Introducing Radioligand Equivalent Terminology

Dr. Hendifar explains the rationale for proposing the term radioligand equivalent (RLE). The existing medical nomenclature framework classifies drugs as either new products or generics, leaving no accurate category for 505(b)(2)-approved radioligand therapies, which are neither novel standalone products nor duplicate generics. The RLE terminology fills this gap by reflecting the nature of these products: similar but not identical to the reference, approved using existing safety data supplemented by independent bridging studies, and requiring their own FDA label review. The term was formally proposed and published in the Journal of Nuclear Medicine and is available open-access.

Dr. Soares provides a concrete example of how an RLE may differ from its reference radioligand therapy while remaining clinically interchangeable. A product using the same somatostatin receptor-targeting ligand, chelator, and radionuclide as the reference therapy might differ in radionuclide production method; for example, carrier-added versus non-carrier-added lutetium-177. Dr. El-Haddad explains this distinction: carrier-added products contain trace amounts of non-radioactive lutetium isotopes introduced during production, while non-carrier-added products are more radiochemically pure. The clinical impact of this difference is negligible. Regulatory specifications limit impurities to amounts with no meaningful patient effect but waste disposal requirements may differ between products, with carrier-added material potentially requiring different storage or disposal protocols. Animal study waste management may be particularly affected.

Dr. El-Haddad clarifies that RLE terminology is designed to complement existing FDA regulatory language, using terms like pharmaceutical equivalent, therapeutic equivalent, and bioequivalent already have precise FDA definitions that should remain unchanged. RLE does not propose a new regulatory category; it provides a practical clinical lexicon allowing multidisciplinary teams to quickly recognize that a given radioligand therapy was approved through the 505(b)(2) pathway and understand the nature of the supporting evidence.