Evaluation of in vitro drug interactions with karenitecin, a novel, highly lipophilic camptothecin derivative in phase II clinical development

The objective of this study was to describe the potential metabolism and protein-binding interactions with karenitecin, a novel computer-engineered, highly lipophilic camptothecin. Individual cloned cytochrome P450 (CYP450) isoenzymes were used to determine, in vitro, the metabolism of karenitecin. Known substrates and inhibitors of each isoenzyme were employed to evaluate CYP450 drug interactions with karenitecin. To assess the extent, variability, and role of various drug-binding proteins, the authors examined, in vitro, the effects of both albumin (Alb) and α-acidic glycoprotein (AAG) on karenitecin plasma protein binding (PPB). Equilibrium dialysis techniques were used to measure the free fraction of karenitecin in the presence of varying ratios of Alb and AAG. Artificial plasma, spiked with karenitecin, was dialyzed for 72 hours at 37°C against a Sorensen's buffer solution using regenerated cellulose membranes having a molecular weight cutoff of 12 to 14 kDa. Additional protein-binding experiments were conducted to assess the potential PPB drug interactions between karenitecin and other highly protein-bound drugs commonly used in the treatment of cancer patients. In vitro experiments suggested that karenitecin is metabolized by CYP450 3A4, 2C8, and 2D6 isoenzymes and is an inhibitor of the CYP450 3A4 and 2C8 isoenzymes. The mean (± SD) percentage of karenitecin bound to plasma proteins was 99.1% ± 0.27%. The extent of karenitecin protein binding was directly proportional to the plasma concentration of AAG. Protein-binding displacement interactions were observed in the in vitro experiments with phenobarbital, phenytoin, mitoxantrone, and salicylic acid. It was concluded that karenitecin has the potential to alter CYP450 3A4 and 2C8 drug-metabolizing activity. In addition, in vitro PPB evaluations have demonstrated that karenitecin may displace other highly PPB drugs and that slight variations in plasma AAG concentration may result in large variations in free drug exposure. Each of these interactions could potentially result in increasing the toxicity or alter the efficacy of combination anticancer drug therapy if they are significant in patients. Future karenitecin clinical trials should include studies to monitor or evaluate the effects of these potential drug interactions on the overall toxicity of karenitecin when used in combination with other drugs.