Predicting drug release and degradation kinetics of long-acting microsphere formulations of tacrolimus for subcutaneous injection.

Today, tacrolimus represents a cornerstone of immunosuppressive therapy for liver and kidney transplants and remains subject of preclinical and clinical investigations, aiming at the development of long-acting depot formulations for subcutaneous injection. One major challenge arises from establishing in vitro-in vivo correlations due to the absence of meaningful in vitro methods predictive for the in vivo situation, together with a strong impact of multiple kinetic processes on the plasma concentration-time profile. In the present approach, two microsphere formulations were compared with regards to their in vitro release and degradation characteristics. A novel biorelevant medium provided the physiological ion and protein background. Release was measured using the dispersion releaser technology under accelerated conditions. A release of 100% of the drug from the carrier was achieved within 7 days. The capability of the in vitro performance assay was verified by the level A in vitro-in vivo correlation analysis. The contributions of in vitro drug release, drug degradation, diffusion rate and lymphatic transport to the absorption process were quantitatively investigated by means of a mechanistic modelling approach. The degradation rate, together with release and diffusion characteristics provides an estimate of the bioavailability and therefore can be a guide to future formulation development.

Novel Dissolution Approach for Tacrolimus-Loaded Microspheres Using a Dialysis Membrane for in Vitro-in Vivo Correlation.

The aim of this study was to establish a novel approach to in vitro dissolution evaluation using a combination of the paddle method and a dialysis membrane, both to predict the overall in vivo performance of tacrolimus microspheres and also to identify a suitable dissolution test method to describe the in vivo initial burst phenomenon. This new dissolution method for evaluating the release of tacrolimus from microspheres consisted of rotating a customized paddle inside a dialysis membrane using a conventional paddle apparatus. Findings were compared with a method in which the paddle was rotated outside the dialysis membrane, the conventional paddle method, and the flow-through cell method. We concluded that the paddle method with a dialysis membrane and internal agitation, which was designed to mimic in vivo conditions, predicted the overall pharmacokinetic (PK) profile of tacrolimus microspheres whereas the conventional paddle method described the initial burst. These findings suggest that it may not be possible to predict both the PK profile and initial burst using a single analysis method. We therefore recommend that evaluation of the initial burst be performed separately. In conclusion, we propose that combination of the paddle method with a dialysis membrane and internal agitation to evaluate the overall PK profile, together with the paddle method to describe the in vivo initial burst, represents a novel approach to in vitro dissolution evaluation for microsphere formulations.

Release mechanisms of tacrolimus-loaded PLGA and PLA microspheres and immunosuppressive effects of the microspheres in a rat heart transplantation model.

The objective of this study was to elucidate the release and absorption mechanisms of tacrolimus loaded into microspheres composed of poly(lactic-co-glycolic acid) (PLGA) and/or polylactic acid (PLA). Tacrolimus-loaded microspheres were prepared by the o/w emulsion solvent evaporation method. The entrapment efficiency correlated with the molecular weight of PLGA, and the glass transition temperature of PLGA microspheres was not decreased by the addition of tacrolimus. These results indicate that intermolecular interaction between tacrolimus and the polymer would affect the entrapment of tacrolimus in the microspheres. Tacrolimus was released with weight loss of the microspheres, and the dominant release mechanism of tacrolimus was considered to be erosion of the polymer rather than diffusion of the drug. The whole-blood concentration of tacrolimus in rats was maintained for at least 2 weeks after a single subcutaneous administration of the microspheres. The pharmacokinetic profile of tacrolimus following subcutaneous administration was similar to that following intramuscular administration, suggesting that the release and dissolution of tacrolimus, rather than the absorption of the dissolved tacrolimus, were rate-limiting steps. Graft-survival time in a heart transplantation rat model was prolonged by the administration of tacrolimus-loaded microspheres. The microsphere formulation of tacrolimus would be expected to precisely control the blood concentration while maintaining the immunosuppressive effect of the drug.

Effect of aminoalkyl methacrylate copolymer E/HCl on in vivo absorption of poorly water-soluble drug.

This study aimed to investigate in vivo absorption of tacrolimus formulated as a solid dispersion using Eudragit E®/HCl (E-SD). E-SD is an aminoalkyl methacrylate copolymer that can be dissolved under neutral pH conditions. E-SD was used alone as a solid dispersion carrier and/or was mixed with tacrolimus primarily dispersed with hydroxypropylmethylcellulose (HPMC). Tacrolimus was formulated with E-SD at several different ratios. Formulations with tacrolimus/E-SD ratio of 1/3 showed higher in vivo absorption, compared to tacrolimus dispersed in the excipients (primarily HPMC) found in commercially available tacrolimus capsules, using a rat in situ closed loop method. Good correlation was observed between in vitro drug solubility and in vivo drug absorption. In vitro solubility tests and rat oral absorption studies of tacrolimus/HPMC solid dispersion formulations were also conducted after mixing the HPMC dispersion with several ratios of E-SD. E-SD/tacrolimus/HPMC formulations yielded high in vitro drug solubility but comparatively low in vivo absorption. Dog oral absorption studies were conducted using capsules containing a formulation of tacrolimus/E-SD at a ratio of 1/5. The E-SD formulation-containing capsule showed higher in vivo drug absorption than tacrolimus dispersed in the standard HPMC capsule. These studies report enhancement of the in vivo absorption of a poorly water-soluble drug following dispersion with E-SD when compared to formulation in HPMC.

Aminoalkyl methacrylate copolymers for improving the solubility of tacrolimus. I: Evaluation of solid dispersion formulations.

The aim of this study was to investigate the effect of Eudragit E/HCl (E-SD) on the reprecipitation of a poorly water-soluble drug, tacrolimus. To evaluate the inhibition of reprecipitation of E-SD, reprecipitation studies on tacrolimus were conducted using a dissolution test method. Solubility of tacrolimus was measured at regular intervals in a dissolution media, in which tacrolimus was dissolved in ethanol, and the test media contained additives for inhibiting precipitation. Supersaturation profiles of tacrolimus were observed, and were maintained for 24h only with E-SD. Solid dispersion formulations of tacrolimus prepared with hydroxypropylmethylcellulose (HPMC) or E-SD in different drug/carrier ratios were also investigated. Solid dispersions prepared with E-SD showed higher solubility of tacrolimus compared with that of HPMC. In the E-SD formulation, the drug solubility influences to drug/carrier ratio. The formulation of drug/E-SD (1/5) showed the highest drug solubility. Thus, it may be inferred that a definite drug/carrier ratio exists to increase drug solubility. Further, by mixing E-SD the solid dispersion prepared with HPMC showed enhanced drug solubility.