Pharmacodynamics of streptavidin-coated cyanoacrylate microbubbles designed for molecular ultrasound imaging.

OBJECTIVES: To assess the pharmacodynamic behavior of cyanoacrylate, streptavidin-coated microbubbles (MBs) and to investigate their suitability for molecular ultrasound imaging. MATERIALS AND METHODS: Biodistribution of MBs was analyzed in tumor-bearing mice using gamma-counting, immunohistochemistry, flow cytometry, and ultrasound. Further, vascular endothelial growth factor receptor 2-antibody coupled MBs were used to image tumor neovasculature. RESULTS: After 1 minute >90% of MBs were cleared from the blood and pooled in the lungs, liver, and spleen. Subsequently, within 1 hour a decent reincrease of MB-concentration was observed in the blood. The remaining MBs were removed by liver and spleen macrophages. About 30% of the phagocytosed MBs were intact after 48 hours. Shell fragments were found in the kidneys only. No relevant MB-accumulation was observed in tumors. In contrast, vascular endothelial growth factor receptor 2-specific MBs accumulated significantly within the tumor vasculature (P < 0.05). CONCLUSIONS: The pharmacokinetic behavior of streptavidin-coated cyanoacrylate MBs has been studied. In this context, the low amount of MBs in tumors after >5 minutes is beneficial for specific targeting of angiogenesis.

A statistical model for the quantification of microbubbles in destructive imaging.

RATIONALE AND OBJECTIVES: Quantification of targeted ultrasound contrast agents allows for the monitoring of endothelial marker expressions on a molecular level. In this study, a statistical correction is provided, which allows for improved precision in estimating the concentration of microbubbles (MBs) from Doppler images. Doppler imaging can be used to display the destruction of single MBs. However, concentrations will generally be too high to distinguish the individual events resulting in an inaccurate microbubble (MB) quantification. Therefore, a mathematical description of destruction events in Doppler images is developed which yields a correction formula for the concentration estimate from the color pixel density. METHODS: The mathematical model is experimentally verified in gelatin phantoms using a high resolution imaging system (Vevo 770) and experimental cyanoacrylate MBs. Sensitive Particle Acoustic Quantification (SPAQ) is used to quantify MB in a defined volume. The SPAQ step size is varied from 32 to 127 µm to demonstrate the validity of the model for high color pixel densities. RESULTS: The corrected acoustic quantification shows the expected linear dependence on the step size and thus the amount of MBs in the images (R = 0.95). At SPAQ step sizes up to 127 µm, a MB concentration of 2.7 × 10 MBs/mL can be quantified. CONCLUSIONS: The results demonstrate the validity of the proposed correction. Quantification results of the SPAQ technique were considerably improved. The resulting formula is readily applied to SPAQ measurements at no additional expense.

Molecular targeting of lymph nodes with L-selectin ligand-specific US contrast agent: a feasibility study in mice and dogs.

PURPOSE: To evaluate the feasibility of using intravenously administered L-selectin ligand-specific polymer-stabilized air-filled microparticles (MPs) for active targeting of peripheral lymph nodes under normal conditions in animal models. MATERIALS AND METHODS: L-selectin ligand-specific MPs and two control substances (immunoglobulin M-isotype MPs and native MPs) were each administered in three conscious mice as a single intravenous bolus injection (1.4 x 10(7) MPs/kg). All mice were sacrificed 30 minutes after administration. Lymph nodes (cervical, inguinal, axillary, popliteal, mesenteric), spleen (positive control), and kidney (blood pool control) were removed and examined for MP-related stimulated acoustic emission (SAE) signals by using harmonic color Doppler ultrasonography (US) in a tank containing degassed water. A second experiment was performed in six anesthetized beagle dogs by using the same MP formulation. Each of the MP formulations was administered in two anesthetized dogs as a single intravenous bolus injection (1 x 10(7) MPs/kg). The popliteal lymph nodes, spleen (positive control), and kidney (blood pool control) were examined in vivo with US for MP-related SAE signals 30 minutes after administration. Fisher exact test for the one-side alternative was used for mouse data analysis. RESULTS: The lymph nodes of all mice (P =.05) and the popliteal lymph nodes of both dogs treated with L-selectin ligand-specific MPs showed clear MP-related SAE signals, whereas the lymph nodes of all mice and the popliteal lymph nodes of four dogs that received the control substances did not show any SAE signals. CONCLUSION: Use of an intravenously administered L-selectin ligand-specific US contrast agent is feasible for active lymph node targeting in mice and dogs.