Effects of a GnRH cytotoxin on reproductive function in peripubertal male dogs

Methods for long-term or permanent disruption of reproductive function via nonsurgical techniques are needed for a variety of species, including companion animals. In a previous study, we demonstrated the ability of a cytotoxin (pokeweed antiviral protein-PAP) conjugated to d-Lys⁶-GnRH, to disrupt reproductive function in adult male dogs. The objective of the present study was to examine the ability of a d-Lys⁶-GnRH-PAP conjugate to disrupt reproductive function in peripubertal male dogs. Peripubertal male dogs (n = 15; approximately 16-32 weeks old) were treated with d-Lys⁶-GnRH-PAP as follows: dogs (n = 7; Group I) received GnRH-PAP (0.1 mg/kg SQ) with a second treatment (0.25 mg/kg) 20 weeks later. An additional group (n = 3; Group II) of peripubertal dogs was treated with GnRH-PAP (0.25 mg/kg) twice (20 weeks apart). Control dogs (n = 5) received d-Lys⁶-GnRH analog (0.0045 mg/kg SQ) without PAP. Efficacy was assessed by monitoring testis size, serum concentrations of testosterone and LH, as well as LH release subsequent to a GnRH (5 μg/kg) stimulus. Dogs in Group I (n = 5) that did not respond to the initial two treatments were given a third GnRH-PAP injection (0.25 mg/kg), 12 months after the initial treatment. The initial GnRH-PAP treatment in peripubertal male dogs did not affect testis growth, LH release or serum testosterone concentrations; however, administration of a higher dose of GnRH-PAP after puberty resulted in a marked and rapid decline in testis size, serum testosterone concentrations and LH responsiveness to GnRH stimulation in 9 of 10 dogs. Suppression of reproductive function was maintained in treated dogs for 18-50 weeks; four dogs had suppression of reproductive activity through the end of the study. In conclusion, GnRH-PAP given after puberty markedly suppressed reproductive activity. Due to variability in the response and duration of suppression after treatment with GnRH-PAP, more research is required to determine its efficacy for nonsurgical sterilization of the male dog.