Modifications to the N-terminus but not the C-terminus of calcitonin gene-related peptide(8-37) produce antagonists with increased affinity.
Seventeen novel analogues of human calcitonin gene-related peptide(8-37) (hCGRP(8-37)) were synthesized by solid-phase methods and purified to apparent homogeneity by semipreparative cation exchange and/or reversed-phase high-performance liquid chromatography. The C-terminal Phe was replaced by Gly, cyclohexylalanine (Cha), Tyr, all four isomers of beta-methylphenylalanine (beta-MePhe), and l- and d-tetrahydroisoquinoline carboxylic acid (Tic), resulting in analogues 3-11. For the synthesis of the beta-MePhe-containing analogues 6-9, crystallization was used to separate a mixture of all four isomers of beta-MePhe into the erythro pair of enantiomers (2S,3S, 2R,3R) and the threo pair of enantiomers (2S,3R, 2R,3S), which were then converted to Fmoc derivatives and used in two separate syntheses. Two diastereomeric peptides were obtained from each synthesis and were separated by RP-HPLC to yield enantiomerically pure 6-9. Substitution of Tyr for Phe caused no change in binding affinity at CGRP receptors. All other substitutions for Phe resulted in substantial reductions in binding affinity. Indeed, no binding was observed for analogues 7, 9, and 11, all of which contained a d-amino acid residue in the C-terminal position, and the binding affinities of the remaining analogues were >10-fold lower than that of h-alpha-CGRP(8-37). These data suggest that a conformationally flexible phenyl ring in the C-terminal position of h-alpha-CGRP(8-37) is preferred for high-affinity binding to CGRP receptors. Acetylation, benzoylation, and benzylation of the N-termini of h-alpha-CGRP(8-37) and h-beta-CGRP(8-37) produced analogues 12-14 and 16-18, respectively. A byproduct was isolated by RP-HPLC from the resin-cleaved crude product of each benzylated analogue, which was characterized as the dibenzylated derivative of h-alpha-CGRP(8-37) and h-beta-CGRP(8-37) (analogues 15 and 19, respectively). Amino acid analysis and (1)H NMR showed that the second benzyl group was located on the C4 carbon of the imidazole ring of His(10). Radioligand binding experiments showed that derivatizing the N-termini substantially increased binding affinities at CGRP receptors. The benzoylated and dibenzylated derivatives had the highest affinities, which were approximately 50-fold greater than those of h-alpha-CGRP(8-37). Functional experiments confirmed that the N-terminally derivatized analogues of h-alpha-CGRP(8-37) are antagonists that are more potent than h-alpha-CGRP(8-37). In conclusion, these studies underscore the importance of Phe(37) of h-alpha-CGRP(8-37) for binding to CGRP receptors and have identified the N-terminus and His(10) as two positions that can be used for the design of antagonists with increased affinity for CGRP receptors.
Smith
D David
DD
Department of Biomedical Sciences, Creighton University, 2500 California Plaza, Omaha, Nebraska 68178, USA. dsmith@creighton.edu
Saha
Shankar
S
Fang
Guoyong
G
Schaffert
Courtney
C
Waugh
David J J
DJ
Zeng
Wanyun
W
Toth
Geza
G
Hulce
Martin
M
Abel
Peter W
PW
D000818:Animals; D015740:Calcitonin Gene-Related Peptide / Q000138:chemical synthesis* / Q000737:chemistry / Q000494:pharmacology; D000077221:Calcitonin Gene-Related Peptide Receptor Antagonists; D003331:Coronary Vessels / Q000378:metabolism / Q000502:physiology; D006801:Humans; D066298:In Vitro Techniques; D008566:Membranes; D009126:Muscle Relaxation / Q000187:drug effects; D009131:Muscle, Smooth, Vascular / Q000187:drug effects / Q000502:physiology; D010446:Peptide Fragments / Q000138:chemical synthesis* / Q000737:chemistry / Q000494:pharmacology; D011869:Radioligand Assay; D018015:Receptors, Calcitonin Gene-Related Peptide / Q000819:agonists; D021241:Spectrometry, Mass, Electrospray Ionization; D013237:Stereoisomerism; D013329:Structure-Activity Relationship; D013552:Swine
D000077221:Calcitonin Gene-Related Peptide Receptor Antagonists; D010446:Peptide Fragments; D018015:Receptors, Calcitonin Gene-Related Peptide; C058636:calcitonin gene-related peptide (8-37); D015740:Calcitonin Gene-Related Peptide