Figure 2. Mechanism of action of cladribine.

Cladribine is taken up into cells by nucleoside transporters and then is phosphorylated to the mono-phosphate (the rate-limiting step) by deoxycytidine kinase, highly expressed in lymphocytes. Subsequent phosphorylation steps produce the active species, the triphosphate. The triphosphate cannot be efficiently degraded by adenosine deaminase, and 5′-nucleotidase has low expression in lymphocytes. This leads to high levels of the cladribine triphosphate, which is toxic to cells by a number of mechanisms, including incorporation into DNA leading to single-stranded breaks.