Theranostic Attributes of Acoustic Cluster Therapy and Its Use for Enhancing the Effectiveness of Liposomal Doxorubicin Treatment of Human Triple Negative Breast Cancer in Mice

INTRODUCTION

Acoustic cluster therapy (ACT) comprises co-administration of a formulation containing microbubble/microdroplet clusters (PS101), together with a regular medicinal drug (e.g., a chemotherapeutic) and local ultrasound (US) insonation of the targeted pathological tissue (e.g., the tumor). PS101 is confined to the vascular compartment and, when the clusters are exposed to regular diagnostic imaging US fields, the microdroplets undergo a phase-shift to produce bubbles with a median diameter of 22 µm when unconstrained by the capillary wall. In vivo these bubbles transiently lodge in the tumor's microvasculature. Low frequency ultrasound (300 kHz) at a low mechanical index (MI = 0.15) is then applied to drive oscillations of the deposited ACT bubbles to induce a range of biomechanical effects that locally enhance extravasation, distribution, and uptake of the co-administered drug, significantly increasing its therapeutic efficacy.

METHODS

In this study we investigated the therapeutic efficacy of ACT with liposomal doxorubicin for the treatment of triple negative breast cancer using orthotopic human tumor xenografts (MDA-MB-231-H.luc) in athymic mice (ICR-NCr-Foxn1nu). Doxil® (6 mg/kg, i.v.) was administered at days 0 and 21, each time immediately followed by three sequential ACT (20 ml/kg PS101) treatment procedures (n = 7-10). B-mode and nonlinear ultrasound images acquired during the activation phase were correlated to the therapeutic efficacy.

RESULTS

Results show that combination with ACT induces a strong increase in the therapeutic efficacy of Doxil®, with 63% of animals in complete, stable remission at end of study, vs. 10% for Doxil® alone (p < 0.02). A significant positive correlation (p < 0.004) was found between B-mode contrast enhancement during ACT activation and therapy response. These observations indicate that ACT may also be used as a theranostic agent and that ultrasound contrast enhancement during or before ACT treatment may be employed as a biomarker of therapeutic response during clinical use.

Therapeutic Dose Response of Acoustic Cluster Therapy in Combination With Irinotecan for the Treatment of Human Colon Cancer in Mice

Introduction: Acoustic Cluster Therapy (ACT) comprises coadministration of a formulation containing microbubble-microdroplet clusters (PS101) together with a regular medicinal drug and local ultrasound (US) insonation of the targeted pathological tissue. PS101 is confined to the vascular compartment and when the clusters are exposed to regular diagnostic imaging US fields, the microdroplets undergo a phase shift to produce bubbles with a median diameter of 22 µm. Low frequency, low mechanical index US is then applied to drive oscillations of the deposited ACT bubbles to induce biomechanical effects that locally enhance extravasation, distribution, and uptake of the coadministered drug, significantly increasing its therapeutic efficacy. Methods: The therapeutic efficacy of ACT with irinotecan (60 mg/kg i.p.) was investigated using three treatment sessions given on day 0, 7, and 14 on subcutaneous human colorectal adenocarcinoma xenografts in mice. Treatment was performed with three back-to-back PS101+US administrations per session with PS101 doses ranging from 0.40-2.00 ml PS101/kg body weight (n = 8-15). To induce the phase shift, 45 s of US at 8 MHz at an MI of 0.30 was applied using a diagnostic US system; low frequency exposure consisted of 1 or 5 min at 500 kHz with an MI of 0.20. Results: ACT with irinotecan induced a strong, dose dependent increase in the therapeutic effect (R2 = 0.95). When compared to irinotecan alone, at the highest dose investigated, combination treatment induced a reduction in average normalized tumour volume from 14.6 (irinotecan), to 5.4 (ACT with irinotecan, p = 0.002) on day 27. Median survival increased from 34 days (irinotecan) to 54 (ACT with irinotecan, p = 0.002). Additionally, ACT with irinotecan induced an increase in the fraction of complete responders; from 7% to 26%. There was no significant difference in the therapeutic efficacy whether the low frequency US lasted 1 or 5 min. Furthermore, there was no significant difference between the enhancement observed in the efficacy of ACT with irinotecan when PS101+US was administered before or after irinotecan. An increase in early dropouts was observed at higher PS101 doses. Both mean tumour volume (on day 27) and median survival indicate that the PS101 dose response was linear in the range investigated.

