Abstract
CD137 is a powerful T cell costimulatory molecule, and CD137 agonists are being evaluated for human cancer immunotherapy. Urelumab and utomilumab, are two agonistic anti-CD137 antibodies that are most advanced in clinical trials but suffer from liver toxicity and low potency, respectively. Here we describe the development of a new type and formulation of a CD137 agonist that combines high potency and a strong safety profile. Methods: The extracellular domain of recombinant human CD137 ligand (rhCD137L) was conjugated onto mesoporous silica nanoparticles (MSNs) of approximately 50 nm in diameter, and the ratio of rhCD137L to MSNs was optimized based on their ability to costimulate the cytotoxic activity of T cells. As nasopharyngeal carcinoma (NPC) cells often express CD137, the in vitro effect of rhCD137L-MSNs on T cell-mediated tumor cytotoxicity was evaluated using the NPC cell lines C666 and HK-1, each tested as CD137-expressing and -deficient variants. Results were compared with those obtained using MSNs conjugated with urelumab (ure-MSNs) or unconjugated urelumab. The biodistribution, therapeutic efficacy and toxicity of rhCD137L-MSNs were subsequently assessed in humanized mouse NPC models. Results: rhCD137L-MSNs were of higher potency than ure-MSNs or unconjugated urelumab in inducing in vitro T cell killing of the NPC cell lines C666 and HK-1, of both CD137-expressing and -deficient phenotypes. C666-CD137 and HK1-CD137 cells were eliminated more efficiently than the CD137-deficient cells. In vivo, in humanized mouse NPC models, both rhCD137L-MSNs and ure-MSNs inhibited tumor growth, with rhCD137L-MSNs being slightly more potent. This was reflected in an increase in T cell activation markers and an increased infiltration of effector memory CD8+ T cells into the tumor. In contrast to ure-MSNs, rhCD137-MSN treatment did not induce liver damage, thereby demonstrating a more favorable safety profile than ure-MSNs. Conclusions: This study identifies a formulation of rhCD137L on MSNs that combines high potency with excellent safety.