Distribution of itopride hydrochloride in the rat gastric muscular layer was determined using radioluminography. To determine the level of radioactivity in the gastric muscular layer, a target organ, after administration of ¹⁴C-itopride hydrochloride, we established the quantitative imaging plate method (IP method). The obtained value in the gastric muscular layer was compared to that in the blood by this method.
As shown in the radioluminogram of 0.5 h after oral administration of ¹⁴C-itopride hydrochloride, the radioactivity was found in the gastric muscular layer. Same result was obtained from the radioluminogram of 0.5 h after intraduodenal administration of ¹⁴C-itopride hydrochloride to the rat. On the other hand, the radioactivity was not detected in the gastric muscular layer, when ¹⁴C-itopride hydrochloride was administered into stomach of the pylorus-ligated rat. These results suggest that ¹⁴C-itopride hydrochloride is distributed to the gastric muscular layer via blood after being absorbed from small intestine.
The level of radioactivity in the gastric muscular layer was about twice of that in blood as determined by this assay method. The radioactivity concentrations of the metabolites in the gastric muscular layer were determined. As the result, the unchanged itopride existed predominantly and other metabolites were found in small quantities in the gastric muscular layer after oral administration of ¹⁴C-itopride hydrochloride to rats.
These results indicate that the distribution of itopride in the gastric muscular layer is good enough to explain its pharmacological effect.

View full abstract

The present study used the published data to examine the relationship between accumulation factor (predicted value) and accumulation ratio (observed value) of a compound as a ratio of tissue concentration at 24 hr after single administration to that after repeated administration in rats. The possibility of the prediction of tissue accumulation is discussed.
Reports on tissue distribution have been published for 197 compounds in six journals from 1980 to 1993, and among them 55 compounds were found in this journal, Xenobiotic Metabolism and Disposition. Detailed investigation of these 55 compounds revealed that 30 compounds showed the accumulation ratios in 350 tissues of more than 3, a tentative critical value in this study. The order of probability of showing an accumulation ratio of more than 3 was: kidney, spleen, blood, liver, adrenal and skin. We tried to obtain reasonably assessed accumulation factor and using it to predict the extent of accumulation based on tissue concentration.
First, the t_1/2 obtained from the concentration at terminal two points was utilized to calculated the accumulation factor and compare it with the accumulation ratio. However, most of the accumulation factors were greater than the corresponding accumulation ratios and no equivalent relationship was obtained. This led us to try another method using the t_1/2 β value obtained by fitting to a multi-compartment equation (two or three exponential function) of tissue concentration after single administration. In this case, a tendency similar to that found with the previous method was obtained. Also, a tendency was found, larger accumulation factors leading to greater discrepancy between the accumulation factors and ratios. When the tissues from studies on 10 compounds were separated into four classes according to the accumulation ratio of 1-3, 3-5, 5-7 and more than 7, the t_1/2 β of these tissues after single administration showed means of 50.2, 65.3, 79.0 and 100.6 hr, respectively, indicating approximately proportional relationships between both values.
Some examples, on the simulation of tissue concentration for repeated administration ba sed on data from a single administration, are presented.

