LHRH antagonists

After almost two decades, the research on LHRH antagonists has produced a number of decapeptides that are currently in clinical studies. The structures of these antagonists, unlike the agonists, differ substantially from that of LHRH. Five of the ten amino acids are unnatural and of D configuration. The structural combination of a hydrophobic N-terminus (residues 1, 2, and 3) and a basic/hydrophilic C-terminus (residues 6 and 8) was thought to be responsible for some HR reactions encountered with the second generation of LHRH antagonists. This side effect was greatly reduced by substituting the appropriate combination of amino acids at positions 5, 6, and 8. The next hurdle in the drug development of LHRH antagonists was solubility and aggregation. In the case of A-75998, water solubility was improved by 12- to 25-fold via substitution of NMeTyr at position 5. However, based on DLS analysis, the aqueous solutions still contained some large aggregates that were not visible to the naked eye. This formation of aggregates was eliminated on formulating A-75998 in Encapsin. In men, a single s.c. dose of 2 mg of A-75998 suppressed T to the castrate levels for over 30 hr. Other LHRH antagonists including ganirelix and cetrorelix are also in phase I/II clinical studies. Clinical studies with cetrorelix in prostate cancer; in vitro fertilization, and benign prostate hypotrophy have been reported.