Acoustic Cluster Therapy (ACT) enhances the therapeutic efficacy of paclitaxel and Abraxane® for treatment of human prostate adenocarcinoma in mice

Acoustic cluster therapy (ACT) is a novel approach for ultrasound mediated, targeted drug delivery. In the current study, we have investigated ACT in combination with paclitaxel and Abraxane® for treatment of a subcutaneous human prostate adenocarcinoma (PC3) in mice. In combination with paclitaxel (12mg/kg given i.p.), ACT induced a strong increase in therapeutic efficacy; 120days after study start, 42% of the animals were in stable, complete remission vs. 0% for the paclitaxel only group and the median survival was increased by 86%. In combination with Abraxane® (12mg paclitaxel/kg given i.v.), ACT induced a strong increase in the therapeutic efficacy; 60days after study start 100% of the animals were in stable, remission vs. 0% for the Abraxane® only group, 120days after study start 67% of the animals were in stable, complete remission vs. 0% for the Abraxane® only group. For the ACT+Abraxane group 100% of the animals were alive after 120days vs. 0% for the Abraxane® only group. Proof of concept for Acoustic Cluster Therapy has been demonstrated; ACT markedly increases the therapeutic efficacy of both paclitaxel and Abraxane® for treatment of human prostate adenocarcinoma in mice.

Acoustic Cluster Therapy: In Vitro and Ex Vivo Measurement of Activated Bubble Size Distribution and Temporal Dynamics

Acoustic cluster technology (ACT) is a two-component, microparticle formulation platform being developed for ultrasound-mediated drug delivery. Sonazoid microbubbles, which have a negative surface charge, are mixed with micron-sized perfluoromethylcyclopentane droplets stabilized with a positively charged surface membrane to form microbubble/microdroplet clusters. On exposure to ultrasound, the oil undergoes a phase change to the gaseous state, generating 20- to 40-μm ACT bubbles. An acoustic transmission technique is used to measure absorption and velocity dispersion of the ACT bubbles. An inversion technique computes bubble size population with temporal resolution of seconds. Bubble populations are measured both in vitro and in vivo after activation within the cardiac chambers of a dog model, with catheter-based flow through an extracorporeal measurement flow chamber. Volume-weighted mean diameter in arterial blood after activation in the left ventricle was 22 μm, with no bubbles >44 μm in diameter. After intravenous administration, 24.4% of the oil is activated in the cardiac chambers.

Acoustic Cluster Therapy (ACT) - pre-clinical proof of principle for local drug delivery and enhanced uptake

Proof of principle for local drug delivery with Acoustic Cluster Therapy (ACT) was demonstrated in a human prostate adenocarcinoma growing in athymic mice, using near infrared (NIR) dyes as model molecules. A dispersion of negatively charged microbubble/positively charged microdroplet clusters are injected i.v., activated within the target pathology by diagnostic ultrasound (US), undergo an ensuing liquid-to-gas phase shift and transiently deposit 20-30μm large bubbles in the microvasculature, occluding blood flow for ~5-10min. Further application of low frequency US induces biomechanical effects that increase the vascular permeability, leading to a locally enhanced extravasation of components from the vascular compartment (e.g., released or co-administered drugs). Results demonstrated deposition of activated bubbles in tumor vasculature. Following ACT treatment, a significant and tumor specific increase in the uptake of a co-administered macromolecular NIR dye was shown. In addition, ACT compound loaded with a lipophilic NIR dye to the microdroplet component was shown to facilitate local release and tumor specific uptake. Whereas the mechanisms behind the observed increased and tumor specific uptake are not fully elucidated, it is demonstrated that the ACT concept can be applied as a versatile technique for targeted drug delivery.

