Noboru Hikichi

Stereoselective blood-brain barrier transport of histidine in rats

The transport characteristics of l- and d-histidine through the blood-brain barrier (BBB) were studied using cultured rat brain microvascular endothelial cells (BMEC). l-Histidine uptake was a saturable process. A decrease in incubation temperature from 37 to 0 degreesC or the addition of metabolic inhibitors (DNP and rotenone) reduced the uptake rate of l-histidine. Ouabain, an inhibitor of (Na+, K+)-ATPase, also reduced uptake of l-histidine. Moreover, the substitution of Na+ with choline chloride and choline bicarbonate in the incubation buffer decreased the initial l- and d-histidine uptake rates. These results suggested that l-histidine is actively uptaken by a carrier-mediated mechanism into the BMEC, with energy supplied by Na+. However, l-histidine uptake at 0 degreesC was not completely inhibited, and it was reduced in the presence of an Na+-independent System-L substrate, BCH, suggesting facilitated diffusion (the Na+-independent process) by a carrier-mediated mechanism into the BMEC. l-histidine uptake in rat BMEC also appeared to be System-N mediated since uptake was inhibited by glutamine, aspargine and l-glutamic acid gamma-monohydroxamate. System-N mediated transport was not pH sensitive. d-histidine transport was also studied in rat BMEC. d-histidine transport by rat BMEC has similar characteristics to l-histidine. However, System-N transport did not play a role in d-histidine uptake. The uptake of l-histidine was also greater than that of the d-isomer, indicating the stereoselective uptake of histidine in rat BMEC.

The Optical Resolution of Racemic Chlorpheniramine and Its Stereoselective Pharmacokinetics in Rat Plasma

Abstract— An ovomucoid‐conjugated column has been developed for the chiral stationary‐phase liquid chromatographic resolution of racemic chlorpheniramine with a quantitation limit of 0·05 μg mL−1. The assay was used to study the stereoselective kinetics of chlorpheniramine enantiomers in rats. After bolus intravenous administration of racemic chlorpheniramine maleate (20 mg kg−1), plasma concentration of the (–)‐form was higher than that of the (+)‐form. In the elimination phase, the concentrations of (+)‐ and (–)‐chlorpheniramine in the plasma declined biexponentially with half‐lives of 18·2 and 50·0 min, respectively. Although there was no significant difference in blood‐to‐plasma concentration ratio of both enantiomers, the apparent total blood clearance of (+)‐chlorpheniramine was twice as large as that of the (–)‐isomer. Binding of (–)‐chlorpheniramine to rat plasma protein was stronger than that of (+)‐chlorpheniramine suggesting stereoselective pharmacokinetics may be due to a difference in the plasma protein binding.

The Disposition of (R)-α-Methylhistamine, a Histamine H3-Receptor Agonist, in Rats

Abstract— Using a modified HPLC method with a fluorescence spectrophotometer and a weak cation exchanger, it was possible to separate (R)‐α‐methylhistamine (α‐methylhistamine) from histamine in plasma and various tissues. The assay was used to study the disposition and pharmacokinetic analysis of α‐methylhistamine after a bolus intravenous administration to rats. After rapid intravenous administration (12·6 mg kg−1), the plasma concentration declined biexponentially with a half‐life of 1·3 min in the elimination phase. The area under the plasma concentration‐time curve and total body clearance were 130 μg min mL−1 and 97 mL min−1 kg−1, respectively. After administration, α‐methylhistamine was immediately transferred to various tissues. The concentration was high in the kidney, lung, and liver (kidney > lung > liver), but low in the brain. The tissue‐to‐plasma concentration ratios in peripheral tissues were greater than 1, suggesting that the transfer of α‐methylhistamine to peripheral tissues was due to a specialized transport mechanism or possibly to tissue binding. However, the finding that the tissue/plasma ratio in the brain was lower than unity suggests that the transport system in this tissue depends on a concentration gradient, and that α‐methylhistamine crosses the blood‐brain barrier in rats with difficulty.

The disposition of thioperamide, a histamine H3-receptor antagonist, in rats

Abstract— An HPLC method using an ovomucoid‐conjugated column has been developed for measurement of thioperamide, a histamine H3 antagonist, with a minimum quantitation limit of 0·05 μg mL−1 The assay was used to study the disposition of thioperamide in rats. After bolus intravenous administration of thioperamide (10 mg kg−1), the plasma concentration decreased monoexponentially with a half‐life of 26·9 min. The apparent total body clearance of thioperamide from rat plasma was 74·6 mL min−1 kg−1. Although thioperamide was quickly transferred to various tissues, its concentrations in peripheral tissues were higher than that in the brain. However, the brain regional tissue/plasma ratios of thioperamide increased continuously after its injection.

Kinetics of buccal absorption of propafenone single oral loading dose in healthy humans

Buccal absorption has an advantage when compared with other administration routes because of its rapid onset of action. We examined the pharmacokinetics of buccal-absorbed propafenone in healthy humans. 1. After a single oral administration of 150 mg of propafenone, the average peak concentration of propafenone was 27.9+/-2.5 ng/ml, and that of 5-hydroxypropafenone was 61.7+/-6.6 ng/ml (n=5). The times to reach peak serum concentrations of propafenone and 5-hydroxypropafenone were 1.8+/-0.1 hr and 1.5+/-0.2 hr, respectively. 2. After a buccal absorption of 150 mg of propafenone, the time to reach peak serum concentration of propafenone was 16.9+/-2.3 min (n=8). The average peak concentration of propafenone was 30.4+/-1.4 ng/ml (n=8), and the concentrations of 5-hydroxypropafenone were below the detection limit in all subjects. The rapid upstroke of serum concentrations of propafenone by buccal administration may cause rapid onset of pharmacological conversion.

