Katsumi Miyazaki

Improving the Oral Bioavailability of Albendazole in Rabbits by the Solid Dispersion Technique

We have investigated the oral bioavailability of granules of albendazole, a drug used for treating echinococcosis in man, prepared by the solid dispersion technique.

Development of a new system for prediction of drug absorption that takes into account drug dissolution and pH change in the gastro-intestinal tract

A new system for prediction of drug absorption that takes into account drug dissolution and pH change in the gastro-intestinal tract was developed. In this new system, a drug (solid form) is added into a drug-dissolving vessel (pH 1.0) and the dissolved drug is transferred to a pH adjustment vessel (pH 6.0). Then the drug solution is transferred to the apical surface of Caco-2 cells, and the permeation rate of the drug across a Caco-2 monolayer is determined. This system was able to predict the oral absorption ratios of ten water-soluble drugs in humans. Using this system, it was predicted that drugs that permeated Caco-2 at a rate of more than 0.1% of the dose in 200 min would be almost completely absorbed after oral administration in humans. For a drug whose permeation ratio was less than 0.03%, the absorption ratio was predicted to be less than 30%. This system also enabled prediction of the absorption rate and variability in the absorption of albendazole, a drug with poor water solubility. It also enabled assessment of the improvement in absorption using a solid dispersion of albendazole-polymers that improved the water solubility. The results suggest that this system is useful for oral absorption screening of new drugs and pharmaceutical products.

Determination of ampicillin, amoxicillin, cephalexin, and cepharadine in plasma by high-performance liquid chromatography using fluorometric detection

Several methods for the assay of aminopenicillins [l-8] and aminocephalosporins [g-16] in the body fluids have been studied. These utilize techniques such as fluorometory [l-6, 9-12] and high-performance liquid chromatography with UV detection [7, 8, X3-16]. These methods including our methods, however, are not sufficient for concentrations below about 50 ng/ml of plasma. And in the case of the experiments which study intestinal absorption behavior of these drugs, a more sensitive method of determination in the body fluids is required. Recently, Uno et al. [17], Lebelle et al. [18], and Rarbhaiya et al. [19] showed that the structure of the fluorescent degradation products employed in some fluorometric assay procedures, including products obtained by our procedures, had a pyrazine‘ ring as a common structural unit. In this study, we have developed a more sensitive and a reproducible high-performance liquid chromatographic (HPLC) assay for ampicillin, amoxicillin, cephalexin, and cephradine based on the common structure of these fluorescent degradation products obtained by our previous assay procedures.

Comparison of transport characteristics of amino β-lactam antibiotics and dipeptides across rat intestinal brush border membrane

The transport characteristics of amino β‐lactam antibiotics, ampicillin and cephradine, have been examined and compared with that of glycylglycine using brush border membrane vesicles isolated from rat small intestine. The initial rate of glycylglycine uptake was markedly stimulated in the presence of an inward H + gradient compared with the uptake rates in the absence of an H + gradient. With the same H + gradient the stimulation of cephradine uptake was lower and ampicillin uptake was not altered. Cephradine uptake, however, was greater than that of glycylglycine in both vesicular conditions ((pH)i > (pH)0 and (pH)i = (pH)0). Inhibitory effects of dipeptides, ampicillin and cephradine on the initial uptake of glycylglycine were also examined. Glycylglycine uptake was significantly decreased in the presence of L‐phenylalanylglycine or carnosine. Ampicillin and cephradine did not alter the uptake of glycylglycine. These results suggest that the contribution of the inward H+ gradient to the permeation of ampicillin, cephradine and glycylglycine across the rat small intestinal brush border membranes is different for each of the substances examined.

An in vitro system for prediction of oral absorption of relatively water-soluble drugs and ester prodrugs

