The absorption, distribution, and excretion of radioactivity were studied in the male rat after a single dermal application of ¹⁴C-labeled FK506 ointment at an FK506 dose level of 1.6 mg/kg. The application period of the ointment was 24 hours.
1. Urinary and fecal excretion of radioactivity were 0.5 and 5.1% of the administered dose, respectively, over 168 hours after application to the intact skin under the non-occlusive dressing condition, 0.4 and 4.2% to the intact skin under the occlusive condition, and 2.4 and 53.8% to the damaged skin under the occlusive condition. Radioactivity remaining at the application site and in the animal body were respectively 14.6 and 0.3% at 168 hours after application to the intact skin under the non-occlusive condition, and were 4.7 and 0.4% after application to the damaged skin under the occlusive condition.
2. Radioactivity was not detected in the whole blood and plasma at any sampling points after ointment application to the intact dorsal skin under occlusive dressing condition. After application to the damaged dorsal skin under the occlusive condition, the pharmacokinetic parameters of radioactivity in the whole blood and plasma were respectively as follows: maximal concentration (C_max), 42.7 and 37.9 ng eq./mL; time to reach Cmax, both 2 hours; area under the concentration-time curve, 559 and 457 ng eq.·h/mL; and half life, 9.0 and 9.3 hours.
3. After ointment application to the damaged dorsal skin under the occlusive dressing condition, rank order of radioactivity detected at 30 minutes after application was as follows: lung and adrenal gland>brown fat, heart, thyroid gland, kidney, liver and spleen>plasma, urinary bladder and eyeball>cerebrum and testis. The maximal level of radioactivity was observed between 1/2 and 2 hours after application in most tissues. The proportion of radioactivity at 72 hours after application of the maximal level was less than 0.1 in all tissues except the urinary bladder where the proportion was 0.94.

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The pharmacokinetics of tacrolimus (FK506) were studied in the male rat after a single dermal application of FK506 ointment over an application period of 24 hours. In the typical experiment, the amount of ointment applied was 100 mg per each rat, and application area was 10 cm². The pharmacokinetic parameters estimated were maximal blood concentration (C_max), time to reach C_max (T_max), and area under the blood concentration-time curve (AUC).
1. Blood concentrations of FK506 were determined after a single dermal application of FK506 ointment under the occlusive or non-occlusive dressing condition. After respective application of 0.5 and 0.1% FK506 ointment to the intact and damaged skin, C_max and AUC values did not differ significantly between either dressing conditions. Blood concentrations of FK506 after a single dermal application of FK506 ointment to the damaged skin were much higher than those applied to the intact skin. The bioavailability of FK506 after application of 0.5% FK506 ointment to the intact skin was 5% and that to the damaged skin was 62%.
2. After a single dermal application of 0.03, 0.1 and 0.5% FK506 ointment to the intact skin, the C_max and AUC values increased in proportion to the strength of the ointment. After a single dermal application of 0.3% FK506 ointment to 2.5, 5 and 10 cm₂ of the intact skin, the C_max and AUC values increased proportionally to the area of the skin. The C_max and AUC values did not the differ significantly after single dermal application of 10, 30, 100 and 300 mg of 0.3% FK506 ointment to the intact skin.

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To examine whether the retention of imidazole analogues in the organ rich with elastic connective tissue can be of toxicological significance, the effect of ¹⁴C labeled 2-methylimidazole (2MI) as a model compound on the mechanical, biochemical, and morphological properties of the rat aorta, together with its irreversible interaction with the tissue macromolecules, was investigated after chronic treatment during maturation (10 and 100 μmol/kg/day for 28 days, from 4 to 8 weeks of age, subcutaneous injection).
1. Aortic extensibility was significantly decreased in 2MI-dose-dependent manner, whereas tensile strength showed an increasing trend.
2. Aortic elastin content was decreased dose-dependently, while collagen content was increased.
3. The low dose of 2MI scarcely altered histological picture of the aortic wall compared to the control group (isotonic saline dosing), but the high dose caused focal disorganization and fragmentation of the medial elastic laminae. Thickness of the aortic media showed a dose-dependent increasing trend.
4. The level of irreversibly bound 2MI-equivalent in elastin fraction was considerably high as compared with that in non-elastin fraction, and was increased dose-dependently. The aortic microautoradiograph showed that [¹⁴C]2MI-derived radioactivity occurred in the media, the region rich with elastic fiber.
These findings indicate that 2MI affects maturation of the elastic organ materially, and that the effect can be closely linked with the binding of 2MI to elastin. It is possible that the same holds true for some other imidazole-based compounds that are retained in large quantities in the elastic connective tissue.

