Approximately of 95 % metastatic colorectal cancer (mCRC) cases are characterized by microsatellite stability (MSS) [1]. In this group, traditional immune-based treatment has consistently failed, giving rise to an unmet medical need regarding effective treatment options in the setting of chemotherapy-refractory MSS mCRC [2-5]. Immunotherapeutic combinations based on novel agents are being evaluated to address this difficult-to-treat patient group.

All three drug combinations discussed here have confirmed the activity of CTLA-4 and PD-(L)1 inhibition in MSS mCRC without hepatic lesions. The combination of botensilimab plus balstilimab has demonstrated ongoing responses. Vilastobart and muzastotug, two CTLA-4 inhibitors that are masked and activated intratumorally, show a theoretical safety advantage, although more mature safety and efficacy data will be required to position them within the treatment landscape.

Botensilimab ± balstilimab vs. SOC

The Fc-enhanced multifunctional CTLA-4 antibody botensilimab displays differential mechanisms of action such as activation of T-cell priming and antigen-presenting cells designed to extend therapy to ‘cold’ or poorly immunogenic solid tumors [6-8]. Botensilimab in combination with the anti-PD-1 antibody balstilimab has shown encouraging clinical activity in a phase I study conducted in the relapsed/refractory MSS mCRC setting, with an objective response rate (ORR) of 22 % in patients without liver metastases and durable responses [6]. At ASCO GI 2025, Fakih et al. presented preliminary results from a randomized, open-label, phase II study exploring botensilimab at two doses as monotherapy or together with balstilimab in the setting of refractory MSS mCRC without active liver metastases [9].

These patients had previously received fluoropyrimidine, oxaliplatin and irinotecan-based chemotherapy and, if medically appropriate, anti-VEGF and/or anti-EGFR targeted treatment. They were randomly assigned to one of five cohorts. Botensilimab 75 mg every 6 weeks (Q6W) was administered either alone (n = 38) or together with balstilimab 240 mg Q2W (n = 62). Two more groups received botensilimab at a dose of 150 mg Q6W alone (n = 40) or combined with balstilimab 240 mg Q2W (n = 61). In the fifth study arm, the standard of care was administered, i.e., either tri­fluridine/tipiracil or regorafenib (n = 33). The primary endpoint was the ORR.

Deep and durable remissions

While the standard-of-care treatment did not give rise to objective responses, botensilimab plus balstilimab induced ORRs of 19 % and 8 % in the botensilimab 75 mg and 150 mg dose groups, respectively, with disease control rates of 55 % and 54 %, respectively (Table 1). The combinations demonstrated greater clinical benefit than single-agent botensilimab. At the time of the analysis, 70 % of responses were ongoing, and the median duration of response had not been reached yet. Several patients were progression-free at two years, with some showing absence of active clinical disease according to ctDNA assessment.

In terms of safety, botensilimab 75 mg/balstilimab elicited immune-related adverse events (AEs) less frequently than botensilimab 150 mg/balstilimab, with rates of 35 % vs. 50 % for diarrhea/colitis, 13 % vs. 25 % for hypothyroidism and 6 % vs. 28 % for skin reactions. Grade ≥ 3 diarrhea/colitis occurred in 18 % vs. 27 %. No new safety signals were identified, and no treatment-related deaths occurred. Based on the ORR of 19 % and superior tolerability, the botensilimab 75 mg dose plus balstilimab was selected for the phase III setting.
Together with the phase I data, balstilimab plus botensilimab has now been evaluated in two prospective studies with concordant findings [6, 9]. As the authors pointed out, these results confirm the contribution of balstilimab to botensilimab, with the combination leading to deep and durable responses. The large sample size in the phase II trial provides a tight confidence interval for the observed ORR, further supporting the development of this combination in phase III trials. Furthermore, the durability of benefits suggests a potential curative outcome in a select patient population.

