Preparation And Physicochemical Characteristics Of Lipid Nanoparticulate Carriers

Solid lipid nanoparticles (SLN) are collolidal carrier system for controlled drug deliveiy and followed by the development of emusion, liposomes, microparticles and nanoparticles based on synthetic polymers. They combine advantages of emusions, liposomes and polymeric nanoparticles. Identical to polymeric nanoparticles, their solid matrix protects incorporated active ingredients against chemical degradation and provides the highest flexibilities in the modulation of the drug release profiles. Similar to emulsions and liposomes, they are composed of well physiologically tolerated excipients and can be produced on large industrial scale by high pressure homogenization. However, there are also some potential limitations associated with SLN, i.e. limited drug loading capacity, drug expulsion during storage due to the crystallization of particles.Nanostructured lipid carners (NLC) composed of a solid lipid matrix with a certain content of liquid lipid are a new generation of lipid nanoparticles. The incorporation of liquid lipids to solid lipids leads to great imperfections in the crystal lattice of nanoparticles, and thus leading to improved drug loading capacity and reduced drug expulsion during storage. The SLN or NLC based on the lipid carriers are mostly fit for entrapping lipophilic drugs. For hydrophilic drugs, their limitedsolubility in lipid matrix could be disadvantageous with respect to drug entrapment efficiency. To overcome this problem, the hydrophobic ion pairing (HIP) of hydrophilic drugs with amphipathic molecules is reported. Major advantages of hydrophobic ion pairing are to increase the lipophilicity of hydrophilic molecules, so as to improve the drug entrapment efficiency for hydrophilic drugs.In present study, NLC and HIP were employed to improve the drug loading capacity of lipid nanoparticles. Solvent diffusion method was firstly used to prepare NLC. Monostearin and caprylic/capric triglycerides (GTCC) were chosen as the solid lipid and liquid lipid. Clobetasol propionate used as a model drug was incorporated into the NLC. The surface structure of nanoparticles were visualized by atomic force microscopy (AFM), and the differential scanning calorimetry (DSC) were used for SLN and NLC crystal order. The effect of liquid lipid content and preparation temperature on particle size, zeta potential, drug loading capacity and release behaviors were investigated. The optimal cryoprotectant was selected by freeze-thaw test and reconstitution of lyophilized products. The stability for nanaparticles suspension and lyophilized products in storage were determined. To prepare hydrophobic ion pairing complex, the leuprorelin (LR) was chosen as the hydrophilic model drug, the sodium stearate (SA-Na) was used as the acidic amphipathic molecules, and they were combined and formed the leuprorelin sodium stearate (LR-SA-Na) complex to increase the lipophilicity of the leuprorelin. The ion pairing behaviors in aqueous solution was investigated by aqueous solubility change and DSC for leuprorelin. After optimization of ion pairing process, the resultant LR-SA-Na complex was freeze-dried. The free leuprorelin and LR-SA-Na complex were encapsulated within stearic acid SLN by using solvent diffusion method and emulsion/evaporation method. The effect of hydrophobic ion pairing process and preparation method on particle size, drug loading capacity and release behaviors wasinvestigated.Monostearin NLC and SLN with particle size in 328.4-413.3 nm were successfully prepared by solvent diffusion method in an aqueous system. AFM results indicated that the addition of liquid lipid to solid lipid led to smooth surface of NLC that compared to SLN with roughness surface, however, after 24 h of in vitro release, the NLC showed roughness surface as well. Compared with SLN, NLC with a certain content of liquid lipid GTCC exhibited improved drug encapsulation efficiency and drug loading capacity. As a result, the drug encapsulation efficiency for SLN obtained by solvent diffusion method at 0°C was 45.15%, while that of NLC with 25% GTCC content obtained in the same condition was 67.17%. These results were explained by DSC investigations. The addition of GTCC to nanoparticles formulation resulted in massive crystal order disturbance and less ordered