Modulation of Photosensitization Processes for an Improved Targeted Photodynamic Therapy

A Bacterial Nanomedicine Combines Photodynamic-Immunotherapy and Chemotherapy for Enhanced Treatment of Oral Squamous Cell Carcinoma

Bacterial therapy is an emerging hotspot in tumor immunotherapy, which can initiate antitumor immune activation through multiple mechanisms. Porphyromonas gingivalis (Pg), a pathogenic bacterium inhabiting the oral cavity, contains a great deal of pathogen associated molecular patterns that can activate various innate immune cells to promote antitumor immunity. Owing to the presence of protoporphyrin IX (PpIX), Pg is also an excellent photosensitizer for photodynamic therapy (PDT) via the in situ generation of reactive oxygen species. This study reports a bacterial nanomedicine (nmPg) fabricated from Pg through lysozyme degradation, ammonium chloride lysis, and nanoextrusion, which has potent PDT and immune activation performances for oral squamous cell carcinoma (OSCC) treatment. To further promote the tumoricidal efficacy, a commonly used chemotherapeutic drug doxorubicin (DOX) is efficiently encapsulated into nmPg through a simple incubation method. nmPg/DOX thus prepared exhibits significant synergistic effects on inhibiting the growth and metastasis of OSCC both in vitro and in vivo via photodynamic-immunotherapy and chemotherapy. In summary, this work develops a promising bacterial nanomedicine for enhanced treatment of OSCC.

Recent advances in fluorescence imaging-guided photothermal therapy and photodynamic therapy for cancer: From near-infrared-I to near-infrared-II

Phototherapy (including photothermal therapy, PTT; and photodynamic therapy, PDT) has been widely used for cancer treatment, but conventional PTT/PDT show limited therapeutic effects due to the lack of disease recognition ability. The integration of fluorescence imaging with PTT/PDT can reveal tumor locations in a real-time manner, holding great potential in early diagnosis and precision treatment of cancers. However, the traditional fluorescence imaging in the visible and near-infrared-I regions (VIS/NIR-I, 400-900 nm) might be interfered by the scattering and autofluorescence from tissues, leading to a low imaging resolution and high false positive rate. The deeper near-infrared-II (NIR-II, 1000-1700 nm) fluorescence imaging can address these interferences. Combining NIR-II fluorescence imaging with PTT/PDT can significantly improve the accuracy of tumor theranostics and minimize damages to normal tissues. This review summarized recent advances in tumor PTT/PDT and NIR-II fluorophores, especially discussed achievements, challenges and prospects around NIR-II fluorescence imaging-guided PTT/PDT for cancers.

Thieno[3,4-d]pyrimidin-4(3H)-thione: an effective, oxygenation independent, heavy-atom-free photosensitizer for cancer cells

All-organic, heavy-atom-free photosensitizers based on thionation of nucleobases are receiving increased attention because they are easy to make, noncytotoxic, work both in the presence and absence of molecular oxygen, and can be readily incorporated into DNA and RNA. In this contribution, the DNA and RNA fluorescent probe, thieno[3,4-d]pyrimidin-4(1H)-one, has been thionated to develop thieno[3,4-d]pyrimidin-4(3H)-thione, which is nonfluorescent and absorbs near-visible radiation with about 60% higher efficiency. Steady-state absorption and emission spectra are combined with transient absorption spectroscopy and CASPT2 calculations to delineate the electronic relaxation mechanisms of both pyrimidine derivatives in aqueous and acetonitrile solutions. It is demonstrated that thieno[3,4-d]pyrimidin-4(3H)-thione efficiently populates the long-lived and reactive triplet state generating singlet oxygen with a quantum yield of about 80% independent of solvent. It is further shown that thieno[3,4-d]pyrimidin-4(3H)-thione exhibits high photodynamic efficacy against monolayer melanoma cells and cervical cancer cells both under normoxic and hypoxic conditions. Our combined spectroscopic, computational, and in vitro data demonstrate the excellent potential of thieno[3,4-d]pyrimidin-4(1H)-thione as a heavy-atom-free PDT agent and paves the way for further development of photosensitizers based on the thionation of thieno[3,4-d]pyrimidine derivatives. Collectively, the experimental and computational results demonstrate that thieno[3,4-d]pyrimidine-4(3H)-thione stands out as the most promising thiobase photosensitizer developed to this date.

