The camptothecins are an important new class of anticancer compounds that are active agents against small cell lung cancer (SCLC). Nevertheless, efficacy is limited by both acquired and intrinsic mechanisms of resistance, and by the substantial toxicities produced by these drugs. To understand better the mechanisms involved in camptothecin cytotoxicity, we established and characterized several camptothecin resistant and partially revertant human epidermoid cell lines derived from KB cells. With this cell model, we showed that exposure to camptothecin resulted in down-regulation of the target enzyme topoisomerase (topo) I, that this down-regulation may contribute to resistance to camptothecin, and that both the down-regulation of topo I and resistance can be inhibited by co-treatment with a proteasome inhibitor. Furthermore, we and others have shown, that down-regulation of topo I is one of several mechanisms of resistance to camptothecin related to enhanced DNA repair. In a previous clinical study, we also showed that down-regulation of topo I occurs in peripheral mononuclear cells in vivo following treatment with topotecan. As a result, the first aim of this proposal will be to perform a phase I and pharmacokinetic study to establish the optimal doses of topotecan combined with PS-341. The efficacy of this regimen will be further evaluated for patients with relapsed SCLC. The second aim is to determine whether addition of PS-341 prevents down-regulation of topo I in vivo. These pharmacodynamic studies will be performed in peripheral mononuclear cells and in disaggregated tumor cells. In addition, we will determine the expression of the XRCC 1 protein in tumor biopsies, since we have established a role for this DNA repair protein in camptothecin resistance. The data generated by these studies will provide additional insight into clinically relevant mechanisms of resistance to camptothecin that occur in SCLC. [unreadable] [unreadable]