Adenovirus-mediated transfer of inducible caspases: A novel "death switch"gene therapeutic approach to prostate cancer

In patients with localized prostate cancer, radical prostatectomy and radia tion therapy, although effective in controlling localized disease, are ofte n associated with significant side effects attributable to injury of adjace nt tissues. Moreover, patients with metastatic disease eventually fail syst emic hormonal or chemotherapy because of the development of progressive, re fractory disease. In this study, we evaluated the safety and efficacy of a novel suicide gene therapy that could potentially spare normal tissue while bypassing molecular mechanisms of apoptosis resistance by using chemically inducible effector caspases to trigger apoptosis in prostate cancer cells. Initially, we compared the ability of a panel of inducible Fas signaling i ntermediates to kill human and murine prostate cancer cell lines. On the ba sis of the superior killing by downstream caspase-1 and caspase-3, replicat ion-deficient adenoviral vectors expressing conditional caspase-1 (Ad-G/iCa sp1) or caspase-3 (Ad-G/iCasp3), regulated by nontoxic, lipid-permeable, ch emical inducers of dimerization (CID), were constructed. Upon vector transd uction followed by CID administration, aggregation and activation of these recombinant caspases occur, leading to rapid apoptosis. In vitro, both huma n (LNCaP and PC-3) and murine (TRAMP-C2 and TRAMP-C2G) prostate cancer cell lines were efficiently transduced and killed in a CID-dependent fashion. b l vivo, direct injection of Ad-G/iCasp1 into s.c. TRAMP-C2 tumors caused fo cal but extensive apoptosis without evidence for a bystander effect at the maximal viral dose (i.e., 2.5 x 10(10) viral particles/25 mul) in host anim als that also received CID compared with control animals. Treatment with Ad -G/iCasp1 plus CID resulted in a transient, yet significant, reduction both in tumor growth and volume compared with tumors treated with vector but no t CID (P < 0.035) or vector-diluent plus CID (P < 0.022), both of which gre w more rapidly. These results demonstrate that CID-regulated, caspase-based suicide gene therapy is safe and can inhibit the growth of experimental pr ostate cancer in vitro and ill vivo through potent induction of apoptosis, providing a rationale for further development.