Radionuclide molecular imaging of human epidermal growth factor receptor type 2 (HER2) expression may help to stratify breast and gastroesophageal cancer patients for HER2-targeting therapies. Albumin-binding domain-derived affinity proteins (ADAPTs) are a new type of small (46-59 amino acids) protein useful as probes for molecular imaging. The aim of this first-in-humans study was to evaluate the biodistribution, dosimetry, and safety of the HER2-specific ^99mTc-ADAPT6. Methods: Twenty-nine patients with primary breast cancer were included. In 22 patients with HER2-positive (n = 11) or HER2-negative (n = 11) histopathology, an intravenous injection of 385 ± 125 MBq of ^99mTc-ADAPT6 was performed, randomized to an injected protein mass of either 500 μg (n = 11) or 1,000 μg (n = 11). Planar scintigraphy followed by SPECT imaging was performed after 2, 4, 6, and 24 h. An additional cohort (n = 7) was injected with 165 ± 29 MBq (injected protein mass, 250 μg), and imaging was performed after 2 h only. Results: Injections of ^99mTc-ADAPT6 were well tolerated at all mass levels and not associated with adverse effects. ^99mTc-ADAPT6 cleared rapidly from the blood and most other tissues. The normal organs with the highest accumulation were the kidney, liver, and lung. Effective doses were 0.009 ± 0.002 and 0.010 ± 0.003 mSv/MBq for injected protein masses of 500 and 1,000 μg, respectively. Injection of 500 μg resulted in excellent discrimination between HER2-positive and HER2-negative tumors as early as 2 h after injection (tumor-to-contralateral breast ratio, 37 ± 19 vs. 5 ± 2; P < 0.01). The tumor-to-contralateral breast ratios for HER2-positive tumors were significantly (P < 0.05) higher for an injected mass of 500 μg than for either 250 or 1,000 μg. Conclusion: Injections of ^99mTc-ADAPT6 are safe and associated with low absorbed and effective doses. A protein dose of 500 μg is preferable for discrimination between tumors with high and low expression of HER2. Further studies are justified to evaluate whether ^99mTc-ADAPT6 can be used as an imaging probe to stratify patients for HER2-targeting therapy in areas where PET imaging is not readily available.

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