Abstract 3089: CDKN2A and p53 status predicts response to CDK4/6 inhibition in melanoma

Abstract

Abstract The p16-CyclinD1-CDK4-RB pathway regulates G1-S cell cycle transition. This pathway is deregulated in 90% of melanomas, through amplification or increased expression of CDK4 or Cyclin D1 (CCND1), or reduced expression of the CDK inhibitor p16INK4A (CDKN2A). To explore if CDK4 may be a viable target for the treatment of human melanoma we screened a panel of melanoma cell lines with the highly selective CDK4/6 inhibitor PD-0332991 (PD-991). A majority of cell lines were found to be sensitive to PD-991, although there was a distinct population of resistant cell lines. In sensitive melanoma cell lines, treatment with PD-991 resulted in loss of hyperphosphorylated RB and a concomitant G0/G1 cell cycle arrest. Treated cells displayed an enlarged flattened morphology and an increase in SA-β-galactosidase staining compared to vehicle controls, indicative of cellular senescence. RB-null cell lines were resistant to CDK4/6 inhibition, and did not display cell cycle arrest or senescence. Expression and mutational analysis was performed to identify additional genomic predictors of sensitivity and resistance. Loss of CDKN2A/p16 correlated with sensitivity to PD-991 (p<0.02), and this was confirmed at the level of protein expression. Mutation of p53 and the corresponding low expression of p53 targets p21 and MDM2 correlated with resistance (p<0.0001). There was no increase in p53 transcriptional targets or evidence of apoptosis in response to PD-991 treatment of sensitive cells, and senescence occurs in both p53 WT and mutant cells, indicating that this response does not require functional p53. However, in p53 WT cells, combination treatment with PD-991 and the p53 pathway activator Nutlin3a results in increased sensitivity to PD-991 compared to treatment with PD-991 alone. This additive effect is not observed in p53 mutant cells, suggesting there is a functional role for WT p53 in the response to PD-991. Conclusions: These preliminary data identify RB, p16 and p53 as predictors of response to PD-991, and support further evaluation of CDK4/6 inhibitors in melanoma. Citation Format: Claire A. Martin, Laura Kirby, Stephen Wong, Grant A. McArthur, Karen E. Sheppard. CDKN2A and p53 status predicts response to CDK4/6 inhibition in melanoma. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 3089. doi:10.1158/1538-7445.AM2015-3089