The Combination of Active‐Targeted Photodynamic Therapy and Photoactivated Chemotherapy for Enhanced Cancer Treatment

ABSTRACT

Scientists have been actively investigating novel therapies that can effectively eradicate cancer cells with negligible side effects in normal tissues when used alone or in a combinatorial approach. Photodynamic therapy has emerged as a promising non‐invasive therapy that integrates photosensitizer, oxygen, and a specific wavelength of light for the treatment of cancer. Despite encouraging outcomes yielded by PDT, conventional PSs are faced with longstanding challenges such as poor water solubility, a short half‐life, and off‐target toxicity. Development of nanotherapeutics has shown great potential in overcoming this issue. The tumor microenvironment is inherently hypoxic, and this promotes tumor resistance to PDT, as it is oxygen‐dependent. Photoactivated chemotherapy, an oxygen‐independent light‐based therapy, utilizes chemotherapeutic regimens that remain inert until exposed to light, allowing target‐specific activation while minimizing off‐target toxicity. Integration of these techniques can improve selectivity and yield synergistic cytotoxic effects that could improve cancer treatment.

Photoactivated chemotherapy (PACT), an oxygen‐independent light‐based therapy, utilizes chemotherapeutic regimens that remain inert until exposed to light, allowing target‐specific activation while minimizing off‐target toxicity. The integration of these techniques has the potential to improve selectivity and yield synergistic cytotoxic effects that could improve cancer treatment.

1. Introduction

Cancer remains a daunting public health issue that claims approximately 8.2 million lives globally. By 2030, projections indicate that cancer will cause over 13.1 million deaths worldwide, making it the second‐leading cause of death after cardiovascular disorders [1]. Current therapeutic interventions for treating cancer include chemotherapy, surgery, and radiotherapy. However, these modalities are impeded by complications relating to bone marrow suppression, gastrointestinal tract issues, hepatic and renal toxicity, and cardiac impairment [2]. Furthermore, medicinal treatments rarely yielded an appreciable prognosis in patients due to the heterogenicity of cancers and the development of therapy resistance mechanisms [2, 3]. Surgery is closely associated with mortality and morbidity, hampering quality of life [2]. The search for alternative treatments remains imperative to improve the prognosis and quality of life of patients without attenuating therapeutic efficacy. Photodynamic therapy (PDT) has been under intense investigation due to its non‐invasiveness and increased potency. PDT is based on the integration of three essentials: PS, light with a suitable wavelength, and the availability of molecular oxygen [4]. The PDT reaction begins with the absorption of the PS by tumor tissues, followed by its activation using a specific wavelength of light, which leads to a series of photochemical reactions that exert cytotoxic activity in cancer cells due to the generation of reactive oxygen species (ROS) [5].

In recent years, scientists have been actively engaged to improving the effectiveness and target‐specificity of cancer therapies. In relation to PDT, the hydrophobic nature of PSs causes them to aggregate in aqueous conditions, thereby drastically hampering the efficacy of PDT [6, 7]. In addition, PSs cannot exclusively differentiate between tumor cells and healthy tissues due to their poor specific affinity for tumor cells, so adjacent healthy tissues may be affected during treatment. In this instance, the integration of nanotechnology and PDT holds great promise for addressing the challenges [7]. Oxygen is an indispensable element for initiating the PDT mechanism of action. However, the excessive proliferation of cancer cells leads to a hypoxic microenvironment of cancer cells, which drastically stifles the PDT activity [8]. Therefore, recent studies have pointed out that hypoxia is a hallmark characteristic of tumor tissue, which significantly reduces intracellular ROS yield [9]. During the process of PDT, the increased usage of PDT may further exacerbate tumor hypoxia, even leading to reduced PDT overall efficacy [8, 9]. Therefore, the combination of PDT with oxygen‐dependent modalities is one of the main strategies to improve cancer treatment through synergistic effects.

PACT is a promising light‐based cancer treatment that has garnered a lot of attention due to its increased selectivity and negligible side effects. Contrary to PDT, PACT is oxygen‐independent and more effective for treating hypoxic tumors [10]. PACT agents undergo a photoinduced change in their chemical formula upon irradiation, which in turn releases cytotoxic compounds that are independent of oxygen in tumor cells [11]. Table 1 summarizes the differences between PDT and PACT. In this review, we examine the potential combination of photoactive agents that could be used in PDT and PACT therapies to produce anticancer synergistic effects in tumor cells, thereby eliminating resistance and hypoxic tumors that are unresponsive to PDT alone.

TABLE 1.

Photodynamic therapy versus photoactivated chemotherapy.

