Pharmacokinetics, Metabolism, Distribution and Excretion of Carfilzomib in Rats.

Carfilzomib, a tetrapeptide epoxyketone that selectively and irreversibly inhibits the 20S proteasome, has demonstrated a favorable safety profile and anti-tumor efficacy in phase II trials in patients with multiple myeloma [1, 2]. In this study, we characterized the pharmacokinetics (PK), metabolism, distribution and excretion of carfilzomib in Sprague Dawley rats.

In vivo studies were conducted following single intravenous (IV) administrations of carfilzomib to rats. For pharmacokinetic evaluations, plasma samples were collected pre-dose and from 0.1 to 60 minutes post-dose. Biliary excretion was evaluated in bile-duct cannulated rats with bile collection for 8 hours. Urine was collected for 24 hours post-dose for evaluation of renal excretion. Metabolism was studied in plasma, bile and urine samples collected from rats treated with carfilzomib and following in vitro incubations of carfilzomib with hepatocytes and tissue homogenates of lung, heart and kidney from rats. Liquid chromatography with tandem mass spectrometric detection was used in determination of carfilzomib and metabolites in different matrices. Noncompartmental analysis was used in calculation of pharmacokinetic parameters.

(1) Carfilzomib was rapidly cleared from the plasma compartment following bolus IV administration with a terminal half life of less than 20 minutes. The plasma clearance was 229, 195 and 296 mL/min/kg, the C_max (the maximum concentration) was 16.4, 39.3 and 42.9 μM and the AUC_last (area under the plasma concentration vs time curve) was 12.6, 28.5 and 37.6 min*μmol/L, at the dose levels of 2, 4 and 8 mg/kg, respectively. At all dose levels, the clearance was much higher than rat hepatic flow. When carfilzomib was given as 30-minute intravenous infusion at 8 mg/kg, the steady state concentration of 1.5 μM was reached within 15 minutes and was 28-fold lower than the C_max with IV bolus at the equivalent dose. Other PK parameters (clearance, AUC and t_1/2) were comparable between bolus and infusion dosing. Rapid and potent proteasome inhibition in blood and tissues (brain excepted) was observed following either bolus or infusion administration, demonstrating wide tissue distribution of carfilzomib. (2) Carfilzomib was rapidly metabolized not only in rat hepatocytes, but also in incubations with tissue homogenates of rat lung, kidney and liver as well as in rat blood, indicating that carfilzomib could be metabolized in tissues and in blood in vivo following IV administration to rats. These results suggested that the high clearance was due to extrahepatic metabolism of carfilzomib in vivo. In plasma, urine and bile collected from rats treated with carfilzomib, as well as in incubations of carfilzomib with rat hepatocytes and tissue homogenates, the most abundant metabolites were found to be peptide fragments and the diol of carfilzomib. These results indicated that peptidase cleavage and epoxide hydrolysis were the principal pathways of metabolism and that P450-mediated mechanisms were not significant in the overall metabolism of carfilzomib. (3) Carfilzomib was eliminated mainly in the form of metabolites following intravenous dosing to rats via renal and biliary excretions.

Carfilzomib is rapidly cleared and widely distributed following IV administrations to rats. Metabolism of carfilzomib is mediated by plasma and tissue resident peptidases and epoxide hydrolases. The irreversible mechanism of proteasome inhibition by carfilzomib overcomes clearance mechanisms and enables equivalent proteasome inhibition with both bolus and infusion administration. These data support ongoing efforts to study the effect of infusion administration of carfilzomib in patients with myeloma.

[1] S. Jagennath, et al. (2008) Blood 112(11): Abstract #864.

[2] R. Vij, et al., (2008) Blood 112(11): Abstract #865.

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