Study On Liposomes With Gelated Interior As Drug Delivery Carriers

Liposomes have broad applications in the field of drug delivery as they are able to entrap both hydrophilic and hydrophobic drugs, enhance the therapeutic index and decrease the toxicity. However, because of the liquid core and the membrane which are easy to be disrupted, the drug is easy to leak out leading to instability of the liposomes. In this thesis, a new vehicle-liposome containing thermosensitive poloxamer gel (gelliposomes, GL) was designed to integrate the advantages of liposomes and in situ gel systems and avoid their disadvantages separately. This system can lower the "fluidity" of liposome core, enhance the membrane rigidity and further improve the physicochemical poperties of the liposomes. A small molecule hydrophilic drug cytarabine (Ara-C) was selected as the model drug and thermosensitive poloxamer gel solution was used as the core of liposomes. Gelliposomes were established to overcome the inherent disadvantages of liposomes. GLs were evaluated both in vitro and in vivo to highlight the effect of the gelled core on their behaviors.The content of this thesis includes the preparation of a series of thermosensitive poloxamer gel solutions and the determination of their sol-gel transition temperatures (Tgels). Ara-C-GL (Ara-C gelliposomes) was prepared, characterized and in vitro release was studied. Pharmacokinetics and tissue distributions of Ara-C-GL were investigated. Long-circulating Ara-C gelliposomes (PEG-Ara-C-GL) were also prepared and characterized to highlight the merits of interior gelation reserving the outer surface for further modification.Stirring method and viscosity method were both applied to determine Tgels of different poloxamer gel solutions and the results showed that both methods were reproducibility and showed no significant difference. A series of Tgels of P407 solutions were tested and it can be concluded that only P407 solutions cannot achieve ideal sol-gel transition temperature. P188 was used to increase the Tgels of P407 solutions and P407/P188 (18%/10%) was selected as the optimum poloxamer solution for the further study.The thin-film dispersion method was used to prepare GLs and the morphology was studied by transmission electron microscope (TEM) and there was no obvious difference between GLs and CLs. The addition of trace of TritonX-100 into the gelated 5(6)-carboxyfluorescein liposomes dispersed on the slide glass destroyed the membrane of liposomes and the existence of interior gel was identified by the fluorescence microscope. The interior gel temperatures of gelliposome (Tgel-in) were determined by both Polarizing Optical Microscope and 5(6)-carboxyfluorescein release from gelliposomes. Ara-C was selected as a model drug to optimize the formulations of GLs.The leakage rates of Ara-C gelliposomes (Ara-C-GL) and Ara-C conventional liposomes (Ara-C-CL) were determined at 4℃,25℃and 37℃separately and the results showed that Ara-C-GL leaked less and established more stability than Ara-C-CL at all temperatures. Results of Ara-C GL stability in plasma at 37℃showed that the leakage of Ara-C-GL was lower than Ara-C-CL and the higher the concentration of gel solution as the core of gelliposomes was, the lower leakage rate of Ara-C-GL was. TritonX-100 was used as a challenging agent to investigate the stability of CF-GL and CF-CL at 25℃and 37℃. For the same concentration of TritonX-100 solution at both 25℃and 37℃, the florescence leakage rate of CF-CL was obviously higher than CF-GL and it can be concluded that CL was more stable than GL.In vitro release of Ara-C-GL was tested by the flow-through cell dissolution method and different factors which may influence the release of GL were investigated. When the flow rate of flow-through cell dissolution increased, the drug amount released from GL increased. Compared with CL, the release rate of GL was faster in the initial period and slower in the later stage. Influence of cholesterol on GL release showed that the amount of cholesterol would affect the rigidity of liposome membrane and further influence the release. At 37℃, the release of GL containing P407/P188 (14%/10%) solution which could not be gelated and P407/P188 (18%/10%) solution which could be gelated were compared and the result showed that the former one released more and quickly than the later one.Pharmacokinetic parameters of Ara-C solution, Ara-C conventional liposome (Ara-C-CL) and Ara-C gelliposome (Ara-C-GL) in rats showed that Ara-C-GL could significantly retard the circulating time and displayed sustained release effect. The results of tissue distributions showed that the Ara-C uptake by different tissues of Ara-C-GL were more than other two formulations and the elimination was the lowest.PEG2000-DSPE modified Ara-C gelliposome (long circulating gelliposome, PEG-Ara-C-GL) was prepared by similar method as Ara-C-GL. Pharmacokinetics of PEG-Ara-C GL in rats showed that the concentration was significantly higher than Ara-C-GL and the resident time was obviously longer than Ara-C-GL. Results of tissue distribution in mice showed that the drug uptake of almost all tissues by injection of PEG-Ara-C GL were obviously lower than Ara-C-GL. It can be concluded that PEG-Ara-C-GL successfully avoided uptake of RES, and the plasma concentration was enhanced significantly.Thermosensitive poloxamer gel was successfully used as the core of liposomes to prepare the new vehicle. The small hydrophilic drug Ara-C was selected as the model drug to optimize the Ara-C-GL formulations. The leakage rate of Ara-C-GL was obviously lower than that of Ara-C-CL at different conditions and showed better stability than Ara-C-CL. The release behavior of Ara-C-GL in vitro and in vivo was investigated and typical sustained release was observed. Ara-C-GL can be modified by PEG2000-DSPE to achieve long-circulating effect in vivo. In all, it can be concluded that the new vehicle can be successfully established and the membrane of gelliposmes can also be modified as conventional liposomes, gelation in liposomes may influence the drug release behaviors of liposomes in vitro and in vivo, and this effect has a great potential to retard the drug release, change drug in vivo pharmacokinetic behavior and tissue distribution.