在生理血流条件下靶向超声微泡对P-选择素的靶向黏附效能

目的平行板流动腔评价在生理血流条件下携抗小鼠P-选择素单抗靶向超声微泡（MBp）的靶向黏附效能。方法采用＂亲和素-生物素＂桥接法构建MBp;在3种浓度（10、100和1000ng/ml）小鼠P-选择素Fc段（PSFc）包被的平行板流动腔和固定剪切应力下,以及最大包被浓度和不同剪切应力（0.2-1.7dyn/cm^2）下分别检测MBp的每分钟结合数量（结合率）,以抗小鼠P-选择素单抗封闭组和空白组为对照。在3种包被浓度下检测MBp达半数解离的剪切应力。所有分组样本数均为3。结果两对照组均未见有明显的MBp结合。实验组MBp结合率随包被浓度的增高而增加（P〈0.05）,然而与剪切应力呈现双向性（P〈0.05）。MBp达半数解离的剪切应力随包被浓度的增加而增大（P〈0.05）。结论MBp在生理条件下可与PSFc特异有效地结合,体外对靶向超声微泡的靶向黏附效能评价将有助于判定超声分子成像的效果和靶向微泡的在体应用环境条件。     

Deformable gas-filled microbubbles targeted to P-selectin.

Ultrasound contrast microbubbles have been successfully targeted to a number of intravascular disease markers. We hypothesized that targeted delivery could be improved further, by making the microbubbles deformable, leading to increased microbubble-endothelium adhesion contact area and stabilized adhesion. Activated leukocytes utilize such strategy; they deform after binding to inflamed endothelium in the vasculature. Lipid-shell microbubbles were targeted to the endothelial inflammatory protein P-selectin with a monoclonal anti-P-selectin antibody attached to the microbubble shell. Deformable microbubbles were created by controlled pressurization with partial gas loss, which generated an average excess shell surface area of approximately 30% and the formation of outward-projected wrinkles and folds. Targeted microbubble adhesion and deformability were assessed in the parallel plate flow chamber under shear flow. Sustained adhesion of deformable microbubbles at wall shear stresses between 0.4 and 1.35 dyn/cm(2) was consistently better than adhesion of wrinkle-free microbubbles. Over this shear range, targeted wrinkled microbubbles were deformed by shear flow, unlike wrinkle-free microbubbles. In a murine cremaster inflammation model, a significant improvement of deformable microbubble targeting was observed by intravital microscopy. Overall, the mechanical aspects of adhesion, such as particle shape, deformability and surface microstructure, are important in engineering efficient site-targeted particle-based agents for medical imaging and therapy.     

In vivo quantification of ultrasound targeted microbubbles to enhance cancer assessment

Contrast‐enhanced ultrasound with targeted microbubble contrast agents is an emerging technique for imaging biological processes at the molecular level. The accumulation of targeted microbubbles at tissue sites overexpressing specific molecular markers increases the backscattered signal for noninvasive evaluations of diseases. The aim of this preliminary study was to combine molecular imaging with an in vivo contrast agent quantification to support the early diagnosis of the pathology and to enhance the assessment of neoplastic tissues. Tumor growth was induced by subcutaneous injection of prostate cancer cells in four rats. Microbubbles targeted to tissue factor (TF) were administered. A vascularized region located in proximity to the tumor and centered around the focus depth was analyzed in each animal. The backscattered signals (i.e. the radio‐frequency data) were acquired during two different perfusion conditions to evaluate the contribution of attached microbubbles. After image generation by means of a multi‐pulse contrast‐enhanced technique, a nonlinear regression method based on the support vector machine was employed to estimate the contrast agent concentrations in cubic voxels (1‐mm side length). The number of attached microbubbles per mm³ was estimated based on a multi‐dimensional vector of features extracted from the processed radio‐frequency signals. A significant correlation (p < 0.05) between the size of the tumors and the estimated microbubble concentration was found, thus opening the possibility for combining molecular imaging and contrast agent concentration mapping to refine pathology evaluation. Copyright © 2016 John Wiley & Sons, Ltd.     

