Cibisatamab and FAP-4-1BBL in microsatellite-stable colorectal cancer: a phase 1b trial

We evaluated cibisatamab, a carcinoembryonic antigen (CEA)-directed CD3 T cell-engaging bispecific antibody, in combination with FAP-4-1BBL, a fibroblast activation protein (FAP)-targeted 4-1BB ligand providing tumor-localized co-stimulation, in an open-label phase 1b dose-escalation study in patients with microsatellite-stable (MSS) metastatic colorectal cancer (mCRC) progressing after two or more prior therapies. Patients received cibisatamab with escalating doses of FAP-4-1BBL weekly or every 3 weeks after obinutuzumab pretreatment to mitigate anti-drug antibody formation. The primary endpoint was safety; secondary endpoints included antitumor activity, pharmacokinetics and biomarker analyses. Among 52 treated patients, the combination showed a manageable safety profile. Dose-limiting toxicities occurred in 2 out of 52 patients (3.8%). Cytokine release syndrome (CRS) occurred in 30 out of 52 patients (57.7%; grade ≥3: 2 out of 52, 3.8%) and was manageable; after a cycle 1 cibisatamab dose reduction to 60 mg, serious CRS occurred in 4 out of 27 patients (14.8%; grade ≥3: 0 out of 27). Gastrointestinal toxicities consistent with CEA-directed T cell engagement were observed. Colitis occurred in 7 out of 52 patients (13.5%), including immune-mediated enterocolitis and one fatal cytomegalovirus colitis. No maximum tolerated dose of FAP-4-1BBL was established. Confirmed partial responses were observed in 7 out of 52 patients (13.5%). Pharmacodynamic analyses demonstrated systemic immune activation, including increased IFNγ, soluble CD25, soluble 4-1BB (CD137) and activated, proliferating CD8 + T cells. Paired tumor biopsies showed increased intratumoral CD8 + and CD8 + Ki67 + T cell infiltration. These findings demonstrate the feasibility of combining tumor antigen-directed T cell engagement with localized co-stimulation, with evidence of immune activation and preliminary antitumor activity supporting further clinical development. ClinicalTrials.gov identifier: NCT04826003 .

Immune checkpoint inhibitors have substantially improved outcomes in CRC with mismatch repair deficiency or high microsatellite instability (dMMR/MSI-H) 1 . However, more than 85% of patients have mismatch repair proficient (pMMR)/MSS disease 2 , which is typically characterized by limited immune infiltration and poor responsiveness to current immunotherapies 3 , 4 , resulting in a persistently poor prognosis in the metastatic setting. Effective therapeutic strategies for this population remain an unmet need.

T cell receptor (TCR)-engaging therapeutics can activate cytotoxic T cells and have demonstrated clinical benefit in hematologic malignancies 5 , 6 and, more recently, in selected solid tumors 7 , 8 . Nonetheless, extending these approaches to non-inflamed tumors such as pMMR/MSS CRC remains challenging.

Effective activation of T lymphocytes requires integration of multiple signals. In the classical two-signal model of T cell activation 9 , signal 1 is provided by recognition of peptide–MHC complexes by the TCR, whereas signal 2 is delivered through co-stimulatory receptors that amplify TCR signaling 10 . In the absence of adequate co-stimulation, TCR engagement alone can result in incomplete activation or functional tolerance 11 .

Among inducible co-stimulatory receptors, 4-1BB (also known as CD137 or TNFRSF9) is upregulated after T cell activation and promotes proliferation, survival, memory differentiation and cytotoxic function upon engagement with its ligand or agonistic antibodies 10 , 12 , 13 , 14 . Combining targeted TCR engagement with complementary co-stimulatory signals may, therefore, enhance the magnitude and durability of antitumor T cell responses.

Cibisatamab is a 2:1 T cell bispecific antibody that targets CEA, expressed in more than 80% of CRCs, and CD3ε on T cells 15 , 16 . By simultaneously binding CEA and CD3ϵ, cibisatamab induces T cell activation independently of native TCR specificity, resulting in lymphocyte-mediated tumor cell killing 16 , 17 . In a previous phase 1 study of cibisatamab monotherapy in CEA-positive solid tumors, preliminary antitumor activity was observed, with confirmed partial responses in 4.0% of evaluable patients and a median duration of response of 6.5 months 18 .

We hypothesized that the antitumor activity of a T cell engager such as cibisatamab could be enhanced by spatially restricted co-stimulation mediated by a FAP-targeted 4-1BB ligand (FAP-4-1BBL). This concept is supported by preclinical co-culture models of tumor organoids and FAP-expressing cancer-associated fibroblasts (CAFs) 19 . FAP-4-1BBL enables selective 4-1BB engagement within FAP-expressing tissues, including the tumor microenvironment and tumor-draining lymph nodes 13 , 20 . In a first-in-human dose-escalation study, FAP-4-1BBL (RO7122290) monotherapy showed modest clinical activity but demonstrated an acceptable safety profile and evidence of increased intratumoral T cell infiltration 13 .

Here we report the results of the phase 1b dose-escalation study evaluating the combination of cibisatamab and FAP-4-1BBL after obinutuzumab pretreatment in patients with heavily pretreated MSS mCRC. Obinutuzumab pretreatment to deplete B cells was required because of induction of anti-drug antibodies (ADAs) by cibisatamab 21 . We further contextualize these findings in the combination setting by comparison with previously reported cibisatamab monotherapy data and associated pharmacodynamic analyses 18 . Our study assessed the impact of tumor-localized 4-1BB co-stimulation on safety, immune activation and clinical activity when combined with cibisatamab.

