Imaging and Monitoring HER2 Expression in Tumors during HER2 Antibody–Drug Conjugate Therapy Utilizing a Radiolabeled Site-Specific Single-Domain Antibody Probe: 68Ga-NODAGA-SNA004-GSC

The overexpression of HER2 is pivotal in the initiation and progression of breast cancer. Developing HER2-targeted radiotracers is crucial for noninvasive assessment of HER2 expression, patient selection for HER2-targeted therapy, monitoring treatment response, and identifying resistance. Here, we reported a nonsite-specific coupled radiotracer, 68Ga-NOTA-SNA004-His6, and a site-specific coupled radiotracer, 68Ga-NODAGA-SNA004-GSC, based on a novel HER2 nanobody, SNA004. Both radiotracers exhibited high affinity, specific targeting, and rapid clearance in vitro and in vivo. Additionally, these tracers and trastuzumab showed noncompetitive binding to HER2. Compared to 68Ga-NOTA-SNA004-His6, 68Ga-NODAGA-SNA004-GSC demonstrated significantly reduced renal and liver uptake. PET/CT imaging with 68Ga-NODAGA-SNA004-GSC sensitively detected the responsiveness of various tumor models to trastuzumab and its antibody-drug conjugates (ADCs). Overall, the site-specific coupled radiotracer 68Ga-NODAGA-SNA004-GSC offered significant advantages in biodistribution and signal-to-noise ratio, making it a valuable tool for monitoring HER2 expression levels before, during, and after trastuzumab and ADC treatment.