Interaction profile and tolerability of barnidipine.

INTRODUCTION: Barnidipine is a new dihydropyridine calcium antagonist that is presented in modified-release capsules containing a single S-S optical isomer of the molecule. Its characteristics are of interest, as there is evidence of differences in kinetics, dynamics, interactions and safety of individual enantiomers in traditional racemic preparations of calcium antagonists. The safety of barnidipine and its interaction profile are reviewed. SAFETY: Adverse events with barnidipine are of mild to moderate intensity, most commonly of type I and occur in the early phase of treatment. Furthermore, safety results in elderly patients are comparable with those in the general population, indicating that barnidipine can be used without dose adjustment in elderly hypertensive patients. INTERACTIONS: Barnidipine has a pharmacokinetic interaction profile that compares favourably with those from other calcium antagonists. The pharmacokinetic properties of barnidipine are unaffected by food. Minor increases in its availability may occur with concomitant use of alcohol or grapefruit juice, but these are unlikely to have clinical relevance. In contrast with several other calcium antagonists, barnidipine does not affect the steady-state kinetics of digoxin, whereas, like other calcium antagonists its bioavailability may be increased by the concomitant administration of cimetidine. In addition, the potential of barnidipine and its major metabolites to affect the metabolism of concomitant medication is unlikely to be of clinical relevance. CONCLUSION: The interaction and tolerability profile of barnidipine is well established in all age groups.

4-Substituted 1-chloro-2-nitrobenzenes: structure-activity relationships and extension of the substrate model of rat glutathione S-transferase 4-4.

In the present study, eleven 4-substituted 1-chloro-2-nitrobenzenes were tested for their GSH conjugation capacity when catalyzed by base or rat glutathione S-transferase (GST) 4-4. Kinetic parameters (ks and K(m), kcat, and kcat/K(m)) were determined and subsequently used for the description of structure-activity relationships (SAR's). For this purpose, eight physicochemical parameters (electronic, steric, and lipophilic) of the substituents and five computer-calculated parameters of the substrates (charge distributions and several energy values) were used in regression analyses with the kinetic parameters. The obtained SAR's are compared with corresponding SAR's for the GSH conjugation of 2-substituted 1-chloro-4-nitrobenzenes, previously determined [Van der Aar et al. (1996) Chem. Res. Toxicol. 9, 527-534]. The kinetic parameters of the 4-substituted 1-chloro-2-nitrobenzenes correlated well with the Hammett sigma p- constant; the Hammett sigma p constant corrected for "through resonance", while the corresponding kinetic parameters of the 2-substituted 1-chloro-4-nitrobenzenes did not. The base- and GST 4-4-catalyzed GSH conjugation reactions of 2-substituted 1-chloro-4-nitrobenzenes depend to a different extent on the electronic properties of the ortho substituents, suggesting the involvement of different rate-limiting transition states. The base- and GST 4-4-catalyzed conjugation of 4-substituted 1-chloro-2-nitrobenzenes, however, showed a similar dependence on the electronic properties of the para substituents, indicating that these substrates are conjugated to GSH via a similar transition state. Multiple regression analyses revealed that, besides electronic interactions, also steric and lipophilic restrictions appeared to play an important role in the GST 4-4-catalyzed GSH conjugation of 4-substituted 1-chloro-2-nitrobenzenes. Finally, the 4-substituted 1-chloro-2-nitrobenzenes were also used to extend the previously described substrate model for GST 4-4 [De Groot et al. (1995) Chem. Res. Toxicol. 8, 649-658], by which a specific steric restriction of substrates for GST 4-4 became clear.

Structure-activity relationships for chemical and glutathione S-transferase-catalysed glutathione conjugation reactions of a series of 2-substituted 1-chloro-4-nitrobenzenes.

