Rate and equilibrium constants for the epimerization of the endothelin receptor antagonist J-104,132 in aqueous solution.

The degradation of [5S-[5alpha,6beta,7alpha(R*)]]-2-butyl-5-(1,3-benzodioxol-5-yl)-7-[(2-carboxypropyl)-4-methoxyphenyl]-6-dihydro-5H-cyclopenta[b]pyridine-6-carboxylic acid (J-104,132) was studied in aqueous solution as a function of temperature and pH. The degradation reaction does not proceed to completion; rather, a stable equilibrium is attained in which approximately 2% of the degradate is produced. Kinetic data for the formation of the degradate are analyzed using an integrated form of the rate law for a reversible first-order reaction, and the forward and reverse rate constants and overall equilibrium constants are presented. Isolation and spectroscopic structural determination indicate that the degradate is the C7 beta-epimer of the drug. A mechanism for the epimerization reaction involving a novel enamine-like intermediate is proposed and shown to be consistent with the kinetic data. The rate and equilibrium constants are used to predict the room temperature stability of an injectable formulation of J-104,132, and these predictions are compared to actual data from long-term stability studies. It is concluded that the preformulation kinetic studies provide essential data needed for optimum drug product development.

Antiarrhythmic efficacy of selective blockade of the cardiac slowly activating delayed rectifier current, I(Ks), in canine models of malignant ischemic ventricular arrhythmia.

BACKGROUND: To date, the lack of potent and selective inhibitors has hampered the physiological assessment of modulation of the cardiac slowly activating delayed rectifier current, I(Ks). The present study, using the I(Ks) blocker L-768,673, represents the first in vivo assessment of the cardiac electrophysiological and antiarrhythmic effects of selective I(Ks) blockade. METHODS AND RESULTS: In an anesthetized canine model of recent (8.5+/-0.4 days) anterior myocardial infarction, 0.003 to 0.03 mg/kg L-768,673 IV significantly suppressed electrically induced ventricular tachyarrhythmias and reduced the incidence of lethal arrhythmias precipitated by acute, thrombotically induced posterolateral myocardial ischemia. Antiarrhythmic protection afforded by L-768,673 was accompanied by modest 7% to 10% increases in noninfarct zone ventricular effective refractory period, 3% to 5% increases in infarct zone ventricular effective refractory period, and 4% to 6% increases in QTc interval. In a conscious canine model of healed (3 to 4 weeks) anterior myocardial infarction, ventricular fibrillation was provoked by transient occlusion of the left circumflex coronary artery during submaximal exercise. Pretreatment with 0.03 mg/kg L-768,673 IV elicited a modest 7% increase in QTc, prevented ventricular fibrillation in 5 of 6 animals, and suppressed arrhythmias in 2 additional animals. CONCLUSIONS: The present findings suggest that selective blockade of I(Ks) may be a potentially useful intervention for the prevention of malignant ischemic ventricular arrhythmias.

Investigation of the polymorphism of the angiotensin II antagonist agent MK-996.

The polymorphism of the angiotensin II antagonist agent MK-996 was studied, with particular emphasis on crystal form stability, solubility, and reproducible crystallization of the drug. X-ray powder diffraction patterns indicated differences in the crystal forms of early research and development lots. Solubility data for the different crystal forms in water at 25 degrees C are in agreement with the solution calorimetry data and indicated that crystalline form I is the thermodynamically stable polymorph of MK-996 under ambient conditions. In contrast to the other polymorphs, form I is reproducibly prepared on both the laboratory and production scale. This study examines methodology to determine the most suitable polymorph for development.

Selection of optimal hydrate/solvate forms of a fibrinogen receptor antagonist for solid dosage development.

