The incidence and mortality of colorectal cancer (CRC) are at the highest level in the world for many years. Early screening improves 5-year survival in patients with colorectal cancer, but approximately 25% of patients remain with stage 4 disease, and another 25-50% have early-stage disease but develop metastatic disease. In the United States, the prognosis for patients with metastatic colorectal cancer (mCRC) remains poor, with a median 5-year survival rate of only 12.5%. Therefore, there is a need to find more effective treatments for colorectal cancer patients.

The immunotherapy approach has gained significant attention over the past decade due to its success in achieving durable efficacy in previously difficult-to-treat solid tumors such as melanoma and lung cancer. High tumor mutational burden has become a hallmark of response to immunotherapy in several tumor types. Pembrolizumab and nivolumab were approved by the FDA in 2017 for the second-line treatment of colorectal cancer with mismatch repair deficiency (dMMR) or microsatellite high instability (MSI-H).

In contrast, current immune checkpoint inhibitors (ICIs) are ineffective against tumors with normal mismatch repair function (pMMR) and microsatellite stable (MSS) or microsatellite low instability (MSI-L). Their mechanisms of immune resistance are generally thought to be the expression of relatively low levels of immunosuppressive ligands, low tumor mutational burden and lack of immune cell infiltration.

However, about 15% of patients in colorectal cancer are dMMR/MSI-H, and in metastatic colorectal cancer, it is lower, only about 4%. This means that at least 85% of pMMR/MSS patients do not benefit from current immune checkpoint inhibitors (ICIs). Related studies suggest that combination therapy with PD1 inhibitors and other immune checkpoint molecular modulators such as CTLA4 may be beneficial in a small subset of patients with pMMR/MSS tumors, but for the majority of patients with this subtype of colorectal cancer， alternative immunomodulatory approaches are needed.

The current researches on immunotherapy for MSS colorectal cancer mainly include the following aspects.

1. Combination of MEK inhibitor and PD-1/PD-L1 inhibition

MEK is a kinase that activates ERK, the final kinase in the RAS/RAF/MEK/ERK pathway. Studies have found that activation of the RAS-MAPK pathway can not only promote tumor cell proliferation, but also be associated with a reduction in T cell infiltration in tumors. Preclinical studies have shown that inhibition of MEK can upregulate the expression of MHCI and increase the infiltration of CD8+ T cells into tumors. The combination of PD-1 and MEK inhibition produced synergistic antitumor activity in preclinical tumor models. Based on these data, the investigators initiated a phase I study of the MEK inhibitor cobimetinib and PDL1 inhibition with atezolizumab.

The study included an expanded cohort of patients with KRAS-mutant colorectal cancer, and of 23 patients recruited when preliminary data were reported in 2016, 4 patients (17%) had a partial response. Three of the four responders had confirmed pMMR/MSS, and the status of the other patient was unknown. Updated results were presented in 2018, showing a tolerable safety profile and partial response in 7 of 84 patients enrolled (8%; 3 pMMR/MSI-L, 1 MSI-L and 3 unknown status). ), and the mean follow-up time was 14.3 months. Several trials of MEK inhibition in combination with PD1 and chemotherapy are still ongoing.

2. Bispecific antibody therapy

Bispecific antibodies are a new class of engineered reagents with the ability to bind two independent targets. CEA-TCB is a T cell bispecific antibody that simultaneously binds carcinoembryonic antigen (CEA) on tumor cells and CD3 on T cells, thereby cross-linking cancer cells and T cells, which lead to T cell engagement and activation , independence of pre-existing immunity, T-cell infiltration, and tumor inflammation. Two ongoing phase I studies are exploring CEA-TCB as monotherapy and in combination with atezolizumab. Encouraging clinical activity reported in patients with metastatic MSS colorectal cancer receiving CEA-TCB monotherapy, enhanced by combination with atezolizumab, as of the March 2017 data point . In the combination arm, the response rate was 18% (n = 2), and stable disease was observed in 7 patients (64%) for an overall disease control rate of 82%. Overall, the toxic response was manageable. CEA-TCB is the first T-cell bispecific antibody to show efficacy in solid tumors, especially multiple sclerosis colorectal cancer. At present, related research is still in progress.

3. Tumor mutational burden

Tumor mutational burden (TMB) refers to the number of somatic nonsynonymous mutations per megabase pair (Mb) within a given genomic region. It measures all non-synonymous coding mutations in the tumor exome. TMB has been shown to be an independent predictor of the efficacy of ICIs in some solid tumors, including colorectal cancer. It is currently known that higher TMB is associated with stronger immunogenicity, which may enhance the antitumor activity of immunotherapy. Notably, high TMB values appeared not only in MSI-H but also in MSS tumors. The efficacy of immunotherapy is initially confirmed in MSS colorectal cancer patients with high TMB values

Studies have shown that high TMB (≥10 muts/mb) can benefit from pembrolizumab treatment regardless of microsatellite status. About 3% of MSS colorectal cancers have a TMB ≥ 12, and about 80% of them do not have a POLE mutation. It can be clearly seen that TMB is a promising biomarker, and its indicative effect on tumors remains to be further explored.

4．POLE and POLD1

POLE and POLD1 are genes critical for the encoding of the polymerases ε and δ, which are involved in proofreading and fidelity in DNA replication. The TCGA Colorectal Cancer Database found that 4 hypermutated tumors were not MSI-H. Nearly 7.4% of colorectal cancer patients harbored mutations in POLE or POLD1, and 74% of POLE or POLD1-mutated tumors were MSS or MSI-L. POLE mutations are mutually exclusive with MSI, have the highest TMB, and are enriched in mutation-related neoantigens. Significant responses to pembrolizumab have been shown in MSS metastatic colorectal cancer patients with POLE mutations, and these findings suggest that colorectal cancer patients with POLE mutations may benefit from immunotherapy.

POLE may become another acceptable MMR/MSI-like and effective biomarker in the near future.

5．Liver metastases

There is growing evidence that the site of metastatic disease affects the efficacy of immunotherapy. In colorectal cancer, several studies have shown that liver metastases are associated with a low response to immunotherapy. In a trial of regorafenib plus the PD-1 inhibitor toripalimab in MSS metastatic colorectal cancer, a higher ORR (30%) was also observed in patients without liver metastases. Liver metastases were associated with significantly lower disease control rates compared with no liver metastases. Liver metastases were the variable most associated with poor outcomes after PD-1/PD-L1 inhibition, and these findings suggest that liver metastases may suppress systemic antitumor immune responses, thereby reducing immunotherapy efficacy.

Conclusion

In recent years, immunotherapy has demonstrated strong and durable clinical benefits, in colorectal cancer, the benefit of immunotherapy is still largely limited to patients with MSI. According to related research reports, patients with POLE mutations and high TMB mutations can benefit from pembrolizumab treatment; immunotherapy in patients with low TMB and multiple sclerosis still faces challenges, and the activity of anti-PD-1 combined with MEK inhibitor therapy may provide some help, especially in patients without liver metastases. Inspired by additional trials aimed at developing effective treatment strategies, new combinations and biomarkers will help guide clinicians in more personalized treatment of colorectal cancer patients. We believe that immunotherapy may soon change the treatment landscape for colorectal cancer.

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