Characteristic	Photodynamic therapy	Photoactivated chemotherapy	References
Oxygen dependence	Less effective in low oxygen concentrations	Remains effective in hypoxic conditions	[12]
Stoichiometry	Catalytic	Stoichiometric	[13]
Mechanism of action	Generates cytotoxic singlet oxygen and ROS	Activates prodrugs to release cytotoxic agents	[14]
Clinical application	Superficial tumors	Deep‐seated tumors	[15, 16]
Cost and availability	Cheap and widely available	Expensive and limited availability	[17]
Drug type	Photosensitizers	Photoactivated prodrugs	[18]

2. Photodynamic Therapy

PDT is a clinically proven advancement in cancer treatment that has garnered significant attention as a potential method of cancer treatment over time [19]. Unlike conventional therapies, PDT is less invasive and demonstrates a strong cytotoxic affinity for malignant cells versus healthy tissues. PDT directly induces cell death via various modes of cell death mechanisms, namely apoptosis, necrosis, and autophagy, which can considerably improve the quality of life and prognosis of cancer patients [4, 20]. Hence, PDT has gained popularity in cancer treatment.

2.1. Mechanism of Action and Modes of Cell Death Induction

To eradicate cancer cells via oxidative stress, the PDT process requires the optimum integration of a specific wavelength, a PS, and tumor molecular oxygen. The success of this treatment hinges on the PS molecule, which can be selectively retained in target cells or tissues relative to surrounding normal tissues upon systemic or local administration [21]. Once the optimal dose of PS is retained, the targeted tissues can then be illuminated with visible light for a predetermined exposure period. When exposed to light corresponding to its action spectrum, the PS transitions from the ground state (single state) to the excited state (triplet state) [22]. In this triplet state, PS combines with molecular oxygen to form reactive oxygen species via two mechanisms [7]. Type I involves the PS interacting with biomolecules via a transfer of hydrogen atoms to form radicals, which then react with molecular oxygen to produce ROS. Whereas in a type II reaction, ions and energy directly react with oxygen to generate extremely oxidizing singlet oxygen (¹O₂) [23]. However, the initiation of the aforementioned mechanisms is influenced by various factors, such as the oxygen level, pH, and the configuration of the PS. When all the oxygen is consumed, the type II mechanism thrives [24]. Figure 1 depicts the rationale behind PDT.

FIGURE 1.

Illustration of photodynamic therapy mechanism of action.

The photodamaging activity of PDT is attributed to three mechanisms, such as ROS‐induced tumor cell death, antitumor immune responses, and tumor‐related vascular system vascular destruction. PDT destroys cancer cells predominantly via apoptosis, necrosis, or autophagy [25]. The occurrence of different types of cell death depends on the accumulation of PS within the cell. The destruction of the mitochondria triggers apoptosis; cell membrane disruption and loss of integrity evoke necrosis; and impairment of the lysosomes or reticulum causes autophagy [7]. Moreover, the efficacy of PDT is also influenced by its ability to evoke systemic antitumor immune reactions. PDT destroys the architecture of the tumor, allowing cancer cells and immune cells to interact directly. The direct destruction of tumor tissue elicits a potent inflammatory reaction, and leukocytes infiltrate the tumor, stimulating a significant release of inflammatory mediators [26]. The overall PDT outcomes largely hinge on the choice of PS based on its characteristics related to solubility and target‐selectivity [7].

2.2. Photosensitizers

PSs are molecules that demonstrate light‐sensitive chemical and/or physical reactions when excited with light correlating with their absorption spectrum [19]. Ideally, A PS should be inactive in the dark, exhibit high chemical purity, selectively accumulate in the target tissue, be readily available and cost‐effective, remain stable at room temperature, be effectively eliminated from the body, exhibit a short drug–light interval, and effectively generate singlet oxygen upon irradiation [7]. Photofrin and hematoporphyrin are categorized as the first‐generation PSs [27]. Photofrin remains the widely used PS, which has been approved for the treatment of various cancers, such as cervical cancer, lung cancer, bladder cancer, and esophageal cancer. However, this PS cannot reach its full potential in clinical trials due to drawbacks related to low chemical purity, skin hypersensitivity, and poor absorption in the therapeutic window [6].

These issues compelled researchers to develop second‐generation PSs with greater chemical purity, a higher singlet oxygen yield, and enhanced tissue penetration depth due to their strong absorption in the therapeutic window. Furthermore, they exhibit negligible side effects due to their higher affinity for cancer tissues and rapid clearance from the body [28]. Several classes of PS have been the subject of extensive research for PDT applications, such as phthalocyanine derivatives, anthraquinone derivatives, porphyrin derivatives, chlorin‐e6 derivatives, and photofrin derivatives [29]. In recent years, third‐generation PSs have garnered significant attention due to their great potential to alleviate the limitations of conventional PSs (poor efficacy, off‐target toxicity, and poor light absorption rate) as they can selectively accumulate in target tissues while sparing normal tissues, which is attributed to their modification with inherently hydrophilic nanomaterials [30]. Most PSs are intrinsically hydrophobic, thereby tending to aggregate with other molecules in an aqueous setting [31]. Therefore, combining these PSs with nanomaterials, including hydrophilic polymers, circumvents this issue. Furthermore, nanomaterials not only alleviate the aggregation of PSs but also prevent their premature degradation before reaching tumor regions, thereby increasing their bioavailability and efficient drug delivery [32]. Table 2 gives an overview of several promising PSs that have garnered significant attention in the clinical setting [25, 33].

TABLE 2.

PSs that have been explored in the clinical setting.

