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Potent and selective inhibitors of thrombin were sought based on the (R)-Phe-Pro-Arg sequence. The objective was to generate similar binding interactions to those achieved by potent competitive inhibitors of the argatroban type, so eliminating the need for covalent interaction with the catalytic serine function, as utilized by aldehyde and boronic acid type inhibitors. Improving the S(1) subsite interaction by substitution of arginine with a 4-alkoxybenzamidine residue provided potent lead 2 (K(i) = 0.37 nM). Though an amide bond, which H-bonds to the active site, is lost, modeling indicated that a new H-bond is generated between the alkoxy oxygen atom and the catalytic Ser-195 hydroxyl group. Substitution of the benzamidine system by 1-amidinopiperidine then gave compound 4, which provided a further gain in selectivity over trypsin. However, previous work had shown that these compounds were likely to be too lipophilic (Log D +0.4 and +0.2, respectively) and to suffer rapid hepatic extraction, presumably via biliary elimination. Accordingly, both proved short-acting when administered intravenously to rats and showed poor activity when given intraduodenally. The aim was then to reduce lipophilicity below a log D of -1.2, which in a previously reported series had been effective in preventing rapid clearance. It was anticipated that compounds of this type would rely on the cation selective paracellular route of absorption from the gastrointestinal tract. Potent polar analogues with selectivity >1000 over trypsin were obtained. The best in vivo activity was shown by compound 12. However, in the final analysis, its oral bioavilability proved poor, relative to analogues with similar physicochemical properties derived from argatroban, consistent with the hypothesis that molecular shape is an additional important determinant of paracellular absorption.

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The regional hemodynamic responses to continuous 4-day infusion of ,7-dihydro-7-oxo-2-(2-pyridylmethyl)-2H-pyrazolo [4,3-d]pyrimidin-5-yl]-2-(2-methoxyethoxy)-5-pyridylsulphonyl}piperazine] (266 g kg Ϫ1 h Ϫ1 ) alone and in combination with a low dose of enalapril (10 g kg Ϫ1 h Ϫ1 ) were measured in conscious spontaneously hypertensive rats to test the hypothesis that the renin-angiotensin system may influence the cardiovascular consequences of inhibition of phosphodiesterase 5 (PDE5) by UK-357,903 or vice versa. UK-357,903 alone caused a fall in mean blood pressure (Ϫ12.1 mm Hg) associated with vasodilatation in the mesenteric and hindquarters vascular beds. The only way in which the effects of enalapril given alone differed significantly from those of the vehicle was in causing mesenteric vasodilatation, which developed over the 4 days of infusion. UK-357,903 given in combination with enalapril caused hypotension (Ϫ17.8 mm Hg) and vasodilatation in the renal, mesenteric, and hindquarter vascular beds. There was evidence of a significant interaction between the effects of the two compounds on renal Doppler shift and vascular conductance with the combined action of the two compounds being greater than the sum of their individual effects. However, although there was a trend for the combination to have greater effects than either of the individual agents on blood pressure and mesenteric vascular conductance, there was no statistical evidence of an interaction. The results indicate that inhibition of the renin-angiotensin system uncovers additional renal vasodilator effects of UK-357,903, and/or inhibition of PDE5 enhances the renal vasodilator effects of angiotensin-converting enzyme inhibition.The effectiveness of antihypertensive therapy is influenced by the high degree of redundancy in cardiovascular control mechanisms, such that pharmacological interference with any particular system may have less functional effect than expected due to compensation by other systems. In this context, we recently described the cardiovascular effects of a 4-day continuous infusion of the phosphodiesterase 5 (PDE5) inhibitor pyrimidin-5-yl]-2-(2-methoxyethoxy)-5-pyridylsulphonyl}-piperazine] in conscious spontaneously hypertensive rats (SHR) (Gardiner et al., 2004) and showed that initially (day 1) UK-357,903 caused modest hypotension together with mesenteric and hindquarters vasodilatation but no significant renal vasodilatation. On the subsequent days of the experiment, the significant effects waned possibly due to activation of vasoconstrictor compensatory mechanisms one of which could have been the renin-angiotensin system (Gardiner et al., 2004). Consistent with this hypothesis, there are reports that PDE5 inhibition in man may lead to activation of the sympathetic nervous system (Phillips et al., 2000) and of the renin-angiotensin system (Chiu and Reid, 2002), although whether or not it is sympathetic activation that drives the renin-angiotensin system, is unknown.In our previous study, we concluded with the hypothesi...

UK-37,248, 4-(2-(lH-imidazol-l-yl)ethoxy)benzoic acid hydrochloride, potently inhibited human blood platelet microsomal thronfooxane (Tx) synthetase, IC50=3xl0-9M. TxB2 was quantitated by a specific radioimmunoassay (RIA). In contrast (PG) endoperoxide synthesis by ram seminal vesicle microsomes and prostacylclin (PGI2) synthesis by pig aortic microsomes/were minimally affected by concentrations of UK-37,248 up to 1x10-4M. In the rabbit isolated perfused lung arachidonic acid (AA) metabolites were quantitated by differential bioassay. Concentrations of UK-37,248 from 10-7-10-6M selectively reduced TxA2 production from AA but increased the release of PGI2 and other PGs.The effect of UK-37,248 in the whole animal was studied by estimating TxB2 levels in blood samples removed before and after dosing. The samples were allowed to clot and the serum TxB2 levels were assessed by RIA. In anaesthetised rabbits, 15 minutes after injection of 0.3mg/kg i.v. UK-37,248, serum TxB2 levels were reduced by 75%. In dogs the compound was similarly effective, lmg/kg p.o. inhibiting TxB2 production by 79% two hours after dosing. Aggregation of human platelet-rich plasma in vitro, initiated by threshold collagen, was inhibited by UK-37,248 (IC50=4.8x10-6M). In rabbits, UK-37,248 at 2mg/kg i.v. prevented the mortality due to pulmonary embolism and reduced the associated thrombocytopenia and elevation of plasma TxB2 caused by i.v. AA. In conclusion, UK-37,248 is a selective inhibitor of platelet Tx-synthetase with antiaggregatory and anti-thrombotic activity.