Glutathione S-transferases (GSTs) constitute an important class of phase II (de)toxifying enzymes, catalysing the conjugation of glutathione (GSH) with electrophilic compounds. In the present study, Km, kcat and kcat/Km values for the rat GST 1-1-, 3-3-, 4-4- and 7-7-catalysed conjugation reactions between GSH and a series of 10 different 2-substituted 1-chloro-4-nitrobenzenes, and the second-order rate constants (ks) of the corresponding base-catalysed reactions, were correlated with nine classical physicochemical parameters (electronic, steric and lipophilic) of the substituents and with 16 computer-calculated molecular parameters of the substrates and of the corresponding Meisenheimer complexes with MeS- as a model nucleophile for GS- (charge distributions and several energy values), giving structure-activity relationships. On the basis of an identical dependence of the base-catalysed as well as the GST 1-1- and GST 7-7-catalysed reactions on electronic parameters (among others, Hammett substituent constant sigma p and charge on p-nitro substituents), and the finding that the corresponding reactions catalysed by GSTs 3-3 and 4-4 depend to a significantly lesser extent on these parameters, it was concluded that the Mu-class GST isoenzymes have a rate-determining transition state in the conjugation reaction between 2-substituted 1-chloro-4-nitrobenzenes and GSH which is different from that of the other two GSTs. Several alternative rate-limiting transition states for GST 3-3 and 4-4 are discussed. Furthermore, based on the obtained structure-activity relationships, it was possible to predict the kcat/Km values of the four GST isoenzymes and the ks of the base-catalysed GSH conjugation of 1-chloro-4-nitrobenzene.

Structure-activity relationships for the glutathione conjugation of 2-substituted 1-chloro-4-nitrobenzenes by rat glutathione S-transferase 4-4.

In the present study structure--activity relationships (SAR's) are described for the experimentally determined kinetic parameters (Km, kappa cat, and kappa cat/Km) of the GST 4-4-catalyzed reaction between GSH and 10 2-substituted 1-chloro-4-nitrobenzenes. Steric, lipophilic, and electronic parameters were correlated with the kinetic parameters. Moreover, charge distributions and several energy values were calculated for the substrates and the corresponding Meisenheimer intermediates with MeS- as a model nucleophile for the thiolate anion of GSH and used in the regression analyses. The correlations obtained were compared with the corresponding SAR's for the base-catalyzed GSH conjugation reaction at pH 9.2. A high correlation coefficient was found between the kinetic parameter kappa s for the base-catalyzed reaction and the Hammett substituent constant (sigma p). Much lower correlation coefficients were obtained with kappa cat and sigma p and with kappa cat/Km and sigma p. Moreover, the reaction constant rho was significantly higher for the base-catalyzed than for the enzyme-catalyzed reaction. Also, high correlations were found between the kinetic parameters and the charges on the p-nitro substituent in the substrates. When kappa s was plotted against these charges, a linear relationship was found in which the slope was larger than the slope of a corresponding plot with kappa cat/Km. The Hammett sigma p can be divided into an inductive (F) and a resonance (R) component. With multiple regression between the kinetic parameters and F and R, higher correlation coefficients were obtained than with sigma p alone. Our observations suggest that the transition states for the base-catalyzed and the GST 4-4-catalyzed GSH conjugation reaction are different. Moreover, single classical physiochemical and computer-calculated molecular parameters and combinations of them can be an alternative approach for examining SAR's for spontaneous and GST-catalyzed GSH conjugation reactions.

Enzyme kinetics and substrate selectivities of rat glutathione S-transferase isoenzymes towards a series of new 2-substituted 1-chloro-4-nitrobenzenes.

1. Four different rat glutathione S-transferase (GST) isoenzymes, belonging to three different classes, were examined for their GSH conjugating capacity towards 11 2-substituted 1-chloro-4-nitrobenzene derivatives. Significant differences were found in their enzyme kinetic parameters Km, kcat and kcat/Km. 2. Substrates with bulky substituents on the ortho-position appeared to have high affinities (low Km's) for the active site of the GST-isoenzymes, suggesting that there is sufficient space in this area of the active site. A remarkably high Km (low affinity) was found for 2-chloro-5-nitropyridine towards all GST-isoenzymes examined. 3. GST 3-3 catalysed the reaction between GSH and the substrates most efficiently (high kcat) compared with the other GST-isoenzymes. Moreover, GST 3-3 showed clear substrate selectivities towards the substrates with a trifluoromethyl-, chlorine- and bromine-substituent. 1-Chloro-2,4-dinitrobenzene and 2-chloro-5-nitrobenzonitrile were most efficiently conjugated by all four GST-isoenzymes examined. 4. When the rate of the conjugation reactions was followed, a linear increase of formation of GS-conjugate could be seen for 2-chloro-5-nitrobenzonitrile during a much longer period of time than for 1-chloro-2,4-dinitrobenzene with all GST-isoenzymes examined. Therefore, it is suggested that 2-chloro-5-nitrobenzonitrile might be recommended as an alternative model substrate in GST-research.

