Clofarabine Exerts Antileukemic Activity Against Cytarabine-resistant B-cell Precursor Acute Lymphoblastic Leukemia with Low Deoxycytidine Kinase Expression

Cytosine arabinoside (Ara-C) is one of the key drugs for the treatment of acute myeloid leukemia. It is also used for consolidation therapy of acute lymphoblastic leukemia (ALL). Ara-C is a deoxyadenosine analog and is phosphorylated to form cytosine arabinoside triphosphate (Ara-CTP) as an active form. In the first step of the metabolic pathway, Ara-C is phosphorylated to Ara-CMP by deoxycytidine kinase (DCK). However, the current cumulative evidence in the association of the Ara-C sensitivity in ALL appears inconclusive. We analyzed various cell lines for the possible involvement of DCK in the sensitivities of B-cell precursor ALL (BCP-ALL) to Ara-C. Higher DCK expression was associated with higher Ara-C sensitivity. DCK knockout by genome editing with a CRISPR-Cas9 system in an Ara-C-sensitive-ALL cell line induced marked resistance to Ara-C, but not to vincristine and daunorubicin, indicating the involvement of DCK expression in the Ara-C sensitivity of BCP-ALL. DCK gene silencing due to the hypermethylation of a CpG island and reduced DCK activity due to a nonsynonymous variant allele were not associated with Ara-C sensitivity. Clofarabine is a second-generation deoxyadenosine analog rationally synthesized to improve stability and reduce toxicity. The IC50 of clofarabine in 79 BCP-ALL cell lines was approximately 20 times lower than that of Ara-C. In contrast to Ara-C, although the knockout of DCK induced marked resistance to clofarabine, sensitivity to clofarabine was only marginally associated with DCK gene expression level, suggesting a possible efficacy of clofarabine for BCP-ALL that shows relative Ara-C resistance due to low DCK expression.