Recent developments in photodynamic therapy and its application against multidrug resistant cancers

AuNPs have played a significant role in the advancement of a synergistic hyperthermia-phototherapy technique because of their surface plasmon resonance (SPR) feature, their capacity to convert photoenergies into thermal energy for PDT applications, and their enzyme mimetic activity [66]. Based on these properties, a metal nano-organic framework was developed, where AuNPs act as catalysts like nano enzymes and the PS chlorin e6 is encapsulated to mitigate tumor hypoxia and reinforces PDT under 660 nm laser irradiation. This nano-delivery platform is stable in blood circulation and can easily penetrate and accumulate in tumors [66]. Similarly, in another study, AuNPs conjugated with 5-ALA showed enhanced PDT in cutaneous squamous cell carcinoma under 621 nm irradiation using LED for 1.5 h [67]. It was observed that 5-ALA-AuNPs not only suppressed cancer cell viability but also increased cell apoptosis and produced more singlet oxygen compared to PDT with 5-ALA alone [67]. In addition, to improve the PS's excitation effectiveness, LSPRs surrounding AuNPs may be utilized. Singlet oxygen generation in several AuNP morphologies has been investigated [68]. These include spherical AuNPs with hollow structures, gold nanocages (AuNCs), gold nanorods (AuNRs), and gold nanospheres [68].

As an optical contrast agent, AuNRs were used by Kuo et al to kill A549 cancer cells [69]. The cancer cells were exposed to PDT and hyperthermia simultaneously after their treatment with AuNR conjugated PS indocyanine green (ICG) under an 808 nm NIR laser for 30 s to 3 min. The results showed that the combined approach successfully eliminated cancer cells when compared to PDT or photothermal treatment (PTT) alone [70]. On the other hand, Wang and Lu hypothesized that coordinating an early-phase PDT impact with a late-phase PTT effect would boost the synergistic therapeutic efficacy of AuNPs [71]. To do this, they conjugated Ce6 molecules into polyethylene glycol (PEG) chains to create PEGylated AuNRs, which they evaluated in breast cancer cells in vitro and a xenograft model in mice using MDA-MB-435 [71]. After 4 h of intratumoral delivery of the nanoconjugates, researchers used a 671 nm laser (1.0 W cm⁻²) to treat the tumor for 6 min. Tumor volume was significantly reduced by laser therapy with Ce6-loaded PEGylated AuNRs compared to the respective control groups [71]. This research proved that the combinatorial method of PDT/PTT of AuNPs is more effective against cancer. A pH-stimulus-responsive drug delivery system based on chitosan-coated AuNPs was developed by Zhang et al [72]. This system too has applications for both PDT and PTT [72]. Due to the SPR absorption in the visible region and its nonlinear features, PTT using gold nanospheres can be accomplished using pulsed or continuous wave lasers in the near-infrared (NIR) and visible (Vis) wavelength spectrums [73–75].

Different SNP varieties, including organically modified silica, mesoporous silica NPs (MSNPs), and Stöber SNPs, have recently been investigated as PDT vectors. This is because SNPs tend to be chemically inactive, have a porous structure that does not grow or change when the pH changes, and have a clear matrix that lets light in and out. They are very useful and can be made using a wide range of ingredients and easy-to-use synthesis methods. Changes can be made to the NPs' density, ability to spread out, size, and shape. Their surfaces can also be changed by several functional chemicals, polymers like PEG, or biomolecules that target cancer cells in particular. In an investigation, Cornell prime dots (C' dots), which are extremely tiny PEGylated fluorescent core-shell SNPs, were employed to provide a potential targeted platform for PDT usage in oncology [76]. MSNPs are better for delivering drugs because they have a large surface area and a high pore volume that can be reached [77]. Spherical and rod-shaped MSNPs were created to be used as third-generation PSs in a trial on the bladder cancer model using UM-UC-3 and HT-1376 cell lines. Cells were exposed to white light at 8.4 mW cm⁻² for 40 min [78].

To target cancer cells precisely, MSN was functionalized by a biomolecule that can bind to cancer cells. The absorption of the NPs by receptors would be enhanced if the particles were functionalized to target certain receptors that are overexpressed on the surface of cancer cells in many malignancies. Based on this idea a unique MSN was made where the water-soluble PS and cancer cell-targeting mannose covalently attached to the mesoporous silica matrix for PDT application in MDA-MB 231 breast cancer cells. Results showed that without irradiation MSN conjugated with mannose and PS had only 19% cytotoxicity but when cells were irradiated under 630–680 nm NIR exposure at 6 mW cm⁻² for 40 min 99% cell death occurred [79]. On the other hand, tests on nude tumor-bearing mice were used to assess in vivo biodistribution of protoporphyrin IX (PpIX)-silica NPs loaded with the DID (dioctadecyl tetramethyl indodicarbocyanine chloro benzene) tracer [80]. Up to 24 h after the NPs' tail vein injection, photographs of the biodistribution were taken. The three cancer models under research (HCT 116, A 549, and glioblastoma) showed significant tumor uptake of PpIX-silica NPs with good phototoxicity under 630 nm NIR irradiation at 4 mWcm⁻², but the peak accumulations occurred at different times: 2 h for glioblastoma models, 16 h for A549, and 20 h for HCT 116 models. Other types of SNPs have been used to encase PS in PDT [80]. In another report, silica nanospheres with a 130 nm diameter that are hollow and porous were synthesized [81]. The NPs were created by carefully hydrolyzing N-(b-aminoethyl)-a-aminopropyl triethoxysilane under the influence of ammonia. Hypocrellin A was not surface adsorbing but rather embedded, according to fluorescence quenching experiments. It was found that the particles have high thermal and light stability. These SNPs were efficiently absorbed by HeLa cancer cells and had higher PDT efficacy than hypocretin A.

