Microsatellite Stable mCRC Research Articles

Tumour mutational burden predicts resistance to EGFR/BRAF blockade in BRAF-mutated microsatellite stable metastatic colorectal cancer

AimTo unveil genomic and immunohistochemical expression profiles associated with primary resistance to EGFR/BRAF targeted therapy in patients with BRAF-mutated and microsatellite stable (MSS) metastatic colorectal cancer. Experimental designIn this multicenter case-control study on patients treated with EGFR/BRAF ± MEK blockade, we compared a primary resistance cohort (N = 20; RECISTv1.1 PD/SD, and progression-free survival [PFS] <16 weeks) versus a sensitive one (N = 19; RECISTv1.1 PR/CR, and PFS ≥16 weeks) in terms of clinical and genomic/expression data by means of comprehensive genomic profiling, tumour mutational burden (TMB), BRAF-mutant transcriptional subtypes (BM) classification and PTEN expression. ResultsLeft-sided tumours (28% of the total) were enriched in the sensitive versus resistant cohort (53% versus 10%, P = 0.010). Genomic alterations in the PIK3CA/MTOR pathway, BM1 status and PTEN loss were similarly distributed among patients with resistant and sensitive tumours. Amplification of CCND1-3 genes were found only in patients with primary resistance (20% versus 0%, P = 0.106). TMB and prevalence of intermediate TMB (TMB-I 6–20 mutations/Mb) were higher in the resistant versus sensitive cohort (median TMB: 6 [IQR, 3–7.29] versus 3 [IQR, 1.26–3.5]; P = 0.013; TMB-I/H: 60% versus 11%; P = 0.001). Patients with TMB-I had shorter PFS (3.3 versus 5.9 months; HR = 2.19, 95%CI, 1.07–4.47, P = 0.031) and overall survival (6.3 versus 10.5 months; HR = 2.22, 95%CI, 1.02–4.81, P = 0.044). ConclusionDespite the small sample size, the association of a relatively higher TMB with limited benefit from EGFR/BRAF blockade in patients with MSS and BRAF-mutated mCRC deserves prospective validation.

Combined Treatment with Immunotherapy-Based Strategies for MSS Metastatic Colorectal Cancer.

Simple SummaryGiven that most patients with MSS mCRC do not respond to immunotherapy agents or do not have durable clinical responses, there is an unmet clinical need to obtain predictors of response to immunotherapy and rational immunotherapy-based combination therapies for this entity. In this review, we present the efforts that have been made to date in the clinical setting to develop immunotherapy-based combinations for MSS mCRC and the rationale that lies behind each strategy.In recent years, deepening knowledge of the complex interactions between the immune system and cancer cells has led to the advent of effective immunotherapies that have revolutionized the therapeutic paradigm of several cancer types. However, colorectal cancer (CRC) is one of the tumor types in which immunotherapy has proven less effective. While there is solid clinical evidence for the therapeutic role of immune checkpoint inhibitors in mismatch repair-deficient (dMMR) and in highly microsatellite instable (MSI-H) metastatic CRC (mCRC), blockade of CTLA-4 or PD-L1/PD-1 as monotherapy has not conferred any major clinical benefit to patients with MMR-proficient (pMMR) or microsatellite stable (MSS) mCRC, reflecting 95% of the CRC population. There thus remains a high unmet medical need for the development of novel immunotherapy approaches for the vast majority of patients with pMMR or MSS/MSI-low (MSI-L) mCRC. Defining the molecular mechanisms for immunogenicity in mCRC and mediating immune resistance in MSS mCRC is needed to develop predictive biomarkers and effective therapeutic combination strategies. Here we review available clinical data from combinatorial therapeutic approaches using immunotherapy-based strategies for MSS mCRC.

Combination Immunotherapies to Overcome Intrinsic Resistance to Checkpoint Blockade in Microsatellite Stable Colorectal Cancer.

Simple SummaryImmune checkpoint inhibitors have revolutionized the treatment landscape of microsatellite instable colorectal cancer. However, their efficacy is very limited in patients with microsatellite stable colorectal cancer. Recent preclinical and clinical studies have suggested the involvement of various tumor cell-intrinsic and tumor microenvironmental mechanisms in primary resistance to immunotherapy in microsatellite stable colorectal cancer. Because microsatellite stable colorectal cancers are immunologically heterogeneous with different immune evasive mechanisms, it is critical to precisely identify the major mechanism of resistance in each patient and to accordingly apply personalized combination immunotherapy strategies.Although immune checkpoint inhibitors (ICIs) have shown promising results in the treatment of treating various malignancies, progress has been severely limited in metastatic colorectal cancer (mCRC). ICIs are effective in a fraction of patients with microsatellite instability-high mCRC but have little clinical efficacy in patients with microsatellite stable (MSS) mCRC, which accounts for 95% of mCRC cases. MSS mCRCs are considered to have intrinsic resistance to ICI monotherapy through multiple mechanisms. (1) They are poorly immunogenic because of their low tumor mutation burden; (2) frequent activation of the WNT/β-catenin signaling pathway excludes intratumoral CD8+ T cell immunity; (3) the tumor microenvironment is immunosuppressive because of the presence of various immunosuppressive cells, including tumor-associated macrophages and regulatory T cells; and (4) frequent liver metastasis in MSS mCRC may reduce the efficacy of ICIs. To overcome these resistance mechanisms, combination approaches using various agents, including STING agonists, MEK inhibitors, VEGF/R inhibitors, WNT/β-catenin inhibitors, oncolytic viruses, and chemo/radiotherapy, are actively ongoing. Preliminary evidence of the efficacy of some has been shown in early clinical trials. This review summarizes novel combination immunotherapy strategies described in recent preclinical and clinical studies to overcome the limitations of ICI monotherapy in MSS mCRC.

