Actions of a novel thromboxane A2-receptor antagonist, S-145, on isolated monkey and cat arteries

Characterization and localization of thromboxane A2 receptor in human and guinea-pig nasal mucosa using radiolabelled (+)-S-145

1. TxA2 receptor (TP-receptor) antagonists such as S-1452 and Bay u 3405 have been shown to be effective in alleviating nasal blockage in patients with allergic rhinitis as well as guinea-pig allergic rhinitis models. The present study was conducted to examine the existence and localization of the TP-receptor in human and guinea-pig nasal mucosa by in vitro receptor binding autoradiography using radiolabelled (+)-S-145, which is a potent and specific TP-receptor antagonist with an extremely slow dissociation rate. 2. We ascertained the binding specificity of [3H]-(+ )-S-145 in human and guinea-pig platelet membranes by comparing the ability of four TP-receptor ligands of U-46619, (+)-S-145, I-(+)-S-145 and Bay u 3405 to displace the specific binding of [3H]-(+)-S-145 and [3H]-U-46619. The rank order of potency (Ki) for the displacement was correlated highly with that determined from [3H]-U-46619 binding to the same preparations. 3. Quantitative autoradiography using a radioluminographic imaging plate system, in which the radioactivity of [3H]-(+)-S-145 is expressed as photostimulated luminescence (PSL) per area (mm2), revealed that specific binding of [3H]-(+)-S-145 to human and guinea-pig nasal mucosa was saturable. Scatchard analysis showed about three fold higher affinity and two fold greater maximal binding to the nasal mucosa of humans than that of guinea-pigs: the KD and Bmax values in human mucosa were 2.82+/-0.35 nM and 6.47+/-0.33 PSL mm(-2) and those in guinea-pig mucosa were 8.23+/-1.93 nM and 3.37+/-0.66 PSL mm(-2), respectively. 4. Specific [3H]-(+)-S-145 binding to cryostat sections of human and guinea-pig nasal mucosa was displaced by another TP-receptor antagonist, Bay u 3405, and a TP-receptor agonist, U-46619. The order of potency (Ki value: nM) was (+)-S-145 (2.5) > Bay u 3405 (15.4) > > U-46619 (359.6) in human nasal mucosa and (+)-S-145 (22.8) > U-46619 (49.8) approximately Bay u 3405 (62.1) in guinea-pig nasal mucosa. These rank orders showed rather good correlation with those obtained for the respective platelet membranes. 5. Autoradiographs of human nasal mucosa demonstrated that specific [125I]-(+)-S-145 binding sites mainly exist on the smooth muscle layers of venous sinusoids and arterioles in the lamina propria, with few or no binding sites in the epithelium and nasal gland. 6. We concluded that radiolabelled (+)-S-145 can be used as a TP-receptor ligand for autoradiographic study, and that the TP-receptor is exclusively located on smooth muscle layers of venous sinusoids and arterioles in the nasal mucosa. The potent vasoconstrictive activity of TxA2 may cause reduction of local blood flow followed by mucosal oedema probably through mechanisms of vascular injury such as ischaemia-reperfusion.

Intrinsic activity of the non-prostanoid thromboxane A2 receptor antagonist, daltroban (BM 13,505), in human platelets in vitro and in the rat vasculature in vivo

