EXAMINATION OF METHODS FOR THE PREPARATION OF BIOLOGICALLY ACTIVE RADIOLABELED MELANOTROPINS.

Abstract

Alpha-melanotropin (alpha-melanocyte stimulating hormone, α-MSH) exerts its biological action by binding to specific receptors on the outer cell membranes of its target tissues with a high degree of affinity and specificity. Current evidence suggests that this takes place both in vitro and in vivo in both normal and malignant melanocytes. Thus, if it were possible to attach a radioisotope (e.g., ¹²⁵I) to α-MSH, or a suitable analogue, without interfering with the receptor affinity of the hormone, then a radioreceptor assay could be developed which would allow hormone-receptor interaction to be studied in detail. In addition, this radio-labeled melanotropin might be expected to accumulate in melanoma tumors in vivo thus facilitating tumor localization by nuclear imaging methods as has been successfully accomplished for thyroid tumors. 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. This loss of hormonal activity was primarily a consequence of oxidation of methinonine and occurred regardless of the oxidant used (chloramine T, lactoperoxidase-hydrogen perioxide, or iodogen). Under similar iodination conditions using 4-norleucine-alpha-melanotropin ([Nle⁴]-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 [Nle⁴]-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). This importance of these studies in the development of a tracer for a radio-receptor assay and for in vivo localization of melanoma tumors is discussed.