Phenethyl Isothiocyanate-Conjugated Chitosan Oligosaccharide Nanophotosensitizers for Photodynamic Treatment of Human Cancer Cells

The aim of this study is to synthesize phenethyl-conjugated chitosan oligosaccharide (COS) (abbreviated as ChitoPEITC) conjugates and then fabricate chlorin E6 (Ce6)-incorporated nanophotosensitizers for photodynamic therapy (PDT) of HCT-116 colon carcinoma cells. PEITC was conjugated with the amine group of COS. Ce6-incorporated nanophotosensitizers using ChitoPEITC (ChitoPEITC nanophotosensitizers) were fabricated by dialysis method. 1H nuclear magnetic resonance (NMR) spectra showed that specific peaks of COS and PEITC were observed at ChitoPEITC conjugates. Transmission electron microscope (TEM) confirmed that ChitoPEITC nanophotosensitizers have spherical shapes with small hydrodynamic diameters less than 200 nm. The higher PEITC contents in the ChitoPEITC copolymer resulted in a slower release rate of Ce6 from nanophotosensitizers. Furthermore, the higher Ce6 contents resulted in a slower release rate of Ce6. In cell culture study, ChitoPEITC nanophotosensitizers showed low toxicity against normal CCD986Sk human skin fibroblast cells and HCT-116 human colon carcinoma cells in the absence of light irradiation. ChitoPEITC nanophotosensitizers showed a significantly higher Ce6 uptake ratio than that of free Ce6. Under light irradiation, cellular reactive oxygen species (ROS) production of nanophotosensitizers was significantly higher than that of free Ce6. Especially, PEITC and/or ChitoPEITC themselves contributed to the production of cellular ROS regardless of light irradiation. ChitoPEITC nanophotosensitizers showed significantly higher PDT efficacy against HCT-116 cells than that of free Ce6. These results indicate that ChitoPEITC nanophotosensitizers have superior potential in Ce6 uptake, ROS production and PDT efficacy. In the HCT-116 cell-bearing mice tumor-xenograft model, ChitoPEITC nanophotosensitizers efficiently inhibited growth of tumor volume rather than free Ce6. In the animal imaging study, ChitoPEITC nanophotosensitizers were concentrated in the tumor tissue, i.e., fluorescence intensity in the tumor tissue was stronger than that of other tissues. We suggest that ChitoPEITC nanophotosensitizers are a promising candidate for the treatment of human colon cancer cells.

Femtosecond intersystem crossing to the reactive triplet state of the 2,6-dithiopurine skin cancer photosensitizer

Site-selected sulfur-substituted nucleobases are a class of all organic, heavy-atom-free photosensitizers for photodynamic therapy applications that exhibit excellent photophysical properties such as strong absorption in the ultraviolet-A region of the electromagnetic spectrum, near-unity triplet yields, and a high yield of singlet oxygen generation. Recent investigations on doubly thionated nucleobases, 2,4-dithiothymine, 2,4-dithiouracil, and 2,6-dithiopurine, demonstrated that these set of dithionated nucleobases outperform the photodynamic efficacy exhibit by 4-thiothymidine-the most widely studied singly substituted thiobase to date. Out of the three dithionated nucleobases, 2,6-dithiopurine was shown to be the most effective, exhibiting inhibition of cell proliferation of up to 63% when combined with a low UVA dose of 5 J cm^-2. In this study, we elucidated the electronic relaxation pathways leading to the population of the reactive triplet state of 2,6-dithiopurine. 2,6-Dithiopurine populates the triplet manifold in less than 150 fs, reaching the nπ* triplet state minimum within a lifetime of 280 ± 50 fs. Subsequently, the population in the nπ* triplet state minimum internally converts to the long-lived ππ* triplet state within a lifetime of 3 ± 1 ps. The relatively slow internal conversion lifetime is associated with major conformational relaxation in going from the nπ* to ππ* triplet state minimum. A unity triplet yield of 1.0 ± 0.1 is measured.

