For the purpose of clarifying in vivo pharmacokinetics of OPB-2045 (1-(3, 4-dichlorobenzyl)-5-octyl-biguanide hydrochloride) in rats, a single subcutaneous administration of ¹⁴C-OPB-2045 was given to male, female and pregnant rats to study the distribution of the compound by whole body autoradiography. Also, a single percutaneous administration of ¹⁴C-OPB-2045 and ¹⁴C-Chlorhexidine was given to rats to study and compare the distribution at the site of administration by microautoradiography.
1. In male and female rats, very high radioactivity was distributed at the site of administration and in the contents of the intestine, and high radioactivity in the adrenals and in the contents of the bladder. Disappearance of the radioactivity at the injection site was slow. Distribution of radioactivity in the brain, blood, testis, seminal vesicle, ovaries and uterus was slight.
2. Distribution in pregnant rats was characterized by slight radioactivity distribution in the ovaries and placenta and almost no present in the fetus and amniotic fluid.
3. When ¹⁴C-OPB-2045 was administered percutaneously, radioactivity was distributed mainly in the corneal layer of epidermis and was slightly distibuted in the hair and hair follicles. When Hibiten solution containing ¹⁴C-Chlorhexidine was administered percutaneously, radioactivity was distributed in the epidermis from the corneal to the basal layers, sebaceous gland and hair, and were slightly distributed in the hair follicles.

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Pharmacokinetics in the plasma, tissue distribution and excretion of TA-510, a novel hepatic agent, were investigated after a single oral (30 mg/kg) or intravenous (3 mg/kg) administration of ¹⁴C-labeled TA-510 to male rats and dogs.
1. The absorption extent of TA-510 across the intestinal barrier was estimated to be about 80% and 100% of the administered dose in rats and dogs, respectively. The absorption extent was calculated from the ratios of the urinary excretion of radioactivity after oral and intravenous administration of ¹⁴C-TA-510.
2. After oral administration, the plasma Cmax and AUC of the unchanged TA-510 in rats were approximately fifty- and eighty-fold lower than those in dogs, respectively. The AUC ratio of unchanged drug to total radioactivity accounted for only 2% in rat plasma, suggesting that TA-510 is susceptible to first-pass metabolism in male rats.
3. The radioactivity in most tissues reached the highest levels 6 hr after oral administration to male rats, the tissues showing higher level of radioactivity than that in blood were liver, a target tissue, and kidney, except gastrointestinal tract. The radioactivity levels in the lung and skin were similar to that in blood. The distribution into the other tissues such as brain, spleen and testis were shown to be extremely low. The radioactivity thereafter disappeared gradually from the tissues and was only detectable in the intestinal contents and liver 72 hr after administration.
4. Within 72 hr after oral administration to rats, 27.5% and 68.9% of the radioactivity was excreted into urine and feces, respectively. The cumulative excretion of radioactivity, within 48 hr after oral administration, in bile and urine of bile-duct cannulated rats were 72.2% and 5.5% of the dose, respectively. After intraduodenal injection of the bile to recipient rats, 35.9% of the radioactivity was reabsorbed. In dogs, 51.4% and 44.7% of the radioactivity was recovered from the urine and feces, respectively, within 72hr after oral administration.

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The metabolites of TA-510, a novel hepatic agent, were investigated by HPLC and LC/MS/MS after oral administration of ¹⁴C-TA-510 (30 mg/kg) to rats and dogs or TA-510 (200 mg) to human. The presence of conjugated metabolites was confirmed by enzymatic hydrolyses.
1. Major radioactive components in the urine of rats and dogs and in the bile of rats were the glucuronides of O-demethylated metabolites which consisted of M2 (3'-O-demethylated form), M3 (4'-O-demethylated form), and M4 (3', 4'-di-O-demethylated form). The unchanged TA-510 was found in trace amount, accounting for 0.25% and 1.38% of the administered dose in the urine of rats and dogs, respectively.
2. Within 24 hr after administration to rats, in urine 4.89% and 4.18% of the dose were excreted into the urine as M3-glucuronide and M4-glucuronide, and 29.15% and 4.82% of the dose were excreted into the bile as M3-glucuronide and M2-glucuronide, respectively. In dog urine, main metabolite was identified as M3-glucuronide, 18.22% of the dose, and the other metabolites accounted for less than 2% of the dose.
3. Within 24 hr after oral administration of TA-510 to human, the unchanged TA-510 was hardly detected in the urine (0.04% of the dose), and three kinds of glucuronides were found to be the main metabolites. The enzymatic hydrolyses of the human urine produced the aglucones M1 (4-ketone reduced form), M5 (4'-O-demethylated form of M1) and M3, which accounted for 7.73%, 11.11% and 17.02% of the dose, respectively.
4. The metabolites of TA-510 in human urine were analyzed using chiral column because TA-510 was administered as a racemate. There was a large difference in the enantiomer ratios [5S, 6R, 7S] / [5R, 6S, 7R] among the metabolites, 4.22-10.41 as M1, 0.99-2.31 as M3 and 0.18-0.34 as M5, suggesting that TA-510 racemate might be metabolized enantioselectively.

