PSA, which is overexpressed in prostate carcinoma, represents a molecular target for selectively
releasing an anticancer agent from a prodrug formulation. In this study, we report on the in vivo
antitumor efficacy of an efficacious albumin-binding prodrug of doxorubicin (PSA9) that
incorporates p-aminobenzyloxycarbonyl (PABC) as a self-immolative spacer in addition to the
heptapeptide, Arg-Ser-Ser-Tyr-Tyr-Ser-Leu, which serves as a substrate for PSA. The prodrug is
cleaved very efficiently by PSA releasing H-Ser-Leu-PABC-doxorubicin and subsequently
doxorubicin in PSA-positive cell lysates and prostate tumor homogenates as the final cleavage
product. PSA9 at 36mgkg1 doxorubicin equivalents (intravenous) was compared with
conventional doxorubicin at equitoxic doses (at 33mgkg1; intravenous) in an orthotopic
mouse model of prostate cancer using LNCaP lentiviral luciferase-neomycin cells transduced with
luciferase. Whereas doxorubicin did not show any efficacy against the primary tumor or metastases,
the prodrug reduced the primary tumor by 30–50% and circulating PSA levels, and in addition,
showed a pronounced reduction in lung and bone metastases by B77% and B96%, respectively,
and a positive trend regarding the activity against liver and lymph-node metastases compared with
control and doxorubicin-treated animals. The incorporation of PABC as a self-immolative spacer
together with a PSA substrate demonstrates superior antitumor effects over doxorubicin attributed
to an efficient cleavage by PSA releasing doxorubicin as the final active agent in prostate tumor
homogenates. Using this approach for developing effective prodrugs against prostate cancer, is
worthy of further preclinical optimization.