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Biomedical subjects

B Testa

Publications and source records attributed to B Testa.

At least 55 records · Page 3Linked to original sources
PubMed1998

Molecular properties and pharmacokinetic behavior of cetirizine, a zwitterionic H1-receptor antagonist.

The ionization and lipophilicity behavior of the antihistamine (H1-receptor antagonist) cetirizine was investigated, showing the drug to exist almost exclusively as a zwitterion in the pH region 3.5-7.5. In this pH range, its octanol/water lipophilicity is constant and low compared to cationic antihistamines (log D = log PZ = 1.5), whereas its H-bonding capacity is relatively large (delta log PZ > or = 3.1). Conformational, electronic, and lipophilicity potential calculations revealed that zwitterionic cetirizine experiences partial intramolecular charge neutralization in folded conformers of lower polarity. Pharmacokinetic investigations have shown the drug to be highly bound to blood proteins, mainly serum albumin, and to have a low brain uptake, explaining its lack of sedative effects. As such, cetirizine does not differ from "second-generation" antihistamines. In contrast, its very low apparent volume of distribution in humans (0.4 L kg-1, smaller than that of exchangeable water) implies a low affinity for lean tissues such as the myocardium and is compatible with the absence of cardiotoxicity of the drug. The zwitterionic nature and modest lipophilicity of cetirizine may account for this pharmacokinetic behavior. The suggestion is offered that cetirizine and analogous zwitterions, whose physicochemical, pharmacokinetic, and pharmacodynamic properties differ from those of "first-" and "second-generation" drugs in this class, could be considered as "third-generation" antihistamines.

Alkanes↗
PubMed1998

Molecular toxicology and the medicinal chemist.

Drug metabolism has a number of pharmacodynamic and pharmacokinetic consequences which cannot be ignored even at the early stages of drug research. A number of aspects of drug metabolism are thus of interest to medicinal chemists, e.g. prodrug and soft drug design. This mini-review focuses mainly on toxication resulting from reactions of functionalization and conjugation. In the former case, oxidoreductases can reduce xenobiotics to nucleophilic radicals, or oxidize them to electrophilic and oxidizable metabolites. Conjugation reactions also play a role in toxication by generating lipophilic residues (e.g. hybrid triglycerides) or adduct-forming metabolites (e.g. some acylglucuronides), or by interfering with physiological pathways (e.g. Coenzyme A conjugates). Functional moieties undergoing such reactions are known as toxophoric groups. Because they are the biochemical endpoint of several toxication reactions, macromolecular adducts are now of special significance in molecular toxicology. But, as discussed in the conclusion, the substrate specificity of drug-metabolizing enzymes, the many biological factors that influence metabolism, and various repair and removal mechanisms all contribute to decrease toxicological risks and to protect organisms.

Biotransformation↗
PubMed1998

Chiral inversion and hydrolysis of thalidomide: mechanisms and catalysis by bases and serum albumin, and chiral stability of teratogenic metabolites.

The chiral inversion and hydrolysis of thalidomide and the catalysis by bases and human serum albumin were investigated by using a stereoselective HPLC assay. Chiral inversion was catalyzed by albumin, hydroxyl ions, phosphate, and amino acids. Basic amino acids (Arg and Lys) had a superior potency in catalyzing chiral inversion compared to acid and neutral ones. The chiral inversion of thalidomide is thus subject to specific and general base catalysis, and it is suggested that the ability of HSA to catalyze the reaction is due to the basic groups of the amino acids Arg and Lys and not to a single catalytic site on the macromolecule. The hydrolysis of thalidomide was also base-catalyzed. However, albumin had no effect on hydrolysis, and there was no difference between the catalytic potencies of acidic, neutral, and basic amino acids. This may be explained by different reaction mechanisms of the chiral inversion and hydrolysis of thalidomide. Chiral inversion is deduced to occur by electrophilic substitution involving specific and general base catalysis, whereas hydrolysis is thought to occur by nucleophilic substitution involving specific and general base as well as nucleophilic catalysis. As nucleophilic attack is sensitive to steric properties of the catalyst, steric hindrance might be the reason albumin is not able to catalyze hydrolysis. 1H NMR experiments revealed that the three teratogenic metabolites of thalidomide, in sharp contrast to the drug itself, had complete chiral stability. This leads to the speculation that, were some enantioselectivity to exist in the teratogenicity of thalidomide, it could result from fast hydrolysis to chirally stable teratogenic metabolites.

