Part II: Influence of dimerization of a modified GnRH-I peptide sequence on a male antifertility vaccine

PROBLEM: In the previous paper, we described how the tetanus toxoid (TT) conjugated monomer, CHWSYGLRPG-NH2, induced high neutralizing antibody titres, which resulted in decreased levels of testosterone and subsequent antifertility. However, its counterpart HWSYGLRPGC, induced low avidity antibody titres. We wanted to know whether peptide dimerization would improve the efficacy of both peptides. METHOD OF STUDY: Male Sprague-Dawley rats were immunized with modified dimerized GnRH-I peptides (HWSYGLRPGCCGPRLGYSWH and GPRLGYSWHCCHWSYGLRPG-NH2),with or without conjugation to TT. RESULTS: The unconjugated dimers were not effective in causing castration, although the first peptide dimer did induce production of antibodies. When conjugated to TT, both dimers showed the same level of efficacy in causing castration as each other. However, there were differences in antibody binding to native GnRH. CONCLUSIONS: Dimerization and conjugation to a carrier improved the antifertility efficacy of HWSYGLRPGC, whereas the conjugated monomer CHWSYGLRPG-NH2 showed a greater level of consistent castration than its conjugated dimer.