From ultrathin capsules to biaqueous vesicles

The layer-by-layer (LbL) adsorption of anionic polyelectrolytes (PE) and tobramycin sulfate (TbS) multilayers on zinc oxide core particles followed by the controlled core-removal process leads to the formation of ultrathin capsules, which gradually convert to biaqueous vesicles and emulsionlike systems depending on the hydrophilicity/hydrophobicity of the PE backbone, PE/TbS ratio, and Zn2+ concentration. The unique characteristics of the PE/TbS multilayer capsules result because of the formation of PE/TbS/H2O biphasic liquid systems unlike the other LbL capsular systems that form stiff PE coacervates when mixed together in water. This paper investigates the PE/TbS ultrathin capsule to biaqueous vesicle transition and its physicochemical properties.

Comparison of artificial neural network and multiple linear regression in the optimization of formulation parameters of leuprolide acetate loaded liposomes

PURPOSE

We planned to optimize the effect of formulation variables on the percent drug entrapment (PDE) of the liposomes encapsulating leuprolide acetate by reverse phase evaporation method using Artificial neural network (ANN) and Multiple linear regression (MLR).

METHOD

Twenty seven formulations were prepared based on 3x3 factorial design. The volume of aqueous phase (X(1)), HSPC/DSPG [negative charge] (X(2)), and HSPC/Cholesterol (X(3)) were selected as the causal factors. Potential variables such as concentration of lipid: drug and hydration medium were kept constant in experimental design. The PDE (dependent variable) and the transformed values of independent variables were subjected to multiple regression analysis to establish a second order polynomial equation (full model). A set of PDE and causal factors was used as tutorial data for the ANN and fed into a computer. The feed forward back propagation (bp) method was optimized. The ANN model and MLR were validated for accurate prediction of PDE.

RESULTS

To simplify the polynomial equation, F-statistic was applied to reduce polynomial equation (reduced model) by neglecting non-significant (P<0.05) terms. The reduced polynomial equation was used to plot three two-dimensional contour plots at fixed levels of -1, 0 and 1 of the variable X(3) to obtain various combination values of the two other independent variables (X(1) and X(2)) at predetermined PDE. The root mean square value of the trained ANN model by feed forward bp method was 0.0000354, which indicated that the optimal model was reached. The optimization methods developed by both ANN and MLR were validated by preparing another six liposomal formulations. The predetermined PDE (from ANN and MLR) and the experimental data were compared with predicted data by paired "t" test, no statistically significant difference was observed. ANN showed less error compared to MLR.

CONCLUSIONS

These findings demonstrate that the ANN model provides more accurate prediction and is quite useful in the optimization of pharmaceutical formulations when compared to multiple regression analysis method. The normalized error (NE) value observed with the optimal ANN model was 0.0211 while it was 0.0658 for the full model in the case of second-order polynomial equation composed of the combination of causal factors (X(1), X(2) and X(3)). Thus the derived equation, contour plots and ANN helps in predicting the values of the independent variables for maximum PDE in the preparation of leuprolide acetate liposomes by reverse phase evaporation technique.

Ultrathin antibiotic walled microcapsules

Ultrathin microcapsules comprised of anionic polyelectrolytes (PE) and a polycationic aminoglycoside (AmG) antibiotic drug were prepared by depositing PE/AmG multilayers on zinc oxide (ZnO) colloid particles using the layer-by-layer self-assembly technique and subsequently dissolving the ZnO templated cores. The polyelectrolytes, dextran sulfate sodium (DxS) and poly(styrenesulfonate) (PSS), were selected owing to their different backbone structure. An aminoglycoside, tobramycin sulfate (TbS), was used for studying DxS/TbS or PSS/TbS multilayer films. The multilayer growth on ZnO cores was characterized by alternating zeta potential values that were different for the DxS/TbS and PSS/TbS multilayers due to the PE chemistry and its interaction with Zn(2+) ions. Transmission and scanning electron microscopy provide evidence of PE/TbS multilayer coating on ZnO core particles. The slow acid-decomposition of the ZnO cores using weak organic acids and the presence of sufficient quantity of Zn(2+) in the dispersion were required to produce antibiotic multilayer capsules. There was no difference in the morphological characteristics of the two types of capsules; although, the yield for [PSS/TbS](5) capsules was significantly higher than for [DxS/TbS](5) capsules which was related to the physicochemical properties of DxS/TbS/Zn(2+) and PSS/TbS/Zn(2+) complexes forming the capsule wall. The TbS quantity in the multilayer films was determined using a quartz crystal microbalance and high performance liquid chromatography techniques which showed less TbS loading in both, capsules and multilayers on planar gold substrate, than the theoretical DxS:TbS or PSS:TbS stoichiometric ratio. The decomposition of the [PE/TbS](6) multilayers was fastest in physiological buffer followed by mannitol and water. The decomposition rate of the [PSS/TbS](6) multilayers was slower than [DxS/TbS](6) monolayers. The incomplete decomposition of DxS/TbS under saline conditions suggests the major role of hydrogen bonding for stability of DxS/TbS multilayers. A combination of hydrogen bonding and hydrophobic interaction between phenyl rings in PSS was responsible for PSS/TbS multilayer stability. In vivo studies in rabbits highlight the safety and sustained drug delivery potential of the PE/AmG microcapsules. The antibiotic walled ultrathin capsules presented here are suitable for sustained ophthalmic antibiotic delivery.

