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

Cyclooxygenase-2 inhibition and thromboxane A(2) receptor antagonism attenuate hypoxic pulmonary vasoconstriction in a porcine model

AIM

Hypoxic pulmonary vasoconstriction (HPV) causes pulmonary hypertension that may lead to right heart failure. We hypothesized that the COX-2 inhibitor nimesulide and the thromboxane A(2) receptor antagonist daltroban would attenuate HPV.

METHODS

Haemodynamic measurements and blood sampling were performed in 18 anaesthetized, mechanically ventilated pigs, with mean ± SEM weights of 31.3 ± 0.6 kg, in normoxia (F(i)O(2)~0.21) and hypoxia (F(i)O(2)~0.10), before and 5, 15 and 45 min after initiation of right atrial infusion of nimesulide (n = 6) or daltroban (n = 6), respectively, and in six control pigs.

RESULTS

Compared with normoxia, hypoxia (n = 18) increased mean pulmonary artery pressure by 15.8 ± 0.8 mmHg (P < 0.001), pulmonary vascular resistance (PVR) by 2.7 ± 0.3 WU (P < 0.05) and mean right atrial pressure by 2.3 ± 0.3 mmHg (P < 0.001). In the control pigs, mean pulmonary artery pressure, PVR and mean right atrial pressure remained stable (P = ns) throughout 45 min hypoxia, compared with hypoxia baseline. Nimesulide decreased mean pulmonary artery pressure by 3.7 ± 1.3 mmHg after 45 min (P < 0.013), as well as PVR by 0.8 ± 0.2 WU (P < 0.05), levelling off after 15 min. Daltroban transiently increased (P < 0.001) mean pulmonary artery pressure and mean right atrial pressure by 7.2 ± 1.2 and 2.7 ± 0.4 mmHg, respectively, but they returned to hypoxia baseline (P = ns) within 5 min. Daltroban then decreased mean pulmonary artery pressure to after 45 min be 4.2 ± 1.6 mmHg lower (P < 0.005) than at hypoxia baseline.

CONCLUSION

COX-2 inhibition and thromboxane A(2) receptor antagonism attenuate HPV by decreasing mean pulmonary artery pressure by approximately 10-11%, as measured 45 min after initiation of nimesulide or daltroban infusion respectively.

Erucamide as a modulator of water balance: new function of a fatty acid amide

The aim of this study was to isolate a compound from blood plasma that inhibits intestinal diarrhea and that appears also to regulate fluid volumes in other organs. The isolation procedure included lipid extraction, liquid chromatography, and gas chromatography. The active substance was identified by mass spectrometry as erucamide (MW 337 Da). The biological effect was reproduced with authentic erucamide. Erucamide is a fatty acid amide, such as oleamide and anandamide, which modulate other physiological functions in a receptor-mediated fashion. All the exact biological functions of erucamide are as yet to be defined, but it is already known to stimulate angiogenesis. Erucamide concentrations were determined in body organs from the pig. The blood plasma level was 3 ng/g, and those of lung, kidney, liver, and brain were 12, 2.5, 1.0, and 0.5 ng/g, respectively. Erucamide was below detection level in the intestine, but is known to be present in the cerebrospinal fluid. In the rat, 3H-erucamide was accumulated in vivo into lung, liver, and spleen and in vitro into lung, liver, brain, and intestine. The in vitro uptake was time and temperature dependent, but not saturable.

Antiplatelet and anticoagulant effects of "HN-11 500," a selective thromboxane receptor antagonist

The antiplatelet and anticoagulant effect of a thromboxane receptor (TX receptor) antagonist developed by Nycomed (Linz) has been studied in a placebo-controlled double-blind phase I study. Sixteen healthy male volunteers received different single oral doses of "HN-11 500" (C(14)H(15)NO(5)S(2); 1, 10, 100, 200, and 400 mg). Eight volunteers received placebo. The washout period between each dosage applied was at least 12 days. Platelet aggregation induced by the thromboxane mimetic "U 46 619" (C(21)H(34)0(4)) and platelet adhesion to siliconized glass were significantly and dose-dependently inhibited. The effect lasted between 3 and 4 h (10 mg) and 8 h (400 mg), respectively, and correlated well with the pharmacokinetic data. Platelet aggregation seems to be more sensitive to monitor the effects of HN-11 500 on platelet function than platelet adhesion. Plasma levels of 300 ng/ml HN-11 500 probably leads to >90% inhibition of platelet aggregation. The template bleeding time slightly increased but did not exceed the normal range. Furthermore, there was a wide variation of results. There were no significant changes in platelet counts, platelet-induced thrombin generation time (PITT), and blood coagulation parameters. All doses of HN-11 500 were well tolerated. HN-11 500 is a potent TX receptor antagonist (TXRA), which inhibits either platelet aggregation or platelet adhesion, which has not yet been described. In clinical routine, TXRAs have to demonstrate the effectiveness in large clinical trials for different clinical indications and to compete with single or combined administrations of cyclooxygenase (COX) inhibitors, thienovridines, thromboxane synthase inhibitors, and GIIb/IIIa inhibitors.