Algorithms↗
PubMed1998

Mechanisms of liposomes/water partitioning of (p-methylbenzyl)alkylamines.

PURPOSE: The objective of this study was to compare and interpret the variations in lipophilicity of homologous (p-methylbenzyl)alkylamines (MBAAs) in isotropic (octanol/water) and anisotropic (zwitterionic liposomes/water) system. METHODS: Two experimental approaches were used, namely the pH-metric method to measure lipophilicity parameters in octanol/water and liposomes/water systems, and changes in NMR relaxation rates to validate the former method and to gain additional insights into the mechanisms of liposomes/water partitioning. RESULTS: For long-chain homologues (N-butyl to N-heptyl), the octanol/water and liposomes/water systems mostly expressed hydrophobicity. In contrast, the lipophilicity of the shorter homologues (N-methyl to N-propyl) in the two systems expressed various electrostatic and polar interactions. CONCLUSIONS: The study sheds light on the molecular interactions between zwitterionic liposomes and amphiphilic solutes in neutral and cationic form.

1-Octanol↗
PubMed1998

Evidence for the existence of [3H]-trimetazidine binding sites involved in the regulation of the mitochondrial permeability transition pore.

1. Trimetazidine is an anti-ischaemic drug effective in different experimental models but its mechanism of action is not fully understood. Data indicate that mitochondria could be the main target of this drug. The aim of this work was to investigate the binding of [3H]-trimetazidine on a purified preparation of rat liver mitochondria. 2. [3H]-trimetazidine binds to two populations of mitochondrial binding sites with Kd values of 0.96 and 84 microM. The total concentration of binding sites is 113 pmol mg(-1) protein. Trimetazidine binding sites are differently distributed. The high-affinity ones are located on the outer membranes and represent only a small part (4%) of total binding sites, whereas the low-affinity ones are located on the inner membranes and are more abundant (96%) with a Bmax=108 pmol mg(-1) protein. 3. Drug displacement studies with pharmacological markers for different mitochondrial targets showed that [3H]-trimetazidine binding sites are different from previously described mitochondrial sites. 4. The possible involvement of [3H]-trimetazidine binding sites in the regulation of the mitochondrial permeability transition pore (MTP), a voltage-dependent channel sensitive to cyclosporin A, was investigated with mitochondrial swelling experiments. Trimetazidine inhibited the mitochondrial swelling induced by Ca2+ plus tert-butylhydroperoxide (t-BH). This effect was concentration-dependent with an IC50 value of 200 microM. 5. Assuming that trimetazidine effectiveness may be related to its structure as an amphiphilic cation, we compared it with other compounds exhibiting the same chemical characteristic both for their ability to inhibit MTP opening and to displace [3H]-trimetazidine bound to mitochondria. Selected compounds were drugs known to interact with various biological membranes. 6. A strong correlation between swelling inhibition potency and low-affinity [3H]-trimetazidine binding sites was observed: r=0.907 (n=24; P<0.001). 7. These data suggest that mitochondrial sites labelled with [3H]-trimetazidine may be involved in the MTP inhibiton.

Animals↗
PubMed1998

Molecular modeling of hen egg lysozyme HEL[52-61] peptide binding to I-Ak MHC class II molecule.

A bound conformation of the antigenic decapeptide hen egg lysozyme HEL[52-61] associated to the mouse MHC class II (MHC II) I-Ak was modeled by homology with the three-dimensional structure of hemagglutinin HA[306-318]-HLA-DR1 complex. HEL peptide Tyr53 could not be aligned with the HA peptide Tyr308 because this resulted in a buried Tyr53 side chain within the I-Ak peptide-binding groove and this conflicted with this side chain being recognized by T cells. Therefore, Asp52 of HEL was fixed as the P1 anchor and aligned on Tyr308 of HA. After molecular dynamics, the modeled complex was stable even in the absence of any constraint. The peptide backbone adopted a polyproline II-like conformation with canonical hydrogen bonding between the peptide backbone and MHC II molecule. Asp52, IIe55, Gin57 and Ser60 were predicted to be deeply buried into P1, P4, P6 and P9 MHC II pockets, and Tyr53, Leu56, Asn59 and Arg61 as TCR contacting residues. The modeling of 15 complexes associating I-Ak with peptides derived from HEL[52-61] by single amino acid substitution proved stable with conserved hydrogen bonds and side chain orientation compatible with their recognition by two T cell hybridomas. Moreover, comparison with the recently solved crystal structure of the related HEL[50-62]-I-Ak complex revealed striking similarities.

