Absorption, distribution, metabolism and excretion of ¹⁴C-T-593 were investigated in rats after a single oral administration.
1. In the in situ loop study in rats, ¹⁴C-T-593 was widely absorbed from the duodenum to the colon.
2. The plasma levels of ¹⁴C T-593 after oral administration at doses of 10, 30 and 100 mg/kg to fasting male rats reached the C_max within 0.31 ?? 0.63 hr, and thereafter declined bi-phasically. Both the C_max and AUC_0-∞ increased in linear proportion to the dose. In non-fasting rats, the C_max and AUC_0-∞ were about 38% and 90% of those in fasting rats, respectively. The absorbed amount of ¹⁴C-T-593 after oral administration was slightly decreased by the food intake. No difference was observed in the plasma concentration-time profile between male and female rats. The plasma levels of ¹⁴C-T-593 after intravenous administration at a dose of 10 mg/kg to male rats declined tri-phasically. The bioavailability was calculated to be about 9% compared the AUC_0-∞ after oral and intravenous administration at a dose of 10 mg/kg.
3. The levels in most tissues after oral administration of ¹⁴C-T-593 at a dose of 10 mg/kg to fasting male rats reached the C_max at 0.5 hr, and thereafter decreased in parallel with plasma level. The radioactivity in the testis was eliminated slowly compared with other tissues.
4. The radioactivity in the gastric mucosa after intraduodenal administration of ¹⁴C-T-593 at a dose of 10 mg/kg to fasting male rats was high followed by the muscular layer and plasma. The gastric mucosa maintained high level that decreased slowly.
5. Most of the radioactivity in the urine, bile and feces after oral administration of ¹⁴C-T-593 at a dose of 10 mg/kg to fasting male rats was the unchanged T-593, and T-593M1, M2, M3 and M5 were found in minor quantities.
6. Within 96 hr after oral administration of ¹⁴C-T-593 at a dose of 10 mg/kg to fasting male rats, the radioactivity excreted in the urine and feces were 2.5% and 98.9% of the dose, respectively. No difference was observed in urinary and fecal excretion between fasting and non-fasting rats, and also between male and female rats. Within 96 hr after intravenous administration of ¹⁴C-T-593 at a dose of 10 mg/kg to male rats, the radioactivity excreted in the urine and feces were 47.2% and 51.2% of the dose, respectively.
7. Within 24 hr after oral administration of ¹⁴C-T-593 at a dose of 10 mg/kg to bile supplied rats, the radioactivity excreted in the bile and urine were 8.4% and 31 YO of the dose, respectively.
8. The plasma concentration-time profile after intravenous and intraportal administration of ¹⁴C-T-593 at doses of 0.1, 1 and 10 mg/kg to male rats indicated that about 30% of the orally absorbed ¹⁴C T-593 was excreted into bile by the first pass effect.

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The transfer of radioactivity into the fetus and milk after oral administration of ¹⁴C-T-593 at a dose of 10 mg/kg were investigated in pregnant and lactating rats.
1. The levels of radioactivity in the whole fetus and fetal tissues at 2 hr, after oral administration of ¹⁴CT-593 at a dose of 10 mg/kg to fasting pregnant rats on 19th day of gestation, accounted for less than 16.8% of that in the maternal plasma. The radioactivity in the fetal tissues was eliminated slowly when compared with the elimination from the maternal tissues.
2. The whole body autoradiograms, after oral administration of ¹⁴C-T-593 at a dose of 10 mg/kg to fasting pregnant rats on 12th and 19th day of gestation, showed that radioactivity was scarcely noted in the fetus.
3. The levels of radioactivity in the milk, after oral administration of ¹⁴C-T-593 at a dose of 10 mg/kg to non-fasting lactating rats, reached the C_max at 6 hr and was 2.4 times higher than that in the plasma. The levels in milk decreased in parallel with that in plasma.

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Blood concentration, distribution, metabolism and excretion of ¹⁴C-T-593 were investigated during and after repeated oral administration to male rats at a dose of 10 mg/kg for 21 days. In addition, the effects of T-593 on hepatic drug metabolizing enzyme system were also examined after repeated oral administration to male rats at the doses of 10, 30 and 100 mg/kg for 7 days.
1. The blood levels of radioactivity at 24 hr after daily dosing increased in parallel with the number of dosing. The blood level after the 21st dosing was 5.8 times higher than that after the 1st dosing.
2. The radioactivity levels in plasma and most tissues at 24 hr after the 1st, 7th, 14th and 21st dosing reached a steady state by the 14th dosing. The levels in the blood cells, hypophysis and testis after the 21st dosing decreased slowly compared with those after the 1st dosing.
3. Most of the radioactivity in the urine excreted within 24 hr after the 21st dosing consisted of the unchanged T-593 and the metabolic profile after the 21st dosing was similar to that after 1st dosing.
4. The cumulative excretion of radioactivity in the urine and feces within 24 hr after daily dosing was almost constant after the 2nd dosing. The cumulative excretion in the urine and feces within 96 hr after the 21st dosing was 1.09% and 96.35%, respectively.
5. Repeated dosing of T-593 had no effect on the drug metabolizing enzyme system in hepatic microsomes.

