Cibisatamab and FAP-4-1BBL in microsatellite-stable colorectal cancer: a phase 1b trial.
Melero I., Tanos T., Calvo Aller E., Qvortrup C., van Dongen M., Baraibar I., Beom S-H., Thistlethwaite F., Riesco MDC., Garcia MM., Woodcock V., Kim TW., Umana P., McIntyre C., Chen L., Heichinger C., Hinton H., Saylan T., Davydov II., Guarin E., Boehnke A., Moreno V.
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 .