Advances on the BRAF Front in Colorectal Cancer

In this issue of Cancer Discovery, Corcoran and colleagues (1) present results of a multicenter dose-escalation phase I study. This study tests combinations targeting BRAF and/or MEK and EGFR in advanced colorectal cancer with BRAF^V600E mutation. A related article by Hazar-Rethinam and colleagues (2) presents comprehensive translational and preclinical studies testing rational combinations to overcome mechanisms of resistance. BRAF^V600E-mutant protein is a druggable target for BRAF inhibitors, which demonstrated salutary activity in advanced melanoma with BRAF^V600E mutation (3). Unfortunately, monotherapy with BRAF inhibitors, such as vemurafenib, in advanced colorectal cancer with BRAF^V600E mutation was found to be disappointing, with an objective response rate of 5% and a median progression-free survival (PFS) of 2.1 months. BRAF^V600E mutation in advanced colorectal cancer, especially in the absence of microsatellite instability (MSI), can indicate poor prognosis, which emphasizes the need for development of new therapies (4). Preclinical studies demonstrated that a lack of response to BRAF inhibition can be explained by the feedback activation of EGFR, which ultimately translates into therapeutic resistance through the downstream activation of the MAPK pathway (Fig. 1; ref. 5). A dose-finding single-center phase I study testing a combination of vemurafenib, the EGFR antibody cetuximab, and irinotecan demonstrated an encouraging objective response rate exceeding 30% and a median PFS of 7.7 months in 19 patients with previously treated colorectal cancer (n = 18) or appendiceal cancer (n = 1) with BRAF^V600E mutation (6). Therefore, it came as no surprise when the multicenter SWOG 1406 phase II study, which randomized 106 patients with metastatic colorectal cancer with BRAF^V600E mutation previously treated with ≤2 lines of systemic therapy to receive irinotecan and cetuximab with or without vemurafenib, demonstrated prolongation of a median PFS [4.4 months; 95% confidence interval (CI), 3.6–5.7 vs. 2.0 months; 95% CI, 1.8–2.1; HR = 0.42; 95% CI, 0.26–0.66; P < 0.001]. This regimen was ultimately included in the latest version of the National Comprehensive Cancer Network guidelines for colorectal cancer (www.nccn.org; ref. 7). Similarly, another BRAF inhibitor, encorafenib, in combination with cetuximab and the PI3Kα-specific inhibitor alpelisib demonstrated an objective response rate of 18% in previously treated patients with colorectal cancer with BRAF^V600E mutation in a phase I dose-finding study (8). Nevertheless, similar to other targeted therapies, adaptive resistance ultimately develops in nearly all responding patients, highlighting the need for novel strategies (6). Kopetz and colleagues reported detection of RAS mutations with low allele frequencies (<1%) in pretreatment tumor samples from patients with advanced colorectal cancer and BRAF^V600E mutation, who were enrolled to receive vemurafenib monotherapy (4). It is plausible that these low-frequency alterations, which are known to activate the MAPK pathway, can become drivers of therapeutic resistance under the selection pressure of BRAF-targeted therapy (9). Molecular testing and dynamic tracking of tumor-derived circulating cell-free DNA (cfDNA) has potential to provide valuable information about response or lack of thereof, clonal evolution in real time, and molecular signals of emerging resistance in colorectal cancer with BRAF^V600E mutation (6, 9, 10). Observations from early clinical studies suggested that patients with cancers harboring the BRAF^V600E mutation in the tumor tissue and who have undetectable BRAF^V600E mutation in plasma cfDNA have a longer median time to treatment failure when treated with BRAF and/or MEK inhibitors than patients with BRAF^V600E mutation detected in plasma cfDNA (13.1 months; 95% CI, 5.0–21.2 vs. 3.0 months; 95% CI, 2.3–3.7; P = 0.001; ref. 10). Furthermore, serial molecular profiling of plasma cfDNA from patients with colorectal cancer and BRAF^V600E mutation treated in a dose-escalation study with vemurafenib, irinotecan, and cetuximab revealed emergence of multiple alterations mostly involving genes such as MAP2K1, GNAS, ARAF, ERBB2, and others, which directly or indirectly activate the MAPK pathway (6).

