2534 Background: The approved topoisomerase I (TopI) inhibitors (irinotecan and topotecan) are camptothecin derivatives. The lactone ring structure of camptothecins negatively impacts their clinical potential.Genz-644282 is a novel non-camptothecin that induces TopI-DNA cleavage complexes at similar as well as unique genomic positions; that are more persistent. This study was designed using a pharmacokinetic-pharmacodynamic (PK-PD) model to predict the maximum tolerated dose (MTD). Methods: A PK-PD relationship of Genz-644282 to inhibit tumor growth was derived (Simeoni, Can Res 2004). Using an accelerated titration design, cohorts of 1, 3 or 6 patients received 3 weekly doses of Genz-644282 on a 28 day schedule starting with 0.5 mg/m2. After MTD on the 28 day schedule was exceeded, 21 day schedule was initiated. Results: 49 patients (N=44 data available: 26M :18F) were dosed. Tumor types were colorectal (15), breast (5), small cell lung (SCLC 3), non-small cell lung (4), others (17). As predicted from the PK-PD model 8 cohorts were evaluated on the 28 day schedule before defining the MTD. The MTD was 8 mg/m2 and 9 mg/m2 for the 28 day and 21 day schedules, respectively. Dose limiting toxicities that determined the MTD were: gr 4 thrombocytopenia (n=2); gr 2 thrombocytopenia (n=1) and gr 4 neutropenia (n=1) both of which resulted in ≥72h delays in initiating cycle 2. Treatment emergent adverse events (>10%) were nausea (45%), fatigue (39%), anorexia (32%), anemia (27%), vomiting (23%), thrombocytopenia (18%), diarrhea (16%), dehydration (16%), hyperglycemia (16%), cough (16%), dyspnea (14%), depression (11%) and hyponatremia (11%). Efficacy data suggest 2 responders with SCLC (1 minor response;1 complete response), and 2 patients (breast, gastric) with stable disease (≥ 6 months). PK data show a Genz-644282 half-life of approximately 50 h, a linear dose-exposure relationship, and no accumulation in between doses. Conclusions: Genz-644282 is a non-camptothecin TopI inhibitor in phase I development with ongoing expansion phase. The emerging pk, efficacy and safety data are suggestive of a distinct clinical profile of Genz-644282 from the camptothecins.