Amino Acid Sequence↗
PubMed1998

Ligand specificity of the genetic variants of human alpha1-acid glycoprotein: generation of a three-dimensional quantitative structure-activity relationship model for drug binding to the A variant.

Human alpha1-acid glycoprotein (AAG) is a mixture of at least two genetic variants: the A variant and the F1 and/or S variant or variants, which are encoded by two different genes. In a continuation of previous studies indicating specific drug transport roles for each AAG variant according to its separate genetic origin, this work was designed to (1) determine the affinities of the two main gene products of AAG (i.e., the A variant and a mixture of the F1 and S variants) for 35 chemically diverse drugs and (2) to obtain meaningful 3D-QSARs for each binding site. Affinities were obtained by displacement experiments, leading to qualitative indications about binding site characteristics. In particular, drugs binding selectively to the A variant displayed some common structural features, but this was not seen for the F1*S variants. Three-dimensional QSAR analyses using the CoMFA method yielded a steric model for binding to the A variant, from which a simplified haptophoric model was derived. In contrast, no statistically sound model was found for the F1*S variants, possibly due (among other reasons) to an insufficient number of high affinity ligands in the set.

Alleles↗
PubMed1997

H3-receptor antagonists: synthesis and structure-activity relationships of para- and meta-substituted 4(5)-phenyl-2-[[2-[4(5)-imidazolyl]ethyl]thio]imidazoles.

We report the synthesis, octanol/water partition coefficient (log P), dissociation constants (pKa), H3-receptor affinity (pKi in rat brain membranes, [3H]-N alpha-methylhistamine), and H3-antagonist potency (pA2 in guinea ileum, (R)-alpha-methylhistamine) of novel H3-receptor antagonists obtained by introducing a para or meta substituent on the phenyl ring of the lead compound 4(5)-phenyl-2-[[2-[4(5)-imidazolyl]ethyl]thio]imidazole (3a). The substituents were chosen to obtain broad and uncorrelated variation in their lipophilic, electronic, and steric properties. The log P values of the neutral species cover almost 3 orders of magnitude (from 1.40 to 4.11). The pKa,2 values (protonation of the 2-thioimidazole fragment) vary from 3.13 to 4.34, indicating that this fragment, which incorporates the so-called polar group common to many H3-receptor antagonists, is neutral at physiological pH. The compounds had pKi values in a range too narrow (from 7.28 to 8.03) to derive QSAR equations. In one case (3g), a biphasic displacement curve was observed (pKi,1 = 8.53; pKi,2 = 6.90). The pA2 values ranged 2 orders of magnitude (from 6.83 to 8.87) and yielded a QSAR model (PLS) indicating that antagonist potency depends parabolically on lipophilicity and is decreased by bulky para substituents. The compounds of this series, therefore, maintain a fair-to-good affinity for rat brain H3-receptor and a fair-to-good H3-antagonist potency on guinea pig ileum, although varying markedly in their lipophilicity. The series thus appears as a good candidate for pharmacokinetic optimization leading to brain-penetrating H3-receptor antagonists.

Animals↗
PubMed1997

A cellular automata model of diffusion in aqueous systems.

A cellular automata model of a solute diffusing in water has been created and studied for the influential attributes. The results with this model are in agreement with experimental results; that is, that lipophilic solutes diffuse faster than do polar solutes. The model reveals that a solution composed of a relatively lipophilic solute permits a greater extent of diffusion of another solute. This observation is in agreement with the model showing a diffusion preference of a solute between two solutions made up of differing polarities. The solute diffuses farther into the lipophilic solution. A temperature-lipophilicity phase diagram shows the influence of these two attributes on the rate of diffusion. A model of diffusion through solutions containing stationary ingredients reveals a faster rate when the ingredient is lipophilic. We are led to a conclusion that the relative lipophilicity of solutes or stationary ingredients in a solution has a direct influence on the rates of diffusion of other solutes in their midst.

Diffusion↗
PubMed1997

Blood-to-brain transfer of various oxicams: effects of plasma binding on their brain delivery.

