[Downregulation of LH in the bitch after application of the GnRH-agonist buserelin as a slow-release implant].

OBJECTIVES: Demonstration of downregulation of luteinizing hormone (LH) using the gonadotropin-releasing hormone (GnRH)-agonist buserelin as the active ingredient in the form of a slow-release implant.

MATERIAL AND METHODS: To eliminate any negative feedback mechanisms of endogenous sex steroids, nine bitches were ovariohysterectomized prior to treatment. The applied drug was the slow-release implant Profact Depot(®) with the active ingredient buserelin; dosages were 3.3, 6.6 or 13.2 mg per dog (n = 3 per group). LH bioavailability was assessed in single blood samples and in sequential blood samples collected over a 6-hour time windows, which allowed determination of the AUC (area under the curve), basal concentration, number of pulses and maximal pulse-amplitude.

RESULTS: No dose dependency was observed, leading to the conclusion that maximum efficiency could already be achieved using the lowest dose of 3.3 mg. Therefore, for further evaluation, the dogs were combined as a single group. An increasing downregulatory effect was observed from weeks 2 to 26, with the pharmacodynamic activity lasting approximately 34 weeks. There was a significant increase 1 hour after implantation to almost twice the pre-treatment value; elevated but continuously declining LH concentrations lasted a further 8 hours.

CONCLUSION: Similar as in male dogs, in the bitch buserelin in the form of a slow-release implant also led to downregulation of LH secretion with a preceding initial increase lasting for several hours. This increase must be seen in relation to the unwanted side effects when using this type of drug for downregulation of ovarian function in the bitch.

CLINICAL RELEVANCE: Inhibition of the initial LH increase appears to be an important factor to allow for routine clinical use of slow-release GnRH-agonist implants in the bitch.