Biodegradable thermosensitive polymers: synthesis, characterization and drug delivery applications
Biodegradable thermosensitive polymers: synthesis, characterization and drug delivery applications
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发表时间：
2006-03
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通讯作者：
O. Soga
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作者：
O. Soga
在本论文中描述的研究的目的是设计聚合物胶束显示控制不稳定性由于“疏水到亲水”的核心转换，并证明其作为药物递送载体的实用性。为此，合成了一类新型的温敏性和可生物降解的聚合物，聚（N-（2-羟丙基）甲基丙烯酰胺单/二乳酸酯）（聚（HPMAm-单/二乳酸酯））。这些聚合物在水中的浊点（CP）根据共聚物组成的比率而变化。由于具有13 °C的CP的聚（HPMAm-二乳酸酯）及时转化为聚（HPMAm-单乳酸酯）（65 °C的CP）或更亲水的pHPMAm，因此该聚合物被认为适合于在体温下“疏水到亲水”的转化。合成了两亲性聚（HPMAm-dilactate）和聚乙二醇AB嵌段共聚物（pHPMAmDL-b-PEG）。这些嵌段共聚物通过将聚合物水溶液从低于临界胶束温度（CMT）快速加热到高于临界胶束温度（CMT）而在水中形成尺寸为约50 nm的聚合物胶束。低温透射电镜分析表明，pHPMAmDL-b-PEG胶束为球形，粒径分布较窄。1H NMR和静态光散射测量表明，胶束具有固体状和致密的核心结构，并且疏水核心被亲水性PEG冠稳定。最重要的是，pHPMAmDL-b-PEG胶束在体温下显示出受控的溶解，这是由于热敏性嵌段中乳酸侧链的水解导致的核心的溶解，表明我们的受控不稳定性的概念可以用pHPMAmDL-b-PEG胶束实现。接下来，研究了紫杉醇（PTX）（一种非常疏水的细胞生长抑制药物）到pHPMAmDL-b-PEG胶束中的负载。利用pHPMAmDL-b-PEG的热敏性，通过简单混合少量的乙醇中的浓缩PTX溶液和聚合物水溶液，随后将所得溶液加热到聚合物的CMT以上来进行装载。PTX可以几乎定量地负载在胶束中高达2 mg/mL。PTX的释放诱导的pH值依赖性的胶束在相对高浓度的PTX的不稳定，而透析对大量的水诱导的PTX的释放通过扩散。载有PTX的胶束对B16 F10细胞显示出与泰素（紫杉醇在Cremophor EL和乙醇的50：50混合物中的临床使用的PTX制剂）相当的细胞毒性。另一方面，空胶束的毒性远低于Cremophor EL载体，这有利于体内应用。当静脉内给药到大鼠中时，pHPMAmDL-b-PEG胶束显示出相对长的血液循环时间，24小时后20%的注射剂量仍在血流中循环。与空胶束相反，在小鼠中静脉内施用后，装载到pHPMAmDL-b-PEG胶束中的PTX被相当迅速地清除。PTX-负载的pHPMAmDL-b-PEG胶束在静脉内和腹膜内给药后显示出与紫杉醇相当的体内抗肿瘤功效。总之，本论文的工作表明，pHPMAmDL-b-PEG聚合物胶束由于其可控的不稳定性而具有作为疏水药物载体的前景。
The aim of the research described in this Thesis is to design polymeric micelles showing controlled instability due to "hydrophobic to hydrophilic" conversion of the core, and to demonstrate its utility as a drug delivery vehicle. For that purpose, a novel class of thermosensitive and biodegradable polymers, poly(N-(2-hydroxypropyl) methacrylamide mono/di lactate) (poly(HPMAm-mono/di lactate)), were synthesized. The cloud point (CP) of these polymers in water varied depending on the ratio of the copolymer composition. Since poly(HPMAm-dilactate), having a CP of 13 °C, is converted in time to poly(HPMAm-monolactate) (CP of 65 °C) or more hydrophilic pHPMAm, this polymer was supposed to be suitable for "hydrophobic to hydrophilic" conversion at body temperature. Therefore, amphiphilic AB block copolymers of poly(HPMAm-dilactate) and poly(ethylene glycol) (pHPMAmDL-b-PEG) were synthesized. These block copolymers formed polymeric micelles in water with a size of around 50 nm by rapidly heating an aqueous polymer solution from below to above the critical micelle temperature (CMT). By cryo-transmission electron microscopy analysis, it was shown that pHPMAmDL-b-PEG micelles have a spherical shape with a narrow size distribution. 1H NMR and static light scattering measurements demonstrated that the micelles have solid-like and dense core structures and that the hydrophobic core is stabilized with a hydrophilic PEG corona. Most importantly, the pHPMAmDL-b-PEG micelles showed controlled dissolution at body temperature as a result of the hydrophilization of the core due to the hydrolysis of the lactic acid side chain in the thermosensitive block, demonstrating that our concept of controlled instability can be achieved with pHPMAmDL-b-PEG micelles. Next, the loading of paclitaxel (PTX), a very hydrophobic cytostatic drug, into pHPMAmDL-b-PEG micelles was studied. Taking advantage of the thermosensitivity of pHPMAmDL-b-PEG, the loading was done by simple mixing of a small volume of a concentrated PTX solution in ethanol and an aqueous polymer solution and subsequent heating of the resulting solution above the CMT of the polymer. PTX could be almost quantitatively loaded in the micelles up to 2 mg/mL. Release of PTX was induced by the pH-dependent destabilization of the micelles at relatively high concentration of PTX, while dialysis against a large volume of water induced the release of PTX by diffusion. PTX-loaded micelles showed comparable cytotoxicity as Taxol (clinically used formulation of PTX in a 50:50 mixture of Cremophor EL and ethanol) against B16F10 cells. On the other hand, the empty micelles were far less toxic than the Cremophor EL vehicle, which is beneficial for in vivo applications. When administered intravenously into rats, pHPMAmDL-b-PEG micelles showed a relatively long blood circulation time with 20 % of the injected dose still circulating in the bloodstream after 24 hours. In contrast to empty micelles, PTX that was loaded into pHPMAmDL-b-PEG micelles was cleared quite rapidly after intravenous administration in mice. PTX-loaded pHPMAmDL-b-PEG micelles showed comparable in vivo antitumor efficacy as Taxol both after intravenous and intraperitoneal administration. In conclusion, the work presented in this Thesis indicates that pHPMAmDL-b-PEG polymeric micelles have promising features as vehicles for hydrophobic drugs owing to their controlled instability.