Temozolomide Combined with Ipilimumab Plus Nivolumab Enhances T Cell Killing of MGMT-expressing, MSS Colorectal Cancer Cells

Colorectal cancer (CRC) is the third most frequently diagnosed cancer and third-deadliest cancer globally. Over 95% of patients with metastatic CRC have tumors that are microsatellite stable (MSS) and do not respond to immune checkpoint inhibitors (ICI). Results from the 2022 MAYA clinical trial suggest that the DNA-damaging agent temozolomide (TMZ), which is usually used to treat glioblastoma (GBM), sensitizes patients with MSS, MGMT-silenced CRC to ipilimumab + nivolumab ICI. The benefit of adding ipilimumab + nivolumab to TMZ and the impact of MGMT silencing remain unclear. Here, we aimed to determine in a controlled system if adding ICI to TMZ enhances T cell killing of MSS CRC cells. We also aimed to determine the contribution of MGMT to this response. Western blot analysis indicated that CRC cells (n = 4) had significantly elevated MGMT expression as compared to GBM cells (n = 4) likely due to MGMT promoter methylation in GBM cells. In line with this, CRC cells were slightly more resistant to TMZ compared to GBM cells after five days of treatment. TMZ + ICI sensitized MGMT-expressing, MSS CRC cells to T cell killing. TMZ alone did not enhance T cell killing of MSS or MSI CRC cells but did slightly enhance T cell killing of T98G GBM cells. Our results indicate that TMZ sensitizes MSS, MGMT-expressing CRC cells to ipilimumab + nivolumab ICI. Importantly, this suggests that TMZ-mediated sensitization to ipilimumab + nivolumab appears independent of MGMT status and the patient cohort that may benefit from TMZ + ipilimumab + nivolumab may be expanded to CRC patients with MGMT-expressing, MSS tumors.