Sonolysis of Albunex-supplemented, 40% hematocrit human erythrocytes by pulsed 1-MHz ultrasound: pulse number, pulse duration and exposure vessel rotation dependence

Therapeutic Ultrasound in Cardiovascular Medicine

An advantage of therapeutic ultrasound (US) is the ability to cause controlled biological effects noninvasively. Depending on the magnitude and frequency of exposure parameters, US can interact in different ways with a variety of biological tissues. The development and clinical utility of therapeutic US techniques are now rapidly growing, especially with regard to the application of US pulses for cardiac pacing and the potential treatment of cardiovascular diseases. This review outlines the basic principles of US-based therapy in cardiology, including the acoustic properties of the cardiovascular tissue, and the use of US in therapeutic cardiovascular medicine.

Hemocoagulase Combined with Microbubble-Enhanced Ultrasound Cavitation for Augmented Ablation of Microvasculature in Rabbit VX2 Liver Tumors

We investigated a new method for combining microbubble-enhanced ultrasound cavitation (MEUC) with hemocoagulase (HC) atrox. Our goal was to induce embolic effects in the vasculature and combine these with an anti-angiogenic treatment strategy. Fourteen days after being implanted with a single slice of the liver VX2 tumor, rabbits were randomly divided into five groups: (i) a control group injected intra-venously with saline using a micropump; (ii) a group given only an injection of HC; (iii) a group treated only with ultrasound cavitation; (iv) a group treated with MEUC; (v) a group treated with MEUC + HC. Contrast-enhanced ultrasound was performed before treatment and 1 h and 7 d post-treatment to measure tumor size, enhancement and necrosis range. QontraXt software was used to determine the time-intensity curve of tumor blood perfusion and microvascular changes. At 1 h and 7 d after treatment with MEUC + HC, the parameters of the time-intensity curve, which included peak value, regional blood volume, regional blood flow and area under the curve value and which were measured using contrast-enhanced ultrasound, were significantly lower than those of the other treatment groups. The MEUC + HC treatment group exhibited significant growth inhibition relative to the ultrasound cavitation only, HC and MEUC treatment groups. No damage was observed in the surrounding normal tissues. These results support the feasibility of reducing the blood perfusion of rabbit VX2 liver tumors using a new method that combines MEUC and HC.

Acoustic Cavitation-Mediated Delivery of Small Interfering Ribonucleic Acids with Phase-Shift Nano-Emulsions

Localized, targeted delivery of small interfering ribonucleic acid (siRNA) has been the foremost hurdle in the use of siRNA for the treatment of various diseases. Major advances have been achieved in the synthesis of siRNA, which have led to greater target messenger RNA (mRNA) silencing and stability under physiologic conditions. Although numerous delivery strategies have shown promise, there are still limited options for targeted delivery and release of siRNA administered systemically. In this in vitro study, phase-shift nano-emulsions (PSNE) were explored as cavitation nuclei to facilitate free siRNA delivery to cancer cells via sonoporation. A cell suspension containing varying amounts of PSNE and siRNA was exposed to 5-MHz pulsed ultrasound at fixed settings (6.2-MPa peak negative pressure, 5-cycle pulses, 250-Hz pulse repetition frequency (PRF) and total exposure duration of 100 s). Inertial cavitation emissions were detected throughout the exposure using a passive cavitation detector. Successful siRNA delivery was achieved (i.e., >50% cell uptake) with high (>80%) viability. The percentage of cells with siRNA uptake was correlated with the amount of inertial cavitation activity generated from vaporized PSNE. The siRNA remained functional after delivery, significantly reducing expression of green fluorescent protein in a stably transfected cell line. These results indicate that vaporized PSNE can facilitate siRNA entry into the cytosol of a majority of sonicated cells and may provide a non-endosomal route for siRNA delivery.