Out of all nanomaterials and NPs, superparamagnetic iron oxide NPs (SPIONS) are the ones that have been studied most profoundly in combination with PDT [82]. In a study, iron oxide (Fe₃O₄) NPs, or MNPs, were conjugated with Zn phthalocyanine derivatives. At first folic acid (FA) and amine-functionalized magnetic NPs were covalently linked to Complexes 1 (Zn mono cinnamic acid phthalocyanine) and 2 (Zn mono carboxyphenoxy phthalocyanine), respectively [83]. Next, they were evaluated in the human breast carcinoma cell line MCF7 for enhanced PDT at a fixed irradiation dosimetry of 170 J cm⁻² at 680 nm. These complexes exert significant singlet oxygen generation and better bioavailability and toxicity on MCF7 cells [83]. Huo et al synthesized iron oxide (Fe₃O₄) NPs in the presence of triphenylphosphine (TPP)-grafted dextran. Here Fe²⁺/Fe³⁺, ions have a magnetic resonance imaging (MRI) effect [84]. Next, the PSs protoporphyrin IX (PpIX) and glutathione (GSH)-responsive mPEG-ss-COOH are grafted on Fe₃O₄@Dex-TPP NPs to form Fe₃O4@Dex/TPP/PpIX/ss-mPEG NPs [84]. After internalization, these nano platforms increase the oxygen concentration in tumor cells by the Fenton reaction and produce ROS around the mitochondrial surface under illumination with a 637 nm laser which leads to disruption of mitochondrial membrane permeability. Therefore, this nano platform can serve as a Fenton reaction-assisted PDT for tumor therapeutic efficacy [83, 84]. In another study, a magnetic core and silica layer were combined to create hybrid nanomaterials. A single or double silica coating was applied for the synthesis of SPIONs. Following its entrapment in the silica layer, the PS molecule methyl blue decomposed on the surface of SPIONs. Upon exposure to light of a visible wavelength (λ = 532 nm or λ = 633 nm), this hybrid nanomaterial successfully generates singlet oxygen and highlights the potential use of MNPs in PDT [85]. The use of ferroptosis inducers in ferroptosis therapy to cause lethal lipid peroxidation and kill tumor cells is an intriguing option for the administration of chemotherapy for cancer. Chen et al created a NP-driven system where ferric oxide (Fe₃O₄) and chlorin E6 (Ce6) are combined in polylactic-co-glycolic acid to create a potent ferroptosis-photodynamic anticancer treatment [86]. The Fe₃O₄-PLGA-Ce6 NP is capable of breaking down in the acidic TME, releasing Ce6, ferrous (Fe⁺²), and ferric ions (Fe⁺³). The Fenton reaction may be carried out by extracellular hydrogen peroxide (H₂O₂) and release ferrous and ferric ions to create hydroxyl radicals (OH‧) and induce ferroptosis in cancer cells [87]. When exposed to laser light under 660 nm, 0.5 W cm⁻² for 3 min, the discharge of Ce6 may promote the production and accumulation of ROS to provide PDT that can improve ferroptosis in 4T1 breast cancer cells [87]. Additionally, this Fe₃O₄-PLGA-Ce6 nanosystem has good MRI characteristics, extremely effective anticancer activity, and high in vivo biocompatibility [87].

Titanium dioxide (TiO₂) is an oxide of titanium that is found in nature. It is also called titania. TiO₂ is used in many different ways, such as photodegradation, photocatalysis, cleaning up the environment, dividing water for hydrogen fuel, reducing CO₂, making surfaces that clean themselves, electrochromic gadgets, sensors, and cheap solar cells. TiO₂ has lately been thought of as a possible photosensitizing compound for PDT due to its low toxicity, biochemical inertness, notable biocompatibility, and unusual photocatalytic properties [88]. Photoinduced hole-electron pairs are made when UV radiation with higher energy than the band gap of TiO₂ excites electrons in TiO₂'s valence band. The strong reduction and oxidation characteristics of these photoinduced electrons and holes allow them to interrelate with nearby oxygen and water molecules to produce ROS [89]. Based on these properties, TiO₂ NPs were utilized for PDT of cancers. The survival of human cervical cancer cells (HeLa) was reduced by TiO₂ NPs upon solar and ultraviolet (UV) irradiation [89]. When these TiO₂ nanocrystals were doped with metal (cobalt) and non-metal (nitrogen) the photoactivation of doped-TiO₂ NPs was significantly enhanced in the Vis/NIR region [89]. However, these nanocrystals were not significantly cytotoxic compared to PEGylated undoped-TiO₂. Therefore, this study showed that water-soluble PEGylated TiO₂ NPs may be a good candidate for the PDT of cervical cancer cells [89]. Afterward, multiple cell lines of human carcinoma, including monocytic leukemia cells (U937), colon carcinoma cells (Ls-174-t), bladder cancer cells (T24), adenocarcinoma cells (SPC-A1), glioma cells (U87), and breast carcinoma cells (MDA-MB-468, MCF-7), were investigated to discover the accurate mechanism of TiO₂ NPs' UV-induced phototoxic effect [90]. TiO₂ NPs have only rarely been used in vivo for PDT applications. This is because TiO₂ is insoluble and forms aggregates in the physiological environment; as a result, it is easily recognized by cells and removed from circulation, which in turn reduces its accumulation in the tumor.