The objective of this work was to compare the physicochemical properties of four crystalline forms of the fibrinogen receptor antagonist L-738,167 [2(S)-[p-toluenesulfonyl amino]-3-[[[5,6,7,8-tetrahydro-4-oxo-5-[2-(piperidin-4-yl)ethyl] -4-H-pyrazolo[1,5-a][1,4] diazepin-2-yl] carbonyl]amino]-propionic acid] to determine the best form for use in the development of oral dosage formulations. Four crystalline forms [form A (trihydrate), form B (pentahydrate), form C, and form D] were compared using x-ray powder diffractometry, thermal analysis, and moisture sorption studies. The trihydrate, form A, was demonstrated to hydrate upon exposure to relative humidity (RH) above 50% at room temperature (25 degrees C) with conversion to the pentahydrate. The pentahydrate, form B, converted to the trihydrate at room temperature when exposed to humidity levels below 25% RH. The crystalline pentahydrate was shown to be stable to dehydration upon storage at 30 degrees C/60% RH and 40 degrees C/75% RH for 3 months. The suspension of form A or form D in water resulted in conversion to form B, the stable hydrated form in an aqueous environment. Form C has a unique crystalline structure that is stable in an aqueous environment and not subject to hydration/dehydration with changes in relative humidity and thus may offer some advantages in pharmaceutical development.

Solid-phase extraction and liquid chromatographic quantitation of the antiarrhythmic drug L-768673 in a microemulsion formulation.

A sensitive and specific method based in solid-phase extraction and reverse-phase liquid chromatography was developed and validated for the quantitation of L-768673 in a microemulsion formulation. Following a water wash, the drug was eluted from the extraction column with acetonitrile and was analyzed on a reverse-phase C18 column with UV detection at 245 nm. The mobile phase consisted of acetonitrile-0.2% trifluoroacetic acid, 0.1% triethylamine (53:47 v/v). The retention time L-768673 was approximately 28 min with a flow rate of 1.5 ml min-1.

Solvent oxygen is not incorporated into N10-formyltetrahydrofolate in the reaction catalyzed by N10-formyltetrahydrofolate synthetase.

The mechanism of the reaction catalyzed by N10-formyltetrahydrofolate synthetase involves the formation of formyl phosphate as an intermediate which then formylates tetrahydrofolate at the N-10 position. Previous studies demonstrated that the non-enzymic formylation of tetrahydrofolate by formyl phosphate occurs exclusively at the more nucleophilic 5-nitrogen in the reduced pyrazine ring. The experiments described in this report were designed to determine whether N5-formyltetrahydrofolate might be the first product to be formed on the enzyme, followed by formyl transfer to the 10-nitrogen via the cyclic intermediate N5,10-methenyltetrahydrofolate. If this were the case, oxygen from solvent H2O would be incorporated into the formyl group of the N10-derivative. By conducting the reaction in a 1:1 mixture of [16O]H2O and [18O]H2O and using 13C NMR spectroscopy we show that no 18O is incorporated into the product and conclude that the reaction proceeds via a direct formylation of the N-10 position by formyl phosphate.

Formylation of tetrahydrofolate by formyl phosphate.

Formyl phosphate is the putative intermediate in the formylation of tetrahydrofolate (THF) catalyzed by N10-formylTHF synthetase. In this study the non-enzymic reaction between formyl phosphate and THF was examined at 5 degrees C. 1H-NMR, HPLC and kinetic analysis of the proton-catalyzed conversion of the product to N5,10-methenylTHF were used to identify the product. In contrast to the enzyme reaction, which produces N10-formylTHF, N5-formylTHF was the only formylated THF derivative formed. The reaction was conducted at pH values of 3, 5, and 7, with the highest yield being obtained at pH 5 (64-85%, based on THF). The enzyme, therefore, changes the regioselectivity of this reaction by increasing the reactivity of the 10-nitrogen and either decreasing the reactivity of the 5-nitrogen or limiting its accessibility to formyl phosphate. 2-Mercaptoethanol, present in the reaction mixture to protect THF from O2, was also formylated by formyl phosphate, at the oxygen position.

Formation and utilization of formyl phosphate by N10-formyltetrahydrofolate synthetase: evidence for formyl phosphate as an intermediate in the reaction.

