Development of radioimmunoassay for a potent luteinizing hormone-releasing hormone antagonist. Evaluation of serum levels after injection of [Ac-3-(2-naphthyl)-D-Ala1, D-Phe(pCl)2, 3-(3-pyridyl)-D-Ala3, D-Cit6, D-Ala10] LHRH.

Highly potent antagonistic analogs of luteinizing hormone-releasing hormone (LHRH), free of edematogenic effects have been developed. These analogs proved to be potent inhibitors of LH, follicle stimulating hormone (FSH) and sex steroid levels in animals and human beings. The clinical utility of these compounds would be greatly enhanced by a sustained delivery system, capable of maintaining therapeutic peptide levels in blood over an extended period of time. Consequently, long acting formulations of microcapsules were prepared from one of the most potent antagonists, [Ac-3-(2-naphthyl)-D-Ala1, D-Phe (pCl)2, 3-(3-pyridyl)-D-Ala3, D-Cit6, D-Ala10] LHRH (SB-75). The microcapsules consisted of 2% w/w SB-75 in poly-DL-lactide-co-glycolide (PLGA), a biocompatible, biodegradable polymer. To facilitate pharmacokinetic studies necessary for experimental and clinical investigation of the microencapsulated analog, a highly sensitive and specific radioimmunoassay was developed. The antibody against SB-75 was generated in rabbits. No significant cross-reaction could be detected with several natural peptides and analogs tested. The sensitivity of the assay is 0.6 pg/tube. The RIA is suitable for direct determination of SB-75 level in 20 microliters serum. The two lots of SB-75 microcapsules exhibited different pharmacokinetic release patterns. Single intramuscular injection of 20 mg SB-75 microcapsules, PLGA batch No. 001, into female rats maintained elevated serum SB-75 levels for three weeks. The suppression of LH secretion during this period was indicated by histological findings. The ovaries in the treated group were polyfollicular and no corpora lutea were present, indicating a prolonged ovarian inactivity due to LH deprivation.(ABSTRACT TRUNCATED AT 250 WORDS)