H3B-6527 clinical biomarker assay development and characterization of HCC patient samples.

Abstract

4121 Background: FGFR4/FGF19 signaling axis is a novel therapeutic target in HCC. Multiple covalent FGFR4 inhibitors, including H3B-6527, are under clinical development. Preclinical efficacy studies in mice (including PDX) have shown that FGF19 expression (FGF19+) is a predictive biomarker for FGFR4 inhibitor response. The mechanisms driving FGF19 expression in HCC is largely unknown however, in some cases, focal amplification of ch11q13.3 containing FGF19 gene is thought to drive the FGF19 expression. Consistent with the preclinical observations, clinical studies have also shown that FGF19+is a predictive biomarker for FGFR4 inhibitor response. However, these trials have also reported a large number of FGF19+patients failing to respond to FGFR4 inhibitors necessitating refinement of patient selection strategies. In an attempt to obtain deeper insights into the role of FGF19+as a predictive biomarker and potentially uncover additional biomarkers that will enable improvement in patient selection strategies, we have characterized a set of 258 HCC patient samples Methods: Samples were acquired from biobanks and utilized to qualify clinical assays including FGF19 copy number (FISH), mRNA expression (qRT-PCR), FGF19 protein (IHC), and a focused NGS panel for assessing both mutations and copy number. A multiplexed protein and mRNA platform enabled assessment of p-ERK and Ki67 (protein) and Cyp7A1 (mRNA) amongst other exploratory PD biomarkers from two FFPE slides. Results: FGF19 positivity rates for IHC and qRT-PCR were 18% (41/225) and 42% (87/209), respectively. The overall correlation was 60%, with 63% (22/235) IHC positive cases also being positive by qRT-PCR. For IHC+/qRT-PCR (-) cases, RNA quality may have impacted assay sensitivity. 4% (9/244) of samples were positive for FGF19 copy number. Among samples with FGF19 copy number gains, 22% (2/9) did not show positive FGF19 expression. Conclusions: Based on our data, FGF19 mRNA is a more inclusive selection strategy and offers an approach to further refine thresholds for efficacy as determined in the clinic. Multiplexed protein-mRNA assays were also validated and implemented to enable a more comprehensive clinical biomarker program.