. Vilastobart is being assessed in a first-in-human phase I/II study in patients with advanced solid tumors and MSS CRC. In the phase IA/B setting, the mono­therapy was generally well tolerated and demonstrated anti-tumor activity in heavily pretreated patients [11]. The combination of vilastobart and atezolizumab was investigated in the phase IC dose-escalation study that is still enrolling. Initial evidence of clinical efficacy was observed in ‘cold’ tumors including MSS CRC [12]. One patient with MSS CRC has an ongoing confirmed partial response with resolution of liver metastases. The phase II part is currently testing the combination of vilastobart 100 mg Q6W and atezolizumab 1,200 mg Q3W in heavily pretreated patients with MSS mCRC. Hecht et al. reported the outcomes for 40 individuals 16 of whom had liver metastases. In 70 %, ≥ 3 prior lines of treatment had been administered [13]. Vilastobart plus atezolizumab showed anti-tumor activity in this population. Three patients without liver metastases developed partial responses (2 confirmed, 1 pending confirmation). All of these were ongoing at the time of the analysis, with normalization of carcinoembryonic antigen (CEA) levels accompanied by undetectable circulating tumor DNA (ctDNA) in one case and substantial decreases in ctDNA in the two others. One additional patient without liver metastases had a 24 % tumor reduction at the initial 9-week assessment and remained on treatment at the time of the analysis. In the response-evaluable group without liver metastases, the ORR was 27 %. The combination was generally well tolerated. Any-grade fatigue and diarrhea were reported in 30 % and 20 %, respectively, followed by infusion-related reactions in 13 %. Six patients experienced grade 3/4 treatment-related AEs (TRAEs), with grade 4 events emerging in two cases (thrombocytopenia and neutropenia, one each). The analysis revealed no grade 5 TRAEs. No dose reductions of vilastobart were necessary due to AEs. In three cases, TRAEs led to discontinuation of treatment. There was a low incidence of immune-related AEs, with 5 % of patients developing colitis. Overall, the clinical data suggest that vilastobart has a differentiated safety profile from systemically active anti-CTLA-4, which is consistent with tumor-selective activation. According to the authors, the initial data support further development of the combination of vilastobart plus atezolizumab in patients with MSS CRC. <h3>Muzastotug plus pembrolizumab: loading dose strategy</h3> The anti-CTLA-4 IgG1 masked antibody muzastotug (ADG126) carries cleavable masking peptides and is preferentially activated in the tumor microenvironment. Binding to a unique epitope blocks CTLA-4 function, primes T cells and depletes regulatory T cells. Compared to the unmasked parental monoclonal antibody, muzastotug shows higher and sustained steady-state tumor-specific engagement of CTLA-4 in the tumor microenvironment and reduced peripheral drug exposure through selective cleavage [14]. At ASCO GI 2025, Li et al. presented an update of the phase IB/II study evaluating muzastotug plus pembrolizumab in patients with advanced solid tumors [15]. Fifty-three of them had advanced MSS CRC without liver metastases; peritoneal involvement was present in 23 %. The scientists employed a loading dose (LD) strategy to assess the possibility of maximizing efficacy while minimizing toxicity. A single muzastotug LD of 20 mg/kg was followed by muzastotug 10 mg/kg Q3W plus pembrolizumab in 12 patients, while 30 and 11 received muzastotug 10 mg/kg Q3W and Q6W, respectively, in addition to pembrolizumab. Compared to the muzastotug 10 mg/kg Q3W regimen, the administration of the LD plus muzastotug 10 mg/kg Q3W indeed enhanced clinical efficacy, resulting in ORRs of 33 % vs. 23 %. Both of these groups had no peritoneal metastases. In the subpopulation receiving muzastotug 10 mg/kg Q6W, none of the patients responded. The data were not mature in the LD group regarding median progression-free survival. At the same time, a manageable safety profile was maintained. The discontinuation rate due to AEs remained low at 6 % in the overall population. No dose-limiting toxicities or grade 4/5 treatment-related AEs occurred. In the LD group, grade 3 key TRAEs were more frequent than in the other groups <strong>(Table 2)</strong> but manageable through dose modification and infrequent use of infliximab or other medical interventions. Dose optimization results demonstrated dose-dependent effects and a clear concordance between muzastotug cleaved exposure in plasma <em>versus</em> efficacy and safety. The authors concluded that the muzastotug LD strategy at 20 mg/kg coupled with dose modification whenever necessary can improve efficacy while maintaining manageable safety in patients with late-stage MSS CRC. Studies that further investigate clinical benefits of muzastostug plus pembrolizumab as an immunotherapy doublet in advanced MSS CRC and/or as an immunotherapy backbone in combination with line- or disease-specific standards of care are being initiated or planned.” content_alignment_medium=”” content_alignment_small=”” content_alignment=”” hide_on_mobile=”small-visibility,medium-visibility,large-visibility” sticky_display=”normal,sticky” class=”” id=”” margin_top=”” margin_right=”” margin_bottom=”” margin_left=”” fusion_font_family_text_font=”” fusion_font_variant_text_font=”” font_size=”” line_height=”” letter_spacing=”” text_transform=”” text_color=”” animation_type=”” animation_direction=”left” animation_color=”” animation_speed=”0.3″ animation_delay=”0″ animation_offset=”” logics=””]

Phase I/II data for vilastobart/atezolizumab

Another novel immunotherapeutic agent is the tumor-activated, high-affinity, Fc-enhanced anti-CTLA-4 antibody vilastobart. This agent blocks the CTLA-4 checkpoint and drives antibody-dependent cellular cytotoxicity against regulatory T-cells that highly express CTLA-4. In vivo studies have shown 10-fold higher potency compared to ipilimumab [10].

