The evolving landscape of BRAF inhibitors in BRAF mutant colorectal cancer and the added value of cytotoxic chemotherapy

Introduction

Our understanding of the molecular profile and associated targeted therapies has revolutionised the approach to treatment of metastatic colorectal cancer (mCRC). Approximately 10% (range, 8–21%) of mCRC carry a BRAF mutation which occurs primarily (>90%) at the V600E codon and leads to overactivation of the RAS/RAF/MEK/ERK signalling [mitogen-activated protein kinases (MAPK)] pathway (1). BRAF^V600E mutant (MT) mCRC are renowned for their poor prognosis with a median overall survival (OS) inferior ranging from 10 to 20 months with resistance to standard systemic therapy, often not even reaching second-line treatment (2,3). The exception is the subset of MSI-H/dMRR patients (up to 30% of BRAF^V600E MT CRC) who benefit from immunotherapy with checkpoint inhibitors (CPI) following the recent Keynote-177 trial (4).

Unlike melanoma, previous trials with BRAF inhibition monotherapy (or combination with MEK inhibition) for BRAF^V600E MT mCRC show only minimal activity. This is due to feedback upregulation of epidermal growth factor receptor (EGFR) that re-activates the oncogenic pathway bypassing BRAF. Benefit of combining BRAF and EGFR inhibitors to overcome this pharmacological escape has been seen in several trials now. The BEACON phase 3 trial (5) is the pivotal study of this approach showing that the combination of the BRAF inhibitor encorafenib with anti-EGFR treatment (cetuximab) with or without a MEK-inhibitor (binimetinib) led to significantly better OS and overall response rates (ORR) compared to irinotecan (FOLFIRI) or irinotecan with cetuximab.

One strategy to ameliorate outcomes of these patients might be to combine multiple mitogen-activated protein kinases (MAPK) targeting agents with cytotoxic agents. Encouraging preclinical data combining irinotecan with anti-BRAF molecules (6,7), led Kopetz et al. (8) to explore the addition of the BRAF inhibitor vemurafenib to a backbone of irinotecan and cetuximab in previously treated BRAF^V600E MT mCRC.

Study design and summary of results

After random allocation, patients received irinotecan + cetuximab (IC), or irinotecan + cetuximab + vemurafenib (VIC). In 100 eligible patients with BRAF^V600E MT mCRC who had one (including progression during or within 6 months after adjuvant) or two prior systemic regimens with the primary outcome being progression free survival (PFS). The primary end point PFS favoured the experimental arm (VIC) 4.2 months vs. the control arm (IC) 2.0 months [hazard ratio (HR) 0.50; 95% confidence interval (CI) 0.32–0.76; P=0.001]. Worth noting that patients were allowed to have had prior irinotecan, which might have contributed to the poor outcomes in the control arm. The improvement in PFS of adding vemurafenib was seen throughout all subgroups such as prior irinotecan treatment, tumour location, microsatellite status, PIK3CA mutations, and RNA profiling. Although there was a trend for improved OS when adding vemurafenib, this was not statistically significant with OS of 9.6 months in the VIC group vs. 5.9 months in the IC group (HR 0.77; 95% CI: 0.50 to 1.18; P=0.23). The crossover of 21 patients (42%) from the IC to VIC group probably influenced this noting the PFS for those crossing over was 5.4 months in keeping with those treated upfront with this combination. The ORR were 17% and 4% in the VIC and IC group, whilst disease control rates were 65% and 21% respectively.

S1406 study also performed retrospective next-generation sequencing (NGS) and RNA sequencing on tumour tissue and prospective circulating tumour DNA (ctDNA) NGS to look for prognostic and predictive biomarkers and identify genomic mechanisms of resistance. A ctDNA analysis was possible in 69 (69%) of patients at baseline, but only in 34% at baseline and at first restaging, which stresses the challenge for collection. Eighty-seven percent of patients in the experimental arm with at minimum of two ctDNA time points demonstrated a downgrading in variant allele frequency (VAF) of BRAF^V600E whereas no patients in the control arm did (P≤0.001), supporting increased activity of the VIC protocol. Regarding mechanisms of resistance, remarkably only one patient in the VIC arm acquired KRAS mutations (G12V and Q61L) on progression.