A Method for Differentiating Targeted Microbubbles in Real Time Using Subharmonic Micro-Ultrasound and Interframe Filtering

This study introduces a new method for differentiating targeted microbubbles in the presence of flowing microbubbles and tissue using micro-ultrasound. The method relies on subharmonic (SH) imaging for segmenting microbubble signals from tissue signals, and low-pass interframe filtering for segmenting bound targeted microbubbles from flowing microbubbles. The method is evaluated with 30 frames per second SH B-mode imaging in vitro, using a wall-less vessel flow phantom. The SH B-mode cineloops were postprocessed using an interframe moving average filter to segment the regions of bound microbubbles on the inner surface of the vessel phantom. The bound bubbles were then disrupted with sufficiently high ultrasound pressures, so that the dynamic process of targeted microbubble binding under flowing conditions could be observed. These preliminary results show that the proposed method is a feasible solution to the challenge of differentiating targeted microbubbles in the presence of tissue and freely flowing microbubbles at high frequencies, which in turn should improve the specificity of targeted microbubble detection. (E-mail: aneedles@visualsonics.com)     

Specific E‐selectin targeting with a superparamagnetic MRI contrast agent

Targeting of the endothelial inflammatory adhesion molecule E‐selectin by magnetic resonance imaging (MRI) was performed with a superparamagnetic contrast agent in the context of in vitro and in vivo models of inflammation. The specific contrast agent was obtained by grafting a synthetic mimetic of sialyl Lewis^x (sLe^x), a natural ligand of E‐selectin expressed on leukocytes, on the dextran coating of ultrasmall particles of iron oxide (USPIO). This new contrast agent, called USPIO‐g‐sLe^x, was tested, in vitro, on cultured human umbilical vein endothelial cells (HUVECs) stimulated to express inflammatory adhesion molecules, and in vivo, on a mouse model of hepatitis. In vitro, HUVECs were stimulated with the pro‐inflammatory cytokine tumor necrosis factor alpha (TNF‐α) and were then incubated with USPIO‐g‐sLe^x or ungrafted USPIO. In vivo, hepatitis was induced on NMRI mice by injection of concanavalin A (Con A). USPIO‐g‐sLe^x and ungrafted USPIO were injected intravenously. In vitro results showed an extensive retention of USPIO‐g‐sLe^x on TNF‐α stimulated HUVECs. Image intensity and R₂ measurements performed on T₂‐weighted MR images demonstrated a significantly higher binding of USPIO‐g‐sLe^x on stimulated HUVECs. In vivo, USPIO are known to pass through the fenestrae of the liver and to be captured by Kupffer cells, inducing a loss of signal intensity on T₂‐weighted MR images. Unexpectedly, when injected to Con A‐treated mice, USPIO‐g‐sLe^x induced a significantly lower attenuation of liver signal intensity than USPIO or USPIO‐g‐sLe^x injected to healthy mice, or USPIO injected to Con A‐treated mice, suggesting that the specific contrast media is retained extracellularly by an interaction with E‐selectin overexpressed on the vascular endothelium. Both in vitro and in vivo results therefore indicate that USPIO‐g‐sLe^x is recognizing endothelial E‐selectin. USPIO‐g‐sLe^x is thus well suited for the MRI diagnosis of inflammation and for the in vitro evaluation of endothelial cells activation. Copyright © 2006 John Wiley & Sons, Ltd.     

