Comparative activity of tamoxifen and N-desmethyltamoxifen in human multidrug resistant leukemia cell lines

Tamoxifen (Tmx) is one of a number of agents that can reverse the multidrug resistant (MDR) phenotype in vitro; it is unique, however, in that both the parent compound and the main metabolite, N-desmethyltamoxifen (NDMTmx), have long plasma half-lives, 7 and 14 days, respectively. To determine whether NDMTmx was as active as the parent compound in restoring sensitivity to daunorubicin (DNR) in vitro, we performed uptake and retention studies in the MDR-positive human leukemia cell lines CEM-VLB and HL-60/RV+ using laser flow cytometry to quantitate intracellular DNR concentration. Cells incubated with DNR and NDMTmx 10 μM demonstrated increased intracellular DNR levels that were very similar to those obtained with DNR and Tmx 10 μM. Cellular retention of DNR was also measured after incubation with Tmx 10 μM or NDMTmx 10 μM and resuspension in fresh medium containing Tmx or NDMTmx in order to stimulate in vivo conditions. Washout curves in both cell lines were similar with both Tmx and NDMTmx. Finally, clonogenic experiments were performed to determine whether Tmx and NDMTmx demonstrated the same degree of cytotoxicity. The combination of Tmx 10 μM plus DNR and that of NDMTmx 10 μM plus DNR each resulted in > 80% growth inhibition. These results suggest that NDMTmx is as active as Tmx in restoring sensitivity to DNR in vitro, a finding which may have important implications in clinical trials that assess the ability of Tmx to reverse the MDR phenotype.