Concerning safety, adding vemurafenib resulted in more grade 3/4 toxicity, but notably anaemia, neutropenia and nausea or vomiting which are not specific to BRAF inhibition. This led to 22% of patients in the VIC group discontinuing treatment vs. 8% in the IC group. This is also in great contrast with the triplet and doublet groups of BEACON (5) where respectively 7% and 8% discontinued therapy due to adverse events and highlights the risk of combination with chemotherapy.

Discussion and commentary

It is not surprising that this study met its primary endpoint of improved PFS as the combination of BRAF and EGFR inhibition in second and further lines treatment of BRAF^V600E mCRC has also been proven to be effective in other recent studies including the larger phase III BEACON trial (5). BEACON randomly assigned patients to receive encorafenib, binimetinib and cetuximab (triplet); encorafenib and cetuximab (doublet); or the investigators’ choice of either cetuximab and irinotecan or cetuximab and FOLFIRI (folinic acid, fluorouracil, and irinotecan) (control). When we look at S1406 and BEACON, both trials showed a statistically significant benefit in median progression-free survival (mPFS) (S1406 VIC 4.2 months vs. IC 2.0 months; HR 0.50 and BEACON triplet 4.3 months and doublet 4.2 months vs. control 1.5 months; HR 0.38 and 4.0) and ORR (S1406 17% vs. 4% and BEACON 26% vs. 20%) reflecting clear activity of BRAF inhibition together with EGFR inhibition in the later lines of treatment of BRAF^V600E MT mCRC. When we look at older trials evaluating outcomes of BRAF MT mCRC, before the implementation of BRAF inhibitors, PFS was indeed worse. Morris et al. (9) retrospectively analysed 127 patients with BRAF MT mCRC who received a median of 2 lines of chemotherapy and reported a mPFS for second-line of 2.5 months (95% CI: 1.8–3.0 months), and for third-line 2.6 months (95% CI: 1.0–4.2 months).

BEACON more importantly also reported a statistically significant benefit in mOS as their primary endpoint; 9.0 months in the triplet group and 8.4 months in the doublet group vs. 5.4 months in the control group, both statistically significant (respectively HR 0.52; 95% CI: 0.39 to 0.70; P<0.001 and HR 0.60; 95% CI: 0.45 to 0.79; P<0.001). mOS benefit in SWOG S1406 was almost 4 months (VIC 9.6 months vs. IC 5.9 months; HR, 0.77, 95% CI: 0.50 to 1.18) comparable to the absolute mOS benefit in BEACON, though was not statistically significant (P=0.23) and not the primary endpoint of this phase II trial. The considerable amount (42%) of cross-over could be an explanation for this, whereas in BEACON crossover was not permitted before the cut-off date.

Notably, both S1406 as well as BEACON used a control arm (IC; FOLFIRI + cetuximab) that could be questioned given the conflicting results regarding the use of an anti-EGFR regimen in BRAF^V600E MT mCR, especially in second or further lines (10,11). In the large FIRE-3 trial (12), a first-line study comparing FOLFIRI plus bevacizumab vs. FOLFIRI plus cetuximab, among BRAF^V600E MT mCRC enrolled in this trial (N=48; 14%), cetuximab led to a higher ORR but no difference in PFS and OS between the two arms. Similarly, second-line PICCOLO trial (13), also could not demonstrate a benefit in OS when adding panitumumab to irinotecan over irinotecan alone in KRAS wild patients who had progressed on oxaliplatin, with a of 1.84 in favour of irinotecan alone in the BRAF^V600E MT subgroup. The value of adding 5FU to this regimen is unclear, as the control arm in BEACON also included FOLFIRI + cetuximab but still reported poor PFS and OS.

There remain a number of questions as to optimal therapy for BRAF^V600E MT mCRC, such as the most favourable BRAF targeting agent. Preclinical models have shown that encorafenib has greater potency compared with other BRAF inhibitors such as vemurafenib an dabrafenib (14). Unfortunately, as yet there are no clinical trials with head-to-head comparison in the management of BRAF^V600E MT mCRC

