Response to osimertinib in a colorectal cancer patient with an T790M mutation: A case report.

Background: Although common in lung cancer, somatic epidermal growth factor receptor () mutations are rarely found in colorectal cancer, occurring in approximately 3% of cases. Treatment with anti-EGFR antibodies is commonplace, but tyrosine kinase inhibitors are not standard treatments in colorectal cancer. Here we report a case of sustained response to osimertinib in a colorectal cancer patient with an T790M mutation on cell-free DNA analysis.

Case Summary: A 72-year old woman with a past medical history of post-polio syndrome confined to a wheelchair, scoliosis and hypothyroidism presented with metastatic sigmoid colon adenocarcinoma with hepatic metastases. Next generation sequencing revealed a wild-type, microsatellite stable, PD-L1 negative malignancy. Mutations in and were also identified, as well as amplification. Cell-free DNA analysis revealed an T790M mutation. She was unable to tolerate first-line treatment with panitumumab, 5-fluorouracil and leucovorin, progressed on second-line treatment with trifluridine/tipiracil plus bevacizumab, and was unable to tolerate third-line treatment with regorafenib. She was started on fourth-line treatment with off-label osimertinib, with clinical response - decrease in size of hepatic metastases and a pericardial effusion. She remained on treatment with osimertinib for seven months.

Conclusion: This case shows the benefit of multi-gene sequencing assays to identify potential therapeutic options in patients with refractory disease.