Determination of tacrolimus in rabbit aqueous humor by liquid chromatography–electrospray ionization tandem mass spectrometry

Hyaluronic acid-coated niosomes facilitate tacrolimus ocular delivery: Mucoadhesion, precorneal retention, aqueous humor pharmacokinetics, and transcorneal permeability

Tacrolimus (FK506) was used to prevent corneal allograft rejection in patients who were resistant to steroids and cyclosporine. However, the formulation for FK506 ocular delivery remained a challenge due to the drug's high hydrophobicity, high molecular weight, and eye's physiological and anatomical constraints. The aim of this project is to develop an ocular delivery system for FK506 based on a combined strategy of niosomes and mucoadhesive hyaluronic acid (HA), i.e., FK506HA-coated niosomes, which exploits virtues of both niosomes and HA to synergistically improve ophthalmic bioavailability. The FK506HA-coated niosomes were characterized with particle size, zeta potential, and rheology behavior. Mucoadhesion of FK506HA-coated niosomes to mucin was investigated through surface plasmon resonance in comparison with non-coated niosomes and HA solution. The results showed that niosomes possessed adhesion to mucin, and HA coating enhanced the adhesion. The in vivo precorneal retention was evaluated in rabbit, and the results showed that HA-coated niosomes prolonged the residence of FK506 significantly in comparison with non-coated niosomes or suspension. Aqueous humor pharmacokinetics test showed that area under curve of HA-coated niosomes was 2.3-fold and 1.2-fold as that of suspension and non-coated niosomes, respectively. Moreover, the synergetic corneal permeability enhancement of the hybrid delivery system on FK506 was visualized and confirmed by confocal laser scanning microscope. Overall, the results indicated that the hybrid system facilitated FK506 ocular delivery on mucoadhesion, precorneal retention, aqueous humor pharmacokinetics and transcorneal permeability. Therefore, HA-coated niosomes may be a promising approach for ocular targeting delivery of FK506.

Ocular safety and pharmacokinetics study of FK506 suspension eye drops after corneal transplantation

PURPOSE

The aim of this study was to investigate the sensitization, pharmacokinetics, and absorption of FK506 after corneal transplantation.

METHODS

New Zealand albino rabbits were divided into normal and corneal transplantation groups. Each group was divided into 5 subgroups--saline, blank matrix, high-dose, medium-dose, and low-dose, respectively. There were 10 rabbits in each subgroup. One drop (25 μL) of FK506 was administered topically to both eyes of the rabbits 4 times daily for 30 days. Thirty days later, 5 rabbits of each subgroup were sacrificed after the administration of the last dose. Both eyes were enucleated; the left eye was used for pathologic examination and the right eye for the determination of FK506 distribution. The other 5 rabbits in each subgroup were sacrificed 14 days after the former 5 rabbits were sacrificed, and their eyes were enucleated for pathologic examination and tissue distribution determination as the former 5 rabbits in each subgroup (the second batch).

RESULTS

Fluorescein staining and local ocular reaction provided evidence that there were no significant differences between control and FK506-instilled eyes in the rabbit model at any of the tested doses. Histologic examination revealed no ocular abnormality in the rabbits instilled with any doses of FK506 eyedrop. The peak serum concentration (C(max)) of systemic absorption ranged from 4.31±0.79 ng/mL to 14.89±6.85 ng/mL.

CONCLUSION

Our study suggests that up to 0.1% FK506 administered 4 times a day (q.i.d.) topically is safe for the rabbit eye. However, further safety studies are required in view of systemic adverse effects.

Study of the effect of Wuzhi tablet (Schisandra sphenanthera extract) on tacrolimus tissue distribution in rat by liquid chromatography tandem mass spectrometry method

A liquid chromatography/tandem mass spectrometry (LC-MS/MS) method was developed and validated for determining tacrolimus (FK506) in rat tissues to study the effect of Schisandra sphenanthera extract on FK506 tissue distribution. After a liquid-liquid extraction with ethyl acetate, FK506 and ascomycin (IS) were subjected to LC-MS/MS analysis using positive electrospray ionization under multiple reactions monitoring mode. Chromatographic separation of FK506 and ascomycin was achieved on a Hypersil BDS C(18) column with a mobile phase consisting of methanol-water (containing 2 mM ammonium acetate, 95 : 5, v/v). The intra- and inter-batch precision of the method were less than 8.8 and 9.8%, respectively. The intra- and inter-batch accuracies ranged from 97.5 to 104.0%. The lowest limit of quantification for FK506 was 0.5 ng/mL. The method was applied to a FK506 tissue distribution study with or without a dose of Wuzhi (WZ) tablet. Most of the FK506 tissue concentrations were slightly increased after a concomitant WZ tablet dose, but the whole blood concentration of FK506 was dramatically increased 3-fold after a concomitant WZ tablet dose. These results indicated that the LC-MS/MS method was rapid and sensitive enough to quantify FK506 in different rat tissues, and strict drug monitoring is recommended when co-administering WZ tablet in clinical use.

Origin of the allyl group in FK506 biosynthesis

FK506 (tacrolimus) is a secondary metabolite with a potent immunosuppressive activity, currently registered for use as immunosuppressant after organ transplantation. FK506 and FK520 are biogenetically related natural products that are synthesized by combined polyketide synthase/nonribosomal peptide synthetase systems. The entire gene cluster for biosynthesis of FK520 from Streptomyces hygroscopicus var. ascomyceticus has been cloned and sequenced. On the other hand, the FK506 gene cluster from Streptomyces sp. MA6548 (ATCC55098) was sequenced only partially, and it was reasonable to expect that additional genes would be required for the provision of substrate supply. Here we report the identification of a previously unknown region of the FK506 gene cluster from Streptomyces tsukubaensis NRRL 18488 containing genes encoding the provision of unusual building blocks for FK506 biosynthesis as well as a regulatory gene. Among others, we identified a group of genes encoding biosynthesis of the extender unit that forms the allyl group at carbon 21 of FK506. Interestingly, we have identified a small independent diketide synthase system involved in the biosynthesis of the allyl group. Inactivation of one of these genes, encoding an unusual ketosynthase domain, resulted in an FK506 nonproducing strain, and the production was restored when a synthetic analog of the allylmalonyl-CoA extender unit was added to the cultivation medium. Based on our results, we propose a biosynthetic pathway for the provision of an unusual five-carbon extender unit, which is carried out by a novel diketide synthase complex.