435P The AGITG Modulate study: Randomised phase II study testing manipulation of the tumour micro environment (TME) to enable synergy with PD1 inhibitors in microsatellite stable (MSS) metastatic colorectal cancer (mCRC)

PD-1 checkpoint inhibitors are active in microsatellite unstable (MSI) mCRC, which have an inflamed TME but are inactive in MSS-mCRC. The resistance of MSS mCRC is potentially related to the TME which excludes immune cells. The Modulate study tested 2 strategies designed to alter the TME in MSS mCRC, thereby enabling synergy with PD1 inhibitors.

Abstract 1656: Effects of chemotherapy on activation of the STING signaling pathway and enhancing immunotherapy responses in colorectal cancer

Abstract Background: Metastatic colorectal cancer (mCRC) is the second leading cause of cancer death in the US. The response rate to current systemic therapies is ~50% and most patients die within 2.5 years from the diagnosis of metastasis. Recently, immune checkpoint inhibitor (ICI) therapies have shown benefit in a small subset (~4%) of patients with mCRC whose tumors were microsatellite instability-high (MSI-H), in contrast to those patients with mCRCs that are microsatellite stable (MSS) where responses are extremely rare. Thus, developing therapeutic approaches to improve the efficacy of ICI therapies in patients with MSS mCRC is extremely important. Aims: Our aim was to identify strategies that can likely increase immune-cell infiltration in MSS CRC as one of the major factors responsible for patients with MSI-H mCRC to respond to ICI therapies is high levels of immune-cell infiltration in the tumors. Methods and Results: Recent studies have shown that activation of the STimulator of INterferon Genes (STING) innate immune pathway results in alterations in cytokine release from cancer cells that, in turn, lead to enhanced recruitment of T-cells in the tumor microenvironment. Thus, activation of STING is likely a means of overcoming resistance to ICI-therapy. We hypothesized that activating the STING pathway, via specific cytotoxic chemotherapeutic agents, could result in improved clinical responses in patients with MSS mCRC. We treated a panel of MSI-H and MSS CRC cell lines with various chemotherapies approved for the treatment of patients with mCRC. STING protein was found to be activated in all cell lines treated with SN38, the active metabolite of irinotecan, but not with oxaliplatin or 5FU. Using western blot analyses, we demonstrated a significant increase in STING protein and its polymerization in CRC cells treated with SN38. Activation of factors downstream of STING signaling was also observed in SN38 treated CRC cells. Finally, performing migration assays using the macrophage cell line THP-1 demonstrated that conditioned media (CM) from CRC cells treated with SN38 could significantly increase migration of THP-1 cells when compared to CM from untreated CRC cells. Conclusions: Our studies indicate that among the three major chemotherapies that are used for treatment of patients with mCRC, SN38 has the strongest effect on activation of the STING pathway in both MSI-H and MSS CRC cells. SN38 treatment of CRC cells results in induction of an immune-modulatory secretome leading to increased migration of immune-cells. These findings indicate that specific chemotherapies can likely modulate ICI-therapy responses and further in depth studies evaluating the efficacy of combining SN38 and ICI therapies in MSS mCRC are warranted. Citation Format: Fan Fan, Susmita Ghosh, Lee Ellis, Rajat Bhattacharya. Effects of chemotherapy on activation of the STING signaling pathway and enhancing immunotherapy responses in colorectal cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1656.

PD-L1 targeting and subclonal immune escape mediated by PD-L1 mutations in metastatic colorectal cancer

BackgroundIn patients with microsatellite stable (MSS) metastatic colorectal cancer (mCRC), immune checkpoint blockade is ineffective, and combinatorial approaches enhancing immunogenicity need exploration.MethodsWe treated 43 patients with predominantly microsatellite stable RAS/BRAF...

