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Analysis of the p53 pathway in clear cell renal cell carcinoma to identify novel drug combinations.
Abstracts that were granted an exception in accordance with ASCO's Conflict of Interest Policy are designated with a caret symbol (^).
Background: p53 is rarely mutated in clear cell renal cell carcinoma (ccRCC) with typical mutation frequencies between 0 and 5%. Recent studies have shown that combined p53/MDM2 expression is significantly linked with reduced survival (P<0.0001, HR=3.2) and is an independent prognostic indicator (Cox multiple regression analysis, P=0.027). Studies have also found that p53 is negatively regulated by MDM2 in RCC cells but that rescue of p53 stability is not effective at inducing apoptosis. Studies of normal kidney have shown that signalling to p53 by genotoxic agents is intact, but fail to elicit an apoptotic response. We have therefore investigated a rational combinatorial approach to drug targeting in RCC cells in which both MDM2 is inhibited and apoptosis is stimulated. Methods: Senescence was quantitated by assaying SA-β-galactosidase. Drug sensitivity was determined by measuring MTT conversion at multiple time points. Data from these were used to generate isobolograms and combination indices. Further details of the drugs used will be presented. Results: Using a p53-MDM2 antagonist (currently in clinical trials), our results indicate that in RCC cells rescue of p53 induces some growth arrest and senescence but not apoptosis (IC70 range 7-30 μM). Similarly, using a pro-apoptotic drug (also in clinical trials) which has been demonstrated to promote apoptosis in a wide range of cells, we find little growth inhibitory effect (IC70 range 2.5-30 μM). However, combining a p53-MDM2 antagonist with the pro-apoptotic drug leads to dramatic synergy (combination indices as low as 0.26 [<1 indicates synergy]) and rapid p53-dependent apoptosis in a range of p53 wild-type RCC cell lines. Crucially, the drug combinations have no impact on non-tumour renal cells (neither IC70 nor IC50 could be determined, but both exceed 30 μM) suggesting these may have a high therapeutic index. Conclusions: Rationally designed combinatorial approaches for the treatment of RCC can lead to dramatic enhancement of RCC cell sensitivity and clearly warrant further investigation in both pre-clinical studies (under development) and in clinical trials for RCC due to the retention of wild-type p53 in this disease.