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To identify TA-510 reductase and obtain information on the stereochemical aspects, the reductase activity has been studied in subcellular fractions from 15 human livers, and finally purified using TA-510 enantiomers as substrates.
1. This activity was present in the cytosolic fraction and exhibited strong product stereoselectivity, i.e. both enantiomers were reduced exclusively to trans-alcohol (M1), but not cis-alcohol. Human liver microsomes showed practically no ketone reductase activity.
2. There was also a remarkable variation between the subjects in the substrate stereoselectivity, suggesting that racemic TA-510 was metabolized by at least two reductive enzymes with different stereochemical requirements.
3. The enzyme responsible for the reduction of (+)-TA-510 enantiomer was co-eluted with carbonyl reductase during purification steps, and separated from another enzyme with preference to (-)-TA-510 enantiomer as a substrate. Identity of (+) -TA-510 reductase and carbonyl reductase was also suggested by comparing the kinetic analysis and susceptibility to inhibitors of human liver cytosols and the purified enzyme.
4. (-)-TA-510 reductase was purified to a homogeneous protein and was shown as a monomeric protein with a molecular weight of 36 kDa. Amino acid sequences of five peptides obtained by proteolytic digestion of the purified enzyme were completely identical to the corresponding regions of previously reported 3α-hydroxysteroid/dihydrodiol dehydrogenase.

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Information concerning the relationship between pharmacokinetics and pharmacodynamics is essential for physicians and pharmacists when deciding a dosage regimen of drug therapy. This article assesses the significance of PK/PD analysis for adjusting dosage in drug therapy and for recognizing a subgroup in a population according to the response to drug therapy. PK/PD analysis can be used as decision-making tools for scientific and strategic decisions in all stages of drug development and it would be of great help in leading to optimized experimental design and thus reductions in development cost and time.

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The population pharmacokinetic analysis is a relatively new approach, which can be used to obtain the pharmacokinetic and/or pharmacodynamic information required to make rational decision about the magnitude of an initial dose for a given patient, and of a subsequent dose if adjustment is needed. The present article reviews our recent effort for population pharmacokinetics, focusing on the analyzes of digoxin disposition in the elderly and of phenytoin disposition in children. The population pharmacokinetic parameters of digoxin were estimated using 689 routine therapeutic drug monitoring data from 243 patients. A three-step approach combining Bayesian regression and nonlinear mixed effect analysis was useful for estimating population parameters of digoxin. On the other hand, a simulation study showed that the one-compartment model with dose-dependent clearance should be considered for estimating Michaelis-Menten elimination kinetics. Then, the population pharmacokinetic parameters of phenytoin were estimated using 531 routine therapeutic drug monitoring data from 116 epileptic patients. In addition, we showed that the genetic polymorphism of CYP2C isoenzymes plays an important role in the pharmacokinetic variability of phenytoin. The population pharmacokinetic approach was useful for analyzing the pharmacogenetics of phenytoin.

