Pharmacological difference between degrader and inhibitor against oncogenic BCR-ABL kinase

Chronic myelogenous leukemia (CML) is characterized by the oncogenic fusion protein, BCR-ABL protein kinase, against which clinically useful inhibitors have been developed. An alternative approach to treat CML is to degrade the BCR-ABL protein. Recently, potent degraders against BCR-ABL have been developed by conjugating dasatinib to ligands for E3 ubiquitin ligases. Since the degraders contain the dasatinib moiety, they also inhibit BCR-ABL kinase activity, which complicates our understanding of the impact of BCR-ABL degradation by degraders in CML growth inhibition. To address this issue, we chose DAS-IAP, as a potent BCR-ABL degrader, and developed a structurally related inactive degrader, DAS-meIAP, which inhibits kinase activity but does not degrade the BCR-ABL protein. DAS-IAP showed slightly weaker activity than DAS-meIAP in inhibiting cell growth when CML cells were treated for 48 h. However, DAS-IAP showed sustained growth inhibition even when the drug was removed after short-term treatment, whereas CML cell growth rapidly resumed following removal of DAS-meIAP and dasatinib. Consistently, suppression of BCR-ABL levels and downstream kinase signaling were maintained after DAS-IAP removal, whereas kinase signaling rapidly recovered following removal of DAS-meIAP and dasatinib. These results indicate that BCR-ABL degrader shows more sustained inhibition of CML cell growth than ABL kinase inhibitor.