Inhibition of ribonucleotide reductase by a new class of isoindole derivatives: Drug synergism with cytarabine (ara-C) and induction of cellular apoptosis

The hydroxyisoindole dione derivatives ISID and MISID are new compounds wit h structures resembling purines and possessing a hydroxamic acid moiety whi ch is the pharmacophore of hydroxyurea (HU), an inhibitor of ribonucleotide reductase (RR). ISID and MISID exhibited 100-to 500-fold higher cytotoxici ty as compared to HU against cell lines sensitive (CEM/0) or resistant to a ra-C (CEM/ara-C/7A; CEM/dCk[-]). Both MISID and ISID showed significant inh ibitory activity of ribonucleotide reductase (RR). Treatment of CEM/0 cells with 10 mu M ISID showed a linear decrease in all the dNTPs leading to a c omplete depletion by 4 hours with no recovery of enzymatic activity of RR u p to 48 hours in the presence of the drug, suggesting an irreversible inhib ition of this enzyme. However, 10 mu M MISID caused a significant time depe ndent, but reversible inhibition of RR in a whole cell assay in CEM/0 cells . Pretreatment of CEM/0 cells with 10 mu M MISID for I hour increased cellu lar ara-CTP concentrations approximately 2-fold as compared to untreated co ntrols. However, a reduction in intracellular ara-CTP concentration was obs erved following a commensurate depletion of ATP in these cells after 4 hrs of ISID pretreatment. Similarly, the ara-CTP concentration was augmented by 1.6-fold following pretreatment of CEM/0 cells with 10 mu M MISID for 4 ho urs.