Between 19 April 2021 and 11 November 2024, patients with pMMR/MSS mCRC who had progressed after at least two prior lines of therapy were screened for this open-label, multicenter, phase 1b dose-escalation study. Fifty-four patients with measurable disease at baseline were enrolled. Fifty-two patients received at least one administration of study drug and were analyzed for safety and efficacy. Baseline characteristics and demographics are summarized in Table 1 , and patient disposition is shown in the CONSORT diagram (Fig. 1a,b ). The safety and efficacy analyses were conducted in all patients who received at least one dose of study treatment ( n = 52).

CONSORT diagrams summarizing the design of the phase 1b study evaluating the CEA-targeted T cell engager cibisatamab in combination with the tumor-targeted 4-1BB agonist FAP-4-1BBL in patients with advanced solid tumors and the disposition of patients included in the present analysis. The diagrams illustrate the dose-escalation cohorts evaluating different FAP-4-1BBL dose levels and schedules administered with cibisatamab, including cohorts receiving a flat cibisatamab dose from cycle 1 and cohorts evaluating a refined step-up regimen (60 mg at cycle 1 followed by 100 mg from cycle 2). C1, cycle 1; C2, cycle 2.

The study comprised two parts. Part 1 evaluated safety, pharmacokinetics and pharmacodynamics during weekly (QW) escalation of FAP-4-1BBL. Part 2 explored doses deemed safe in part 1 using every-3-week (Q3W) dosing (Extended Data Fig. 1a,b ). Cibisatamab was administered Q3W in both parts.

Initially, cibisatamab was given at 100 mg from cycle 1 (part 1 cohorts 1 and 2; part 2 cohort 1). After a grade 3 CRS event, the cycle 1 dose was reduced to 60 mg from part 1 cohort 3 and part 2 cohort 2 onward, with escalation to 100 mg from cycle 2 (60 → 100 mg).

Study design and dosing schedules are shown in Extended Data Fig. 1a,b , with baseline characteristics and cohort dose levels summarized in Table 1 and in the CONSORT diagrams (Fig. 1 ). Dose escalation of FAP-4-1BBL in combination with cibisatamab was discontinued for strategic reasons before a maximum tolerated dose (MTD) was defined.

Treatment with cibisatamab plus FAP-4-1BBL was associated with immune-mediated adverse events consistent with the known safety profile of T cell-engaging therapies and prior clinical experience with cibisatamab, with most events occurring during the early treatment cycles. Patients routinely received low-dose corticosteroid and non-steroidal anti-inflammatory premedication prior to infusions. The highest FAP-4-1BBL doses administered were 130 mg QW and 90 mg Q3W. Two dose-limiting toxicities (DLTs) were observed: presyncope (grade 3 serious adverse event (SAE), related to FAP-4-1BBL) in the cibisatamab 100 mg plus FAP-4-1BBL 35 mg QW cohort and CRS (grade 3 SAE, related to both cibisatamab and FAP-4-1BBL) in the cibisatamab plus FAP-4-1BBL 50 mg QW plus cohort. A summary of treatment-emergent adverse events is presented in Table 2 , and details by cohort are provided in Extended Data Tables 1 and 2 .

All patients (52 out of 52) experienced at least one adverse event. The most common (≥30%) were fatigue (63.5%), CRS (57.7%), diarrhea (55.8%), pyrexia (50.0%), decreased appetite (44.2%), cough (32.7%), anemia (32.7%), nausea (30.8%) and arthralgia (30.8%) (Table 2 and Extended Data Tables 1 and 2 ). Grade 3 or higher adverse events occurred in 38 patients (73.1%), and SAEs occurred in 34 patients (65.4%). Four patients (7.7%) experienced grade 5 events (sepsis, sudden death, general physical health deterioration and cytomegalovirus (CMV)-related colitis); the former two were not considered treatment related, whereas general physical health deterioration and CMV-related colitis were. The event coded as ‘general physical health deterioration’ reflects progressive clinical decline in the context of advanced disease without a clearly identifiable alternative cause.

Dose interruptions or modifications of FAP-4-1BBL occurred in 21 patients (40.4%), and adverse events resulted in treatment discontinuation in eight patients (15.4%). Adverse events considered related to FAP-4-1BBL led to dose modification or interruption in 17 patients (32.7%) and to treatment discontinuation in one patient (1.9%) (Table 2 and Extended Data Table 1 ). Adverse events by dose level showed no clear dose-dependent increase in grade 3 or higher toxicity, although higher-dose weekly cohorts demonstrated a trend toward increased incidence of gastrointestinal and immune-mediated events (Extended Data Table 1 )

CRS occurred predominantly during cycle 1 (median onset, day 2) and was grade 1 in 25 patients (48.1%), grade 2 in 4 patients (7.7%) and grade 3 in 2 patients (3.8%). Serious CRS events were reported in 13 patients (25.0%) overall, including 4 out of 27 patients (14.8%) treated with cibisatamab 60 mg in cycle 1 followed by 100 mg from cycle 2 (Table 2 and Extended Data Table 3 ). CRS was managed per protocol and institutional guidelines with supportive care (antipyretics, intravenous fluids and corticosteroids as indicated); some patients required hospitalization, and tocilizumab was used in selected cases. Additional details on management of CRS are provided in the Methods .

Gastrointestinal toxicity, a known on-target/off-tumor effect of cibisatamab 18 , was also observed with combination therapy, including nausea (30.5%), vomiting (23.1%) and diarrhea (55.8%). Diarrhea occurred throughout treatment (median onset, 36 days; range, −2 to 393 days relative to first dose). The negative onset reflects the study design, in which treatment was initiated on study day 1, and patients received obinutuzumab lead-in dosing on study day −13 or −8 depending on the protocol version. Colitis was reported in seven patients (13.5%), including cases of immune-mediated enterocolitis and one case of CMV colitis.