Photosensitizer	Approval	Cancer type	Wavelength (nm)
5‐Aminolevulinic acid (5‐ALA, Levulan)	Worldwide	Actinic keratosis	635
Methyl aminolevulinate (Metvix)	Worldwide
Talaporfin sodium/NPe6 (Laserphyrin)	Japan	Lung cancer	664
Bremachlorin (Radachlorin)	Russia	Sarcoma, nasopharyngeal, brain	660
Temoporfin/mTHPC (Foscan)	Europe	Head and neck, brain, skin, lung, bile	652
Verteporfin (Visudyne)	Worldwide	Skin, ophthalmic, pancreatic	690
Porfimer sodium (photofrin)	Worldwide	Lung, brain, esophagus, bile duct, ovarian	630
Hexaminolevulinate hydrochloride (Cysview)	USA, Europe, Canada	Bladder cancer detection	360–450
Redaporfin (LUZ 11)	Pending approval in Europe	Bile duct	749
Padoporfin (TOOKAD)		Prostate cancer	762

2.3. Nanotechnology in Photodynamic Therapy

In recent years, nanoparticles (NPs) have been extensively investigated in the field of PDT to address various challenges hindering its progress in both preclinical and clinical studies. NPs are commonly described as submicroscopic particles with dimensions ranging from 1 to 100 nm. They are derived from a wide range of naturally occurring or synthetic materials and can be designed as cargo for various theranostic agents in a targeted manner [3]. NPs are endowed with large surface‐to‐volume ratios, which allow for effective delivery of adequate amounts of PS at targeted regions [34]. They may prevent untimely PS release and its potential for inactivation by biological barriers, thereby preventing unwanted accumulation in healthy tissues, and reducing overall photosensitivity [35]. NPs improve the amphilicity of PSs, which allows them following freely travel through the bloodstream and localize in the tumorous tissue [36]. They typically capitalize on the enhanced permeability and retention (EPR) effect, a phenomenon attributed to impaired and leaky tumor neovasculature and poor lymphatic drainage of the tumor, which promotes both the diffusion of PS nanocarriers into and absorption by tumor tissue [37]. Furthermore, their highly modifiable surface chemistry facilitates the attachments of targeting moieties (i.e., antibodies, peptides, folic acid, and carbohydrates), which enhances biodistribution, pharmacokinetics, cellular uptake, and target‐selectivity [38]. They can be formulated as multifunctional nanocarriers that carry diverse components, such as imaging agents and chemotherapeutics, which allow for effective combinatorial regimens for cancer treatment [39]. Various nanocarrier systems have been used in PDT applications, such as liposomes, dendrimers, nanomicelles, upconversion NPs, metal–organic, mesoporous silica, graphene‐based nanomaterials, and metal–organic frameworks [7].

2.4. Active‐Targeted Nanocarrier Delivery Systems in Photodynamic Therapy Applications

Nanocarrier systems can promote either passive or active localization of PS in tumor regions Figure 2. PS incorporated in NPs (passive uptake) are not target‐specific and, to some extent, accumulate in adjacent normal tissues [40]. In order to tackle these problems, nanocarriers are modified with targeting moieties for precise spatial control in vivo, thereby enhancing the effectiveness of PS molecules compromised by passive accumulation and their incapacity to differentiate between tumor cells and normal cells. Various biomarkers are exclusively or highly expressed on the surfaces of cancer cells, and the incorporation of ligand onto the surface of the nanocarrier systems can effectively identify tumor cells by binding to these markers, which will significantly mitigate collateral damage to healthy neighboring tissues [41]. Active‐targeting is facilitated by receptor‐mediated endocytosis, which enhances the internalization of nanocarriers by cancer cells, consequently improving PS bioavailability within the cells [38].

FIGURE 2.

Passive and active‐targeted delivery techniques.

Several studies have shown promising results with active‐targeted nanocarrier delivery systems in PDT applications. Shi et al. [42] investigated the phototoxic effect of diiodostyryl bodipy‐conjugated hyaluronic acid NPs (DBHA‐NPs) on colon cancer cells in vivo. DBHA‐NPs significantly inhibited tumor growth due to their target selectivity and EPR effect. Wang et al. [43] modified pyropheophorbide (Pyro) with folic acid and reported that this conjugated Pyro showed remarkable PDT efficacy due to improved cellular uptake of Pyro compared to free Pyro. Similar observations were also reported in ovarian cancer [44]. Another study evaluated the efficacy of hypericin‐loaded transferrin nanoformulations on colorectal cancer cells. The nanoformulation was stable during PDT and significantly yielded cytotoxic ROS compared to free hypericin [45]. Pan et al. [46] designed a photoactive antibody‐Chlorin e6 conjugate to target tumor cells in vitro and in vivo. Both in vitro and in vivo studies demonstrated that the conjugate could rapidly and efficiently target tumors, generate ROS, and destroy tumor cells. In addition, Li et al. [47] investigated the effect of a nanocarrier system composed of indocyanine green (ICG), hollow gold nanospheres, and TNYL peptide to actively target EphB4 receptors expressed by CT‐26 tumor cells. Li and colleagues noted that the nanoconjugate showed an increased accumulation in CT‐26 tumor cells compared to free ICG.

