ower frequency region in C is plotted with a y -scale larger than that in A in order to display the peaks of the vitamin E moiety in TPGS. As a result, the methylene peak of DSPE at 1.28 ppm was truncated. DMSO sample. The FWHH of methylene (CH 2 ) protons of DSPE at 1.28 ppm was increased from 4.95 Hz in DMSO-d 6 ( Fig. 5 A) to 41.45 Hz in the aqueous micelle sample ( Fig. 5 B). Moreover, 1 H sig- nals of 17-AAG Stanozolol were no longer detectable in the micelle sample. These results clearly demonstrate that the DSPE moiety and 17-AAG in the micelle sample experience much slower motions compared to the DMSO-d 6 control, indicating that they are buried within the hydrophobic core of the micelles. Next, 17-AAG-incorporating PEG-DSPE/TPGS mixed micelles were studied, and identical amounts of PEG-DSPE, TPGS and 17- AAG dissolved in DMSO-d 6 were used as a control. The proton signals from both copolymers and 17-AAG were observed with sharp line widths in the DMSO-d 6 sample ( Fig. 5 C). In the aque- ous sample, the 1 H signals from the vitamin E portion of TPGS were broadened in the micelle sample and those from 17-AAG not detectable ( Fig. 5 D), indicating that the vitamin E moiety of TPGS and 17-AAG molecules are incorporated into the micelle core.

Importantly, the addition of TPGS in the micelle composition fur- ther reduced the motion of methylene protons of DSPE as evidenced had little impact on the biodisposition of 17-AAG in rats ( Xiong et al., 2009 ). In another recent study, 17-AAG was formulated into vasoactive intestinal peptide-conjugated PEG-DSPE micelles, which exhibited similar cytotoxicity against human breast can- cer MCF-7 cells as free 17-AAG. The optimal buy Stanozolol loading of 17-AAG was determined to be about 0.5 mM in 5 mM PEG-DSPE micelles ( Onyüksel et al., 2009 ). Yet the release of 17-AAG from these micelles was not characterized. In our current study, PEG-DSPE/TPGS mixed micelles were investigated as nanocarriers for 17-AAG. Compared to pure PEG- DSPE micelles, we found that the addition of TPGS in the micelle composition significantly reduced the release rate constant of 17- AAG. This is most likely because PEG-DSPE/TPGS mixed micelles are thermodynamically more stable than pure PEG-DSPE micelles. Owing to the very low transition temperature (12 ◦ C) of PEG-DSPE molecules, PEG-DSPE micelles are fluid and dynamic complexes that undergo rapid exchange with monomer molecules in the aqueous solution at 37 ◦ C ( Kastantin et al., 2009 ).

This thermal motion may destabilize the drug-copolymer association within the micelles, resulting in a fast release of the loaded drug from the micelles. Being approximately half of the molecular size of PEG-DSPE, TPGS molecules are believed to fill the “void” between amphiphilic PEG-DSPE chains, as they spontaneously orient and form spheroidal “core–shell” micelle structure ( Fig. 6 ). As sup- ported by our 1 H NMR results, the insertion of TPGS into PEG-DSPE micelles strengthened the hydrophobic interactions within the micelle core, as well as partially restricted dynamic motion of PEG chains in the corona region, which may collectively elevate the acti- vation energy required for monomer desorption and thus decrease monomer exchange kinetics of the micelles. Furthermore, the incorporation of TPGS in the micelle compo- by the broader peak at 1.28 purchase Stanozolol ppm ( 1/2 = 57.98 Hz in Fig. 5 D) than sition strikingly enhanced the encapsulation capacity for 17-AAG. that of the PEG-DSPE micelles ( 1/2 = 41.45 Hz in Fig. 5 B), suggest- This is believed to be a result of increased hydrophobic interac- 6 176 T. Chandran et al.

International Journal of Pharmaceutics 392 (2010) 170–177 quence in extending the circulation time of the drug-incorporating micelles and achieving targeted drug delivery to the tumor tissue. In summary, we have demonstrated the feasibility of utilizing beef PEG-DSPE/TPGS mixed micelles as novel nanocarriers for 17-AAG. The incorporation of TPGS in the micelle composition restricted molecular m