Another important question is the added value of chemotherapy in these regimens. The rationale for adding irinotecan in S1406 came from preclinical and clinical data using patient-derived xenografts to illustrate the potential of irinotecan to add to the activity of BRAF and EGFR inhibition. In a phase IB study from Hong et al. (7), the combination of vemurafenib with cetuximab and irinotecan demonstrated a 35% response rate and promising PFS for setting up this phase II trial. This theory is further reinforced by older data, including the BOND (15) study, that confirmed the synergistic effect of combining irinotecan and cetuximab in patients with irinotecan-refractory mCRC (16). They reported a significantly higher ORR in the cetuximab-irinotecan group compared with the cetuximab group (22.9% vs. 10.8%; P=0.007), as well as a longer median time to progression (TTP) (4.1 vs. 1.5 months, respectively). This demonstrates the augmentation irinotecan offers to anti EGFR blockade in a BRAF unselected population, which when considering the synergism anti EGFR has shown to BRAF blockade, strengthens the argument for combining targeting agents with this chemotherapy backbone. However, in the case of S1406 the addition of irinotecan seems to lead to similar outcomes to those reported for the non-chemotherapy experimental groups in BEACON (5). Furthermore, there is increased toxicity in this trial with the addition of irinotecan.

Next steps for BRAF MT mCRC

Although recent approval of the combination of cetuximab and encorafenib in second or further-lines has improved outcomes of BRAF^V600E MT mCRC, still only 60% of these patients make it to second-line treatment due to the combative nature of this disease (3). Currently, there is ongoing debate regarding first-line treatment recommendations for BRAF^V600E MT mCRC. FOLFOXIRI plus bevacizumab has been considered more beneficial than doublet regimens plus bevacizumab, as shown in the subgroup analysis of phase III TRIBE study (17). However, this advantage was not confirmed in the subsequent TRIBE 2 study (18) and also the meta-analysis (19) from the same group did not report a benefit of triplet over doublet for patients with BRAF^V600E mCRC.

It is therefore crucial to optimise first-line options by investigating the role of BRAF inhibitors, including enrolment in clinical trials. Founded on the results from BEACON, the ongoing (though not recruiting) phase II ANCHOR-CRC trial (NCT03693170) is evaluating the role of BRAF inhibition in first-line setting. Their preliminary results have already shown a favourable overall response rate (ORR) of 47.8% in the single-arm group receiving encorafenib, binimetinib plus cetuximab in first-line setting for BRAF^V600E MT mCRC. However, the mPFS of only 4.9 months (95% CI: 4.4–8.1 months) seems to be quite poor compared to other first-line trials. For example, mPFS of the BRAF subgroup in TRIBE (17) was 5.5 months (95% CI: 1.6–11.2 months) in the FOLFIRI + bevacizumab arm and 7.5 months (95% CI: 5.1–15.0 months; HR 0.57) in the FOLFOXIRI + bevacizumab arm. To improve outcomes in first-line setting, cytotoxic chemotherapy could be associated to these regimens. The ongoing three-arm phase III BREAKWATER trial (NCT04607421) will hopefully answer the questions regarding the value of adding chemotherapy in the upfront setting. This trial is comparing first-line encorafenib + cetuximab ± chemotherapy (FOLFOX or FOLFIRI) vs. investigator’s choice of standard chemotherapy. The strengths of this trial are that it will also compare irinotecan vs. oxaliplatin in the experimental arm and that there is a head-to-head comparison with current standard of care chemotherapy + bevacizumab. We may expect to see a better safety profile in the experimental groups with chemotherapy compared to S1406, as patients in first-line are often in a better general condition and less prone to accumulated haematological toxicity.

Role of prognostic and predictive biomarkers and possible mechanisms of resistance or the molecular complexity of BRAF ^V600E CRC

ctDNA via liquid biopsy in CRC, as in many other malignancies, can aid in early diagnosis, detection of minimal residual disease and monitoring treatment response, assess different types of molecular alterations responsible for tumour transformation and heterogeneity that give rise to resistance and assist the decision of the most suitable therapies (20). As it was shown in the phase 1B study from the same group, in S1406, serial ctDNA BRAF^V600E testing appeared to be a sensitive indicator of treatment response with 87% of the experimental arm demonstrating a downgrade in VAF of BRAF^V600E while no patients in the control arm did (P≤0.001). However, it should be noted that in S1406, correlative studies were underpowered due to limited patients providing ctDNA. Remarkably, ctDNA response rate was very high, 87% compared to only 17% radiologically. This also correlated with Hong’s findings that decrease in ctDNA predicts radiological response (7). Furthermore, Corcoran et al. (21) demonstrated in their phase I study investigating the combination of BRAF and EGFR inhibition with dabrafenib + panitumumab ± MEK inhibition with trametinib in BRAF^V600E MT mCRC, that the percentage of ctDNA dropping seemed to correlate with response.

