several mechanisms of resistance have been reported. cytarabine is rapidly deaminated by cytidine deaminase in the serum into the inactive uracil derivative. cytarabine-5´-monophosphate is deaminated by deoxycytidylate deaminase, leading to the inactive uridine-5´-monophosphate analog. cytarabine-5´-triphosphate is a substrate for samdh1. furthermore, samhd1 has been shown to limit the efficacy of cytarabine efficacy in patients.