PSA-specific and non-PSA-specific conversion of a PSA-targeted peptide conjugate of doxorubicin to its active metabolites

Tumor-selective delivery of doxorubicin by a prostate-specific antigen (PSA )-targeted peptide conjugate prodrug of doxorubicin was demonstrated in a n ude mouse xenograft model of human prostate cancer. The prodrug (referred t o as doxorubicin conjugate) contains doxorubicin linked to a seven-amino ac id peptide conjugate that was designed to increase delivery of doxorubicin to tumor sites through the hydrolytic properties of PSA, which prostate tum ors express in high amounts. Following i.p. administration of the doxorubic in conjugate to mice, tumor exposure to doxorubicin was increased 2.5-fold as compared with that achieved after an equimolar dose of doxorubicin itsel f. However, in heart tissue, the site of clinical dose-limiting toxicity, d oxorubicin concentrations observed after administration of doxorubicin conj ugate were substantially lower than those in mice that received doxorubicin itself. While the prodrug provided selective delivery of doxorubicin to tu mor tissue, there was substantial non-PSA-specific formation of doxorubicin in laboratory animals, a factor that would limit the extent of therapeutic gain of the prodrug. Following i.v. administration to mice, rats, dogs, an d monkeys, about one-third of the dose was metabolized to doxorubicin. In t umor-bearing mice, the fraction of the dose metabolized to doxorubicin appe ared even higher. This is likely the result of conjugate conversion to doxo rubicin by both PSA-specific (in tumor) and non-PSA-specific proteolytic ac tivities. In vitro studies provided further support for the PSA specificity of metabolism; LNCaP cells mediated rapid metabolism of the conjugate, whi le DuPRO-1 cells, which are deficient in PSA, were incapable of metabolism.