Why is concomitant use of abacavir + lamivudine with cladribine not recommended?

Concomitant use of abacavir + lamivudine with cladribine is not recommended because lamivudine inhibits cladribine’s intracellular phosphorylation in vitro, potentially reducing cladribine’s clinical efficacy; some clinical findings also support a possible interaction, although the interaction has not been studied.