Sublingual absorption of leuprolide: comparison between human and animal models

Leuprolide is a potent luteinizing hormone releasing hormone agonist used for the treatment of hormone-dependent diseases. It is a decapeptide drug currently administered by subcutaneous and intramuscular injection because it is not orally bioavailable. In the present study, sublingual gel formulations of leuprolide were administered to dogs, monkeys and humans. Plasma samples were analyzed by radioimmunoassay. Absorption and pharmacokinetics of leuprolide following sublingual administration were compared and evaluated. It was found that the extent and rate of absorption were similar between humans and monkeys following sublingual dosing of leuprolide formulations. A prolonged absorption of up to approximately 6 h after dosing was observed in both humans and monkeys. The rate and extent of absorption were significantly higher in dogs than in humans. The estimate of absolute bioavailability of leuprolide was 46.7% in dogs compared with 2.7% in monkeys at an equivalent dose of 0.45 mg/kg. Absolute bioavailabilities in humans were 2.0, 2.3 and 2.4% at doses of 1.125, 2.25 and 4.5 mg, respectively. Based on these results, the dog is not an appropriate animal model for evaluating sublingual absorption of leuprolide. This work suggests that monkey is a preferred model for the development and assessment of sublingual formulations of leuprolide.