A membrane model of the human oral mucosa as derived from buccal absorption performance and physicochemical properties of the beta-blocking drugs atenolol and propranolol.

The buccal absorption characteristics and physicochemical properties of the beta-adrenoceptor blocking agents propranolol and atenolol have been investigated to evaluate their permeation properties across biological lipid membranes. The dissociation constants, solubilities of free base, and n-heptane partition coefficients show that propranolol in its unionized form is much more lipophilic than atenolol, both drugs being bases with a similar pKa. Buccal absorption was studied under conditions of varying drug concentration, contact time, and pH, and controlled through the use of a non-absorbable marker. The absorption findings are in general agreement with the pH-partition theory. A new compartmental diffusional model that includes membrane storage and a hypothetical "aqueous pH-buffering surfaces system" allowed a more exhaustive interpretation to be made. A method for the estimation of the intrinsic pH and buffer capacity of the postulated surface system from drug pH-absorption data and partition coefficients alone is described. With human oral mucosa the intrinsic pH was near 6.7, and the buffering capacity of the system (expressed as the ratio deltapH/deltapH eff) about 2.86. The method was validated using published absorption data from the rat small intestine. Absorption of unionized drug through pores is shown to be negligible in the buccal absorption situation. The time course of absorption suggests membrane storage of lipophilic compounds; the in vivo partition coefficient of unionized propranolol relative to the mucous membrane could be calculated for the peusdo-steady state of absorption, i.e. the partition equilibrium between mouth content and membrane, to be approximately 776; this value is of the same order as the in vitro partition coefficient for the erythrocyte/plasma system. The lipid biophase of the buccal membrane is estimated semiquantitatively to be of intermediate polarity (epsilon = 3-4).     

Synthetic peptides derived from the β2−β3 loop of Raphanus sativus antifungal protein 2 that mimic the active site

Abstract: Rs‐AFPs are antifungal proteins, isolated from radish (Raphanus sativus) seed or leaves, which consist of 50 or 51 amino acids and belong to the plant defensin family of proteins. Four highly homologous Rs‐AFPs have been isolated (Rs‐AFP1–4). The structure of Rs‐AFP1 consists of three β‐strands and an α‐helix, and is stabilized by four cystine bridges. Small peptides deduced from the native sequence, still having biological activity, are not only important tools to study structure?function relationships, but may also constitute a commercially interesting target. In an earlier study, we showed that the antifungal activity of Rs‐AFP2 is concentrated mainly in the β2?β3 loop. In this study, we synthesized linear 19‐mer peptides, spanning the entire β2?β3 loop, that were found to be almost as potent as Rs‐AFP2. Cysteines, highly conserved in the native protein, are essential for maintaining the secondary structure of the protein. Surprisingly, in the 19‐mer loop peptides, cysteines can be replaced by α‐aminobutyric acid, which even improves the antifungal potency of the peptides. Analogous cyclic 19‐mer peptides, forced to adopt a hairpin structure by the introduction of one or two non‐native disulfide bridges, were also found to possess high antifungal activity. The synthetic 19‐mer peptides, like Rs‐AFP2 itself, cause increased Ca²⁺ influx in pregerminated fungal hyphae.     

Dependence of the absorption through the human oral mucosa of antihistaminic (H1) preparations on their physicochemical properties

Antihistaminic (H1) drugs pipolphen, suprastin and dimedrol are characterized according to absorption through human oral mucosa and distribution between phosphate buffer solution and organic solvents at various medium pH values. Their constants of ionization and coefficients of distribution in the n-octanol--phosphate buffer system, pH 7.4, as well as the bound fraction at the interaction of these drugs with human serum albumin were determined. The relationship between absorption through human oral mucosa of antihistaminic drugs and the hydrophobic character of their molecules and constants of ionization was found. An attempt was made to predict for them on the basis of the authors' own results and literature data possible pharmacokinetic and pharmacodynamic behavior in the organism.     

