In vivo targeted killing of prostate tumor cells by a synthetic amoebapore helix 3 peptide modified with two γ-linked glutamate residues at the COOH terminus.

We previously designed a pro-cytolytic peptide to target prostate-specific membrane antigen (PSMA)-positive prostate tumor cells. The backbone of the peptide was derived from the cell lytic amoebapore H-3 domain, which becomes completely inactive upon modification by two glutamate residues linked to the ε-amide group of the COOH-terminal lysine through γ-linkages (H-3Glu2). This modified H-3 domain regains its lytic activity against PSMA-positive cells (LNCaP) after the γ-linked glutamate residues are cleaved by PSMA. Our previous in vitro results demonstrate that the modified amoebapore peptide has strong cytolytic activity towards PSMA-positive cells and very little activity towards PSMA-negative cells. In the present study, the in vivo efficacy of this modified peptide was examined in human LNCaP prostate tumor xenografts in nude mice. The results showed significantly decreased tumor size and PSA levels in treated mice as compared to control mice. As well, 5/12 of the treated mice were tumor-free. Peptide distribution studies showed that peptide levels in the prostate tumors maintained a steady concentration for approximately 6 hours. Single-dose toxicity studies showed no toxic effects of the peptide when administered intraperitoneally or intravenously at a dose of 30 mg/kg.