The synthesis, magnetic purification and evaluation of 99mTc-labeled microbubbles

Effect of Poly(ethylene glycol) Configuration on Microbubble Pharmacokinetics

Microbubbles (MBs) hold substantial promise for medical imaging and therapy; nonetheless, knowledge gaps persist between composition, structure, and in vivo performance, especially with respect to pharmacokinetics. Of particular interest is the role of the poly(ethylene glycol) (PEG) layer, which is thought to shield the MB against opsonization and rapid clearance but is also known to cause an antibody response upon multiple injections. The goal of this study was, therefore, to elucidate the role of the PEG layer in circulation persistence of MBs in the naïve animal (prior to an adaptive immune response). Here, we directly observe the number and size of individual MBs obtained from blood samples, unifying size and concentration into the microbubble volume dose (MVD) parameter. This approach enables direct evaluation of the pharmacokinetics of intact MBs, comprising both the lipid shell and gaseous core, rather than separately assessing the lipid or gas components. We examined the in vivo circulation persistence of 3 μm diameter phospholipid-coated MBs with three different mPEG2000 content: 2 mol % (mushroom), 5 mol % (intermediate), and 10 mol % (brush). MB size and concentration in the blood were evaluated by a hemocytometer analysis over 30 min following intravenous injections of 20 and 40 μL/kg MVD in Sprague-Dawley rats. Interestingly, pharmacokinetic analysis demonstrated that increasing PEG concentration on the MB surface resulted in faster clearance. This was evidenced by a 1.6-fold reduction in half-life and area under the curve (AUC) (p < 0.05) in the central compartment. Conversely, the AUC in the peripheral compartment increased with PEG density, suggesting enhanced MB trapping by the mononuclear phagocyte system. This was supported by an in vitro assay, which showed a significant rise in complement C3a activation with a higher PEG content. In conclusion, a minimal PEG concentration on the MB shell (mushroom configuration) was found to prolong circulation and mitigate immunogenicity.

Quantitative Pharmacokinetics Reveal Impact of Lipid Composition on Microbubble and Nanoprogeny Shell Fate

Microbubble-enabled focused ultrasound (MB-FUS) has revolutionized nano and molecular drug delivery capabilities. Yet, the absence of longitudinal, systematic, quantitative studies of microbubble shell pharmacokinetics hinders progress within the MB-FUS field. Microbubble radiolabeling challenges contribute to this void. This barrier is overcome by developing a one-pot, purification-free copper chelation protocol able to stably radiolabel diverse porphyrin-lipid-containing Definity® analogues (pDefs) with >95% efficiency while maintaining microbubble physicochemical properties. Five tri-modal (ultrasound-, positron emission tomography (PET)-, and fluorescent-active) [64 Cu]Cu-pDefs are created with varying lipid acyl chain length and charge, representing the most prevalently studied microbubble compositions. In vitro, C16 chain length microbubbles yield 2-3x smaller nanoprogeny than C18 microbubbles post FUS. In vivo, [64 Cu]Cu-pDefs are tracked in healthy and 4T1 tumor-bearing mice ± FUS over 48 h qualitatively through fluorescence imaging (to characterize particle disruption) and quantitatively through PET and γ-counting. These studies reveal the impact of microbubble composition and FUS on microbubble dissolution rates, shell circulation, off-target tissue retention (predominantly the liver and spleen), and FUS enhancement of tumor delivery. These findings yield pharmacokinetic microbubble structure-activity relationships that disrupt conventional knowledge, the implications of which on MB-FUS platform design, safety, and nanomedicine delivery are discussed.

