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 alpha-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 degrees 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 karentiecin 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.