ZnO has a similar photocatalytic activity and band gap energy (3.2 eV) as TiO₂. It has been established that ZnO quantum dots, which have an average diameter of 11.6 nm, produce singlet oxygen as well as other forms of ROS when exposed to blue (400–500 nm) light [91]. There was no triplet signal in the ZnO samples that had not been exposed to radiation. Considering application in combination photothermal theory (PTT) as well as PDT for cancer, Vasuki and Manimekalai investigated the anticancer effect of ternary modified ZnO nanocomposites with NIR absorbance on the MCF-7 breast carcinoma cell line [92]. The nanocomposites showed significant cytotoxicity against cancer cells, and therefore, they can be used for combined cancer therapy [92]. In another study, it was shown that ZnO nanorods (NRs) can be used as a PS and biomarker for PDT on cervical cancer cell lines. This is because ZnO NRs are a source of NIR light for deep tissue penetration and have more surface plasmon resonance light absorption than ZnO NPs [93]. Through the use of laser scanning confocal microscopy and fluorescence spectroscopy with excitation at 488 and 514 nm wavelengths, it was possible to analyze the fluorescence and light activation of ZnO NRs in cancer cells. The fluorescence of the ZnO NRs was conjugated with the well-known PDT PS 5-ALA, and the results demonstrated that ZnO NRs are potent tumor necrosis candidates under the right light activation and are a good candidate for PDT of malignancies [93]. According to studies, Ultraviolet A radiation (20 mW cm² for 15 min) combined with ZnO NP doses of 0.2 and 2 g ml⁻¹ can effectively destroy cancer cells by causing late apoptosis and necrosis. In this aspect, Hariharan et al synthesized PEGylated ZnO NPs conjugated with doxorubicin (DOX), which generates ROS under UV irradiation for potential anti-tumor activity [94]. It has also been reported that ROS are produced when light energy is absorbed by PEG-functionalized Ag-doped semiconductor NPs of ZnO and transferred to water and molecular oxygen molecules in the biological environment [95, 96].

Graphene sheets roll up CNTs into hollow cylindrical structures with both ends open. The carbon atoms are exclusively organized in rings similar to those found in benzene. The armchair, zigzag, and chiral structural representations of CNT include allotropic forms of both sp2 planar and sp3 cubic [98]. CNTs are classified as single-walled CNTs (SWCNTs), which have an inner diameter of 1–3 nm and an outer diameter of 2–100 nm, and multi-walled CNTs (MWCNTs), which have an inner diameter of 1–3 nm and an outer diameter ranging from 0.2 to several micrometers, respectively, based on the layer formation [97]. CNTs play a special function as nanocarriers for medicines, polymers, PSs, and particular ligands that target siRNA and DNA [97]. SWCNTs penetrate the cells directly, whereas MWCNTs enter through the endocytosis pathway. Based on this property, CNTs are widely used in cancer therapy as novel drug carriers. To treat cancer effectively and specifically, researchers are now concentrating on combination therapies using CNTs and PDT. To improve solubility and bioavailability and to target only cancer cells, PSs are combined with CNTs. For efficient PDT of colon cancer cells, Sundaram and Abrahamse created SWCNTs combined with hyaluronic acid (HA) and chlorin e6 (Ce6) [98]. This nanocomposite induces morphological changes in colon cancer cells using PDT at a fluence of 5 J cm⁻² and 10 J cm⁻² after 24 h followed by LDH cytotoxicity and cell death [98].

A 2D network of carbon atoms with sp2 hybridization makes up graphene. Graphene undergoes an oxidation-reduction process to produce rGO and GO. The finding that graphene can be used as PSs in PDT and effectively kill cancer cells has been demonstrated in several studies [99, 100]. Considering its capacity to absorb light in the NIR range, graphene has been the subject of extensive in vivo and in vitro studies into its potential use in cancer imaging and phototherapy [101, 102]. Hosseinzadeh et al developed a nanocomposite for PDT using GO and MB and evaluated its cytotoxicity on the triple-negative breast cancer cell line MDA-MB-231 by MTT assay [103]. In dark conditions cell viability was reduced by up to 60% for the PS using a concentration of 20 µg ml⁻¹; however, under irradiation with a red LED light of 630 nm for 30 min, a reduction of up to 80% was obtained using the same dose of PS [103]. This is because, in the presence of red LED light, MB-GO nanocomposites could penetrate more inside the tumor cells and exert toxicity. rGO NPs have also been used in PDT for cancer in the last few years. To test the photodynamic activity in the human breast cancer cell line MCF7, Vinothini and coworkers (2020) created an rGO-based nanocomposite with magnetic NPs (Fe₃O₄), camptothecin, 4-hydroxycoumarin, and allylamine [104]. The rGO hybrid exerts cytotoxicity by producing ROS against MCF7 cells under 365 nm of laser irradiation of 20 mW cm⁻² for 3 min [104].

