Examination of methods for the preparation of biologically active radiolabeled melanotropins

The present studies were initiated to develop a radioactive melanotropin with full, or nearly full, biological activity. This labeled melanotropin must be of sufficient specific radioactivity to be suitable as a tracer in a radioreceptor assay and ultimately as a marker for in vivo tumor localization. The studies described herein provide information concerning: chloramine T induced iodination, lactoperoxidase catelyzed iodination, and iodogen induced iodination of α-MSH and certain structural analogues. Radio-labeled derivatives of various melanotropins were prepared using a variety of iodination techniques. Under conditions commonly used for the iodination of other peptides a substantial loss of biological activity of the native hormone (α-MSH) was observed. Under similar iodination conditions using 4-norleucine-alpha-melanotropin ([NIe4]-α-MSH), satisfactory incorporation of label into the peptide was accomplished without significant loss of biological activity. Data are presented suggesting that this peptide is far superior to α-MSH for use in the preparation of a radioactive melanotropin. Although some success was achieved using [NIe4]-α-MSH with all three iodination methods, the simplest and most consistent method involved the use of iodogen followed by purification of the labeled product using high performance liquid chromatography (HPLC)