General treatment for metastatic colorectal cancer: From KEYNOTE 177 study

In the palliative treatment of metastatic colorectal cancer (mCRC), doublet chemotherapy (FOLFOX or FOLFIRI) or triplet chemotherapy (FOLFOXIRI) combined with targeted drugs (cetuximab or bevacizumab) is the main regimen. Recently, microsatellite instability-high (MSI-H) or DNA mismatch repair deficient (dMMR) was discovered as a biomarker to distinguish immunotherapy-benefited populations. In this context, recently published randomized phase III clinical trials tested the efficacy and safety of immunotherapy and traditional chemotherapy with or without targeted drugs as first-line treatment for patients with MSI-H/dMMR mCRC.Here, we briefly analyze this article and further discuss immune monotherapy or double immunotherapy for patients with MSI-H/dMMR mCRC, the immunotherapy for patients with BRAF V600E mutant mCRC, and the immunotherapy for patients with microsatellite stable mCRC.

Repeat sequential oxaliplatin-based chemotherapy (FLOX) and nivolumab versus FLOX alone as first-line treatment of microsatellite-stable (MSS) metastatic colorectal cancer (mCRC): Initial results from the randomized METIMMOX study.

3556 Background: Immune checkpoint blockade (ICB) has revolutionized patient outcome for the small mCRC subgroup with highly immunogenic disease. The majority of mCRC cases, however, are MSS without innate ICB susceptibility. In our ongoing METIMMOX study, we hypothesize that MSS mCRC can be transformed into an immunogenic condition by short-course oxaliplatin-based therapy (FLOX), enabling patients with unresectable, previously untreated metastases to obtain durable disease control when adding ICB therapy. Here we present the protocol-planned interim analysis. Methods: Eligibility criteria include infradiaphragmatic metastasis and C-reactive protein &lt; 60 mg/L. At analysis 15 January 2021, 54 patients stratified according to primary tumor sidedness and mutational status and evaluable for the primary end point (progression-free survival; PFS) had been randomly assigned to a standard-of-care schedule of 8 FLOX cycles Q2W (control arm) or repeat sequential 2 FLOX cycles and 2 nivolumab cycles (240 mg Q2W) to a total of 8 cycles (experimental arm), for both arms before treatment break until disease progression and reintroduction of a new treatment sequence. Radiologic response assessment is every 8 weeks. Safety, tolerability, objective response rate, and duration of response are among secondary end points. Results: At median follow-up of 6.4 (range, 0.5-20) months, patients were well balanced between the treatment arms with regard to the predefined strata and single-organ or multiple-organ metastases. Median PFS for the entire groups of control and experimental arm patients was 5.6 (range, 0.5-15; n = 26) and 6.6 (range, 0.5-20; n = 28) months, respectively. The number of FLOX-related CTCAE grade 3 or higher adverse events, including 2 deaths after initial FLOX administration, was comparable in the two arms. Twelve immune-related grade 3-4 adverse events (no new safety signals) were recorded. In the experimental arm, 4 (16%) patients, all RAS/BRAF-mutant cases, had experienced complete response and 9 (32%) patients had ongoing objective response at 8 months. The control arm cases had 0 with complete response and 6 (23%) with ongoing objective response at 8 months, 1 of whom had proceeded to curative-intent liver surgery. Conclusions: MSS mCRC patients may hold the opportunity of ICB responsiveness evoked by short-course oxaliplatin-based chemotherapy. The search for predictive biomarkers of ICB responsiveness is ongoing in the specifically designed METIMMOX correlative study program. Clinical trial information: NCT03388190.

Concurrent BRAFV600E and BRCA mutations in microsatellite stable (MSS) metastatic colorectal cancer (mCRC): Prevalence and case series of mCRC (pts) with prolonged overall survival (OS).

3561 Background: BRAF V600E+, MSS mCRC patients comprise up to 10% of advanced CRC. They have a poor prognosis with median survivals typically &lt;1 year. Despite use of multi-agent first-line chemotherapy regimens and combination targeted therapies, outcomes are still poor. In our Institutional Molecular Tumor Board database, we identified 3 consecutive mCRC pts with MSS/ BRAF V600E who also had a BRCA1 or BRCA2 co-mutation and had prolonged overall survival. Prior studies suggested that BRCA mutations are uncommon in CRC and we queried the Foundation Medicine (FM) genomic database to evaluate the prevalence of these cases as well as those with co-mutations in other homologous recombination genes. Methods: 36,966 CRC pts were sequenced by FMI using hybrid capture comprehensive genomic profiling (CGP) to evaluate all classes of genomic alterations (GA) for pathogenic BRAF mutations and/or a mutation in BRCA1/2 or a co-mutation in other homologous recombination (HR) genes ( BARD1, CDK12, FANCL, PALB2, ATM, RAD54L, CHEK2, BRAF, BRIP1, RAD51D, RAD51C, RAD51B, CHEK1). Selected cohort analysis were BRAF V600E co-mutated with BRCA1 and BRCA2, separated into MSI-H and MSS cohorts. The clinicopathological features and genomic loss of heterozygosity (gLOH) of those with a BRAF V600E and a BRCA1/BRCA2 mutation are described along with 3 consecutive cases of CRC patients, identified through the Inova Schar Cancer Institute (ISCI) molecular tumor board (MTB) registry, whom had prolonged OS. Results: Of 36,966 colorectal cancer pts, 6.6% were BRAF V600E+ and 1.5% had any co-occurring HR gene mutation(s) with 0.6% having co-mutations of BRAF V600E and BRCA1/2. BRCA co-mutations were higher in MSI-High BRAF V600E, however 24.1% of these occurred in MSS BRAF V600E. BRCA1 co-mutated was more commonly associated with MSS BRAF V600E and was associated with a higher gLOH than MSI-H BRAF V600E (18.7% vs 2.8%; p &lt;0.001 ). In our institutional MTB database, (3/241;1.2%) CRC patients were MSS, BRAF V600E+ with BRCA1 or BRCA2 co-mutations, one confirmed germline and 2 somatic in origin, and had average gLOH of 21.4% with overall survivals of 72+(alive), 17+(alive), and 30 months, respectively. Conclusions: Co-existence of BRAF V600E/ BRCA1/2 may represent a unique subset of advanced MSS CRC that may have a better prognosis and represent an opportunity to test novel targeted therapies. Larger prospective clinical validation trials in this subset is warranted.[Table: see text]