Fullerenes (also known as bucky balls) are closed convex polyhedra composed of an even number -of carbon nanostructures with a high triplet yield, and a high molar absorption coefficient and are made of sp2 hybridized carbon atoms. Fullerenes have multiple types but the most renowned forms are C60, C70, etc because of their important uses. Fullerenes are particularly effective at creating photoexcited singlet oxygen (₁O²) in organic solvents or hydrophobic environments, but in an aqueous environment, they convert to type 1 photochemistry and produce HO‧ and superoxide anions [105]. Although pristine C60 fullerenes are insoluble in water, the cage can be easily functionalized with polar groups such as carboxylic acids and quaternary amino groups to increase solubility and biological compatibility [105]. Various pristine C60 fullerenes as well as their functionalized derivatives, have been extensively used as PSs for in vitro cancer cell killing [106]. Fullerene C70 is also used in PDT of cancers due to its extended π system [107]. Guan and his team created fullerene C70 nanovesicles (abbreviated FCNVs) using C70-oligo ethylene glycol-Ce6 and comprised of both hydrophilic and hydrophobic components [46]. These constructs efficiently absorb light in the NIR region within 660 nm (20 mW cm⁻²) and exert strong anticancer activity in the lung carcinoma cell line A549 [46]. Fullerene C60 dyads are also reported for PDT in cancers. In one study four novel glucose-BODIPY-fullerene dyads with styryl units were created. The organic detergent Tween 80 helps to produce nanomicelles, which measure 14–17 nm [108]. When exposed to UV light, glucose-BODIPY-fullerene nanomicelles (14–17 nm) effectively produce singlet oxygen and ROS [108]. K562 human chronic myelogenous leukemia suspension cells were used to examine the molecules' in vitro anti-cancer efficacy under the influence of light and the absence of light for PDT [108].

Porphyrin-fullerene dyads have gained increasing interest due to their interesting optoelectronic properties and their potential applications in PDT. These dyads are created by joining organic molecules that donate electrons, like porphyrins, with molecules that accept them, like fullerene derivatives [109]. The efficient intramolecular photoinduced charge separation that the donor-acceptor dyads can undergo has sparked extensive research on the practical use of these materials. Additionally, it has been demonstrated that fullerene functions as a quencher, minimizing the photobleaching of the porphyrin component during PDT. A PS's therapeutic efficacy may be diminished due to photobleaching, which is the irreversible destruction of PSs upon exposure to light. In overextended PDT treatment sessions, the fullerene aids in preserving the stability and activity of the PS by quenching the excited states of the porphyrin. In 2021, Vallecorsa et al assessed the in vitro photoactivity of a porphyrin coupled to a fullerene and four meso-substituted porphyrins. The results suggest that the porphyrin-fullerene dyad TCP-C 604 has potential in PDT application as compared to fullerene dyad alone under 600 nm NIR radiation with a fluence rate between 10 and 220 mJ cm⁻² and the power density 0.5 mW cm⁻² [110] Although porphyrin-fullerene dyads show great promise in anticancer applications, it is important to note that further study is required to improve their characteristics and guarantee their safety and efficacy in clinical settings.

Chitosan is a polymer. It is obtained from chitin, which is usually found in crab and insect shells. It is made up of N-acetyl-glucosamine and glucosamine repeating units. Chitosan has a positive charge and is unable to dissolve at pH 7, but it dissolves at an acidic pH [122]. The amphiphilic nature of chitosan has promoted them to serve as carriers for hydrophobic anticancer therapeutics [123]. For PDT, the PS was either confined inside the inner core of self-constructed NPs of chitosan or it formed a covalent or ionic connection with chitosan, which was then assembled into a NP. Chitosan NPs (CNPs) can encapsulate protoporphyrin IX (PpIX), a PS, and vitamin B9 (PpIX-B9) and transport it to tumor cell masses for effective PDT [124]. In a different investigation, ICG-loaded hydrophilic sulfhydryl NPs of chitosan (SA-CS-NAC@ICG NPs) were developed by Yang et al using a self-assembly and self-crosslinking method for PDT [125]. Traditional PS carriers have drawbacks due to their poor chemical stability, inadequate loading, and single-responsive PS release. These drawbacks are resolved by these unique pH/GSH multi-responsive chitosan NPs. ROS are reduced by the SA-CS-NAC@ICG NPs when exposed to 808 nm laser light. In vitro cell experiments verified that these novel NPs had a great capacity for cellular absorption, low toxicity, and effective cancer inhibition [125]. Chitosan NPs with ALA derivatives, such as prodrugs 5-ALA and 8-ALA, have the synergistic effect of combining conventional PDT with electrochemotherapy for the treatment of melanoma cancer [126]. In a different study, a nano-code delivery of chitosan/tripolyphosphate (CS-TPP) was made by combining a PS 5-ALA and methylenetetrahydrofolate dehydrogenase 1-like (MTHFD1L) shRNA for PDT. By triggering apoptosis and producing ROS in vitro under 635 nm, 10 J cm⁻², this CS-TPP-(shMTHFD1L-ALA)-PDT demonstrated potent anticancer efficacy in oral squamous cell carcinoma (OSCC) [127].

