PET Imaging of Macrophage Mannose Receptor–Expressing Macrophages in Tumor Stroma Using 18F-Radiolabeled Camelid Single-Domain Antibody Fragments

Tumor-associated macrophages constitute a major component of the stroma of solid tumors, encompassing distinct subpopulations with different characteristics and functions. We aimed to identify M2-oriented tumor-supporting macrophages within the tumor microenvironment as indicators of cancer progression and prognosis, using PET imaging. This can be realized by designing 18F-labeled camelid single-domain antibody fragments (sdAbs) specifically targeting the macrophage mannose receptor (MMR), which has been identified as an important biomarker on this cell population. Methods: Cross-reactive anti-MMR sdAbs were generated after immunization of an alpaca with the extracellular domains of both human and mouse MMR. The lead binder was chosen on the basis of comparisons of binding affinity and in vivo pharmacokinetics. The PET tracer 18F-fluorobenzoate (FB)-anti-MMR sdAb was developed using the prosthetic group N-succinimidyl-4-18F-fluorobenzoate (18F-SFB), and its biodistribution, tumor-targeting potential, and specificity in terms of macrophage and MMR targeting were evaluated in mouse tumor models. Results: Four sdAbs were selected after affinity screening, but only 2 were found to be cross-reactive for human and mouse MMR. The lead anti-MMR 3.49 sdAb, bearing an affinity of 12 and 1.8 nM for mouse and human MMR, respectively, was chosen for its favorable in vivo biodistribution profile and tumor-targeting capacity. 18F-FB-anti-MMR 3.49 sdAb was synthesized with a 5%–10% radiochemical yield using an automated and optimized protocol. In vivo biodistribution analyses showed fast clearance via the kidneys and retention in MMR-expressing organs and tumor. The kidney retention of the fluorinated sdAb was 20-fold lower than a 99mTc-labeled counterpart. Compared with MMR- and C-C chemokine receptor 2–deficient mice, significantly higher uptake was observed in tumors grown in wild-type mice, demonstrating the specificity of the 18F tracer for MMR and macrophages, respectively. Conclusion: Anti-MMR 3.49 was denoted as the lead cross-reactive MMR-targeting sdAb. 18F radiosynthesis was optimized, providing an optimal probe for PET imaging of the tumor-promoting macrophage subpopulation in the tumor stroma.

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