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The aim of this study was to investigate the ocular distribution of tacrolimus (FK506) and absorption into the systemic circulation after a single or repeated topical instillation of FK506 ophthalmic suspension in male New Zealand white rabbits.In the single instillation study (group 1), 29.1-34.8 microL of a 0.1, 0.3, and 1% suspension was administered to each of the 15 rabbits. In the repeated instillation study (group 2), 27.1-39.5 microL of a 0.3% suspension was administered to 27 rabbits q.i.d. (i.e., at 3-h intervals) for 14 days. In the intravenous (i.v.) dose study (group 3), 1 mg/kg of FK506 was administered to 3 rabbits. The amount of FK506 was measured by using a competitive enzyme immunoassay.The results for single and repeated instillation studies were similar. In the single instillation study, blood T(max) after an instillation of the 0.1, 0.3, and 1% suspensions (at 0.8, 1.0, and 1.0 hours) did not differ significantly among these doses. One (1) h after an instillation of the 1% suspension, ocular tissue concentrations, except the retina/choroid, vitreous body, and lens, were higher than the blood concentration (C(max): 2.7 ng/mL). In particular, concentrations in the conjunctiva, cornea, iris, and anterior sclera were much higher than the blood concentration (148, 900, 120, and 145 ng/g tissue). In the repeated instillation study, concentrations in the blood and ocular tissues (except the lens) reached a steady state by the 7th day. In the i.v. dose study, AUC(0-24h) and T(1/2) were 1643 ng h/mL and 18.5 h, respectively.The high-level distribution of FK506 was observed in the conjunctiva, which is desirable because the conjunctiva is the target tissue for pharmacologic effect (i.e., efficacy).

The aim of this study was to investigate the absorption, distribution, and excretion of radioactivity in male rabbits after a single or repeated instillation of (14)C-labeled tacrolimus (FK506) ophthalmic suspension or an intravenous (i.v.) administration of (14)C-FK506.The 0.3% (14)C-FK506 suspension was administered in single and repeated (three times, 5-min intervals) instillation studies, and 1 mg/kg of (14)C-FK506 was administered in the i.v. dose study.Results for single and repeated instillation studies were similar. In eyeball microautoradiograms, 15 min after dosing, the level of radioactivity in the cornea was the highest, followed by conjunctiva. After 1 h, little specific distribution was detected in the corneal epithelium, stroma, or Descemet's membrane. At 24 h, the level of radioactivity in the cornea decreased. Whole-body autoradiograms showed that the radioactivity was distributed to the digestive tract through the nasal meatus and esophagus and then was excreted into the feces. In the i.v. dose study, the distribution of radioactivity in whole-body autoradiographs was similar to that in quantitative tissue distribution measurements. The excretion of radioactivity in the urine and feces up to 168 h were 4.5 and 94.9%, respectively.After the ocular instillation, FK506 is first absorbed in the cornea, conjunctiva, and nasolacrimal duct, and then the rest is distributed to digestive tract through the nasal meatus and esophagus, after which it is excreted mainly into the feces.

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 (Cmax), 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.

The penetration and absorption of tacrolimus (FK506) in the skin of rat after a single topical application of 14C-labeled FK506 (14C-FK506) ointment under occlusive dressing was studied by semimicroautoradiography of the skin and by counting radioactivity in the sliced sections of the skin. Stability of FK506 in the skin was investigated in vivo and in vitro. 1. The distribution of the radioactivity of 14C-FK506 in rat skin was observed by semi-microautoradiography after a single dermal application of the ointment. In the intact skin, radioactivity of the ointment was mainly distributed in the stratum corneum of the epidermis during application for 24 hours. In the damaged skin, radioactivity was observed in the epidermis and dermis after application for 2 hours, and was evenly distributed in the dermis and subcutaneous connective tissues after application for 8 hours. Radioactivity was detected in the hair follicles of both the intact and damaged skin, but that was negligible in the sebaceous gland of the intact skin or was less than in the follicles of the damaged skin. 2. After a single dermal application of 14C-FK506 ointment to rats, the penetration of FK506 was investigated by counting radioactivity in the sliced section of skin. In the intact skin, maximal radioactivity was observed in the section of the epidermis throughout the application period for 24 hours. In the damaged skin, maximal radioactivity was detected in the dermis after application for 2 hours, and radioactivity in the sections was decreased thereafter and only a small amount of radioactivity was observed after application for 24 hours. 3. After application of 14C-FK506 ointment for 24 hours, levels of radioactivity were compared with those of FK506 in the skin, and the unchanged FK506 accounted for about 90% of radioactivity detected. 4. The oxidative metabolism of FK506 was studied in the skin and liver microsomes. Formation of the in vitro major metabolite, the 13-O-demethylated metabolite, in the skin microsomes was several-fold less intensive than in the liver microsomes.

The absorption, distribution, metabolism and excretion of tacrolimus (FK506) were studied in the rat after intravenous (i.v.) and oral administration of 14C-labeled FK506 (14C-FK506). 1. After i.v. injection at a dose level of 1.0 mg/kg, the pharmacokinetic parameters of FK506 in the whole blood were as follows: elimination half life, 6.4 hours; total body clearance, 1.59 l/h ·kg; and volume of distribution at steady state, 11.8 l/kg. After oral administration at a dose level of 3.2 mg/kg, the parameters in the whole blood were as follows: maximal blood concentration (Cmax), 41 ng/ml; time to reach Cmax, 0.25 hour; and area under the concentration-time curve (AUC0-24h), 255 ng·h/ml. Absorption of radioactivity and FK506 was 37 and 14%, respectively, calculated from the values of AUC0-24 h in the whole blood. Distribution of FK506 in the blood changed depending on its levels. The ratio of whole blood to plasma levels was more than 2.5 at less than the whole blood levels of 50 ng/ml but decreased above these levels. 2. Radioactivity was distributed throughout the body after i.v. injection of 14C-FK506 at a dose of 0.32 mg/kg. Radioactivity was highest in most of tissues at 5 minutes after injection, the first sampling point after injection, and the levels were higher than those in the plasma except that in the white fat, testis, cerebellum and cerebrum. At 72 hours after injection, radioactivity in most tissues decreased to less than 10% of the maximal concentrations except that in the cerebrum, testis and urinary bladder, in which about half of the concentration was detected. After oral administration at a dose of 1.0 mg/kg, radioactivity was distributed mainly in the gut and liver and was hardly detected in the other tissues. 3. During 72 hours after administration of 14C_ FK506 at a dose of 1.0 mg/kg, 8 and 95%of the dosed radioactivity were respectively excreted to the urine and feces after i.v. injection and 4 and 96% after oral administration. During 48 hours after i.v. injection to the bile duct-cannulated rats, 3, 8 and 82% of the dosed radioactivity were recovered in the urine, feces and bile, respectively, and 3, 6 and 35% during the same period after oral administration. 4. Less than 0.4% of the dosed radioactivity was excreted as unchanged FK506 in the urine, feces and bile, and the elution pattern of excreted radioactivity was very complex on high performance liquid chromatography and no major but many small radioactive peaks were observed. The major in vitro metabolite, 13-O-mono-demethylated metabolite, was detected in the urine, feces and bile but its portion to total radioactivity was very small.