Regulation of fatty acid biosynthesis by intermediates of the cholesterol biosynthetic pathway.

The biosynthesis of cholesterol and fatty acid (FA) proceeds by independent pathways. Information is lacking on potential interaction that could provide feedback regulation between these pathways. In an attempt to search for a new approach to produce a dual effect on regulation of these two processes, we have identified mevalonate-5-diphosphate (MevPP) decarboxylase, an enzyme of the cholesterol biosynthesis pathway, the inhibition of which leads not only to the suppression of cholesterol but also FA biosynthesis. Mechanistic studies with Hep G2 cells using specific inhibitors for MevPP decarboxylase and related enzymes reveal that the inhibitory effect on FA biosynthesis is mainly due to the accumulation of MevPP, resulting from MevPP decarboxylase inhibition. The present study proposes a new mechanism through which interpathway regulation could take place between the cholesterol and FA cascades.

Characterization of CGS 8515 as a selective 5-lipoxygenase (5-LO) inhibitor.

1. CGS 8515 selectively inhibited 5-LO (IC50 = 0.1 microM) with negligible effect on CO, 12-LO, 15-LO and TxS at concentrations up to 100 microM. 2. CGS 8515 selectively inhibited A23187-induced formation of 5-LO products in rat and human whole blood with a 20-70 fold separation of effects over the formation of CO products. 3. Ex vivo and in vivo studies with rats showed that CGS 8515, at an oral dose of 2-50 mg/kg, significantly inhibited A23187-induced formation of LTs in whole blood and in the lung. The effect persisted for at least 6 h in the ex vivo blood model. 4. CGS 8515, at oral doses as low as 5 mg/kg, significantly suppressed exudate volume and leukocyte migration in the carrageenan-induced pleurisy and sponge models in the rat.

Characterization of CGS 8515 as a selective 5-lipoxygenase inhibitor using in vitro and in vivo models.

CGS 8515 inhibited 5-hydroxyeicosatetraenoic acid (5-HETE) and leukotriene B4 synthesis in guinea pig leukocytes (IC50 = 0.1 microM). The compound did not appreciably affect cyclooxygenase (sheep seminal vesicles), 12-lipoxygenase (human platelets), 15-lipoxygenase (human leukocytes) and thromboxane synthetase (human platelets) at concentrations up to 100 microM. CGS 8515 inhibited A23187-induced formation of leukotriene products in whole blood (IC50 values of 0.8 and 4 microM, respectively, for human and rat) and in isolated rat lung (IC50 less than 1 microM) in vitro. The selectivity of the compound as a 5-lipoxygenase inhibitor was confirmed in rat whole blood by the 20-70-fold separation of inhibitory effects on the formation of leukotriene from prostaglandin products. Ex vivo and in vivo studies with rats showed that CGS 8515, at an oral dose of 2-50 mg/kg, significantly inhibited A23187-induced production of leukotrienes in whole blood and in the lung. The effect persisted for at least 6 h in the ex vivo whole blood model. CGS 8515, at oral doses as low as 5 mg/kg, significantly suppressed exudate volume and leukocyte migration in the carrageenan-induced pleurisy and sponge models in the rat. Inhibitory effects of the compound on inflammatory responses and leukotriene production in leukocytes and target organs are important parameters suggestive of its therapeutic potential in asthma, psoriasis and inflammatory conditions.

An alternate mechanism for regulation of leukotriene production in leukocytes: studies with an anti-inflammatory drug, sodium diclofenac.

Sodium diclofenac, a potent cyclooxygenase inhibitor, was recently shown to inhibit arachidonic acid conversion to leukotriene products in human leukocytes. This activity was confirmed by radioimmunoassay in calcium ionophore A 23187-stimulated leukocytes isolated from the rat peritoneal cavity and human peripheral blood. Studies with rat peritoneal leukocytes revealed that this effect was not mediated by inhibition of 5-lipoxygenase or phospholipase A2, but rather through modulation of arachidonic acid uptake and release. The potency of this effect was dependent upon cell type; macrophages being more sensitive to the drug than neutrophils. In leukocytes treated with sodium diclofenac, arachidonic acid released from phospholipids in response to A 23187 challenge was reincorporated into triacylglycerols. The drug enhanced the spontaneous uptake of arachidonic acid into the cellular triacylglycerol pool and, in this manner, decreased the availability of intracellular arachidonic acid. Therefore, sodium diclofenac, in addition to inhibition of cyclooxygenase, regulates leukotriene production of inflammatory cells by a mechanism mediated in part through the redistribution of arachidonic acid in lipid pools.