The Added Value of Baseline Circulating Tumor DNA Profiling in Patients with Molecularly Hyperselected, Left-sided Metastatic Colorectal Cancer.

The routine use of liquid biopsy is not recommended for the choice of initial treatment for patients with metastatic colorectal cancer (mCRC). We included patients with left-sided, RAS/BRAF wild-type, HER2-negative, and microsatellite stable mCRC, treated with upfront panitumumab/FOLFOX-4 in the Valentino study. We performed amplicon-based genomic profiling of 14 genes in baseline plasma samples and compared these data with tumor tissue ultra-deep sequencing results. Specific gene mutations in circulating tumor DNA (ctDNA) and their clonality were associated with progression-free survival (PFS), overall survival (OS), and radiological dynamics. Ten and 15 of 120 patients had a mutation of RAS and PIK3CA in ctDNA, with a positive concordance with tissue deep sequencing of only 31.3% and 47.1%, respectively. Presence of RAS or PIK3CA mutations in baseline ctDNA was associated with worse median PFS [8 vs. 12.8 months; HR, 2.49; 95% confidence interval (CI), 1.28-4.81; P = 0.007 and 8.5 vs. 12.9 months; HR, 2.86; 95% CI, 1.63-5.04; P < 0.001] and median OS (17.1 vs. 36.5 months; HR, 2.26; 95% CI, 1.03-4.96; P = 0.042 and 21.1 vs. 38.9 months; HR, 2.18; 95% CI, 1.16-4.07; P = 0.015). RAS mutations in ctDNA were associated with worse RECIST response, early tumor shrinkage, and depth of response, while PIK3CA mutations were not. Patients with higher levels of RAS/PIK3CA variant allele fraction (VAF) in ctDNA had the worst outcomes (VAF ≥ 5% vs. all wild-type: median PFS, 7.7 vs. 13.1 months; HR, 4.02; 95% CI, 2.03-7.95; P < 0.001 and median OS, 18.8 vs. 38.9 months; HR, 4.07; 95% CI, 2.04-8.12; P < 0.001). Baseline ctDNA profiling may add value to tumor tissue testing to refine the molecular hyperselection of patients with mCRC for upfront anti-EGFR-based strategies.

Tissue and plasma tumor mutation burden (TMB) as predictive biomarkers in the CO.26 trial of durvalumab + tremelimumab (D+T) versus best supportive care (BSC) in metastatic colorectal cancer (mCRC).