NIR Fluorescent Imaging and Photodynamic Therapy with a Novel Theranostic Phospholipid Probe for Triple-Negative Breast Cancer Cells

New exogenous probes are needed for both imaging diagnostics and therapeutics. Here, we introduce a novel nanocomposite near-infrared (NIR) fluorescent imaging probe and test its potency as a photosensitizing agent for photodynamic therapy (PDT) against triple-negative breast cancer cells. The active component in the nanocomposite is a small molecule, pyropheophorbide a-phosphatidylethanolamine-QSY21 (Pyro-PtdEtn-QSY), which is imbedded into lipid nanoparticles for transport in the body. The probe targets abnormal choline metabolism in cancer cells; specifically, the overexpression of phosphatidylcholine-specific phospholipase C (PC-PLC) in breast, prostate, and ovarian cancers. Pyro-PtdEtn-QSY consists of a NIR fluorophore and a quencher, attached to a PtdEtn moiety. It is selectively activated by PC-PLC resulting in enhanced fluorescence in cancer cells compared to normal cells. In our in vitro investigation, four breast cancer cell lines showed higher probe activation levels than noncancerous control cells, immortalized human mammary gland cells, and normal human T cells. Moreover, the ability of this nanocomposite to function as a sensitizer in PDT experiments on MDA-MB-231 cells suggests that the probe is promising as a theranostic agent.

Synthesis, modification and bioapplications of nanoscale copper chalcogenides

Copper chalcogenides have a simple general formula, variable atomic ratios, and complicated crystal structures, which lead to their wealth of optical, electrical, and magnetic properties with great potential for wide applications ranging from energy conversion to the biomedical field. Herein, we summarize the recent advances in (1) the synthesis of size- and morphology tunable nanostructures by different methods; (2) surface modification and functionalization for different purposes; and (3) bioapplications for diagnosis and treatment of tumors by different imaging and therapy methods, as well as antibacterial applications. We also briefly discuss the future directions and challenges of copper chalcogenide nanoparticles in the biomedical field.

Thionated organic compounds as emerging heavy-atom-free photodynamic therapy agents

This minireview focuses on recent progress in developing heavy-atom-free photosensitizers based on the thionation of nucleic acid derivatives and other biocompatible organic compounds for prospective applications in photodynamic therapy. Particular attention is given to the use of thionated nucleobase derivatives as "one-two punch" photodynamic agents. These versatile photosensitizers can act as "Trojan horses" upon metabolization into DNA and exposure to activating light. Their incorporation into cellular DNA increases their selectivity and photodynamic efficacy against highly proliferating skin cancer tumor cells, while simultaneously enabling the use of low irradiation doses both in the presence and in the absence of molecular oxygen. Also reviewed are their primary photochemical reactions, modes of action, and photosensitization mechanisms. New developments of emerging thionated organic photosensitizers absorbing visible and near-infrared radiation are highlighted. Future research directions, as well as, other prospective applications of heavy-atom-free, thionated photosensitizers are discussed.

Triphenylamine-substituted zinc porphyrin nanoparticles with photodynamic/photothermal activity for cancer phototherapy in vitro

An amphiphilic zinc porphyrin complex with a typical donor–acceptor (D–A) structure was synthesized, where the triphenylamine acted as a donor unit while the porphyrin was used as an electronic acceptor. Due to the presence of triethylene glycol moieties on the parent structure, Zn-TPAP could spontaneously assemble to the related nanoparticles (Zn-TPAP NPs) with improved hydrophilicity. The as-prepared Zn-TPAP NPs presented relatively uniform spherical particles with the average particle sizes around 160 nm, which was suitable for tumor accumulation benefiting from the EPR effect. Due to the aggregation of the porphyrin molecules in the assembled nanostructures, Zn-TPAP NPs displayed broadened and red-shifted absorption and quenched fluorescence relative to that of Zn-TPAP. In addition to ROS generation, Zn-TPAP NPs exhibited moderate photothermal effects and the photothermal conversion efficiency was measured as 29%. Zn-TPAP NPs showed good biocompatibility and could generate ROS in the A549 cells. Under light irradiation, Zn-TPAP NPs can efficiently kill cancer cells. Thus, Zn-TPAP NPs could be used as potential nanoagents for cancer treatment through the photothermal/photodynamic synergistic modes.