Analysis of the pulmonary hypertensive effects of the isoprostane derivative, 8-iso-PGF2alpha, in the rat

1. We analysed the pulmonary hypertensive effects of the F2-isoprostane derivative, 8-iso-prostaglandin F2alpha (8-iso-PGF2alpha), in comparison with those of the high efficacy thromboxane A2/prostanoid (TP) receptor agonist, U-46619, in pentobarbitone-anaesthetized, open-chest rats (n=4-15 per group). 2. 8-iso-PGF2alpha produced dose-dependent increases in mean pulmonary arterial pressure, with an ED50 of 39.0 (31.4-50.6) microg kg(-1), i.v. (geometric mean with 95% confidence limits in parentheses) compared to 1.4 (1.1-2.3) microg kg(-1), i.v., for U-46619. The maximum responses evoked by U-46619 and 8-iso-PGF2alpha were not statistically significantly different (21.0+/-1.0 and 25.8+/-1.9 mmHg at 10 microg kg(-1) of U-46619 and 630 microg kg(-1) of 8-iso-PGF2alpha, respectively). 3. The TP receptor antagonist, SQ 29,548 (0.63 mg kg(-1), i.v. + 0.63 mg kg(-1) h(-1)) fully antagonised both U-46619 and 8-iso-PGF2alpha-induced pulmonary hypertensive responses. 4. Further experiments were carried out to determine whether 8-iso-PGF2alpha antagonized the pulmonary hypertensive responses evoked by U-46619, or those induced by itself, as would be predicted for a partial agonist. However, ED10 or ED25 doses of 8-iso-PGF2alpha (10 or 20 microg kg(-1), i.v.) failed to reduce the pulmonary hypertensive responses induced either by U-46619 or by itself. 5. The data suggest that in the pulmonary vascular bed of the rat, 8-iso-PGF2alpha acts as an agonist of high intrinsic activity at SQ 29,548-sensitive (probably TP) receptors.

Nitric oxide regulation of TP receptor-mediated pulmonary vasoconstriction in the anesthetized, open-chest rat

We investigated the influence of endothelial nitric oxide (NO) on the pulmonary pressor activity of the stable thromboxane A2 analogue, U-46619 (9,11-dideoxy-9 alpha-(methanoepoxy) prostaglandin F2 alpha), in anesthetized open-chest Sprague-Dawley rats (n = 6-9 per group). NO synthase inhibition, as obtained by N omega-nitro-L-arginine methyl ester (L-NAME; 0.63 mg/kg i.v. + 20 mg/kg/h), induced sustained systemic hypertension (mean maximal increase, delta, in mean systemic arterial pressure = 38 +/- 6 mmHg; P < 0.05 vs. vehicle) associated with slight bradycardia (delta heart rate = -42 +/- 8 beats/min; P < 0.05 vs. vehicle) and delayed- (> 30 min) onset pulmonary hypertension (delta mean pulmonary arterial pressure = 10 +/- 3.4 mmHg; P < 0.05 vs. vehicle). In separate experiments, when mean systemic arterial pressure was maximally increased by L-NAME, the difference between mean pulmonary arterial pressure and mean left atrial pressure was greater in L-NAME-treated rats (41 +/- 16% compared to 10 +/- 1% in the vehicle group; P < 0.05), strongly suggesting that spontaneously released NO modulated pulmonary vascular resistance. L-Arginine at a dose which reduced by approximately 50% the L-NAME-associated systemic hypertension did not alter the late rise in mean pulmonary arterial pressure (delta mean pulmonary arterial pressure = 12 +/- 4 mmHg; P = NS vs. L-NAME alone). U-46619, elicited rapid, dose-dependent, and transient increases in mean pulmonary arterial pressure (delta = 8.8 +/- 2.0 and 21.2 +/- 1.9 mmHg at 1.25 and 20 micrograms/kg i.v. respectively; both P < 0.01 vs. vehicle). U-46619 (1.25 micrograms/kg)-induced increases in mean pulmonary arterial pressure were fully antagonized by the thromboxane A2/prostanoid (TP) receptor antagonist, SQ 29,548 ([1S-[1 alpha,2 alpha(5Z),3 alpha,4 alpha]]-7-[3-[[2-[(phenyl-amino)-carbonyl] hydrazino] methyl]-7-oxabicyclo [2.2.1]hept-2-yl]-5-heptenoic acid) (0.63 mg/kg i.v. + 0.63 mg/kg/h). Injection of U-46619 (1.25 micrograms/kg), 15 min after L-NAME administration, evoked a 24.7 +/- 0.9 mmHg increase in mean pulmonary arterial pressure (P < 0.01 vs. U-46619 in control rats), which was (i) greater than that produced by a 16-fold higher dose of U-46619 alone, (ii) fully antagonized by SQ 29,548, (iii) significantly attenuated during coadministration of L-NAME and L-arginine (10 mg/kg i.v. + 160 mg/kg/h; delta mean pulmonary arterial pressure = 14.6 +/- 4.3 mmHg; P < 0.05 vs. U-46619 following L-NAME alone and P = NS vs. U-46619 in control rats). These results indicate that, under normal circumstances, pulmonary vasomotor tone is regulated by spontaneously released NO. Moreover, pulmonary vascular NO attenuates TP receptor-mediated pressor responses, strongly suggesting that in addition to mediating pulmonary vasoconstriction, TP receptor activation also concomitantly releases NO within the pulmonary vasculature.