Targeted photodynamic therapy for prostate cancer: Inducing apoptosis via activation of the caspase-8/-3 cascade pathway

Abstract

The limitation of specific delivery of photosensitizers to tumor sites, represents a significant shortcoming of photodynamic therapy (PDT) application at present. Prostate-specific membrane antigen (PSMA), a validated biomarker for prostate cancer, has attracted considerable attention as a target for imaging and therapeutic applications for prostate cancer. The present study focuses on the investigation of a PSMA inhibitor-conjugate of pyropheophorbide-a (Ppa-conjugate 2.1) for a targeted PDT application and the mechanism of its mediated-cell death in prostate cancer cells. Multiple fluorescence labeling methods were employed to monitor PDT-treated prostate cancer cells by confocal laser scanning microscopy. Our results demonstrate that Ppa-conjugate 2.1 mediated apoptosis is specific to PSMA+ (positive) LNCaP cells, but not PSMA- (negative) PC-3 cells. Furthermore, these results indicate that following PDT, the activation of caspase-8, -3, -9, cleavage of poly(ADP-ribose) polymerase (PARP) and DNA fragmentation is sequential. The appearance of cleaved beta-actin further supported involvement of caspase-3. Specific caspase inhibitor blocking studies reveal that the caspase-8/-3 cascade pathway plays a key role in apoptosis of LNCaP cells induced by Ppa-conjugate 2.1. The demonstrated selective targeting and efficacy of this agent suggests that targeted PDT could serve as an alternative treatment option for prostate cancer.