Serum progesterone and oxytocinase, and endometrial and luteal gene expression in pregnant, nonpregnant, oxytocin, carbetocin and meclofenamic acid treated mares.

Our objectives were to examine changes in endometrial and luteal gene expression during estrus, diestrus, pregnancy and treatments to induce luteolysis and putatively induce luteostasis. Groups were: Diestrus (DIEST), Estrus (ESTR), Pregnant (PREG), Oxytocin (OXY), Carbetocin (CARB), and Meclofenamic acid (MFA). Blood was obtained from day (D)12 to D15 for measurement of oxytocinase, also referred to as leucyl-cysteinyl aminopeptidase (LNPEP) and progesterone. Luteal biopsies were obtained on D12 and D15 and an endometrial biopsy on D15. Real-time RT-PCR was performed for the following genes: PGR, ESR1, OXTR,OXT, LNPEP, PTGS2, PTGFR, PLA2G2C, PTGES, SLC2A4, and SLC2A1. Regarding serum LNPEP, PREG and OXY (p-value<0.001) had higher concentrations than DIEST mares. Endometrial PTGES expression was higher (p-value <0.04) in DIEST, PREG and OXY than other groups. Endometrium from ESTR had increased expression of OXT (p-value < 0.02) compared to MFA and OXY mares. Carbetocin treatment: decreased serum progesterone and LNPEP; increased endometrial PLA2G2C; decreased endometrial PTGES; and decreased luteal aromatase and PTGES. Treatment with MFA: decreased endometrial PLA2G2C, increased endometrial PTGES; and resulted in less OXTR and OXT luteal abundance on D12 compared to D15. Endometrial and luteal expression of LNPEP is affected by physiologic stage and treatment and is involved in luteal function and pregnancy recognition pathways through effects on oxytocin and prostaglandin synthesis in the horse.

TRPM4 inhibition by meclofenamate suppresses Ca2+-dependent triggered arrhythmias.

AIMS: Cardiac arrhythmias are a major factor in the occurrence of morbidity and sudden death in patients with cardiovascular disease. Disturbances of Ca2+ homeostasis in the heart contribute to the initiation and maintenance of cardiac arrhythmias. Extrasystolic increases in intracellular Ca2+ lead to delayed afterdepolarizations and triggered activity, which can result in heart rhythm abnormalities. It is being suggested that the Ca2+-activated nonselective cation channel TRPM4 is involved in the aetiology of triggered activity, but the exact contribution and in vivo significance are still unclear. METHODS AND RESULTS: In vitro electrophysiological and calcium imaging technique as well as in vivo intracardiac and telemetric electrocardiogram measurements in physiological and pathophysiological conditions were performed. In two distinct Ca2+-dependent proarrhythmic models, freely moving Trpm4-/- mice displayed a reduced burden of cardiac arrhythmias. Looking further into the specific contribution of TRPM4 to the cellular mechanism of arrhythmias, TRPM4 was found to contribute to a long-lasting Ca2+ overload-induced background current, thereby regulating cell excitability in Ca2+ overload conditions. To expand these results, a compound screening revealed meclofenamate as a potent antagonist of TRPM4. In line with the findings from Trpm4-/- mice, 10 µM meclofenamate inhibited the Ca2+ overload-induced background current in ventricular cardiomyocytes and 15 mg/kg meclofenamate suppressed catecholaminergic polymorphic ventricular tachycardia-associated arrhythmias in a TRPM4-dependent manner. CONCLUSION: The presented data establish that TRPM4 represents a novel target in the prevention and treatment of Ca2+-dependent triggered arrhythmias.

Nickel(II)-meclofenamate complexes: Structure, in vitro and in silico DNA- and albumin-binding studies, antioxidant and anticholinergic activity.