Evaluation of galantamine transbuccal absorption by reconstituted human oral epithelium and porcine tissue as buccal mucosa models: Part I

Over the last decade, interest in delivering drugs through buccal mucosa has increased. As a major limitation in buccal drug delivery could be the low permeability of the epithelium, the aim of this study was to evaluate the aptitude of galantamine, useful in Alzheimer’s disease, to penetrate the buccal mucosa. The evaluation of the ability of galantamine to permeate through the buccal epithelium was investigated using two permeation models. Firstly, in vitro permeation experiments were carried out using reconstituted human oral non-keratinised epithelium and Transwell diffusion cells system. Results were validated by ex vivo experiments using porcine buccal mucosa as membrane and Franz type diffusion cells as permeation model. The entity of buccal permeation was expressed in terms of drug flux (J_s) and permeability coefficients (K_p). Data collected by in vitro and ex vivo experiments were in agreement and suggested that buccal mucosa does not block diffusion of galantamine. The effects of drug application on histology of tissue specimens used in every experiment were also studied: no sign of flogosis and no significant cytological or architectural changes were highlighted.     

Evaluation of galantamine transbuccal absorption by reconstituted human oral epithelium and porcine tissue as buccal mucosa models: Part I

Over the last decade, interest in delivering drugs through buccal mucosa has increased. As a major limitation in buccal drug delivery could be the low permeability of the epithelium, the aim of this study was to evaluate the aptitude of galantamine, useful in Alzheimer’s disease, to penetrate the buccal mucosa. The evaluation of the ability of galantamine to permeate through the buccal epithelium was investigated using two permeation models. Firstly, in vitro permeation experiments were carried out using reconstituted human oral non-keratinised epithelium and Transwell diffusion cells system. Results were validated by ex vivo experiments using porcine buccal mucosa as membrane and Franz type diffusion cells as permeation model. The entity of buccal permeation was expressed in terms of drug flux (J_s) and permeability coefficients (K_p). Data collected by in vitro and ex vivo experiments were in agreement and suggested that buccal mucosa does not block diffusion of galantamine. The effects of drug application on histology of tissue specimens used in every experiment were also studied: no sign of flogosis and no significant cytological or architectural changes were highlighted.     

The use of liposomes as a model for drug absorption: β-lactam antibiotics

Liposomes were prepared from egg phosphatidylcholine-cholesterol-diacetylphosphate (80: 20: 5) and total lipid extracts of rat intestinal mucosa, and the permeability of the liposomal membrane to eight β-lactam antibiotics was studied by using a dynamic dialysis method. Although all the antibiotics used here are ionized and poorly lipid-soluble at pH 6·5, some of them are orally active and efficiently absorbed from the small intestine. The release rate constants from the aqueous dispersion of drug-entrapped liposomes were approximately in the order of their absorbability. Intestinal lipid liposomes were more permeable to the antibiotics than egg lecithin liposomes and the release rate constants for the drugs from intestinal lipid liposomes were strongly correlative with their absorption rate constants, except for cephalothin and ampicillin, the deviations of which could be explained by their surface activity. It is suggested that lipid components of the intestinal mucosa and the bilayer structure may play an important role in the absorption process of the antibiotics. The validity of liposomes as a model for the intestinal absorption of drugs is also discussed.     

Synthesis and properties of human γ-lipotropin

The synthesis of human γ-lipotropin by the solid-phase method is described. The synthetic product was characterized by R_f in partition chromatography on Sephadex G-50, paper electrophoresis, polyacrylamide gel electrophoresis, thin-layer chromatography, HPLC, end group determination, peptide mapping of a tryptic digest, and amino acid analyses of acid and enzymic digests. The synthetic material is identical to natural human γ-lipotropin when assayed against natural human β-lipotropin for lipolytic activity.     

β-Endorphin: synthesis and properties of human β-endorphin-(1–28) and its analogs

Three analogs of human β-endorphin (β_h-EP) were synthesized by the solid-phase method: β_h-EP-(1–28) (II), [D-Ala², Gln⁸] - β_h-EP-(1–28) (III). Radioreceptor binding assay with use of tritiated β_h-EP as primary ligand gave relative potencies as follows: β_h-EP, 100; I, 85; II, 380; III, 146. Relative potencies in an analgesic assay were: β_h-EP; 100; I, 18; II, 36; III, 13.     

