Evaluation of the In Vivo PET Generator ¹⁴⁰Nd/¹⁴⁰Pr Using a HER2-Targeting Agent

Theranostic strategies using targeted radiopharmaceuticals frequently use f-block therapeutic radionuclides, such as ²²⁵Ac, ¹⁷⁷Lu, and ¹⁶¹Tb; however, there are limited PET imaging counterparts within this chemical space. The in vivo PET generator pair ¹⁴⁰Nd (half-life, 3.4 d; electron capture)/¹⁴⁰Pr (half-life, 3.4 min; 51.0% β⁺) represents a promising imaging analog for these therapeutic radionuclides. The purpose of this study was to assess the feasibility and preclinical performance of the ¹⁴⁰Nd/¹⁴⁰Pr in vivo generator using a human epidermal growth factor receptor 2 (HER2)-targeted monoclonal antibody. High-purity ¹⁴⁰Nd was produced via the ¹⁴¹Pr(p,2n)¹⁴⁰Nd reaction at the University of Alabama at Birmingham cyclotron facility and purified using an in-house diglycolamide-based separation method. Trastuzumab was conjugated with pSCN-Bn-DOTA or SCN-MACROPA chelators and radiolabeled with ¹⁴⁰Nd to generate [¹⁴⁰Nd]Nd-DOTA-trastuzumab or [¹⁴⁰Nd]Nd-MACROPA-trastuzumab. Radiolabeling of DOTA-trastuzumab and MACROPA-trastuzumab with ¹⁴⁰Nd achieved quantitative radiochemical yields of greater than 95% corresponding to specific activity of 74.0 kBq/µg (2.0 μCi/µg) and 114.7 kBq/µg (3.1 μCi/µg), respectively. [¹⁴⁰Nd]Nd-DOTA-trastuzumab remained more than 95% intact in phosphate-buffered saline, saline, mouse serum, and human serum for up to 7 d. [¹⁴⁰Nd]Nd-MACROPA-trastuzumab remained more than 95% intact in phosphate-buffered saline and saline but showed significant decomplexation in mouse and human sera. Both radiotracers demonstrated significantly higher uptake in HER2-positive BT474 cells than in HER2-negative MDA-MB-468 cells (P < 0.0001) and HER2-blocked BT474 cells. PET/CT imaging with [¹⁴⁰Nd]Nd-DOTA-trastuzumab demonstrated substantial tumor accumulation in HER2-positive tumors (SUV_mean, 4.90 ± 0.66 at 7 d) compared with HER2-negative tumors (SUV_mean, 0.69 ± 0.11 at 7 d). Ex vivo biodistribution supported the imaging findings, with HER2-positive tumor uptake increasing from 26.08 ± 3.93 %ID/g to 48.87 ± 22.18 %ID/g between 24 h and 7 d, whereas HER2-negative tumors showed lower uptake (11.52 ± 0.71 %ID/g to 10.54 ± 1.00 %ID/g). In contrast, [¹⁴⁰Nd]Nd-MACROPA-trastuzumab exhibited minimal tumor accumulation in both models, as demonstrated by PET/CT (SUV_mean at 7 d: HER2-positive, 0.49 ± 0.10; HER2-negative, 0.14 ± 0.06) and biodistribution analysis (4.60 ± 1.40 %ID/g vs. 3.63 ± 0.77 %ID/g, respectively), likely attributable to in vivo decomplexation. These results demonstrate that [¹⁴⁰Nd]Nd-DOTA-trastuzumab exhibits excellent stability, HER2-specific targeting, and sustained tumor uptake, supporting the feasibility of the ¹⁴⁰Nd/¹⁴⁰Pr in vivo generator as a PET imaging platform for antibody-based theranostic applications.