavage of β-aminoacrylate bond and release of drugs and other products 91, 92 after oxidative degradation.

5.2. Subsequent dual-releasing phototriggers

5.2.1. Carbazole-combined to o-hydroxycinnamate

Carbazoles are of great importance among nitrogen containing heterocycles mostly due to efficient luminescence property³⁵¹, various biological performances³⁵² and easy modification and functionalization of core frame³⁵³. Accordingly, Venkatesh et al.³⁵⁴ designed a carbazole-combined to o-hydroxycinnamate derivative 93 for the subsequent dual release of similar and different alcohols upon one- and two-photon excitation (Scheme 22). The mechanism for the dual release upon irradiation progresses through excitation of 93 to its singlet state 94 and then a trans‒cis isomerization leading to the release of first alcohol and arrangement of the newly coumarin carbazole Cou-CBZ. The second LG also release by following a similar mechanism. The first and second release confirmed by an increase in fluorescence intensity and fluorescence color change, respectively. Since, in the Cou-CBZ internal charge transfer (ICT) take places between one carbazole moiety (as donor) and one ester-carbonyl moiety (as an acceptor) the fluorescence intensity is higher than Cou-CBZ-Cou. Furthermore, the fluorescence color change (from green to blue) is because, in Cou-CBZ-Cou, ICT does not occur. Although, the limitation of this work is the coumarin carbazole byproducts can operate as an inner filter.

Scheme 22.

The structure of dual-releasing phototrigger 93 and subsequent release of alcohols.

5.2.2. Acetyl carbazole

The same group³⁵⁵ synthesized a fluorescent dual-releasing phototrigger substrate (CBZ-CA-Cbl) based on acetyl carbazole chromophore with two arms to photocaging and subsequent release of both caffeic acid (CA) and Cbl simultaneously after UV light irradiation. Irradiation of CBZ-CA-Cbl causes to a singlet excited state 96 that undergoes ISC to their triplet state 97 and then cleavage of the C–O bond in carbazole continues 97 to form anion-pair intermediate 98. Solvation of the ion-pair intermediate 98, gives hydroxyacetyl 99 and released first drug. The second drug also release by related mechanism. By using the natural product CA, this dual-releasing phototrigger displayed improved anticancer effect (lower 45% viability, IC₅₀ at 15 μmol/L) in comparison to single-releasing phototrigger CBZ-Cbl (60% viability) and even CBZ-CA (above 75% viability) (Scheme 23).

Scheme 23.

The CBZ-CA-Cbl structure and subsequent dual-release of CA and Cbl.

5.2.3. Functionalized acridines

Acridine derivatives with the planer structure are a DNA intercalator and topoisomerase II inhibitor^356,357 that widely considered for their anticancer and antibacterial properties358, 359, 360, 361, 362. These results afforded the opportunity to develop acridine-based phototriggers for controlled release of drugs. Zhuang et al.³⁶³ first described C9-functionalized acridine phototrigger for the release of different alcohols under UV light irradiation. Furthermore, Jana et al.³⁶⁴ and Piloto et al.³⁶⁵ utilized the same C9-functionalized acridine phototrigger to the liberation of carboxylic acids and neurotransmitter amino acids. However, this phototrigger can liberate just one LG, limiting its application in combination treatment. Ray et al.³⁶⁶ designed a C4- and C5-functionalized acridine with dual arm to protection and then release of two anticancer drugs Cbl and valproic acid (Vpa) simultaneously under visible light irradiation (λ ≥ 410 nm) (Scheme 24). The photorelease pathway upon visible light irradiation is as follows: firstly, Acr-Cbl-Vpa excited to its singlet state and then undertakes a heterolytic C–O bond dissociation at the C4 or C5 benzyl substituent 101 to yield anion pair 102 that after solvation, one equivalent of Cbl or Vpa liberated. Subsequently, the attained acridine 103 once more excited to its singlet state and generates another equivalent of Cbl or Vpa. Irradiated Acr-Cbl-Vpa-incubated cells with visible light showed approximately double cytotoxicity (EC₅₀ = 12.59 μmol/L) than free Cbl (EC₅₀ = 20.23 μmol/L) with a synergistic efficacy of Vpa upon Cbl. This dual-releasing phototrigger also illustrated the real-time monitoring ability from green (uncleaved nano-drug) to blue (cleaved nano-drug) color change through photolysis.

5.2.4. Salicylaldazine

Biswas et al.³⁶⁷ have selected salicylaldazine (SDA) as the fundamental chromophore, which displays AIE and ESIPT process simultaneously. As previously mentioned (Section 3.2), the ESIPT incorporated with AIE provide further views to extend PTDDSs to drug photorelease with more efficiency in the aggregated state and dispelling the nonfluorescent character of the phototriggered molecules such as pHP groups. In this study, as displayed in Scheme 25, salicylaldazine moiety functionalized with two different drugs ferulic acid (FA) and Cbl on both sides (SDA-FA-Cbl) by simple merging of salicylaldazine to pHP. By virtue of which, it supplies the fast photorelease of two drugs sequentially inside the cell (5 min) with distinct fluorescent color change (from yellow to blue) using visible light irradiation (>410 nm) which confirmed the high efficiency of this phototriggered DDS.