A year later, Li and colleagues [48] conjugated pyropheophorbide (Pyro), a cyclic cRGDfK (cRGD) peptide, which significantly increased the targeted‐specificity of Pyro and its tumoricidal effects in murine models versus unbound Pyro. Table 3 summarizes active‐targeted nanocarrier systems that have shown promising results in PDT applications. Despite the promising results shown by active‐targeted nanocarrier systems, the hypoxic nature of solid cancer remains a daunting challenge for PDT efficacy. The scarcity of O₂, the most paramount fundamental of the PDT process, which serves as the main facilitator for the energy transfer light, can significantly reduce phototoxic damage through singlet oxygen generation and therefore lower the therapeutic efficacy [49]. Therefore, it remains imperative to circumvent this issue by integrating PDT with modalities that are not reliant on O₂ to function so that they are supplementary when oxygen in the tumor depletes during the PDT process.

TABLE 3.

Various active‐targeted nanoplatforms explored in cancer therapy using photodynamic therapy.

Targeting biomolecule	PS‐nanocarrier	Cancer	Outcome	References
cRGDyk peptide	Ferrous chlorophyllin (Fe‐CHL) modified polylactic‐co‐glycolic acid NPs	Melanoma cells in vitro (B16‐F10)	The nanobioconjugate showed enhanced cellular uptake of the PS and increased singlet oxygen yield	[50]
Anti‐HER‐2 antibodies (Ab)	5‐Aminolevulinic acid (5‐ALA) conjugated to Gold‐Silver NPs	Breast cancer cells in vitro (MCF‐7)	The active nanocarrier system significantly reduced cell viability to 13% versus free PS with 49%	[51]
Folic acid	Chlorin e6 loaded with thermosensitive liposomes (TSL) with copper sulfide (CuS) NPs	Cervical cancer cells in vivo & in vitro (HeLa)	The active nanocarrier increased cellular uptake of Ce6 and controlled its release, which resulted in remarkable phototoxicity and inhibition of tumor growth	[52]
Transferrin	Hypocrellin A (HA) loaded onto poly(d,l‐Lactide‐co‐glycolide) (PLGA) & carboxymethylchitosan (CMC) NPs	Lung cancer cells in vitro & vivo (A549)	The nanobioconjugate induced apoptosis, which resulted in tumor delay at a 63% tumor inhibition rate for 15 days with negligible side effects on normal tissues	[53]
Hyaluronic acid	Chlorin e6 conjugated to β‐cholanic acid (5β‐CA) NPs	Colon cancer cells in vitro & in vivo (HT29)	The nanobioconjugate‐PDT considerably inhibited tumor growth by 10 fold versus control groups, which showed no alterations in tumor growth. In vivo tumor models presented no off‐target toxicity	[54]
DNA Aptamer	Chlorin e6 conjugated to gold NPs	Liver cancer cells in vitro & in vivo (HepG2)	The nanobioconjugate demonstrated remarkable PDT and chemotherapeutic synergistic effects and showed improved cellular uptake	[55]

2.5. Hypoxia Limitation in Photodynamic Therapy

Hypoxia is a common feature exhibited by solid tumors, which influences a series of physiological anomalies in the tumor microenvironment, including impaired tumor vasculature, erratic cellular proliferation, and a defective lymphatic system [56, 57, 58]. In this instance, tumor cells generate energy through various metabolic pathways that are accelerated by consuming endogenous O₂, particularly the oxidative phosphorylation (OXPHOS) metabolic pathway. As a result, the oxygen‐dependent nature of PDT makes it difficult for ROS generation in tumor cells. Furthermore, this insufficient oxygen supply leads to limited ROS generated in a cell even after continuous PDT processes [59, 60, 61]. Thus, at the cancer cell level (where PDT occurs), cancer cell characteristics (hypoxia) are not conducive to PDT (Figure 3). When it comes to the PDT process (Figure 3), oxygen consumption during PDT can worsen hypoxia in cancer cells and ultimately impede therapeutic outcomes by inducing changes in the proteome and genome of neoplastic cells. Nonetheless, anti‐apoptotic factors get stimulated to attenuate the cytotoxic effects of PDT‐generated ROS. In addition, tumor hypoxia promotes survival and tumor development by allowing cells to surmount nutrient scarcity or evade their hostile environment. Given this, tumor metastasis and angiogenesis are directly attributed to hypoxia and promote cellular proliferation, which significantly reduces the therapeutic effect of PDT. Therefore, tumor hypoxia counteracts PDT processes via ROS production and shields tumor cells from PDT‐induced destruction by evoking a series of pro‐survival reactions, ultimately promoting PDT resistance. In this regard, this poses a significant challenge in PDT [9, 62].

FIGURE 3.

Hypoxia activating signal transduction pathways linked to cell survival in cancer cells following photodynamic therapy [9]. Copyright 2022, Elsevier.