1. We evaluated the effects of daltroban on (i) human platelet shape change and aggregation in vitro, and (ii) mean systemic and pulmonary arterial pressures (MAP and MPAP, respectively) as well as haematocrit, in anaesthetized, open-chest Sprague-Dawley rats, compared with those of a chemically distinct prostanoid thromboxane A2 (TxA2) receptor antagonist, SQ 29,548, and agonist, U-46619. 2. In human platelets in vitro, daltroban (10 nM-100 microM; n = 6 per group) concentration-dependently induced shape change, attaining at 50 microM, a maximum amplitude of 0.83 +/- 0.09 mV representing 46.4 +/- 4.8% of that evoked by U-46619 (1.78 +/- 0.20 mV at 0.2 microM; n = 9); and inhibited U-46619-induced platelet aggregation with an IC50 of 77 (41-161)nM. SQ 29,548 (10 nM-100 microM; n = 6 per group) failed to evoke any platelet shape change, but potently inhibited U-46619-induced platelet aggregation with an IC50 < 10 nM. 3. In anaesthetized rats in vivo, daltroban (10-2500 micrograms kg-1, i.v. infused over 2 min; n = 4-8 per group) produced a bell-shaped dose-response curve for MPAP and haematocrit, and evoked maximal increases of 12.7 +/- 2.1 mmHg and 5.8 +/- 1.5% at 80 micrograms kg-1 (n = 6) and 630 micrograms kg-1 (n = 8), respectively (both P < 0.05) with ED50s of 20 (16-29) and 217 (129-331) micrograms kg-1, respectively. By comparison, U-46619(0.16-20 microg kg-1, i.v.), induced dose-dependent increases in MPAP and haematocrit (25.4 +/- 1.0 mmHg and 16.1 +/- 2.9% at the highest dose; n = 12, both P<0.01), with ED50s of 1.8 (1.3-2.5) and 3.9(3.5- 5.4) microg kg- 1, respectively. Daltroban dose-dependently increased MAP with a maximum amplitude of 42.2 +/- 4.4 mmHg at a dose of 80 microg kg-1 [ED50 = 94 (64-125) microg kg-1], similar to that induced by U-46619 (41.3 +/- 9.6 mmHg) at a dose of 0.63 microg kg-1 [ED50= 0.22 (0.13-0.24) microg kg-1]. SQ 29,548(10-2500 microg kg-1, i.v.; n =4 per group) failed to modify significantly any of these parameters.4. Our results clearly demonstrate that daltroban, in a similar manner to the TxA2 analogue, U-46619,but unlike the TxA2 receptor antagonist, SQ 29,548, exhibits significant intrinsic activity in human platelets in vitro and in the rat vasculature in vivo, possibly through TxA2 receptor activation.

Potentiation of norepinephrine-induced contraction by primary prostaglandin receptor activation in rat aorta

This study investigates the role of primary prostaglandin receptor activation in the modulation of agonist-induced vascular smooth muscle contraction. Prostaglandin F2 alpha induced a concentration-dependent contraction of the rat aorta that was nearly abolished by the thromboxane A2 receptor antagonist, SQ29548. Prostaglandin F2 alpha in the presence of SQ29548 induced leftward shifts of the norepinephrine and KCl concentration-response curves. Nifedipine abolished the leftward shift of the norepinephrine concentration-response curve observed in the presence of prostaglandin F2 alpha and SQ29548. These results suggest that a function of primary prostaglandin receptor activation may be to potentiate agonist-induced contraction. The potentiation is dependent upon the opening of dihydropyridine-sensitive Ca2+ channels.

Application of lipid microspheres to prepare a thromboxane A2 receptor antagonist aerosol for inhalation

The methyl ester of a new thromboxane A2 receptor antagonist, (+)S-145, i.e. (1R,2S,3S,4S)-(5Z)-7-(3-phenyl-sulfonyl-aminobicyclo[2,2,1]he pt-2-yl)heptenoic acid, was incorporated into lipid microspheres (lipo S-145-Me) and its pharmacological effect and tissue distribution were examined in guinea pigs following aerosol delivery. Bronchoconstrictive responses induced by intravenous injection of U46619 or the inhalation of ovalbumin were suppressed in a dose-dependent manner by aerosol inhalation of lipo S-145-Me, which was 3-10 times more potent that the unencapsulated calcium dihydrate of the original drug (S-1452). There was no significant difference in the airway tissue distribution of labelled lipo S-145-Me versus S-1452 after 2 or 5 min of inhalation, but the encapsulated drug showed marked accumulation in the lungs after 30 min of inhalation. The in vitro uptake of lipo [14C] S-145-Me by fresh human neutrophils and an eosinophil cell line was respectively 7 times and 3.5 times higher than that of [14C] S-1452. These results suggest that lipo S-145-Me has the potential to be used as an inhalational antiasthma agent, and that its effect may be partly attributable to a for inflammatory cells which are responsible for allergic airway inflammation.

