School of Biomedical Engineering Science and Health Systems , Drexel University , Philadelphia , Pennsylvania 19104 , United States.
Department of Radiology , Thomas Jefferson University , Philadelphia , Pennsylvania 19107 , United States.
Langmuir. 2019 Aug 6;35(31):10068-10078. doi: 10.1021/acs.langmuir.8b03725. Epub 2019 Mar 14.
Ultrasound contrast agents consist of stabilized microbubbles. We are developing a surfactant-stabilized microbubble platform with a shell composed of Span 60 (Sorbitan monostearate) and an emulsifying agent, water-soluble vitamin E (α-tocopheryl poly(ethylene glycol) succinate, abbreviated as TPGS), named SE61. The microbubbles act both as an imaging agent and a vehicle for delivering oxygen to hypoxic areas in tumors. For clinical use, it is important that a platform be stable under storage at room temperature. To accomplish this, a majority of biologicals are prepared as freeze-dried powders, which also eliminates the necessity of a cold chain. The interfaces among the surfactants, gas, and liquids are subject to disruption in both the freezing and drying phases. Using thermocouples to monitor temperature profiles, differential scanning calorimetry to determine the phase transitions, and acoustic properties to gauge the degree of microbubble disruption, the effects of the freezing rate and the addition of different concentrations of lyoprotectants were determined. Slower cooling rates achieved by freezing the samples in a -20 °C bath were found to be reproducible and produce contrast agents with acceptable acoustical properties. The ionic strength of the solutions and the concentration of the lyoprotectant determined the glass-transition temperature (') of the frozen sample, which determines at what temperature samples can be dried without collapse. Crucially, we found that the shelf stability of surfactant-shelled oxygen microbubbles can be enhanced by increasing the lyoprotectant (glucose) concentration from 1.8 to 5.0% (w/v), which prevents the melt temperature () of the TPGS phase from rising above room temperature. The increase in glucose concentration results in a lowering of of the emulsifying agent, preventing a phase change in the liquid-crystalline phase and allowing for more stable bubbles. We believe that preventing this phase change is necessary to producing stabilized freeze-dried microbubbles.
超声造影剂由稳定的微泡组成。我们正在开发一种由 Span 60（失水山梨醇单硬脂酸酯）和乳化剂水溶性维生素 E（聚乙二醇琥珀酸生育酚酯，简称 TPGS）组成的表面活性剂稳定的微泡平台，命名为 SE61。这些微泡既可以作为成像剂，也可以作为向肿瘤缺氧区域输送氧气的载体。对于临床应用，重要的是平台在室温下储存时保持稳定。为了实现这一目标，大多数生物制剂被制备成冻干粉末，这也消除了冷链的必要性。表面活性剂、气体和液体之间的界面在冷冻和干燥阶段都会受到破坏。通过使用热电偶监测温度曲线、差示扫描量热法确定相变以及测量微泡破坏程度的声学特性，确定了冷冻速率和添加不同浓度的冷冻保护剂的影响。通过将样品在-20°C 的浴中冷冻来实现较慢的冷却速率，发现其结果可重复，并且产生的造影剂具有可接受的声学特性。溶液的离子强度和冷冻保护剂的浓度决定了冷冻样品的玻璃化转变温度（T g ），这决定了样品在不坍塌的情况下可以干燥到什么温度。至关重要的是，我们发现通过将冷冻保护剂（葡萄糖）的浓度从 1.8%（w/v）增加到 5.0%（w/v），可以增强表面活性剂壳氧微泡的货架稳定性，从而防止 TPGS 相的熔融温度（T m ）上升到室温以上。葡萄糖浓度的增加会降低乳化剂的 T g ，防止液晶相发生相变，并允许更稳定的气泡。我们认为，防止这种相变为生产稳定的冻干微泡是必要的。