61 Background: Pembrolizumab was recently granted tissue agnostic FDA accelerated approval for metastatic cancers with TMB≥10 mut/Mb. However, limited data supports immunotherapy in microsatellite stable (MSS) mCRC with TMB≥10 mut/Mb. We assessed tissue TMB and contrasted it to plasma derived TMB in the CO.26 trial. Methods: CO.26 was a phase 2 trial (2-sided ⍺ = 0.1 and 80% power) that randomized 180 patients (pts) 2:1 to D+T or BSC in refractory mCRC. Pre-treatment plasma was sequenced with the GuardantOMNI assay and archival tissue underwent exome sequencing with TMB assessed per the TMB harmonization project. MSI-H cases were excluded. For plasma TMB, we used a previously published cut point (≥28). Results: Overall survival (OS) but not progression free survival (PFS) was improved with D+T in the entire population. Of 180 pts, 163 were evaluable for plasma and 110 for tissue TMB. Median time between archival tissue and plasma collection was 3.1 yrs (IQR 1.9-5.1). Median tissue TMB was 6.6 muts/Mb (IQR 4.1-12.0), while median plasma TMB was 16.3 muts/Mb (IQR 9.4-25.9). Tissue and plasma TMB (r = -0.039, P = 0.69) were not correlated. Tissue TMB≥10 was not prognostic in the BSC arm (HR 1.01, 90%CI 0.52-1.92, P = 0.99) and OS was not improved in pts with tissue TMB≥10 (32/110 pts) following D+T vs BSC. A test of interaction suggested this threshold was not predictive (P = 0.85). Using a minimum P-value approach, no threshold supported high tissue TMB as predictive in MSS mCRC. In fact, the optimal cut point suggested low tissue TMB ( &lt; 4.1 muts/Mb) had the greatest benefit from D+T (P-interaction = 0.048) and pts with TMB ≥4.1 mut/Mb (HR 0.50, 90%CI 0.26-0.96, P = 0.083) trended to better OS in the BSC arm. In contrast, 35/163 pts (21%) were identified in a high plasma TMB group associated with worse OS (HR 2.56, 90%CI 1.45-4.54, P = 0.007) in the BSC arm but improved OS following D+T compared to BSC with P-interaction = 0.082. Only 1 response was noted following D+T in a pt with tissue TMB = 16 mut/Mb and plasma TMB = 13 mut/Mb. Conclusions: Archival tissue TMB≥10 mut/Mb does not appear predictive of D+T benefit in MSS mCRC. Plasma derived TMB may better reflect evolutionary changes following intervening therapy than archival tissue. Clinical trial information: NCT02870920. [Table: see text]

Phase II study of pembrolizumab plus capecitabine and bevacizumab in microsatellite stable (MSS) metastatic colorectal cancer (mCRC): Interim analysis.

77 Background: MSS mCRC rarely responds to pembrolizumab monotherapy, but capecitabine and bevacizumab may induce immune-stimulatory effects. This study evaluates the safety, tolerability and preliminary efficacy of pembrolizumab in combination with capecitabine and bevacizumab in MSS mCRC. We present results at the planned interim analysis. Methods: Design:single-arm, open-label, single-site phase 2 trial with a safety lead-in to confirm the recommended phase 2 dose (RP2D) for capecitabine and expansion cohorts. Per the Simon’s 2-stage design, ≤1 response in 29 patients (pts) requires trial suspension. Key eligibility criteria: MSS mCRC with stable disease (SD) or progressive disease (PD) on prior fluoropyrimidine-based therapy. Treatment: RP2D PO capecitabine on days 1-14 plus 200 mg IV pembrolizumab and 7.5 mg/kg IV bevacizumab on day 1 in 21-day cycles. Pts are followed for toxicity and radiographic response. Results: From 04/2018-09/2020, 29 pts were enrolled, of whom 15 (52%) were female; 21 (72%) white; and median age was 55 years (range 36-77 years). Prior therapies: 2 (7%) pts had SD and 27 (93%) pts had PD on fluoropyrimidine-containing regimens; 24 (83%) pts had prior exposure to bevacizumab. The RP2D for capecitabine was 1000 mg/m2 PO BID, with no dose limiting toxicities observed. Complete toxicity data are available for 25 off-treatment pts. The most common related adverse events (AEs) were palmar-plantar erythrodysesthesia (PPE) (64%) and fatigue (68%). Grade ≥3 related AEs occurred in 9 (36%) pts, including immune-related AEs of Grade 3 dyspnea, hypophosphatemia, and pancreatitis in 1 pt each. Treatment related AEs leading to dose interruptions, reductions, or delays occurred in 15 (60%) pts, most commonly PPE in 13 (52%) pts. No pt had a related AE leading to treatment discontinuation or death. Disposition: of 29 pts enrolled, 24 were removed for PD and 1 was removed for an unrelated AE. Best response by RECIST 1.1 in 23 evaluable pts: partial response (PR) in 2 (9%); SD in 14 (61%); PD in 7 (30%). Median time on treatment was 6 months (range 2-26 months). Conclusions: Combination of pembrolizumab with capecitabine and bevacizumab was found to be tolerable with an expected toxicity profile in MSS mCRC pts. With 2 responses, the study met interim analysis criteria to continue accrual. Tissue and blood-based immune correlatives are planned. Clinical trial information: NCT03396926.

Response to PD-1 and PD-L1 based immunotherapy in MSS advanced colorectal cancer is impacted by metastatic disease sites.

