PET imaging of differentiated thyroid cancer with thyrotropin-alfa

Thyrotropin-alfa is an FDA approved recombinant human TSH agonist. This study represents a preclinical evaluation of thyrotropin-alfa as a thyroid-stimulating hormone receptor (TSHR)-targeted PET radiopharmaceutical, [⁸⁹Zr]Zr-thyrotropin-alfa. [⁸⁹Zr]Zr-thyrotropin-alfa was synthesized by conjugating p-SCN-Bn-deferoxamine (DFO) to thyrotropin-alfa in a molar ratio of 3:1 (DFO:thyrotropin-alfa) and radiolabeling with ⁸⁹Zr (t_1/2 = 78.4 h, β⁺ = 22.7%) at a molar activity of 25.9 MBq/nmol. [⁸⁹Zr]Zr-thyrotropin-alfa uptake was assessed in THJ529T and FTC133 cells stably transduced with the TSHR and compared to their low-expressing wild-type. Studies included a combination of in vitro cell uptake, in vivo PET imaging, and ex vivo biodistribution on Days 1-3 post-injection in male and female mice. In vitro uptake was significantly higher (P < 0.0001) in TSHR + THJ529T (6.6 ± 1.3% bound/mg) and FTC133 (3.5 ± 0.5% bound/mg) cells over low-expressing wild-type counterparts (2.9 ± 1.3% bound/mg and 2.0 ± 0.4% bound/mg, respectively). Blocking uptake with excess DFO-thyrotropin-alfa showed specificity for TSHR (P < 0.0001). In vivo PET imaging showed the highest uptake in TSHR + xenografts on Day 1 post-injection. Ex vivo biodistribution demonstrated significantly higher uptake in the TSHR + female FTC133 xenograft model (P < 0.0001) and TSHR + male FTC133 xenograft model (P < 0.0001) compared to TSHR- xenografts. Uptake of [⁸⁹Zr]Zr-thyrotropin-alfa supports continued preclinical optimization and potential studies in clinical trials.