Targeting CD6 in solid tumors: safety, early efficacy, and mechanistic insights from a proof-of-concept trial of itolizumab

Background CD6 is a promising target in cancer immunotherapy, with preclinical evidence supporting its role in modulating immune responses. This warrants further investigation into current CD6-targeting treatments, which could lead to new anti-tumor therapies. However, the clinical effects of targeting CD6 in solid tumors remain unexplored. Methods This proof-of-concept study is the first in-human evaluation of the safety and early effectiveness of anti-CD6 monoclonal antibody itolizumab in patients with advanced, highly heterogeneous solid tumors. Treatment had two phases: induction and maintenance. In induction, group A received 100 mg weekly for nine doses; group B received 200 mg biweekly for five doses. After completing induction, patients moved to maintenance 21 days later, receiving 100 mg every 21 days for up to six doses. The primary endpoint was safety; secondary objectives included disease control, survival, and exploratory molecular mechanisms and blood biomarkers. Results Itolizumab demonstrated a favorable safety profile, with no safety-related treatment discontinuations, no related serious adverse events, and no unexpected toxicities. In this heavily treated, non-biomarker-selected population, disease control was achieved in 25% in group A and 20% in group B, and the 1-year survival rate was 26% in group A and 15% in group B. Mechanistically, ex vivo analysis demonstrated that itolizumab downregulates CD6, increasing soluble CD6 levels and promoting a cytotoxic phenotype on CD8 and NK cells. Additionally, patient-derived peripheral blood mononuclear cells demonstrated boosted in vitro killing capacity specifically against CD318+ cancer cells. The antibody also modulates PD-1 expression in peripheral lymphocytes, promoting NK cell activity while maintaining inhibitory signals on CD8+ T cells and decreasing T helper overactivation. These findings support the dual role of itolizumab, enhancing anti-tumor responses while preventing autoimmune-like effects by reducing off-target inflammation and avoiding immune-related adverse effects associated with immune checkpoint therapies. Conclusions Overall, these findings support further development of itolizumab as a rational immunomodulatory strategy with a favorable safety profile for advanced solid tumors, particularly in CD318-expressing tumors and as part of rational combination regimens. Trial registration number RPCEC00000444.