A novel, mild, specific and indirect maleimido‐based radioiodolabeling method: Radiolabeling of analogs derived from parathyroid hormone (PTH) and PTH‐related protein (PTHrP)

In an effort to design a mild, non‐oxidative and site‐specific means of radiolabeling bioactive molecules we have employed maleimido‐sulfhydryl chemistry to produce bioactive hormone radioligands. We have prepared two novel radioiodolabeled reagents, 3′‐maleimidopropanoyl‐3‐ ¹²⁵ I‐tyramide and its retro analog, N‐maleoyl‐N′‐3‐(4‐hydroxy‐3‐ ¹²⁵ I‐phenyl)propanoy1 ethylenediamide, by either oxidative radioiodination of the precursors or radiolabeling of the phenolic component prior to its incorporation into the radiolabeling reagents. These reagents were then used to radiolabel analogs of parathyroid hormone (PTH) and parathyroid hormone‐related protein (PTHrP) in an efficient way, yielding reaction mixtures which were easily purified. The radioligands obtained are stable upon storage and bind in a reversible manner to a single population of binding sites displaying affinity in the low nanomolar range. The potencies of these analogs are comparable to the non‐modified PTH and PTHrP analogs. This study demonstrates the utility of the novel maleimido‐based indirect radioiodination approach and highlights some of its advantages over either direct oxidative procedures or acylation using the Bolton‐Hunter reagent.