Charged groups on pyropheophorbide-based photosensitizers dictate uptake by tumor cells and photodynamic therapy efficacy

Recent Advances in the HPPH-Based Third-Generation Photodynamic Agents in Biomedical Applications

Photodynamic therapy has emerged as a recognized anti-tumor treatment involving three fundamental elements: photosensitizers, light, and reactive oxygen species. Enhancing the effectiveness of photosensitizers remains the primary avenue for improving the biological therapeutic outcomes of PDT. Through three generations of development, HPPH is a 2-(1-hexyloxyethyl)-2-devinyl derivative of pyropheophorbide-α, representing a second-generation photosensitizer already undergoing clinical trials for various tumors. The evolution toward third-generation photosensitizers based on HPPH involves structural modifications for multimodal applications and the combination of multifunctional compounds, leading to improved imaging localization and superior anti-tumor effects. While research into third-generation HPPH is beneficial for advancing PDT treatment, equal attention should also be directed toward the other two essential elements and personalized diagnosis and treatment methodologies.

Tumor Cell-Specific Retention and Photodynamic Action of Erlotinib-Pyropheophorbide Conjugates

To enhance uptake of photosensitizers by epithelial tumor cells by targeting these to EGFR, pyropheophorbide derivatives were synthesized that had erlotinib attached to different positions on the macrocycle. Although the addition of erlotinib reduced cellular uptake, several compounds showed prolonged cellular retention and maintained photodynamic efficacy. The aim of this study was to identify whether erlotinib moiety assists in tumor targeting through interaction with EGFR and whether this interaction inhibits EGFR kinase activity. The activity of the conjugates was analyzed in primary cultures of human head and neck tumor cells with high-level expression of EGFR, and in human carcinomas grown as xenografts in mice. Uptake of erlotinib conjugates did not correlate with cellular expression of EGFR and none of the compounds exerted EGFR-inhibitory activity. One derivative with erlotinib at position 3, PS-10, displayed enhanced tumor cell-specific retention in mitochondria/ER and improved PDT efficacy in a subset of tumor cases. Moreover, upon treatment of the conjugates with therapeutic light, EGFR-inhibitory activity was recovered that attenuated EGFR signal-dependent tumor cell proliferation. This finding suggests that tumor cell-specific deposition of erlotinib-pyropheophorbides, followed by light triggered release of EGFR-inhibitory activity, may improve photodynamic therapy by attenuating tumor growth that is dependent on EGFR-derived signals.

Tumor cell-specific retention of photosensitizers determines the outcome of photodynamic therapy for head and neck cancer

Pheophorbide-based photosensitizers have demonstrated tumor cell-specific retention. The lead compound 3-[1'-hexyloxyethyl]-2-devinylpyropheophorbide-a (HPPH) in a clinical trial for photodynamic therapy of head and neck cancer lesions indicated a complete response in 80% of patients. The question arises whether the partial response in 20% of patients is due to inefficient retention of photosensitizers by tumor cells and, if so, can the photosensitizer preference of individual cancer cases be identified prior to photodynamic therapy. This study determined the specificity of head and neck cancer cells and tumor tissues for the uptake and retention of diffusible pheophorbides differing in peripheral groups on the macrocycle that contribute to cellular binding. The relationship between photosensitizer level and light-mediated photoreaction was characterized to identify markers for predicting the effectiveness of photodynamic therapy in situ. The experimental models were stromal and epithelial cells isolated from head and neck tumor samples and integrated into monotypic tissue cultures, reconstituted three-dimensional co-cultures, and xenografts. Tumor cell-specific photosensitizer retention patterns were identified, and a procedure was developed to allow the diagnostic evaluation of HPPH binding by tumor cells in individual cancer cases. The findings of this study may assist in designing conditions for photosensitizer application and photodynamic therapy of head and neck cancer lesions optimized for each patient's case.

On the mechanisms of photodynamic action of photosensitizers based on polycationic derivatives of synthetic bacteriochlorin against human lung cancer cells A549 (in vitro study)

BACKGROUND

One of the tasks of anticancer photodynamic therapy is increasing the efficacy of treatment of cancer nodes with large (clinically relevant) sizes using near-infrared photosensitizers (PS). We study the photodynamic action against A549 human lung cancer cells using PS based on polycationic derivatives of synthetic bacteriochlorin.

METHODS

The efficacy and mechanisms of the photodynamic action of PS based on polycationic derivatives of synthetic bacteriochlorin against A549 lung cancer cells were studied in vitro using immunocytochemical and morphological methods.

RESULTS

It was found that PS based on tetracationic and octacationic derivatives of synthetic bacteriochlorin induce necrosis, apoptosis, decreasing of proliferative and mitotic activity, as well as reducing the number of ALDH1-positive cancer cells with signs of stem cells in A549 human lung cancer cell culture. The IC₅₀ values (concentration of a PS that reduces cells survival by 50%) were about 0.69 μM for tetracationic PS and 0.57 μM for octacationic PS under irradiation at 30 J/cm² while in the "dark" control they were higher than 100 μM for both PSs.

CONCLUSIONS

Photosensitizers based on polycationic derivatives of synthetic bacteriochlorin have high phototoxicity against A549 cancer cells caused by the induction of necrosis and apoptosis of cancer cells, including cells with signs of stemness, and a sharp decrease of mitotic and proliferative activity.