Mechanisms of prostate permeability triggered by microbubble-mediated acoustic cavitation

The objective of this research was to study the mechanisms of opening of the blood-prostate barrier and increased permeability of prostate tissue induced by microbubble cavitation. Thirty-five rabbits were randomly divided into four study groups: (1) control group and groups exposed to (2) microbubble alone, (3) ultrasound alone, or (4) combined intervention (ultrasound + microbubble group). Evans blue (EB) tracer was used to gauge the changes of permeability of prostate tissue. Furthermore, light and electron microscopy analyses were conducted, as well as the western blot analysis of expression of gap junction (Cx43) protein. We observed that EB concentration in prostate tissue was significantly greater in the ultrasound + microbubble group compared with either intervention alone (p < 0.05, both comparisons). Furthermore, light microscopy of tissue samples from animals exposed to ultrasound + microbubble showed epithelial cell disarrangement, loss of interstitial structure, and thickness of fibrous stroma. In line with these findings, electron microscopy analysis demonstrated widening of cell gaps and broken cell connections, as well as more dense lysosomes and secretary granules, and mitochondrial swelling. These changes were absent in the animals exposed to microbubble or ultrasound alone. Finally, only combined treatment with microbubble or ultrasound significantly elevated expression of Cx43 (p < 0.05 vs. control group). In conclusion, increases of permeability of prostate tissue by acoustic cavitation appear to involve opening of tight junctions, widening of intracellular spaces, changes in the structure of acinar cell membrane, enhancement of vesicular transport, and loosening of fibrous stroma. Increased expression of cell gap junction protein will help to restore normal connections between cells and the blood-prostate barrier after the treatment.

Vascular effects of microbubble-enhanced, pulsed, focused ultrasound on liver blood perfusion

The purpose of this study was to investigate the vascular effects of microbubble-enhanced pulsed high-pressure ultrasound on liver blood perfusion. In the presence of circulating lipid-shell microbubbles, a focused ultrasound transducer was used to transcutaneously treat eight livers of healthy rabbits for perfusion analysis and to treat three livers with the abdomen open for histologic analysis. Twenty-two livers treated with the ultrasound only (n = 11) or microbubbles only (n = 11) served as the controls. The focused ultrasound was operated at a frequency of 1.22 MHz with a peak negative pressure of 4.6 MPa. The liver blood perfusion was estimated by performing contrast-enhanced ultrasound and gray-scale quantification on the livers before and after treatment. A temporary, nonenhanced region occurred in all of the experimental livers. The regional contrast gray-scale values of the experimental group dropped significantly from 88.4 before treatment to 2.7 after treatment. The liver perfusion also demonstrated a gradual recovery over a 60-min period. The liver perfusion of the control groups remained the same after treatment. We found microvascular rupture, hemorrhage and swelling hepatocytes upon histologic examination of the experimental group. Regional liver blood perfusion can be temporarily blocked by microbubble-enhanced focused ultrasound with high-pressure amplitude. These vascular effects can be explained as acute microvascular injury of the liver and may have clinical implications.

Ultrasound-biophysics mechanisms

Ultrasonic biophysics is the study of mechanisms responsible for how ultrasound and biological materials interact. Ultrasound-induced bioeffect or risk studies focus on issues related to the effects of ultrasound on biological materials. On the other hand, when biological materials affect the ultrasonic wave, this can be viewed as the basis for diagnostic ultrasound. Thus, an understanding of the interaction of ultrasound with tissue provides the scientific basis for image production and risk assessment. Relative to the bioeffect or risk studies, that is, the biophysical mechanisms by which ultrasound affects biological materials, ultrasound-induced bioeffects are generally separated into thermal and non-thermal mechanisms. Ultrasonic dosimetry is concerned with the quantitative determination of ultrasonic energy interaction with biological materials. Whenever ultrasonic energy is propagated into an attenuating material such as tissue, the amplitude of the wave decreases with distance. This attenuation is due to either absorption or scattering. Absorption is a mechanism that represents that portion of ultrasonic wave that is converted into heat, and scattering can be thought of as that portion of the wave, which changes direction. Because the medium can absorb energy to produce heat, a temperature rise may occur as long as the rate of heat production is greater than the rate of heat removal. Current interest with thermally mediated ultrasound-induced bioeffects has focused on the thermal isoeffect concept. The non-thermal mechanism that has received the most attention is acoustically generated cavitation wherein ultrasonic energy by cavitation bubbles is concentrated. Acoustic cavitation, in a broad sense, refers to ultrasonically induced bubble activity occurring in a biological material that contains pre-existing gaseous inclusions. Cavitation-related mechanisms include radiation force, microstreaming, shock waves, free radicals, microjets and strain. It is more challenging to deduce the causes of mechanical effects in tissues that do not contain gas bodies. These ultrasonic biophysics mechanisms will be discussed in the context of diagnostic ultrasound exposure risk concerns.