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WinPBPK is a computer program for the analysis of drug distribution in the body which utilizes a physiologically-based pharmacokinetic model, where linear ordinary differential equations are numerically solved by the Runge-Kutta-Gill method. This program is written in Visual BASIC (VB) language. Since VB runs under Windows and has a high level user interface, WinPBPK is available to all DOS/V IBM compatible computers. Numerically solved tissue or organ drug concentrations vs. time data are shown on the CRT display as concentrations vs. time curves and thereafter are saved on hard disk. These numerical data can also be shown as simulated image whole body autoradiograms where drug concentrations in all the tissues or organs are converted to monochrome or full color image data. These image data can be visually compared to that obtained by real autoradiography using ¹⁴C-labelled drug. The system was applied to the pharmacokinetic analysis of tacrolimus, an immunosuppressant, in rats. The PBPK model for tacrolimus was developed with 17 tissues and organs including lung, blood, brain, heart, liver, pancreas, spleen, intestine, kidneys, gall bladder, thyroid, fat tissue, muscle, testes, bone and skin. The exchange rate of the amount of drug in each tissue or organ was described with differential equations using physiological parameters, biochemical parameters and pharmacokinetic parameters. The simulated image autoradiograms of tacrolimus at 15 min, 1 hr, 3 hr, and 8 hr after iv bolus injection to rats, 1 mg/kg, were compared with the autoradiograms of ¹⁴C tacrolimus being injected to rats, 1 mg/kg. There was good correlation between the simulated image autoradiograms and the real ones. By means of this software, the predicted pattern of drug distribution to the tissues and organs based on the PBPK model analysis can be compared visually to the real autoradiogram.

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The absorption, distribution and metabolism of bromfenac sodium were investigated after single or repeated application of ¹⁴C-bromfenac sodium ophthalmic solution to both eyes of male albino rabbits at a dose of 0.1 mg/animal (0.05 mg/50 μl/eye).
1. After single instillation, plasma level of radioactivity reached a maximum of 113.2 ng eq./ml at 30 min and then declined with a half-life of 2.2 hr from 1 hr to 12 hr. The radioactivity levels, in almost half of the ocular tissues, reached the maxima at 30 min after administration. Radioactivity distribution to the corneas was the highest, followed by the anterior sclera and conjunctiva. Radioactivity levels in the lens and vitreous body were the lowest. At 72 hr after administration, radioactivity level in the lens was 7% of the maximum but radioactivity in the other tissues was below the detection limit.
2. During repeated instillation, radioactivity level in plasma was below the detection limit at 24 hr after a single dose, but increased with the number of doses after 7 doses and then reached a steady state by 21 doses. Radioactivity levels in ocular tissues increased with the number of doses except a part of tissues, but then reached a steady state by 21 doses. Radioactivity levels in the lens and posterior sclera after 21 doses were 4.0 and 1.6 times higher than those after a single dose, respectively. Elimination of radioactivity from the plasma and ocular tissues after 21 doses was slightly slower than that after a single dose.
3. Parent compound in the aqueous humor collected at 1, 2 and 4 hr after single instillation accounted for 73.0-79.5% of radioactivity in aqueous humor. Four metabolites, a benzoic acid derivative AHR-11665, formed by oxidation of the carboxymethyl group of bromfenac, its precursor WAY-127039-A-1, and 2-oxyindole derivative AHR-10240 and isatine derivative AHR-11652 formed by intramolecular cyclization were detected. Parent compound in the plasma accounted for 76.7-80.5% of radioactivity in plasma at 1, 2 and 4 hr after single instillation. Unknown metabolites 1RaP1, 1RaP2 and 1RaP3 were detected in addition to the metabolites detected in the aqueous humor.

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Radioluminography (RLG) has been developed as one of the two-dimensional radiodetection techniques and widely applied to the drug evaluation studies. As the validation items of RLG, 1) the uniformity examination in fl-ray sensitivity, 2) the evaluation of background PSL value (PSL_BG) and monitoring radioactive contamimination of IP, and 3) the accurate expression of radioactivity in Bq instead of PSL were pointed out. The uniformity in β-ray sensitivity was examined by analyzing the IPs exposed to a planar ¹⁴⁷Pm radiation source. A small but definite non-unifomiry in sensitivity was observed, which could be attributed to the mechanics of BAS. New methods for accurately evaluating PSL_BG and monitoring radioactive contamination of IP were proposed. The usefulness of microplate-RLG as off-line counting of HPLC was emphasized. A new radiographic method that images the density distribution of lyophilized animal sections of WBA was described. This technique made it possible to measure the thickness (mg/cm²) of each tissue and correct the self absorption of β-ray, and thereby opened a road to a genuine quantitative WBA.

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