Stereoselective N‐Demethylation of Chlorpheniramine by Rat‐liver Microsomes and the Involvement of Cytochrome P450 Isozymes

Previous studies have suggested that degradation of the two stereoisomers of chlorpheniramine in the liver might be catalysed by different types of cytochrome P450. Stereoselective N‐demethylation of chlorpheniramine and the involvement of cytochrome P450 (CYP) isozymes have, therefore, been investigated in the liver microsomes of eight‐week‐old male rats.

In vivo microdialysis measurement of histamine in rat blood effects of compound 48/80 and histamine receptor antagonists

An in vivo microdialysis method combined with a highly sensitive HPLC method which was developed for the analysis of the mediators in the CNS has been applied to assay histamine concentrations in the blood. The technique was used to study the effects of compound 48/80 and histamine receptor antagonists on histamine release in the blood of rats. The mean basal level of histamine in the blood measured by in vivo microdialysis was 177.8 +/- 11.1 pmol/mL. This level was not affected significantly by intraperitoneal (i.p.) injection of saline, and remained at the constant level for at least 8 hr after injection of saline. After i.p. injection of histamine (0.5 mg/kg), histamine was quickly detected in the blood of the jugular vein. Moreover, because the recovered histamine in the dialysate is directly proportional to the free fraction in the blood, the in vivo microdialysis method of blood is a reliable method of examining histamine release into the blood. In our experiments, the histamine level in dialysates from rat jugular vein was markedly increased by compound 48/80 (2.0 mg/kg, i.p.), demonstrating the histamine release into the blood from mast cells. However, there was no increase in histamine concentration after an i.p. injection of histamine receptor antagonists, such as pyrilamine (2.0 mg/kg), d-chlorpheniramine (2.0 mg/kg), cimetidine (10 mg/kg), or thioperamide (10 mg/kg). Thus, the present results suggested that these histamine receptor antagonists might not have an influence on histamine release into the blood.

In‐vivo Microdialysis Measurement of 5‐Hydroxytryptamine and its Metabolites, 5‐Hydroxyindoleacetic Acid and N‐Acetyl 5‐Hydroxytryptamine, in Rat Blood: Effects of Histamine‐receptor Antagonists

The blood concentrations of 5‐hydroxytryptamine (5‐HT) and its metabolites, 5‐hydroxyindoleacetic acid (5‐HIAA) and N‐acetyl 5‐HT were assayed by in‐vivo microdialysis and a highly sensitive HPLC procedure that was originally developed to analyse CNS mediators. We investigated the effects of histamine‐receptor antagonists on 5‐HT metabolism and its release into the blood of rats.

Enantioselective Pharmacokinetics of α‐Fluoromethylhistidine in Rats and Its Comparison with Histidine

Abstract— The enantiomer‐specific pharmacokinetics of histidine and its analogue, α‐fluoromethylhistidine (FMH), were investigated in rats. After bolus intravenous administration of each enantiomer of histidine or FMH at a dose of 40·3 mg kg−1 as free base equivalents, the plasma concentrations of l‐histidine, d‐histidine, (S)‐FMH and (R)‐FMH decreased biexponentially with half‐lives of 39·2, 20·8, 32·8 and 25·0 min, respectively, in the elimination phase. Although the concentration of l‐histidine in the plasma was lower than that of d‐histidine, there was no large difference in plasma concentration‐time curves of the enantiomers of FMH. The apparent total clearance of l‐histidine from rat plasma was about 4 times that of d‐histidine or the enantiomers of FMH. l‐Histidine was quickly transferred to the peripheral tissues where the concentrations also decrease biphasically. l‐Histidine penetrated more rapidly into the brain than either its d‐enantiomer or a compound closely related in structure such as FMH. However, the disappearance of l‐histidine from the various brain regions was very rapid. In contrast, brain/plasma ratios of d‐histidine and (S)‐FMH increased continuously after injection of these compounds, indicating that d‐histidine or (S)‐FMH partitioned into the brain and was very slowly removed from the brain; (R)‐FMH was not distributed to the brain. These results suggested stereoselectivity in disposition of histidine and FMH enantiomers in rats.

ラット肝ミクロソームによるクロルフェニラミンのN-脱メチル化の立体選択性 : 年齢差, 性差に関する検討

We have established a simple method for the chiral stationary-phase liquid chromatographic resolution of racemic chlorpheniramine (Chp) and its two N-demethylated metabolites, monodesmethylchlorpheniramine (DMChp) and didesmethylchlorpheniramine (DDMChp), using an ovomucoid-conjugated column with respective quantitation limits of 5 ng/ml. The assay was used to study the age and sex difference in stereoselective N-demethylation of Chp by rat liver microsomes. The formation rate of each DMChp from racemic Chp was about 2.2 times faster with the S-(+)-enantiomer than the R-(-)-enantiomer in male rats at the age of only 8 weeks, whereas the rate of N-demethylation in male rats was not different between S-(+)- and R-(-)-enantiomers at the age of 3 and 24 weeks. The Vmax/Km value for the formation of S-(+)-DMChp increased with age in male rats, but the value in 8-week-old rats was lower than that in 3-week-old rats in the formation of R-(-)-DMChp. In female rats, on the other hand, the formation rate of DMChp from racemic Chp did not differ between S-(+)- and R-(-)-enantiomers at each age, suggesting a lack of stereoselectivity in the microsomal N-demethylation. Moreover, the Km and Vmax values for N-demethylation of Chp enantiomers were nearly identical between two consecutive ages in female rats. Further metabolism of DMChp to DDMChp was not observed between both enantiomers in male and female liver microsomes at the age of 3, 8 and 24 weeks.