Vascular effects induced by combined 1-MHz ultrasound and microbubble contrast agent treatments in vivo

Previous in vivo studies have demonstrated that microvessel hemorrhages and alterations of endothelial permeability can be produced in tissues containing microbubble-based ultrasound contrast agents when those tissues are exposed to MHz-frequency pulsed ultrasound of sufficient pressure amplitudes. The general hypothesis guiding this research was that acoustic (viz., inertial) cavitation, rather than thermal insult, is the dominant mechanism by which such effects arise. We report the results of testing five specific hypotheses in an in vivo rabbit auricular blood vessel model: (1) acoustic cavitation nucleated by microbubble contrast agent can damage the endothelia of veins at relatively low spatial-peak temporal-average intensities, (2) such damage will be proportional to the peak negative pressure amplitude of the insonifying pulses, (3) damage will be confined largely to the intimal surface, with sparing of perivascular tissues, (4) greater damage will occur to the endothelial cells on the side of the vessel distal to the source transducer than on the proximal side and (5) ultrasound/contrast agent-induced endothelial damage can be inherently thrombogenic, or can aid sclerotherapeutic thrombogenesis through the application of otherwise subtherapeutic doses of thrombogenic drugs. Auricular vessels were exposed to 1-MHz focused ultrasound of variable peak pressure amplitude using low duty factor, fixed pulse parameters, with or without infusion of a shelled microbubble contrast agent. Extravasation of Evans blue dye and erythrocytes was assessed at the macroscopic level. Endothelial damage was assessed via scanning electron microscopy (SEM) image analysis. The hypotheses were supported by the data. We discuss potential therapeutic applications of vessel occlusion, e.g., occlusion of at-risk gastric varices.

Role of intracellular calcium ions and reactive oxygen species in apoptosis induced by ultrasound

Recently, we have reported that ultrasound (US)-induced apoptosis is due to inertial cavitation and that extracellular reactive oxygen species (ROS) generated by inertial cavitation are not directly correlated with the apoptosis (Honda et al. 2002). The molecular mechanism of apoptosis induced by US is not yet sufficiently clear. Here, we examine the role of intracellular calcium ions and the intracellular ROS on apoptosis induced by US. Human myelomonocytic lymphoma U937 cells were exposed to continuous 1-MHz US at an intensity of 4.9 W/cm(2) (I(SPTA)) in the presence of air, and changes of intracellular calcium ion concentration ([Ca(2+)]i) in individual cells by digital imaging, various flow cytometric analyses of endpoints of apoptosis (early apoptosis, secondary necrosis, loss of mitochondria membrane potential, superoxide formation, caspase-3 activation) and DNA fragmentation were explored. Furthermore, the effects of an intracellular calcium ion chelator (BAPTA-AM), an antioxidant (N-acetyl-L-cysteine, NAC), a calcium channel blocker (verapamil), Ca(2+)-free buffer and Levovist were also investigated. These results indicate that: 1. the mitochondria-caspase pathway and the Ca(2+)-dependent pathway play cardinal roles in apoptosis induced by US because BAPTA-AM partially inhibited DNA fragmentation, loss of mitochondria membrane potential and caspase-3 activation; 2. intracellular ROS generated from mitochondria, rather than extracellular ROS (which were directly produced by inertial cavitation in the medium), are involved in the regulation of apoptosis induced by US because addition of NAC after sonication showed effective suppression of the apoptosis; and 3. increase of [Ca(2+)]i appears to be due to nonspecific influx from outside the cells because verapamil is not effective and no increase of [Ca(2+)]i due to sonication could be observed in the Ca(2+)-free buffer.

Quantification of optison bubble size and lifetime during sonication dominant role of secondary cavitation bubbles causing acoustic bioeffects

