Background: PS-341, a selective inhibitor of the proteasome, currently is under evaluation as an anticancer agent in multiple phase I clinical trials. In animal-model studies, PS-341 was rapidly removed from the vascular compartment and distributed widely, quickly approaching the limits of detection. An accurate pharmacodynamic assay has been developed as an alternative or complement to pharmacokinetic measurements.

Methods: Fluorogenic kinetic assays for both the chymotryptic and tryptic activities of the proteasome have been optimized for both whole blood and blood cells. Using the ratio of these activities and the catalytic mechanism of the proteasome, we developed a novel method of calculating percentage of inhibition, using two structurally unrelated inhibitors (PS-341 and lactacystin).

Results: This ratio method was demonstrated to be sensitive (detection limit of 13% inhibition with 10 μg of cell lysate), specific to the proteasome (PS-341 provides >98% inhibition), accurate (112% analyte recovery), and precise (0% ± 5% inhibition at 0 nmol/L PS-341 and 74.5% ± 1.7% inhibition at 200 nmol/L PS-341). Using these assays, we found that both erythrocytes and leukocytes contain proteasome at 3 μmol/L. Pharmacodynamic results for PS-341 obtained from the whole-blood ratio method were comparable to those using leukocytes determined by another method.

Conclusions: The described assay provides a reliable method for studying the pharmacodynamics of proteasome inhibitors and is now in use in concurrent phase I clinical trials with PS-341.