PURPOSE: The objective of this work was to assess the influence of binding to plasma proteins and to serum on the brain extraction of four antiinflammatory oxicams. METHODS: The brain extraction of isoxicam, tenoxicam, meloxicam and piroxicam was investigated in rats using the carotid injection technique. Blood protein binding parameters were determined by equilibrium dialysis using human serum, human serum albumin (HSA) and alpha-l-acid glycoprotein (AAG) solutions at various concentrations. RESULTS: All oxicams had low values of brain extraction, between 19% and 39% when dissolved in serum, i.e. under physiological conditions. Brain efflux rate constants calculated from the wash-out curves were the same in the absence or presence of serum. Brain efflux was inversely related to the polarity of the oxicams, such that the higher their H-bonding capacity, the lower their brain efflux. The free dialyzable drug fraction was inversely related to protein concentration. However, rat brain extraction was always higher than expected from in vitro measurements of the dialyzable fraction. CONCLUSIONS: Except for piroxicam whose brain extraction was partially decreased in the presence of proteins, the serum unbound and initially bound fractions of oxicams both seem available for transfer into the brain. Modest affinities for AAG rule out any related effect. More surprising is the apparent lack of effect on brain transfer of the high-affinity binding to HSA and serum. The enhanced brain uptake of meloxicam in the presence of AAG could be a result of interactions between this globular protein and the endothelial wall.

Adult↗
PubMed1997

Pharmacokinetics of beta-adrenoceptor blockers in obese and normal volunteers.

AIMS: Obesity can modify the pharmacokinetics of lipophilic drugs. As beta-adrenoceptor blockers (BB) are often prescribed for obese patients suffering from hypertension or coronary heart disease, this study compares the pharmacokinetics of lipophilic beta-adrenoceptor blockers in obese and control subjects. METHODS: Nine obese (157 +/- 24% of ideal body weight (IBW) mean +/- s.d.) and nine non-obese healthy volunteers (98 +/- 10% IBW), aged 32 +/- 9 years, were included in the study. Subjects were randomly given a single i.v. infusion of one of the following racemic beta-adrenoceptor blockers, whose doses (expressed as base per kg of IBW) were: propranolol (0.108 mg), labetalol (0.99 mg) and nebivolol (0.073 mg). The plasma concentrations of unchanged drugs were measured by h.p.l.c. The ionisation constants and lipophilicity parameters of beta-adrenoceptor blockers were assessed. RESULTS: The pharmacokinetic data for the three drugs were qualitatively similar. There was a trend towards a greater total distribution volume (Vss) in obese patients than in controls. However, Vss expressed per kg body weight was slightly smaller in obese patients. The relationship between Vss and lipophilicity of five beta-adrenoceptor was studied by combining the current results with those previously obtained with a moderately lipophilic drug (bisoprolol) and a hydrophilic one (sotalol). The Vss of the five drugs was positively and well-correlated (r2 = 0.90; P < 0.01) with their distribution coefficient at pH 7.4 (log D7.4), but not with their partition coefficients. The linear regression coefficients for lean and obese subjects were very similar. CONCLUSIONS: Lipophilic beta-adrenoceptor blockers seem to diffuse less into adipose than into lean tissues. All electrical forms of the drugs (i.e. cations, neutral forms, or zwitterions) present at physiological pH contribute to their tissue distribution, in both obese and lean subjects. Their tissue distribution in obese patients could be restricted by the sum of hydrophobic forces and hydrogen bonds they elicit with macromolecules in lean tissues.

Adipose Tissue↗
PubMed1997

Esterase-like activity of human serum albumin toward prodrug esters of nicotinic acid.

The esterase-like activity of human serum albumin (HSA) toward esters of nicotinic acid was investigated under a variety of conditions such as protein concentration, temperature, pH, ionic strength, nature of buffers, and presence of organic solvents. Initial rate constants of hydrolysis of 18 nicotinates in the presence of 50 microM HSA were measured at pH 7.4 and 37 degrees C. The substrates displayed half-lifes ranging from less than 15 min (2-butoxyethyl nicotinate) to more than 95 hr (methyl nicotinate). The hydrolysis of tert-butyl nicotinate was too slow to be measurable, whereas 1-carbamoylethyl nicotinate was stabilized against hydrolysis by the presence of HSA. The rate constants of HSA-catalyzed hydrolysis were well correlated (r2 = 0.85; N = 12) with previously published data obtained in human plasma, indicating similar substrate specificities in the two biological preparations. All evidence points to serum albumin as the possible major catalyst of hydrolysis of nicotinate esters in human plasma.

Esterases↗
PubMed1996

Inhibition of [3H]dopamine uptake into striatal synaptosomes by isoquinoline derivatives structurally related to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine.