Secondary Bjerknes Forces Deform Targeted Microbubbles

In this study, we investigated the effect of secondary Bjerknes forces on targeted microbubbles using high-speed optical imaging. We observed that targeted microbubbles attached to an underlying surface and subject to secondary Bjerknes forces deform in the direction of their neighboring bubble, thereby tending toward a prolate shape. The deformation induces an elastic restoring force, causing the bubbles to recoil back to their equilibrium position; typically within 100 μs after low-intensity ultrasound application. The temporal dynamics of the recoil was modeled as a simple mass-spring system, from which a value for the effective spring constant k of the order 10⁻³ Nm⁻¹ was obtained. Moreover, the translational dynamics of interacting targeted microbubbles was predicted by a hydrodynamic point particle model, including a value of the spring stiffness k of the very same order as derived experimentally from the recoiling curves. For higher acoustic pressures, secondary Bjerknes forces rupture the molecular adhesion of the bubbles to the surface. We used this mutual attraction to quantify the binding force between a single biotinylated microbubble and an avidin-coated surface, which was found to be between 0.9 and 2 nanonewtons (nN). The observation of patches of lipids left at the initial binding site suggests that lipid anchors are pulled out of the microbubble shell, rather than biotin molecules unbinding from avidin. Understanding the effect of ultrasound application on targeted microbubbles is crucial for further advances in the realm of molecular imaging.     

Dynamic Visualization of Lymphatic Channels and Sentinel Lymph Nodes Using Intradermal Microbubbles and Contrast‐Enhanced Ultrasound in a Swine Model and Patients With Breast Cancer

Objective. Sentinel lymph node (SLN) identification using intradermal micro‐bubbles and contrast‐enhanced ultrasound (CEUS) has been recently reported in swine models and patients with breast cancer. The objective of this study was to investigate the dynamics of intradermally administered microbubbles as they travel to draining SLNs in pigs. We also performed a detailed study of the passage of microbubbles through breast lymphatic channels in a small group of patients with breast cancer. Methods. Nine anesthetized healthy pigs were used for the study, and 5 female patients with primary breast cancer were recruited. Pigs received intradermal injections of a microbubble contrast agent in several territories to access lymphatic drainage to regional lymph nodes. Patients had periareolar intradermal injection of the microbubble contrast agent. Ultrasound examination was performed in the real‐time contrast pulse sequencing mode with a commercial scanner. Results. Sentinel lymph nodes were identified rapidly (<1 minute) and consistently in pigs. Intradermal microbubble injection and CEUS were found to have perfect concordance with the Evans blue dye method in locating swine SLNs. In all 5 patients with breast cancer, the microbubble contrast agent entered breast lymphatic channels and traveled to draining ipsilateral axillary SLNs within 3 minutes. Conclusions. Intradermally injected microbubbles traverse readily though lymphatic channels in pigs and human breast tissue. The ability to rapidly identify SLNs in the diagnostic period would enable targeted biopsy and may facilitate preoperative axillary staging in patients with early breast cancer.     

Activation of microbubbles by short-pulsed ultrasound enhances the cytotoxic effect of cis-diamminedichloroplatinum (II) in a canine thyroid adenocarcinoma cell line in vitro

Ultrasound targeted microbubble destruction has succeeded in delivering drugs and genes. This study was designed to explore characteristics of ultrasound targeted microbubble destruction using short-pulsed diagnostic ultrasound. Canine thyroid adenocarcinoma cells were exposed to short-pulsed diagnostic ultrasound in the presence of cis-diamminedichloroplatinum (II) (cisplatin) and ultrasound contrast agent Sonazoid^® microbubbles. The cytotoxic effect of cisplatin was enhanced by short-pulsed diagnostic ultrasound and microbubbles. Incubation time with microbubbles influenced the cytotoxic effect of cisplatin. However, exposure duration did not affect the cytotoxic effect of cisplatin. Therefore, short-pulsed diagnostic ultrasound may activate microbubbles near cells and deliver cisplatin into cells. In addition, activation of microbubbles may be concluded in a short time. Our results suggest that short exposure duration could be potentially sufficient to induce efficient drug delivery by ultrasound targeted microbubble destruction using short-pulsed diagnostic ultrasound.     

