Preclinical Evaluation of ^99mTc-MAG₃-5-Fab Targeting TREM2 in Lung Cancer Mouse Models: A Comparison with...

Triggering receptor expressed on myeloid cells-2 (TREM2), which is expressed on the surface of tumor-associated macrophages (TAMs), has been found to play a major role in the diagnosis and treatment of tumors. TREM2 expression is significantly upregulated in tumor tissues, and therefore, targeting TREM2 for tumor imaging may be of value. Previously, we performed TREM2 targeting imaging by using ⁶⁸Ga-NOTA-COG1410 or a ¹²⁴I-labeled monoclonal antibody (mAb) and F(ab')₂ in mouse models of colon and gastric tumors. However, some of the shortcomings of these probes (i.e., the high uptake of ⁶⁸Ga-NOTA-COG1410 in the liver, the difficulty of obtaining iodine-124, and the long half-life of iodine-124) have hindered their clinical use. Herein, we sought to synthesize novel molecular probes targeting TREM2 that are more conducive to clinical translation, eliminating the interference of isotope availability and in vivo probe biodistribution issues. Therefore, we established A549 cell lines with negative human TREM2 (hTREM2) expression (GFP tag; hTREM2^- A549) or upregulated hTREM2 expression (GFP tag; hTREM2⁺ A549) using lentiviral transfection and confirmed these with Western blotting and immunocytochemistry. We then prepared a mouse anti-human TREM2 (5-mAb) by immunizing with the hTREM2 antigen. The antibody fragments 5-F(ab')₂ and 5-Fab were prepared from 5-mAb, and ^99mTc-MAG₃-5-F(ab')₂ and ^99mTc-MAG₃-5-Fab were then synthesized with excellent stability and specificity. ^99mTc-MAG₃-5-F(ab')₂ had a slightly higher in vitro affinity than ^99mTc-MAG₃-5-Fab (K_d = 3.32 ± 0.05 nmol versus 4.62 ± 0.85 nmol). ^99mTc-MAG₃-5-F(ab')₂ and ^99mTc-MAG₃-5-Fab both showed excellent specificity: after adding a 100-fold precursor, the two probes binding to the cells were almost blocked. In vivo pharmacokinetics showed that the distribution and elimination half-lives of ^99mTc-MAG₃-5-Fab (T_1/2α = 1.25 ± 0.30 min and T_1/2β = 21.98 ± 2.80 min, respectively) were significantly reduced compared to those of ^99mTc-MAG₃-5-F(ab')₂ (T_1/2α = 2.64 ± 0.37 min and T_1/2β = 86.55 ± 26.86 min, respectively). In micro single-photon emission computed tomography/computed tomography (micro-SPECT/CT) imaging, the tumor was clearly displayed at 1 h after ^99mTc-MAG₃-5-Fab injection, while the blood background was extremely low at 3 h, and the probe was mainly excreted through the kidneys and biliary tract. ^99mTc-MAG₃-5-F(ab')₂ uptake was also detected at the tumor site, although the blood background was consistently high. The biodistribution results were consistent with the micro-SPECT/CT imaging results. ^99mTc-MAG₃-5-Fab could clearly display hTREM2⁺ A549 tumors in a short time (1 h) with low uptake in nontumor organs and tissues and thus has clinical application prospects.