Clinical trial screening of CDKN2A genomic alterations in patients with pancreatic cancer and hepatobiliary cancers requires greater precision than somatic sequencing alone.

Abstract

287 Background: The TAPUR Study is a phase II multi‐basket study that evaluates the anti‐tumor activity of commercially available targeted agents in pts with advanced cancers with genomic alterations known to be drug targets. Results in two cohorts of PC and GBC pts each with CDKN2A loss or mutation were reported at ASCO 2018. The conclusion was that monotherapy with palbocicilib is not associated with clinical activity in these patients. This may be a false conclusion if the genomic targets were absent in these patients. Methods: A total of 158 GI pts (P = 123, GB = 20, Bile Duct = 15) with deep whole exome sequencing (WES) of tumor and blood samples, and whole transcriptomic sequencing (RNA-Seq) (∼200x106 reads per tumor) were available for this analysis from a commercial database. Variant calling was performed through joint probabilistic analysis of tumor and normal DNA reads, with germline status of variants being determined by heterozygous or homozygous alternate allele fraction in the germline sample. Results: 26 somatic variants and 12 germline variants were detected, with one sample overlapping with a germline and a somatic variant (p.A148T and p.A76Rfs∗44). Counting all 11 discrete germline variants as false positives, a total 37 of 158 samples would be positive for CDKN2A mutant status, a rate of 23% (17%-31% CI). However, if the 8 common germline variants are excluded, the call rate is 29/158 = 18% (12%-25% CI). The false positive rate is 4/158 = 14% (4%-31% CI). By RNAseq, true somatic CDKN2A variants had significantly higher TPM counts than germline variants (T-test p = 0.0002). RB expression was not significantly different between the two groups. Conclusions: The failure of palbociclib to show benefit in CDKN2A mutated PC and GBC patients in the 20 patient cohort of the TAPUR study could possibly be explained by patient selection rather than solely drug failure. It is unlikely related to RB loss.