Acoustic cavitation has been shown to deliver molecules into viable cells, which is of interest for drug and gene delivery applications. To address mechanisms of these acoustic bioeffects, this work measured the lifetime of albumin-stabilized cavitation bubbles (Optison) and correlated it with desirable (intracellular uptake of molecules) and undesirable (loss of cell viability) bioeffects. Optison was exposed to 500 kHz ultrasound (acoustic pressures of 0.6-3.0 MPa and energy exposures of 0.2-200 J/cm2) either with or without the presence of DU145 prostate cancer cells (10(6) cells/ml) bathed in calcein, a cell-impermeant tracer molecule. Bubble lifetime was determined using a Coulter counter and flow cytometer, while bioeffects were evaluated by flow cytometry. The lifetime of Optison cavitation nuclei was found to decrease and bioeffects (molecular uptake and loss of cell viability) were found to increase with increasing acoustic energy exposure. These bioeffects correlated well with the disappearance of bubbles, suggesting that contrast agent destruction either directly or indirectly affected cells, probably involving unstabilized cavitation nuclei created upon the destruction of Optison. Because Optison solutions presonicated to destroy all detectable bubbles also caused significant bioeffects, the indirect mechanism involving secondary cavitation bubbles is more likely.

The relation between cavitation and platelet aggregation during exposure to high-intensity focused ultrasound

Our previous study showed that high-intensity focused ultrasound (HIFU) is capable of producing "primary acoustic hemostasis" in the form of ultrasound (US)-induced platelet activation, aggregation and adhesion to a collagen-coated surface. In the current study, 1.1 MHz continuous-wave HIFU was used to investigate the role of cavitation as a mechanism for platelet aggregation in samples of platelet-rich plasma. A 5 MHz passive cavitation detector was used to monitor cavitation activity and laser aggregometry was used to measure platelet aggregation. Using spatial average intensities from 0 to 3350 W/cm2, the effects of HIFU-induced cavitation on platelet aggregation were investigated by enhancing cavitation activity through use of US contrast agents and by limiting cavitation activity through use of an overpressure system. Our results show that increased cavitation activity lowers the intensity threshold to produce platelet aggregation and decreased cavitation activity in the overpressure system raises the intensity threshold for platelet aggregation.

Inertial cavitation dose and hemolysis produced in vitro with or without Optison

Gas-based contrast agents (CAs) increase ultrasound (US)-induced bioeffects, presumably via an inertial cavitation (IC) mechanism. The relationship between IC dose (ICD) (cumulated root mean squared [RMS] broadband noise amplitude; frequency domain) and 1.1-MHz US-induced hemolysis in whole human blood was explored with Optison; the hypothesis was that hemolysis would correlate with ICD. Four experimental series were conducted, with variable: 1. peak negative acoustic pressure (P-), 2. Optison concentration, 3. pulse duration and 4. total exposure duration and Optison concentration. P- thresholds for hemolysis and ICD were approximately 0.5 MPa. ICD and hemolysis were detected at Optison concentrations >/= 0.01 V%, and with pulse durations as low as four or two cycles, respectively. Hemolysis and ICD evolved as functions of time and Optison concentration; final hemolysis and ICD values depended on initial Optison concentration, but initial rates of change did not. Within series, hemolysis was significantly correlated with ICD; across series, the correlation was significant at p < 0.001.

The pulse length-dependence of inertial cavitation dose and hemolysis

Gas-based ultrasound (US) contrast agents increase erythrocyte sonolysis, presumably via enhancing inertial cavitation (IC) activity. The amount of IC activity (IC "dose") and hemolysis generated by exposure to 1.15 MHz US were examined with different US pulse lengths, but with the same delivered acoustic energy, for Optison and Albunex. The hypotheses were that 1. at longer pulse lengths, IC would generate more bubbles that could nucleate additional IC activity; 2. if the interval between pulse pairs were short enough for the next pulse to hit derivative bubbles before their dissolution, more IC could be induced; and 3. hemolysis would be proportional to IC activity. Two types of studies were performed. In the first, bubble generation after each burst of IC activity was quantified using an active cavitation detector (ACD), for different pulse lengths (5, 10, 20, 30, 50, 100 or 200 cycles), but the same pressure level (3 MPa) and total "on" time (173.16 ms). Low concentrations of either Optison or Albunex were added into the tank with high-intensity and interrogating transducers orthogonal to each other. For pulse lengths > 100 cycles, and pulse repetition intervals < 5 ms, a "cascade" effect (explosive bubble generation) was observed. In the second, IC was measured by passive detection methods. IC dose and hemolysis were determined in whole blood samples at a pressure level (3 MPa) and interpulse interval (5 ms) that induced the "cascade" effect. Each blood sample was mixed with the same number of contrast microbubbles (Optison approximately 0.3 v/v % and Albunex approximately 0.5 v/v %), but exposed to different pulse lengths (5, 10, 20, 30, 50, 100 or 200 cycles). With Optison, up to 60% hemolysis was produced with long pulses (100 and 200 cycles), compared with < 10% with short pulses (5 and 10 cycles). Albunex generated considerably less IC activity and hemolysis. The r(2) value was 0.99 for the correlation between hemolysis and IC dose. High pulse-repetition frequency (PRF) (500 Hz) generated more hemolysis than the low PRF (200 Hz) at 3 MPa. All experimental results could be explained by the dissolution times of IC-generated bubbles.

