In Vitro/In Vivo Correlation For C-14-Methylated Lysozyme Release From Poly(Ether-Ester) Microspheres

Purpose. The purpose of this study was to obtain an in vitro/in vivo correlation for the sustained release of a protein from poly(ethylene glycol) terephthalate (PEGT) / poly(butylene terephthalate) (PBT) microspheres.Methods. Radiolabeled lysozyme was encapsulated in PEGT/PBT microspheres via a water-in-oil-in-water emulsion. Three microsphere formulations varying in copolymer composition were administered subcutaneously to rats. The blood plasma was analyzed for radioactivity content representing released lysozyme at various time points post-dose. The in vitro release was studied in phosphate-buffered saline.Results. The encapsulation efficiency, calculated from the radioactivity in the outer water phase of the emulsion, varied from 60-87%. Depending on the PEG segment length and wt% PEGT, the lysozyme was released completely in vitro within 14 to 28 days without initial burst. C-14-methylated lysozyme could be detected in the plasma over the same time courses. The in vitro/in vivo correlation coefficients obtained from point-to-point analysis were greater than 0.96 for all microsphere formulations. In addition, less then 10% of administered radioactivity remained at dose site at 28 days for the microsphere formulations, indicating no notable retention of the protein at the injection site.Conclusion. The in vitro release in phosphate-buffered saline and the in vivo release in rats showed an excellent congruence independent of the release rate of C-14-methylated lysozyme from PEGT/PBT microspheres.