AR-67 is a lipophilic third generation camptothecin analogue, currently under early stage clinical trials. It acts by targeting Topoisomerase 1 (Top1), a nuclear enzyme essential for DNA replication and transcription and is present in two forms, the pharmacologically active lipophilic lactone and the charged carboxylate. In oncology patients participating in a phase I clinical trial, AR-67 lactone was the predominant species in plasma. Similarly to other camptothecins, the identified dose-limiting toxicities for AR-67 were neutropenia, thrombocytopenia and fatigue. In addition, in vitro metabolism studies indicated AR-67 lactone as a substrate for CYP3A4/5 as well as the UGT1A7 and UGT1A8 enzymes localizing in the liver and the gut. Numerous studies have demonstrated the over-expression of transporters in certain tumor types. Here, the effect of interactions between AR-67 and efflux or uptake transporters on the antitumor efficacy of AR-67 in vitro was studied. We showed that BCRP and MDR1 overexpression confers resistance to AR-67. Moreover, we demonstrated the therapeutic superiority of protracted dosing over more intense dosing regimens of AR-67 using xenografts models. Our studies indicated the schedule-dependent expression of Top1 and the preferential partitioning of AR-67 in the tumor tissue. We reason that these are factors that need to be taken into consideration when designing dosing schedules aiming to maximize efficacy. As most cytotoxic drugs, AR-67 has a narrow therapeutic window. Thus, it is essential to identify the variables influencing exposure to this camptothecin analogue. A thorough compartmental pharmacokinetic analysis was performed on the patient data obtained in a phase 1 clinical trial on AR-67. Moreover, sources of intersubject variability associated with obtaining pharmacokinetic parameter estimates were identified and a population covariate pharmacokinetic model was developed. In conclusion, the drug development of AR-67 is a work in process. Findings presented above provide an insight on the factors contributing to its efficacy and toxicity when given to cancer patients.

The third generation camptothecin analog, AR-67, is undergoing early phase clinical trials as a chemotherapeutic agent. Like all camptothecins it undergoes pH dependent reversible hydrolysis between the lipophilic lactone and the hydrophilic carboxylate. The physicochemical differences between the lactone and carboxylate could potentially give rise to differences in transport across and/or entry into cells. In vitro studies indicated reduced intracellular accumulation and/or apical to basolateral transport of AR-67 lactone in P-gp and/or BCRP overexpressing MDCKII cells and increased cellular uptake of carboxylate in OATP1B1 and OATP1B3 overexpressing HeLa-pIRESneo cells. Pharmacokinetic studies were conducted in rats to study the disposition and oral bioavailability of the lactone and carboxylate and to evaluate the extent of the interaction with uptake and efflux transporters. A pharmacokinetic model accounting for interconversion in the plasma was developed and its performance evaluated through simulations and in vivo transporter inhibition studies using GF120918 and rifampin. The model predicted well the likely scenarios to be encountered clinically from pharmacogenetic differences in transporter proteins, drug-drug interactions and organ function alterations. Oral bioavailability studies showed similarity following lactone and carboxylate administration and indicated the significant role ABC transporters play in limiting the oral bioavailability.