Targeting gastrin-releasing peptide receptors of prostate cancer cells for photodynamic therapy with a phthalocyanine-bombesin conjugate.

Sulfonated aluminum phthalocyanines (AlPcS) are potent photosensitizers for the photodynamic therapy (PDT) of cancer. In this study we evaluate the possibility to improve the efficacy of AlPcS-PDT for prostate cancer by targeting tetrasulfonated aluminum phthalocyanines (AlPcS(4)) to the gastrin-releasing peptide receptor (GRPR) through coupling to bombesin. A mono-carbohexyl derivative of AlPcS(4) is attached to 8-Aoc-bombesin(7-14)NH(2) via an amide bridge to yield a bombesin-AlPcS(4) conjugate linked by a C-14 spacer chain. The conjugate is characterized by mass spectroscopy and shown to bind to the GRPR with a relative binding affinity (RBA) of 2.3, taking bombesin (RBA=100) as unity. The in vitro photodynamic efficacy of the conjugate against PC-3 human prostate cancer cells is improved by a factor 2.5 over the non-conjugated mono-carbohexyl derivative of AlPcS(4).