Bioavailability of amoxicillin and clavulanic acid from extended release tablets depends on intragastric tablet deposition and gastric emptying.

The rate and extent of amoxicillin and clavulanic acid absorption from pharmacokinetically enhanced extended release (ER) tablets is strongly influenced by the intake conditions. In order to investigate the cause of the food effects, a pharmacokinetic study with simultaneous imaging of the in vivo behaviour of the ER tablets by magnetic marker monitoring (MMM) was performed. Under fasting conditions the amoxicillin AUC (1854+/-280microg min ml(-1)) was significantly lower than after intake at the beginning of the breakfast (2452+/-354microg min ml(-1)) or after the breakfast (2605+/-446microg min ml(-1)). In contrast, clavulanic acid AUC was well comparable after tablet intake under fasting conditions and intake at the beginning of a breakfast (191+/-46 and 189+/-44microg min ml(-1), respectively) but significantly lower following a breakfast (126+/-71microg min ml(-1)). The localization data showed that the reduced bioavailability of amoxicillin under fasting conditions is due to early gastric emptying in combination with poor absorption from deeper parts of the small intestine. Prolonged gastric residence of clavulanic acid caused by intragastric tablet deposition in the proximal stomach was identified as the reason for the decreased bioavailability of clavulanic acid after tablet intake following the meal.

Efficiency of enterohepatic circulation, its determination and influence on drug bioavailability.

A new approach has been developed to determine the factors that influence the balance of drug elimination from the body. This approach is based on 1. a six-compartment-model with compartments connected by different flow rates assuming kinetic processes of first order, 2. on solutions of geometric series and 3. on numerical solutions of a system of non-linear equations. In the model, different ways of drug elimination have been considered: renal, liver and fecal elimination of the drug and its metabolism in the liver. The organs have been characterized by their drug availabilities. Further, the metabolic activity of the liver, the efficiency of drug absorption and re-absorption from the gastrointestinal (GI) tract have been included. This paper identifies three events, characterized by their efficiencies--1. hepatic excretion, 2. elimination of drug from liver into the gall bladder in its non-metabolized form and 3. the re-absorption of the drug from GI tract--as necessary conditions of enterohepatic circulation of (EHC). The product of these efficiencies has been introduced as the efficiency of enterohepatic circulation. Further, the drug bioavailability as a function of the efficiency of EHC is presented. The study shows that--based on the total amounts of non-metabolized drug in urine after p.o. and i.v. administration to animals with and without cannulated bile duct and in the bile of cannulated animals--the efficiency of EHC, bioavailability of the drug, renal and hepatic availability of the drug, metabolic activity of the liver and efficiency of drug absorption and re-absorption from the gut can be determined. Additionally, it has been shown that, depending on the efficiency of enterohepatic circulation, small variabilities in drug pharmacokinetic properties can cause high variance of drug bioavailability. The publication points towards the efficiency of EHC as on a factor that plays a key role in establishing in vitro-in vivo correlation.

A mathematical model for protein-induced lipid vesicle leakage: interaction of the intermediate filament protein vimentin and its isolated N-terminus with phosphatidylinositol vesicles.

A mathematical model was developed to analyze the leakage of phosphatidylinositol small unilamellar vesicles induced by the intermediate filament protein vimentin and its isolated N-terminal polypeptide. This model describes the kinetic and steady-state characteristics of this vesicle leakage as a direct action of protein on the lipid bilayer. Moreover, qualitative information at the molecular level can be deduced about protein-protein or protein-lipid interactions from the derived initial rate of vesicle leakage and the value of vesicle leakage at steady-state condition as a function of the protein concentration. Additionally, quantitative data on the inhibitory effect of various substances (here Ca2+ or Mg2+) can also be derived. This approach offers a possibility to compare interactions occurring within different protein-lipid systems by determining the characteristic parameters for the respective kinetic and steady-state conditions.

