Photosensitizer Prodrug Research Articles

Selective Photodetection and Photodynamic Therapy for Prostate Cancer through Targeting of Proteolytic Activity

Frequent side effects of radical treatment modalities and the availability of novel diagnostics have raised the interest in focal therapies for localized prostate cancer. To improve the selectivity and therapeutic efficacy of such therapies, we developed a minimally invasive procedure based on a novel polymeric photosensitizer prodrug sensitive to urokinase-type plasminogen activator (uPA). The compound is inactive in its prodrug form and accumulates passively at the tumor site by the enhanced permeability and retention effect. There, the prodrug is selectively converted to its photoactive form by uPA, which is overexpressed by prostate cancer cells. Irradiation of the activated photosensitizer exerts a tumor-selective phototoxic effect. The prodrug alone (8 μmol/L) showed no toxic effect on PC-3 cells, but upon irradiation the cell viability was reduced by 90%. In vivo, after systemic administration of the prodrug, PC-3 xenografts became selectively fluorescent. This is indicative of the prodrug accumulation in the tumor and selective local enzymatic activation. Qualitative analysis of the activated compound confirmed that the enzymatic cleavage occurred selectively in the tumor, with only trace amounts in the neighboring skin or muscle. Subsequent photodynamic therapy studies showed complete tumor eradication of animals treated with light (150 J/cm(2) at 665 nm) 16 hours after the injection of the prodrug (7.5 mg/kg). These promising results evidence the excellent selectivity of our prodrug with the potential to be used for both imaging and therapy for localized prostate cancer.

Thrombin-sensitive dual fluorescence imaging and therapeutic agent for detection and treatment of synovial inflammation in murine rheumatoid arthritis

We have developed a thrombin-sensitive polymeric photosensitizer prodrug (T-PS) to selectively image and eradicate inflammatory lesions in rheumatoid arthritis (RA). Thrombin is a serine protease up-regulated in synovial tissues of rheumatoid arthritis (RA) patients. T-PS consists of a polymeric backbone, to which multiple photosensitizer (PS) units are tethered via short thrombin-cleavable peptide linkers.Fluorescence emission and phototoxicity of the prodrug are efficiently quenched due to the interaction of neighboring photosensitizer units. The prodrug is passively delivered to the inflammation site via the enhanced permeability and retention (EPR) effect. Subsequent site-selective proteolytic cleavage of the peptide linkers restores its photoactivity by increasing the mutual distance between PS.Whole animal imaging in murine collagen-induced arthritis, an experimental model of RA revealed a dose-dependent fluorescence increase in arthritic paws after systemic prodrug injection. In addition, administration of T-PS resulted in much higher fluorescence selectivity for arthritic joints as compared to the free PS. Irradiation of the arthritic joints induced light dose dependent phototoxic effects such as apoptosis, vascular damage and local hemorrhage. Long-term observations showed complete regression of the latter. Irradiated non-arthritic tissues or non-irradiated arthritic tissues showed no histological effects after photodynamic therapy with T-PS.This illustrates that T-PS can localize inflammatory lesions with excellent selectivity and induce apoptosis and vascular shut down after irradiation.

Enhanced Prostate Cancer Targeting by Modified Protease Sensitive Photosensitizer Prodrugs

Prodrugs combining macromolecular delivery systems with site-selective drug release represent a powerful strategy to increase selectivity of anticancer agents. We have adapted this strategy to develop new polymeric photosensitizer prodrugs (PPP) sensitive to urokinase-like plasminogen activator (uPA). In these compounds (to be referred to as uPA-PPPs) multiple copies of pheophorbide a are attached to a polymeric carrier via peptide linkers that can be cleaved by uPA, a protease overexpressed in prostate cancer (PCa). uPA-PPPs are non-phototoxic in their native state but become fluorescent and produce singlet oxygen after uPA-mediated activation. In the present work, we studied the influence of side-chain modifications, molecular weight, and overall charge on the photoactivity and pharmacokinetics of uPA-PPPs. An in vitro promising candidate with convertible phototoxicity was then further investigated in vivo. Systemic administration resulted in a selective accumulation and activation of the prodrug in luciferase transfected PC-3 xenografts, resulting in a 4-fold increase in fluorescence emission over time. Irradiation of fluorescent tumors induced immediate tumor cell eradication as shown by whole animal bioluminescence imaging. PDT with uPA-PPP could therefore provide a more selective treatment of localized PCa and reduce side effects associated with current radical treatments.