靶向性声学造影剂与血管内皮细胞相互作用的实验研究

目的 制备携抗人VCAM-1单克隆抗体的白蛋白声学造影剂，观察其与损伤血管内皮细胞的相互作用，探讨评价血管内皮功能的新方法。 方法 采用交联法将抗人VCAM-1单克隆抗体共价偶联到自制氟碳气体为核心的白蛋白微气泡表面，制备靶向性声学微气泡；倒置显微镜下分别观察普通白蛋白微气泡、靶向性微气泡与正常内皮细胞、损伤内皮细胞的结合作用，高倍视野下计数内皮细胞及黏附的微气泡的数目，通过计算微气泡与内皮细胞的比值对两者之间的结合作用进行定量分析。 结果 无论是正常内皮细胞，或损伤内皮细胞，仅见少量的对照组微气泡的黏附作用；而镜下可见大量携VCAM-1单抗的白蛋白微气泡黏附在损伤内皮细胞表面，黏附数目显著高于黏附于正常内皮细胞表面的数目。 结论 携VCAM-1单抗的靶向性声学造影剂能够特异性结合在损伤内皮细胞表面，开拓了超声成像技术检测血管内皮损伤、评价血管内皮功能新的研究领域。     

靶向超声介导抗细胞间黏附因子-1单克隆抗体对大鼠心肌缺血再灌注损伤的实验性研究

目的探讨靶向超声能否提高抗细胞间黏附因子-1单克隆抗体对大鼠心肌缺血再灌注损伤的保护作用。方法将60只心肌缺血的雄性SD大鼠分为3组,第1组为超声介导抗细胞间黏附因子-1单克隆抗体定向于受损心肌组,第2组为单纯抗细胞间黏附因子-1单克隆抗体组;第3组为对照组。每组又分为24h后处死组和14d后处死质量分数组,每小组为10只。处死后分别行坏死区中性粒细胞记数,梗死区质量分数测定,免疫组织化学检测缺血心肌中的VEGF蛋白表达,缺血心肌区域毛细血管记数,观察用药前后心电图ST段改变。结果24h处死组超声介导组坏死区中性粒细胞平均记数少于单纯用药组,超声介导组梗死区面积明显小于单纯用药组。14d后处死组超声介导组梗死区重量比明显小于单纯用药组,且有大量VEGF蛋白阳性反应的棕褐色颗粒。结论超声介导微泡造影剂携带抗细胞间黏附因子-1单克隆抗体能提高局部药物浓度,减少中性粒细胞在缺血心肌的浸润,减少组织再损伤,促进缺血区毛细血管再生,减少心肌梗死面积。     

Quantitative Guidelines for the Prediction of Ultrasound Contrast Agent Destruction During Injection

Experiments and theory were undertaken on the destruction of ultrasound contrast agent microbubbles on needle injection, with the aim of predicting agent loss during in vivo studies. Agents were expelled through a variety of syringe and needle combinations, subjecting the microbubbles to a range of pressure drops. Imaging of the bubbles identified cases where bubbles were destroyed and the extent of destruction. Fluid-dynamic calculations determined the pressure drop for each syringe and needle combination. It was found that agent destruction occurred at a critical pressure drop that depended only on the type of microbubble. Protein-shelled microbubbles (sonicated bovine serum albumin) were virtually all destroyed above their critical pressure drop of 109 ± 7 kPa Two types of lipid-shelled microbubbles were found to have a pressure drop threshold above which more than 50% of the microbubbles were destroyed. The commercial lipid-shelled agent Definity was found to have a critical pressure drop for destruction of 230 ± 10 kPa; for a previously published lipid-shelled agent, this value was 150 ± 40 kPa. It is recommended that attention to the predictions of a simple formula could preclude unnecessary destruction of microbubble contrast agent during in vivo injections. This approach may also preclude undesirable release of drug or gene payloads in targeted microbubble therapies. Example values of appropriate injection rates for various agents and conditions are given.     