Characterization of the recombinant human neuronal nicotinic acetylcholine receptors α3β2 and α4β2 stably expressed in HEK293 cells

HEK293 cells were stably transfected with the cDNAs encoding full-length human neuronal nicotinic acetylcholine receptor (nAChR) subunit combinations α3β2 or α4β2. [³H]-(±)Epibatidine ([³H]-(±)EPI) bound to membranes from A3B2 (α3β2) and A4B2.2 (α4β2) cells with K_d values of 7.5 and 33.4 pM and B_max values of 497 and 1564 fmol/mg protein, respectively.Concentration-dependent increases in intracellular free Ca²⁺ concentration were elicited by nAChR agonists with a rank order of potency of EPI>1,1-dimethyl-4-phenylpiperazinium (DMPP)>nicotine (NIC)=suberyldicholine (SUB)>cytisine (CYT)=acetylcholine (ACh) for A3B2 cells and EPI>CYT=SUB=NIC=DMPP>ACh for A4B2.2 cells. Antagonists of nAChRs blocked NIC-induced responses with a rank order of potency of d-tubocurarine (d-Tubo)=mecamylamine (MEC)>dihydro-β-erythroidine (DHβE) in A3B2 cells and MEC=DHβE>d-Tubo in A4B2.2 cells.Whole-cell patch clamp recordings indicate that the decay rate of macroscopic ACh-induced currents is faster in A3B2 than in A4B2.2 cells and that A3B2 cells are less sensitive to ACh than A4B2.2 cells. ACh currents elicited in α3β2 and α4β2 human nAChRs are maximally potentiated at 20 and 2 mM external Ca²⁺, respectively.Our results indicate that stably expressed α3β2 and α4β2 human nAChRs are pharmacologically and functionally distinct.     

Inhibition of human β‐tryptase by Bowman–Birk inhibitor derived peptides

Abstract: Four 11‐residue peptides based on the Bowman–Birk inhibitor (BBI) structure were synthesized. These were tested for their ability to inhibit human β‐tryptase. Peptides with a basic residue at P₁ inhibited tryptase even though the intact BBI protein is inactive. This result is interpreted in terms of the unique structural arrangement of active sites in tryptase which prevent access by large protein inhibitors.     

Theoretical π-π* absorption and circular dichroic spectra of β-turn model peptides

The dipole interaction model, treated by the partially dispersive normal mode method, is used to calculate π-π* absorption and circular dichroic spectra of β-turn model peptides in certain conformations. These include Ac-Gly-Gly-NHMe, Ac-L-Ala-L-Ala-NHMe, and Ac-L-Ala-Gly-NHMe in the standard β-turn conformations I, II, and III of Venkatachalam and cyclo(L-Ala-Gly-ε-aminocaproyl), cyclo(L-Ala-L-Ala—aminocaproyl), and cyclo(L-Ala-D-Ala-ε-aminocaproyl) in the minimum-energy conformations of Nemethy et al. Boltzmann average circular dichroic spectra of the cyclic compounds agree with experimental spectra in most respects. The results are compared with previous theoretical CD spectra for these molecules and with conformational assignments based on other evidence. Absorption spectra in the π-π* band are predicted to be moderately sensitive to conformation.     

The β-adrenoceptor blocking properties of the α-methyl analogues of propranolol and practolol in the anaesthetized dog

1. The beta-adrenoceptor blocking properties of alpha-methyl propranolol and alpha-methyl practolol were determined in anaesthetized dogs according to their abilities to modify the isoprenaline-induced effects on diastolic pressure, heart rate, myocardial contractile force, femoral arterial blood flow and pulmonary airway resistance.2. alpha-Methyl propranolol shifted the isoprenaline dose-response curves for the fall in diastolic pressure and the positive inotropic and chronotropic responses to the right in a parallel manner yielding pA(2) and slope values of 6.66 (0.92), 6.34 (0.77) and 6.59 (0.61) respectively. The slopes of the graphs for determining pA(2) values for the cardiac beta-adrenoceptor blocking properties of alpha-methyl propranolol were significantly less than 1 and indicated a mechanism other than simple, competitive, reversible antagonism.3. alpha-Methyl propranolol exerted a much weaker blockade of respiratory smooth muscle beta-adrenoceptors than has been reported for propranolol.4. alpha-Methyl practolol exerted a weaker blocking effect on myocardial beta-adrenoceptors than has been reported for practolol. No significant blockade of vascular or respiratory smooth muscle beta-adrenoceptors occurred after a total cumulative dose of 10 mug/kg of alpha-methyl practolol.5. alpha-Methyl substitution of propranolol and practolol reduces the potency but increases the selectivity of their beta-adrenoceptor blocking properties.6. The beta-adrenoceptors subserving cardiac stimulation, vasodilatation and bronchodilatation are representative of three different beta-adrenoceptor sub-types in the dog.     

