CDK4/CDK6 inhibition as a novel strategy to suppress the growth and survival of BCR-ABL1(T315I)+ clones in TKI-resistant CML

PURPOSE: Ponatinib is the only approved tyrosine kinase inhibitor (TKI) suppressing BCR-ABL1(T315I)-mutated cells in chronic myeloid leukemia (CML). However, due to side effects and resistance, BCR-ABL1(T315I)-mutated CML remains a clinical challenge. Hydroxyurea (HU) has been used for cytoreduction in CML for decades. We found that HU suppresses or even eliminates BCR-ABL1(T315I)+ sub-clones in heavily pretreated CML patients. Based on this observation, we investigated the effects of HU on TKI-resistant CML cells in vitro. METHODS: Viability, apoptosis and proliferation of drug-exposed primary CML cells and BCR-ABL1+ cell lines were examined by flow cytometry and (3)H-thymidine-uptake. Expression of drug targets was analyzed by qPCR and Western blotting. FINDINGS: HU was more effective in inhibiting the proliferation of leukemic cells harboring BCR-ABL1(T315I) or T315I-including compound-mutations compared to cells expressing wildtype BCR-ABL1. Moreover, HU synergized with ponatinib and ABL001 in inducing growth inhibition in CML cells. Furthermore, HU blocked cell cycle progression in leukemic cells, which was accompanied by decreased expression of CDK4 and CDK6. Palbociclib, a more specific CDK4/CDK6-inhibitor, was also found to suppress proliferation in primary CML cells and to synergize with ponatinib in producing growth inhibition in BCR-ABL1(T315I)+ cells, suggesting that suppression of CDK4/CDK6 may be a promising concept to overcome BCR-ABL1(T315I)-associated TKI resistance. INTERPRETATION: HU and the CDK4/CDK6-blocker palbociclib inhibit growth of CML clones expressing BCR-ABL1(T315I) or complex T315I-including compound-mutations. Clinical studies are required to confirm single drug effects and the efficacy of `ponatinib+HU´ and ´ponatinib+palbociclib´ combinations in advanced CML. FUNDING: This project was supported by the Austrian Science Funds (FWF) projects F4701-B20, F4704-B20 and P30625.