72 Background: Microsatellite stable (MSS) metastatic colorectal cancer (mCRC) has been associated with immunotherapy resistance. However, the impact of metastatic diseases sites on clinical outcome with checkpoint inhibition has not be adequately investigated. Methods: Following IRB approval (14361), we performed a retrospective study to assess the response and progression-free survival (PFS) to PD-1 or PD-L1 based therapy in patients with MSS mCRC who progressed following standard chemotherapy and targeted therapy. Patients were included if they received an FDA approved anti-PD1 or anti-PD-L1 agent, alone or in combination with other investigational agents. Exclusion criteria included concurrent cytotoxic therapy. Response and PFS were determined by RECIST guidelines. Results: 97 patients with refractory MSS mCRC satisfied the inclusion criteria were evaluable for analysis. 57, 17, 13, and 10 received nivolumab, atezolizumab, pembrolizumab, and durvalumab, respectively. 47, 8, 10, 10, 17 received concurrent VEGFR, MEK, CTLA-4, radiation, or other targeted therapies. Among tested variables, including age, gender, primary tumor location, ECOG status, RAS, BRAF, APC, TP53, TMB, and sites of metastasis, only ECOG status (0 vs. 1) and metastatic disease to the liver were associated with disease control (partial response and stable disease; p = 0.005 and &lt; 0.001 respectively). Disease control rates in patients without liver metastases (n = 43) was 56% (95% CI: 40-71%), compared with 2% (95% CI: 0.1-10%) in patients with liver metastases (n = 54) (p &lt; 0.001). Also, liver involvement was predictive for PFS based on multivariate Cox regression model (p &lt; 0.001). The median PFS in patients with liver involvement was 1.5 vs 4.5 months for no liver involvement (HR = 4.41, p = &lt; 0.001). Amongst patients without liver metastases, 35% (15/43) remained without progression at 6 months. These patients were characterized by lung metastases (n = 8) and/or lymph node metastases (n = 4). Conclusions: The presence of liver metastases is MSS mCRC is predictive for lack of benefit from PD-1/PD-L1 inhibitors. Patients without liver metastases can derive a meaningful clinical benefit, with 35% being progression-free at 6 months. Future PD-1/PD-L1 drug development should consider metastatic sites of disease in study design and development.

Interim analysis of the AVETUXIRI Trial: Avelumab combined with cetuximab and irinotecan for treatment of refractory microsatellite stable (MSS) metastatic colorectal cancer (mCRC)—A proof of concept, open-label, nonrandomized phase IIa study.

80 Background: Immune checkpoint inhibitors have demonstrated poor efficacy in MSS mCRC. Previous research indicate that cetuximab (anti-EGFR chimeric monoclonal antibody) could initiate, independently from RAS mutation, an immunogenic tumor cell death and mediate antitumor immune response. In this trial, we aim to explore the clinical efficacy and safety of anti-PDL1 avelumab (AVE) combined with cetuximab (CET) and irinotecan (IRI) for treatment refractory MSS mCRC. Methods: AVETUXIRI (NCT03608046) is a multicenter academic study recruiting MSS, BRAFV600E wt, mCRC patients (pts) refractory to standard treatment (fluoropyrimidine, oxaliplatin, irinotecan and anti-EGFR treatment if RAS wt tumor) in 2 cohorts (cohort A: RAS wt – cohort B: RAS mut). In both cohorts, patients receive CET (400 mg/m2 W1, 250 mg/m2 W2, 500 mg/m2/2 weeks from W3), IRI (180 - 150 mg/m2/2 weeks from W1) and AVE (10 mg/kg/2 weeks starting from W3). Primary endpoints are overall response rate (ORR), defined as partial or complete response (PR or CR) according (i)RECIST1.1, and safety. Secondary endpoints include disease control rate (DCR), PFS and OS. Based on a Simon 2-stage design for ORR in each cohort (cohort A: P0=0.15, P1=0.33 / cohort B: P0=0.09, P1=0.25 / α = 0.1, β = 0.2 in both cohorts), 10 and 13 patients are required in the first stage of cohort A and B respectively. At least 2 pts have to reach PR or CR in each cohort to allow the continuation of the trial in the 2nd stage. Results: Between Oct 2018 and Jan 2020, 23 patients (median age 62 y-old, 86.9% male 78.3% left-sided, 91.3% synchronous mCRC) have been included in the first stage of the trial. No major or unexpected safety events were observed. 21.7% (5/23) of pts presented grade 3 diarrhea, all related to IRI, with complete resolution after IRI dose reduction or interruption. A reduced starting dose of IRI (150 mg/m2) was amended (09/2019) for the last included 8 pts without any grade 3-4 diarrhea occurrence. Grade 1-2 hypothyroidism was the only immune-related side effect. 3 PR were observed in cohort A and none in cohort B. DCR was 60.0% (6/10) and 61.5% (8/13) in cohort A and B respectively. Median PFS and OS were respectively 4.2 and 12.7 months (cohort A) and 3.8 and 14.0 months (cohort B). 6 months-PFS rate was 40.0% and 38.5% in cohort A and B. 12 months-OS rate was 53.3% and 57.7% in cohort A and B. The median follow-up of patients was 9.2 months. Conclusions: The AVETUXIRI trial met its primary efficacy endpoint for RAS wt mCRC pts justifying the study continuation in cohort A (2nd stage). No PR was observed in RAS-mut cohort. Nevertheless, encouraging data of DCR, PFS and OS observed in RAS mut cohort allow the opening of a new cohort for RAS-mut mCRC (cohort C) with PFS as primary endpoint. Clinical trial information: NCT03608046.