matrix of NLC, and hence increased the drug loading capacity. The in vitro release behaviors of NLC were dependent on the production temperature and GTCC content. NLC obtained at 70 °C exhibited biphasic drug release pattern with burst release at the initial stage and prolonged release afterwards;whereas NLC obtained at 0 °C showed basically sustained drug release throughout the release time. The drug release rates were increased with increasing the GTCC content. These results were probably related with the liquid lipid distribution behaviors in NLC. When solvent diffusion method at 70°C was used to prepare NLC, most of liquid lipid was located at the shell of nanoparticles. In contrast, the liquid lipid was homogenously distributed in NLC matrix when 0 °C production temperature was used to produce NLC.The particle sizes of both NLC and SLN were increased significantly when the SLN and NLC dispersion were stored at 25 °C in the dark over a period of 30 days, the particle size of NLC increased from 433.3 nm to708.3 nm, and the rate of particle growth for NLC was higher than that for SLN. In contrast, the particle growth wasslower when SLN and NLC dispersion were stored at 4 °C in the dark over a period of 30 days. Due to the instabilities of the SLN and NLC dispersion, the freeze-dried SLN and NLC formulations were employed to improve the storage stability. 0.3% poloxamer 188 (w/v) and 2% mannitol(w/v) were chosen as the cryoprotectants to decrease the aggregation of nanoparticles during the freeze-drying process. Even the lyophilized powders of SLN and NLC were stored for 6 months, the particle sizes only slightly increased. For the case of NLC, the particle size increased from 528 nm to 558 nm. NLC displayed a good ability to reduce the drug expulsion in storage compared to SLN. After 6 months of storage, the drug loading of SLN was reduced from 2.27% to 1.24%, about 45.37% drug was expulsed. In contrast, the drug loading of NLC only reduced about 7.65% under the same storage condition.The LR-SA-Na complex was obtained by hydrophobic ion pairing process. The amount of leuprorelin which was hydrophobically ion paired with sodium stearate was related with the sodium stearate/leuprorelin molar ratios. As a result, the highest amount of LR-SA-Na complex (88.5% leuprorelin hydrophobically ion paired with sodium stearate) was obtained when the sodium stearate/leuprorelin molar ratios reached to 2:1. However, the amount of LR-SA-Na complex decreased with increasing sodium stearate/leuprorelin molar ratios beyond 2:1. The DSC results confirmed the formation of LR-SA-Na complex. About 90% leuprorelin was dissociated from complex at 0 h when the LR-SA-Na complex was dispersed in 0.2% SDS solution with pH 6.8, and the number of leuprorelin dissociated from complex increased in the initial 4 h, howerver, afterwards, the partial of dissociated leuprorelin was hydrophobically ion paired with sodium stearate and formed of complex again.The solvent diffusion method and emulsion/evaporation method were employed to prepare leuprorelin and its complex SLN. As a result, the mean particle size wasabout 400 nm for SLN obtained by solvent diffusion method or emulsion/evaporation method. Compared to the free leuprorelin, the LR-SA-Na complex SLN showed higher drug entrapment efficiency, lower drug burst release and longer drug release time due to the increased lipophilicity of complex. Take the SLN obtained by solvent diffusion method for example, the drug entrapment efficiency for free leuprorelin SLN was 28.0%, and about 57.8% drug was released at initial 2h, the whole release time continued obout 12 h. In contrast, while the drug was hydrophobically ion paired with sodium stearate and formed LR-SA-Na complex, the drug entrapment efficiency increased to 46.1%, the drug burst release at the initial 2 h decreased to 13.5 %, and the whole drug release time extended to 24 h. Compared to SLN obtained by solvent diffusion method, the SLN produced by emulsion/evaporation method displayed higher drug entrapment efficiency and longer drug release time as well .The drug entrapment efficiency for LR-SA-Na complex SLN produced by emulsion/evaporation method was 74.6%, and its drug burst release at initial 2 h was 23.7%, the whole drug release time continued 96 h compared that of 24 h for LR-SA-Na complex SLN obtained by solvent diffusion method.