The molecular complexity of BRAF^V600E MT CRC and the dynamic changes under biological pressure of targeted agents could also be captured by serial ctDNA testing and may reveal mechanisms of resistance via alternations of the MAPK signalling pathway. In the trial by Corcoran et al. (21), almost half of patients (48%) developed RAS mutations in ctDNA at the time of progression, and this might even have been an underestimation. Moreover, multiple subclonal RAS mutations were revealed in many (33%) of these patients at progression, emphasizing the ability of tumour heterogeneity in the context of acquired resistance to therapy. Also, Kopetz and colleagues suggested that many BRAF^V600E MT CRCs may already possess tumour subclones with 1 or more RAS mutations prior to therapy, with the potential for rapid development of heterogeneous resistant subclones under treatment pressure. Yet, in S1406, only 1/50 patients on the experimental arm showed acquired KRAS mutations (G12V and Q61L) on progression. The authors assume that the mechanism of resistance induced by a combination of cytotoxic chemotherapy with targeted therapy may vary from regimens consisting of only targeted therapy. The currently ongoing first-line BREAKWATER trial will further explore this hypothesis by investigating the correlation between ctDNA genetic alterations and clinical outcome.

Conclusion and future strategies

The S1406 trial confirms the previously proven benefit of BRAF inhibitors in combination with anti-EGFR treatment in second- (and third-line) treatment of BRAF^V600E MT CRC, more specific in combination with chemotherapy. The added value of chemotherapy remains unclear and may result in increased toxicity given more than double of patients discontinued the experimental group in S1406 compared to triplet and doublet group in BEACON (5). However, we should take caution when comparing these two trials directly. Ultimately, further clinical trials are required to answer this question.

Despite there being unanswered questions in second- and third-line therapy, many patients with BRAF^V600E MT mCRC still do not reach second-line. Hence, the key question is whether to move these regimens to first-line treatment to improve prognosis. The ongoing BREAKWATER trial (NCT04607421) will provide further exploration of this hypothesis and will also assess the added value of chemotherapy in first-line, including a comparison of irinotecan vs. oxaliplatin based chemotherapy in the initial safety run in phase.

As ctDNA BRAF^V600E MT evolution appears to be predictive of treatment response and given the tumour heterogeneity and emergence of MAPK pathway alterations known to drive resistance in BRAF^V600E MT mCRC during treatment, the inclusion of ctDNA analysis in future treatment trials and ideally normal practice will assist management decisions for this complex group of patients.

Acknowledgments

Funding: None.

Footnote

Provenance and Peer Review: This article was commissioned by the editorial office, Digestive Medicine Research. The article did not undergo external peer review.

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://dmr.amegroups.com/article/view/10.21037/dmr-21-99/coif). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