Five novel nickel(II) complexes with the non-steroidal anti-inflammatory drug sodium meclofenamate (Na-mclf) have been synthesized and characterized in the absence or co-existence of the nitrogen-donors imidazole (Himi), 2,2'-bipyridylamine (bipyam), 2,2'-bipyridylketoxime (Hpko) and 2,9-dimethyl-1,10-phenanthroline (neoc); namely [Ni(mclf-O)2(Himi)2(MeOH)2], [Ni(mclf-O)2(MeOH)4], [Ni(mclf-O)(mclf-O,O')(bipyam)(MeOH)]·0.25MeOH, [Ni(mclf-O,O')2(neoc)] and [Ni(mclf-O)2(Hpko-N,N')2]·MeOH·0.5H2O. The affinity of the complexes for calf-thymus (CT) DNA was investigated by various techniques and intercalation is suggested as the most possible interaction mode. The interaction of the complexes for bovine and human serum albumins was also investigated in order to determine the binding constants, concluding that the complexes bind reversibly to albumins for the transportation towards their target cells or tissues and their release upon arrival at biotargets. The antioxidant activity of the compounds was evaluated via their ability to scavenge 1,1-diphenyl-picrylhydrazyl and 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) free radicals and to reduce H2O2. For the determination of the anticholinergic ability of the complexes the in vitro inhibitory activity against the enzymes acetylcholinesterase and butyrylcholinesterase was evaluated and presented promising results. The in silico molecular modeling calculations employed provide useful insights for the understanding of the mechanism of action of the studied complexes at a molecular level. This applies on both the impairment of DNA by its binding with the studied complexes and transportation through serum albumins, as well as the ability of these compounds to act as anticholinergic agents.

Copper(II) complexes with meclofenamate ligands: Structure, interaction with DNA and albumins, antioxidant and anticholinergic activity.

The interaction of copper(II) with the non-steroidal anti-inflammatory drug sodium meclofenamate (Na-mclf) in the presence or absence of the nitrogen-donor co-ligands pyridine (py) or 2,2'-bipyridylamine (bipyam), yielded the novel Cu(II) complexes [Cu2(mclf-O,O')4(MeOH)2]·2MeOH (1·2MeOH), [Cu(mclf-O)2(py)3]·H2O·0.5MeOH (2·H2O·0.5MeOH) and [Cu(mclf-O,O')2(bipyam)] (3). The characterization of the complexes was achieved by various techniques, including single-crystal X-ray crystallography. In order to study the binding mode and strength of the complexes to calf-thymus (CT) DNA, various techniques were employed which suggested intercalation between the DNA-bases as the most possible interaction mode. Competitive studies with ethidium bromide (EB) revealed the ability of the complexes to displace the EB from the EB-DNA adduct, verifying the intercalative binding mode. The affinity of the complexes to bovine and human serum albumin proteins (SAs) was investigated by fluorescence emission spectroscopy and the corresponding binding constants bear relatively high values, showing that the complexes bind tightly and possibly reversibly to SAs. The antioxidant activity of the complexes against 1,1-diphenyl-picrylhydrazyl (DPPH), 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) radicals and the ability to reduce H2O2 proved to be of significant magnitude. The in vitro inhibitory activity against the enzymes acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE) was evaluated, in order to assess the anticholinergic ability of the complexes, which appeared promising.

Modifications of diflunisal and meclofenamate carboxyl groups affect their allosteric effects on GABAA receptor ligand binding.

Gamma-aminobutyric acid type A receptors (GABAAR) are allosterically modulated by the nonsteroidal anti-inflammatory drugs diflunisal and fenamates. The carboxyl group of these compounds is charged at physiological pH and therefore penetration of the compounds into the brain is low. In the present study we have transformed the carboxyl group of diflunisal and meclofenamate into non-ionizable functional groups and analyzed the effects of the modifications on stimulation of [(3)H]muscimol binding and on potentiation of γ-aminobutyric acid-induced displacement of 4'-ethenyl-4-n-[2,3-(3)H]propylbicycloorthobenzoate. N-Butylamide derivative of diflunisal modulated radioligand binding with equal or higher potency than the parent compound, while diflunisalamide showed reduced allosteric effect as compared to diflunisal. Amide derivative of meclofenamate equally affected radioligand binding parameters, while both diflunisal and meclofenamate methyl esters were less active than the parent compounds. Our study clearly demonstrates that an intact carboxyl group in diflunisal and meclofenamate is not indispensable for their positive GABAAR modulation. Further derivatization of the compound might yield compounds with higher selectivity for GABAARs that could be utilized in drug development.

Crystal structures of three classes of non-steroidal anti-inflammatory drugs in complex with aldo-keto reductase 1C3.

