Development and clinical validation of circulating tumor cell (CTC) biomarkers in clear cell renal cell carcinoma (ccRCC) for the OMNIVORE clinical trial.

Abstract

4579 Background: New therapeutic strategies and biomarkers of treatment resistance are needed for metastatic ccRCC patients (pts) progressing on Nivolumab (Nivo). The upcoming OMNIVORE phase II clinical trial will evaluate whether adding Ipilumimab improves rPFS for pts with SD or PD on Nivo alone. We report development and clinical validation of predictive CTC biomarkers that will be evaluated in the OMNIVORE trial. Methods: We tested blood samples from ccRCC pts collected on biomarker trials at Dana Farber and U. of Wisconsin. Carbonic Anhydrase (CA) IX was used to capture CTCs in the VERSA platform to compare protein and gene expression signatures of resistance to Nivo. HLA and PD-L1 expression on CTC was quantified with independent confirmation with multicolor flow cytometry (FC). Results: We identified CTCs in 26/27 pts using RCC-specific CA IX antibody. Staining with CAXII and PAX8 confirmed CTC were of renal origin. The range of captured CK+/CAXII+/CD45- CTCs was 3-279 (median 15) from 18 pts. Multicolor FC found 8/9 pts with triple positive events for renal specific markers CAIX/CAXII/PAX8 [0-14.9%, median 0.32]. PDL1 and HLA staining was validated in cell line and pts samples with high reproducibility. PDL1 was expressed in < 10% of CTCs while HLA expression had high intra- and interpatient heterogeneity. Three pts had CTCs positive for both HLA and PDL1 that correlated with a high frequency of CTCs that were positive for 3 RCC markers (1.3, 5.1 and 14.9% of events) for an R² value of 0.92 (p < 0.0001). In 3 pts with ongoing response to Nivo (9-13 moths) triple positive events were 0, 0.2 and 0.3% (median 0.25%) and absent expression of PDL1/HLA. In 3 pts with early progression on Nivo ( < 5 mo), triple positive CTCs ranged 0.33-14.9% (median 5.05%) with CTCs from 2/3 pts positive for PDL1/HLA. Conclusions: We report the first identification of ccRCC CTCs using CAIX, CAXII and PAX8 as confirmatory markers. CTC frequency and PDL1/HLA expression is lower in Nivo responders versus pts with early progression. High but variable HLA expression suggests variant resistance mechanisms. The utility of theses predictive and pharmacodynamic biomarkers will be tested in the OMNIVORE trial.