BackgroundRegulatory T cells (T_regs) contribute to an immunosuppressive tumor microenvironment. They play an important role in the establishment and progression of tumors with high T_regs infiltration and present a major obstacle to tumor eradication by immunotherapies. Numerous strategies have been attempted to deplete or block T_regs, although their success has been limited.MethodsA CD25-targeted, pyrrolobenzodiazepine (PBD) dimer-based antibody–drug conjugate (ADC) was investigated for its ability to deplete T_regs and induce antitumor immunity. Antitumor activity of CD25-ADC either alone or in combination with an anti-programmed cell death protein 1 (PD-1) antibody was evaluated in CD25-negative syngeneic models that exhibit tumor infiltration of CD25-expressing T_regs, and its pharmacodynamics and pharmacokinetics were assessed.ResultsSingle low doses of CD25-ADC resulted in potent and durable antitumor activity in established syngeneic solid tumor models and the combination of a suboptimal dose was synergistic with PD-1 blockade. Tumor eradication by the CD25-targeted ADC was CD8+ T cell-dependent and CD25-ADC induced protective immunity. Importantly, while CD25-ADC mediated a significant and sustained intratumoral T_regs depletion, accompanied by a concomitant increase in the number of activated and proliferating tumor-infiltrating CD8+ T effector cells, systemic T_regs depletion was transient, alleviating concerns of potential autoimmune side effects.ConclusionsThis study shows that a PBD dimer-based, CD25-targeted ADC is able to deplete T_regs and eradicate established tumors via antitumor immunity. This represents a novel approach to efficiently deplete T_regs via a very potent DNA damaging toxin known to induce immunogenic cell death. Moreover, this study provides proof of concept for a completely new application of ADCs as immunotherapeutic agents, as the main mode of action relies on the ADC directly targeting immune cells, rather than tumor cells. These strong preclinical data warrant the clinical evaluation of camidanlumab tesirine (ADCT-301), a PBD-based ADC targeting human CD25, either alone or in combination with checkpoint inhibitors in solid tumors with known T_regs infiltration. A phase I trial (NCT03621982) of camidanlumab tesirine in patients with selected advanced solid tumors is ongoing.