Dynamics of Targeted Microbubble Adhesion Under Pulsatile Compared with Steady Flow

Hemodynamic flow variations at low fluid shear stress are thought to play a critical role in local atherosclerotic plaque initiation and development and to affect plaque instability. Targeted microbubbles are being developed as intravascular agents for identifying atherosclerotic lesions using ultrasound. How variations in local hydrodynamic flow influence the adhesiveness of targeted microbubbles is not well understood. We postulated that rates of targeted microbubble binding and accumulation differ when subjected to steady flow (SF) as compared with oscillatory or pulsatile flow (PF), because PF imposes non-uniform blood rheology and periodic acceleration and deceleration of blood velocity, when compared with SF. We assessed the binding rates of targeted microbubbles in seven randomly assigned PF and seven matched SF replicate runs at low (<1 Pa) and intermediate (≥1 and <2.5 Pa) wall shear stress (WSS) by drawing 4.8 × 10⁶ microbubbles mL⁻¹ over streptavidin-coated substrates, immobilized within a parallel plate flow chamber at a calculated density of 81 binding sites μm^-2. Selective binding and accumulation of targeted microbubbles was recorded in a single field of view using real-time video microscopy. Microbubble accumulation was modeled to obtain flow-mediated microbubble binding kinetics (amplitude, A, and rate constant, k). PF elicited higher microbubble accumulation rates, in comparison to SF. The rates of microbubble accumulation differed significantly between PF and SF (p < 0.05) at intermediate WSS but not at low WSS (p > 0.05). The rate of microbubble accumulation decreased as WSS increased.     

Antitumor Effect of Docetaxel‐Loaded Lipid Microbubbles Combined With Ultrasound‐Targeted Microbubble Activation on VX2 Rabbit Liver Tumors

Objective. The purpose of the study was to explore the antitumor effect of docetaxel‐loaded lipid microbubbles combined with ultrasound‐targeted microbubble activation (UTMA) on VX2 rabbit liver tumors. Methods. Docetaxel‐loaded lipid microbubbles were made by a mechanical vibration technique. VX2 liver tumor models were established in 90 rabbits, which were randomly divided into 6 groups, including control, docetaxal‐loaded lipid microbubbles alone, docetaxal alone, docetaxal combined with ultrasound, pure lipid microbubbles combined with ultrasound, and docetaxel‐loaded lipid microbubbles combined with ultrasound (DOC+MB/US). The tumor volume and inhibition rate (IR) of tumor growth were calculated and compared. Apoptosis was detected by terminal deoxyuridine nick end labeling. Proliferating cell nuclear antigen and matrix metalloproteinase 2 (MMP2) protein expression was detected by immunohistochemistry. Caspase 3 and MMP2 messenger RNA (mRNA) expression was detected by in situ hybridization histochemistry. The tumor metastasis rate and survival time of the animals were compared. Results. The IR and apoptotic index of the DOC+MB/US group were the highest among all groups, and the proliferating labeling index was the lowest. Matrix metalloproteinase 2 protein and mRNA expression in the DOC+MB/US group was the lowest among all groups, and caspase 3 mRNA expression in the DOC+MB/US group was the highest. The extensive metastasis rate in the DOC+MB/US group was the lowest, and the survival time of the animals in the DOC+MB/US group was the longest. Conclusions. Docetaxel‐loaded lipid microbubbles combined with UTMA could inhibit the growth of VX2 rabbit liver tumors by deferring proliferation and promoting apoptosis, which may provide a novel targeted strategy for chemotherapy of liver carcinoma.     

超声微泡连接特异性抗体的制备方法及靶向化处理因素对超声微泡物理性质的影响

目的 探讨超声微泡连接特异性抗体的制备方法及靶向化处理因素对超声微泡物理性质的影响.方法 应用生物素-链霉亲和素连接系统,使注射用六氟化硫微泡(SonoVue)与特异性抗血管细胞黏附分子-1(VCAM-1)抗体相连接,用间接免疫荧光法检测抗体与微泡的连接,用马尔文激光粒径分析仪分别测定生物素化处理前后微泡的粒径,采用超声诊断仪评价微泡靶向化构建前后的显像效果.结果 SonoVue与特异性抗体成功连接,间接免疫荧光检测呈阳性.生物素修饰后的微泡粒径分布变窄,与普通微泡的超声显像效果略有差异.结论 通过生物素-链霉亲和素系统可以成功靶向化构建超声微泡,靶向化处理因素对微泡的粒径有一定影响,对微泡的超声显影效果略有影响.     

