靶向超音波於血栓溶解之研究

Abstract

缺血性中風、心肌梗塞與深層靜脈栓塞等常見疾病與血栓的形成息息相關，臨床上注射血栓溶解劑所伴隨的內出血等副作用，使得發展安全以及更有效率的血栓溶解治療方式成為目前重要的研究課題。直至今日，歷經約30年的研究，超音波應用於血栓溶解治療為目前極具潛力的治療模式。許多研究團隊利用低頻超音波 (20 kHz~2 MHz) 與超音波對比劑所誘發的穴蝕效應，幫助血栓溶解劑更深入血栓內部進而增加血栓溶解的治療成效。也有研究指出，即使無血栓溶解劑的存在，超音波與對比劑的作用機制亦可造成血栓的溶解破壞。也因此，本研究的主要目的為嘗試提升超音波應用於血栓溶解治療的效能以及安全性，分別從超音波的使用頻率與靶向性超音波對比劑兩方面著手：(1) 靶向性超音波對比劑：超音波對比劑的加入可以降低誘發穴蝕效應所需要的能量，此外隨著新興生物科技的發展，藉由超音波對比劑的表面修飾，可以達到專一性辨認血栓、增加血栓局部對比劑濃度的效果。在靶向誘發穴蝕效應的作用下，希望可藉此提升體外低頻超音波輔助血栓溶解治療的安全性以及效能。(2) 超音波的使用頻率：不同於先前的研究，本研究配合臨床上的需求，嘗試發展結合觀測血栓生成以及即時溶解治療的高頻超音波系統(> 20 MHz)，並使用雙頻激發方式企圖使高頻超音波成功誘發穴蝕效應，以期將具有高空間解析度的血管內超音波應用於臨床上的血栓溶解治療，達到診治合一的目的。本研究的實驗結果顯示：1. 實驗室自製靶向性超音波對比劑的確具有標靶能力，並且具有增強血塊局部區域穴蝕效應的潛能，在未來的臨床應用上，可望在避免血管損傷的安全操作下提升血栓溶解效能。2. 聲場強度大小是影響高頻超音波能否誘發穴蝕效應的主因。因此，高頻超音波系統礙於探頭孔徑大小的限制而無法提供足夠能量誘發穴蝕效應，將造成血管內超音波應用於血栓溶解作用上的困難，也因此未來仍會以體外低頻超音波做為血栓溶解治療的研究主軸。

Ischemic stroke, myocardial infarction and deep vein thrombosis are common diseases which are related to thrombosis. Systemic thrombolytic therapy is one of the most widely used treatments for recanalization of the occluded vessel. However, the administration of thrombolytic agents has a major disadvantage of inducing comprehensive hemorrhage and may cause death. Therefore, there is a clinical need for treatment methods to increase the effectiveness of complete reperfusion without increasing the probability of bleeding. Several studies have shown that low-frequency ultrasound (20 kHz~2 MHz) enhances enzymatic thrombolysis by increasing the transport of thrombolytic agents into clots through cavitation-related mechanisms. Besides, there were some studies shown a thrombolytic potential of ultrasound in combination with ultrasound contrast agent even without the presence of thrombolytic agents. Thus, the use of ultrasound to enhance and accelerate the thrombolysis of occluded vessels is an area of active investigation. The purpose of this study was to develop two approaches for improving the efficiency and safety of ultrasound-assisted thrombolysis which were base upon the therapeutic frequency we chosen and the use of targeted contrast agents：(1). Targeted ultrasound contrast agent：Ultrasound contrast agents, mainly microbubbles, are good nuclei for reducing the threshold for cavitation induction. By the virtue of the rapid discovery of novel biomarkers and affinity ligands, targeted ultrasound contrast agents are developed to recognize thrombi and thereby increasing the local concentration of microbubbles. One hypothesis of the present study was that a targeted ultrasound contrast agent can enhance both the thrombolytic efficacy and safety in transcutaneous (noninvasive) ultrasound for the same reason. (2). The ultrasound frequency： For clinical need, we were trying to develop a high-frequency ultrasound system with high-resolution for imaging and a possible combined diagnostic and therapeutic in vivo application using the same transducer. Thus, one component of this study is the investigation of the feasibility of sonothrombolysis by using intravascular ultrasound (IVUS, > 20 MHz). Here we used the dual high-frequency method to induce cavitation and estimate its intensity by measuring differential ICD, acoustic intensity of the low frequency component. According to the experimental results, we conclude that：1. Our thrombus-targeted ultrasound contrast agent was capable of targeting specific sites and has the potential for inducing localized cavitation thus avoiding the possible endothelial damage in the clinical application. 2. The magnitude of acoustic intensity plays a major rule in the cavitation induction using high-frequency ultrasound. Besides, due to the limitation of high-frequency ultrasound aperture size, sonothrombolysis using intravascular ultrasound may not be feasible.