Corcoran and colleagues (1) are to be commended for taking efforts to develop effective combination therapies for advanced colorectal cancer with BRAF^V600E mutation to the next level by testing combinations of a BRAF inhibitor, dabrafenib, with an EGFR antibody, panitumumab, with and without the MEK inhibitor trametinib and a combination of trametinib and panitumumab. The triple combination of dabrafenib, panitumumab, and trametinib attained a higher number of objective responses relative to dabrafenib or trametinib with panitumumab (21% vs. 10% vs. 0%, respectively). Even though this is an early-phase study with a relatively small sample size, these findings are intriguing and warrant further exploration. The triple combination of dabrafenib, trametinib, and panitumumab had a higher incidence of grade ≥3 adverse events than a combination of dabrafenib with panitumumab (70% vs. 45%); however, the triple combination did not fare worse than a combination of trametinib and panitumumab (70% vs. 67%). Also, as expected, a combination of trametinib and panitumumab had the highest number of skin-related side effects. Furthermore, pharmacodynamic studies demonstrated the greatest percentage reduction in pERK levels (surrogate marker for the MAPK pathway suppression) measured by IHC in posttreatment tumor biopsy specimens relative to baseline for the triple combination of dabrafenib, trametinib, and panitumumab compared with panitumumab with dabrafenib or trametinib (60% vs. 37% vs. 41%). Molecular testing utilizing BEAMing digital PCR of serially collected plasma cfDNA samples revealed that nearly half of the patients (48%) treated with dabrafenib, trametinib, and panitumumab, who had an objective response or stable disease, had in addition to a rebound in BRAF^V600E mutation levels an emergence of KRAS and/or NRAS mutations in cfDNA at the time of disease progression. This suggests MAPK activation as a key driver of adaptive therapeutic resistance. A related article by Hazar-Rethinam and colleagues (2) takes a deep dive into mechanisms of resistance in advanced colorectal cancer with BRAF^V600E mutation treated with novel combinations such as dabrafenib, trametinib, and panitumumab; dabrafenib and panitumumab; or encorafenib, cetuximab, and alpelisib. Evaluations of postprogression biopsies and cfDNA sxamples revealed heterogeneity of emergent mutations involved in adaptive resistance, such as KRAS, NRAS, MAP2K1, or MAP2K2. Of interest, in some patients, even multiple emergent resistance mutations within the same genes were detected in postprogression cfDNA samples, but not in postprogression biopsies from all sites, underscoring tumor heterogeneity. Detection of treatment-emergent alterations known to activate the MAPK pathway prompted investigators to further study rational combinations combating adaptive resistance. The ERK reactivation has been hypothesized as a potential mechanism of resistance in cancers treated with MAPK pathway–targeted therapies. Therefore, ERK inhibition can provide an attractive approach to limit therapeutic resistance (11). Indeed, in vitro models using the VACO432 colorectal cancer cells with BRAF^V600E mutation engineered to also harbor seven distinct mutations in KRAS, NRAS, MAP2K1, and MAP2K2 genes, which are associated with acquired resistance, demonstrated that unlike other strategies such as BRAF and EGFR inhibition, BRAF and MEK inhibition, BRAF, MEK, and EGFR inhibition, ERK inhibition, or BRAF and ERK inhibition, a triple inhibition of BRAF, EGFR, and ERK can completely abrogate tumor growth. These findings were also replicated in in vivo xenograft models (2).

Understanding the underlying biology has been critical in the development of effective combination strategies to target colorectal cancer with BRAF^V600E mutation. Although a combination of vemurafenib, cetuximab, and irinotecan outperformed cetuximab and irinotecan in a randomized trial, an objective response rate of 16% remained relatively low with a median PFS of less than 5 months (7). The presented combination of dabrafenib, trametinib, and cetuximab can offer an attractive alternative, which has the potential to further improve therapeutic outcomes (1). In addition, replacing a MEK inhibitor with an ERK inhibitor can be more effective in combating adaptive resistance driven through MAPK pathway activation and warrants further testing in future clinical studies (2). Furthermore, colorectal cancer with BRAF^V600E mutation is often associated with MSI, which is a known key factor for favorable outcomes of treatment with the immune checkpoint inhibitors, suggesting a potential value of combinations including immunotherapy.

F. Janku reports receiving commercial research grants from Bayer, BioMed Valley Discoveries, Deciphera, FujiFilm Corporation, Genentech, Novartis, Piqur, Plexxikon, Symphogen, and Upsher-Smith Laboratories; has ownership interest (including patents) in Trovagene; and is a consultant/advisory board member for Guardant Health, IFM Therapeutics, Immunomet, Novartis, Synlogic, and Trovagene.

The author would like to thank Ms. Jennifer Herold for editorial and grammar assistance.