View full abstract

The absorption, distribution, metabolism and excretion of Ritipenem acoxil (RIPM-AC), an oral prodrug of antibiotic Ritipenem (RIPM), were investigated in rats and dogs after a single oral administration of ¹⁴C-RIPM-AC.
1. A fter oral administration of ¹⁴C-RIPM-AC to male rats at a dose of 30 mg/kg, the radioactivity level peaked at 30 min in blood, indicating rapid absorption from intestinal tract. In the in situ loop study, RIPM-AC was mainly absorbed from proximal region of the small intestine. About 50% of the administered dose was absorbed. The feeding condition did not affect the absorption. RIPM-AC was not detected in the plasma. RIPM-AC was hydrolyzed to RIPM in the intestinal tract. Maximum concentration of RIPM was found at 30 min.
2. The radioactivity levels in the kidney, urinary bladder and intestine were high and those in the other tissues were lower than 71% of that in the plasma. RIPM was detected by bioassay in the kidney and plasma up to 30 min but not in the lung and liver. The binding ratio of radioactivity to plasma proteins was 55% at 30 min after administration and increased with time. Irreversible binding also increased. The elimination of radioactivity from the fat, central nervous system, thyroid gland, adrenal gland, spleen, thymus, skeletal muscle, bone marrow and brown fat was slow.
3. The excretion of radioactivity in the urine, feces and expired air was completed within 48 hr after administration of ¹⁴C-RIPM-AC to male rats at a dose of 30 mg/kg, accounting for 47.7, 40.6 and 7.1% of the dose within 120 hr, respectively. The biliary excretion of radioactivity was 1.2% of the dose. RIPM accounted for more than 50% of the radioactivity excreted in the urine within 4 hr.
4. The metabolic fate of RIPM-AC was dose-dependent ranging from 10 to 100 mg/kg and independent of sex.
5. Species differences were observed in the metabolic fate of RIPM-AC between rats and dogs.

View full abstract

The absorption, distribution, metabolism and excretion of Ritipenem acoxil (RIPM-AC), an oral prodrug of antibiotic Ritipenem (RIPM), were investigated in rats after repeated oral administration of ¹⁴C-RIPM-AC.
1. The radioactivity levels in blood and tissues 24 hr after daily dosing reached a steady state by the 21st dosing. The radioactivity accumulated in the fat. The radioactivity levels in the other tissues were less than 8 times of those after single dosing.
2. The elimination of rad ioactivity from the blood and tissues after the 21st dosing was slower than that after a single dosing, especially from the central nervous system, aorta, fat, skeletal muscle, eyeball, skin and trachea.
3. Major metabolites in plasma after the 21st dosing were RIPM and polar metabolites. The concentration of the polar metabolites 8 hr after the 21th dosing was 3 times higher than that after single dosing.
4. The urinary excretion of radioactivity decreased with the number of doses. Major urinary metabolites were RIPM and P1 but the concentration of P1 decreased and the concentration of polar metabolites increased by repeated dosing.

View full abstract

1. Placental transfer of Ritipenem acoxil (RIPM-AC), an oral prodrug of antibiotic Ritipenem (RIPM), was investigated in rats on the 12th and 18th days of gestation after oral administration of ¹⁴C-RIPM-AC. Minor quantity of radioactivity was transferred to fetuses soon after administration, however, the elimination of radioactivity from fetuses was slow.
2. The radioactivity level in the milk was about 4 times higher than that in the plasma 8 hr after oral administration of ¹⁴C-RIPM-AC to lactating rats but declined more rapidly than that in the plasma.

View full abstract

Receptor-mediated endocytosis is one of the most important system for determining the disposition of physiologically active peptides. We reviewed the pharmacokinetic properties of granulocyte colonystimulating factor (G-CSF), one of the hematopoietic growth factors, and also the contribution of the receptor-mediated endocytosis in the disposition of G-CSF was discussed. At present, three kinds of recombinant human G-CSF have been on the market in Japan, i.e., rhG-CSF produced by CHO cells, rhG-CSF by E. coli and mutant G-CSF by E. coli which has a different amino acid sequence in the N-terminal region. These G-CSFs commonly show the non-linear pharmacokinetic properties in no relation to the sugar chain or N-terminal sequence. That is, the total clearances of G-CSFs in the experimental animals and humans decrease as the dose increases and those increase after the repeated administrations. Pharmacokinetic analysis of the plasma concentration-time profiles reveals that the total clearance of G-CSF consists of both saturable and nonsaturable clearances. The mean apparent bioavailability of G-CSF after subcutaneous administration is about 60%. The role of a sugar chain in the disposition of G-CSF is not so important, but the negative charges of sialic acids of the sugar chain effect the total clearance and the absorption from the subcutaneous sites. The clinical observations that the total clearance of G-CSF well correlates with the absolute neutrophil counts in the peripheral blood, suggest the contribution of the G-CSF receptor to the saturable clearance of G-CSF. In fact, the several studies using mutant G-CSF have evaluated that the receptor-mediated clearance of the G-CSF receptor in the bone marrow mainly contributes to the saturable G-CSF clearance. On other hand, the glomerular filtration in the kidney is the major mechanism of the nonsaturable clearance of G-CSF.