A Quantitative Comparison of 5‐Aminolaevulinic Acid‐ and Methyl Aminolevulinate‐Induced Fluorescence, Photobleaching and Pain During Photodynamic Therapy

The characteristics of protoporphyrin IX (PPIX) fluorescence in superficial basal cell carcinoma (sBCC) and carcinoma in situ (Bowen's Disease, BD) following application of 5-aminolaevulinic acid (5-ALA) and its methyl ester (methyl aminolevulinate [MAL]) before, during and after photodynamic therapy (PDT) were investigated in 40 patients. Photosensitizer prodrug penetration can limit PDT efficacy and understanding the characteristics of PPIX fluorescence through fluorescence spectroscopy, may improve knowledge of photosensitizer delivery. Fluorescence intensity was assessed quantitatively, and the rate of photobleaching was determined by fitting an exponential decay. As a secondary end-point, PDT-induced pain was also measured continuously during treatment using a novel hand-held device, known as a pain logger. In vivo PPIX fluorescence was shown to decrease during irradiation, allowing the in vivo photobleaching of PPIX to be monitored. No significant difference was found between ALA- or MAL-induced PPIX fluorescence in lesions of sBCC and BD (P>0.05), indicating no detectable difference in PPIX kinetics for the two prodrugs as assessed by these measures. Pain, as assessed by the logger device, showed high interindividual variability and pain levels tended to be higher initially, decreasing during treatment. No difference was seen in pain experienced during ALA-or MAL-PDT (P>0.05).

Thrombin-sensitive photodynamic agents: A novel strategy for selective synovectomy in rheumatoid arthritis

Protease-sensitive macromolecular prodrugs have attracted interest for bio-responsive drug delivery to sites with up-regulated proteolytic activities such as inflammatory or cancerous lesions. Here we report the development of a novel polymeric photosensitizer prodrug (T-PS) to target thrombin, a protease up-regulated in synovial tissues of rheumatoid arthritis (RA) patients, for minimally invasive photodynamic synovectomy. In T-PS, multiple photosensitizer units are tethered to a polymeric backbone via short, thrombin-cleavable peptide linkers. Photoactivity of the prodrug is efficiently impaired due to energy transfer between neighbouring photosensitizer units. T-PS activation by exogenous and endogenous thrombin induced an increase in fluorescence emission by a factor of 16 after in vitro digestion and a selective fluorescence enhancement in arthritic lesions in vivo, in a collagen-induced arthritis mouse model. In vitro studies on primary human synoviocytes showed a phototoxic effect only after enzymatic digestion of the prodrug and light irradiation, thus demonstrating the functionality of T-PS induced PDT. The developed photosensitizer prodrugs combine the passive targeting capacity of macromolecular drug delivery systems with site-selective photosensitizer release and activation. They illuminate lesions with pathologically enhanced proteolytic activity and induce cell death, subsequent to irradiation.

Urokinase-plasminogen-activator sensitive polymeric photosensitizer prodrugs: design, synthesis and in vitro evaluation