Biological and environmental factors affecting ultrasound-induced hemolysis in vitro: 2. Medium dissolved gas (pO2) content

The data collected in this project supported the a priori hypothesis that the concentration of dissolved oxygen in whole human blood in vitro affected the extent of ultrasound (US)-induced hemolysis under conditions conducive to the occurrence of inertial cavitation. Aliquots of whole human blood in vitro with a relatively high O(2) level had statistically significantly more 1-MHz US-induced hemolysis than aliquots with a relatively low O(2) level in the presence of controlled gas nucleation (Albunex or ALX, supplementation), with US-induced hemolytic yields being substantially less at 2.2- and 3.5-MHz exposures or in the absence of ALX-supplementation at otherwise comparable acoustic pressures, pulse lengths and duty factors. Passive cavitation detection (pcd) measures indicated a linear relationship for hemolysis up to about 70% and pcd values (R(2) = 0.99).

Biological and environmental factors affecting ultrasound-induced hemolysis in vitro: 1. HIV macrocytosis (cell size)

This paper reports the results of a further test of the hypothesis that the extent of ultrasound (US)-induced cell lysis in the presence of a US contrast agent to enhance cavitational effects is a function of cell size. The present data support the hypothesis. Human adult erythrocytes in vitro derived from patients with HIV (n = 15) and apparently healthy individuals (n = 15) were compared for US-induced hemolysis in vitro. The anticoagulated whole blood from patients with HIV and macrocytic erythrocytes had significantly greater (p <0.0001) mean corpuscular volume (MCV) and a significantly greater (p <0.03) extent of US-induced hemolysis in vitro relative to blood from apparently normal, healthy individuals. As a control to determine if disease state (i.e., HIV infection per se) might be a contributing factor in US-induced hemolysis in vitro, the blood from patients with HIV and apparently normal MCVs (n = 15) was also tested against an additional population of apparently normal, healthy individuals (n = 15); there were no statistically significant differences in MCVs or US-induced hemolysis between the two groups (p >> 0.05). There were also no statistically significant differences in viscosities or hematocrits of the whole blood or plasma in vitro from HIV-macrocytic or apparently healthy individuals but, for all blood types, a pooled correlation existed between hematocrit and whole blood viscosity.

Arrhythmias in rat hearts exposed to pulsed ultrasound after intravenous injection of a contrast agent

OBJECTIVE

To develop an animal model suitable for characterizing electrocardiographic arrhythmias in hearts exposed to ultrasound after injection of a microbubble contrast agent.

METHODS

Conduction complex and heart lesion data were recorded from 20 rats that received intravenous injections of 0.25 mL of a contrast agent and were exposed to pulsed ultrasound (frequency, 3.1 MHz; pulse duration, 1.3 microseconds; pulse repetition frequency, 1700 Hz; and in situ peak rarefactional pressure, 15.9 MPa). The volume of the contrast agent based on body weight and the mechanical index (ultrasonic pressure) exceeded those used in echocardiography by 14 to 345 and 3 to 29 times, respectively.

RESULTS

Premature atrial complexes, premature ventricular complexes, or polymorphic ventricular tachycardia occurred in 10 rats. When ultrasound exposure was halted, arrhythmias ceased but reoccurred in 4 of the 10 rats when exposure resumed. Myocardial degeneration identified by histochemical staining (hematoxylin-basic fuchsinpicric acid) was observed in 16 rats; however, only 10 rats had arrhythmias. There was no significant difference in the amount of histochemical staining in hearts from rats with arrhythmias when compared with rats without arrhythmias.