Human serum albumin (HSA) is an acidic, positively charged plasma protein with a wide variety of roles. HSA is a globular, amphoteric protein that maintains its structure between pH 4 and 9 [128]. NPs created using HSA have recently attracted attention as prospective theranostic drug carriers, especially for the administration of lipophilic drugs. Hydrophobic medicines can be transported by HSA-based NPs without the use of potentially hazardous solvents [128]. Additionally, HSA-based NPs can target the tumor by endogenous albumin pathways with massive doses of chemotherapeutic medications and can avoid the problem of the accrual of drug NPs elsewhere in the body. Abraxane^® (albumin-bound paclitaxel NPs) is another example of a medication that utilizes HSANP-binding technology. In a medical facility, this technique is employed to provide paclitaxel for the treatment of pancreatic, breast, and pulmonary malignancies [128]. For the combined phototherapy and chemotherapy of breast cancer, flexible hollow human serum albumin NPs (HHSA) were produced and coupled with two distinct DOX therapeutics and the PS chlorin e6 (ce6) [129]. In in vitro conditions, it reduces the survival of 4T1 breast cancer cells without irradiation but it has remarkably higher toxicity under 660 nm irradiation. Additionally, these HHSADOX-ce6 NPs lower tumor metastasis in vivo [129]. ICG, a NIR fluorescent dye, has demonstrated significant possibilities in cancer PDT and PTT [130]. However this dye has some limitations that make it difficult to use effectively in PDT/PTT, such as the fact that it is insoluble in water, has a brief half-life, and has non-targeting accumulations. This is why Liu et al developed PEGylated-HSA-ICG-TAT to penetrate the nucleus of cancerous cells and get around ICG's drawbacks in the treatment of tumors [130]. They demonstrated that compared to free ICG, these ICG-loaded NPs had a greater cellular absorption rate and a stronger PDT/PTT impact. Moreover, these ICG-loaded NPs were easily metabolized in normal mice with no toxicity but in tumor-bearing mice, these NPs showed a significant reduction in tumor growth [130]. A similar NIR dye, IR780, and HSA NPs (HSA@IR780@DTX), which are loaded with docetaxel (DTX), were created for targeted imaging and PTT/PDT together with chemotherapeutic treatment of castration-resistant prostate cancer [131]. In this study, prostate cancer cells were irradiated with a 1 W cm⁻² 808 nm laser for 2.5 min and produced a large amount of ROS. In another study, a tumor-targeted multifunctional theranostic medication for PTT and PDT applications was made using four therapeutically approved substances: artesunate (Arte), folic acid (FA), HSA, and ICG under a single NIR irradiation. The produced nanocomposites (FA-IHA NPs) displayed good physiological and photo-stability, excellent cellular absorption, and tumor accumulation in vitro and in vivo [132]. Additionally, when this nanocomposite (FA-IHA NPs) were irradiated under 808 nm NIR (1 W cm⁻²) Arte was released in the tumor cell and showed a chemotherapeutic effect [132]. C086 is a derivative of Curcumin and it is an inhibitor of heat shock protein 90 (HSP90). It has stronger antitumor activity than Curcumin. Based on this property He et al developed a C086-loaded HSA NP for PDT against cancer cells [133]. The anti-tumor PDT activity of C086@HSA NPs is applied to the human cervical cancer cell line Hela. Upon 10 min blue light exposure at 450 nm (5 mW cm⁻²), it was found that C086@HSA NPs caused apoptosis and cell cycle arrest at the S and G2/M phase in HeLa cells. C086@HSA NPs were also tested for in vivo anticancer activity employing bright blue light (80 mW cm⁻²) using PDT in a 4T1 tumor-bearing mouse model [133].

Gelatin is a proteinaceous polymer that can be partially hydrolyzed from a pig or bovine collagen. It is a water-soluble macromolecule that is frequently used as a food additive. Gelatin NPs are now applied for PS delivery [134]. For in vivo PDT, the hydrophobic PS's water solubility must be increased, and its accumulation in tumor tissue must be increased. So Son et al have conjugated choline e6 (Ce6) with gelatin NPs for PDT applications [135]. The resulting conjugates were more water-soluble for an elongated period than hydrophobic Ce6, and upon laser irradiation under 658 nm (0.3 W, 200 J), they could produce singlet oxygen and destroy tumor cells. Additionally, the conjugates displayed an extended presence in the circulatory system and an exceedingly amplified assemblage in tumor tissue in vivo. Lee et al have developed self-assembled gelatin NPs containing both pheophorbide a (Pba) and thiabendazole (TPZ) to study the synergistic effect of drugs and their efficient delivery in tumor cells in vivo [136]. Gelatin polymer chains were coupled with PEG and Pba to create amphiphilic structures for self-assembly in aquatic conditions. These NPs were then loaded with TPZ to create TPZ-Pba-NPs. After laser irradiation at 95.1 J cm⁻², the efficacy of combination therapy employing TPZ-Pba-NPs was assessed in SCC7 tumor cells, and it was found that TPZ exerts toxicity in hypoxia. Additionally, in tumor-bearing mice, the in vivo therapeutic efficacy of combination therapy using TPZ-Pba-NPs was assessed [136].

Biopolyesters are biocompatible naturally occurring or synthetic polymers. PLA made from D- and/or L-lactic acid monomers (PLLA, PDLLA), poly(-caprolactone), poly(hydroxyalkanoates), and poly(lactic acid with glycolic acid) copolymers (PLGA) are examples of naturally occurring polymers. Poly(orthoesters), poly(-amino esters) (PbAE), and poly(-hydroxy esters) are examples of synthetic polymers. Hydrophobic PLGA or PCL NPs that trap hydrophobic PSs are examples of simple polyester-based NPs produced for PDT applications. When second-generation PSs were coupled with PLGA NPs, they demonstrated good photodynamic activity, tumor-inhibiting activity, improved ₁O² production, and increased plasma circulation duration. In one work, PLGA NPs were synthesized and conjugated with IR780 PSs for increased PDT against osteosarcoma HOS cells. These constructed NPs induce apoptosis and ferroptosis in HOS cells via excessive accumulation of ROS [137]. Similarly, in another study, Toluidine Blue was coupled as PSs in PLGA NPs (TB@PLGA), and it was proven that TB@PLGA NPs may trigger apoptosis via a photodynamic mechanism and cause widespread necrosis of tumor cells when exposed to 660 nm laser irradiation. Furthermore, it was discovered that these TB@PLGA NPs have an excellent tumor suppression effect in vivo [138]. For increased PDT of malignancies, PLGA NPs are employed for the regulated release of PS MB with a PARP inhibitor, veliparib in a very low concentration [139]. These VMB NPs showed no toxicity in the dark but when they were irradiated with visible light they showed significant toxicity against B16F10-Nex2 cells [139].

