Temperature-Sensitive Liposomal Hydrogel For Sustained Release Of Indapamide

Indapamide is an effective antihypertensive diuretic agent with little side effect, mainly used to treat mild or moderate hypertension. The patient often need long time administration of drug to relieve the symptoms. Currently, Indapamide is only available in its oral dosage form. In order to reduce the frequency of its administration, we brought forward liposomal encapsulation of lauroyl-indapamide, the prodrug of indapamide, which was then incorporated into temperature-sensitive hydrogel to obtain a long-term sustained release of indapamide after its i.m. administration. According to this aim, a series of experiments were conducted.Indapamide is a water-insoluble highly lipophilic drug, which is difficult to be encapsulated into liposomes with its entrapment efficiency much lower. In order to encapsulate it into liposomes, a modified indapamide substituted at the free amino with a long carbonic chain, lauroyl-indapamide, was synthesized, whose structure was confirmed by FTIR, ¹H-NMR and UV after its purification with silica column.The pKa values of lauroyl-indapamide at different concentrations were determined when lauroyl-indapamide were dissolved in a mixed acetone/buffer solutions at different ratios, they were then extrapolated to obtain the characteristic pKa. value of the drug at extremely diluted concentration in purified water. The w/o partition coefficient of lauroyl-indapamide between butyl alcohol and water was determined with bottle shaking method. The results proved that the pKa value is 3.87 and lgP is 2.588 respectively, which demonstrate that the drug is a lipophilic weak acid. The degradation kinetic parameters of drug in mixture of acetone/buffer solution (9:1, v/v) at different pH were determined, and the results proved that lauroyl-indapamide is most stable at pH6.8, and a good correlation of the degradation rates of free drug and that encapsulated in liposomes was showed.Lauroyl-indapamide is a highly lipophilic drug, but showed much lower entrapment efficiency in liposomes when prepared with ethanol injection method, only in the presence of divalent, trivalent or higher valent cations can it be encapsulated in liposomes, and obtained a much higher entrapment efficiency (more than 90%). It was demonstrated by ¹⁵N-NMR, ¹H-NMR and ¹H-¹⁵N-NMR(HMBC) that Ca²⁺ can eliminate the adsorption of solvent molecule on drug molecule and the hydrogen bond between them. The hydrogen at the N linked with lauroyl group substituted by Ca²⁺ may also favor the liposomal encapsulation of drug lauroyl-indapamide.The temperature-sensitive hydrogel was prepared with cold method, and the factors that may influence the gel temperature have been explored, and proved that gel temperature decreased with the concentration of PF127 increased, and the addition of PF68 and PEG could increase the gel temperature, while the addition of sodium chloride and Tris buffer solution showed an opposite effect. Based on the formulation of temperature-sensitive hydrogel, a liposomal lauroyl-indapamide hydrogel was prepared, which also showed reversible gel properties, and the liposomal lauroyl-indapamide was imbedded in the semisolid gel when its temperature rose, and the gel process has little influence on the liposomes size and entrapment efficiency of drug.The membraneless-dissolution model was used to explore the release behavior of drug in dissolution apparatus, and the results proved that the drug was released in liposome rather than in free form from the liposomal hydrogel with its size and entrapment efficiency of drug changing little, and the morphologies of liposomes have also been observed under Transmission Electronic Micrograph (TEM). Both drug release and liposomal hydrogel dissolution rates were followed zero-order kinetics, the later controlled the former and a good correlation between them was showed. Both drug release and liposomal hydrogel dissolution rate were influenced by such as the stirring speed and the height from the bottom of vessel of the stirring peddles, release area, sort of buffer solution, osmotic pressure and composition of the formulation and so on. The boundary layer thickness and mass transfer coefficient of liposomes across this boundary layer were the determinants that controlled the liposomal hydrogel dissolution and drug release.The diffusion of drug in the gel was measure by a free-diffusion model, and found that the drug is always diffused, following a Fickian law, in liposome rather than in free form in the hydrogel, which was demonstrated by the near constant molar ratios of drug to lipid in each gel slice. And also found the more flexible the liposomal membrane was, the faster the drug diffused, the smaller the liposome was the fast the drug diffused. The drug diffused fast with water channel increased in the liposomal hydrogel, while the diffusivity decreased when the polymer concentration was increased, due to the distorted aqueous channels and higher microviscosity.The results of liposomal lauroyl-indapamide incubated with rat blood at 37℃proved that lauroyl-indapamide degraded into indapamide almost quantitatively (more than 90%), and the correlation between the percentage of lauroyl-indapamide degraded and that of indapamide produced showed a good linearity.The concentrations of lauroyl-indapamide and indapamide afterâ…°.m. administration of the temperature-sensitive liposomal lauroyl-indapamide hydrogel can be followed simultaneous, and found that their half-lives were 15.298h and 21.19h respectively, compared withâ…°.m. administration of liposomal lauroyl-indapamide, their half-lives were extended 3.37 times and 4.49 times, and their peak times were extended 8.94 times and 7.09 times, their AUC(area under the curve) were increased 7.95 times and 6.58 times respectively, and the formulation can sustained-release lauroyl-indapamide and indapamide for 192h.Temperature-sensitive liposomal lauroyl-indapamide hydrogel can sustained-release liposomes mainly through the slowly dissolution of the liposomal hydrogel by biological fluids in muscle, the released liposomes then quickly enter blood stream where they are not stable and disrupted, and followed by the release of lauroyl-indapamide that they contained, which is then experienced chain broken and degraded into its parent drug indapamide in blood.The overall result is that the temperature-sensitive liposomal lauroyl-indapamide hydrogel can release indapamide in a sustained manner to exert its long-term antihypertensive diuretic activity.