We developed an in vitro system simulating the physiological condition in the gastrointestinal (GI) tract for prediction of oral absorption of relatively water-soluble drugs and ester prodrug pivampicillin. This evaluation system includes a drug-dissolving vessel (DDV, assumed stomach), a pH adjustment vessel (PAV, assumed intestine) and a side-by-side diffusion chamber that is mounted by a Caco-2 monolayer, which is grown on a polycarbonate filter, or by a rat intestine between the donor and receiver compartments. Our proposed system can accommodate large amounts of solid drugs, simulating a drastic pH change process in GI tract, that is, an orally administered solid drug is dissolved in the stomach (pH 1-2) and transferred to the intestine (pH 6), and that dissolution process can also be monitored. The optimal flow rates for our system are 0.35-1.10 ml/min. Using this system, cumulative permeations of eight relatively water-soluble drugs were compared, and these cumulative permeations indicated the ability of drug absorption in humans. Drugs that permeated across a Caco-2 monolayer at cumulative permeation of more than 0.03% or over 0.04% in rat intestine can be almost completely absorbed in humans. If the cumulative permeation across a Caco-2 monolayer is lower than 0.03% or below 0.04% in the rat intestine, there was a good linear correlation between cumulative permeation across a Caco-2 monolayer and oral absorption in humans, or between cumulative permeation across a rat intestine and oral absorption in humans. In the case of relatively water-soluble drugs, a good linear correlation was obtained between cumulative permeation across a Caco-2 monolayer and cumulative permeation across a rat intestine. This result indicates that it is possible to predict the oral absorption of a relatively water-soluble drug in humans based on the cumulative permeation of the drug across a Caco-2 monolayer and/or a rat intestine. The time course of permeation of the ester prodrug pivampicillin, which is metabolized in a Caco-2 monolayer or in a rat intestine, was also evaluated. It stated clearly that it is also possible to predict the oral absorption of pivampicillin in humans based on the cumulative permeation across a Caco-2 monolayer or rat intestine. Our newly developed system enables more kinds of oral preparations and also pH-dependent soluble drugs to be evaluated.

Uptake characteristics of polyamines into rat intestinal brush-border membrane

The uptake characteristics of polyamines, such as spermine, spermidine and putrescine, have been investigated using brush-border membrane vesicles isolated from the small intestine of rats. The uptake of these polyamines into the membrane vesicles was high and the order of uptake was spermine greater than spermidine greater than putrescine at medium pH 7.5, respectively. The medium pH considerably affected the uptake of these polyamines and the amount of uptake increased remarkably with an increase of the medium pH (pH 7.5 or 8.0 greater than pH 5.5). An inward Na+ gradient did not stimulate the uptake rate of any of these polyamines. We have also examined the binding behaviour to the membrane lipid, phospholipids and total lipid, and there was a good correlation in the binding properties, pH-dependency and uptake activity, between the liposomes and brush-border membrane vesicles. These results suggest that the uptake of the polyamine into the vesicles consisted of rapid binding to the outside intestinal surface and slower binding to the inside membrane after permeation. Furthermore, findings from experiments concerning the mutual inhibition among these polyamines and concerning the effect of other polycations, having 2-5 amines in number, on the uptake of spermine, suggest that the number of amino groups in the polyamine molecules plays an important role in the uptake process into the brush-border membrane vesicles.

Contribution of Passive Transport Mechanisms to the Intestinal Absorption of β-Lactam Antibiotics

Abstract— The transport characteristics of aminopenicillins (ampicillin and amoxicillin), aminocephalosporins (cephalexin, cephradine and cefadroxil) and cefazolin have been compared with those of an actively transported substance (D‐glucose) and a passively transported substance (L‐glucose). Although the initial uptake of the aminocephalosporins was stimulated in the presence of an inward H+ gradient, there was no overshoot in the uptake of any of the drugs tested, even in the presence of an H+ gradient. Also, the time course and the degree of uptake of these drugs were similar to those of L‐glucose, especially in the absence of an H+ gradient. These results suggest that the β‐lactam antibiotics tested, like L‐glucose, pass through the rat intestinal brush border membrane mainly by passive diffusion. However, the differences in absorption between these drugs, like the differences in their disappearance from a proximal loop of rat intestine, cannot be explained by a simple permeation process alone.

Liquid chromatographic method for the determination of ganciclovir and/or acyclovir in human plasma using pulsed amperometric detection.