PEG is a polymer that is used to conjugate polyester NPs and micelles for drug entrapment in both hydrophobic and hydrophilic environments. PEGylated polyesters, such as PEG-PLGA, PEG-PCL, and PEG-PLA, are used to make a variety of NPs. PEGylated polyesters in nano PDT have been tried as a delivery strategy for hydrophobic PSs by several researchers [111]. Trimethylene carbonate-based monomers were used to create light-responsive polycarbonates (LrPC) and PEGylated LrPC (LrPC-PEG) before being loaded with the PSs 5, 10, 15, and 20-tetrakis(m-hydroxyphenyl)chlorin (mTHPC) [140].

Polymerization of a nanoemulsion matrix yields hydrogel-like nanostructures known as polyacrylamide (PAA) NPs. Biodegradable polyacrylamide NPs have become prevalent in cancer therapeutics such as PDT, fluorescence imaging, and cancer targeting [141]. Both biodegradable and non-biodegradable cross-linkers can be used to produce hydrophilic PAA NPs and can have their surfaces functionalized with targeted ligands. The two simplest approaches for conjugating a PS into PAA NPs are encapsulation and post-loading, and depending on the PS's water solubility, both procedures may have varying effects on PDT activity. When PAA NPs are coupled with hydrophilic PSs like MB and its derivatives, they produce more ₁O² under 808 nm laser at a power density of 1.5 W cm⁻² for 2 min and increase cell death [142]. In another work amine-functionalized PAA NPs were loaded with hydrophobic PSs such as HPPH and NIR fluorescent organic dyes (NIRFDs). These NIRFDs can be excited in the first or second NIR windows of tissue optical transparency (NIR-I, ∼700–950 nm and NIR-II, ∼1000–1350 nm) but the PS HPPH cannot absorb light. The result shows that these nano-shell conjugates produced the most ₁O² and exhibited the highest phototoxicity in vitro and in vivo [143]. In an earlier study, tetrasulfonato-aluminum phthalocyanine-entrapped NPs coated with a second porphyrin-based PS and polylysine-bound tetrasulfonato-aluminum phthalocyanine (PCNP-P) were synthesized and were loaded with photodynamic PS. These two polyacrylamide-based NPs produce huge ROS in HT29 cells when irradiated with 7 J cm⁻² laser light and have good therapeutic potential [144].

Concentrated phospholipid vesicles known as liposomes have several bilayered membranes composed of lipids derived from either natural or artificial sources. They make ideal therapeutic carriers since they have the unique capacity to store hydrophilic medicines in their aqueous interior and substances that are hydrophobic in their inner layers [145, 146]. Different PS has been used in specific commercial liposomal formulations, including benzoporphyrin derivative (BPD)-MA, mTHPC, and zinc phthalocyanine (Zn-Pc) [147]. A PS-lipid combination called porphyrin-phospholipid leads to persistent, non-exchangeable inclusion into the liposome bilayer. Liang et al used nanoscale (20 nm) iron oxide-loaded porphyrin-grafted NPs of lipid to create a theranostic nano platform (Fe₃O₄@PGL NPs) [148]. These nano platforms generate ROS to kill cancer cells and transfer Fe ions into tumor cells via the Fenton reaction [148]. In a different study, DOX, an effective chemotherapeutic agent, was added to porphyrin-imbedded lipid NPs for cooperative chemo-PDT. PGL-DOX NPs established exceptional cellular absorption, chemo-photodynamic reactions under 650 nm laser (0.2 W cm⁻², 10 min), and fluorescent imaging abilities in HeLa and PC3 cell lines. Porphyrin's usual ability to generate ROS with lower irradiation meaningfully blocked tumor growth in vivo once it united with the lethal potentials of the anticancer drug DOX [149]. Therefore from these studies, it can be stated that liposomal/lipid NPs can be used in PDT of cancers with suitable PSs.

Dendrimers have received a considerable amount of interest as PS vectors because of their unique architecture. It contains several functional groups on the surface that can connect various molecules or functional moieties. The size of these molecules and their lipophilicity can be modified to improve cellular absorption and tissue biodistribution. PAMAM dendrimers are the most promising and well-characterized dendrimers. PAMAM dendrimers are used in PDT for the transportation of PS in tumor tissues. They are frequently used as templates for preparing tiny NPs. To create self-assembled NPs, PAMAM dendrimers are treated with poly(ethylene glycol) cholesterol. The hydrophobic core of the NPs is enclosed with PS chlorin e6 (Ce6) and MnO₂ to increase the PDT. When exposed to a 670 nm laser, the developed DPCCM NPs act like enzymes that can catalyze the oxidation of H₂O₂ to produce O₂ and exert an enhanced PDT effect [150]. Kojima et al used PEG-attached dendrimers to create nanocapsules with PSs for use in PDT [151]. They created two PEG-attached dendrimers from poly(amidoamine) (PEG-PAMAM) and poly(propylene imine) (PEG-PPI) and employed two PSs, RB and protoporphyrin IX (PpIX) at the same time. When compared to free PpIX, the combination of PpIX with PEG-PPI demonstrated efficient cytotoxicity by producing high levels of singlet oxygen under 530 nm light using a Xe lamp (400 W m⁻²) and efficiently delivering this singlet oxygen to mitochondria [151]. Along with PAMAM dendrimers, other dendrimers like PPI, ALA, and anionic phosphorous dendrimers are also studied for PDT applications [152].

