Background: Immune checkpoint inhibitors have revolutionized outcomes for patients with a spectrum of cancer types, including the first tissue/site-agnostic FDA approvals (in 2017 for 2nd-line and 2020 for 1st-line) for advanced cancers with DNA microsatellite instability-high/mismatch repair deficiency (MSI-High/MMRd).

However, the factors associated with patients’ access to MSI/MMR testing are unknown. Methods: Patients ≥20-years-old who newly presented with histopathologically-confirmed stage 4 colorectal adenocarcinoma from 2010-2016 were identified from the National Cancer Database, which comprises >70% of all newly-diagnosed cancer patients in the U.S. Patients were excluded if they lacked data about MSI/MMR testing or were initially diagnosed at another institution. The primary outcome was receiving MSI/MMR testing, either via immunohistochemistry or molecular diagnostic. Patient demographic, socioeconomic, and care setting characteristics were evaluated for association with MSI/MMR testing, as well as between MSI/MMR testing and immunotherapy receipt, using multivariable logistic regression. Results: Of 45,326 newly-diagnosed stage 4 colorectal adenocarcinoma patients, only 26.5% (n=11,998) received MSI/MMR testing – rising from 14.4% in 2010 to 41.1% in 2016 (adjusted odds ratio [aOR] 1.26/year, 95% confidence interval [95%CI] 1.25-1.29, p<0.001). Overall, patients who were older (referent 60-69-years- old; 70-79-years-old aOR 0.83, 95%CI: 0.77-0.89, p<0.001; 50-59-years-old aOR 1.25, 95%CI: 1.16-1.33, p<0.001), male (aOR 0.94, 95%CI: 0.90-0.99, p=0.01), or of Black non-Hispanic race/ethnicity (aOR 0.87 vs. White non-Hispanic, 95%CI: 0.82-0.94, p<0.001) were independently less likely to receive testing. Additionally, patients who were either uninsured (referent private insurance, aOR 0.78, 95%CI: 0.70-0.86, p<0.001), Medicaid-insured (aOR 0.87, 95%CI: 0.80-0.94, p=0.001), or Medicare- insured (aOR 0.87, 95%CI: 0.81-0.93, p<0.001); or diagnosed at community (referent academic/NCI-comprehensive cancer program, aOR 0.60, 95%CI: 0.56-0.66, p<0.001) or comprehensive community (aOR 0.76, 95%CI: 0.72-0.80, p<0.001) cancer programs, were also significantly less likely to be tested. Even for patients diagnosed in 2016, untested patients received independently less immunotherapy than tested patients (aOR 0.61, 95%CI: 0.53-0.68, p<0.001). Conclusions: MSI/MMR testing rates for stage 4 colorectal cancer patients have dramatically improved in recent years, but appeared underutilized in patients that were older, of Black non-Hispanic race/ethnicity, uninsured or Medicaid-insured, or diagnosed at community programs. Our findings suggest that socioeconomic and care setting opportunities exist for improving access to testing of this important biomarker.

Citation Format: Nayan Lamba, Bryan Iorgulescu. Disparities in microsatellite instability/mismatch repair biomarker testing for patients with advanced colorectal cancer [abstract]. In: Proceedings of the AACR Virtual Conference: Thirteenth AACR Conference on the Science of Cancer Health Disparities in Racial/Ethnic Minorities and the Medically Underserved; 2020 Oct 2-4. Philadelphia (PA): AACR; Cancer Epidemiol Biomarkers Prev 2020;29(12 Suppl):Abstract nr PO-091.