DESCRIPTION: (Applicant's Abstract) Two topoisomerase I (topo I) poisons, topotecan (TPT) and irinotecan (CPT-l1), are currently licensed for use in the U.S. Clinical responses to these agents are highly variable. Although preclinical studies have identified numerous factors that can affect the action of topo I poisons in vitro, few of these parameters have been examined in tumor samples or correlated with clinical response. Studies supported by this grant have investigated 1) mechanisms of resistance to topo I poisons in clinical leukemia specimens and 2) effects of combining topo I poisons with other agents. In pursuing the first goal, we have demonstrated during the current funding period that topo I content in acute myelogenous leukemia specimens varies widely but correlates with other markers of proliferation (e.g., PCNA); that the TPT concentration required to stabilize topo I-DNA complexes in acute leukemia specimens ex vivo varies over a 30-fold range irrespective of topo I content; and that this variation in TPT concentration required to stabilize topo I-DNA cleavage complexes can been recapitulated in a tissue culture model. Additional tissue culture studies have demonstrated enhanced sensitivity to TPT or SN-38 (the active metabolite of CPT-11) when activity of the DNA damage checkpoint kinase ATR is inhibited. While addressing the second goal, we have demonstrated that cytotoxic effects are more than additive when SN-38 is combined with the quinazoline-based kinase inhibitor CI1033 or with gemcitabine. Further studies have demonstrated that the SN-38/CI1033 synergy reflects enhanced drug accumulation as a result of CI1033-mediated inhibition of the ABC cassette transporter BCRP. To build on these results, we now propose to 1) evaluate the relationship between response of three well-defined cohorts of solid tumor patients receiving single-agent CPT-11 or TPT and various tumor cell parameters that have been implicated in drug resistance in preclinical models (including topo I content, p53 status, proliferative index, levels of anti-apoptotic Bcl-2 family members, and expression of replication checkpoint proteins); 2) use the recently developed tissue culture model to determine the mechanistic basis for the observation that different TPT concentrations are required to stabilize topo I-DNA complexes in different leukemia specimens; and 3) examine the mechanistic basis for the unanticipated synergy of the gemcitabine + SN-38 combination and determine the effect of combining SN-38 with other agents currently undergoing early clinical testing. Collectively, these studies should provide insight into factors that affect response to topo I poisons in the clinical setting and aid in the rational integration of topo I poisons into multidrug regimens.