Physicochemical characteristics of Sonazoid, a new contrast agent for ultrasound imaging

The objective of the current work is to describe the physicochemical characteristics of Sonazoid, a new ultrasound contrast agent for detection and characterisation of focal liver lesions. It has been demonstrated that Sonazoid powder for injection consists of microspheres of perfluorobutane (PFB) stabilised by a monomolecular membrane of hydrogenated egg phosphatidyl serine, embedded in an amorphous sucrose structure. Upon reconstitution with sterile water, stabilised microspheres of PFB are released in a predefined amount and size into a low viscosity, isotonic sucrose solution with a neutral pH. Sonazoid reconstituted product contains approximately 8 microl microspheres/ml with volume median diameter of approximately 2.6 microm. The product contains approximately 1.2 billion microspheres/ml of which less than 0.1% are larger than 7 microm. The acoustic properties of Sonazoid such as attenuation efficacy, fundamental and second harmonic backscatter efficacy are all well correlated to the microsphere volume concentration. The stability of Sonazoid after reconstitution is good, with no significant changes in physicochemical properties 2 h after reconstitution. Pressure stress is well tolerated by both concentrated and diluted Sonazoid with no permanent effects of pressures up to 300 mm Hg. The level and consistency of the investigated physicochemical properties demonstrate that Sonazoid should be well suited as a contrast agent for medical imaging with ultrasound.

In vitro stability analyses as a model for metabolism of ferromagnetic particles (Clariscan), a contrast agent for magnetic resonance imaging

Clariscan is a new contrast agent for magnetic resonance imaging. It is an aqueous suspension of ferromagnetic particles injected for blood pool and liver imaging. Previous experiments showed that particles made of 59Fe were taken up by the mononuclear phagocytic system and then solubilised. The present work aims at explaining a possible mechanism for the dissolution of these ferromagnetic particles in the body. The particles were diluted in 10-mM citrate or 10-mM acetate buffers at pH 4.5, 5.0 and 5.5 and incubated at 37 degrees C for up to 22 days, protected from light. The mixtures were analysed at different times during this incubation period using photon correlation spectroscopy, magnetic relaxation, visible spectroscopy and reactivity of the iron with the chelator, bathophenanthroline disulphonic acid. The data obtained with these techniques showed that the particles were almost completely solubilised within 4-7 days when incubated in 10 mM citrate, pH 4.5. Incubation in 10 mM citrate buffer, pH 5.0 revealed a slower solubilisation of the particles, as the changes observed after 72 h of incubation at pH 5.0 were 43-71% of the changes observed at pH 4.5. Incubation in 10 mM citrate, pH 5.5 revealed an even slower solubilisation of the particles, as the changes observed after 72 h of incubation at pH 5.5 were 12-34% of those observed at pH 4.5. Incubation of the particles in 10 mM acetate at pH 4.5, 5.0 and 5.5, as well as incubation of the particles in water pH adjusted to pH 5.1, resulted in only minor or no solubilisation of the particles. The results indicate that the low pH of endosomes and lysosomes, as well as endogenous iron-complexing substances, may be important for the solubilisation of these ferromagnetic particles following i.v. injection of Clariscan.

