A double antibody RIA was developed for the measurement of the long-acting
GnRH agonist D-Ser(TBU)6EA10GnRH (
buserelin). The antibody, raised in rabbits against a
buserelin-
hemocyanin conjugate, reacted with the intact molecule and also molecular fragments containing the C6-9 tetrapeptide sequence and permitted the measurement of
buserelin activity in serum and urine. Natural
GnRH, LH, and FSH did not cross-react in this assay system. The assay was applied to samples obtained from children receiving
buserelin for the management of
central precocious puberty either by once daily injection of 30 micrograms/kg or by
nasal spray (in; 200 micrograms every 8 h). Urine and serum samples, chromatographed on
Sephadex G-25, contained immunoreactive material corresponding closely in molecular size to [125I]
buserelin. In unextracted serum samples taken at intervals after sc
therapy in 11 girls, the peak immunoreactive
buserelin levels of 52.2 +/- 14.8 ng/ml (mean +/- SEM) occurred at 30 min. The half-time of elimination was 74.9 +/- 36.9 min. Approximately 30% of the dose was detected in urine collected for 3 h after injection. Similar data were obtained in 3 normal adults given 10 micrograms/kg
buserelin, iv. By contrast, after the administration of 200 micrograms
buserelin by metered
nasal spray, the mean peak serum concentration in 10 girls was 100-fold less (0.65 +/- 0.14 ng/ml), although the halftime of elimination was almost identical. Only 0.73% of the nasal dose was excreted by 3 h. Calculated relative bioavailability data indicated maximal nasal absorption of 6%. However, absorption after
nasal administration varied greatly, and in 2 children, serum and urinary concentrations of
buserelin after supervised administration were negligible. We conclude that in
buserelin therapy, in the dose used in this study, does not represent optimal treatment for the initial management of patients with
precocious puberty. The success of in
therapy in sustaining initial effects of
buserelin given by sc administration presumably reflects changes in receptor sensitivity induced by sc treatment.