Scheme 25.

The structure and photorelease of the dual PTDDS SDA-FA-Cbl.

The photochemical release of drugs from SDA-FA-Cbl initiates with AIE phenomena, ESIPT movement from pHP moiety to the imine group and then continues during photo-Favorskii rearrangement by forming the putative spirodiketone intermediate similar to what was expressed in Scheme 1. This PTDDS demonstrated a highest level of cytotoxicity towards HeLa cells (above 90%) upon irradiation compared with free Cbl due to effective biodistribution and the synergistic effect of FA and Cbl.

6. Hazard assessment and clinical studies of PTDDS methods

Despite the great attention on the design and effectiveness of PTDDS, it is important to pay attention to the side effects of this method due to the poor understanding of drug–tissue interactions and material properties^368,369. In addition, it should be considered that the biomaterials and micro- or nanoparticulate formulations are not necessarily inert. Therefore, before use for clinical trials, several issues such as the assessment of formulation's biocompatibility via in vitro and in vivo sights, the safety of the all components of the PTDDS, including the drugs to be delivered locally and systemically, and mitigating tissue reaction directly via decoration and surface modification of particles with appropriate materials, should be addressed, accurately. In addition, it is still troubles to penetrate internal body parts even with IR. In current research, there are few in vivo phototargeted evaluations of PTDDSs, for example, Carling et al.¹⁵⁹, Shen et al.¹⁷⁹, and He et al.³⁴⁹ investigated the in vivo pharmacokinetic activity of prepared phototriggered structures in mice tissue. However, PDT clinically approved as an effective method to cancer treatment³⁷⁰, while PTT and PTDDS are still in clinical studies to treat cancer³⁷¹.

7. Conclusions and future challenge

Various directions have been successfully extended for triggering the release of chemically or biologically moieties using peripheral stimuli, such as light that is a principally noteworthy stimulus. In this review, we have covered latest phototriggered molecules on DDSs together with their main mechanism for triggered drug release. The unique properties of all the mentioned PTDDSs in this review are given in Table 1. Phototrigered molecules are excellent and versatile tools for the release of various chemicals especially bioactive molecules in living tissue. Although various phototriggers have been developed, there are important challenges to be addressed for sensible demands. One of the major challenges to expand the biomedical applications of phototriggers is to increase the diversity of the phototriggers family. In continuation to our prior work248, 249, 250, 251 and despite this study, there has been an upsurge in interest in our group on design and exploitation of photochromic 1,3-diazabicyclo[3.1.0]hex-3-en derivatives to fulfill the demands for increase the variety of the phototriggers. Recently, we (Shamsipur et al.²⁴⁷) found that these photochromic structures are able to bind with charged molecules under sunlight. The capacity of these structures for photo-induced binding with active molecules might bring new opportunities to promote the development of innovative and new phototriggers. Furthermore, this family of photochromic derivatives is an excellent photo-responsive connection by short-term exposure to sunlight that may significantly influences their ability to form safe and ultrasensitive PTDDSs.

Table 1.

The unique properties of the foresaid PTDDSs in biomedical application.