A chemometrics method for separation of size dependent and size independent attributes of microspheres for medical ultrasound imaging

The size dependent and size independent contributions to the mechanical properties of poly(ethyleneglycol) microspheres with encapsulated air, made for medical ultrasound imaging, have been studied. A relation between the size of microspheres in a reference group and their acoustic attenuation efficacy was derived by partial least square regression. The developed model was used to estimate acoustic attenuation as a function of the measured size distribution for various preparations made during formulation and process studies. The ratio between the measured attenuation and model estimated attenuation was then used to evaluate variations in the mechanical properties of the polymer shell and the contribution of such variations to the acoustical properties of the substance. The statistical parameter was compared with results for the mechanical compressibility derived from a theoretical approach and the results from the two methods mutually confirmed each other. The statistical procedure outlined has been found applicable for studies of other particulate products with size dependent in vitro or in vivo properties.

Oscillations of polymeric microbubbles: effect of the encapsulating shell

A model for the oscillation of gas bubbles encapsulated in a thin shell has been developed. The model depends on viscous and elastic properties of the shell, described by thickness, shear modulus, and shear viscosity. This theory was used to describe an experimental ultrasound contrast agent from Nycomed, composed of air bubbles encapsulated in a polymer shell. Theoretical calculations were compared with measurements of acoustic attenuation at amplitudes where bubble oscillations are linear. A good fit between measured and calculated results was obtained. The results were used to estimate the viscoelastic properties of the shell material. The shell shear modulus was estimated to between 10.6 and 12.9 MPa, the shell viscosity was estimated to between 0.39 and 0.49 Pas. The shell thickness was 5% of the particle radius. These results imply that the particles are around 20 times more rigid than free air bubbles, and that the oscillations are heavily damped, corresponding to Q-values around 1. We conclude that the shell strongly alters the acoustic behavior of the bubbles: The stiffness and viscosity of the particles are mainly determined by the encapsulating shell, not by the air inside.

Acoustic properties of NC100100 and their relation with the microbubble size distribution

RATIONALE AND OBJECTIVES

NC100100 is a contrast agent for medical imaging with ultrasonography consisting of stabilized gas microbubbles in an aqueous suspension. The objective of this article is to explore the acoustic properties of NC100100 and their relation with the microbubble size distribution. The results are used to motivate the choice of a suitable assay/dosage parameter for precise control of product efficacy.

METHODS

The concentration and size distribution of microbubbles in > 50 preparations of NC100100 were determined by Coulter counting, and the acoustic attenuation and backscatter efficacy were determined for all samples. The in vivo efficacy of the product was investigated by harmonic imaging of the heart in a dog model.

RESULTS

The results demonstrated that the attenuation and backscatter efficacy per microbubble volume vary strongly with size, showing distinct maxima with respect to microbubble diameter. Sizes for optimal attenuation per volume ranged from 2.6 to 5.8 microns, depending on ultrasound frequency. The contribution of the smaller end tail of the microbubble distribution was shown to be negligible. From the observed size dependency for the acoustic properties, the volume concentration of microbubbles was chosen as the assay/dosage parameter for NC100100. The accuracy of this parameter as a descriptor of product efficacy was demonstrated by precise, linear relations between volume, concentration, and attenuation/backscatter. In comparison, the correlation between the microbubble number and acoustic properties was not significant. Results from the in vivo study showed a precise, linear relation between injected microbubble volume and the observed in vivo efficacy.

CONCLUSIONS

The acoustic properties of NC100100 are dependent on microbubble size. The observed batch-to-batch variance in the acoustic properties of the product may be fully explained by variation in concentration and size. Microbubble volume is a more precise predictor of in vitro/in vivo efficacy than microbubble number and consequently was chosen as the assay/dosage parameter for NC100100.

Relationship between size of injected microspheres and ultrasound imaging of heart by PLSR

PURPOSE

To investigate the enhancement in ultrasound signal (US) in heart, for diagnostic purposes, as a function of size and number of injected air-filled microspheres by multivariate statistical methods.

METHODS

The US enhancement in left ventricle was measured after injection of 31 suspensions of air-filled albumin microspheres divided on and injected in six dogs. The relationship between the size of microspheres between 1-38 microns and the US enhancement was explored by Pearson's product moment correlation analysis, ordinary least-squares regression, principal component regression (PCR) and partial least-squares regression (PLS).

