Long term disease control with low and infrequent doses of pembrolizumab for metastatic dMMR rectal adenocarcinoma – A case report

Abstract

Pembrolizumab is a first-line treatment for metastatic colorectal cancer (mCRC) with microsatellite instability (MSI-H) or deficient in mismatch repair protein (dMMR). Based on the results of the Keynote 177 study, the standard dosing for pembrolizumab is 200 mg every 3 weeks, or 400 mg every 6 weeks. Due to the significant cost incurred, many patients are not able to afford this treatment, especially in a resource-constrained healthcare setting. Here, we reported the outcome of a patient with dMMR mCRC who was treated with first-line pembrolizumab at low and infrequent dosing. The patient received 2 cycles of pembrolizumab 3 weekly at the standard dose of 200 mg, followed by 2 cycles of 100 mg every 3 months, and then 100 mg every 6 months. He demonstrated significant improvement with the treatment, and remained in partial response 2 years into therapy. This case illustrates that infrequent and reduced dosage of pembrolizumab is a viable treatment option for some patients with dMMR mCRC, presenting to us a possible opportunity for more efficient resource utilisation. However, more research is required to help identify those patients who may benefit from such low-intensity regimens.