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References

1. Davies H, Bignell GR, Cox C, et al. Mutations of the BRAF gene in human cancer. Nature 2002;417:949-54. [Crossref] [PubMed]
2. Taieb J, Lapeyre-Prost A, Laurent Puig P, et al. Exploring the best treatment options for BRAF-mutant metastatic colon cancer. Br J Cancer 2019;121:434-42. [Crossref] [PubMed]
3. Seligmann JF, Fisher D, Smith CG, et al. Investigating the poor outcomes of BRAF-mutant advanced colorectal cancer: analysis from 2530 patients in randomised clinical trials. Ann Oncol 2017;28:562-8. [Crossref] [PubMed]
4. André T, Shiu KK, Kim TW, et al. Pembrolizumab in microsatellite-instability-high advanced colorectal cancer. N Engl J Med 2020;383:2207-18. [Crossref] [PubMed]
5. Van Cutsem E, Huijberts S, Grothey A, et al. Binimetinib, encorafenib, and cetuximab triplet therapy for patients with BRAF V600E-mutant metastatic colorectal cancer: safety lead-in results from the phase III BEACON colorectal cancer study. J Clin Oncol 2019;37:1460-9. [Crossref] [PubMed]
6. Yaeger R, Cercek A, O’Reilly EM, et al. Pilot trial of combined BRAF and EGFR inhibition in BRAF-mutant metastatic colorectal cancer patients. Clin Cancer Res 2015;21:1313-20. [Crossref] [PubMed]
7. Hong DS, Morris VK, El Osta B, et al. Phase IB study of vemurafenib in combination with irinotecan and cetuximab in patients with metastatic colorectal cancer with BRAFV600E mutation. Cancer Discov 2016;6:1352-65. [Crossref] [PubMed]
8. Kopetz S, Guthrie KA, Morris VK, et al. Randomized trial of irinotecan and cetuximab with or without vemurafenib in BRAF-mutant metastatic colorectal cancer (SWOG S1406). J Clin Oncol 2021;39:285-94. [Crossref] [PubMed]
9. Morris V, Overman MJ, Jiang ZQ, et al. Progression-free survival remains poor over sequential lines of systemic therapy in patients with BRAF-mutated colorectal cancer. Clin Colorectal Cancer 2014;13:164-71. [Crossref] [PubMed]
10. Rowland A, Dias MM, Wiese MD, et al. Meta-analysis of BRAF mutation as a predictive biomarker of benefit from anti-EGFR monoclonal antibody therapy for RAS wild-type metastatic colorectal cancer. Br J Cancer 2015;112:1888-94. [Crossref] [PubMed]
11. Pietrantonio F, Petrelli F, Coinu A, et al. Predictive role of BRAF mutations in patients with advanced colorectal cancer receiving cetuximab and panitumumab: a meta-analysis. Eur J Cancer 2015;51:587-94. [Crossref] [PubMed]
12. Heinemann V, von Weikersthal LF, Decker T, et al. FOLFIRI plus cetuximab or bevacizumab for advanced colorectal cancer: final survival and per-protocol analysis of FIRE-3, a randomised clinical trial. Br J Cancer 2021;124:587-94. [Crossref] [PubMed]
13. Seymour MT, Brown SR, Middleton G, et al. Panitumumab and irinotecan versus irinotecan alone for patients with KRAS wild-type, fluorouracil-resistant advanced colorectal cancer (PICCOLO): a prospectively stratified randomised trial. Lancet Oncol 2013;14:749-59. [Crossref] [PubMed]
14. Stuart DD, Li N, Poon DJD, et al. Abstract 3790: preclinical profile of LGX818: a potent and selective RAF kinase inhibitor. Cancer Res 2012;72.
15. Cunningham D, Humblet Y, Siena S, et al. Cetuximab monotherapy and cetuximab plus irinotecan in irinotecan-refractory metastatic colorectal cancer. N Engl J Med 2004;351:337-45. [Crossref] [PubMed]
16. Prewett MC, Hooper AT, Bassi R, et al. Enhanced antitumor activity of anti-epidermal growth factor receptor monoclonal antibody IMC-C225 in combination with irinotecan (CPT-11) against human colorectal tumor xenografts. Clin Cancer Res 2002;8:994-1003. [PubMed]
17. Cremolini C, Loupakis F, Antoniotti C, et al. FOLFOXIRI plus bevacizumab versus FOLFIRI plus bevacizumab as first-line treatment of patients with metastatic colorectal cancer: updated overall survival and molecular subgroup analyses of the open-label, phase 3 TRIBE study. Lancet Oncol 2015;16:1306-15. [Crossref] [PubMed]
18. Cremolini C, Antoniotti C, Rossini D, et al. Upfront FOLFOXIRI plus bevacizumab and reintroduction after progression versus mFOLFOX6 plus bevacizumab followed by FOLFIRI plus bevacizumab in the treatment of patients with metastatic colorectal cancer (TRIBE2): a multicentre, open-label, phase 3, randomised, controlled trial. Lancet Oncol 2020;21:497-507. [Crossref] [PubMed]
19. Cremolini C, Antoniotti C, Stein A, et al. Individual patient data meta-analysis of FOLFOXIRI plus bevacizumab versus doublets plus bevacizumab as initial therapy of unresectable metastatic colorectal cancer. J Clin Oncol 2020; [Crossref] [PubMed]
20. Dasari A, Morris VK, Allegra CJ, et al. ctDNA applications and integration in colorectal cancer: an NCI colon and rectal-anal task forces whitepaper. Nat Rev Clin Oncol 2020;17:757-70. [Crossref] [PubMed]
21. Corcoran RB, André T, Atreya CE, et al. Combined BRAF, EGFR, and MEK inhibition in patients with BRAFV600E-mutant colorectal cancer. Cancer Discov 2018;8:428-43. [Crossref] [PubMed]