Encapsulation of hydrophobic drugs in polymeric micelles through co-solvent evaporation: the effect of solvent composition on micellar properties and drug loading.

This study was designed to develop an optimized co-solvent evaporation procedure for the efficient encapsulation of hydrophobic drugs in polymeric micelles of methoxy poly(ethylene oxide)-block-poly(epsilon-caprolactone) (MePEO-b-PCL). MePEO-b-PCL block copolymers having varied MePEO and PCL molecular weights were synthesized, assembled to polymeric micelles, and used for the encapsulation of cyclosporine A (CyA) by a co-solvent evaporation method. The co-solvent composition was varied by changing the type of organic co-solvent (using acetone, acetonitrile and tetrahydrofuran), the ratio of organic to aqueous phase, and their order of addition. Carrier size, morphology and encapsulated CyA levels were defined by dynamic light scattering (DLS), transmission electron microscopy (TEM) and HPLC, respectively, and the effect of co-solvent composition on micellar properties and loaded CyA levels was evaluated. Application of acetone and acetonitrile as the selective co-solvent for the core-forming block led to a decrease in the average diameter of self-assembled structures. When acetone was added to water, a decrease in the ratio of organic to aqueous phase led to an increase in the loading efficiency of CyA in MePEO-b-PCL micelles. A similar trend in CyA loading was observed for MePEO-b-PCL micelles of varied MePEO and PCL block lengths. The ratio of organic to aqueous phase did not affect CyA loading when water was added to acetone. Irrespective of the order of addition, the decrease in the organic to aqueous phase ratio caused a reduction in the average diameter of the empty and CyA loaded micelles. We conclude that the co-solvent evaporation method may be optimized to improve the efficiency of drug encapsulation in polymeric micelles. For CyA encapsulation in MePEO-b-PCL micelles, addition of acetone to water at lower organic to aqueous phase ratio is shown to be the optimum procedure leading to higher drug encapsulation and smaller average diameter for the self-assembled structures.

In vivo temporal and spatial distribution of depolarization and repolarization and the illusive murine T wave.

This study assessed in vivo temporal and spatial electrophysiological properties of murine hearts and the effect of manipulation of transmural action potential durations (APDs) on T wave morphology. Monophasic action potentials (MAPs) were acquired from multiple left ventricular sites. All MAPs exhibited a plateau phase, with a spike and dome appearance being present in epicardial recordings. Activation occurred from endocardial apex to epicardial apex and apex to base while repolarization occurred from base (shortest 90 eta0 level of repolarization (MAP90), 95.4 +/- 8.9 ms) to apex and epicardium to endocardium (longest MAP90, 110.77 +/- 10.6 ms). The peak of phase 0 of the epicardial base MAP correlated with the return to baseline of the initial and usually dominant waveform of the QRS and the onset of the second usually smaller wave, which clearly occurred in early repolarization, thus establishing where depolarization ended and repolarization began on the murine ECG. This second waveform was similar to the J wave seen in larger animals. Despite temporal and spatial electrophysiological similarities, a T wave is frequently not seen on a murine ECG. There are several determinants of T wave morphology, including transmural activation time, slope of phase 3 repolarization and differences in epicardial, endocardial and M cell APDs. Experimental manipulation of murine transmural gradients by shortening epicardial MAP(90) to 84% of endocardial MAP90 the epicardial/endocardial ratio in larger mammals when a positive T wave is present, resulted in a positive murine T wave. Thus, manipulation of the transmural gradients such that they are similar to larger mammals can result in T waves with similar morphology.