New Targeted Gold Nanorods for the Treatment of Glioblastoma by Photodynamic Therapy

This study describes the employment of gold nanorods (AuNRs), known for their good reputation in hyperthermia-based cancer therapy, in a hybrid combination of photosensitizers (PS) and peptides (PP). We report here, the design and the synthesis of this nanosystem and its application as a vehicle for the selective drug delivery and the efficient photodynamic therapy (PDT). AuNRs were functionalized by polyethylene glycol, phototoxic pyropheophorbide-a (Pyro) PS, and a "KDKPPR" peptide moiety to target neuropilin-1 receptor (NRP-1). The physicochemical characteristics of AuNRs, the synthesized peptide and the intermediate PP-PS conjugates were investigated. The photophysical properties of the hybrid AuNRs revealed that upon conjugation, the AuNRs acquired the characteristic properties of Pyro concerning the extension of the absorption profile and the capability to fluoresce (Φf = 0.3) and emit singlet oxygen (ΦΔ = 0.4) when excited at 412 nm. Even after being conjugated onto the surface of the AuNRs, the molecular affinity of "KDKPPR" for NRP-1 was preserved. Under irradiation at 652 nm, in vitro assays were conducted on glioblastoma U87 cells incubated with different PS concentrations of free Pyro, intermediate PP-PS conjugate and hybrid AuNRs. The AuNRs showed no cytotoxicity in the absence of light even at high PS concentrations. However, they efficiently decreased the cell viability by 67% under light exposure. This nanosystem possesses good efficiency in PDT and an expected potential effect in a combined photodynamic/photothermal therapy guided by NIR fluorescence imaging of the tumors due to the presence of both the hyperthermic agent, AuNRs, and the fluorescent active phototoxic PS.

The application of titanium dioxide, zinc oxide, fullerene, and graphene nanoparticles in photodynamic therapy

Nanoparticles (NPs) have been shown to have good ability to improve the targeting and delivery of therapeutics. In the field of photodynamic therapy (PDT), this targeting advantage of NPs could help ensure drug delivery at specific sites. Among the commonly reported NPs for PDT applications, NPs from zinc oxide, titanium dioxide, and fullerene are commonly reported. In addition, graphene has also been reported to be used as NPs albeit being relatively new to this field. In this context, the present review is organized by these different NPs and contains numerous research works related to PDT applications. The effectiveness of these NPs for PDT is discussed in detail by collecting all essential information described in the literature. The information thus assembled could be useful in designing new NPs specific for PDT and/or PTT applications in the future.

Iron Chelators and Exogenic Photosensitizers. Synergy through Oxidative Stress Gene Expression

In non-invasive anticancer photodynamic therapy (PDT), a nontoxic photosensitizer (PS), which is activated by visible light, is used as a magic bullet that selectively destroys cancer cells. Recently, we described the combined therapy of 5-aminolevulinic acid (ALA-PDT) with thiosemicarbazone (TSC), i.e. an iron-chelating agent. This resulted in a strong synergistic effect. Herein, we investigated a novel strategy using a combination of PDT consist of the xenobiotic-porphyrin type PS with TSC. We observed a synergistic effect for all of the pairs of TSC-PS. This approach can be rationalized by the fact that both chlorin and TSC can affect the generation of reactive oxygen species (ROS). In order to elucidate the plausible mechanism of action, we also combined the investigated PSs with DFO, which forms complexes that are redox inactive. We detected a slight antagonism or additivity for this combination. This may suggest that the ability of an iron chelator (IC) to participate in the production of ROS and the generation of oxidative stress is important.

Reactive oxygen species generating systems meeting challenges of photodynamic cancer therapy

The reactive oxygen species (ROS)-mediated mechanism is the major cause underlying the efficacy of photodynamic therapy (PDT). The PDT procedure is based on the cascade of synergistic effects between light, a photosensitizer (PS) and oxygen, which greatly favors the spatiotemporal control of the treatment. This procedure has also evoked several unresolved challenges at different levels including (i) the limited penetration depth of light, which restricts traditional PDT to superficial tumours; (ii) oxygen reliance does not allow PDT treatment of hypoxic tumours; (iii) light can complicate the phototherapeutic outcomes because of the concurrent heat generation; (iv) specific delivery of PSs to sub-cellular organelles for exerting effective toxicity remains an issue; and (v) side effects from undesirable white-light activation and self-catalysation of traditional PSs. Recent advances in nanotechnology and nanomedicine have provided new opportunities to develop ROS-generating systems through photodynamic or non-photodynamic procedures while tackling the challenges of the current PDT approaches. In this review, we summarize the current status and discuss the possible opportunities for ROS generation for cancer therapy. We hope this review will spur pre-clinical research and clinical practice for ROS-mediated tumour treatments.

