Protracted dosing of the lipophilic camptothecin analogue AR-67 in non-small cell lung cancer xenografts and humans

Purpose: Although preclinical studies on camptothecin antitumor effect have demonstrated the superiority of low-dose protracted dosing, these findings were not replicated in the clinic. 7-t-butyldimethylsilyl-10-hydroxycamptothecin (AR-67) is a camptothecin analogue currently under investigation in early phase clinical trials. To maximize the therapeutic potential of AR-67, we sought to identify factors that affect response to treatment. Methods: After determining the maximum tolerated dose using neutropenia as a toxicity endpoint, xenografts received AR-67 under varying dosing schedules and were monitored for survival. On the last treatment day, tumor tissue was collected and topoisomerase 1 (Top1), γH2AX, caspase 3 and PARP protein content was evaluated. AR-67 plasma and tumor pharmacokinetics were also studied in mice and cancer patients who were administered AR-67 as a 1-h intravenous infusion on days 1, 4, 8, 12 and 15 every 21 days. Results: Low-dose protracted dosing schedules increased animal survival compared to less frequent, but higher-dose courses and the expression of Top1 and γH2AX were schedule dependent. Fatigue and neutropenia were the dose-limiting toxicities identified in patients receiving AR-67. Finally, elimination of AR-67 from the tumor site was slower in both xenografts and tumor of a patient enrolled in the pilot clinical trial. Conclusions: We demonstrated that low-dose protracted dosing schedules of AR-67 are therapeutically effective and Top1 reflects the biological activity of AR-67 in xenografts. Moreover, the tumor pharmacokinetics as well as the efficacy and safety of AR-67 given intermittently to cancer patients warrant further investigation.