Oxime-based 19-Nortestosterone-Pheophorbide a Conjugate: Bimodal Controlled Release Concept for PDT

Photodynamic therapy has become a feasible direction for the treatment of both malignant and non-malignant diseases. It has been in the spotlight since FDA regulatory approval was granted to several photosensitizers worldwide. Nevertheless, there are still strong limitations in the targeting specificity that is vital to prevent systemic toxicity. We report the synthesis and biological evaluation of a novel bimodal oxime conjugate composed of a photosensitizing drug, red-emitting pheophorbide a, and nandrolone, a steroid specifically binding the androgen receptor (AR) commonly overexpressed in various tumors. We fully characterized the physico-chemical properties of the nandrolone-pheophorbide a conjugate (NT-Pba) together with singlet oxygen generation. As light-triggered therapies have the potential to provide important advances in treatment of hormone sensitive cancer, the biological potential of this novel specifically targeted photosensitizer was assessed in prostatic cancer cell lines in vitro using one AR-positive (LNCaP) and one AR-negative/positive cell line (PC-3). U-2 OS cells, both with and without stable AR expression, were used as a second cell line model. Interestingly, we have found that the NT-Pba conjugate was not only photodynamically active and AR-specific, but also that its phototoxic effect was more pronounced compared to pristine pheophorbide a. Further, we examined the intracellular localization of NT-Pba. Live-cell fluorescence microscopy showed a clear evidence that the NT-Pba conjugate localized in the endoplasmic reticulum and mitochondria. Moreover, we performed a competitive localization study with the excess of nonfluorescent nandrolone, which was able to displace fluorescent NT-Pba from the cell interior, which further confirmed the binding specificity. The oxime ether bond degradation was assayed in living cells by both live-cell microscopy and steroid receptor reporter assay using AR U-2 OS cells. In summary, NT-Pba is a promising candidate for both selective targeting and eradicating of AR-positive malignant cells by photodynamic therapy.