Peptide-substituted phthalocyanine photosensitizers: design, synthesis, photophysicochemical and photobiological studies

Activatable molecular beacons were synthesized bearing phthalocyanine, peptide sequence and fluorophore groups. The phototoxicity and cytotoxicity of the systems were studied against the cervical cancer cell line named HeLa for evaluation of their suitability for photodynamic therapy.

Polymeric nanocarrier systems for photodynamic therapy

Photodynamic therapy (PDT) is an emerging treatment modality that involves the combined action of photosensitizers (PSs) and light for treatment of solid tumor and other diseases. Although this therapeutic method has been considered as an alternative to classical cancer treatments, clinical PDT requires further advances in selectivity and therapeutic efficacy to overcome numerous shortages related to conventional PDT. In this regard, great efforts have been devoted to the development of polymeric nanocarrier-encapsulated PSs for targeted PDT, aiming at improvement of water solubility and tumor-specificity of hydrophobic PSs. Here, we discuss the general concepts and considerations of polymeric nanocarriers for efficient delivery of PSs. In recent, the amphiphilic PS-polymer conjugate-based self-quenchable nanoparticles and PS-polymer-conjugate/quencher nanocomplexes have emerged as an attractive delivery platform for efficient and reliable PDT. They can incorporate and deliver the PS in a photodynamically inactive state but demonstrate cytotoxic effects by tumor environment-sensitive activation mechanisms, so that the photodynamic cancer treatment can achieve maximum target specificity. Here, we report the recent achievements on the development of activatable PS formulations based on PS-polymer conjugates.

Antioxidant-photosensitizer dual-loaded polymeric micelles with controllable production of reactive oxygen species

Poly(ethylene glycol)-b-poly(caprolactone) (PEG-b-PCL) micelles dually loaded with both pheophorbide a (PhA) as a photosensitizer and β-carotene (CAR) as a singlet oxygen ((1)O2) scavenger were designed to control photodynamic therapy (PDT) activity in cancer treatment. The CAR in the PhA/CAR micelles significantly diminished PhA-generated (1)O2 through direct (1)O2 scavenging, whereas the CAR molecules lost their (1)O2 scavenging activity when the PhA and CAR were spatially isolated by the disintegration of the PEG-b-PCL micelles. In cell-culture systems, light irradiation at a post-treatment time that corresponded to the presence of the micelles in the blood environment induced negligible phototoxicity, whereas light irradiation at a post-treatment time that corresponded to the presence of the micelles in the intracellular environment induced remarkable phototoxicity. In addition, a longer post-treatment time induced greater internalization of PhA/CAR micelles, which resulted in higher phototoxicity, suggesting an increase in photo killing activity against the tumor cells of interest. Thus, the co-loading of a (1)O2 generator and a (1)O2 scavenger into a single micelle is a potential strategy that may be useful in facilitating more accurate and reliable PDT with site-specific controllable production of singlet oxygen species for cancer treatment.

A dual activatable photosensitizer toward targeted photodynamic therapy

An unsymmetrical bisferrocenyl silicon(IV) phthalocyanine has been prepared in which the disulfide and hydrazone linkers can be cleaved by dithiothreitol and acid, respectively. The separation of the ferrocenyl quenchers and the phthalocyanine core greatly enhances the fluorescence emission, singlet oxygen production, intracellular fluorescence intensity, and in vitro photocytotoxicity. The results have been compared with those for the two symmetrical analogues which contain either the disulfide or hydrazone linker and therefore can only be activated by one of these stimuli. For the dual activatable agent, the greatest enhancement can be attained under a slightly acidic environment (pH = 4.5-6.8) and in the presence of dithiothreitol (in millimolar range), which can roughly mimic the acidic and reducing environment of tumor tissues. This compound can also be activated in tumor-bearing nude mice. It exhibits an increase in fluorescence intensity in the tumor over the first 10 h after intratumoral injection and can effectively inhibit the growth of tumor upon illumination.

