Using novobiocin as a specific inhibitor of breast cancer resistant protein to assess the role of transporter in the absorption and disposition of topotecan.

Purpose: To investigate the role of intestinal breast cancer resistant protein (BCRP) in the absorption and disposition of topotecan (TPT) using novobiocin (NOV) as a specific inhibitor.

Methods: Transporter inhibition specificity of NOV was assessed in cells overexpressing BCRP or Pgp. Sprague-Dawley rats were orally or intravenously dosed with TPT (2 and 1 mg/kg for p.o. and i.v., respectively) with or without oral co-administration of NOV (50 mg/kg). Pharmacokinetic parameters of TPT were obtained by noncompartmental analysis. To assess the role of BCRP in TPT intestinal permeation, rat ileal segment was perfused with 10 microM TPT in the presence or absence of NOV (500 microM), TPT permeability was calculated based on drug appearance in mesenteric blood. To assess the role of BCRP in TPT intestinal secretion, rat ileal segment was perfused with saline in the presence or absence of NOV (500 microM), while TPT was i.v. infused into rat. Intestinal secretion of TPT was calculated based on drug appearance in the perfusate.

Results: NOV significantly inhibited efflux activity of BCRP, but not Pgp. Coadministration of NOV markedly increased oral TPT AUC(0-720) and Cmax by 3- and 4.5-fold, respectively, and decreased systemic clearance of i.v. injected TPT (from 44.40+/-7.28 without NOV to 29.44+/-1.99 ml/min/kg with NOV). The inclusion of NOV in perfusate significantly increased TPT permeability from 0.81+/-0.30 x10(-6) to 1.26+/-0.12 x10(-6) cm/s, while, the intestinal secretion of TPT was reduced by ~50% when NOV was included in perfusate.

Conclusions: The present study establishes in vitro and in vivo inhibition potency and specificity of NOV on BCRP and provides direct evidence that intestinal BCRP plays an important role in limiting the oral absorption and influencing the systemic elimination of TPT.