Background: MODUL is an adaptable, phase 2, signal-seeking trial testing novel agents as first-line therapy for predefined subgroups of patients with metastatic colorectal cancer (mCRC). Previously reported findings demonstrated that adding atezolizumab to fluoropyrimidine (FP)/bevacizumab as first-line maintenance treatment after induction with FOLFOX + bevacizumab did not improve efficacy outcomes in BRAF^wt mCRC. Given these efficacy results, exploratory assessments on tumour samples were conducted to provide insights into factors that might affect efficacy of maintenance treatment and provide guidance on appropriate therapeutic strategies for BRAF^wt mCRC. Methods: In patients with BRAF^wt tumours (Cohort 2), experimental treatment was FP/bevacizumab + atezolizumab. Primary efficacy endpoint: progression-free survival (PFS). Overall survival (OS) was a secondary endpoint. Archival tissue samples from 104 patients were analysed by immunohistochemistry (IHC) at HistoGeneX (PD-L1; CD8/GrB/FoxP3). SP142 antibody was used for PD-L1 IHC analysis, which evaluated PD-L1^low (IC 0–1) vs PD-L1^high (IC > 1) in correlation with PFS and OS in the control and experimental arms. CD8/GrB/FoxP3 triplex staining was also performed to evaluate potential correlations with efficacy. Results: 445 patients with BRAF^wt mCRC were randomised (2:1 ratio) to maintenance treatment in Cohort 2. Archival samples from 104 patients were analysed: FP/bevacizumab + atezolizumab (n = 82); FP/bevacizumab (n = 22). The biomarker evaluable population (BEP) for PD-L1 was n = 81 for FP/bevacizumab + atezolizumab [PD-L1^low n = 35 (43%); PD-L1^high n = 46 (57%)] and n = 22 for FP/bevacizumab [PD-L1^low n = 16 (72%); PD-L1^high n = 6 (28%)]. The BEP for CD8/GrB was n = 50 for FP/bevacizumab + atezolizumab and n = 16 for FP/bevacizumab. No difference in PFS or OS was observed in the FP/bevacizumab + atezolizumab vs FP/bevacizumab arms for PD-L1^high [PFS: HR = 1.5 (95% CI 0.45−4.8), p = 0.51; OS: HR = 1.3 (95% CI 0.38−4.1), p = 0.71] or PD-L1^low [PFS: HR = 0.92 (95% CI 0.47−1.8), p = 0.81; OS: HR = 0.78 (95% CI 0.4−1.5), p = 0.48]. Similar results were observed with CD8/GrB^high [PFS: HR = 0.73 (95% CI 0.27−2.0), p = 0.55; OS: HR = 0.66 (95% CI 0.24−1.8), p = 0.44], CD8/GrB^low [PFS: HR = 1.0 (95% CI 0.42–2.5), p = 0.96; OS: HR = 0.73 (95% CI 0.3–1.8), p = 0.5], FoxP3^high [PFS: HR = 0.97 (95% CI 0.37−2.5), p = 0.95; OS: HR = 0.95 (95% CI 0.36−2.5), p = 0.91] and FoxP3^low [PFS: HR = 0.73 (95% CI 0.29−1.9), p = 0.53; OS: HR = 0.5 (95% CI 0.19−1.3), p = 0.18]. Conclusions: These findings suggest that PD-L1, CD8/GrB and FoxP3 might not be predictive biomarkers in BRAF^wt mCRC. Further analyses are needed to further evaluate potential predictive and prognostic factors of response in this setting. Clinical trial information: NCT02291289