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We studied the pharmacokinetics of iv and intranasally administered buserelin, a LHRH agonist peptide, in 14 women with endometriosis. Serum and urinary buserelin concentrations were determined by specific RIA (buserelin antiserum AS-639). Intact buserelin and the metabolites in urine were separated by reverse phase high performance liquid chromatography and measured by RIA. The mean serum buserelin concentrations were 101 +/- 33 (+/- SD) ng/mL 20 min and 1.12 +/- 0.12 ng/mL 360 min after its iv injection in 6 women, and the mean elimination half-life between 20 and 360 min was 51 min. In serum, intact buserelin was the main constituent (10 min, 90%; 120 min, 74%; 360 min, 52%), and the major metabolite was the buserelin-(5-9) pentapeptide (10 min, 0.6%; 120 min, 19%; 360 min, 12%). In the urine collected 0-1 h after buserelin administration, intact buserelin was 66% and the 5-9 pentapeptide was 28% of the total excretion. In the urine collected between 6-24 h after buserelin administration, intact buserelin accounted for 67% and the 5-9 pentapeptide for 32% of the total excretion. The urinary buserelin concentration was 1345 +/- 156 micrograms/g creatinine 1 h and 25 +/- 5 micrograms/g creatinine 6-24 h after buserelin administration. Serum LH, FSH, and estradiol concentrations increased acutely up to 10-fold above basal values; the mean peak LH, FSH, and estradiol values occurred at 180-240 min, 240 min, and 24 h, respectively. In therapeutic studies with buserelin nasal spray in 5 women, serum concentrations of 0.9-1.4 ng/mL were found 15 min after a single dose of 300 micrograms, intranasally, and the urinary excretion was 2.52-3.68 micrograms/24 h during daily administration of 3 doses of 300 micrograms at intervals of 8 h. These results confirm that buserelin is slowly inactivated and remains available to pituitary receptors for a prolonged period after its iv or intranasal administration.

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