64 CuCu-Labeled αCD11b Diabody as a Novel PET Tracer for the Detection of Immunosuppression in Glioblastoma

Glioblastoma multiforme (GBM) is one of the deadliest types of cancer that occurs in people of all ages; 15 months is the average survival time. While treatments for GBM are mostly unsuccessful, immunotherapy has the potential to be an effective strategy for glioblastoma. However, the immunosuppressive influence of tumor-associated myeloid cells (TAMCs) results in poor responses to immunotherapy. As TAMCs are CD11b-positive, the potential of a radiolabeled αCD11b diabody was investigated to assess immunosuppression mediated by TAMCs in an immunocompetent mouse model of glioblastoma. An αCD11b diabody (Db) was constructed with the VH and VL sequences of an αCD11b IgG that resulted in thermal stability and high affinity. αCD11b Db was conjugated with a cross-bridged chelator, CB-TE1K1P, through click chemistry. The resulting conjugate was radiolabeled with 64Cu and investigated in vitro and in a model of glioblastoma. 64CuCu-αCD11b Db visualized TAMCs in a syngeneic mouse glioblastoma, achieving optimal uptake within 4 h post administration with a %ID/g of 1.06 and 0.18 for tumor and healthy brain tissue. In correlating molar activity (8.51, 4.26, 2.12, and 1.06 MBq/nmol) with uptake (%ID/g of 0.162, 0.825, 1.06, and 0.445, respectively), we demonstrated that 2.12 MBq/nmol gave optimal uptake, since tracer pharmacokinetics was modulated by αCD11b Db occupation of the CD11b antigen sink. In conclusion, 64CuCu-αCD11b Db is a high-affinity and stable diabody, which can quantify CD11b-positive TAMCs in the tumor microenvironment, particularly when the molar activity of the administered 64CuCu-αCD11b Db is optimized for managing the CD11b antigen sink in the spleen, liver, and bone marrow.