The new allosteric inhibitor asciminib is susceptible to resistance mediated by ABCB1 and ABCG2 overexpression <i>in vitro</i>

Abstract

Asciminib (previously 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. Asciminib sensitivity was evaluated in asciminib naïve <i>BCR-ABL1+</i> cell lines K562 (negligible ABCB1/ABCG2 expression), K562-Dox (ABCB1-overexpressing through doxorubicin exposure) and K562-ABCG2 (ABCG2 overexpression via transduction) with results demonstrating asciminib efflux by both ABCB1 and ABCG2 transporters. K562-Dox and K562-ABCG2 cells demonstrated increased LD50^asciminib vs K562 control cells: 256 and 299 nM respectively vs 24 nM, <i>p <</i> 0.001. Sensitivity was completely restored with specific inhibitors cyclosporine (ABCB1) and Ko143 (ABCG2): K562-Dox LD50^{asciminib+cyclosporine} = 13 nM, K562-ABCG2 LD50^{asciminib+Ko143} = 15 nM (<i>p</i> < 0.001). When asciminib resistance was modelled <i>in vitro</i>, ABCB1 and ABCG2 overexpression was integral in the development of asciminib resistance. In K562 asciminib-resistant cells, ABCG2 expression increased prior to <i>BCR-ABL1</i> overexpression and remained high (up to 7.6-fold greater levels in resistant vs control cells, <i>p <</i> 0.001). K562-Dox asciminib-resistant cells had increased ABCB1 expression (2.1-fold vs control cells <i>p</i> = 0.0033). KU812 asciminib-resistant cells overexpressed ABCB1 (5.4-fold increase, <i>p <</i> 0.001) and ABCG2 (6-fold increase, <i>p <</i> 0.001) before emergence of a novel myristate-binding pocket mutation (F497L). In all three cell lines, asciminib resistance was reversible upon chemical inhibition of ABCB1, ABCG2 or both (<i>p <</i> 0.001). When K562 asciminib-resistant cells were treated with asciminib in combination with clinically achievable doses of either imatinib or nilotinib, reversal of the resistance phenotype was also observed (<i>p <</i> 0.01). Overexpression of efflux transporters will likely be an important pathway for asciminib 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 an additional rationale for using asciminib in combination with either TKI.