2.6. Photoactivated Chemotherapy

The search for new chemotherapeutic drugs for cancer has been the main preoccupation of researchers for decades. As a result, transition metal‐based agents have garnered significant attention, which is basically attributed to the long‐term clinical application and foundation laid by cisplatin [cis‐Pt (NH₃)₂Cl₂] [63, 64, 65, 66, 67, 68]. Cisplatin and its analogues (carboplatin, oxaliplatin, nedaplatin, heptaplatin, and lobaplatin) have yielded good results in the clinical treatment of various cancers [69]. However, these cisplatin and its derivatives are hampered by poor target‐specificity and off‐target toxicity, which cause severe side effects such as hair loss, neuropathy, and myelotoxicity in patients [70, 71, 72, 73]. Great strides have been made to address these issues, which include the incorporation of targeting nanocarrier systems and investigating reduction‐responsive Pt(IV) prodrugs [74, 75]. Furthermore, researchers are actively engaged in developing photoactivated Pt(IV) agents, which are inactive in the dark but can release tumoricidal Pt(II) species upon exposure to light irradiation. These prodrugs are referred to as PACT agents, which allow for controlled timing and location of radiation exposure to confined drug activity within tumor tissues with negligible side effects on normal tissues [15, 18, 76].

PACT is oxygen‐independent, which makes it suitable for treating hypoxic tumors [77, 78]. In contrast, the efficacy of PDT mainly hinges on the availability of tumor molecular oxygen. However, tumor masses are deprived of oxygen due to their uncontrolled growth and suboptimal vascularization, as well as increased distances for oxygen diffusion [77]. PACT holds great promise to circumvent this limitation [18]. The tremendous potency exhibited by Pt complexes in cancer therapy has propelled the investigation of other transition metal complexes, in particular Ruthenium (Ru) complexes, due to their potential tumoricidal properties and high affinity for cancer cells [79]. Researchers in modern medicinal inorganic chemistry are now focused on developing new Ru‐based [80, 81, 82, 83]. Several Ru(III) complexes, such as NAMI‐A, KP1019, and KP1339, have reached the clinical trial stage thus far [84, 85, 86, 87, 88]. Being the first Ru complex authorized for use in clinical trials, NAMI‐A demonstrated improved efficacy in phase I trials but unfavorable outcomes in phase II, which led to the failure of clinical trials [81, 89]. One study investigated NAMI‐A in humans in phase I trials [88]. The study entailed 12 dose levels (2.4–500 mg/m²/day). Twenty‐four adult patients diagnosed with different types of solid tumors, were treated with NAMI‐A (3 h) for five consecutive days every 3 weeks. The side effects of NAMI‐A were less severe compared to platinum anticancer drugs: moderate renal toxicity, which was noted at the highest doses, was completely reversible, whereas hematological toxicity was minimal. Doses ≥ 400 mg/m²/day caused painful blisters on fingers and toes. The maximum tolerated dose (MTD) of 300 mg/m²/day resulted in treatable mild to moderate general malaise, nausea, vomiting, and diarrhea. Due to its high affinity for blood proteins, ruthenium's total body retention was greater than predicted from preclinical studies [90].

A study (2008–2011) conducted on 32 patients with advanced Non‐small cell lung cancer (NSCLC) [89]. Gemcitabine was administered at a typical dose of 1 g/m² on, whereas dose escalation of NAMI‐A was administered intravenously (3 h) on days 1 and 8 of a 3‐week cycle—was performed in phase I of the study. CTC (common terminology criteria) grade 2–4 neutropenia and anemia were reported at the highest doses, whereas the already mentioned blisters (dose‐limiting toxicity) were observed at 600 mg/m². The MTD was found to be 450 mg/m², where the main non‐hematological adverse events were elevated liver enzymes, transient creatinine elevation, renal toxicity, constipation, and fatigue. In phase II of the study, the antitumor activity determined by RECIST (response evaluation criteria in solid tumors) criteria for solid tumors [91] was evaluated on 15 patients treated with the MTD of NAMI‐A.

The expansion of the phase II cohort with additional patients was not undertaken due to the treatment's efficacy being lower than anticipated for gemcitabine alone, resulting in one case of partial remission and 10 patients exhibiting stable disease for a duration of 6–8 weeks. Furthermore, the patients found the combination treatment to be very exhausting, primarily because of the severe nausea, vomiting, and diarrhea [89].

Subsequently, another Ru chemotherapeutic, KP1019, showed poor water compatibility in phase I, which is now superseded by a more soluble sodium salt, KP1339, currently explored in clinical trials [81]. In 2006, a preliminary phase I dose‐escalation study was conducted with KP1019 (25–600 mg) on only eight patients diagnosed with advanced solid tumors [87, 92]. The complex was administered intravenously two times a week for 3 weeks and showed mild toxicity with the investigated dose range. The condition of five out of six examined patients remained stable, which was unrelated to the dose. Due to poor solubility (too large a volume of infusion solution needed for further dose escalation), the MTD of KP1019 could not be established, which resulted in a full‐scale phase I study conducted on 34 patients using a more soluble sodium derivative Na[trans‐RuCl₄(Ind)₂] (KP1339/IT‐139). The study used nine dose levels (20–780 mg/m²/day), and the complex was administered intravenously on days 1, 8, and 15 in a 28‐day cycle [93]. The treatment caused negligible side effects. Grade 2–3 nausea accompanied by elevated levels of creatinine was found to be DLT (dose‐limiting toxicity) at the maximum dose. Seven patients with various types of tumors (including two cases of NSCLC) remained stable for up to 88 weeks, and one patient with a neuroendocrine tumor exhibited a partial response (PR).

