Whole-body PET imaging of simian immunodeficiency virus using gp120-targeting probes fails to reveal regions of specific uptake in rhesus macaques.

Purpose: Following the initial reports demonstrating the feasibility of immunoPET imaging of simian immunodeficiency virus (SIV) using gp120-targeting monoclonal antibodies in non-human primates, replication efforts of the imaging system in human immunodeficiency virus (HIV)-infected individuals have yielded conflicting results. Herein, we used two anti-gp120 antibodies, 7D3 and ITS103.01LS-F(ab')₂, to interrogate the reproducibility of gp120-targeting probes for immunoPET imaging of SIV in rhesus macaques.

Methods: The binding affinity estimates of ⁸⁹Zr radiolabeled 7D3 and ITS103.01LS-F(ab')₂ to SIV gp120, and the in-vitro and ex-vivo binding specificities of [⁸⁹Zr]Zr-7D3 and [⁸⁹Zr]Zr-ITS103.01LS-F(ab')₂ to SIV Env expressing cells, primary cells, and tissue sections from uninfected and SIV-infected macaques were obtained through competition assays. The biodistributions of [⁸⁹Zr]Zr-7D3 and [⁸⁹Zr]Zr-ITS103.01LS-F(ab')₂ were performed with static PET scans up to 6 days post-injection in 20 rhesus macaques and the standardized uptake values in various tissues were compared between SIV-infected and uninfected controls.

Results: Despite the demonstrated nanomolar affinity of [⁸⁹Zr]Zr-7D3 and [⁸⁹Zr]Zr-ITS103.01LS-F(ab')₂ to SIV gp120, and strong binding specificity to SIV gp120 cell lines, we observed no discernible differences in their binding in primary cells, tissue sections of secondary lymphoid organs, in-vivo probe uptake between SIV-infected and uninfected macaques, or ex-vivo validation necropsies. While the probes remained stable in-vivo, only [⁸⁹Zr]Zr-ITS103.01LS-F(ab')₂ in chronic plasma retained its binding specificity to SIV gp120, with [⁸⁹Zr]Zr-7D3 experiencing a > 97% reduction in binding to gp120 due to competition from endogenous antibodies at the 7D3 binding site.

Conclusion: The overall absence of specific uptake suggests inadequate binding potential (ligand affinity x target molarity) for these probes to effectively image SIV or HIV in-vivo, warranting further investigation into the lack of reproducibility observed with earlier non-human primate SIV imaging and conflicting human studies.