Wild-type APC Is Associated with Poor Survival in Metastatic Microsatellite Stable Colorectal Cancer.

The prognostic implication of wild-type APC (APC-WT) in microsatellite stable (MSS) metastatic colorectal cancer (mCRC) is not well defined. APC prognostic value was evaluated retrospectively in two independent cohorts of patient with MSS mCRC with a confirmatory analysis from a public data set from Memorial Sloan Kettering Cancer Center (MSKCC). In comparison with the APC-mutant (APC-MT) population (n = 255), APC-WT patients (n = 86) tended to be younger (59% of age < 40 vs. 26% of age > 50), right-sided (41.7% vs. 27%), BRAFV600E mutated (23.3% vs. 0.8%), and KRAS wild type (65.1% vs. 49.8%). Alternative WNT pathway alterations, RNF43 and CTNNB1, were over-represented in the APC-WT versus APC-MT population (7% vs. 0.4% and 4.7% vs. 0.4%, respectively). APC-WT patients had a worse overall survival (OS) than APC-MT patients (22.6 vs. 45.6 months, p < .0001). Using a multivariate model correcting for primary tumor location, RAS and BRAF status, APC-WT was predictive of poor survival (APC-MT vs. APC-WT, hazard ratio [HR], 0.62; 95% confidence interval [CI], 0.44-0.86, p = .0037). The prognostic implication of APC-WT on OS was confirmed further in a similar multivariate model of 934 stage IV patients from MSKCC public database (APC-MT vs. APC-WT, HR, 0.63, 95% CI, 0.49-0.81, p < .0001). APC-WT is associated with poor OS in MSS mCRC regardless of RAS and BRAF status. Compared with APC-MT mCRC tumors, APC-WT tumors were associated with other Wnt activating alterations, including RNF43 and CTNBB1. Our data suggest alternative therapy needs to be investigated in APC-WT patients. Patients with microsatellite stable metastatic colorectal cancer with wild-type APC had a worse overall survival than patients with mutated APC regardless of RAS/RAF status. APC status should be considered as a stratification factor in prospective trials, and novel therapeutic strategies need to be developed for this subgroup of patients.

The Efficacy and Safety of Regorafenib in Combination With Anti-PD-1 Antibody in Refractory Microsatellite Stable Metastatic Colorectal Cancer: A Retrospective Study

BackgroundMicrosatellite stable (MSS) or mismatch repair proficient (pMMR) metastatic colorectal cancer (mCRC) is resistant to immune checkpoint inhibitors. However, a recent Japanese trial showed that regorafenib plus nivolumab had encouraging anti-cancer activity in MSS or pMMR mCRCs.Materials and MethodsWe retrospectively reviewed the efficacy and safety data of combination therapy with regorafenib plus anti-PD-1 antibody in patients with refractory MSS or pMMR mCRC in the medical centers of Shandong Province in China.ResultsTwenty-three patients with MSS or pMMR mCRC received regorafenib plus anti-PD-1 antibody. Eighteen (78.3%) patients experienced stable disease as best response, five (21.7%) patients had progressive disease, and no partial response was observed. The disease control rate (DCR) was 78.3% (18/23), and the median progression-free survival (PFS) was 3.1 months (95% CI, 2.32-3.89). Four of five (80.0%) patients with progressive disease had baseline liver metastasis, while nine of 18 (50.0%) patients with stable disease displayed no liver metastasis. One patient receiving radiofrequency ablation treatment for liver and abdominal wall metastases prior to combination treatment experienced a remarkably prolonged PFS of 9.2 months with SD. Neither liver metastasis status nor previous exposure to regorafenib was associated with treatment outcome. Treatment-related grade 3 toxicities were observed in 5/23 (21.7%) patients.ConclusionNo objective response was observed with the combination of regorafenib plus anti-PD-1 antibody, suggesting its little clinical activity in unselected Chinese patients with pMMR/MSS mCRC. Meanwhile, it exhibited some potential benefit in this cohort in terms of DCR and PFS. Adverse events were generally tolerable and manageable. Prospective studies with large sample sizes are needed to verify the findings. This combination strategy plus local ablative therapy might be worthy of further exploration.

495P Lack of liver metastases identifies a group of MSS metastatic colorectal cancer with potential benefit from PD-1/PD-L1 targeting

Microsatellite stable (MSS) metastatic colorectal cancer (mCRC) represent 95% of all mCRC cases and are characterized by resistance to PD-1 or PD-L1 checkpoint inhibition. Liver metastatic disease has been associated with decreased responses to immunotherapy in non-small cell lung cancer and melanoma. However, the impact of metastatic disease to the liver in MSS mCRC on treatment response has not been investigated in the setting of PD-1 or PD-L1 targeting.

