Preclinical in vitro and in vivo evaluation of the cyclin dependent kinase 2 inhibitor NU6102 and a water pro-drug NU6301

Abstract

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Cell cycle progression driven by CDK/cyclin complexes is deregulated in cancer cells. CDK2/cyclin E promotes S-phase and has been widely studied as a potential anticancer drug target. NU6102 (4-(6-cyclohexylmethoxy-9H-purin-2-ylamino)benzenesulfonamide) is a potent (IC50 = 5 nM) and specific inhibitor of CDK2 (IC50 values for CDKs 1/4/5/7/9 = 0.25/1.5/0.48/4.4/1.1 µM, respectively) in enzyme assays. NU6102 is rapidly taken up by cells and inhibits phosphorylation of pRb at threonine 821, consistent with CDK2 inhibition. In growth inhibition studies, in 14 human tumour cell lines, NU6102 inhibited cell growth (96 hr exposure) by 50% (GI50) at 4.5 ± 2.2 µM (range 1.9-10), and the GI50 for CDK2 knockout MEFs was 4.2-fold higher than for WT MEFs. The maximum administrable in vivo dose of NU6102 was 10 mg/kg, due to limited solubility. The N-acetyl pro-drug of NU6102, NU6301 (N-acetyl-4-(6-cyclohexylmethoxy-9H-purin-2-ylamino)benzenesulfonamide), was synthesized and shown to degrade rapidly in mouse plasma in vitro to liberate NU6102 (t_1/2 = 24 minutes). Pharmacokinetic studies in mice confirmed that NU6102 was generated following either i.p. or i.v. administration of 10 mg/kg of NU6301, and the plasma half-life of NU6102 liberated from NU6301 was 42 min following i.p. and 10 minutes following i.v. administration. Following administration of 120 mg/kg NU6301 i.p. (equivalent to 100 mg/kg NU6102) tumour levels of NU6102 in SKUT 1 B human tumour xenografts were maintained at >15 µM for 6 hours. Repeated dosing of NU6301 for 10 days caused a small but significant (Mann-Whitney) increase in the median time for tumour volumes to reach 4 times the starting volume when administered every 12 hours at 120 mg/kg per dose (2 day growth delay, p=0.038) or every 8 hours at 120 mg/kg per dose (3 day growth delay, p=0.007). These studies demonstrate that pharmacological inhibition of CDK2 alone does not result in pronounced antitumour activity in vivo.