Entry	PTDDSs	Unique property/ies		Ref.
1	pHP-Benz-Cbl	PTDDSs based on photochemical bond cleavage	1. Very fast photorelease of Cbl (15 min) upon visible light with distinct fluorescence discolor	108
2	pHP-Naph-Cbl		1. Two-photon absorption in the phototherapeutic window (700 nm)	109
			2. Liberation of Cbl only in their aggregated state under visible light irradiation
			3. High real-time monitoring ability (from greenish-yellow to blue)
3	TPE-pHP-Cbl		1. Liberation of Cbl only in their aggregated state under visible light irradiation	124
			2. High real-time monitoring ability
			3. PDT activity
4	TPE(Cbl)4 NPs		1. Release of 4 eq Cbl only in their aggregated state under visible light irradiation	125
			2. High real-time monitoring ability, PDT activity
5	HBT@o-hydroxycinnamate		1. Rapid and shortest release (60 min) of methyl salicylate	129
			2. Distinct fluorescence color change from orange to blue following photorelease
6	Carbazole-coumarin derivatives 23,24		1. The phototrigger activates upon visible light irradiation	144
			2. A synergic effect of phototriggered drug release and photosensitization of the carbazole-coumarin segment
7	π-Extended coumarin 26-31		1. A two-photon uncaging sensitivity	145
8	Click-and-release system based on coumarin		1. The phototrigger activates upon the blue visible light	146
			2. An extra level of spatial and temporal control for the release of the caged compounds
9	Dex@PP52	PTDDSs based on photoinduced disruption of nanoscale structures	1. The phototrigger activates upon the blue visible light	159
			2. The strong visible light absorption above 500 nm
			3. Photo-reactivity in hydrophobic surroundings
10	PPID		1. A ROS-responsive drug carrier	177
			2. Synergistic effects of PTT/PDT and chemotherapy treatment
11	PEG-polyTNB		1. Resistant to high concentrations of ROS unlike reported thioacetal bonds and preventing non-triggered release	178
12	ZnPC@polySCage		1. Ultra-small size of ZnPC@polySCage	179
			2. Hydrophobic ROS-responsive thioketal MOCs
			3. PDT efficacy
13	Maleimide-anthracene@PEG-b-PCL micelles		1. An ultra light-sensitive characteristics (10 s)	218
			2. A small critical micellar concentration (below 10−5 mg/mL)
			3. Strong hydrolytic dissociation of the PCL segment in the aqueous surroundings
14	IPN		1. The drugs releasing without disruption of the system	265
			2. Increasing or halting of drugs release upon illumination
15	Hypoxic-responsive Azo bridge		1. Synergistic effects of chemotherapy DOX with PDT	300
16	Azo@β-CD		1. Smart capture of MCF-7 cells in a mixture of cells under UV irradiation	301
17	SP-PNIPAM@DOX		1. A more enhance in release of DOX under UV light irradiation and at temperatures	314
18	SP-PDMAEMA@DOX		1. The DOX release by light irradiation, temperature changes and pH	317
19	β-CD-g-PDMAEMA@Azo-PCL		1. Dual stimuli response to changes in light and pH	323
			2. The very fast release of DOX due to synergistic effects of pH and UV light irradiation
20	Supramolecular polymer brush		1. Dual stimuli response to changes in light and temperature	326
			2. Self-assemblies of unimolecular, multi-molecular micelles and vesicles through the host-guest interaction with special sizes
21	Five bilayers nanospheres		1. Dual stimuli response to changes in light and MMP concentration	337
			2. The ability to load two different drugs simultaneously
22	Mono-chromophoric structure 82	Dual-releasing phototriggers	1. Sequential release of two different LG in the presence of a chemical activator and UV light irradiation	345
23	Phototrigger 88		1. The sequential release of two different LG under UV light irradiation no need for an activator	348
			2. High real-time monitoring ability
24	Poly@Ru/PTX		1. The sequential release of two different LG under red light irradiation	349
			2. Synergistic effects of chemotherapy PTX with PDT Ru
			3. High biosafety under red light irradiation
25	Carbazole-combined to o-hydroxycinnamate derivative 93		1. The subsequent dual release of similar and different LG upon one- and two-photon excitation	354
			2. Confirmation of the first and second release by an increase in fluorescence intensity and fluorescence color change, respectively
26	CBZ-CA-Cbl		1. Fluorescent subsequent dual-releasing substrate	355
27	Acr-Cbl-Vpa		1. The sequential release of two different LG under visible light irradiation	366
			2. High real-time monitoring ability
28	SDA-FA-Cbl		1. The sequential release of two different LG under visible light irradiation	367
			2. Display AIE and ESIPT process simultaneously
			3. High real-time monitoring ability

The type of light employed is another challenge to be improved for phototriggers. Especially NIR light is still worth of note due to deep penetration and safety. Although, sensitized methodology, multiphoton and UCNPs technologies are good examples of long wavelength with adequate energy to stimulate bond breaking, isomerization or rearrangement responses in triggered drug release, however these successes restricted for phototriggers due to their low efficiency values. Other criteria for the appropriate design of PTDDSs, include the following items:

• •Strong absorption of phototriggered molecules at wavelengths above 300 nm.
• •Fast responses of phototriggered molecules by a short-term light treatment.
• •Enhancement of their water solubility.
• •Biocompatibility of photo-produced byproducts.
• •The phototriggered intermediates or byproducts should not have absorption spectrum in the range of released bioactive components wavelengths to avoid competitive absorption.

In addition, combination of light into dual/multi stimuli-sensitive DDSs and development of single chromophoric PTDDS with simultaneous release of two drugs can lead to unprecedented and precision control over drug delivery, drug release and therapeutic efficacy. If all these requirements can be considered, the prospect of PTDDSs is very brilliant and we can hope that PTDDSs with avoiding unwanted side effects could be implemented in future studies.

Acknowledgments

The support of this work by the Razi University is gratefully acknowledged.

Author contributions

Mojtaba Shamsipur and Atefeh Ghavidast conceived the idea. Atefeh Ghavidast performed the literature search, wrote the original draft and revised the manuscript. Mojtaba Shamsipur and Afshin Pashabadi edited the manuscript. All authors read and approved the final manuscript.

Conflicts of interest

The authors have no conflicts of interest to declare.