CGS 15435A, a thromboxane synthetase inhibitor with an extended duration of action: a comparison with dazoxiben.

CGS 15435A, a novel thromboxane (Tx) synthetase inhibitor (5-chloro-1-methyl-2-(3-pyridyl)-3-indolhexanoic acid HCl), had a selectivity for Tx synthetase 100,000-fold greater than that for cyclooxygenase, PGI2 synthetase and lipoxygenase enzymes. In conscious beagles, 1 h following a single 3 mg/kg p.o. dose, serum TxB2 was inhibited 95% by CGS 15435 and 82% by dazoxiben (DAZ). Unlike the short acting Tx synthetase inhibitor DAZ, CGS 15435A significantly inhibited TxB2 formation 4, 6, 12 and 24 h after dosing. Serum levels of 6-keto PGF1 alpha and PGE2 were significantly increased following the administration of either drug. CGS 15435A and DAZ were further examined in a model with known Tx involvement. Thrombotic sudden death, produced in anesthetized rabbits by injection of 0.75 mg/kg arachidonic acid (AA) i.v. resulted in a 45% fall in the platelet count and 0% survival. Pretreatment with DAZ (8.6 mumol/kg i.v.) at 0.25 or 2 h pre-AA resulted in 3 and 42% thrombocytopenia and 100 and 0% survival respectively. CGS 15435A (8.6 mumol/kg i.v.) prevented the increases in plasma TxB2 levels, thrombocytopenia and sudden death with pretreatment at 0.25 h (0% thrombocytopenia and 100% survival) or 24 h (11% thrombocytopenia and 83% survival) before AA. These data indicate that CGS 15435A is a potent and selective Tx synthetase inhibitor with a long duration of action, and suggest that the compound could be useful in chronic, non-symptomatic indications of Tx involvement.

Effect of diclofenac sodium on the arachidonic acid cascade.

The anti-inflammatory activity of nonsteroidal anti-inflammatory drugs is primarily attributed to inhibition of distinct steps in the arachidonic acid cascade, particularly, the cyclo-oxygenase pathway. Diclofenac sodium, a compound of this class of drugs, appears to have a dual effect since it also regulates the lipoxygenase pathway. Study of appropriate cell systems (leukocytes and whole blood in rats) demonstrates that diclofenac's potent inhibition of cyclo-oxygenase activity causes a sharp reduction in the formation of prostaglandin, prostacyclin, and thromboxane products, all key mediators of inflammation. Recent work discloses that at higher concentrations, diclofenac sodium also reduces the formation of products of the lipoxygenase pathway (5-hydroxyeicosatetraenoic acid, leukotrienes). The mechanism by which this evolves, however, appears to be unrelated to direct inhibition of lipoxygenase. Instead, by enhancing its reincorporation into triglycerides, diclofenac sodium reduces the intracellular level of free arachidonic acid.

Pharmacology of diclofenac sodium.

Diclofenac sodium is the active ingredient in Voltaren, a nonsteroidal anti-inflammatory drug designed by selection of appropriate physicochemical and steric properties. Its pharmacologic activity, specifically its effects in acute and subchronic inflammation, and its analgesic activity have been assessed in animal models. The tolerability of the compound as judged by several parameters (i.e., ratio between the acute lethal dose or the dose inducing gastrointestinal blood loss and the desired pharmacologic activity) is favorable in comparison with other nonsteroidal anti-inflammatory drugs. Diclofenac sodium acts by potent cyclo-oxygenase inhibition, reduction of arachidonic acid release, and enhancement of arachidonic acid uptake. It thereby results in a dual inhibitory effect on both the cyclo-oxygenase and lipoxygenase pathways.

The effects of diclofenac sodium on arachidonic acid metabolism.

