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.

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Thesis (MMed)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: Background Occult blood loss from the gastrointestinal tract (GIT), causing iron deficiency often with anaemia, can be diagnostically and therapeutically challenging. This is because the endoscopic and radiologic tests may be negative due to the slow, chronic and intermittent nature of the gastrointestinal bleeding, making timing key in detection and localisation of the bleed. These limitations can be approached using two different radioactive isotopes. Firstly, we tested the sensitivity of extending Cr-51 RBC for 21 days relative to 5 days to detect GIT bleeding and its use to optimise timing of a Tc-99m RBC study for GIT blood loss localisation. Finally, we tested if the information provided by the Tc-99m RBC study aided gastroenterologic intervention for anatomical localisation of a lesion. Method In this retrospective review, after obtaining institutional and ethics committee approval, records of patients referred for evaluation of possible GIT blood loss were reviewed. In each; daily appearance of radiochromium in stool was measured in the whole body counter. In those cases exceeding 50 ml/day, a technetium-99m (Tc-99m) localization study was performed. These studies were correlated with clinical findings. Results A total of 59 Cr-51 RBC studies were carried out in 36 females and 21 males (n = 57). In 32 (54%) the radiochromium results were positive with 75% of the bleeding incidences occurring after 5 days of stool collection. Of 17 cases in whom Tc-99m RBC imaging studies were performed, 14 (82%) were positive with specific anatomical sites successfully defined in twelve. In all patients with blood loss of >100 ml/24h, Tc-99m RBC were positive and localised. Ten of the 17 Tc-99m RBC studies were further investigated and half diagnosed with small-bowel angiodysplasia. Conclusion This sequential twin isotope method is practical in revealing otherwise silent intestinal haemorrhage. Although it has good patient acceptability and clinical as well as diagnostic utility in management, further studies are required to clearly establish a cut-off level of blood loss for performing imaging studies and the impact of the findings on the overall patient management. / AFRIKAANSE OPSOMMING: Agtergrond Die evaluasie van okkulte bloedverlies uit die gastro-intestinale kanaal (GIT), met gevolglike ystertekort anemie, kan diagnosties en terapeuties uitdagend wees. Dit is omdat endoskopiese en radiologiese ondersoeke negatief mag wees as gevolg van die stadige, chroniese en intermitterende aard van die gastro-intestinale bloeding, wat die presiese tydstip van opsporing en lokalisering van die bloeding krities belangrik maak. Hierdie beperkings kan aangespreek word deur twee verskillende radioaktiewe isotope te gebruik. Eerstens is die sensitiwiteit van die verlenging van die Cr-51 RBS studie tot 21 dae in plaas van 5 dae om die GIT bloeding op te spoor, getoets, asook die gebruik daarvan om die optimale tyd vir ‘n Tc-99m RBS studie om die GIT bloedverlies te lokaliseer, vas te stel. Laastens is getoets of die inligting van die Tc-99m RBS studie wel bygedra het tot die gastroenterologiese ingreep om die letsel anatomies te lokaliseer. Metode Na institusionele en etiese komitee toestemming is inligting van pasiënte wat vir die evaluering van ‘n moontlike GI bloedverlies verwys is, in hierdie retrospektiewe oorsig nagegaan. Die daaglikse voorkoms van radioaktiewe chroom in stoelgangmonsters is in ‘n heelliggaamteller gemeet. In gevalle waar dit 50 ml/dag oorskry het, is ‘n tegnesium 99m (Tc 99m) studie gedoen. Hierdie studies is met die kliniese bevindinge gekorreleer. Resultate ‘n Totaal van 59 Cr-51 RBS studies is in 36 vroue en 21 mans (n = 57) gedoen. Die gemerkte chroomstudies was positief in 32 (54%), met 75% van die bloedings wat meer as 5 dae na versameling van die stoelgang plaasgevind het. In veertien (82%) van die 17 gevalle waar Tc-99m RBS studies gedoen is, was die studies positief. Spesifieke anatomiese gebiede van bloeding kon in 12 hiervan suksesvol bevestig word. Tc-99m RBS studies was positief in al die pasiënte met ‘n bloedverlies van >100 ml/24h, en kon gelokaliseer word. Tien van die 17 Tc-99m RBS studies is verder ondersoek en die helfte daarvan gediagnoseer met dunderm angiodisplasie. Gevolgtrekking Die opeenvolgende twee isotoopmetode om andersins asimptomatiese dermbloeding op te spoor, is prakties uitvoerbaar. Alhoewel die studies goed deur pasiënte aanvaar is, en ook van kliniese en diagnostiese waarde in die hantering van die pasiënte is, is verdere studies nodig om die afsnypunt vir die hoeveelheid bloedverlies om beeldingstudies uit te voer, sonder twyfel vas te stel, asook om die impak van die bevindings op ‘n groter pasiëntpopulasie vas te stel.