RESULTS

Relative advanced algorithms such as PCR and PLS were required to achieve accurate in vivo/in vitro correlation. The most effective microsphere sizes contributing to US enhancement in left ventricle in dogs were estimated to be in the 7-15 microns range.

CONCLUSIONS

In general, the effective in vivo sizes are dependent on the type of formulation due to the surprisingly large active in vivo sizes found for the tested concept. PCR and PLS are suitable methods for in vivo/in vitro correlation, especially for covariated and noisy data.

Effect of microsphere size distribution on the ultrasonographic contrast efficacy of air-filled albumin microspheres in the left ventricle of dog hearts

RATIONALE AND OBJECTIVES

The in vitro ultrasonographic contrast efficacy of air-filled albumin microspheres has been found to depend on the size distribution of microspheres. The objective of the current study was to empirically describe the relationship between the size distribution of injected air-filled albumin microspheres and the in vivo contrast efficacy after lung capillary filtration in a dog model.

METHODS

Twenty different air-filled microspheres with large and well-defined differences in size distribution were prepared from nine different batches of Albunex (Molecular Biosystems Inc.) and subsequently characterized by Coulter counting. The in vivo ultrasonographic contrast enhancement of these preparations was investigated with a VingMed CFM750 in closed chest model in six mongrel dogs. The observed contrast efficacy, measured as gray-level enhancement in the left ventricle (LV), was correlated to the microsphere size distribution, using both univariate and multivariate approaches.

RESULTS

The results demonstrated a significant contribution to LV contrast efficacy from microspheres larger than approximately 7 microm, and a lack of contribution from microspheres smaller than approximately 7 microm. Linear relationships were found between LV contrast efficacy, and both the number concentration of microspheres between 8 to 12 microm and the total microsphere volume concentration. No significant covariance between in vivo contrast efficacy and the number concentration between 1 to 38 microm or 4 to 10 microm was observed. The multivariate model showed a significant contribution to the in vivo gray-level enhancement from microspheres in the size range 7 to 15 microm, with optimal efficacy per microsphere at approximately 13 microm.

CONCLUSIONS

Large microspheres (> 7 microm), which had been expected to be trapped in the lung capillary bed, contribute most of the observed ultrasound contrast in the LV of the heart.

Photon correlation spectroscopy applied to characterisation of denaturation and thermal stability of human albumin

Photon correlation spectroscopy and light absorption measurements have been applied for characterisation of denaturation kinetics and thermal stability of human albumin in solution. The hydrodynamic size of the molecules has been studied as a function of pH, and the denaturation rate of ten different lots of 5% (w/v) human albumin solution has been measured at various temperatures. In the native (pH 7) state, the hydrodynamic molecular diameter was found to 6.3 nm. The molecular size was relatively stable between pH 10 and 5, but increased with decreasing pH to approximately 20 nm at pH 3. The denaturation rate, measured as change in hydrodynamic diameter per min, was strongly dependent on temperature and increased 3-fold per degree in the 73-75 degrees C range. The investigated lots of albumin solution showed large variations in stability at 74 degrees C, with denaturation rates ranging from 10 to 100 nm min-1. The observed thermal stability for the lots investigated was ranked identically with both the employed techniques. In an effort to explain the observed lot to lot variations in denaturation rate, a broad chemical characterisation including determination of free SH content, fatty acid content and composition and metal content, was performed. However, lot to lot variations in these parameters was not found to fully elucidate the observed variations in thermal stability.

