ABCG2 and leukemia: 32,33 Tryprostatin A was also found to inhibitthe efflux of topotecan and doxorubicin from ABCG2-expressing myelomacells.34 On the other hand, it was unableto reverse ABCG2-mediated resistance to SN-38 in human leukemia cells.35 No structure–activity relationship (SAR)data have been published with respect to inhibition of ABCG2 or reversalof ABCG2-mediated MDR by tryprostatin A (4); more importantly,the structural basis of the interaction of tryprostatin A (4) or related analogs with ABCG2 has not been elucidated.