Specific interaction of the intermediate filament protein vimentin and its isolated N-terminus with negatively charged phospholipids as determined by vesicle aggregation, fusion, and leakage measurements.

The interaction of the intermediate filament protein vimentin and its non-alpha-helical N-terminus with phosphatidylserine and phosphatidylinositol small unilamellar vesicles was investigated by measuring vesicle aggregation, fusion, and leakage. While the N-terminus suppressed Ca2(+)-induced fusion of phosphatidylserine vesicles, it caused their rapid aggregation in the absence of Ca2+; at a molar ratio of lipid to polypeptide of 25:3, the polypeptide/lipid complexes precipitated from the reaction mixture. This aggregation was efficiently diminished by NaCl. The phosphatidylinositol vesicles, on the other hand, became leaky when interacting with the N-terminus of vimentin, even at a molar ratio of lipid to polypeptide of 500:1. The leakage of phosphatidylinositol vesicles was suppressed by the addition of Ca2+ or NaCl to the reaction mixture. Intact vimentin also caused leakage of phosphatidylinositol vesicles, at low and high salt concentration. The results indicate specific and differential interactions of the N-terminus of vimentin with various negatively charged lipid species, although there is an electrostatic component common to these interactions.

LH/hCG receptor in pig follicular fluid.

Follicular fluid isolated from large porcine follicles has been shown to have a specific water soluble 125I-hCG binding sites. After the binding of hCG to follicular fluid the receptor was stabilized against thermal denaturation. The release of LH/hCG receptor into follicular fluid was found to be dependent on its level in follicular cells which was increased together with the maturation of the follicle matures and, in part, could be related to membrane fluidity of the cells. The degree of fluorescence polarization of DPH and the order parameter of 5-doxyl-stearic acid labelled granulosa cells were lower in cells harvested from large follicles (6-12 mm) than in cells from small follicles (1-2 mm). It is suggested that soluble LH/hCG receptor may be actively released from follicular cells into follicular fluid. Specific hCG binding sites were found in the follicular fluid of the pig, rabbit, rat and cow, but not in that of sheep.

Modulation of rat testicular LH/hCG receptors by membrane lipid fluidity.

The specific binding of [125I]hCG to rat testicular membrane preparations was investigated as a function of membrane fluidity changed by lipids. Membrane fluidity was measured either by fluorescence depolarization of diphenylhexatriene or ESR spectra of I(1,14), I(12,3) and CAT 16 incorporated into the membrane. Incubation of membrane with cholesteryl-hemisuccinate increased both the rigidity of membrane lipids and the specific binding of [125I]hCG. A similar rigidifying action of saturated fatty acids was, however, not accompanied by increased accessibility of LH/hCG receptors. Treatment of testicular membranes with unsaturated fatty acids enhanced membrane fluidity but specific binding of the gonadotropin disappeared. In spite of the increase of LH/hCG receptors in cell membranes treated with cholesteryl-hemisuccinate, Leydig cells showed decreased sensitivity to cAMP response to LH stimulation. The results indicate that newly exposed LH/hCG receptors are not coupled with the adenylate cyclase system.

Influence of cholesterol and unsaturated fatty acids on [125I]hCG binding to rat testicular membranes.

Incubation of testicular membranes with cholesteryl-hemisuccinate resulted in an increase of both the membrane microviscosity and [125I]hCG specific binding. 14C-labelled cholesterol as well as fatty acids were effectively incorporated into membrane preparations. Insertion of unsaturated fatty acids (cis-vaccenic, linoleic, oleic, linolenic and arachidonic) into the membrane decreased its microviscosity under simultaneous disappearance of [125I]hCG specific binding. The latter phenomenon seems to be caused by an increase of nonspecific binding which was not due to the uptake of radio-activity from labelled gonadotropin by fatty acids or double bonds of fatty acids incorporated into testicular membranes or taken up by a resin Amberlite IRA-400. The disappearance of specific binding of [125I]hCG by the fluidizing action of unsaturated fatty acids can be reversed by the subsequent action of cholesterol.