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The present paper reports a new approach to building a population pharmacokinetic and pharmacodynamic model (PPK/PD) and interpreting clinical data.
In design of PK studies for the bridging strategy, a simple formula which gives the 90 per cent confidence interval to prove no ethnic PK difference is proposed, which derives the approximate estimation method for calculation of power and sample size for the parallel group design.
In clinical studies, it is important to apply a technique to analyze the linking model with the drug concentration (pharmacokinetics, PK) and the efficacy (pharmacodynamics, PD) which are related closely. Effect compartment model and Jusko-type indirect PD model examples are shown using PPK/PD model analyzed with NONMEM.
Recently the validation method of NONMEM is concerned. The bootstrap approach has been to use to assess the stability and predictive performance of a pharmacokinetic model developed using NONMEM. ‘Guidance for Industry: Population Pharmacokinetics’ suggested the use of the bootstrap procedure as a satisfactory method for the validation and checking of population models in the drug approval process. Although validation methods are still being evaluated and may require further testing.
The PPK/PD approaches are hopeful about the future in the field of drug development and drug evaluation.

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Development of new drugs in recent years is expected to meet the global standards and takes aim at enlarging their market not only in Japan but also in the world wide circumstances. The global assessment standards based on the scientific evidences means all guidelines of each region and standard criteria of ICH (International Conference on Harmonization).
There was no guideline in relation to clinical pharmacokinetic studies in Japan. Therefore, we had to refer to the overseas editions in order to conduct such studies.
In last decade, ICH has been discussed among three regions, and many guidelines, especially pre-clinical and clinical ones have been agreed. According to ICH harmonization agreement, many local guidelines have been revised in each region.
In Japan, ‘Guidance on Clinical pharmacokinetic studies’ has been discussed among experts in this area and will be published shortly, after any important issues are clarified.
In this article, I will report any discussions about this Guidance in Japan as a member of the expert group, and consider the design and analysis method of clinical pharmacokinetic studies from the global viewpoint of new drug development.

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The ICH-E5 guideline issued in August, 1998 brought great change concept and strategy of drug development not only for Japanese pharmaceutical companies but also US and European companies. As only two years have passed since then, Pfizer's Viagra is so far the only product that obtained NDA approval in Japan with ICH-E5-based development. However many companies are now developing various products based on diverse ICH-E5 strategies and plans, and many successful cases will continuously appear within several years. Incidentally, what is important would be that complete clinical data package consisting of Japanese and foreign clinical data should clearly elucidate clinical efficacy and safety, and provide rationale for dosage and dose regimen. It is reported that number of OPSR consultation with bridging study is increasing year by year. A great level of disclosure of information for OPSR consultation on bridging manners is highly desirable, because many people are strongly concerned with what strategies and plans would be possible. The ICH-E5 guideline highlighted the importance of PK and PD studies in clinical development. They are essentially needed not only for analyzing similarity/difference in PK and PD between Japaneses and Americans/Europeans, but also elucidating correlation of dosage/dose regimen and clinical efficacy/safety. Further great progress of PK and PD studies is highly expected.

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For the quantitative estimation of the therapeutic/pharmacological effects and adverse/toxic reactions of drugs act on a specific receptor, we developed an integrated pharmacokinetic system incorporating pharmacokinetics, receptor occupancy theory and a ternary complex model. The developed system enabled to predict the optimal dosage for an investigational drug by using pre-clinical and clinical data (phase I study) under development, and to assess quantitatively the intensity of its adverse/toxic reactions, which may not be detectable in the trial stage. As for commercially available drug, the developed system may be useful to provide the optimal dosage regimen, which offers the desired therapeutic effects with minimum adverse/toxic effects. This methodology may be applied for the evaluation of the therapeutic and/or adverse effects of drugs with other mechanisms of action, such as an enzyme inhibitors/inducers, in future.

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Advances in the technology of human cell and tissue culture and the increasing availability of human materials for laboratory studies have made the use of in vitro human models in metabolism, transport, toxicology and pharmacodynamics studies much increased, and the obtained results can be used for quantitative modeling of pharmacokinetic behavior and estimating in vivo drug-drug interactions, pharmacological effects and/or toxicities. Bearing in mind the usefulness of such in vitro models in toxicological risk assessment, a workshop was held by the Society of Toxicology to evaluate the current status of in vitro human models and to make common recommendations on the use of such models especially to develop the PK basis risk assessment. This report summarizes the evaluation and recommendations on the application and improvement of in vitro models using human tissues, cells or subcellular fractions together with physiologically based pharmacokinetic (PBPK) modeling for quantitatively predicting human metabolism, potential drug-drug interactions and pharmaco-toxicodynamic behavior.

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