Homing and Detection of Unknown Primary Head-Neck Cancer by Acid-Sensing Nanoparticles.

Precise localization of cancer is essential for curative surgery but remains a major clinical challenge when tumors are small or anatomically concealed. While tumor-targeted imaging with nanomaterials has shown promise in preclinical models, mechanistic understanding and clinical translation in cancer patients remain limited. Here, in a prospective Phase 2 clinical trial (NCT05576974), we constructed a single-cell spatial atlas of Pegsitacianine, an acid-sensing nanoparticle with activation pH threshold of 5.3, in human squamous carcinoma of the head and neck. Mechanistically, Pegsitacianine preferentially accumulate in immune-infiltrative severely acidic milieus (iSAM) within tumor stromal regions adjacent to metabolically active cancer cells. Clinically, Pegsitacianine illuminated iSAM regions and achieved fluorescence-guided resection of tumors in 14 of 16 patients with unknown primary cancer, where conventional diagnostic tools failed to locate the tumor. These findings establish the mechanistic link between tumor metabolism, immune infiltration, and nanoparticle delivery, and underscore the clinical value of targeting severe tumor acidosis for cancer detection and therapy.