View full abstract

The absorption enhancers improve drug absorption affecting both of transcellular and paracellular routes, and the effect of the enhancers is larger in the paracellular route opening the tight junction. Theses findings are based on the effects on depolarization of fluorescence in the brush border membrane, Solomon's equation, voltage clamp method and impedance analysis of the colonic membrane. The mechanism of tight junction opening by caprate (C10) that is one of representative enhancers is revealed that C10 releses Ca ion from the intracellular store and that the released Ca ion contracts the actin filament through the activation of myosin light chain kinase. But for other representative enhancers such EDTA and palmitoyl carnitine, other different mechanism is needed but not cleared yet. Glucose transporter is found to work for the glycoside absorption, and sugar coupling with some oligo peptides enhance the absorption not only by transporter use but also by stabilization of peptides. Vitamin A protects the damage of small intestine by methotrexate and improves the absorption of nutrients.
Fluorescence spectra and protein aggregates which are produced during lipid peroxidation in the membrane such as microsome are measured directly from the membrane. They are used as indexes of the extent and site of lipid proxidation. The lipid peroxidation of heart mitochondria by doxorubicin (DXR) is demonstrated with these indexes. Chemiluminescence that is weak but measured very sensitively is also used as an index of lipid peroxidation. The chemiluminescence spectrum shows the production of the activated carbonyl and single oxygen during DXR oxidation. Naproxan which is known comparatively safe is revealed by the above indeces to give lipid peroxidation in liver during its metabolism. It is discussed that the lipid peroxidation in liver is a probable cause of liver damage which is occasionally reported for naproxen treatment.

View full abstract

Liver microsomal carboxylesterases (CES) function in the hydrolysis of a wide variety of endogenous and xenobiotic compounds, and play an important role in drug and lipid metabolism in many mammalian species. For past three decades, we have undertaken the diverse studies on CES from the view points of enzymology and drug metabolism. First, we have purified twenty-seven CES isozymes to electrophoretic homogeneity from liver microsomes of ten mammalian species and humans, and their physical, enzymological and immunological properties were compared each other. The CES isozymes from various species examined showed considerable similarities in physicochemical and immunological properties, but not similar in substrate specificities. Ramda Zap II library from mouse liver, and λ gt 11 libraries from human and rat liver were screened with antibodies raised to a purified liver CES isozymes. The deduced amino acid sequence of the clone possessed many structural characteristics that are highly conserved among rat RL1, RH1, RHlec, RS1 and RS2, mouse ML1, MH1, MS1 and human HU1, including active site sequence (GESAGG, NKQEXG, GDHXD), and four cysteines which may be involved in the specific disulfide bond. It is well known that proteins which are retained in the endoplasmic reticulum lumen contain the retention signal at their carboxyl-terminal of the tetrapeptide. The five CES clones (RL1, RH1, RHlec, MH1 and HU1) also contained ER-retention signal (HXEL). When clone was expressed in COS cells, the plasmid-coded protein was retained. The cells expressing CES were found to have very highly activity towards xenobiotic esters and amides. In conclusion, liver microsomal CES in mammalian and humans are closely involved in drug and lipid metabolism in the endoplasmic reticulum, and it is noteworthy that the isozymes from various species examined showed considerable similarities in amino acid sequences, but not similar in substrate specificities. These reasons may be, at least in part, due to the variances of substrate binding site.

View full abstract