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The oxidative metabolism of tacrolimus (FK506) and its in vitro major metabolite was studied using rat liver microsomes. The microsomes were incubated with ¹⁴C-labeled (¹⁴C-) substrates and NADPH under aerobic condition, and the reaction products were analyzed by high-performance liquid chromatography. The 13-O-mono-demethylated metabolite (M-I) was detected as the major metabolite of 14-CFK506 at the early stage of incubation and many polar metabolites including di-demethylated and unidentified metabolites were detected at the late stage. The 13, 15 and 13, 31-O-di-demethylated metabolites (M-VII and M-VI, respectively) and more polar metabolites were produced from ¹⁴C-M-I. The oxidative metabolism of M-I by rat liver microsomes was higher in adult males than in females, was inhibited by the addition of polyclonal antibody against cytochrome P450 3A2, and was induced by the pretreatment of rats with phenobarbital and dexamethasone. These results indicate that FK506 is metabolized to polar metabolites via M-I, and the metabolism of M-I is catalyzed by rat cytochrome P450 3A subfamily, the same as in the case of FK506 metabolism.

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The absorption, distribution and excretion of tacrolimus (FK506) was studied in the baboon after intravenous (i.v.) and oral administration of ¹⁴C-labeled FK506 (¹⁴C-FK506) at respective dose levels of 1 and 10 mg/kg.
1. After i.v. injection of ¹⁴C-FK506, both total radioactivity and FK506 were eliminated bi or tri-exponentially from the body. The whole blood levels of radioactivity were higher than those in the plasma, and were almost the same as the FK506 levels in the whole blood, which were much higher than those in the plasma. Model-independent pharmacokinetic parameters of FK506 in the whole blood were as follows: elimination half-life (T_1/2), 12.3 hours; total body clearance, 0.265 L/h·kg; and volume of distribution at steady state, 3.93 L/kg. After oral administration, the parameters were as follows: T_1/2, 10.4 hours; maximal blood concentration (C_max), 190 ng/ml; time to reach C_max, 2 hours; and area under the concentration-time curve (AUC_0-∞), 2007 ng·h/ml. The bioavailability of radioactivity and FK506 was 7.0 and 5.1% respectively. The levels of radioactivity and FK506 were much lower in the plasma than those of the whole blood at FK506 level less than 250 ng/ml in the whole blood.
2. Tissue distribution of radioactivity was studied at 1, 24 and 120 hours after dosing with ¹⁴C-FK506. After i.v. injection, the radioactivity in all tissues collected was higher than that in the plasma 1 hour after injection, and was highest in the lungs, liver, pancreas, and small intestine. Radioactivity decreased gradually and was not detected in almost any tissues 120 hours after injection, except that maximal radioactivity in the large intestine was observed at 24 hours after injection. After oral administration, the radioactivity was detected in a limited number of the tissues such as thymus, liver, spleen, pancreas, bone marrow, and alimentary tract 1 hour after dosing and was not observed in almost any tissues 120 hours after dosing. The radioactivity in the body was mainly excreted in the bile to the feces after both administrations.
3. For 240 hours after administration of ¹⁴C-FK506, 5 and 86% of the dosed radioactivity were excreted in the urine and feces, respectively, after i.v. injection, and 4 and 79% after oral administration. The majority of radioactivity dosed was excreted in the urine and feces during the first 72 hours after both administrations.