CONCLUSIONS

An animal model suitable for characterizing electrocardiographic arrhythmias in rat hearts exposed to ultrasound after injection of a microbubble contrast agent was developed. Because arrhythmias were induced principally when the contrast agent interacted with ultrasound during exposure, the presence of myocardial degeneration alone was not a sufficient explanation for ectopic electrical activity. Under these extreme exposure conditions, the data suggest that pulsed ultrasound through its biomechanical interactions with contrast agents has the potential to induce arrhythmias.

Effects of dissolved gases and an echo contrast agent on apoptosis induced by ultrasound and its mechanism via the mitochondria-caspase pathway

Human histiocytic lymphoma U937 cells were exposed to continuous 1-MHz ultrasound (US) for therapeutic use, (0 approximately 6.5 W/cm(2) (I(SPTA)). Apoptosis and its related end points were examined by flow cytometry. Fraction of cells with low mitochondria membrane potential were observed after sonication and significant superoxide and peroxide formation, increased activity of caspase-3, and DNA fragmentation revealed biochemically, were also found. The fraction of early apoptosis and secondary necrosis increased with the incubation time after sonication. Early apoptosis observed at 6 h after sonication reached its maximum at 2 min of sonication and gradually decreased. On the other hand, secondary necrosis increased with the duration of sonication. When the effects of dissolved gases, Ar, N(2), O(2), air, N(2)O and CO(2), on free radical formation due to inertial cavitation were investigated by electron spin resonance (ESR) spin trapping, formation of hydroxyl radicals and hydrogen atoms was found in solutions saturated with Ar, N(2), O(2) and air, but not with N(2)O and CO(2). Apoptosis induced by US was also dependent on the dissolved gases in the order Ar = N(2) = O(2) = air >> N(2)O = CO(2) approximately 0. These results suggest that US-induced apoptosis, which is mitochondria-caspase dependent, was linked to inertial cavitation. However, quantities of free radicals did not influence the fraction of early apoptosis and secondary necrosis. When the cells were sonicated in the presence of an echo contrast agent, Levovist; synergistic enhancement of secondary necrosis induced by US was observed at concentrations of more than 20 mg/mL. In contrast, an additive increase of early apoptosis was observed in the combined treatments. These results suggest that Levovist; acting as cavitation nuclei, enhances secondary necrosis induced by US due to an increase in the membrane damage.

Ultrasonic contrast agents: safety considerations reviewed

Ultrasonic contrast agents are usually comprised of a stabilised shell encapsulating a gas bubble. When these are introduced in the body they increase the acoustic scattering from the tissues through which they pass, and especially from the vasculature. Their primary uses lie in cardiological and oncological imaging. However, these microbubbles have the potential to act as centres for acoustic cavitation activity, and so it is important to consider the safety of their use from an acoustic standpoint. The addition of ultrasonic contrast agents to in vitro suspensions of red blood cells has been shown to lead to haemolysis when the sample is exposed to ultrasound at levels which leave the cells unharmed in their absence. In vivo the infusion of gas bubble contrast agents into experimental animals has been shown to enhance the incidence of petechiae and haemorrhage in the intestine. The Mechanical Index (MI) thresholds for the effects seen in vitro lie within the range of MIs available with diagnostic clinical scanners, but in vivo the thresholds lie at the top end of the exposure levels available clinically. No adverse effects in humans arising from the ultrasonic exposure of these contrast agents have been reported to date.

[Effect of ultrasound on membrane surface potentials]

The electrostatic equilibrium on the surface of an ion-impermeable membrane was not influenced by ultrasound fields. Only after incorporation of an ion transporter did the ultrasound induce changes of the membrane surface potential. Because the ultrasound effect was completely reversible, measurements of the surface potential of a flat lipid bilayer membrane containing the calcium transporter calcimycin were performed, simultaneously to the ultrasound exposure. The ultrasound-induced volume flow, also called quartz wind, favored the mass transfer through the diffusion boundary close to the membrane, thereby leading to increased calcium concentrations in the immediate vicinity of the membrane. This, in turn, became manifest as a reduction of the negative surface charge density.

