**6. Conclusions**

**5.2 Immunotherapy resistance**

*Immune checkpoint inhibitors in mCRC.*

CT-refractory MSI-H/d-MMR mCRC

1st line MSI-H/d-MMR

CT-refractory MSI-H/d-MMR mCRC

1st line MSI-H/d-MMR

mCRC

*Colorectal Cancer*

mCRC

*response rate.*

**Table 5.**

patient outcomes [139].

acceptable toxicity [143].

**5.3 Biomarkers**

**148**

Most mCRC patients are MSS/p-MMR and results with ICI have been unsatisfactory, with immune resistance mechanisms not clearly elucidated yet. Several trials have been developed exploring ways to overcome this resistance, including by modulating tumor microenvironment, reducing tumor-specific antigen expression, altering immunosuppressive pathways, and activating other immune checkpoint pathways, immune regulatory cells, and cytokines [138]. Combining immunotherapy with CT, radiotherapy, bispecific antibody therapy, other immune checkpoint modulators, and other targeted agents are among strategies explored. The rationale behind this multimodal approach is the potential synergistic effect of targeting different immune escape pathways, resulting in improved response to ICI and

**Setting Study Treatment RR PFS OS Approval**

Nivolumab + ipilimumab

Nivolumab + ipilimumab

*CT, chemotherapy; dMMR, deficient mismatch repair, FDA, Food and Drug Administration; mCRC, metastatic colorectal cancer; MSI-H, microsatellite instability-high; NR, not reached; OS, overall survival; PFS, progression-free survival; RR,*

Pembrolizumab 33% 2.1 m 31.4 m FDA (1st line, CT-

Nivolumab 31% 50% 73% FDA (CT-

55% 71% 85%

60% 77% 83% Not approved

Pembrolizumab 43.8% 16.5 m NR

refractory)

refractory)

Phase II Keynote 164

Phase III Keynote 177

Phase II CheckMate-142

Phase II CheckMate-142

Phase II CheckMate-142

CT has anti-tumor activity due to the direct cytotoxic effect on cancer cells and to stimulating host immune response, and several clinical trials are ongoing investigating the combination of immunotherapy with CT and targeted agents [140]. Radiotherapy can activate the host immune response by upregulating expression of tumorspecific neoantigens through cell damage and increasing membrane MHC class I expression, and several studies are ongoing in CRC combining radiotherapy with ICI. Another combined strategy is ICI and MEK blockers, considering that MEK blockade seems to increase T cell response via upregulation of PD-L1 expression [141]. Following a phase Ib trial of atezolizumab and the MEK inhibitor cobimetinib in MSS CRC, other trials were conducted, with no significant survival improvement [142]. The CEA CD3 TCB (RG7802, RO6958688) is a novel T-cell bispecific antibody targeting the carcinoembryonic antigen (CEA) on tumor cells and CD3 on T cells, which displays anti-tumor activity, leading to increased intra-tumoral T cell infiltration and activation and PD-1/PD-L1 upregulation. CEA-TCB antibody was tested in phase I trials of MSS CRC plus atezolizumab, showing antitumor activity with

Considering immune side effects associated with ICI and their variable efficacy, it is important to identify biomarkers that help predict response to ICI and select

potentially sensitive patients that can be candidates for these agents.

CRC treatment has changed over the last decades, not only by including different chemotherapy agents and combinations, but mainly because new targeted agents have emerged.

In metastatic setting, anti-EGFR and anti-VEGF drugs are widely used and have shown gains in survival and response rate, an important marker in CRC potentially resectable liver metastases. In contrast, several trials with targeted agents have been conducted in the adjuvant setting, without survival benefit. Immunotherapy emerged as a new treatment option with survival benefit, but at the moment it is only effective in a small portion of patients. Several other agents targeting other pathways are emerging, such as NTRK, c-MET, ALK, ROS1, and FGFR inhibitors, with promising results.

In conclusion, patients with CRC are living longer with targeted treatments, but more information about resistance mechanisms and biomarkers is necessary to extend even more their survival gains.

## **Acknowledgements**

The authors gracefully acknowledge Joana Cavaco-Silva (jo.cvsilva@gmail.com) for manuscript English language revision and Inês Gomes (ines.gomes@medicina. ulisboa.pt) for **Figure 1** graphic drawing.

### **Conflict of interest**

L. Costa performed consulting activities for Amgen, Novartis and Servier outside the scope of this manuscript. The remaining authors declare no conflicts of interest.

*Colorectal Cancer*
