Structure-activity relationship of LHRH antagonists with emphasis on substitution of phenylalanine or its derivatives at positions 2,3,5,6,8

Abstract

It has been 20 years since the chemical structure of hypothalamic luteinizing hormone-releasing hormone (LHRH) was published. In the history of developing highly potent and specific antagonists of LHRH, as many as eight positions in the molecule of the decapeptide have been frequently substituted with unnatural amino acids. Positions 2 and 3 were demonstrated to be the most important positions for the agonistic function of the hormone. Together with the modification in position 6, D-Phe* substitution led to the first antagonist [D-Phe*,D-Ala6]LHRH (I) which showed in vivo antiovulatory activity (AOA) in rats (1). Further development involved the modification in positions 1,3 and 10; the design considerations included the abolishing of the agonistic function, the increasing of the enzyme resistance and the adjusting of the global hydrophilic/hydrophobic property of the molecule. As a result, highly potent LHRH antagonists were discovered; [N-Ac-D-p-Cl-Phe’,D-p-Cl-Phe2,D-3-Trp3,D-Arg3,D-Ala’O]LHRH (II) was a typical example (2). After that, D-2-Nal’ was introduced to balance the hydrophilicity caused by D-Arg6 (3). The introducing of D-3-Pal3 led to even more potent LHRH antagonists, such as [N-AC-D-2-Nal’, D-pClPhe*, D-3-Pa13, D-Arg6, D-Ala’qLHRH (III) which showed 100% antiovulatory activity (AOA) at 0.5 u&at in corn oil (4). one of the most potent antagonists in the class of [D-2-Nal’, D-Arg6] analogs. However, soon after that, a toxicology study showed histamine-related side effects when rats were administered such LHRH antagonists containing D-2-Nal’ and D-Arg6 (5). Therefore, the design turned to reduction of the histamine-releasing activity @IRA). Based on the topological similarity with substance P, a neuropeptide responsible for histamine release in the body, our design was focused on the basic area in the C-terminus and the aromatic acids in the N-terminus. [N-AC-D-2-Nal’, D-pClPhe*, D-3-Pa13, Arg’, D-3-Pa16, D-Ala”]LHRH (V) was chosen as the parent compound because it showed very high AOA and only moderate HRA (6). A series of novel derivatives of D- or L- phenylalanine were synthesized through chloromethylation and amination with secondary amines, and inserted in positions 3,5,6 and 8 of (V). Meanwhile, positions 2 and 3 were also fine-adjusted with D-phenylalanine or its novel