Carvedilol-liposome interaction: evidence for strong association with the hydrophobic region of the lipid bilayers

Carvedilol (Kredex, Coreg) is a multiple action antihypertensive drug that has been shown to protect cell membranes from lipid peroxidative damages. In this study the physical and structural effects of carvedilol on lipid bilayers are investigated by fluorescence techniques, differential scanning calorimetry and other physical methods. Carvedilol binds to liposomal membranes (9:1 DMPC:DMPG) strongly with an apparent binding constant on the order of 10(4) M-1 in PBS (pH 7.4). The characteristic changes in its intrinsic fluorescence properties when bound to liposomes suggest that this compound is situated in a non-polar environment. The Stern-Volmer and bimolecular quenching constants, determined using nitrate as the fluorescence quencher, for the free and bound carvedilol indicate that the carbazole moiety is at a depth of > 11 A in the lipid bilayer. Fluorescence anisotropy measurements show that, unlike the membrane probes DPH and TMA-DPH, carvedilol is relatively mobile, and does not have a rigidly-defined molecular orientation in the bilayers. Differential scanning calorimetry results indicate that carvedilol is an effective membrane "fluidizer' as it dose-dependently lowers the gel to liquid crystalline transition temperature and broadens the endothermic transition. Comparative studies of interactions of carbazole, 4-OH carbazole and carvedilol with the model liposomal membranes reveal a possible role of membrane-partitioning in their antioxidant efficacy. These findings are discussed in perspective with the membrane biophysical properties of different classes of therapeutic significant lipid antioxidants in mind.

Micellar aggregation of poloxamer 213 and its interaction with cholesterol derivatives

The micellar properties of Poloxamer 213 (1), a Pluronic copolymer shown to affect lipid absorption and serum cholesterol level in experimental animals, are investigated by surface tension measurements, photon correlation spectroscopy (PCS), Reichardt's dye solubilization technique, differential scanning calorimetry (DSC), electron paramagnetic resonance (EPR) spectrometry, and fluorescence spectroscopy. The clear inflection point at 3 x 10(-6) M (25 degrees C) observed in the surface tension-concentration curve may not represent the CMC for the formation of multimolecular aggregates. In the 10(-4) to 10(-2) M concentration range, the temperature-dependent transition (as the concentration of 1 increases) from the larger (hydrodynamic radius, Rh approximately 20-40 nm), highly hydrated aggregates to the contracted (Rh approximately 6-7 nm), less polar form occurs as a discontinuity. This process is endothermic, with an average delta H of 34.5 kcal/mol. At 25 degrees C, both the 25-(NBD-methylamino)-27-norcholesterol (fluorescence probe) and 3-Doxyl-5 alpha-cholestane (EPR spin probe) begin to show significant interaction with 1 in the 10(-3) M range. The sequestration of the fluorescent cholesterol probe by 1 aggregates begins at approximately 2 x 10(-4) M at 35 degrees C. Analysis of EPR and fluorescence data indicates that the cholesterol analogues are in a nonpolar micellar environment of low fluidity. The significance and implication of the data are discussed in the context of the hypothesized cholesterol sequestration by 1 under physiological conditions.

Automated spectrophotometric assay of cefazolin

An automated, stability-indicating, UV spectrophotometric assay for cefazolin is presented. The method employs a reaction with hydroxylamine and derives its stability-indicating power through comparison of reacted and unreacted aliquots of the sample. A double-probe sampling procedure is used. Good agreement with microbiological assays is obtained, and the coefficient of variation is about 1%.