O(6)-Cyclohexylmethylguanine (
NU2058) was developed as an inhibitor of CDK2 and was previously shown to potentiate
cisplatin cytotoxicity in vitro. The aim of this study was to investigate the mechanism of
cisplatin potentiation by
NU2058. SQ20b,
head and neck cancer cells were treated for 2h with
NU2058 (100 microM) and then for a further 2h with
cisplatin and
NU2058.
NU2058 increased
cisplatin cytotoxicity, by clonogenic assay, with a dose modification factor (DMF) of 3.1.
NU2058 increased total intracellular
platinum levels 1.5-fold, and
platinum-
DNA adduct levels twofold. Furthermore, the
cisplatin-
DNA adducts formed were more toxic in the presence of
NU2058. To investigate whether the effects of
NU2058 on
cisplatin adduct levels and toxicity were dependent on CDK2 activity, additional CDK2 inhibitors were tested. NU6230 (CDK2 IC(50) 18 microM) was equipotent to
NU2058 (CDK2 IC(50) 17 microM) as a CDK2 inhibitor in cell-free and cell-based assays, yet did not potentiate
cisplatin cytotoxicity. Furthermore,
NU6102 was >1000-fold more potent than
NU2058 as a CDK2 inhibitor (CDK2 IC(50) 5 nM) yet was no more active than
NU2058 in potentiating
cisplatin.
NU2058 also potentiated
melphalan (DMF 2.3), and
monohydroxymelphalan (1.7), but not
temozolomide or ionising radiation. Whilst
NU2058 increased
melphalan cytotoxicity, it did not increase
melphalan-
DNA adduct formation. These studies demonstrate that
NU2058 alters the transport of
cisplatin, causing more Pt-
DNA adducts, as well as sensitizing cells to
cisplatin- and
melphalan-induced DNA damage. However, the effects of
NU2058 are independent of CDK2 inhibition.