Aldo-keto reductase 1C3 (AKR1C3) catalyses the NADPH dependent reduction of carbonyl groups in a number of important steroid and prostanoid molecules. The enzyme is also over-expressed in prostate and breast cancer and its expression is correlated with the aggressiveness of the disease. The steroid products of AKR1C3 catalysis are important in proliferative signalling of hormone-responsive cells, while the prostanoid products promote prostaglandin-dependent proliferative pathways. In these ways, AKR1C3 contributes to tumour development and maintenance, and suggest that inhibition of AKR1C3 activity is an attractive target for the development of new anti-cancer therapies. Non-steroidal anti-inflammatory drugs (NSAIDs) are one well-known class of compounds that inhibits AKR1C3, yet crystal structures have only been determined for this enzyme with flufenamic acid, indomethacin, and closely related analogues bound. While the flufenamic acid and indomethacin structures have been used to design novel inhibitors, they provide only limited coverage of the NSAIDs that inhibit AKR1C3 and that may be used for the development of new AKR1C3 targeted drugs. To understand how other NSAIDs bind to AKR1C3, we have determined ten crystal structures of AKR1C3 complexes that cover three different classes of NSAID, N-phenylanthranilic acids (meclofenamic acid, mefenamic acid), arylpropionic acids (flurbiprofen, ibuprofen, naproxen), and indomethacin analogues (indomethacin, sulindac, zomepirac). The N-phenylanthranilic and arylpropionic acids bind to common sites including the enzyme catalytic centre and a constitutive active site pocket, with the arylpropionic acids probing the constitutive pocket more effectively. By contrast, indomethacin and the indomethacin analogues sulindac and zomepirac, display three distinctly different binding modes that explain their relative inhibition of the AKR1C family members. This new data from ten crystal structures greatly broadens the base of structures available for future structure-guided drug discovery efforts.

Synthesis, crystal structures and spectroscopy of meclofenamic acid and its metal complexes with manganese(II), copper(II), zinc(II) and cadmium(II). Antiproliferative and superoxide dismutase activity.

Some new complexes of meclofenamic acid (N-(2,6-dichloro-m-tolyl)anthranilic acid), Hmeclo (1), with potentially interesting biological activities are described. Complexes [Mn(meclo)(2)] (2), [Cu(meclo)(2)(H(2)O)(2)] (3), [Zn(meclo)(2)(H(2)O)(2)] (4) and [Cd(meclo)(2)(H(2)O)(2)] (5) were prepared and structurally characterized by means of vibrational, electronic and (1)H and (13)C NMR spectroscopies. The crystal structure of complexes [Cu(4)(meclo)(6)(OH)(2)(DMSO)(2)]2DMSO (3a) and [Cd(meclo)(2)(DMSO)(3)] (5a) have been determined by X-ray crystallography. Complex (3a) is a centrosymmetric tetramer built up around the planar cyclic Cu(2)(OH)(2) unit. Complex 5a is mononuclear seven-coordinated complex with the meclofenamato ligand behaving as a bidentate deprotonated chelating ligand. Intra and intermolecular hydrogen bonds stabilize these two structures, while the crystal packing is determined by π-π and C-H--π interactions. Meclofenamic acid and its metal complexes have been evaluated for antiproliferative activity in vitro against the cells of three human cancer cell lines, MCF-7 (breast cancer cell line), T24 (bladder cancer cell line), and A-549 (non-small cell lung carcinoma), and a mouse fibroblast L-929 cell line. Complex 5 exhibits the highest selectivity against MCF-7 and 4 shows the highest selectivity against T-24. Complexes 2-5 were found to be more potent cytotoxic agents against T-24 and complex 5 against MCF-7 cancer cell lines than the prevalent benchmark metallodrug, cis-platin. The superoxide dismutase activity was measured by the Fridovich test which showed that complex [Cu(meclo)(2)(H(2)O)(2)] is a good superoxide scavenger.

Development and in-vitro evaluation of sustained-release meclofenamic acid microspheres.

Meclofenamic acid (MFA) sustained-release microspheres were prepared by the solvent evaporation method using cellulose propionate (CP) polymer and acetone as the polymer solvent. Polyethylene glycol (PEG) was used as a channelling agent to improve the release properties of MFA at 1:2:1 drug to polymer to PEG ratio. The microspheres prepared at three different speeds (600, 800 and 1000 rpm) were characterized with regard to their surface morphology, average drug content, particle size distribution and release profiles in phosphate buffer, pH 8.0 at 37 degrees C. The microspheres were stored under accelerated conditions for 3 months and the effect of storage on the different characteristics was studied. Spherical particles with essentially smooth surface and few residual drug crystals on the surface were formed. Smaller particles were formed at higher agitation speeds. The release rate of MFA from these microspheres was not affected by the molecular weight of CP polymer. PEG 2000 was found to have a more enhancing effect on the rate of the release than PEG 4000. The physical properties of the microspheres and their release characteristics were not altered by storing the product at 40 degrees C/80% relative humidity (R.H.) for 3 months.