Isoquinoline derivatives structurally related to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) or 1-methyl-4-phenylpyridinium (MPP+) may be endogenous neurotoxins causing nigral cell death in Parkinson's disease. These compounds inhibit mitochondrial function but, like MPP+, require accumulation in dopaminergic neurones via the dopamine reuptake system to exert toxicity. We, now, examine the substrate affinity of 14 neutral and quaternary isoquinoline derivatives (7 isoquinolines, 2 dihydroisoquinolines and 5 1,2,3,4-tetrahydroisoquinolines) for the dopamine reuptake system by their ability to inhibit the uptake of [3H]dopamine into rat striatal synaptosomes. Ten isoquinoline derivatives and MPP+ inhibited [3H]dopamine uptake in a concentration-dependent manner. Only 5 isoquinoline derivatives produced 50% inhibition of [3H]dopamine uptake (IC50 = 8.0-50.0 microM), none of which were as potent as MPP+ (IC50 = 0.33 microM). These findings suggest that isoquinoline derivatives are moderate to poor substrates for the dopamine reuptake system and that high concentrations of, or prolonged exposure to, isoquinoline derivatives may be necessary to cause neurodegeneration.

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine↗
PubMed1996

Effects of isoquinoline derivatives structurally related to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) on mitochondrial respiration.

Isoquinoline derivatives exert 1-methyl-4-phenylpyridinium (MPP+)-like activity as inhibitors of complex I and alpha-ketoglutarate dehydrogenase activity in rat brain mitochondrial fragments. We now examine the ability of 19 isoquinoline derivatives and MPP+ to accumulate and inhibit respiration in intact rat liver mitochondria, assessed using polarographic techniques. None of the compounds examined inhibited respiration supported by either succinate + rotenone or tetramethylparaphenylenediamine (TMPD) + ascorbate. However, with glutamate + malate as substrates, 15 isoquinoline derivatives and MPP+ inhibited state 3 and, to a lesser extent, state 4 respiration in a time-dependent manner. None of the isoquinoline derivatives were more potent than MPP+. 6,7-Dimethoxy-1-styryl-3,4-dihydroisoquinoline uncoupled mitochondrial respiration. Qualitative structure-activity relationship studies revealed that isoquinolinium cations were more active than isoquinolines in inhibiting mitochondrial respiration; these, in turn, were more active than dihydroisoquinolines and 1,2,3,4-tetrahydroisoquinolines. Three-dimensional quantitative structure-activity relationship studies using Comparative Molecular Field Analysis showed that the inhibitory potency of isoquinoline derivatives was determined by steric, rather than electrostatic, properties of the compounds. A hypothetical binding site was identified that may be related to a rate-limiting transport process, rather than to enzyme inhibition. In conclusion, isoquinoline derivatives are less potent in inhibiting respiration in intact mitochondria than impairing complex I activity in mitochondrial fragments. This suggests that isoquinoline derivatives are not accumulated by mitochondria as avidly as MPP+. The activity of charged and neutral isoquinoline derivatives implicates both active and passive processes by which these compounds enter mitochondria, although the quaternary nitrogen moiety of the isoquinolinium cations favours mitochondrial accumulation and inhibition of respiration. These findings suggest that isoquinoline derivatives may exert mitochondrial toxicity in vivo similar to that of MPTP/MPP+.

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine↗
PubMed1996

Toxicity to PC12 cells of isoquinoline derivatives structurally related to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine.

Isoquinoline derivatives structurally related to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine or 1-methyl-4-phenylpyridinium (MPP+) are inhibitors of mitochondrial function and substrates for the dopamine re-uptake system, but their neuronal toxicity is unclear. In this study, the effects of exposing PC12 cells to four isoquinoline derivatives (isoquinoline, N-methylisoquinolinium, 6,7-methylenedioxyisoquinoline and 1,2,3,4-tetrahydroisoquinoline) and MPP+ (100-1000 microM) were examined. All compounds exhibited concentration-dependent toxicity as determined by lactate dehydrogenase release, but none of the isoquinoline derivatives were more toxic than MPP+. Cytotoxicity of these compounds appears to be directly correlated with their substrate affinity for the dopamine reuptake system, but not mitochondrial inhibition. Thus, the low toxicity of isoquinoline derivatives towards PC12 cells suggests that high concentrations of or prolonged exposure to these compounds may be necessary to cause the neurodegenerative changes related to Parkinson's disease.

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine↗