N10-Formyltetrahydrofolate synthetase from bacteria and yeast catalyzes a slow formate-dependent ADP formation in the absence of H4folate. The synthesis of formyl phosphate by the enzyme was detected by trapping the intermediate as formyl hydroxamate. That the "formate kinase" activity was part of the catalytic center of N10-formyltetrahydrofolate synthetase was shown by demonstrating coordinate inactivation of the "kinase" and synthetase activities by heat and a sulfhydryl reagent, similar effects of monovalent cations, similar Km values for substrates, and similar Ki values for the inhibitor phosphonoacetaldehyde for both activities. The relative rates of the kinase activities for the bacterial and yeast enzymes are about 10(-4) and 4 x 10(-6) of their respective synthetase activities. These slow rates for the kinase reaction can be explained by the slow dissociation of ADP and formyl phosphate from the enzyme. This conclusion is supported by rapid-quench studies where a "burst" of ADP formation (6.4 s-1) was observed that is considerably faster than the steady-state rate (0.024 s-1). The demonstration of enzyme-bound products by a micropartition assay and the lack of a significant formate-stimulated exchange between ADP and ATP provide further evidence for the slow release of the products from the enzyme. The synthesis of N10-CHO-H4folate when H4folate was added to the E-formyl phosphate-ADP complex is also characterized by a "burst" of product formation. The rate of this burst phase at 5 degrees C occurs with a rate constant of 18 s-1 compared to 14 s-1 for the overall reaction at the same temperature. These results provide further evidence for formyl phosphate as an intermediate in the reaction and are consistent with the sequential mechanism of the normal catalytic pathway. Positional isotope exchange experiments using [beta,gamma-18O]ATP showed no evidence for exchange during turnover experiments in the presence of either H4folate or the competitive inhibitor pteroyltriglutamate. The absence of scrambling of the 18O label as observed by 31P NMR suggests that the central complex may impose restraints to limit free rotation of the P beta oxygens of the product ADP.

Substrate activity of synthetic formyl phosphate in the reaction catalyzed by formyltetrahydrofolate synthetase.

Formyl phosphate, a putative enzyme-bound intermediate in the reaction catalyzed by formyltetrahydrofolate synthetase (EC 6.3.4.3), was synthesized from formyl fluoride and inorganic phosphate [Jaenicke, L. v., & Koch, J. (1963) Justus Liebigs Ann. Chem. 663, 50-58], and the product was characterized by 31P, 1H, and 13C nuclear magnetic resonance (NMR). Measurement of hydrolysis rates by 31P NMR indicates that formyl phosphate is particularly labile, with a half-life of 48 min in a buffered neutral solution at 20 degrees C. At pH 7, hydrolysis occurs with P-O bond cleavage, as demonstrated by 18O incorporation from H2(18)O into Pi, while at pH 1 and pH 13 hydrolysis occurs with C-O bond cleavage. The substrate activity of formyl phosphate was tested in the reaction catalyzed by formyltetrahydrofolate synthetase isolated from Clostridium cylindrosporum. Formyl phosphate supports the reaction in both the forward and reverse directions. Thus, N10-formyltetrahydrofolate is produced from tetrahydrofolate and formyl phosphate in a reaction mixture that contains enzyme, Mg(II), and ADP, and ATP is produced from formyl phosphate and ADP with enzyme, Mg(II), and tetrahydrofolate present. The requirements for ADP and for tetrahydrofolate as cofactors in these reactions are consistent with previous steady-state kinetic and isotope exchange studies, which demonstrated that all substrate subsites must be occupied prior to catalysis. The k cat values for both the forward and reverse directions, with formyl phosphate as the substrate, are much lower than those for the normal forward and reverse reactions. Kinetic analysis of the formyl phosphate supported reactions indicates that the low steady-state rates observed for the synthetic intermediate are most likely due to the sequential nature of the normal reaction.