From several clinical studies on identifying molecular markers and studying genetic patterns in these years, it has been well established that genetic variability in patients accounts for a major difference in drug response among individuals. At the same time, gene variations in cancer cells also play an important role and are responsible for drug resistance to chemotherapy. Phase I drug metabolizing enzymes inactivate various chemotherapeutic drugs by hydrolysis, reduction, and oxidation [161]. Humanoid cytochrome P450 (CYP) consists of more than 50 enzymes that play a role in inactivation via the metabolism of Phase I drugs among which CYP2D6 is one of the key enzymes. Tamoxifen which is a commonly used chemotherapeutic drug for breast cancer is metabolized by CYP2D6 to produce endoxifen and 4-hydroxytamoxifen which are therapeutically ineffective and thus lose their therapeutic activity against breast cancer [162]. From research findings, it is evident that patients with a combination of some non-functional allele could decrease the metabolism of the breast cancer chemotherapeutic drug tamoxifen which could reduce the chances of disease-free survival (DFS) or recurrence-free survival and thus lowers the therapeutic efficacy [162]. Plasma membrane transporters are essential for both the absorption and efflux of several chemotherapeutic medicines, and they are closely related to drug resistance. Most drug's intracellular concentration is controlled by the big solute carrier (SLC) family of membrane transporters, which affects drug absorption. Contrarily, drug efflux is caused by the ATP-binding cassette (ABC) superfamily, and some of its members also take part in intracellular drug accumulation. These ABC transporter proteins which are controlled by ABCC1, ABCB1, or P-glycoprotein (p-gp) play a significant role in the development of drug resistance as they determine the drug distribution and absorption by limiting the passage of drugs across the cell membranes [162]. Among different drug efflux proteins, ABCB1 has shown the highest clinical significance for the development of drug resistance from patient samples.

Many chemotherapy drugs, including platinum compounds, have cytotoxic effects that are caused by the production of various DNA adducts [163]. These adducts cause intra and inter-strand breaks, which stop DNA replication and cause cell death. However, various DNA damage repair mechanisms, such as nucleotide-excision repair and base-excision repair (BER), are available to cells to fix these defects. The clinical effectiveness of various medications, including cisplatin (CP), depends on the cancer cell's capacity to repair DNA damage caused by chemotherapy drugs. One of the members of the BER protein family 'XRCC1' can detect and repair DNA damage induced by CP [164]. As a consequence, cancer patients having upregulation of XRCC1 have shown decreased therapeutic responses towards CP and drug unresponsiveness. However, there are few cases of clinical studies where there is no connection observed between CP resistance and XRCC1 or genotypic relevance [164, 165]. In conclusion, several genetic markers have been studied for biomarkers of drug resistance in cancers, but the findings suggest that although some of them are linked with genetic variants but not all could be linked to it. Additionally, there must be other factors that have a key role in the development of drug resistance.

Drug effectiveness or side effects are observed in case of parallel uptake of another medication which is linked to the pharmacodynamic and pharmacokinetic effects. Since cancer patients frequently take multiple medications, including anticancer medications, supportive care medications (such as antibiotics, anxiety relievers, gastrointestinal acid-reducing agents, and cholesterol-lowering statins), and other medications, they are particularly vulnerable to show these drugs non-responsiveness due to their inactivation by other [166]. Multiple studies have shown that cancer patients taking intravenous or oral administration of chemotherapy have shown drug inactivation due to another drug which is also a category of drug resistance or unresponsiveness observed [166–168]. Proton pump inhibitors (PPIs) are the most often prescribed stomach acid-reducing medications, and it is known that many cancer patients take them to treat their gastroesophageal reflux disease and dyspepsia symptoms. Tyrosine kinase inhibitors (TKIs) have poor bioavailability and solubility when taken orally. The uptake of TKIs largely depends on the pH and it decreases as stomach pH is raised by PPIs. Thus, PPIs have a role to play in reduced uptake and efficacy of TKIs class of drugs [169]. On the other hand, uptake of the TKIs is observed to enhance when taken alongside acidic beverages.

Although the gastrointestinal tract may be a site for drug absorption, the liver is the main location for drug biotransformation. Drugs that can activate human cytochrome P450 (CYPs) through higher transcription can lower the blood concentration of the induced enzymes' drug substrates. On the other hand, CYP inhibitors reduce the activity of a prodrug that is converted into a pharmacologically active drug. Additionally, several CYP stimulants or inhibiting agents are also ABC transporter stimulants or inhibiting agents. In summary, DDIs should be crucially considered for cancer patients receiving medications as this leads to drug resistance and decreased effectiveness.

