The present study was undertaken to compare the cellular transport characteristics of [(3)H]NPI-0052 (1R,4R,5S)-4-(2-chloroethyl)-1-((S)-((S)-cyclohex-2-enyl)(hydroxy)methyl)-5-methyl-6-oxa-2-azabicyclo[3.2.0]heptane-3,7-dione (marizomib; salinosporamide A) and [(3)H]NPI-0047 (1R,4R, 5S)-1-((S)-((S)-cyclohex-2-enyl)(hydroxy)methyl)-4-ethyl-5-methyl-6-oxa-2-azabicyclo[3.2.0]heptane-3,7-dione in RPMI 8226 multiple myeloma and PC-3 prostate adenocarcinoma cells to determine whether these properties explain differences in the cytotoxic potencies of these chemical analogs. The results indicate that marizomib, which possesses a chemical-leaving group, is more cytotoxic to both cell lines and inhibits proteasome activity more completely at lower concentrations than NPI-0047, a nonleaving-group analog. Moreover, it was found that both compounds accumulate in these cells by simple diffusion and the same carrier-mediated transport system. Although the rate of uptake is similar, the cellular efflux, which does not seem to be mediated by a major ATP-binding cassette (ABC)-efflux transporter, is more rapid for NPI-0047 than for marizomib. Experiments revealed that the irreversible binding of marizomib to the proteasome is responsible for its slower efflux, longer duration of action, and greater cytotoxicity compared with NPI-0047. The discovery that major ABC transporters of the multidrug resistance-associated protein family do not seem to be involved in the accumulation or removal of these agents suggests they may not be affected by multidrug resistance mechanisms during prolonged administration.