Coulter counting and light diffraction analysis applied to characterisation of oil-water emulsions

Coulter counting and light diffraction techniques were successfully applied to the characterisation of the droplet concentration and size distribution in camphene-water and cyclohexane-water emulsions. Both of these techniques required a dilution of the emulsion prior to analysis, and it was found that the destabilizing effect on the droplets of such dilution could be overcome by performing the analyses at temperatures below the melting point of the oil phase. The storage stability of the camphene-water samples at 60 degrees C was reasonably good with a 5-20% change in the investigated parameters over a 24 h period. At room temperature camphene-water samples left to stand undisturbed were unaffected after 24 h, while continuous mixing of the emulsion on a roller board brought about a rapid amalgamation of the particles into larger aggregates. This fusion process was fully described only with the light diffraction analysis due to the broader measuring range of this technique. However, analysing emulsions with a droplet size range covered by both techniques gave identical results.

Crystal size and properties of superparamagnetic iron oxide (SPIO) particles

The properties of a superparamagnetic iron oxide (SPIO) model contrast agent have been studied. The test material, HEP-SPIO, contained iron oxide multicrystal agglomerates coated with heparin, polyanionic, naturally occurring glycosaminoglycan. Fractionation of the HEP-SPIO suspension showed the existence of colloidally stable particles ranging from approx. 100 nm down to single crystal sizes. The small (< 20 nm) particles represented the major number fraction of particles present, but only approx. 2% of the total iron oxide mass. The volume weighted average diameter of the individual iron oxide crystals forming the multicrystal agglomerates was found to be 11-12 nm using transmission electron microscopy and vibrating sample magnetometry (VSM) techniques. Comparable results were obtained with X-ray diffraction and Mössbauer spectroscopy. A number of additional SPIO properties could also be determined on a routine VSM, such as the distribution standard deviation for the log-normal distribution of crystal sizes, the magnetic susceptibility, the magnetic remanence, and the intrinsic magnetization (magnetic moment) of the iron oxide. These parameters are useful tools for evaluation of the magnetic characteristics and contrast efficacy of SPIO contrast agents.

Precision and accuracy of analysis of air-filled albumin microspheres using Coulter Multisizer Mark II

The suitability of the electrical sensing zone technique for routine analysis of air-filled albumin microspheres has been thoroughly investigated using three Coulter Multisizer Mark II instruments. The precision of the method, expressed as repeatability relative standard deviation (RSD), was found to be 1-2% for number distribution parameters and 3-4% for volume distribution parameters. Significant instrument-to-instrument variation as high as 11% was, however, also observed. Accuracy was evaluated from analyses of commercially available latex standards and from comparison of results from Coulter analysis with results from the following alternative techniques: light diffraction, optical microscopy and gravimetry. Accuracy, expressed as the difference from either certified values or values obtained with the alternative techniques, was found to be 100-106%.

Physical and biochemical characterization of Albunex, a new ultrasound contrast agent consisting of air-filled albumin microspheres suspended in a solution of human albumin

Albunex is a new ultrasound contrast agent for medical imaging. The product consists of air-filled albumin microspheres suspended in a solution of 5% (w/v) human albumin. The suspension is sterile, non-pyrogenic and isotonic, with a pH of 7.0 and a viscosity of 1.4 relative to water. The contrast effect is caused by the air-filled microspheres, which range in diameter from 1 to 15 microns, with less than 5% being larger than 10 microns. The product contains a total of about 7 x 10(8) microspheres/ml of suspension. The number concentration of microspheres with diameters between 4 and 10 microns is about 2 x 10(8)/ml. The latter microsphere fraction is assumed to give the main contribution to the ultrasound signal in the left ventricle of the heart after intravenous injection. The air-filled microspheres are prepared by sonication of a heated solution of 5% (w/v) human albumin. During the sonication process, microbubbles of air are formed which become encapsulated in a thin shell of aggregated albumin about 15 nm in thickness. Due to the stabilizing effect of the albumin shell, the air-filled microsphere suspension is stable for at least 2 years when stored refrigerated. The microsphere protein represents about 1.5% of the total protein in the suspension. The remaining protein is soluble albumin molecules which behave like the albumin molecules in the starting material when analysed by a number of biochemical techniques.