We have developed a simple, rapid and highly sensitive method for determining plasma concentrations of ganciclovir and/or acyclovir by using reversed-phase chromatography followed by pulsed amperometric detection. A linear relationship between the amount of ganciclovir (0.05-10 microg/ml plasma) or acyclovir (0.1-20 microg/ml plasma) and peak height ratio was obtained. The relative standard deviations of all standard curves were greater than or equal to 0.999. The limits of detection for ganciclovir and acyclovir quantitation were 10 ng/ml and 50 ng/ml (signal/noise >3), respectively. Daily fluctuations of plasma standard curves (n=5) for the ganciclovir and acyclovir samples were small, with relative standard deviations (RSD) of 3.3 and 4.5% (n=5), respectively. The intra-assay precision for the ganciclovir and acyclovir samples were 6.9 (n=5) and 5.5% (n=5), respectively. Inter-assay precision of ganciclovir (n=3) and acyclovir (n=3) ranged from 2.6 to 6.8% and 3.5 to 5.0%, respectively. Using this method, the pharmacokinetics and removal of ganciclovir during continuous hemodiafiltration (CHDF) in a liver transplant recipient being treated for severe cytomegalovirus infection was investigated. The mean (+/-SD) ratio of ganciclovir concentrations at the inlet and outlet of the dialyzer (C(outlet)/C(inlet)) was 0.56+/-0.09. The areas under the curves of ganciclovir up to 12 h postdosing (AUC(0-->12)) at the inlet and outlet of the dialyzer were 12.54 microg h/ml and 7.16 microg h/ml, respectively. The ultrafiltrate of ganciclovir was 16.6 mg. The terminal elimination half-life (T(1/2)) of ganciclovir during CHDF was 3.6 h. These results demonstrate that CHDF effectively removes ganciclovir. Until formal guidelines have been established, ganciclovir or acyclovir dosage should be adjusted according to the results of monitoring of plasma drug concentration. The method described here is suitable for clinical monitoring of plasma ganciclovir or acyclovir levels in solid organ transplant recipients and for use in studies involving pharmacokinetics.

Evaluation of pH-independent sustained-release granules of dipyridamole by using gastric-acidity-controlled rabbits and human subjects

The solubility of dipyridamole at pH 2.5 was about 6000-fold greater than that at pH 7.0. A commercial powder of dipyridamole showed pH-dependent dissolution. Two kinds of sustained-release granules of dipyridamole were prepared. The release rate of pH-dependent sustained-release granules was controlled by ethylcellulose (EC) and decreased with increasing medium pH. The release rate of pH-independent sustained-release granules was regulated by carboxymethylethylcellulose (CMEC), hydroxypropyl methylcellulose (TC-5) and Eudragit RS100, and was not influenced by varying pH of the medium. We used gastric-acidity-controlled rabbits to evaluate the variability in absorption after oral administration of these formulations. An extremely large difference in bioavailability between the high and low gastric acidity groups was observed after oral administration of the commercial powder. There were no statistically significant differences in the values of Cmax, Tmax, AUC0–12h and MRT between the high and low gastric acidity groups after administration of pH-independent sustained-release granules, while statistically significant differences in Cmax and AUC0–12 h were found between the two groups after administration of pH-dependent sustained-release granules. Furthermore, this pH-independent sustained-release granule preparation was administered orally to human subjects and compared with the commercial powder. There was no significant difference in the AUC0–12 h between the two preparations. It was also shown that the Cmax for the granules was about 50% of that for the commercial powder and that the plasma levels after oral administration of the granules were maintained over a longer duration than those of the commercial powder. It was found that the bioavailability was not influenced by variations in gastric acidity in rabbits and high bioavailability was achieved in human subjects after oral administration of the pH-independent sustained-release granule preparation, indicating that this preparation should be a useful dosage form for the potential reduction of interindividual variabilities in absorption.

Spermine uptake by rat intestinal brush-border membrane vesicles

The uptake of spermine by isolated rat intestinal brush-border membrane vesicles was studied. Uptake was biphasic, with an initial rapid uptake followed by a prolonged slower phase. Spermine uptake was not affected by a Na+ electrochemical gradient. The equilibrium uptake of spermine was considerably dependent upon the medium pH. At pH 7.5 the degree of uptake was higher than that at pH 6.5 and was inversely proportional to the extravesicular osmolarity with a relatively high binding, which was estimated by extraporation to infinite extravesicular osmolarity (zero intravesicular space), while the uptake at pH 6.5 was not altered under the various medium osmolarities. A kinetic analysis of the initial uptake rate of spermine at 37 degrees C gave a Km of 24.2 microM and Vmax of 206.1 pmol/mg protein per min. Furthermore, the uptake at 4 degrees C was nonlinear, providing evidence for saturability. These findings suggest that spermine was associated with intestinal brush-border membrane vesicles in two ways, by binding to the outside and inside of membrane vesicles. The interaction of spermine and the apical membrane can be a contributory factor in the accumulation of this polyamine in the intestine of the intact animal.