• cetuximab
• metastatic colorectal cancer
• triplet

The therapeutic landscape of metastastic colorectal cancer (mCRC) is rapidly evolving thanks to the introduction of new active agents and the improved selection of patients candidate for the different treatment options. The choice of the upfront regimen is a crucial step in the therapeutic route of mCRC patients, able to substantially influence subsequent interventions and to deeply condition long-term prognosis.
While the combination of an intensified chemotherapy backbone, the triplet FOLFOXIRI, with the antiangiogenic bevacizumab is now regarded by all major guidelines as a safe and efficacious option for selected mCRC patients, the combination of the triplet with the other class of targeted drugs available for the first-line treatment of mCRC, the anti-EGFR monoclonal antibodies cetuximab and panitumumab, has been investigated in preliminary phase II trials. In spite of encouraging activity results, though with the bias of an inappropriate molecular selection of included patients, the high incidence of potentially serious adverse events, with special regard to gastrointestinal toxicities, limited the development of this combination.
Moreover, the optimal duration of the upfront treatment with chemotherapy plus a targeted agent for mCRC is a debated issue. Recent trials highlight the possibility to move from continuing the treatment until progression to the development of other strategies including off-therapy breaks or phases of de-intensified therapy in order to spare side effects and to improve patients’ quality of life, without compromising their clinical outcome. Whereas a bev-based maintenance therapy is regarded as a preferred option following upfront chemotherapy plus bev, the role of maintenance after chemotherapy plus an anti-EGFR is not well-established, as well as the optimal de-intensified regimen to be adopted. Preclinical data show that the selective pressure of EGFR inhibitors might lead to the activation of proangiogenic pathways. Based on this rationale, anti-EGFR-treated tumours may be more sensitive to anti-angiogenic agents, thus providing a sound biological rationale to investigate the potential efficacy of the sequential administration of an anti-EGFR and an anti-VEGF.
Drawing from this considerations, the MACBETH study was designed and 116 patients with RAS and BRAF wild-type tumors were randomized to receive up to 8 cycles of a modified schedule of FOLFOXIRI (mFOLFOXIRI) plus cetuximab as induction treatment, followed by cetuximab or bevacizumab as maintenance.
MACBETH study: clinical results
Even if neither of the two arms of the MACBETH trial met the primary endpoint of demonstrating a relevant increase in the rate of patients alive and progression free 10 months after the randomization against literature data with doublets plus anti-EGFR, however the trial provided interesting clinical messages. Firstly, a short induction with mFOLFOXIRI plus cetuximab presents with a feasible and manageable safety profile, with an incidence of adverse events similar to those reported in the literature with doublets plus an anti-EGFR. Secondly, this regimen allows achieving impressive activity results, thus emerging as an appealing treatment option especially when a rapid and consistent tumour shrinkage is required. Thirdly, while switching to bevacizumab in maintenance does not seem a strategy worth of further investigation, the continuation of cetuximab may positively affect the duration of progression-free survival.
Translational projects moving from the MACBETH study
Tissue specimens of patients enrolled in the MACBETH study were centrally collected and analyzed in an extensive plan of translational analyses.
Histopathologic characterization of liver metastases resected after triplets plus cetuximab or bevacizumab
The high response rate and the depth of tumor response achieved with mFOLFOXIRI plus cetuximab translated into a high radical resection rate. These tissue samples represent a unique opportunity to provide a comprehensive characterization of histopathological events occurring during the treatment in a well-annotated series of patients included in a controlled clinical trial. Moreover, in the same timeframe of the MACBETH study, three other prospective studies adopting triplets plus cetuximab or bevacizumab were conducted.
An extensive histopathologic evaluation of colorectal cancer liver metastases resected after triplets plus either bevacizumab or cetuximab was performed in order to evaluate differences in histopathologic parameters of response according to administered targeted agents (bevacizumab versus cetuximab), and to assess the independent prognostic impact of histopathologic parameters. Moreover, three different histopathological growth patterns (HGPs) of liver metastases have been described: desmoplastic (i.e. with a capsule of stroma separating tumour and normal cells), pushing (i.e. with limited infiltration of normal hepatic plates by tumour cells) and replacement (i.e. with abundant infiltration of normal hepatic plates by tumour cells and vessel cooption). By a biologic point of view, while metastases with desmoplastic and pushing HGPs rely on angiogenesis for their vascular supply, those with a replacement HGP co-opt pre-existing sinusoidal vessels, so that tumors with prevalence of this HGP may be intrinsically resistant to antiangiogenic drugs. Based on this rationale, the potential prognostic or predictive role of HGPs was explored. We observed that the histopathologic response is the only independent determinant of survival in patients resected after triplets plus a biologic. Bevacizumab induces a higher histopathologic response rate than cetuximab. The assessment of HGPs may be useful to predict benefit from available targeted agents. In particular, the pushing HGP seems to derive more benefit from the antiangiogenic. Therefore, in order to deepen and hopefully translate to clinical practice the predictive power of HGPs, additional valuable information could be obtained through liver biopsies performed before starting the conversion/neoadjuvant treatment. To this purpose, the possibility to accurately classify HGPs by means of pre-treatment imaging parameters should be investigated.
Exploring the role of rare alterations: BRAF codon 594/596 mutations
The assessment of RAS and BRAF mutations by means of Sequenom revealed the presence of rare BRAF mutations, affecting codons 594 and 596, of uncertain clinical meaning, as compared with the well-known negative prognostic impact of BRAF V600E mutation. When comparing clinical and pathological characteristics, as well as the clinical outcome of patients bearing BRAF codon 594 or 596 alterations, with those of patients with BRAF V600E mutated and BRAF wild-type mCRCs, we found that BRAF codon 594 or 596 mutated mCRCs present peculiar molecular features, pathological characteristics and clinical outcome. In particular, they do not seem to confer a poor prognostic impact, differently form BRAF V600E mutation, consistently with preclinical evidences of a kinase inactivating effect of these rare mutations. Moreover, the role of CRAF in transducing the intracellular signal downstream BRAF 594 or 596 mutated proteins might open the way to further preclinical investigation.