Evaluation of the Specific Adsorption of Biotinylated Microbubbles Using a Quartz Crystal Microbalance

Specific adsorption of biotinylated microbubbles to streptavidin was evaluated by measuring the resonant frequency of an AT-cut quartz crystal microbalance (QCM). Streptavidin was fixed via self-assembled monolayers coated onto the QCM electrode. The resonant frequency of the QCM decreased as a result of specific adsorption of the biotinylated microbubbles, compared with the results for microbubbles containing no biotin. Additionally, there was significant evidence indicating that the frequency shift was caused by the internal gas of the microbubble, as well as the mass of the outer-shell material surrounding the gas. These results suggest that the QCM measurement system can be used effectively to evaluate the specific adsorption of targeted microbubbles.     

Dual-Targeted Microbubbles Specific to Integrin αVβ3 and Vascular Endothelial Growth Factor Receptor 2 for Ultrasonography Evaluation of Tumor Angiogenesis

Aggressive tumors are characterized by angiogenesis that promotes the migration and dissemination of tumor cells. Our aim was to develop a dual-targeted microbubble system for non-invasive evaluation of tumor angiogenesis in ultrasound. Avidinylated microbubbles were conjugated with biotinylated arginylglycylaspartic acid and vascular endothelial growth factor receptor 2 (VEGFR2) antibodies. Subcutaneous MHCC-97H liver carcinoma models were established. Non-targeted, αvβ3-targeted, VEGFR2-targeted and dual-targeted microbubbles was intravenously injected in series while acquiring ultrasound images of the tumor. The microbubbles were destroyed by a high-mechanical-index pulse 4?min after the injection. Peak intensity (PI) before and after the destructive pulse was recorded to compare contrast enhancement by different microbubbles. The targeting rates of the integrin-targeted, VEGFR2-targeted and dual-targeted groups were 95.02%, 96.04% and 94.23%, respectively, with no significant differences. Tumors in all groups were significantly enhanced. The time–intensity curve indicated no significant differences in arrival time, PI, area under the curve, amplitude and mean transit time. The difference in ultrasound signal intensity before and after the destructive pulse (⊿PI) for all targeted microbubble groups was significantly greater than that for the non-targeted microbubble group (all p values?<?0.05), and the difference for the dual-targeted microbubble group was significantly greater than those of both mono-targeted groups (p?<0.05).     

Effect of surface architecture on in vivo ultrasound contrast persistence of targeted size-selected microbubbles

Ultrasound molecular imaging is a powerful diagnostic modality using microbubbles coated with targeting ligands specific for endothelial biomarkers. The circulation persistence of ligand-bearing contrast agents is a key determinant in their contrast enhancement and targeting capability. Prior studies have shown that targeted microbubbles with ligands attached to the shell using the conventional exposed-ligand architecture (ELA) could trigger undesired ligand-induced complement activation and decreased circulation time. Microbubbles with the buried-ligand architecture (BLA), however, were found to inhibit complement activation and prolong circulation time. In the present study, we extended the stealth BLA microbubble design to size-selected (4 to 5-μm diameter) microbubbles targeted with cyclic RGD peptide using the postlabeling technique. Microbubble circulation persistence was measured in the healthy mouse kidney using a Visualsonics Vevo 770 scanner operating at 40 MHz in fundamental mode. The circulation persistence for targeted BLA microbubbles was significantly longer compared with their ELA counterparts and similar to no-ligand controls. Use of the BLA instead of the ELA increased the circulation half-life approximately two-fold. Analysis of the time-intensity and time-fluctuation curves with a two-compartment pharmacokinetic model showed a minimal degree of nonspecific vascular adhesion for any group. These results demonstrate the importance of surface architecture in the design of targeted microbubbles for ultrasound molecular imaging.     

