First-in-Human SPECT Imaging of Trop2 in Breast Cancer and TNBC Patients Using 99mTc-Trop2-VHH

Introduction: Triple-negative breast cancer (TNBC) is an aggressive malignancy with high metastatic potential and limited therapeutic options. Notably, over 88% of TNBC cases display an abundant expression of Trop2 protein, which correlates with unfavorable prognosis. This study aimed to develop and evaluate the potential of ^99mTc-labeled Trop2 targeting nanobody (Trop2-VHH) as a SPECT tracer to assess Trop2 expression levels in breast cancer patients, thereby providing a valuable tool for precise detection and guiding targeted treatment strategies.

Methods: The Trop2-VHH was labeled with ^99mTc via GGGC tag at its C-terminal. Subsequently, SPECT/CT imaging and biodistribution studies were carried out to evaluate the tracer’s performance in different TNBC tumor models. The first human application involved the intravenous administration of 740 MBq of ^99mTc-Trop2-VHH, followed by whole-body SPECT/CT imaging for Trop2 expression detection, with a comparative analysis to ¹⁸F-FDG PET results.

Results: The ^99mTc-Trop2-VHH achieved a radiochemical purity of over 95% after purification. In vivo imaging and biodistribution studies demonstrated that HCC70 exhibited the highest tumor uptake, followed by HCC1937 and MDA-MB-468. Conversely, the Trop2-negative MDA-MB-231 tumor showed minimal uptake of the radiotracer. Linear regression analysis established a correlation between tumor uptake of ^99mTc-Trop2-VHH and Trop2 expression, with an R² value of 0.9883. These findings suggest that ^99mTc-Trop2-VHH SPECT imaging can accurately reflect Trop2 expression levels in various tumor models. Initial human imaging results involving 5 breast cancer patients have revealed apparent uptake of ^99mTc-Trop2-VHH in those breast cancer lesions detected by ¹⁸F-FDG PET scans, and even lesions with low ¹⁸F-FDG uptake can be more effectively detected by ^99mTc-Trop2-VHH imaging, indicating improved detectability using ^99mTc-Trop2-VHH imaging. These lesions were confirmed as Trop2-positive through immunohistochemical (IHC) analysis. Furthermore, ^99mTc-Trop2-VHH exhibits favorable pharmacokinetic properties, with dominant renal metabolism and relatively low uptake in normal thoracic organs. Although the tracer also accumulates in the salivary gland, lacrimal gland, submandibular gland, and pancreas, its safety profile has been established, as no significant changes in vital signs or adverse events were observed during the investigation. These findings emphasize the potential of ^99mTc-Trop2-VHH as a promising and safe SPECT imaging agent for assessing Trop2 in clinical settings.

Conclusions: In summary, this study introduces a promising molecular imaging probe that allows for visualization and evaluation of Trop2 levels in tumors. It holds potential for enhancing the efficacy of TNBC treatment through precise diagnosis and targeted treatment guidance.

• Download figure
• Open in new tab
• Download powerpoint