To overcome drug resistance in cancers, escalating therapeutic drug dosage, and repeated administration has been major factor in the efficacy of present cancer therapies. Anticancer medications must be proactive and have high bioavailability in the tumor tissues to be therapeutically effective. Drugs that have targets within the cell must penetrate the plasma membrane to reach tumor cells [170]. Early research on drug resistance suggested that dysfunctional plasma membrane transporter proteins cause the inability of drug accumulation inside cells and this leads to drug resistance [171]. As discussed previously, cancer cells with drug resistance have an active efflux pump that ejects out hydrophobic molecules or metabolites including the drugs through the plasma membrane which leads to decreased intracellular concentration of the drug and low therapeutic effect. One such drug efflux pump is the ABC transporter proteins [172, 173]. Cells exhibit lower intracellular drug concentration during the development of drug resistance and malignant transformation, which is primarily attained by decreased drug inflow, and increased outflow. Since the majority of common anticancer medications are weak bases with pKa between 7.4–8.4 and are hydrophobic, they permeate the cell membrane passively, but the rate of the same drugs is significantly impaired when the extracellular tumor surroundings are acidic because these medications become strongly protonated and pass the cellular membrane considerably less effectively when charged [174]. The decreased drug inflow via influx transporter proteins, which include SLCs, is one of the causes of the reduced intracellular drug levels found in chemo-resistant cancer cells. Additionally, it is widely acknowledged that ABC transporters like P-gp, MDR-associated protein 1 (MRP1), and BCRP are primarily responsible for increased drug efflux. Drug resistance has been linked to the overexpression of proteins of the ABC superfamily, including BCRP (ABCG2), MDR-associated protein 7 (MRP7/ABCC10) and P-gp, MRP1 (ABCC1) (figure 6) [175, 176]. Many neutral and cationic hydrophobic antitumor chemotherapy drugs, such as paclitaxel, DTX, etoposide, and teniposide acquire MDR when P-gp is over-expressed [177]. Patients with non-small cell lung cancer who were administered with paclitaxel had poor response rates when ABCB1 (P-gp) upregulation was present. During up-regulation of MRP1 cancer cells demonstrate MDR towards etoposide, teniposide, vinblastine, vincristine, and DOX [178]. MDR towards methotrexate, DOX, and camptothecin is caused by upregulation in the BCRP gene (ABCG2) [179]. Since the upregulation of ABC transporters causes poor drug responses and outcomes in patients with many types of cancers, so inhibiting these transporter proteins could be an effective strategy to tackle multidrug resistant cancers and improve the therapeutic response in clinics.

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EVs have been well studied as a route for the development of MDR in cancers. EVs are phospholipid bilayer-enclosed NPs (25–1000 nm) which are incapable of replication. In the payload of EVs, drug efflux pumps like as P-gp, MRP1, and BCRP has been discovered [180]. It is thought that some of the membranes of the EVs ejected by MDR cells may have these drug transporters oriented inverted, which would facilitate the entry of medicines into the EVs [181]. The ability of EVs to enclose chemotherapy medicines in their payload is undoubted. EVs may expel medications into the extracellular environment once they have been liberated from tumor cells thus the bioavailability of the chemotherapeutic drug in the tumor cells is reduced and the therapeutic or drug response is inhibited [181, 182]. This unique mechanism of EVs-mediated therapeutic resistance is supported by in vitro research on the absorption of chemotherapy medicines within EVs. However, there is not much clinical evidence to support the fact that EVs play a role in the development of drug resistance. Rituximab was administered to lymphoma patients in research that used fewer clinical samples [183]. Rituximab was shown to bind to EVs extracted from the patient's plasma, indicating a reduction in the drug's availability for therapeutic effect. In another clinical research, it was observed that EVs isolated from early-stage breast cancer patients of human epidermal growth factor receptor 2 (Her2) showed reduced absorption of trastuzumab compared to patients with advanced stages of the disease, indicating the role of EVs in reducing the availability of the drug and assisting in the development of chemotherapy resistance and poor outcome [184].

Any cell must have an effective DNA damage repair mechanism to sustain genomic stability, retain cellular homeostasis, and stop the growth of cancer. Mutations build up when the DDR's regular control is compromised, resulting in carcinogenesis, rapidly evolving tumors, and resistance to various DNA-damaging drugs. Platinum medicines including carboplatin, CP, and oxaliplatin are among the chemotherapy drugs used in the majority of traditional cancer therapies that cause DNA damage [197]. Some drugs, like nitrogen mustard or chloroethylnitrosoureas, create DNA adducts that prevent cancer cells from actively replicating DNA [197]. Replication stress is created when DNA replication fails and DNA damage cannot be repaired, which causes cells to undergo apoptosis. Deregulated activity in DNA damage repair pathways and significantly enhanced capacity of the cell to restore DNA damage and prevent apoptosis is closely linked with the emergence of drug resistance [198]. When a clinical examination was performed for almost 62 susceptible genes in a cohort composed of 15 000 breast cancer patients and normal people, it was observed that almost 57 of the breast cancer patients were found to have lost function mutation in RAD51D which is a DNA damage repair gene [199]. On the other hand, less than 15 healthy people showed a similar mutation in RAD51D thus indicating the role of DDR genes in developing drug resistance [199].

Alterations in DNA structure are known as epigenetic modifications, which do not involve sequence changes yet are persistently passed down from cell to cell. The epigenetic controls that contribute to treatment resistance in cancer include histone, DNA methylation, microRNA, and chromatin alterations [200]. Several MDR genes, pro-apoptotic genes (APAF-1) [201], including drug transporters (ABCB1) [202], histone modifiers [203], and DNA-repair proteins (MGMT) may have their expression altered after chemotherapy due to epigenetic processes.

One significant epigenetic change seen in many malignancies is DNA methylation which is carried out by DNA methyltransferase [204]. During methylation of the genome, a methyl group is bound to the cytosine residues at the 'CpG islands' thus helping to silence genes by inhibiting gene transcription. The majority of abnormal DNA methylation in cancer is connected to genes that regulate cell proliferation and differentiation, Wingless/Integrated (WNT), Mitogen-activated protein kinase (MAPK), p53, and Vascular endothelial growth factor (VEGF) signaling, or the production of cell cycle inhibitors [205]. Hypermethylation and MDR are highly associated with many malignancies [206]. Different promoter methylation patterns in testicular cancer can be identified from non-seminomas, reflecting particular clinical traits for MDR in patients [207]. Thus, to improve the efficacy of various treatment regimens and combat MDR, therapeutic targeting of epigenetic changes emerges as a crucial and promising method [208].