⁶⁸Ga-Labeled LLP2A for PET Imaging of Very Late Antigen-4 in Acute Cardiac Rejection

Acute rejection (AR) remains the most common complication after heart transplantation (HT). AR is an acute immune response mediated by lymphocytes, and very late antigen-4 (VLA-4) plays a vital role in lymphocyte differentiation and transport, making it an attractive target for developing molecular diagnostic agents. In this study, we proposed the construction of a radionuclide probe ⁶⁸Ga-LLP2A and explored the role of the VLA-4 receptor distribution in AR. The in vitro and in vivo targeting efficiency and specificity of the tracer were validated by the cell uptake assay of monocyte cells and cardiomyocytes, PET/CT imaging, and biodistribution of VLA-4-positive B16F10 tumor-bearing mice models, respectively. ⁶⁸Ga-LLP2A (5.5-7.4 MBq/rat) were injected into each group of rats 5 days after transplantation, including allograft (ALL), allograft treatment (ALL-T), allograft blocking (ALL-B), isograft (ISO), and normal Lewis (NOR) rats. PET/CT images, ex vivo autoradiography, and biodistribution were performed 1 h after injection. Expression levels of VLA-4 in the hearts of rats in each group were assessed by immunochemical staining. The tracer exhibited high accumulation with good contrast in the graft heart of ALL rats in static PET/CT images, autoradiography, and biodistribution studies and lower accumulation of radioactivity in the ALL-T and ALL-B groups, while no uptake was seen in the ISO and NOR groups. Histological analysis confirmed the abundant infiltration of VLA-4-positive cells in the graft heart of ALL rats. In conclusion, ⁶⁸Ga-LLP2A exhibited high VLA-4 targeting efficacy in vivo and could accumulate specifically in the graft heart, rendering it a promising candidate for noninvasive imaging of acute cardiac rejection.