Determination of phase II drug metabolites in equine urine by micellar electrokinetic capillary chromatography.

Micellar electrokinetic capillary chromatography (MECC) using diode array detection has been investigated for the determination of phase I and phase II metabolites of drugs in biofluids. Methods were optimised for the determination of morphine, morphine-3-glucuronide, morphine-6-glucuronide, normorphine, meclofenamic acid and its metabolites in equine urine. Solid-phase extraction procedure were developed to concentrate and purify the analytes from spiked and post administration urines for MECC analysis. A simple on-line procedure for monitoring the kinetics of hydrolysis of morphine-glucuronide conjugates by beta-glucuronidase was demonstrated.

Effect of inhibitor time-dependency on selectivity towards cyclooxygenase isoforms.

Cyclooxygenase (Cox) is a key enzyme in the biosynthesis of prostaglandins and, as such, is the target of non-steroidal anti-inflammatory drugs (NSAIDs). Two isoforms exist, being expressed constitutively (Cox-1), or inducibly in response to inflammatory mediators (Cox-2). Currently available NSAIDs inhibit both isoforms somewhat equipotently but selective Cox-2 inhibition may eliminate unwanted side effects. We have characterized the kinetic mechanisms of the interactions of purified recombinant human cyclooxygenase-1 and -2 (hCox-1, hCox-2) with the selective Cox-2 inhibitor N-(2-cyclohexyloxy-4-nitrophenyl)methanesulphonamide (NS-398) and some classical non-selective NSAIDs. NS-398, flurbiprofen, meclofenamic acid and indomethacin are time-dependent, irreversible inhibitors of hCox-2. The inhibition is consistent with a two-step process, involving an initial rapid equilibrium binding of enzyme and inhibitor, characterized by Ki, followed by the slow formation of a tightly bound enzyme-inhibitor complex, characterized by a first-order rate constant kon. NS-398 is a time-independent inhibitor of hCox-1, consistent with the formation of a reversible enzyme-inhibitor complex. Flurbiprofen, meclofenamic acid and indomethacin are also time-dependent inhibitors of hCox-1 and hence show little selectivity for one isoform over the other. Flufenamic acid is time independent towards both isoforms and is also non-selective. The high degree of selectivity of NS-398 towards Cox-2 results therefore from the difference in the nature of the time-dependency of inhibition of the two isoforms.

A single and multiple dose pharmacokinetic and metabolism study of meclofenamate sodium.

A single and multiple oral dose administration study of meclofenamate sodium (Meclomen) was conducted in ten healthy male volunteers. An initial 300 mg oral dose on day 1 was followed by a 100 mg every 8 h dosage regimen on study days 4 through 18. Intensive plasma and urine sample collection was carried out over the first three study days, and for 120 h following administration of the final dose on day 18. Plasma and urine specimens were analyzed by a specific HPLC assay for unconjugated meclofenamic acid and metabolites I and II of meclofenamic acid before and after sample incubation with beta-glucuronidase. Meclofenamic acid was rapidly absorbed following oral dose administration. Concentrations of meclofenamic acid existed primarily as unconjugated drug in plasma, with only a small amount present in the conjugated form. Meclofenamic acid was rapidly eliminated, with an elimination half-life of approximately 1.3 h. This resulted in no detectable accumulation upon multiple dose administration. Metabolite I, which is one-fifth as active as meclofenamic acid in in vitro inhibition of cyclooxygenase, was present in unconjugated form at steady state in concentrations approximately 50 per cent of those of meclofenamic acid, as unconjugated drug. The majority of metabolite I in plasma existed as glucuronide conjugate. Metabolite II, which is inactive, was present in very significant concentrations in unconjugated form. Plasma protein binding determinations conducted on meclofenamic acid and metabolite I indicated that the free fraction of metabolite I was 8.7 to 10.9 times higher than that of meclofenamic acid. When the lower activity and lower steady state concentrations, but higher free fraction, are considered, it would appear that metabolite I may contribute significantly to the in vivo inhibition of cyclooxygenase activity seen after administration of meclofenamic acid.

