**3. Clinical evidence of immune checkpoint inhibitors in colorectal cancer**

Immunotherapy based on ICIs has changed the treatment paradigm in various tumor types, including lung cancer, melanoma, renal cell carcinoma, etc. These strategies showed minimal clinical activity in nonselected CRC patients [26]. The first glimpse of hope came from a phase I clinical trial investigating the efficacy of the anti-PD-1, nivolumab, in advanced solid tumors, including CRC. Of 14 CRC patients, only one with an MSI-H/dMMR phenotype had a durable complete response (CR) [27]. Further, extensive research has been developed to understand the immune contexture of MSI-H/dMMR tumors, their response to ICIs, and possible combinatorial strategies (**Tables 1** and **2**).

### **3.1 Metastatic setting**

### *3.1.1 Pembrolizumab*

Pembrolizumab is an anti-PD-1 humanized IgG4 Kappa monoclonal antibody (mAb). Its role is to target PD-1 molecules from the T cell's surface and, therefore, to prevent the interaction with its ligands, PD-L1 and PD-L2. By blocking this interaction, pembrolizumab can resuscitate the cytotoxic activity of T cells and promote the recruitment of other immune cells in the tumor microenvironment [28].

The phase Ib KEYNOTE-028 trial (NCT02054806) investigated pembrolizumab's clinical efficacy in patients with PD-L1-positive advanced solid tumors. Only one accomplished a partial response among the 23 PD-L1-positive mCRC patients. Once again, this patient reportedly had an MSI-H/dMMR phenotype, suggesting that this feature could further predict the response to ICIs [29]. Starting from the hypothesis that tumors with an increased number of somatic mutations due to dMMR might be susceptible to ICIs, the phase II KEYNOTE-016 trial investigated the clinical efficacy of pembrolizumab in MSI-H/dMMR CRC, MSS/pMMR CRC, and MSI-H/ dMMR non-CRC. Among the MSI-H/dMMR CRC patients, the progression-free survival (PFS) rate and overall response rate (ORR) were 79% (seven out of nine patients) and 40% (four out of 10 patients), respectively. Similar positive results


*Immunotherapy for Colorectal Cancer in the Era of Precision Medicine DOI: http://dx.doi.org/10.5772/intechopen.105377*

> **Table 1.** *Key clinical trials of ICIs in MSI-H/dMMR mCRC.*


**Table 2.**

*Selected ongoing clinical trials of ICIs in MSI-H/dMMR CRC.*

### *Immunotherapy for Colorectal Cancer in the Era of Precision Medicine DOI: http://dx.doi.org/10.5772/intechopen.105377*

were observed in MSI-H/dMMR non-CRC cohort, with 71% ORR (five out of seven patients). Contrarily, in the MSS/pMMR cohort, the ORR was 0% and the PFS rate was 11%. In the MSI-H/dMMR CRC cohort, the median OS and PFS were not reached. Moreover, a high somatic mutational load was significantly associated with a longer PFS (p = 0.02) [30]. These preliminary results inspired the initiation of the KEYNOTE-164 trial. This phase II study investigated the efficacy of pembrolizumab in two cohorts of previously treated MSI-H/dMMR advanced CRC patients. Cohort A included the patients previously treated with ≥2 lines of standard therapy, while cohort B included the patients treated with ≥1 line of therapy. With a median followup of 31.3 months (mo) for cohort A and 24.2 mo for cohort B, the results showed an ORR of 33% (95% CI; 21–46%). The median OS was 31.4 mo (95% CI; 21.4 mo to not reached) in cohort A, and it was not reached in cohort B [31]. Furthermore, another phase II trial, KEYNOTE-158, investigated the antitumor activity of pembrolizumab in previously treated MSI-H/dMMR non-CRC patients. The ORR was 34.3%, the median PFS was 4.1 mo, and the media OS was 34.5 mo [32]. Considering these results, in May 2017, the Food and Drug Association (FDA) approved pembrolizumab to treat unresectable or metastatic MSI-H/dMMR CRC patients who progressed after conventional chemotherapy with oxaliplatin, fluorouracil, and irinotecan, and for previously treated metastatic or unresectable MSI-H/dMMR solid tumors that have no other satisfactory treatment option [33].

Based on the robust and sustained results seen in refractory mCRC, the phase III KEYNOTE-177 trial investigated the clinical efficacy of pembrolizumab as first-line treatment compared to standard chemotherapy in MSI-H/dMMR mCRC patients. At a median follow-up of 32 mo, pembrolizumab doubled the PFS compared to chemotherapy (8.2 mo versus 16.5 mo; p = 0.0002). The ORR was significantly higher with pembrolizumab than with standard chemotherapy (44% versus 33%). Moreover, the grade 3–5 AEs (adverse events) rate was 66% for standard chemotherapy and only 22% for pembrolizumab [34]. Even if the OS data are not mature yet, a high crossover rate to the immunotherapy arm has been reported. Based on these results, which demonstrate the superiority of pembrolizumab over standard chemotherapy, in June 2020, the FDA approved the treatment with pembrolizumab as first-line treatment in MSI-H/dMMR mCRC patients [35].