Abstract Polymeric photosensitizer prodrugs (PPPs) to be selectively activated by urokinase plasminogen activator (uPA) have been developed. They are composed of uPA-cleavable photosensitizer-peptide-conjugates covalently attached to a poly-lysine backbone (25 kDa). Phototoxicity of PPPs is efficiently reduced by energy transfer between closely positioned photosensitizers (PSs) on the polymeric backbone. Enzymatic cleavage by uPA, a serine protease that is up-regulated in certain types of cancer, leads to the release of PS-peptidyl-fragments and restoration of phototoxicity. In the current study, conjugates with different e-lysine side chain modifications of the polymer-backbone were synthesized and characterized with respect to in vitro properties such as fluorescence and reactive oxygen species (ROS) quenching, as well as enzymatic cleavage by uPA. Based on these in vitro results and a cell screening experiment for photo- and dark toxicity, two compounds, PEG20-N-uPA-PPP and N-uPA-PPP, were selected as the most promising candidates for further evaluation in two uPA-expressing cell lines, DU-145 and PC-3. Here, the pegylated compound PEG20-N-uPA-PPP showed enhanced selective prodrug activation by uPA in comparison to N-uPA-PPP, pinpointing the need for prolonged extracellular residence time of uPA-sensitive prodrugs. In preliminary PDT-experiments, a dose-dependent phototoxic effect of this compound was found in both tested cell lines. In conclusion, this study highlights the fact that backbone substitution units may impact not only photochemical and physicochemical properties of the PPPs, but play an important role for cellular prodrug localization and thus site selective enzymatic activation.

Polymeric Photosensitizer Prodrugs for Photodynamic Therapy

A targeting strategy based on the selective enzyme-mediated activation of polymeric photosensitizer prodrugs (PPP) within pathological tissue has led to the development of agents with the dual ability to detect and treat cancer. Herein, a detailed study of a simple model system for these prodrugs is described. We prepared "first-generation" PPP by directly tethering the photosensitizer (PS) pheophorbide a to poly-(L)-lysine via epsilon amide links and observed that by increasing the number of PS on a polymer chain, energy transfer between PS units improved leading to better quenching efficiency. Fragmentation of the PPP backbone by trypsin digestion gave rise to a pronounced fluorescence increase and to more efficient generation of reactive oxygen species upon light irradiation. In vitro tests using the T-24 bladder carcinoma cell line and ex vivo experiments using mouse intestines illustrated the remarkable and selective ability of these PPP to fluoresce and induce phototoxicity upon enzymatic activation. This work elucidated the basic physicochemical parameters, such as water solubility and quenching/activation behavior, required for the future elaboration of more adaptable "second-generation" PPP, in which the PS is tethered to a proteolytically stable polymer backbone via enzyme-specific peptide linkers. This polymer architecture offers great flexibility to tailor make the PPP to target any pathological tissue known to over-express a specific enzyme.

Tailoring Protease-Sensitive Photodynamic Agents to Specific Disease-Associated Enzymes

We have developed novel polymeric photosensitizer prodrugs (PPPs) for improved photodynamic therapy. In PPPs, multiple photosensitizer units are covalently coupled to a polymeric backbone via protease-cleavable peptide linkers. These initially non-photoactive compounds become fluorescent and phototoxic after specific enzymatic cleavage of the peptide linkers and subsequent release of the photosensitizer moieties. Tethering the photosensitizer via a short and easily modified amino acid sequence to the polymeric backbone allows for the targeting of a wide variety of proteases. Model compounds, sensitive to trypsin-mediated cleavage, with different pheophorbide a-peptide loading ratios and backbone net charges were evaluated with respect to their solubility, "self-quenching" capacity of fluorescence emission, and reactive oxygen species (ROS) generation. In addition, linker sequence impaired selectivity toward enzymatic cleavage was demonstrated either by incubating PPPs with different enzymes having trypsin-like activity or by introducing a single d-arginine mutant in the peptide sequence. In vitro cell culture tests confirmed dose-dependent higher phototoxicity of enzymatically activated PPPs compared to the nonactivated conjugate after irradiation with white light. These data suggest that similar compounds adapted to disease-associated proteases can be used for selective photodynamic therapy.

5-Aminolevulinic Acid-Induced Fluorescence Endoscopy for the Detection of Lower Urinary Tract Tumors

The use of a photodynamic fluorescence marker for diagnosis of tumors is an intriguing concept to improve the thoroughness of surgical tumor resection. Complete surgical resection of multifocal bladder tumors and flat lesions such as carcinoma in situ is known to be difficult, and thus a source of recurrences. We report on the recent experience with the intravesical application of the photosensitizer prodrug 5-aminolevulinic acid (5-ALA), which is a nontoxic physiological heme substrate. Initial results from fluorescence diagnosis using krypton laser light, and recent results with a modified incoherent light source constantly showed a 25% increase in urothelial tumor detection in comparison to white light endoscopy. Due to the high sensitivity, the number of biopsies could be decreased constantly in comparison to routine random biopsies taken under white light endoscopy. The results show about 25–30% of cases with fluorescent lesions which are histologically benign. 5-ALA is a promising tool for the diagnosis of bladder cancer. The outcome of the admission study for 5-ALA in urology in Germany is positive, and is being continued by prospective multicenter clinical studies to prove the hypothesis of reduction of tumor recurrence with this method. 5-ALA-based fluorescence endoscopy is strongly recommended for further clinical testing.

