Buffers burst’ release

Release kinetics of taxol from OCL worm micelles was studied by dialysis method under sink condition . Figure 7 shows the percentage of taxol released from OCLl and OCL3 worm micelles versus time at 37 C, under pH 5 Hepes and pH 7 PBS physiological buffers respectively. After an initial burst release, typical for polymeric micelle systems (7), a much slower and sustained release... 

The in vitro release profiles of DOX from the polymeric micelles was studied in in PBS (0.1 M, pH 7.4) and acetate buffer solutions (0.1 M, pH 5.4) at 37 °C. The results showed an initial burst release of DOX and followed by a sustained release for about 48 h. The initial burst release of DOX from micelles could be attributed to the diffusion of DOX located close to the surface of particles or within the hydrophilic shell [6]. The total release of DOX in a period of 48 h with pH 7.4 and 5.4 was 25% and 37% of total DOX concentration, respectively. However, the release of DOX at a pH value of 5.4 was found faster than that at a pH value of 7.4. These results could be attributed to the re-protonation of the amino group of DOX and the faster degradation of the micelle core at lower pH values. This pH-dependent release profile is of particular interest. It is expected that the greater part of DOX-loaded micelles will remain in the micelles cores for a considerable time period in plasma after intravenous administration and have the potential for prolonged DOX retention time in the blood circulatioa However, a faster release may occur at low local pH surrounding the tumor site or by the more acidic environment inside the endosome and lysosome of tumor cells after cellular uptake of micelles through endocytosis. 

Recently, Tsakala et al. (90) formulated pyrimethamine systems based on several lactide/glycolide polymers. These studies were conducted with both microspheres (solvent evaporation process) and implants (melt extrusion process). In vitro studies indicated that pyrimethamine-loaded implants exhibited apparent zero-order release kinetics in aqueous buffer whereas the microspheres showed an initial high burst and considerably more rapid drug release. In vivo studies in berghi infected mice confirmed that the microspheres did not have adequate duration of release for practical application. However, the implants offer promise for future clinical work as more than 3 months protection was observed in animals. 

In disagreement with the above indications was the finding of Aldridge et al. (146) that for enzyme which was phosphorylated at pH 5.5 with inorganic phosphate and rapidly mixed with buffer at pH 8.4, the rate of dephosphorylation was twice as fast as the turnover of the enzyme at pH 8.0. Also, transient state kinetic studies by Femley and Walker (99, 110) showed a rapid release (burst) of phenol followed by a steady state release of phenol, only at pH < 7. Thus, these data would seem to indicate that at pH >7 the rate determining step is phosphorylation. 

The function of parvalbumin has long been assumed to be that of buffering Ca in muscle cells, i.e., taking up Ca + ions released from Ca " "-troponin complexes, thereby ensuring that the cytoplasmic levels of free Ca + are always kept very low, even during short bursts of muscle activity.The widespread occurrence of parvalbumin in non-muscle tissue indicates that it probably has other roles as well. 

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