Impaired bortezomib binding to mutant ß5 subunit of the proteasome is the underlying basis for bortezomib resistance in leukemia cells.

Proteasome inhibition is a novel treatment for several hematological malignancies. However, resistance to the proteasome inhibitor bortezomib (BTZ, Velcade) is an emerging clinical impediment. Mutations in the ß5 subunit of the proteasome, the primary target of BTZ, have been associated with drug resistance. However, the exact mechanism by which these mutations contribute to BTZ resistance, is still largely unknown. Toward this end, we here developed BTZ-resistant multiple myeloma (8226) and acute lymphoblastic leukemia (CCRF-CEM) cell line models by exposure to stepwise increasing concentrations of BTZ. Characterization of the various BTZ-resistant cells revealed upregulation of mutant ß5 subunit of the proteasome. These newly identified ß5-subunit mutations, along with previously described mutations, formed a mutation cluster region in the BTZ-binding pocket of the ß5 subunit, that of the S1 specificity pocket in particular. Moreover, we provide the first evidence that the mechanism underlying BTZ resistance in these tumor cells is impaired binding of BTZ to the mutant ß5 subunit of the proteasome. We propose that proteasome subunit overexpression is an essential compensatory mechanism for the impaired catalytic activity of these mutant proteasomes. Our findings further suggest that second-generation proteasome inhibitors that target the α7 subunit of the proteasome can overcome this drug resistance modality.

Disposition and toxicity of trabectedin (ET-743) in wild-type and mdr1 gene (P-gp) knock-out mice.

Trabectedin is a novel anticancer drug active against soft tissue sarcomas. Trabectedin is a substrate for P-glycoprotein (P-gp), which is encoded by mdr1a/1b in rodents. Plasma and tissue distribution, and excretion of [(14)C]-trabectedin were evaluated in wild-type and mdr1a/1b(-/-) mice. In parallel, we investigated the toxicity profile of trabectedin by serial measurements of blood liver enzymes and general pathology. [(14)C]-trabectedin was extensively distributed into tissues, and rapidly converted into a range of unknown metabolic products. The excretion of radioactivity was similar in both genotypes. The plasma clearance of unchanged trabectedin was not reduced when P-gp was absent, but organs under wild type circumstances protected by P-gp showed increased trabectedin concentrations in mdr1a/1b(-/-) mice. Although hepatic trabectedin concentrations were not increased when P-gp was absent, mdr1a/1b(-/-) mice experienced more severe liver toxicity. P-gp plays a role in the in vivo disposition and toxicology of trabectedin.