Background Immunotherapy has emerged as a standard treatment for deficient mismatch repair (dMMR) metastatic colorectal cancer (mCRC). Pembrolizumab became widely available as a first-line (1L) option in Australia following the Pharmaceutical Benefits Scheme (PBS) listing in August 2021. The uptake of new treatment options can be lengthy.

Methods The Treatment of Recurrent and Advanced Colorectal Cancer mCRC registry data at participating Australian sites was analysed from January 2015 (when MMR testing became routine). 1L treatment of dMMR cancers was compared with pre- and post-PBS funding.

Results Out of 2819 patients, 2344 (83%) had known MMR status. Of these, 162 (7%) were dMMR, which was associated with older age (median age 69 vs 63 years, P = 0.001), a right-side primary (68% vs 31%, P < 0.001) and a BRAF V600E mutation (49% vs 11%, P < 0.001). Prior to August 2021, 85 out of 117 (73%) patients with dMMR received 1L treatment: 63 out of 85 (74%) chemotherapy and 20 out of 85 (24%) immunotherapy. Following approval, 39 out of 45 (87%) received 1L treatment and 39 out of 39 (100%) pembrolizumab. Of the patients 75 years and older, a significantly higher proportion of patients were treated with any 1L therapy post-PBS listing (89% vs 60%, P = 0.036).

Conclusion Previously reported associations of dMMR were observed. The higher-than-expected proportion of patients with dMMR is likely driven by the inclusion of older patients in this real-world study. Many patients were able to access immunotherapy prior to PBS listing, potentially through trials or access programs. Early uptake of pembrolizumab following PBS listing has been high, and this effective and well-tolerated option has increased the proportion of elderly patients receiving active therapy.