Abstract

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.

Results from MultiScan™ LFER PET/CT

• In-vivo performance of [⁸⁹Zr]Zr-7D3: no marked differences of [⁸⁹Zr]Zr-7D3 uptake in the PET images were evident visually and semi-quantitatively in any organs, including LNs, spleen, and the gut between the uninfected and the chronic SIV-infected RMs
• In-vivo performance of [⁸⁹Zr]Zr-ITS103.01LS-F(ab’)2 : Both qualitative and semi-quantitative PET SUV analysis of various lymphoid tissues and the gut failed to show higher uptake in the chronic SIV-infected RMs compared to the uninfected controls

Fig.1 Maximum intensity projection in-vivo PET images of a representative chronically SIV-infected viremic rhesus macaque (RM3) and a representative uninfected control (RM10) following administration of ~ 1 mg mass of [⁸⁹Zr]Zr-7D3 (~ 30MBq of ⁸⁹Zr) and scanned at 40 h and Day 5 post-injection (a). Tissue uptakes were converted to RAINBOW color map as shown in the color bar, where red color indicates the high standardized uptake value (SUV). Comparison of maximum SUV (SUV_max) in tissues among the chronically SIV-infected viremic (red), pre-acutely SIV-infected (orange), and uninfected controls (blue) at 40 h (b) and Day 5–6 (c) post-injection. Both visual and semi-quantitative SUV analysis showed no higher uptake in the SIV-infected RMs compared to the uninfected controls. Because of intraluminal uptake in some animals at 40 h, the gut uptake is not plotted. Plots are mean values and error bars are standard deviation.

Full article on springer.com