Ultrasound-mediated disruption of cell membranes. II. Heterogeneous effects on cells

Ultrasound has been shown to reversibly and irreversibly disrupt membranes of viable cells through a mechanism believed to involve cavitation. Because cavitation is both temporally and spatially heterogeneous, flow cytometry was used to identify and quantify heterogeneity in the effects of ultrasound on molecular uptake and cell viability on a cell-by-cell basis for suspensions of DU145 prostate cancer and aortic smooth muscle cells exposed to varying peak negative acoustic pressures (0.6-3.0 MPa). exposure times (120-2,000 ms), and pulse lengths (0.02-60 ms) in the presence of Optison (1.7% v/v) contrast agent. Cell-to-cell heterogeneity was observed at all conditions studied and was classified into three subpopulations: nominal uptake (NUP), low uptake (LUP), and high uptake (HUP) populations. The average number of molecules within each subpopulation was generally constant: 10(4)-10(5) molecules/cell in NUP, approximately 10(6) molecules/cell in LUP, and approximately 10(7) molecules/cell in HUP. However, the fraction of cells within each subpopulation showed a strong dependence on both acoustic pressure and exposure time. Varying pulse length produced no significant effect. The distribution of cells among the three subpopulations correlated with acoustic energy exposure, which suggests that energy exposure may govern the ability of ultrasound to induce bioeffects by a nonthermal mechanism.

Ultrasound-mediated disruption of cell membranes. I. Quantification of molecular uptake and cell viability

Ultrasound-mediated drug delivery is a nonchemical, nonviral, and noninvasive method for targeted transport of drugs and genes into cells. Molecules can be delivered into cells when ultrasound disrupts the cell membrane by a mechanism believed to involve cavitation. This study examined molecular uptake and cell viability in cell suspensions (DU145 prostate cancer and aortic smooth muscle cells) exposed to varying peak negative acoustic pressures (0.6-3.0 MPa), exposure times (120-2000 ms), and pulse lengths (0.02-60 ms) in the presence of Optison (1.7% v/v) contrast agent. With increasing pressure and exposure time, molecular uptake of a marker compound, a calcein, increased and approached equilibrium with the extra cellular solution, while cell viability decreased. Varying pulse length produced no significant effect. All viability and molecular uptake measurements collected over the broad range of ultrasound conditions studied correlated with acoustic energy exposure. This suggests that acoustic energy exposure may be predictive of ultrasound's nonthermal bioeffects.

A comparison of the hemolytic potential of Optison and Albunex in whole human blood in vitro: acoustic pressure, ultrasound frequency, donor and passive cavitation detection considerations

This project tested the hypothesis that a "second-generation" ultrasound (US) contrast agent (Optison), offering extended echogenicity over that of its "first-generation" predecessor (Albunex), would have the greater potential for sonolysis of human erythrocytes in vitro. Whole human blood, obtained from apparently healthy donors, was anticoagulated and subsequently exposed in vitro to US in the presence of one of each or neither of the two US contrast agents. The US exposures were for 30 s and involved frequency (1.0, 2.2 and 3.4 MHz) and amplitude (approximately 2.8 to 0.38 MPa P(-)) regimens; pulse duration (200 micros) and interpulse interval (20 ms) were held constant. The data supported the hypothesis, with an overall ratio of approximately 2.5 for relative extent of background-corrected US-induced hemolysis of the Optison/Albunex regimens. Passive cavitation detection analyses corroborated the results obtained with hemolysis.

Comparative sensitivity of human fetal and adult erythrocytes to hemolysis by pulsed 1 MHz ultrasound

Human fetal and adult erythrocytes differ significantly in mean corpuscular volume (MCV), the fetal cells being larger than adult cells and diminishing in MCV as gestational age (GA) increases. Previous studies have shown that the sensitivity of erythrocytes from different species to lysis by mechanically applied shear stress increases as MCV increases. The tested hypotheses in the present project were: 1. fetal erythrocytes would be more sensitive to sonolysis than adult erythrocytes because of the former's larger size, and 2. erythrocyte sonolytic sensitivity would scale with MCV. Fetal and adult erythrocytes were resuspended to 40% hematocrit in oxygenated isotonic saline solution and 500 microL aliquots were exposed for 60 s to 200 micros bursts of 1-MHz ultrasound (US) (peak pressures: approximately 4.8 MPa positive, approximately 2.7 MPa negative; duty factor = 0.01), either with or without 3.6 volume % Albunex (ALX) present. Background-corrected hemolysis was indistinguishable from zero in sham-exposed fetal or adult erythrocyte suspensions. Without ALX, mean background-corrected US-induced hemolysis was significantly greater than zero for fetal and adult cells (0.42 +/- 0.15% vs. 0.62 +/- 0.15), but fetal cell lysis was not significantly greater than adult cell lysis. With ALX, US-induced hemolytic yields increased approximately 80-fold (fetal: 50.53 +/- 2.14; adult: 46.40 +/- 1.85%), and were significantly higher for fetal than for adult cells. There was also a statistically significant correlation between MCV and US-induced background-corrected hemolysis. Thus, the two hypotheses were supported.