Photodynamic Cystoscopy for Detection of Bladder Tumors

The use of a photodynamic fluorescence marker for diagnosis of tumors is an intriguing concept to improve thoroughness of surgical tumor resection. Complete surgical resection of multifocal bladder tumors and flat lesions as carcinoma in situ is known to be difficult, and thus a source of recurrencies. We report on the recent experience with the intravesical application of the photosensitizer prodrug 5-aminolevulinic acid (5-ALA), which is a nontoxic physiological heme substrate. Initial results from fluorescence diagnosis using krypton laser light and recent results with a modified incoherent light source constantly showed a 25% increase in urothelial tumor detection compared with white light endoscopy. Because of the high sensitivity, the number of biopsies could be decreased constantly compared with routine random biopsies taken under white light endoscopy. The results show about 25% to 30% of cases with fluorescent lesions, which are histologically benign. 5-ALA is a promising tool for diagnosis of bladder cancer. The outcome of the initial study of 5-ALA in urology in Germany is positive and is continued by prospective multicenter clinical studies to prove the hypothesis of reduction of tumor recurrence with this method. 5-ALA-based fluorescence endoscopy is strongly recommended for further clinical testing.

5-Aminolevulinic acid-based photodynamic therapy. Clinical research and future challenges.

Photodynamic therapy (PDT) for cancer patients has developed into an important new clinical treatment modality in the past 25-years. PDT involves administration of a tumor-localizing photosensitizer or photosensitizer prodrug (5-aminolevulinic acid [ALA], a precursor in the heme biosynthetic pathway) and the subsequent activation of the photosensitizer by light. Although several photosensitizers other than ALA-derived protoprophyrin IX (PpIX) have been used in clinical PDT, ALA-based PDT has been the most active area of clinical PDT research during the past 5 years. Studies have shown that a higher accumulation of ALA-derived PpIX in rapidly proliferating cells may provide a biologic rationale for clinical use of ALA-based PDT and diagnosis. However, no review updating the clinical data has appeared so far. A review of recently published data on clinical ALA-based PDT and diagnosis was conducted. Several individual studies in which patients with primary nonmelanoma cutaneous tumors received topical ALA-based PDT have reported promising results, including outstanding cosmetic results. However, the modality with present protocols does not in general, appear to be superior to conventional therapies with respect to initial complete response rates and long term recurrence rates, particularly in the treatment of nodular skin tumors. Topical ALA-PDT does have the following advantages over conventional treatments: it is noninvasive; it produces excellent cosmetic results; it is well tolerated by patients; it can be used to treat multiple superficial lesions in short treatment sessions; it can be applied to patients who refuse surgery or have pacemakers and bleeding tendency; it can be used to treat lesions in specific locations, such as the oral mucosa or the genital area; it can be used as a palliative treatment; and it can be applied repeatedly without cumulative toxicity. Topical ALA-PDT also has potential as a treatment for nonneoplastic skin diseases. Systemic administration of ALA does not seem to be severely toxic, but the advantage of using this approach for PDT of superficial lesions of internal hollow organs is still uncertain. The ALA-derived porphyrin fluorescence technique would be useful in the diagnosis of superficial lesions of internal hollow organs. Promising results of ALA-based clinical PDT and diagnosis have been obtained. The modality has advantages over conventional treatments. However, some improvements need to be made, such as optimization of parameters of ALA-based PDT and diagnosis; increased tumor selectivity of ALA-derived PpIX; better understanding of light distribution in tissue: improvement of light dosimetry procedure; and development of simpler, cheaper, and more efficient light delivery systems.