A predictive substrate model for rat glutathione S-transferase 4-4.

Molecular modeling techniques have been used to derive a substrate model for class mu rat glutathione S-transferase 4-4 (GST 4-4). Information on regio- and stereoselective product formation of 20 substrates covering three chemically and structurally different classes was used to construct a substrate model containing three interaction sites responsible for Lewis acid--Lewis base interactions (IS1, IS2, and IS3), as well as a region responsible for aromatic interactions (IS4). Experimental data suggest that the first protein interaction site (pIS1, interacting with IS1) corresponds with Tyr115, while the other protein interaction sites (pIS2 and pIS3) probably correspond with other Lewis acidic amino acids. All substrates exhibited positive molecular electrostatic potentials (MEPs) near the site of conjugation with glutathione (GSH), as well as negative MEP values near the position of groups with Lewis base properties (IS1, IS2, or IS3), which interact with pIS1, pIS2, or pIS3, respectively. Obviously, complementarity between the MEPs of substrates and protein in specific regions is important. The substrate specificity and stereoselectivity of GST 4-4 are most likely determined by pIS1 and the distance between the site of GSH attack and Lewis base atoms in the substrates which interact with either pIS2, pIS3, or a combination of these sites. Interaction between aromatic regions in the substrate with aromatic amino acids in the protein further stabilizes the substrate in the active site. The predictive value of the model has been evaluated by rationalizing the conjugation to GSH of 11 substrates of GST 4-4 (representing 3 classes of compounds) which were not used to construct the model. All known metabolites of these substrates are explained with the model. As the computer-aided predictions appear to correlate well with experimental results, the presented substrate model may be useful to identify new potential GST 4-4 substrates.

The lipid peroxidation product 4-hydroxy-2,3-trans-1 nonenal decreases rat intestinal smooth muscle function in-vitro by alkylation of sulphydryl groups.

The effects of the lipid peroxidation product 4-hydroxy-2,3-trans-1 nonenal (HNE) on intestinal smooth muscle function have been studied. Exposure of rat isolated small intestinal segments to HNE (0.1-0.5 mM) led to decreased muscarinic and beta-adrenergic responses. The maximal response to the muscarinic agonist methacholine and its pEC50 decreased in a dose dependent manner. The response to the beta-adrenoceptor agonist isoprenaline was affected in a similar manner, but at slightly higher concentrations of HNE. As HNE has been described to be sulphydryl-reactive these effects were compared with the effects of the sulphydryl-reactive agent N-ethylmaleimide (NEM). Incubation of intestinal segments with NEM had similar effects on pharmacological responses to methacholine, indicating that the effects of HNE like that of NEM are likely to be caused by alkylation of sulphydryl groups. Dithiothreitol, a compound which reduces oxidized sulphydryl groups, was unable to restore the effects of HNE or NEM, which suggests that the effects of HNE and NEM are irreversible.

4- or 5- (omega-aminoalkyl) thiazoles and derivatives; new selective H2-receptor agonists.

It is well known that both histamine and dimaprit show moderate H2-receptor agonistic activity (guinea pig right atrium). Quantum chemical calculations indicated that 2-aminothiazole derivatives that might be regarded as cyclic dimaprit analogues, should possess H2-receptor agonistic activity as well. In the present study a series of 4- or 5-(omega-aminoalkyl) thiazoles has been synthesized, showing a moderate to strong H2-receptor agonistic activity as compared to histamine whereas no activity on H1- and H3-receptors could be detected. In contrast to histamine and derivatives, which are supposed to "trigger" the H2-receptor via a tautomeric shift involving two protons viz. one proton of the active site of the receptor and one proton of the heteroaromatic ring system, the thiazole derivatives seem to stimulate the H2-receptor via a one proton mechanism. In a series of impromidine analogues the 3- [4(5)-imidazolyl]propyl moiety was replaced by the more H2-receptor specific 3-(4- or 5-thiazolyl)propyl fragment resulting in potent and selective full H2-receptor agonists.