Development of an ⁸⁹Zr-labeled High-Density Lipoprotein Nanoparticle as a PET Agent to Track Efficacy of Immunotherapy

Take-Away Points

• ■ Major Focus: Development of a radiolabeled lipoprotein as a noninvasive PET/CT agent to monitor the effects of immunotherapy on macrophage levels in the aggressive tumor microenvironment.

• ■ Key Results: The developed zirconium 89 (⁸⁹Zr)–labeled high-density lipoprotein (HDL) nanoparticle was capable of delineating between subjects with differential tumor-macrophage levels following pexidartinib-based immunotherapy both in vivo and ex vivo.

• ■ Impact: ⁸⁹Zr-HDL is established as a potential clinically translatable PET/CT agent that is capable of noninvasively monitoring anti–tumor-associated macrophage immunotherapies.

The immunosuppressive nature of aggressive, metastatic cancers due to dense tumor microenvironments contributes to the dismal outlooks observed in a clinical setting. Accumulation of tumor-associated macrophages (TAMs) within the tumor microenvironment results in suppression of the adaptive immune response as well as onset of other critical hallmarks of cancer such as angiogenesis. As such, therapeutic approaches to target TAMs are intriguing; reduction of TAM levels within the tumor could potentially reinitialize the previously blocked immune response. Information pertaining to the efficacy of anti-TAM therapeutics could prove critical for treatment alterations; however, there are currently no accepted biomarkers that track efficacy of immunotherapy.

Recently, Mason et al developed high-density lipoproteins (HDLs) labeled with zirconium 89 ⁽⁸⁹Zr) as a high-affinity macrophage nano-radiotracer in efforts to track TAM level subsequent to pexidartinib-based immunotherapy using PET/CT. HDL particles are well documented as having high affinity for macrophages, a highly upregulated component often associated with immunotherapy and thus, the base of this study. Following evaporation of the phospholipid-containing solution, HDL particles were labeled with ⁸⁹Zr and isolated to greater than 99% purity. Female MMTV-PyMT transgenic mammary adenocarcinoma mice were treated with pexidartinib, followed by an intravenous injection of synthesized ⁸⁹Zr-HDL nanoparticles. ⁸⁹Zr-HDL biodistribution was used to evaluate TAM levels with PET/CT. Secondary confirmation of TAMs occurred using immunofluorescence and autoradiographic analysis ex vivo.

⁸⁹Zr-HDL uptake in tumor cells was significantly reduced (2.9% ± 0.3 vs 3.7% ± 0.2, P < .01) in mice that underwent immunotherapy with pexidartinib based upon PET/CT images. These results were confirmed using IBA-1 staining which showed lower macrophage density in pexidartinib-treated mice and corroborated further using autoradiography.

This study provides evidence that supports ⁸⁹Zr-HDL as a viable radiotracer for PET/CT monitoring of TAM levels, most notably, following anti-TAM immunotherapy. The sensitivity of ⁸⁹Zr-HDL to TAM levels, which is closely associated with the immunosuppressive nature of the tumor microenvironment, establishes this nano-radiotracer as a potential noninvasive tool to quickly monitor efficacy of immunotherapy in a clinical setting.

Highlighted Article

Mason CA, Kossatz S, Carter LM, et al. An ⁸⁹Zr-HDL PET tracer monitors response to a CSF1R inhibitor. J Nucl Med 2020 Mar 1;61(3):433–436. doi: 10.2967/jnumed.119.230466