Tumor mutational burden of microsatellite stable metastatic colorectal tumors by patient factors and KRAS, BRAF, and PIK3CA mutation status

627 Background: Microsatellite instability (MSI) can predict response to pembrolizumab in metastatic colorectal cancer (mCRC). Benefit to PD-1 blockade is limited in microsatellite stable (MSS) mCRC yet MSS tumors represent >95% of all mCRC cases. Evidence suggests that high mutational load remains an important predictor of benefit to checkpoint inhibition across several tumors. We aimed to explore associations between tumor mutational burden (TMB) and various patient factors and mutations of interest to identify features of patients (pts) with MSS mCRC of potential significance to prognosis and PD-1 targeting. Methods: We conducted a retrospective study based on next-generation sequencing (NGS) of MSS metastatic colorectal tumors at our single institution using FoundationOne. TMB was categorized as high ≥ 20 mutations/megabase (Mb), intermediate 6-19 mutations/Mb, and low ≤ 5 mutations/Mb. TMB frequencies were analyzed according to age (≤50, >50, >65, and >70 years old), gender, race, location of primary tumor, and KRAS, BRAF, and PIK3CA mutations. Results: Among 194 MSS tumors with available TMB data, frequencies of tumors with intermediate TMB were significantly different in pts ≤50 years (24.6%, n=69) than in pts >65 (48.9%, n=47, Fisher’s exact (two-tailed) p=0.0096) and >70 (53.1%, n=32, p=0.0067). Frequencies of tumors with low TMB were significantly different in pts ≤50 years (75.4%, n=69) than in pts >65 (46.8%, n=47, p=0.0029) and >70 (43.8%, n=32, p=0.0032). Frequencies of high TMB tumors by age were 0% (≤50 years), 1.6% (>50), 4.3% (>65), and 3.1% (>70) and did not reach significance. Frequencies of high, intermediate, or low TMB tumors were not significantly different based on gender, race, and location of primary tumor. Frequencies of KRAS, BRAF, and PIK3CA mutations were similar across all TMB categories (p=0.7995, 0.8774, and 0.053, respectively). Conclusions: Older individuals with mCRC are less likely to have a low mutation burden by FoundationOne assay. Further validation of these findings in larger cohorts may indicate a potential higher likelihood for elderly colorectal cancer patients to benefit from immune-based strategies.