Years later, Burris et al., 2016 conducted another phase I clinical trial on 46 patients, with the same dose levels and treatment schedule. The MTD was found to be 625 mg/m² [94, 95]. Similarly, the tolerability and safety profile showed no significant hematological toxicity or neurotoxicity, the main adverse events being clinically manageable grade ≤ 2 nausea, fatigue, and vomiting. Overall, the complex resulted in moderate antitumor activity, with 26% disease control efficacy in three of the five patients with carcinoid neuroendocrine tumors, and one PR was observed in a patient with colon cancer.

In recent years, more focus has shifted from the chemotherapeutic activity of Ru(III) complexes to the phototoxic effects of Ru(II) complexes. The first Ru(II)‐based PS for PDT applications, TLD1433, has been approved for human clinical trials (Figure 4) [96]. Furthermore, studies have also pointed out that Ru(II) complexes have the potential to serve as PACT agents (Figure 4) [97, 98, 99, 100]. They are capable of photoinduced ligand dissociation through appropriate structural design, and the resulting Ru(II) aqua species can covalently bind to DNA in a way that is comparable to cisplatin. Ru(II)‐based PACT agents show some prospective advantages over Pt(IV) complexes. They have extensive photophysical and photochemical capabilities, a variety of easily modifiable structures, and potentially high efficacy against cisplatin‐resistant cancer cells due to their octahedral structures, which differ from those of cisplatin [71, 101, 102]. Ru(II) complexes typically exhibit the lowest energy absorption band due to ¹MLCT (Metal‐to‐Ligand Charge Transfer) transitions [76, 103]. When exposed to suitable light, Ru(II) complexes transition from the ¹MLCT state to the ³MLCT state via rapid intersystem crossing (Figure 5). This results in the dissociation and generation of Ru(II) aqua species, which have a high DNA‐binding affinity, showing potential in PACT therapeutic effects [76].

FIGURE 4.

Ruthenium metallodrugs explored in clinical trials [104]. Copyright 2025, Elsevier.

FIGURE 5.

Jablonski diagram of Ru(II) complexes with photolabile ligands.

2.7. Prodrugs Outcomes in Cancer Therapy

Studies by van Geest et al. [11] developed a ruthenium‐based complex capable of revealing itself after cellular uptake. This prodrug was embedded in liposomes and showed no dark toxicity in human skin melanoma cells (A375). Upon light irradiation, this prodrug destroyed cancer cells by releasing STF‐31, a cytotoxic nicotinamide phosphoribosyltransferase (NAMPT) inhibitor, as well as cytotoxic singlet oxygen [11].

In another study, Elias and colleagues [10] investigated the anticancer effects of the Ru complex, [Ru(bpy)₂BC]Cl₂ on melanoma cells. Elias et al. revealed that cell viability results showed a mean phototoxicity index of 340‐fold, which was attributed to aquated photoproducts rather than dissociating ligands. In addition, a significant increase in ROS yield and DNA damage was also observed. Elias and colleagues noted an increase in the early apoptotic cell population at 48 h, which proceeded to the late apoptotic/necrotic cell population at 72 h. On further analysis, western blot results revealed that pro‐ and anti‐apoptotic proteins were facilitated by an interplay between the intrinsic and extrinsic pathways, as well as autophagy and via attenuation of the MAPK and PI3K pathways. Therefore, this study proved that [Ru(bpy)₂BC]Cl₂ is a diverse PACT drug endowed with promising anticancer potential.

Boerhan et al. [105] investigated the phototoxicity of three fluorinated dppz ligand‐coordinated Ru(II) complexes (2–4) containing four monodentate pyridine ligands. These complexes released one pyridine and were covalently bound to DNA after exposure to 470 nm laser irradiation. Boerhan and colleagues complexed noted that compared to the parent complex [Ru(dppz)(py)₄]²⁺ (1), 2–4 exhibited improved phototoxic effects but negligible dark cytotoxicity, making them more ideal for PACT application. Complex 3 showed was the most efficient with IC₅₀ values of about 8 μM toward HeLa and SKOV‐3 cell lines and also had a much greater IC₅₀ value toward normal L‐02 cells. Recent studies conducted by Dao et al. [16] investigated the antitumor efficacy of Ru(II) complex (Ru2) on 4T1 cancer cells. Dao and colleagues noted that with the addition of a donor‐acceptor‐donor (D‐A‐D) linker, the intramolecular charge transition was significantly improved, leading to a high molar extinction coefficient and a long triplet excited state lifespan in the near‐infrared spectrum. Interestingly, Ru2 demonstrated distinct slow photodissociation kinetics upon activation by 700 nm NIR light, which enable cooperative photosensitization and photocatalytic activity to degrade various intracellular biomolecules. In vitro and in vivo studies demonstrated that the exposure of Ru2 to 700 nm NIR light caused nanomolar photocytotoxicity on 4T1 cancer cells through the induction of calcium overload and endoplasmic reticulum (ER) stress.

