The new allosteric inhibitor ABL001 is susceptible to resistance mediated by ABCB1 and ABCG2 overexpression in vitro

Abstract

ABL001, which binds the myristate-binding pocket of the Bcr-Abl kinase domain, is in phase I clinical trials as monotherapy and in combination with imatinib, nilotinib and dasatinib for the treatment of patients with refractory CML or Ph+ ALL. ABL001 sensitivity was evaluated in ABL001 naïve BCR-ABL1+ cell lines K562 (negligible ABCB1/ABCG2 expression), K562-Dox (ABCB1-overexpressing) and K562-ABCG2 (transduced ABCG2 overexpression) with results demonstrating ABL001 efflux by both transporters. K562-Dox and K562-ABCG2 cells demonstrated increased LD50ABL001 vs. K562 control cells: 256 and 299 nM respectively vs. 23 nM, p<0.001. Sensitivity was completely restored with specific inhibitors cyclosporine (ABCB1) and Ko143 (ABCG2): K562-Dox LD50ABL001+Cyclosporine=13 nM, K562-ABCG2 LD50ABL001+Ko143=15 nM (p<0.001). When ABL001 resistance was modelled in vitro, ABCB1 and ABCG2 overexpression was integral in the development of ABL001 resistance. In K562 ABL001-resistant cells, ABCG2 expression increased prior to BCR-ABL1 overexpression and remained high (up to 7.6-fold greater levels in resistant vs. control cells, p<0.001). K562-Dox ABL001-resistant cells had increased ABCB1 expression (2.1-fold vs. control cells p=0.0033). U812 ABL001-resistant cells overexpressed ABCB1 (5.4-fold increase, p<0.001) and ABCG2 (6-fold increase, p<0.001) before emergence of a novel myristate-binding pocket mutation (F497L). In all three cell lines, ABL001 resistance was reversible upon chemical inhibition of ABCB1, ABCG2 or both (p<0.001). When K562 ABL001-resistant cells were treated with ABL001 in combination with clinically achievable doses of either imatinib or nilotinib, reversal of the resistance phenotype was also observed (p<0.01), potentially through inhibition of ABCG2. Overexpression of efflux transporters will likely be an important pathway for ABL001 resistance in the clinical setting. Given the lack of evidence for ABCG2-mediated transport of nilotinib or imatinib at clinically relevant concentrations, our data provide a strong rationale for using ABL001 in combination with either TKI