Gonadotropin-releasing hormone (GnRH) pharmacokinetics: peptide hormone pharmacokinetics needs clarification

The plasma level curves of the peptide hormone gonadotropin-releasing hormone (GnRH) after its intravenous, intramuscular, and intraperitoneal administration into rats were fitted according to a two- (i.v.) and one-compartment model (i.m., i.p.), respectively. From the pharmacokinetic parameters it is concluded that urinary excretion and proteolytic degradation by kidney and liver are not sufficient to fully account for the clearance of the hormone and that, therefore, proteolytic degradation by tissues may play a role for the elimination of GnRH. This may be generally true with other short peptide hormones. The GnRH pharmacokinetics is shown as an example to underline that there presently exist problems of interpreting pharmacokinetic data of peptide hormones and that there is a need for a close interplay between biochemical and pharmacokinetic studies on peptide hormones for their pharmacokinetic behaviour to be understood.