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Simple Summary: Prostate and breast cancers are the most common malignancies. Accurate diagnosis and staging of diseases are important for the prognosis and determination of treatment tactics. The gastrin-releasing peptide receptor is overexpressed in over 80% of estrogen receptor-positive breast cancers and in up to 100% of primary prostate cancers, particularly in prostate cancers of lower grades and smaller sizes. Our group has developed an imaging agent [99mTc]Tc-maSSS-PEG2-RM26 suitable for the detection of gastrin-releasing peptide receptors' expression using SPECT. We aimed to perform a first-in-human study to test the safety of [99mTc]Tc-maSSS-PEG2-RM26 administration, to study its biological distribution in normal organs, and to evaluate the agent's targeting of receptors in tumors. This phase I study was performed in six prostate and seven breast cancer patients. Single injections of the new agent were well tolerated and a number of prostate and breast cancer primary tumors as well as metastases were visualized with SPECT/CT shortly after administration. The gastrin-releasing peptide receptor (GRPR) is overexpressed in prostate cancer (PCa) and in hormone-driven breast cancer (BCa). The aim of this phase I clinical trial was to evaluate safety, biodistribution, and dosimetry after the administration of the recently developed GRPR-targeting antagonistic bombesin analogue [99mTc]Tc-maSSS-PEG2-RM26 in PCa and BCa patients. Planar and whole-body SPECT/CT imaging was performed in six PCa patients and seven BCa patients 2, 4, 6, and 24 h post the intravenous administration of 40 µg of [99mTc]Tc-maSSS-PEG2-RM26 (600–700 MBq). No adverse events or pathological changes were observed. The rapid blood clearance of [99mTc]Tc-maSSS-PEG2-RM26 was observed with predominantly hepatobiliary excretion. The effective doses were 0.0053 ± 0.0007 for male patients and 0.008 ± 0.003 mSv/MBq for female patients. The accumulation of [99mTc]Tc-maSSS-PEG2-RM26 in tumors was observed in four out of six PCa and in seven out of seven BCa patients. In four BCa patients, a high uptake of the agent into the axillary lymph nodes was detected. Immunohistochemistry revealed positive GRPR expression in 60% of primary PCa, 71.4% of BCa tumors, and 50% of examined BCa lymph nodes. In conclusion, a single administration of [99mTc]Tc-maSSS-PEG2-RM26 was safe and well tolerated. [99mTc]Tc-maSSS-PEG2-RM26 SPECT may be useful for tumor detection in PCa and BCa patients, pending further studies. [ABSTRACT FROM AUTHOR]