Heynic acid (6-hydrazinonicotinic acid, HYNIC) is a specialty chemical and bifunctional chelating agent primarily used in radiopharmaceutical research and development. It serves as a key intermediate for labeling biomolecules, such as peptides, antibodies, and proteins, with radioisotopes like ^99mTc and ^186/188Re. This enables the creation of targeted radiotracers for diagnostic imaging (e.g., SPECT) and therapeutic applications in nuclear medicine, including cancer diagnostics and therapy. Its hydrazino group facilitates conjugation to biological vectors, while the nicotinic acid moiety provides stable coordination to metal isotopes. Heynic acid is also employed in biochemical studies for site-specific modification of proteins and in the synthesis of novel imaging agents. As a niche additive in pharmaceutical and biotech industries, it supports precision medicine but is not a commodity chemical.