α-Amino acids derived from ornithine as building blocks for peptide synthesis

Recently great interest has arisen in the synthesis of combinatorial libraries, and this technology provides a significant partner to contemporary strategies in rational design and lead discovery. By simple combination of a given set of building blocks, high numbers of different molecules are produced simultaneously, increasing the possibility of discovery of a lead compound in a limited time. One direction of research in this field focuses on the synthesis of libraries composed of modified amino acids. Here, the synthesis and characteristics of some building blocks derived from ornithine are described. The synthesis is based on the acylation/sulfonation of the copper complex of ornithine by aroyl and arylsulfonyl chlorides exemplified by 2-thiophenecarbonyl chloride, p-toluenesulfonyl chloride and 8-quinolinesulfonyl chloride. To evaluate the potential use of these modified α-amino acids as component in an oligopeptide library, all three derivatives were incorporated in a hexapeptide with a random sequence using a standard coupling procedure (DIC/HOBt/DIEA). Depending upon the acidity of the amido hydrogen on the δ-nitrogen, competition between intramolecular cyclization and peptide bond formation was observed. The higher the acidity, the more pronounced is this side reaction. Coupling conditions for peptide formation were optimized so that the newly described amino acid based building blocks are suitable for incorporation into libraries consisting of unnatural amino acids. The outlined procedures open up a broad avenue of possibilities for creation of diversity into peptidic libraries.     

Effect of oxidation of β-amyloid precursor protein on its β-secretase cleavage. A model study with synthetic peptides and candidate β-secretases

As the amyloidogenic processing of β-amyloid precursor protein (βAPP) proceeds under conditions of oxidative stress, the methionine-596 residue at the β-secretase cleavage point is likely in an oxidized state. In the present work, possible consequences of the oxidation of Met-596 for the generation of the N-terminus of amyloid p protein were modeled using synthetic peptide substrates, matching 587-606 sequence fragment of βAPP and containing either intact methionine or methionine sulfoxide. Patterns and rates for the cleavage of these substrates by purified mast cell chymase, cathepsin G, cathepsin D, matrix metallopro-teinase-3 and neutrophil elastase, were compared. Only the three first proteases, all previously suggested as candidate β-secretases, preferentially cleaved the “intact” substrate after Met-596. For chymase and cathepsin G, the specificity of this cleavage increased upon a shift from optimal alkaline pH to acidic pH, which is also more compatible with the plausible intracellular localization of amyloidogenic βAPP processing. The substitution of methionine sulfoxide for methionine in the substrate slowed down the cleavage rate for all the enzymes tested, by a factor of 6-15. This was associated with shifts of cleavage preferences to points of minor importance for the “intact” peptide, suggesting a specific resistance of the peptide bond after MetSO-596 against proteolysis.     

Agonist function of the recombinant α4β3δ GABAA receptor is dependent on the human and rat variants of the α4‐subunit