携细胞间黏附分子1抗体的Sono Vue微泡造影剂靶向识别受损内皮的实验研究

目的 探讨静电吸附法制备的携细胞间黏附分子1抗体靶向微泡在体外和体内的寻靶能力.方法 采用静电吸附法制备携ICAM-1抗体的靶向SonoVue微泡.体外培养经IL-1β刺激大量表达ICAM-1的人血管内皮细胞ECV304,采用免疫荧光法检测靶向微泡与其结合能力.构建兔急性心肌梗死模型,进行靶向微泡的心肌造影,采用冷冻切片和免疫荧光法榆测靶向微泡与受损血管内膜结合能力.结果 在体外实验中,可见大量携ICAM-1抗体靶向微泡与刺激后ECV304细胞紧密结合,仅有少量靶向微泡与正常ECV304细胞结合.体内实验中,可见大量携ICAM-1抗体靶向微泡在兔梗死心肌受损血管内膜处黏附,仅可见少量靶向微泡在正常血管内膜处黏附.结论 携ICAM-1抗体靶向微泡在体内、体外均能与受损血管内皮特异性结合,不仅有利于靶向超声造影显像,更为微泡携带药物或基因在局部定向释放开辟良好前景.     

Investigating the subharmonic response of individual phospholipid encapsulated microbubbles at high frequencies: a comparative study of five agents

There are a range of contrast ultrasound applications above 10 MHz, a frequency regime in which nonlinear microbubble behavior is poorly understood. Lipid-encapsulated microbubbles have considerable potential for use at higher frequencies because they have been shown to exhibit pronounced nonlinear activity at frequencies up to 40 MHz. The objective of this work was to investigate the influence of agent formulation on the subharmonic response of lipid-encapsulated microbubbles at high frequencies with a view to providing information relevant to improving contrast agent design and imaging performance. An optical-acoustical setup was used to measure the subharmonic emissions from small (d < 3 μm) individual lipid-encapsulated microbubbles as a function of transmit pressure, size and composition. In this study, five agent formulations (Definity™, MicroMarker™ and three in-house agents manipulated to exhibit different levels of shell microstructure heterogeneity) were insonified at 25 MHz over a peak negative pressure (P_n) range of 0.02–1.2 MPa. All agents exhibited distinctly different subharmonic behavior, both in terms of amplitude and active sizes. MicroMarker™ exhibited the strongest, broadest and most consistent subharmonic response, 22% greater in power than that of Definity™ and as much as 50% greater than the in-house formulations. No clear relation between in-house agents’ shell microstructure and nonlinear response was found, other than the variability in the nonlinear response itself. An analysis of the response of MicroMarker™ bubbles suggests that these bubbles exhibit “expansion-dominated” oscillations, in contrast to “compression-only” oscillations observed for similar bubbles at lower frequencies (f < 11 MHz).     

Development of a Translatable Ultrasound Molecular Imaging Agent for Inflammation

This study details the development, characterization and non-clinical efficacy of an ultrasound molecular imaging agent intended for molecular imaging of P-selectin in humans. A targeting ligand based on a recently discovered human selectin ligand was manufactured as fusion protein, and activity for human and mouse P- and E-selectin was evaluated by functional immunoassay. The targeting ligand was covalently conjugated to a lipophilic anchor inserted into a phospholipid microbubble shell. Three lots of the targeted microbubble drug product, TS-07-009, were produced, and assays for size distribution, zeta potential and morphology were established. The suitability of TS-07-009 as a molecular imaging agent was evaluated in vitro in a flow-based adhesion assay and in vivo using a canine model of transient myocardial ischemia. Selectivity for P-selectin over E-selectin was observed in both the human and murine systems. Contrast agent adhesion increased with P-selectin concentration in a dynamic adhesion assay. Significant contrast enhancement was observed on ultrasound imaging with TS-07-009 in post-ischemic canine myocardium at 30 or 90 min of re-perfusion. Negligible enhancement was observed in resting (no prior ischemia) hearts or with a control microbubble 90 min after ischemia. The microbubble contrast agent described here exhibits physiochemical properties and in vivo behavior suitable for development as a clinical imaging agent.