Clofarabine excretion in the isolated perfused rat kidney (IPK)

The overall goal of this dissertation was to characterize the renal excretion of clofarabine and to identify the role of renal transport systems on clofarabine disposition utilizing the isolated perfused rat kidney (IPK) model. In an attempt to accomplish this goal, dose-linearity studies were performed to obtain baseline excretion. Known inhibitors of renal transport systems were used to probe mechanisms of clofarabine excretion in the IPK. The effect of other medications on clofarabine excretion was also evaluated. Based on results obtained from transport inhibition studies in the IPK, follow up studies were conducted in vivo in rats to further evaluate possible drug-drug interactions identified in the IPK Finally, the relevance of the IPK results to in vivo disposition was tested.;A validated HPLC method was used to analyze clofarabine concentrations in urine, perfusate, plasma and ultrafiltrate samples.;Preliminary studies were performed to assess clofarabine binding to IPK perfusate by an ultrafiltration technique. Clofarabine binding to IPK apparatus and ultrafiltration device were also estimated. Clofarabine binding to IPK apparatus and ultrafiltration device were determined to be negligible. However, clofarabine binding to IPK perfusate was estimated at 11%, fraction unbound (fu) value used for pharmacokinetic parameter (XR) estimation was 0.89.;Clofarabine excretion was studied at four doses (160microg, 800microg, 2000microg and 4000microg) in the IPK, targeting perfusate levels of 2, 10, 25, 50 microg/mL respectively. Clofarabine clearance was non-linear in the IPK, although the concentrations tested were approximately 10 to 100-fold higher than maximum concentrations observed in humans. Excretion ratio data indicated a shift from net secretion (XR = 1.2 +/- 0.33) to net reabsorption (XR = 0.78 +/- 0.40) at the highest dose tested. Clofarabine (2 microg/ml) was also co-perfused with known inhibitors of the organic cation (cimetidine, ranitidine and tyramine) and organic anion (probenecid, ellagic acid) transport systems. Additionally, the effect of medications including itraconazole, digoxin, fludarabine and cytarabine (Ara-C) on clofarabine excretion was evaluated.;Co-administration with cimetidine significantly reduced clofarabine clearance (0.83 +/- 0.22 → 0.32 +/- 0.0058 ml/min) and XR (1.2 +/- 0.22 → 0.42 +/- 0.045) more than two fold compared with clofarabine alone in the IPK. The data are consistent with net reabsorption of clofarabine when administered with cimetidine. Of the other known transport inhibitors studied, ellagic acid increased clofarabine clearance and XR. However, probenecid and tyramine had no significant effect on clofarabine excretion in the IPK. Results from IPK studies also pointed to a possible role of nucleoside transporters on the luminal reabsorption of clofarabine. IPK results suggest that clofarabine excretion involves a combination of filtration, secretion, and reabsorption. Transport inhibition studies demonstrate a role of OCT2 on clofarabine renal clearance.;The effect of cimetidine and Ara-C on clofarabine renal disposition was also studied utilizing a series of dosing schemes (pre-treatment, co-administration, and post-dose). Results from these studies indicate all dosing schemes have an effect on clofarabine administration when dosed together with cimetidine. However, the co-administration dosing scheme was determined to have the maximum effect with a 1.7 fold increase in plasma exposure. Clofarabine excretion increased and AUC decreased when clofarabine was administered together with Ara-C. Clofarabine half-life also increased but this was not clinically significant.;Taken together, the results suggest that carrier-mediated transport of clofarabine is by a cimetidine sensitive transporter possibly OCT2. Nucleoside transporters appear to be involved in clofarabine transport.;A comparison was made between clofarabine clearance data generated from IPK and in vivo studies. Although parameter estimates obtained in the IPK were lower than those observed in vivo, similar reduction in clearance was observed in both cases. There was a 61% reduction in clofarabine clearance in the IPK when clofarabine was co-perfused with cimetidine (0.83 mL/min → 0.32 mL/min). Likewise, total clearance and renal clearance were both reduced 61% (20.2 mL/min/kg → 7.8 mL/min/kg) and 73% (10.2 mL/min/kg → 2.76 mL/min/kg) respectively, when clofarabine was co-administered with cimetidine in vivo.;Although good correlation was observed between IPK and in vivo data, predictions may not be applicable at clinical levels. However, patients may need to be cautioned on taking clofarabine together with cimetidine.