Chemotherapy beyond immune checkpoint inhibitors in patients with metastatic colorectal cancer

BackgroundImmune checkpoint inhibitors (ICIs) alone or in combination with chemotherapy (CT) are the current standard of therapy in several cancer types. Patients (pts) with lung cancer display higher response rates to CT when given after ICIs. Although ICIs have failed to demonstrate antitumour activity in microsatellite stable (MSS) metastatic colorectal cancer (mCRC), little is known about CT effect after ICIs. We aimed to assess whether sequential ICIs followed by CT may be an alternative therapeutic approach in a population of pts with mCRC. Material and methodsWe retrospectively assessed CT after ICI (CAICI) failure in pts with mCRC. The ICI regimen consisted of anti-PD(L)1 alone or in combination. The primary end-point was objective response rate. Progression-free survival (PFS) and overall survival (OS) were secondary end-points. ResultsBetween 2014 and 2018, 29 pts with mCRC received CAICI (MSS tumours, 27 pts [86%]). The median number of previous lines was 4 (range, 2–7). Regimens included TAS-102 (n = 14), FOLFIRI (irinotecan, leucovorin, and fluorouracil; n = 6) or FOLFOX (oxaliplatin, leucovorin, and fluorouracil; n = 4), regorafenib (n = 3) and carboplatin (1 pt with BRCA mutation). Partial response and stable disease were observed in 4 (19%) and 9 (43%) pts, respectively (disease control rate, 62%). The median PFS and OS were 3.8 months (95% confidence interval [CI] = 1.5–5.4) and 8.0 months (95% CI = 4.2–14.0), respectively. ConclusionICIs administered before CT might enhance cytotoxic effects even in pts with immunorefractory MSS mCRC. The results of this small cohort need to be validated in independent prospective cohorts. The role of ICIs as modifiers of both tumour cells and microenvironment in mCRC deserves further research.

P-247 Real-world clinical outcomes for third-line standard of care regimens in deficient mismatch repair or microsatellite instability-high metastatic colorectal cancer in France

Evidence suggests that deficient mismatch repair (dMMR)/micrsatellite-High (MSI-H) metastatic colorectal cancer (mCRC) patients are less responsive to conventional treatment (chemotherapy +/- targeted therapy) and have a worse prognosis than those with MMR-proficient or microsatellite stable mCRC. Immune checkpoints inhibitors (ICIs) have rapidly emerged as a key treatment modality for dMMR/MSI-H tumors including mCRC. However, these data were based on single-arm clinical trials and clinical outcomes data with standard of care (SOC) regimens are lacking since dMMR/MSI-H advanced CRC represents only ∼5% of all CRC.

Consensus molecular subtype (CMS) as a novel integral biomarker in colorectal cancer: A phase II trial of bintrafusp alfa in CMS4 metastatic CRC.

4084 Background: Consensus Molecular Subtype 4 (CMS4) colorectal cancer (CRC) features increased TGFβ signaling, which may account for de novo resistance to immunotherapy for patients (pts) with microsatellite stable mCRC. To date, no prior trial has incorporated CMS status as an integral biomarker. Bintrafusp alfa (M7824) is a dual PD-L1 antibody/TGFβ trap with acceptable safety. Methods: Primary tumors from pts with metastatic CRC underwent CMS testing in a CLIA setting. In this Simon two-stage phase II trial (Ho: p &lt; .05; Ha: p≥.25) for CMS4 mCRC, pts received bintrafusp alfa 1200mg IV every 14 days. RT (8Gy/day x 3 days) to a single metastatic lesion with abscopal intent was administered between doses 2 and 3. The primary objective was to estimate response rate (RR) per iRECIST. Correlative studies including RNA sequencing were performed on pre- and on-treatment biopsies. Results: 53 of 137 tested pts (39%) between June 2018-December 2019 had CMS4 mCRC. 13 of 15 treated pts received the agent with RT. All pts were evaluable for toxicity, and 13 for response. Median number of doses was 3 (IQR, 2-4). There was one grade 3 immune-related adverse event (colitis) requiring study discontinuation. There were 2 pts with stable disease and 11 with progressive disease as best response (RR 0%, 95% CI 0-22%). Enrollment was stopped after first stage for futility. Median PFS and OS were 1.6 months and 5.0 months, respectively. In paired samples, treatment with bintrafusp alfa resulted in an increase in the expression of IFNγ signature in nonirradiated metastatic lesions ( p&lt; .001, q&lt; .001). Updated results will be presented. Conclusions: This is the first reported clinical trial to utilize CMS status as an integral biomarker for pts with metastatic CRC and capitalizes on treating CRC subpopulations with targeted agents based upon validated RNA-based signatures. Although the efficacy for bintrafusp alfa and RT is low, changes in IFNγ signature provides a potential signal for refining therapeutic strategies based upon TGFβ enrichment in pts with mCRC. Clinical trial information: NCT03436563 .