Efficient induction of apoptosis in HeLa cells by a novel cationic porphycene photosensitizer

In the present study we analyze the photobiological properties of 2,7,12-tris(α-pyridinio-p-tolyl)-17-(p-(methoxymethyl)phenyl) porphycene (Py3MeO-TBPo) in Hela cells, in order to assess its potential as a new photosensitizer for photodynamic therapy of cultured tumor cells. Using 0.5 μM Py3MeO-TBPo, flow cytometry studies demonstrated an increase of intracellular drug levels related to the incubation time, reaching a maximum at 18 h. LysoTracker(®) Green (LTG) and MitoTracker(®) Green (MTG) probes were used to identify the subcellular localization. Upon exposure to ultraviolet excitation, red porphycene fluorescence was detected as red granules in the cytoplasm that colocalized with LTG. No significant toxic effects were detected for Py3MeO-TBPo in the dark at concentrations below 1 μM. In contrast, Py3MeO-TBPo combined with red-light irradiation induced concentration- and fluence-dependent HeLa cells inactivation. Besides, all photodynamic protocols assayed induced a clear effect of cell detachment inhibition after trypsin treatment. Both apoptotic and necrotic cell death mechanisms can occur in HeLa cells depending on the experimental protocol. After 18 h incubation with 0.5 μM Py3MeO-TBPo and subsequent red light irradiation (3.6 J/cm(2)), a high number of cells die by apoptosis, as evaluated by morphological alterations, immunofluorescent relocalization of Bax from cytosol to mitochondria, and TUNEL assay. Likewise, immunofluorescence techniques showed that cytochrome c is released from mitochondria into cytosol in cells undergoing apoptosis, which occurs immediately after relocation of Bax in mitochondria. The highest amount of apoptosis appeared 24 h after treatment (70%) and this cell death occurred without cell detachment to the substrate. In contrast, with 0.75 μM Py3MeO-TBPo and 3.6 J/cm(2) irradiation, morphological changes showed a preferential necrotic cell death. Singlet oxygen was identified as the cytotoxic agent involved in cell photoinactivation. Moreover, cell cultures pre-exposed to the singlet oxygen scavenger sodium azide showed pronounced protection against the loss of viability induced by Py3MeO-TBPo and light. Different changes in distribution and organization of cytoskeletal elements (microtubules and actin microfilaments) as well as the protein vinculin, after apoptotic and necrotic photodynamic treatments have been analyzed. Neither of these two cell death mechanisms (apoptosis or necrosis) induced cell detachment. In summary, Py3MeO-TBPo appears to meet the requirements for further scrutiny as a very good photosensitizer for photodynamic therapy: it is water soluble, has a high absorption in the red spectral region (where light penetration in tissue is higher), and is able to induce effective high apoptotic rate (70%) related to the more widely studied photosensitizers.

Photocaged permeability: a new strategy for controlled drug release

Light is used to release a drug from a cell impermeable small molecule, uncloaking its cytotoxic effect on cancer cells.

Long-distance energy transfer photosensitizers arising in hybrid nanoparticles leading to fluorescence emission and singlet oxygen luminescence quenching

This paper presents energy transfer occurring in small organically modified core-shell nanoparticles (core lanthanide oxide, shell polysiloxane) (diameter < 10 nm) conjugated with photosensitizers designed for photodynamic therapy applications. These nanoparticles covalently encapsulate a photosensitizing PDT drug in different concentrations. Stable dispersions of the nanoparticles were prepared and the photophysical properties of the photosensitizers were studied and compared to those of the photosensitizers in solution. Increasing the photosensitizer concentration in the nanoparticles was not found to cause any changes in the absorption properties while fluorescence and singlet oxygen quantum yields decreased. As a possible explanation, we have suggested that both long distance energy transfer such as FRET and self-quenching could occur into the nanoparticles. A simple "trend" model of this kind of energy transfer complies with results of experiments on steady state fluorescence and singlet oxygen luminescence.

4-Thiothymidine sensitization of DNA to UVA offers potential for a novel photochemotherapy

Photochemotherapy, in which ultraviolet radiation (UVR: 280-400 nm) or visible light is combined with a photosensitizing drug to produce a therapeutic effect that neither drug or radiation can achieve alone, is a proven therapeutic strategy for a number of non-malignant hyperproliferative skin conditions and various cancers. Examples are psoralen plus UVA (320-400 nm) radiation (PUVA) and photodynamic therapy (PDT). All existing photochemotherapies have drawbacks - for example the association of PUVA with the development of skin cancer, and pain that is often associated with PDT treatment of skin lesions. There is a clear need to develop alternative approaches that involve lower radiation doses and/or improved selectivity for target cells. In this review, we explore the possibility to address this need by exploiting thionucleoside-mediated DNA photosensitisation to low, non toxic doses of UVA radiation.

mTHPC--a drug on its way from second to third generation photosensitizer?