### *3.1.2 Nivolumab +/− Ipilimumab*

Nivolumab is an anti-PD-1 humanized IgG4 mAb that, similar to pembrolizumab, disrupts the interaction between the PD-1 receptor and its ligands (PD-L1 and PDL2). The clinical benefit of nivolumab has been documented in many tumor types as melanoma, lung cancer, and renal cell carcinoma [28].

Ipilimumab is a mAb directed against the surface protein CTLA-4, expressed on activated and regulatory T cells. The CTLA-4 molecule negatively regulates T-cell function by inducing T cell anergy and tolerance [36]. Therefore, CTLA-4 blockade intends to counteract the immune tolerance to cancer cells. To support this idea, James Allison and colleagues showed that antibodies against CTLA-4 enhance the antitumor activity of immune cells in mice transplanted with fibrosarcoma and colon cancer [37].

The phase II CheckMate-142 trial was a large initiative to evaluate the clinical benefit of nivolumab alone or associated with other anticancer therapies in mCRC patients with or without MSI-H/dMMR phenotype. The study has an atypical design which initially included six cohorts: 1—nivolumab monotherapy; 2—nivolumab + ipilimumab (every 3 weeks, for four doses, followed by nivolumab monotherapy every

2 weeks); 3—nivolumab + ipilimumab (every 6 weeks, for four doses, followed by nivolumab monotherapy every 2 weeks); 4—nivolumab + ipilimumab + cobimetinib; 5—nivolumab + BMS-986016; and 6—nivolumab + daratumumab.

Out of 74 MSI-H/dMMR mCRC patients from cohort 1, 31.1% (23 out of 74) achieved an OR, while 69% (51 out of 74) had disease control for ≥12 weeks. Moreover, responses have been obtained in patients with or without KRAR, BRAF mutations, or a history of Lynch syndrome. Additionally, this study reported an OR of 25% in BRAF-mutated patients. These results outperform the ones obtained using standard chemotherapy (<10%) and combination strategies with EGFR, BRAF, or MEK inhibitors (10–16%) [38].

The results from cohort 3, including MSI-H/dMMR mCRC patients treated with nivolumab and ipilimumab, have also been released. At a median follow-up of 13.4 mo, the ORR was 55% (65 out of 119 patients), the median PFS was not reached, and a durable response (≥6 weeks) was seen in 83% of patients. Nonetheless, it is important to note that this trial was not randomized, and the direct comparison could be, at some point, misleading. The phase II CheckMate-142 trial results guided the FDA approval of nivolumab +/− ipilimumab in previously treated MSI-H/dMMR mCRC patients [39].

The CheckMate-142 trial further investigated the clinical benefit of nivolumab + ipilimumab as first-line treatment in MSI-H/dMMR mCRC patients. The trial's primary endpoint was ORR. With a median follow-up of 29 mo, the ORR was 69% and CR 13%. The median PFS and OS were not yet reached. Based on these results, nivolumab is recommended as front-line treatment in MSI-H/dMMR mCRC patients as monotherapy or with ipilimumab [40].

In recent years, other ICIs have made their way into oncological practice and are under clinical investigation, including atezolizumab (anti-PD-L1), avelumab (anti-PD-L1), and durvalumab (anti-PD-L1).

### **3.2 Neoadjuvant and adjuvant setting**

Preclinical studies hypothesized that ICIs might be more effective in neoadjuvant and adjuvant settings. In this regard, the phase II NICHE trial included 40 stages I–III CRC cancer patients with or without MSI-H/dMMR phenotype. All the patients were treated with two doses of nivolumab and one of ipilimumab. All the patients obtained pathological responses in the MSI-H/dMMR group, suggesting that immunotherapy warrants further investigations in the neoadjuvant setting [41]. The ATOMIC study, a phase III randomized controlled trial, is currently investigating Atezolizumab + FOLFOX regimen compared to FOLFOX alone in 700 patients with MSI-H/dMMR stage III CRC. The study's primary endpoint is disease-free survival (DFS), and the results are highly expected [42]. Currently, two sizeable ongoing phase III clinical trials are investigating the addition of anti-PD-L1 avelumab (NCT03827044) or anti-PD-1 pembrolizumab (NCT02912559) to standard adjuvant chemotherapy regimes in stage III MSI-H/dMMR CRC patients.