Effect of glomerulopressin on GFR of rats, role of cyclooxygenase inhibitors and saralasin

En trabajos anteriores se ha demostrado que la glomerulopresina aumenta la filtración glomerular (FG) en sapos, ratas y perros. El efecto de la glomerulopresina es bloqueado por inhibidores de la ciclooxigenasa en varios sistemas. Este trabajo se llevó a cabo con el propósito de estudiar si los inhibidores de la ciclooxigenasa y el bloqueador de los receptores de Ang II, saralasina, impiden el efecto de la glomerulopresina sobre la FG de la rata. Se midió la depuración de inulina en ratas infundidas durante todo el experimento con indometacina o meclofenamato o con saralasina por la vena yugular. Se consideró un período de control durante el cual se infundió Krebs-Ringer-Bicarbonato (KRB) por la arteria femoral y un período experimental durante el cual, en alguns grupos, se cambió la infusión de KRB por la de glomerulopresina. Los inhibidores de la ciclooxigenasa y la saralasina impidieron el aumento de la FG producido por la glomerulopresina. Parece razonable proponer que la glomerulopresina puede aumentar la FG sólo cuando los receptores de la Ang II están libres y que su acción es mediada por la síntesis de prostaglandinas

Effect of glomerulopressin on GFR of rats, role of cyclooxygenase inhibitors and saralasin

En trabajos anteriores se ha demostrado que la glomerulopresina aumenta la filtración glomerular (FG) en sapos, ratas y perros. El efecto de la glomerulopresina es bloqueado por inhibidores de la ciclooxigenasa en varios sistemas. Este trabajo se llevó a cabo con el propósito de estudiar si los inhibidores de la ciclooxigenasa y el bloqueador de los receptores de Ang II, saralasina, impiden el efecto de la glomerulopresina sobre la FG de la rata. Se midió la depuración de inulina en ratas infundidas durante todo el experimento con indometacina o meclofenamato o con saralasina por la vena yugular. Se consideró un período de control durante el cual se infundió Krebs-Ringer-Bicarbonato (KRB) por la arteria femoral y un período experimental durante el cual, en alguns grupos, se cambió la infusión de KRB por la de glomerulopresina. Los inhibidores de la ciclooxigenasa y la saralasina impidieron el aumento de la FG producido por la glomerulopresina. Parece razonable proponer que la glomerulopresina puede aumentar la FG sólo cuando los receptores de la Ang II están libres y que su acción es mediada por la síntesis de prostaglandinas (AU)

Prostaglandin I2 and glucocorticoid-induced rise in arterial pressure in the rat.

The mechanism of glucocorticoid-induced hypertension is not known. Although glucocorticoids can exert an inhibitory effect on prostaglandin synthesis in vitro, their in vivo influence on this system is controversial. The goal of the present study was to determine whether dexamethasone-induced hypertension in Wistar rats is due to inhibition of the synthesis of the vasodilator prostaglandin I2 (PGI2) in vivo. Dexamethasone caused a profound reduction (7 +/- 1 versus 21 +/- 5 ng per 24 h) in the urinary excretion of PGI-M (PGI-M), a major metabolite of PGI2, and a sustained rise in systolic arterial pressure which was maximal after 5 days (144 +/- 9 versus 103 +/- 3 mmHg). A study of the metabolism of [3H]-labeled 6-oxo-PGF1 alpha and PGI2 revealed that dexamethasone exerted a dual action on the prostaglandin system in vivo: an inhibition of PGI2 biosynthesis and an alteration of its metabolism, both effects contributing to the observed reduction in urinary levels of PGI-M. Exogenous arachidonic acid induced a fourfold increase in urinary PGI-M in normal rats (from 14 +/- 3 to 61 +/- 6 ng per 24 h). Despite a large decrease upon addition of dexamethasone, urinary PGI-M remained in the high-normal range in arachidonic acid-treated rats (21 +/- 8 ng per 24 h). Arachidonic acid exerted antihypertensive effects which were marginal initially but significant in the later phase of dexamethasone-induced hypertension (124 +/- 8 versus 139 +/- 8 mmHg in arachidonic acid-treated versus control rats after 7 days of dexamethasone).(ABSTRACT TRUNCATED AT 250 WORDS)

Stoichiometry and kinetics of the interaction of prostaglandin H synthase with anti-inflammatory agents.