Effect of pancreatic type phospholipase A2 on isolated porcine cerebral arteries via its specific binding sites

The addition of porcine pancreatic group I phospholipase A2 (PLA2-I) produced a transient contraction followed by a relaxation in helical strips of porcine cerebral arteries. Its ED50 value (2.3 nM) was almost identical to the Kd value (3.9 nM) calculated from the specific binding of 125I-labeled porcine PLA2-I in cultured porcine cerebral arterial smooth muscle cells. Type-specific action of PLA2s and homologous desensitization strongly implicated the involvement of PLA2-I-specific sites in the response. The transient contraction was abolished by treatment with indomethacin as well as by the removal of endothelium, indicating the dependence of vasoconstrictor prostaglandins synthesized by PLA2-I in endothelium. The PLA2-I-induced relaxation response was also observed in bovine and cat cerebral arteries, thus providing a new aspect of PLA2-I as a vasoactive substance.

Kinetic studies on stereospecific recognition by the thromboxane A2/prostaglandin H2 receptor of the antagonist, S-145

1. The mechanism for the stereospecific recognition of the antagonist S-145 by the thromboxane A2 (TXA2)/prostaglandin H2 (PGH2) receptor was examined by ligand-binding techniques in rat vascular smooth muscle cells (VSMCs) and in human platelet membranes. 2. Scatchard analysis revealed the existence of a single class of binding sites with the same maximum number for both [3H]-(+)-S-145 and [3H]-(-)-S-145 in both cell types. The dissociation constants (Kd) for the binding of the (+)-isomer in rat VSMCs and human platelet membranes were, respectively, 0.40 +/- 0.03 and 0.20 +/- 0.02 nM, each value being lower than that for the (-)-isomer (3.57 +/- 0.74 and 2.87 +/- 0.08 nM, respectively). 3. The rank orders of potency (Ki) for a series of TXA2/PGH2 ligands at inhibiting [3H]-(+)-S-145 binding were highly correlated with those determined for [3H]-(-)-S-145 binding in both cell preparations. 4. Kinetic analysis of the binding of both radioligands revealed a much lower dissociation rate constant (k-1) and a slightly greater association rate constant (k1) for the (+)-isomer compared to those for the (-)-isomer. 5. These results suggest that it is at the stage of dissociation from the TXA2/PGH2 receptor that the stereochemistry of the optical isomers of S-145 confers their difference in affinity for these receptors in rat VSMCs and human platelet membranes.

Regional differences in the prostanoid receptors mediating prostaglandin F2 alpha-induced contractions of cat isolated arteries

U46619, a stable thromboxane A2 (TXA2) mimetic, and prostaglandin F2 alpha (PGF2 alpha) contracted helical strips of cat coronary, renal and mesenteric arteries in a concentration-dependent manner. The EC50 values for U46619 did not differ significantly in these arteries, but those for PGF2 alpha were in the order of coronary less than renal less than mesenteric arteries. Contractions induced by U46619 were antagonized by S-145, a selective TXA2 receptor antagonist, with similar activity in these arteries. On the other hand, contractions induced by low concentrations of PGF2 alpha (10(-9) to 10(-7) M) were not influenced by treatment with S-145 in coronary arteries, although those induced by high concentrations (5 x 10(-7) to 10(-5) M) were partially attenuated. These contractions resistant to the TXA2 antagonist were antagonized by diphloretin phosphate (DPP), a non-selective PG antagonist. Contractions induced by PGF2 alpha (5 x 10(-7) to 5 x 10(-5) M) in mesenteric arteries were inhibited by S-145 in a concentration-dependent manner. Contractions induced by PGF2 alpha in renal arteries were partially inhibited by S-145. The inhibitory activity of S-145 to PGF2 alpha-induced contractions at EC50 was in the order of coronary less than renal less than mesenteric arteries. Treatment with indomethacin slightly potentiated the contractions induced by PGF2 alpha in mesenteric arteries. Removal of the endothelium did not affect the contractile responses induced by PGF2 alpha and the inhibitory activity of S-145 in the arteries. These results suggest that the contractile responses induced by low concentrations of PGF2 alpha (up to 10(-7) M) are associated with their action via PG receptor(s), which is different from TXA2 receptor, and those induced by high concentrations of PGF2 alpha (5 x 10(-7) M or higher) interact with TXA2 receptors in cat vascular smooth muscles. It appears that the functional expression of this PG receptor(s) is greater in coronary arteries than in renal arteries, and that it is not found in mesenteric arteries.