5,10,15,20-Tetrakis(3-hydroxyphenyl)chlorin (mTHPC, Temoporfin) is a widely investigated second generation photosensitizer. Its initial use in solution form (Foscan®) is now complemented by nanoformulations (Fospeg®, Foslip®) and new chemical derivatives related to the basic hydroxyphenylporphyrin framework. Advances in formulation, chemical modifications and targeting strategies open the way for third generation photosensitizers and give an illustrative example for the developmental process of new photoactive drugs.

Targeted and multifunctional arene ruthenium chemotherapeutics

The introduction of multifunctionalities for tumour targeting is becoming a popular strategy toward the development of new therapeutic agents. In particular, the multifaceted potential of ruthenium(II)-arene complexes show great promise as chemotherapeutics. An ever-increasing number of papers dealing with the integration of ruthenium complexes with biologically active molecules to derive bioorganometallic molecules of chemotherapeutic significance have been published in recent years. This perspective review presents a short overview of multifunctional ruthenium-based drugs, especially those containing arene ruthenium complexes, with the emphasis on the combination of photosensitizers with ruthenium complexes for the preparation of novel multifunctional photodynamic therapy agents.

Transport processes of radiopharmaceuticals and -modulators

Radiotherapy and radiology have been indispensable components in cancer care for many years. The detection limit of small tumor foci as well as the development of radio-resistance and severe side effects towards normal tissues led to the development of strategies to improve radio-diagnostic and -therapeutic approaches by pharmaceuticals. The term "radiopharmaceutical" has been used for drugs labeled with radioactive tracers for therapy or diagnosis. In addition, drugs have been described to sensitize tumor cells to radiotherapy (radiosensitizers) or to protect normal tissues from detrimental effects of radiation (radioprotectors). The present review summarizes recent concepts on the transport of radiopharmaceuticals, radiosensitizers, and radioprotectors in cells and tissues, e.g. by ATP-binding cassette transporters such as P-glycoprotein. Strengths and weaknesses of current strategies to improve transport-based processes are discussed.

Targeted photodynamic therapy--a promising strategy of tumor treatment

Targeted therapy is a new promising therapeutic strategy, created to overcome growing problems of contemporary medicine, such as drug toxicity and drug resistance. An emerging modality of this approach is targeted photodynamic therapy (TPDT) with the main aim of improving delivery of photosensitizer to cancer tissue and at the same time enhancing specificity and efficiency of PDT. Depending on the mechanism of targeting, we can divide the strategies of TPDT into "passive", "active" and "activatable", where in the latter case the photosensitizer is activated only in the target tissue. In this review, contemporary strategies of TPDT are described, including new innovative concepts, such as targeting assisted by peptides and aptamers, multifunctional nanoplatforms with navigation by magnetic field or "photodynamic molecular beacons" activatable by enzymes and nucleic acid. The imperative of introducing a new paradigm of PDT, focused on the concepts of heterogeneity and dynamic state of tumor, is also called for.

Stability of peptides and therapeutic success in cancer

INTRODUCTION

Although naturally occurring peptides have been widely used as drugs, their rapid in vivo degradation by proteolysis and their interactions at multiple receptors are part of the reason for the limitation of their clinical applications.

AREAS COVERED

This paper reviews peptide-metabolizing enzymes in the brain and intestinal brush-border membranes, and discusses potential strategies to improve biological activity, specificity and stability of peptides. The reader will gain, via some examples, an appreciation of the challenges involved in identifying peptides stability to improve their biological properties such as selectivity.

EXPERT OPINION

Due to the metabolic process, it is crucial to follow the biodistribution of a peptide drug and/or a peptidic moiety in order to improve its biological properties such as selectivity. To these purposes, pseudopeptides and peptidomimetics preserving the biological properties of native peptides have been developed to increase their resistance to degradation and elimination, bioavailability and selectivity to become good drug candidates.