Evidence has been presented that inhibition by diclofenac sodium of the production of leukotrienes by cells participating in the inflammatory process is due to a decreased availability of intracellular arachidonic acid which results from enhanced uptake of the substrate into triglyceride pools. The diminished leukotriene production does not result from direct inhibition of 5-lipoxygenase or phospholipase A2. Reduced availability of arachidonic acid would also limit production of prostaglandins, although in this case manifestation is obscured by the potent inhibitory effect of diclofenac sodium on cyclooxygenase. This recently discovered action of diclofenac sodium, which has been characterized by studies on isolated leukocytes, appears to be operative in vivo. Consistent with this mechanism, and not explainable by classical cyclooxygenase inhibition, diclofenac sodium inhibited leukotriene production in whole blood from drug-treated animals and also suppressed leukocyte infiltration of subcutaneously implanted sponges. The latter effect contrasts with increased infiltration frequently obtained with other NSAIDs and thought to reflect enhanced production of leukotrienes. In conclusion, the findings suggest that patient acceptance or preference for diclofenac sodium is not merely subjective but has a logical scientific basis.

CGS 12970: a novel, long acting thromboxane synthetase inhibitor.

CGS 12970 is a potent selective inhibitor of human platelet thromboxane synthetase in vitro (IC50 = 12 nM). It is four orders of magnitude less potent as an inhibitor of sheep seminal vesicle cyclooxygenase, bovine aorta prostacyclin synthetase and human leucocyte 15-lipoxygenase. The compound inhibited collagen-induced thromboxane B2 production by human platelets in vitro without an effect on the accompanying platelet aggregation induced by collagen, ADP, platelet activating factor, thrombin, arachidonic acid or the prostaglandin mimetic, U 46619. Administration of CGS 12970 to rats inhibited collagen-induced thromboxane B2 production but had no effect on platelet aggregation ex vivo. It also had no effect on platelet aggregation induced by thrombin or on plasma clotting times. A single oral dose of 1 or 3 mg kg-1 to rabbits inhibited thromboxane B2 production in clotting blood ex vivo for 12 or 24 h respectively. CGS 12970 inhibited thromboxane B2 production in vivo induced by intravenous administration of collagen to rats or calcium ionophore to guinea-pigs. In both cases there was a concomitant elevation of immunoreactive 6-keto-prostaglandin F1 alpha but no effect on the induced thrombocytopenia. As with other thromboxane synthetase inhibitors, CGS 12970 prolonged tail bleeding time in the rat. However, CGS 12970 was not as potent as other thromboxane synthetase inhibitors in this test. In addition to these usual effects of thromboxane synthetase inhibitors, CGS 12970 inhibited the thrombocytopenia induced by the Forssman reaction or ADP administration. In these tests the effect of the compound was of short duration. 8 CGS 12970 had no effect on the thrombocytopenia associated with the Arthus reaction which distinguishes it from cyclo-oxygenase inhibitors. It also had no effect on thrombus formation on a cotton thread in an arteriovenous shunt in the rat.

Characterization of imidazo[1,5-a]pyridine-5-hexanoic acid (CGS 13080) as a selective thromboxane synthetase inhibitor using in vitro and in vivo biochemical models.

CGS 13080 inhibited cell-free thromboxane synthetase with an IC50 of 3 nM. It was at least five orders of magnitude less potent toward other key enzymes involved in arachidonic acid metabolism. Submicromolar concentrations inhibited calcium ionophore-induced formation of thromboxane B2 by intact human platelets with concomitant accumulation of prostaglandin E2. Oral doses lower than 1 mg/kg in rats suppressed the elevations of plasma thromboxane B2 induced by calcium ionophore. This was attended by shunting of endoperoxide substrate to 6-keto-prostaglandin F1 alpha and prostaglandin E2. CGS 13080 is one of the most potent and selective thromboxane synthetase inhibitors yet identified.

Inhibition of prostaglandin synthase by pirprofen. Studies with sheep seminal vesicle enzyme.

1. Pirprofen, racemic 2-[3-chloro-4(3-pyrrolinyl) phenyl] propionic acid, was evaluated for its ability to inhibit the conversion of arachidonic acid into prostaglandin E2 by sheep seminal vesicle prostaglandin synthase in vitro. 2. The compound proved to be a potent inhibitor with a Ki value of about 1.2 muM. Like indomethacin, aspirin and certain other non-steroidal anti-inflammatory drugs, pirprofen inhibited the enzyme competively with respect to substrate. Unlike most non-steroidal anti-inflammatory drugs, however, pirprofen did not promote time-dependent inactivation of the enzyme. It behaved as a competitive, reversible inhibitor, whereas most of the other agents acted as competitive, irreversible inhibitors. 3. The results suggest that inhibition of prostaglandin synthesis accounts in large part for the pharmacological effects of pirprofen.