Preparation, characterization, and biodistribution study of technetium-99m -labeled leuprolide acetate-loaded liposomes in Ehrlich ascites tumor-bearing mice

The purpose of this study was to prepare conventional and sterically stabilized liposomes containing leuprolide acetate in an attempt to prolong the biological half life of the drug, to reduce the uptake by reticuloendothelial system (RES), and to reduce the injection frequency of intravenously administered peptide drugs. The conventional and sterically stabilized liposomes containing leuprolide acetate were prepared by reverse phase evaporation method and characterized for entrapment efficiency and particle size. Radiolabeling of leuprolide acetate and its liposomes was performed by direct labeling with reduced technetium-99m. Its biodistribution and imaging characteristics were studied in ehrlich ascites tumor (EAT)-bearing mice after labeling with technetium-99m. The systemic pharmacokinetic studies were performed in rabbits. A high uptake by tumor was observed by sterically stabilized liposome containing leuprolide acetate compared with free drug and conventional liposomes. The liver/tumor uptake ratio of free drug, conventional (LL), and sterically stabilized liposomes (SLL5000 and SLL2000) was found to be 20, 7.99, 1.63, and 1.23, respectively, which showed the increased accumulation of sterically stabilized liposomes in tumor compared with the free drug and conventional liposomes at 24 hours postinjection. Liver uptake of sterically stabilized liposomes was still 7-fold less than the conventional liposomes. The marked accumulation of liposomes in the tumor-bearing mice was also documented by gamma scintigraphic studies. The findings demonstrate the distribution of these liposomes within solid tumor and prove that the sterically stabilized liposomes experience increased tumor uptake and prolonged circulation half life. Hence these findings will be relevant for the optimal design of long circulating liposomes for the peptide drugs and for targeting of liposomes toward tumor.

Preparation, characterisation and biodistribution of 99mTc-labeled liposome encapsulated cyclosporine

The present study investigated the effect of charge (neutral, negative and positive) on liposomal membrane on the distribution of cyclosporine encapsulated in it to various organs. Liposomes were prepared by using different phospholipids by thin film hydration followed by sequential extrusion through polycarbonate membranes to achieve a desired particle size, with high entrapment efficiency and then lyophilised using sucrose as cryoprotectant. The possible in vivo distribution of cyclosporine and its liposomes after direct labeling with reduced technetium-99m has been studied in mice. The blood kinetics and biodistribution study of these labeled complexes shows prolonged circulation of positive and neutral charged liposomes in blood compared to free drug and negative charged liposomal formulation. The biodistribution of the tagged liposomes confirms that increased radioactivity was seen in liver and spleen, with minimal involvement of the kidney. At 4 h post injection the biodistribution data in kidney reveals approximately 1-2% of the injected dose was present for cyclosporine loaded liposomes, which elicits the possibility of reducing the nephrotoxicity, generally seen in free cyclosporine. Interestingly, the biodistribution and gamma imaging studies of the charged cyclosporine liposomes indicated that an appreciable amount of these labeled complexes goes to bone marrow when compared to the free cyclosporine. The findings demonstrate the distribution of these liposomes within various organs and proved that the positively charged liposomes experience increased bone uptake and prolonged circulation half-life. Hence this finding implies the possibility of using these formulations for liver and bone marrow transplantation.