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NK-104 is a synthetic inhibitor of 3 hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase with very potent lipid-lowering effect. Absorption, distribution, metabolism and excretion of NK-104 were investigated after a single oral or intravenous administration of ¹⁴C-NK-104 at a dose of 1 mg/kg to rats, mainly males.
1. The total radioactivity in plasma reached maximum levels within 1 hr after oral administration to male and female rats, and was followed by the elimination process consinting of three exponential. The T_1/2 in the terminal phase was relatively long (12 ?? 18 hr). Although the C_max was 325 and 415 ng eq./ml in males and females, respectively, the AUC was similar indicating no sex-related difference.
2. The tissue distribution was examined by both whole body autoradiography and quantitative radioassay. Following oral administration to rats, the radioactivity was selectively distributed to the liver, a target organ of this drug. The C_max was approximately 54 times higher in the liver than in plasma.
3. Almost all the radioactivity was excreted into feces after oral or intravenous administration to male and female rats, respectively. Biliary excretion was 75% up to 48 hr after oral administration and most of the drug was subjected to entero-hepatic circulation.
4. After oral administration to rats, the unchanged NK-104 was detected as the main component and a small amount of several metabolites was also detected in plasma and various tissues. In particular, the C_max and AUC of NK-104 in the liver were approximately 20 and 30 times, respectively, higher than those in plasma. In the present study, it is concluded that NK-104 is hardly metabolized via P-450 mediated oxidation resulting in the formation of few products of β-oxidation.

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NK-104 is a very potent competitive inhibitor of HMG-CoA reductase. In this study, the absorption, distribution, excretion and metabolism of ¹⁴C NK-104 were investigated during and after repeated oral administration to male rats at a daily dose of 1 mg/kg for nine days. The levels of radioactivity in the plasma at 0.5 and 24 hr after daily dosing reached a steady state after the 4th administration, and levels were about 2 times higher than those after the 1st administration. High levels of radioactivity were detected in the liver during the repeated administration with maximum levels being 35-51 times higher than those in plasma. The level of radioactivity in other tissues after final administration was about 3 times that after the 1st administration. The elimination of radioactivity from the tissues was rapid and complete by 144 hr after the final administration. The excretion ratio of radioactivity in the urine and feces up to 144 hr after the 9th administration was 0.14 and 97.91%, respectively. Unchanged NK-104 was mainly found in plasma during the repeated oral administration. Two major metabolites, M-6 (pentenoic acid derivative) and M-8 (propenoic acid derivative), as β-oxidation products of NK-104, were found in heart and skeletal muscle after the repeated oral administration at levels 2-4 times than after single administration. The radioactivity present in feces predominantly considered of the unchanged drug during the repeated administration. In conclusion, there was no significant accumulation of radioactivity due to repeated administration of ¹⁴C-NK-104 to rats.

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NK-104 is a very potent competitive inhibitor of HMG-CoA reductase. In this study, the transfer of radioactivity into the fetus and milk were investigated in pregnant and lactating rats after a single oral administration of ¹⁴C-NK-104 at a dose of 1 mg/kg. Whole body autoradiograms of rats on the 18th day of gestation showed a low distribution of radioactivity to amniotic fluid and fetus. On the 13th and 18th day of gestation, the levels of radioactivity in the placenta tissue were less than those in the maternal plasma. Therefore little radioactivity was transferred into the fetus, accounting for less than 36% of that in maternal plasma. The radioactivity in the milk reached a maximum level at 6 hr after administration to the lactating rats on the 14th day after delivery, being seven times higher than that in the maternal plasma. Although the composition of radioactivity in the milk consisted mostly of the unchanged NK-104, its concentration was below the detection limit at 48 hr.

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We developed a new vehicle system that can be encapsulated in soft gelatin capsules and utilized both in preclinical and clinical trials. The particle size distribution of the basic formulation (PEG 400: water: medium chain triglyceride (Miglyol 810®): polyoxyethylene (20) cetylether (BC-20TX®) =73.95 : 13.05 : 10 : 3) of the new vehicle system was very narrow and the particle size was smaller than 1μm immediately after preparation. The stability of the new vehicle was evaluated at room temperature, based on changes in the particle size distribution up to 12 days after preparation. The volume frequency profile of the particle size distribution became slightly broader after 1-day storage, thereafter remained constant during 5-day storage, and was again broadened and shifted to a range of lager diameters after 12 days. In the case of a formulation enriched with a hydrophobic phase (20 w/w% of medium chain triglyceride), the result was similar to that for the above basic formulation. When polyoxyethylene (20) cetylether (surfactant) content was decreased to 1.5 w/w%, however, the volume frequency profile after 12 days was further broadened, and two peaks were observed. Similar results were obtained in terms of parameters calculated from the cumulated frequency profiles of the particle size distribution. These results suggest that this new vehicle system can contain as much as 20 w/w% of hydrophobic phase, and requires 3 w/w% of surfactant (1.5 w/w% is insufficient).
When polyoxyethylene (20) cetylether was partially (10 w/w% of total surfactant) replaced with milk fat globule membrane (MFGM) for each of above three formulations, the volume frequency profile after 12 days was comparable with or narrower than that of the original one. It seems that MFGM is a co-surfactant useful to maintain the particle size distribution of new vehicles, preventing coagulation of particles.

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