Sun et al. [106] designed a novel phototoxic Ru‐containing block copolymer. The polymer was composed of a hydrophobic Ru‐containing block and a hydrophilic poly(ethylene glycol) block that contains drug‐Ru complex conjugates that are red light cleavable (650–680 nm). The block copolymer was encapsulated in micelles to efficiently enhance its bioavailability in cancer cells. Red light initiated the release of the drug‐Ru complex nanoformulation from the micelles, and inhibited tumor growth even in a hypoxic tumor environment. Furthermore, no off‐target toxicity was observed in mice treated with this nanoformulation, indicative of the system's excellent biocompatibility with healthy tissues and blood. Sun et al. [88] concluded that this novel red‐light‐responsive Ru‐containing polymer paves new avenues for phototherapy against hypoxic tumors. Mari et al. [98] incorporated Ru^II‐polypyridyl complexes with a peptide carrier (bombesin), which has a high affinity for receptors overexpressed by the cell membrane of HeLa cells. As anticipated, this peptide showed a high affinity for HeLa cells compared with normal MRC‐5 cells. As a result, no cytotoxicity was observed in normal cells, while a significant phototoxicity was observed in HeLa cells upon irradiation. Table 4 summarizes prodrugs that have shown promising results in PACT applications.

TABLE 4.

Summary of prodrugs studies in cancer therapy.

Prodrugs	Cancer	Wavelength (nm)	Outcomes	References
[Ru(3)(biq)(STF‐31)](PF6)2	A375 human skin melanoma cells in vitro	520	The prodrug encapsulated in liposomes showed increased cellular uptake and significant anti‐proliferative effects on cancer cells when compared to free	[11]
Ru‐STF31	Glioblastoma cancer cell line, U87MG in vitro	630	Ru‐STF31 exhibited increased phototoxicity in normoxic and hypoxic U87MG cells	[107]
Ru polypyridyl complexes	H23 (breast cancer) and T47D (lung cancer) cell lines in vitro	450	Ru complexes exhibited cytotoxicity in cancer cells	[108]
[Ru(bipy)2(dpphen)]Cl2	A549 (lung) cancer cells in vitro	460	Ru prodrug showed potent anticancer effects against A549 cells, which resulted in cell rounding and detachment, loss of membrane integrity, and DNA damage, representing apoptosis	[109]
Ru(7‐OCH3‐dppz)(4‐OCH3‐py)4(PF6)2 (Ru1)	Cisplatin‐resistant A549 (lung) 3‐D multicellular tumor spheroids	800	Ru conjugated to NPs showed more efficient PACT activity towards 3‐D‐cultured cancer cells under both normoxic and hypoxic conditions versus free Ru	[110]
[Ru(η6‐p‐cymene)pta‐(N3)2]	Human cervical carcinoma (HeLa)	450	Ru complex revealed a modest cytotoxicity in the dark, which drastically increased upon exposure to light	[111]

2.8. The Combined Effect of Photodynamic Therapy and Photoactivated Chemotherapy on Solid Tumors

The combinatorial approach of PDT and PACTS is a promising and novel synergistic strategy for improving cancer treatment outcomes. By leveraging the efficacy of both light‐based therapies, this approach holds great promise for circumventing hurdles faced by conventional therapies, allowing for more efficient and target‐specific cancer therapy [112]. PDT, which relies on light to excite a photoactivable molecule that induces cell death due to the production of ROS, is effective at selectively targeting tumor cells but can be hampered by the hypoxic tumor environment Figure 6 [9].

FIGURE 6.

The hypoxic tumor region is farther away from feeding vessels, which play a key role in tumor resistance to photodynamic therapy.

A longstanding obstacle of cancer therapies, including PDT and PACT, is related to the poor bioavailability of PSs and prodrugs in tumor tissues, reducing the complete diffusion of these photoactive molecules throughout the tumor. This may significantly diminish the number of active molecules in the targeted tumor regions as well as in various layers of the tumor, resulting in incomplete tumor elimination [113, 114]. Moreover, unfavorable biodistribution may result in prolonged periods of unwanted retention in healthy tissues. To address this issue, researchers are actively engaged in improving the targeting specificity of PS and prodrugs to reduce their off‐target accumulation as well as enhance their overall therapeutic efficacy [7, 113]. This is attained by incorporating smart delivery systems, such as antibodies, liposomes, and NPs, that may promote excellent biodistribution [115]. The encapsulation or incorporation with smart delivery systems confers to some extent control in the distribution of the PS/prodrug‐carrier complex.