We have examined the kinetics, stoichiometry, and chemical nature of the interaction of three anti-inflammatory agents (indomethacin, flurbiprofen, and meclofenamic acid) with pure ovine prostaglandin H synthase. The kinetics of the interaction with the synthase for each of the three agents, monitored by the decrease in cyclooxygenase activity, was consistent with the model proposed by Rome and Lands (Rome, L.H., and Lands, W.E.M. (1975) Proc. Natl. Acad. Sci. U.S.A. 72, 4863-4865): a rapid and reversible initial binding, followed by a first-order decay of the synthase-inhibitor complex. A relatively stable form of the cyclooxygenase, which had 4-10% of the initial activity, was the eventual product of this decay process. The dissociation constants evaluated for the initial binding were 1.7 +/- 1.5 microM for indomethacin, 0.2 +/- 0.1 microM for flurbiprofen, and 0.08 +/- 0.06 microM for meclofenamic acid. The values of the first order rate constants for the subsequent decay process were 14.9 +/- 11.3 min-1 for indomethacin, 3.4 +/- 0.7 min-1 for meclofenamic acid, and 16.6 +/- 6.2 min-1 for flurbiprofen. In repeated titrations of the cyclooxygenase with the three agents, 1.3 +/- 0.3 mol of indomethacin, 1.2 +/- 0.1 mol of meclofenamic acid, and 1.2 +/- 0.1 mol of S-(+)-flurbiprofen/mol of synthase dimer were found to result in maximal inhibition of the enzyme. Racemic flurbiprofen required 2.4 +/- 0.3 mol/mol synthase dimer for full effect, and the R-(-)-isomer was not inhibitory. Inhibition of the cyclooxygenase activity by these agents thus appears to result from a stereospecific binding to only one of the subunits of the synthase. Intact indomethacin could be recovered quantitatively after prolonged incubation (in stoichiometric quantities) with the synthase had resulted in maximal inhibition of the cyclooxygenase activity. The time-dependent effect of indomethacin on the cyclooxygenase is thus likely to involve a conformational change in the synthase rather than a covalent interaction.

Factors affecting absorption of non-steroidal anti-inflammatory agents in the horse.

The absorption of orally administered phenylbutazone (5 mg/kg) was studied in 10 thoroughbreds, eight ponies and four pony foals. Large variations in area under the curve (AUC) and peak plasma concentrations were found both within an animal and within groups of animals. Administration of phenylbutazone (5 mg/kg) following an overnight fast resulted in no difference among the three groups of animals with respect to AUC (0 to 24 hours), mean (+/- sd) values of which were 132 +/- 68, 107 +/- 48 and 98 +/- 6, respectively. Absorption characteristics of two oral phenylbutazone preparations (Equipalazone; Arnolds and Prodynam; Leo Laboratories) (5 mg/kg) given after an overnight fast were similar. Feeding before drug administration decreased AUC and peak plasma concentration and extended the range of the time taken to attain the latter. Repeated twice daily administration of phenylbutazone (5 mg/kg) resulted in more rapid absorption following the morning than the afternoon dose. Investigations with meclofenamic acid (4.4 mg/kg) also resulted in highest peak plasma concentrations and AUC following overnight fast. It was found that absorption was slower with intramuscular injections of phenylbutazone (2.5 mg/kg) than after oral administration in the fasted animal.

The pharmacokinetics of meclofenamic acid in the horse.

The pharmacokinetics of meclofenamic acid were studied in Thoroughbred horses and in ponies. After intravenous (i.v.) administration of either 2 mg/kg or 4 mg/kg sodium meclofenamate the elimination half-life was of the order of 0.9 h while the volume of distribution was found to be 0.128 litre/kg. Elimination was in accordance with a one-compartment model. Following oral administration of either meclofenamic acid (4 mg/kg) or sodium meclofenamate (4 mg/kg) a much longer terminal half-life than that calculated for Kel from i.v. data was found. This anomaly indicated that the 'flip-flop' phenomenon was present, i.e. Ka exceeded Kel. More rapid and higher peak levels occurred following sodium meclofenamate than meclofenamic acid, although total bioavailability was similar. Studies in ponies with meclofenamic acid showed a lower absorption than that found in Thoroughbreds. Overnight fasting before meclofenamic acid administration did not alter the rate or extent of absorption. Intramuscular administration of sodium meclofenamate resulted in low plasma concentrations and after 25 h only 46% of the drug had been absorbed.