Vilastobart is being assessed in a first-in-human phase I/II study in patients with advanced solid tumors and MSS CRC. In the phase IA/B setting, the mono­therapy was generally well tolerated and demonstrated anti-tumor activity in heavily pretreated patients [11]. The combination of vilastobart and atezolizumab was investigated in the phase IC dose-escalation study that is still enrolling. Initial evidence of clinical efficacy was observed in ‘cold’ tumors including MSS CRC [12]. One patient with MSS CRC has an ongoing confirmed partial response with resolution of liver metastases. The phase II part is currently testing the combination of vilastobart 100 mg Q6W and atezolizumab 1,200 mg Q3W in heavily pretreated patients with MSS mCRC. Hecht et al. reported the outcomes for 40 individuals 16 of whom had liver metastases. In 70 %, ≥ 3 prior lines of treatment had been administered [13].

Vilastobart plus atezolizumab showed anti-tumor activity in this population. Three patients without liver metastases developed partial responses (2 confirmed, 1 pending confirmation). All of these were ongoing at the time of the analysis, with normalization of carcinoembryonic antigen (CEA) levels accompanied by undetectable circulating tumor DNA (ctDNA) in one case and substantial decreases in ctDNA in the two others. One additional patient without liver metastases had a 24 % tumor reduction at the initial 9-week assessment and remained on treatment at the time of the analysis. In the response-evaluable group without liver metastases, the ORR was 27 %.

The combination was generally well tolerated. Any-grade fatigue and diarrhea were reported in 30 % and 20 %, respectively, followed by infusion-related reactions in 13 %. Six patients experienced grade 3/4 treatment-related AEs (TRAEs), with grade 4 events emerging in two cases (thrombocytopenia and neutropenia, one each). The analysis revealed no grade 5 TRAEs. No dose reductions of vilastobart were necessary due to AEs. In three cases, TRAEs led to discontinuation of treatment. There was a low incidence of immune-related AEs, with 5 % of patients developing colitis. Overall, the clinical data suggest that vilastobart has a differentiated safety profile from systemically active anti-CTLA-4, which is consistent with tumor-selective activation. According to the authors, the initial data support further development of the combination of vilastobart plus atezolizumab in patients with MSS CRC.

Muzastotug plus pembrolizumab: loading dose strategy

The anti-CTLA-4 IgG1 masked antibody muzastotug (ADG126) carries cleavable masking peptides and is preferentially activated in the tumor microenvironment. Binding to a unique epitope blocks CTLA-4 function, primes T cells and depletes regulatory T cells. Compared to the unmasked parental monoclonal antibody, muzastotug shows higher and sustained steady-state tumor-specific engagement of CTLA-4 in the tumor microenvironment and reduced peripheral drug exposure through selective cleavage [14].

At ASCO GI 2025, Li et al. presented an update of the phase IB/II study evaluating muzastotug plus pembrolizumab in patients with advanced solid tumors [15]. Fifty-three of them had advanced MSS CRC without liver metastases; peritoneal involvement was present in 23 %. The scientists employed a loading dose (LD) strategy to assess the possibility of maximizing efficacy while minimizing toxicity. A single muzastotug LD of 20 mg/kg was followed by muzastotug 10 mg/kg Q3W plus pembrolizumab in 12 patients, while 30 and 11 received muzastotug 10 mg/kg Q3W and Q6W, respectively, in addition to pembrolizumab.

Compared to the muzastotug 10 mg/kg Q3W regimen, the administration of the LD plus muzastotug 10 mg/kg Q3W indeed enhanced clinical efficacy, resulting in ORRs of 33 % vs. 23 %. Both of these groups had no peritoneal metastases. In the subpopulation receiving muzastotug 10 mg/kg Q6W, none of the patients responded. The data were not mature in the LD group regarding median progression-free survival.

At the same time, a manageable safety profile was maintained. The discontinuation rate due to AEs remained low at 6 % in the overall population. No dose-limiting toxicities or grade 4/5 treatment-related AEs occurred. In the LD group, grade 3 key TRAEs were more frequent than in the other groups (Table 2) but manageable through dose modification and infrequent use of infliximab or other medical interventions. Dose optimization results demonstrated dose-dependent effects and a clear concordance between muzastotug cleaved exposure in plasma versus efficacy and safety.

The authors concluded that the muzastotug LD strategy at 20 mg/kg coupled with dose modification whenever necessary can improve efficacy while maintaining manageable safety in patients with late-stage MSS CRC. Studies that further investigate clinical benefits of muzastostug plus pembrolizumab as an immunotherapy doublet in advanced MSS CRC and/or as an immunotherapy backbone in combination with line- or disease-specific standards of care are being initiated or planned.

REFERENCES

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