In order to treat deep‐seated cancers and evade melanin absorption, a variety of strategies have been used in recent years to overcome cancer resistance to phototherapeutic modalities. One such strategy is the use of photoactive molecules excited by light in the far‐red and near‐infrared ranges. Melanin typically scatters and absorbs throughout the 400–700 nm spectral range that is utilized in PDT [116]. Most clinically approved PS absorb below 700 nm, calling for the need for developing new PSs that absorb in the NIR region, such as bacteriochlorins and Si(IV)‐naphthalocyanine. Bussetti et al. [117] reported that photoactivated Si(IV)‐naphthalocyanine with a 1064 nm laser at 520 J/cm² resulted in a significant 16‐day delay of tumor growth. In another study by Mroz et al. [118], it was reported that 1 μM bacteriochlorin and a 730 nm laser at 120 J/cm² significantly inhibited the cell viability of B16F10 cells by 98%. Another approached used to overcome melanin absorbance and reach deep‐rooted tumors is through the functionalization of PSs and prodrugs with NPs that have a strong absorption in the NIR region, such as upconversion and mesoporous silica NPs [117, 118]. Lee et al. [119] functionalized indocyanine green (ICG) with chitosan‐coated liposomes to significantly augment cellular uptake and photocytotoxicity of ICG in B16‐F10 melanoma cells. Lee and colleagues noted the nanoformulation enhanced skin permeation of ICG, and 40 μM ICG‐chitosan‐coated liposomes, and a 785 nm laser at a power intensity of 100 mW/cm² showed a significant increase in the inhibitory effect when compared to free ICG. Ru(II)‐based PACT agents are effective in treating cancer, but their limited in vivo uses stem from their short wavelength absorption (usually less than 550 nm) and undesirable tumor accumulation.

Zhang et al. [120] modified Ru(II)‐based PACT agent with bovine serum albumin (BSA) attached on the surface of lanthanide‐doped upconversion NPs (UCNP@BSA@Ru) to improve Ru(II) cellular uptake and target‐selectivity in tumor cells. Zhang and colleagues revealed that the nanobioconjugate showed a highly effective and selective affinity for tumor cells in vitro and in vivo due to the targeting abilities of BSA and the EPR effect conferred by NPs. Following light irradiation at 980 nm, the nanobioconjugate significantly exerted cytotoxic effects in cancer with negligible effects on the normal IOSE80 cells. Furthermore, in vivo results showed that the nanobioconjugate significantly inhibited tumor growth after photoactivation using 980 nm. Therefore, dual delivery of PSs and prodrugs both encapsulated into nanoparticles functionalized with targeting moieties (e.g., antibodies, folic acid, and peptides) could selectively target cancer while minimizing damage to normal tissues and exerting PDT‐PACT synergistic cytotoxic effects that prevent tumor resistance. Furthermore, the modification of PSs and prodrugs with NPs that absorb light within the NIR region could not only allow for the treatment of deep‐seated tumors but also evade light absorption by endogenous chromophores such as hemoglobin, myoglobin, and cytochrome or melanin [5], which counteract the PDT‐PACT processes by competing with PS and prodrugs in the absorption process.

3. Conclusion

The combination of PACT and active‐targeted PDT represents a novel and promising approach to enhancing cancer prognosis in patients. Despite PDT showing great potential as a minimally invasive treatment modality, its therapeutic efficacy is often hampered by the hypoxic tumor microenvironment and limitations of traditional PSs. Similarly, PACT provides a controlled and localized chemotherapeutic effect, though it faces challenges related to solubility, target selectivity, and systemic toxicity [107, 110]. The combination of these two modalities allows for a synergistic cytotoxic effect, enhancing selectivity, reducing side effects, and attenuating tumor‐resistant mechanisms.

Recent advances in nanotechnology have paved new avenues for enhancing the efficacy of PDT and PACT. Nanocarrier‐mediated drug delivery platforms have significantly improved PS and prodrug bioavailability, target selectivity, and therapeutic effect [7]. Functionalized NPs can selectively accumulate in targeted tumor regions, improve cellular uptake, and control drug release, thereby mitigating off‐target toxicity and improving treatment specificity [31]. Furthermore, the introduction of near‐infrared (NIR) light‐responsive nanomaterials holds great promise in the therapy of deep‐seated tumors and therefore broadens the clinical utility of this synergy approach [114].

Despite these advances, there is still a significant research gap concerning the co‐delivery of PSs and prodrugs using an active‐targeted nanocarrier platform. More research is needed to optimize the formation of delivery efficiency and clinical translational potential of such systems. Future directives include refining the design of multifunctional nanocarriers, discovering new PS‐prodrug combinations, and conducting extensive preclinical and clinical trials to determine the full potential of PDT‐PACT synergy in cancer [121].

The overall findings from this study suggest that there are currently no studies that have investigated the combination therapy in relation to the active‐targeted nanocarrier platforms incorporating PDT and PACT to prevent drug resistance and improve the overall treatment outcomes. Thus, this study warrants further investigations of active‐targeted nanocarrier systems for dual delivery of PSs and prodrugs.

Author Contributions

Conceptualization: N.W.N. and H.A. Writing – original draft preparation: N.W.N. Writing – review and editing: H.A. Supervision: H.A. All authors have read and agreed to the published version of the manuscript.

Conflicts of Interest

The authors declare no conflicts of interest.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.