1. It is known that the α₄‐subunit is likely to occur in the brain predominantly in α₄β₃δ receptors at extrasynaptic sites. Recent studies have revealed that the α₁‐, α₄‐, γ₂‐ and δ‐subunits may colocalize extrasynaptically in dentate granule cells of the hippocampus. In the present study, we characterized a series of recombinant GABA_A receptors containing human (H) and rat (R) α₁/α₄‐, β₂/β₃‐ and γ_2S/δ‐subunits in Xenopus oocytes using the two‐electrode voltage‐clamp technique. 2. Both Hα₁β₃δ and Hα₄β₃γ_2S receptors were sensitive to activation by GABA and pentobarbital. Contrary to earlier findings that the α₄β₃δ combination was more sensitive to agonist action than the α₄β₃γ_2S receptor, we observed extremely small GABA‐ and pentobarbital‐activated currents at the wild‐type Hα₄β₃δ receptor. However, GABA and pentobarbital activated the wild‐type Rα₄β₃δ receptor with high potency (EC₅₀ = 0.5 ± 0.7 and 294 ± 5 μmol/L, respectively). 3. Substituting the Hα₄ subunit with Rα₄ conferred a significant increase in activation on the GABA and pentobarbital site in terms of reduced EC₅₀ and increased I_max. When the Hα₄ subunit was combined with the Rβ₃ and Rδ subunit in a heteropentameric form, the amplitude of GABA‐ and pentobarbital‐activated currents increased significantly compared with the wild‐type Hα₄β₃δ receptor. 4. Thus, the results indicate that the Rα₄β₃δ, Hα₁β₃δ and Hα₄β₃γ_2S combinations may contribute to functions of extrasynaptic GABA_A receptors. The presence of the Rα₄ subunit at recombinant GABA_A receptors containing the δ‐subunit is a strong determinant of agonist action. The recombinant Hα₄β₃δ receptor is a less sensitive subunit composition in terms of agonist activation.     

Pharmacological profiles of β-funaltrexamine (β-FNA) and β-chlornaltrexamine (β-CNA) on the mouse vas deferens preparation

The profiles of action of β-funaltrexamine (β-FNA) and β-chlornaltrexamine (β-CNA) have been assessed in the mouse vas deferens preparation. β-FNA, but not β-CNA, demonstrated a reversible agonist action that appeared to be mediated via κ-receptor interaction. β-CNA produced an irreversible antagonism of μ-, κ- and δ-mediated agonist actions, whereas β-FNA irreversibly antagonized μ-mediated agonist effects only. This selective action of β-FNA could also be seen following administration in vivo. β-CNA and particularly β-FNA should prove valuable in the elucidation of multiple opioid receptors.     

The Proteolytic Stability and Cytotoxicity Studies of l‐Aspartic Acid and l‐Diaminopropionic Acid derived β‐Peptides and a Mixed α/β‐Peptide

The use of peptides as drugs in pharmaceutical applications is hindered by their susceptibility to proteolysis and therefore low bioavailability. β‐Peptides that contain an additional methylene group in the backbone, are gaining recognition from a pharmaceutical stand point as they are considerably more resilient to proteolysis and metabolism. Recently, we reported two new classes of β ‐peptides, β ³‐ and β²‐peptides derived from l ‐aspartic acid and l ‐diaminopropionic acid, respectively. Here, we report the proteolytic stability of these β‐peptidic compounds and a mixed α /β‐peptide against three enzymes (pronase, trypsin and elastase), as well as, human serum. The stability of these peptides was compared to an α‐peptide. Peptides containing β‐linkages were resistant to all conditions. The mixed α /β‐peptide, however, exhibited proteolysis in the presence of trypsin and pronase but not elastase. The rate of degradation of the mixed α /β‐peptide was slower than that would be expected for an α‐peptide. In addition, these β‐peptides were not toxic to HeLa and COS‐1 cell lines as observed by MTT cytotoxicity assay. These results expand the scope of mixed α /β‐peptides containing β‐amino acids or small β‐peptide fragments as therapeutic peptides.     

Synthesis and properties of human β-endorphin-(1–17) and its analogs

Four analogs of human β-endorphin (β_h-EP) were synthesized by the solid-phase method: β_h-EP-(1–17) (I), [D-Ala²]-β_h-EP-(1–17) (II), [Gln⁸]-β_h-EP-(1–17) (III) and [D-Ala², Gln⁸]-β_h-EP-(1–17) (IV). Measurement in a radio-receptor binding assay with use of tritiated β_h-EP as primary ligand gave relative potencies as follows: Met-enkephalin, 100; I, 33; II, 47; III, 889; IV, 123; β_h-endorphin, 2253.