The new allosteric inhibitor asciminib is susceptible to resistance mediated by ABCB1 and ABCG2 overexpression in vitro
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Laura N. Eadie1,2, Verity A. Saunders1, Susan Branford2,3,4,5, Deborah L. White1,2,6 and Timothy P. Hughes1,2,7
1Cancer Theme, South Australian Health and Medical Research Institute (SAHMRI), Adelaide, South Australia
2School of Medicine, University of Adelaide, Adelaide, South Australia
3School of Biological Sciences, University of Adelaide, Adelaide, South Australia
4Department of Genetics and Molecular Pathology, Centre for Cancer Biology, SA Pathology, Adelaide, South Australia
5School of Pharmacy and Medical Science, University of South Australia, Adelaide, South Australia
6School of Paediatrics, University of Adelaide, Adelaide, South Australia
7Division of Haematology, SA Pathology, Adelaide, South Australia
Timothy P. Hughes, email: email@example.com
Keywords: asciminib; ABL001; ABCB1; ABCG2; resistance
Received: October 11, 2017 Accepted: January 20, 2018 Published: February 03, 2018
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 BCR-ABL1+ 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 LD50asciminib vs K562 control cells: 256 and 299 nM respectively vs 24 nM, p < 0.001. Sensitivity was completely restored with specific inhibitors cyclosporine (ABCB1) and Ko143 (ABCG2): K562-Dox LD50asciminib+cyclosporine = 13 nM, K562-ABCG2 LD50asciminib+Ko143 = 15 nM (p < 0.001). When asciminib resistance was modelled in vitro, ABCB1 and ABCG2 overexpression was integral in the development of asciminib resistance. In K562 asciminib-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 asciminib-resistant cells had increased ABCB1 expression (2.1-fold vs control cells p = 0.0033). KU812 asciminib-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, asciminib resistance was reversible upon chemical inhibition of ABCB1, ABCG2 or both (p < 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 (p < 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.
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