Comparative sensitivity of human and bovine erythrocytes to sonolysis by 1-MHz ultrasound

This project tested the hypothesis that human erythrocytes, being larger than bovine erythrocytes, would be the more sensitive to sonolysis induced by inertial cavitation. The rationale behind this hypothesis was an earlier demonstration that, among sized populations of erythrocytes, an inverse relation existed between erythrocyte volume and mechanically-induced shear forces in the surrounding medium; viz, the larger the cell, the less shear force required to rupture the cell's membrane. At low erythrocyte densities (i.e., approximately 5% hematocrit) the hypothesis was supported; at high cell densities (i.e., approximately 35% hematocrit) it was not supported. The data are consistent with an ultrasound (US)-induced symmetric implosion of affected gas nuclei as causing the effect at low cell densities; under such conditions there is ample spacing among cells for US-induced symmetric growth and collapse of gas nuclei and the concomitant production of radially-expanding shock waves (which lyse the cells); at high cell densities there is not sufficient spacing among cells for US-induced symmetric growth and collapse of bubbles and an alternative mechanism, possibly asymmetric bubble collapse, becomes operational.

Ultrasound enhancement of liposome-mediated cell transfection is caused by cavitation effects

Cationic liposomes (CL) are widely used vectors for gene transfer. Recently, ultrasound (US) was reported to enhance liposome-mediated gene transfer to eucaryotic cells in culture. The present study was aimed at studying the effects of 2-MHz pulsed Doppler US on malignant brain tumor cells transfection by cationic liposome/plasmid-DNA complexes (lipoplexes). Cationic liposomes consisting of DOSPA/DOPE were complexed with a plasmid carrying the cDNA encoding green autofluorescent protein (EGFP). Rodent (9L) and canine (J3T) glioma cells were exposed to pulsed US in the presence of EGFP-lipoplexes. A diagnostic transcranial Doppler device (MultiDop L) was used for insonation for 30, 60, and 90 s at 2 MHz/0.5 W/cm(2). To eliminate US reflection and cavitation, a custom-made absorption chamber was designed, where US is applied through a water tank before interacting with the cells and is fully absorbed after passing through the cell layer. Expression of the marker gene EGFP was quantified by FACS analysis and intravital fluorescent microscopy. Cell viability was accessed by Trypan Blue staining. US treatment of tumor cells on microplates for 60 s yielded a significant increase in transfection rates without damaging the cells, but 90-s treatment killed most of the cells. In the absorption chamber, no significant effects of US on transfection were noted. Additional experiments employed US contrast agent (Levovist, Schering) which was able to significantly increase tumor cell transfection rate by enhancing cavitation effects, and also severely damaged most cells when applied at a concentration of 200 mg/mL. In conclusion, our results support the assumption that US effects on lipoplex transfection rates in brain tumor cells in culture are mediated by cavitation effects.

Erosion of artificial endothelia in vitro by pulsed ultrasound: acoustic pressure, frequency, membrane orientation and microbubble contrast agent dependence

The erosion of cells from fibroblast monolayers simulating the vascular endothelium by 20 micros pulses of ultrasound at 500 Hz PRF was studied in relation to the peak negative acoustic pressure (P-; 0.0-2.5 MPa), ultrasound (US) frequency (1.0, 2.1 or 3.5 MHz), orientation of the monolayer (i.e., simulating the sites of ultrasound entry/exit from a blood vessel) and the presence or absence of a microbubble contrast agent (3 Vol% Albunex). The a priori hypotheses were that erosion of the monolayers would: 1. arise due to insonation treatment, 2. arise as a consequence of cavitation activity and, thus, increase with increasing P- at constant frequency, and decrease with increasing frequency at constant P-, 3. be significantly increased by the presence of a microbubble contrast agent, and 4. have a weak dependence on monolayer orientation. The data support these hypotheses. Under the most severe exposure conditions used, most of the affected cells appeared to have been lysed; however, a substantial number of viable cells were dislodged from the monolayer surface.