IN-VITRO AND IN-VIVO STUDIES WITH A SERIES OF HEXAPEPTIDE ENDOTHELIN ANTAGONISTS

The effects of different amino acids incorporated into the 16 and 17 positions of the C-terminal hexapeptide of ET-1 were examined. Structure-activity relationships (SAR) of the ET receptor antagonists PD 142893 [Ac-(D-Dip16-L-Leu17-L-ASp-L-Ile-L-Ile-L-Trp) (D-Dip = 3,3-D-diphenylalanine)] and PD 145065 [Ac-(D-Bhg16-L-Leu17-L-Asp-L-Ile-L-Ile-L-Trp) (D-Bhg = 5H-dibenzyl[a,d]cycloheptene-10,11-dihydro-glycine)] uncovered certain requirements for high potency. The disodium salt of PD 145065 has 4.0 and 15 nM binding affinity (IC50 values) for the ET(A) (rabbit renal artery vascular smooth-muscle cells) and ET(B) receptor (rat cerebellum), respectively. The compound is also an antagonist of ET-1- and SRTX-6c-stimulated vasoconstrictor activity, with pA2 values of 6.9 (rabbit femoral artery, ET(A) assay) and 7.1 (rabbit pulmonary artery, ET(B) assay). The tripeptidic ET(A) antagonist FR 139317 was found to be less active in the rabbit femoral artery, with a pA2 value of 6.0, and inactive in the rabbit pulmonary artery. Substitution of acidic and basic residues at position 17 in PD 142893 and PD 145065 indicates differences in selectivity. Incorporation of bulky non-natural amino acids at position 16 has led to potent nonselective analogues, including Ac-D-Bheg16-L-Leu-L-Asp-L-Ile-L-Ile-L-Trp [D-Bheg (5H-dibenzo[a,d]cycloheptene glycine)]. The in vivo effects of single-bolus doses of selected ET antagonists on depressor and pressor responses to ET-1 in anesthetized ganglion-blocked rats were evaluated. The disodium salt of PD 145065 dose dependently inhibited the depressor and pressor responses induced by ET-1 infusion, whereas PD 142893 inhibited only the depressor component up to 10 mumol/kg. Duration of action studies were carried out with PD 142893, PD 145065, and FR 139317 in conscious, chronically prepared, normotensive rats over 5 days. Five groups of animals were dosed with ET-1 (1.0 nmol/kg) at 5, 20, 60, and 120 min after administration of the antagonist (10 mumol/kg i.v. bolus). The results indicate that FR 139317 has a very short duration of action on the pressor component, whereas PD 142893 and PD 145065 both showed some blockade of the depressor component of the ET challenge at 2 h postdose.