Therapeutic oxygen delivery by perfluorocarbon-based colloids

After the protocol-related indecisive clinical trial of Oxygent, a perfluorooctylbromide/phospholipid nanoemulsion, in cardiac surgery, that often unduly assigned the observed untoward effects to the product, the development of perfluorocarbon (PFC)-based O₂ nanoemulsions ("blood substitutes") has come to a low. Yet, significant further demonstrations of PFC O₂-delivery efficacy have continuously been reported, such as relief of hypoxia after myocardial infarction or stroke; protection of vital organs during surgery; potentiation of O₂-dependent cancer therapies, including radio-, photodynamic-, chemo- and immunotherapies; regeneration of damaged nerve, bone or cartilage; preservation of organ grafts destined for transplantation; and control of gas supply in tissue engineering and biotechnological productions. PFC colloids capable of augmenting O₂ delivery include primarily injectable PFC nanoemulsions, microbubbles and phase-shift nanoemulsions. Careful selection of PFC and other colloid components is critical. The basics of O₂ delivery by PFC nanoemulsions will be briefly reminded. Improved knowledge of O₂ delivery mechanisms has been acquired. Advanced, size-adjustable O₂-delivering nanoemulsions have been designed that have extended room-temperature shelf-stability. Alternate O₂ delivery options are being investigated that rely on injectable PFC-stabilized microbubbles or phase-shift PFC nanoemulsions. The latter combine prolonged circulation in the vasculature, capacity for penetrating tumor tissues, and acute responsiveness to ultrasound and other external stimuli. Progress in microbubble and phase-shift emulsion engineering, control of phase-shift activation (vaporization), understanding and control of bubble/ultrasound/tissue interactions is discussed. Control of the phase-shift event and of microbubble size require utmost attention. Further PFC-based colloidal systems, including polymeric micelles, PFC-loaded organic or inorganic nanoparticles and scaffolds, have been devised that also carry substantial amounts of O₂. Local, on-demand O₂ delivery can be triggered by external stimuli, including focused ultrasound irradiation or tumor microenvironment. PFC colloid functionalization and targeting can help adjust their properties for specific indications, augment their efficacy, improve safety profiles, and expand the range of their indications. Many new medical and biotechnological applications involving fluorinated colloids are being assessed, including in the clinic. Further uses of PFC-based colloidal nanotherapeutics will be briefly mentioned that concern contrast diagnostic imaging, including molecular imaging and immune cell tracking; controlled delivery of therapeutic energy, as for noninvasive surgical ablation and sonothrombolysis; and delivery of drugs and genes, including across the blood-brain barrier. Even when the fluorinated colloids investigated are designed for other purposes than O₂ supply, they will inevitably also carry and deliver a certain amount of O₂, and may thus be considered for O₂ delivery or co-delivery applications. Conversely, O₂-carrying PFC nanoemulsions possess by nature a unique aptitude for ¹⁹F MR imaging, and hence, cell tracking, while PFC-stabilized microbubbles are ideal resonators for ultrasound contrast imaging and can undergo precise manipulation and on-demand destruction by ultrasound waves, thereby opening multiple theranostic opportunities.

Single Photon Emission Computed Tomography Tracer

Single photon emission computed tomography (SPECT) is the state-of-the-art imaging modality in nuclear medicine despite the fact that only a few new SPECT tracers have become available in the past 20 years. Critical for the future success of SPECT is the design of new and specific tracers for the detection, localization, and staging of a disease and for monitoring therapy. The utility of SPECT imaging to address oncologic questions is dependent on radiotracers that ideally exhibit excellent tissue penetration, high affinity to the tumor-associated target structure, specific uptake and retention in the malignant lesions, and rapid clearance from non-targeted tissues and organs. In general, a target-specific SPECT radiopharmaceutical can be divided into two main parts: a targeting biomolecule (e.g., peptide, antibody fragment) and a γ-radiation-emitting radionuclide (e.g., ^99mTc, ¹²³I). If radiometals are used as the radiation source, a bifunctional chelator is needed to link the radioisotope to the targeting entity. In a rational SPECT tracer design, these single components have to be critically evaluated in order to achieve a balance among the demands for adequate target binding, and a rapid clearance of the radiotracer. The focus of this chapter is to depict recent developments of tumor-targeted SPECT radiotracers for imaging of cancer diseases. Possibilities for optimization of tracer design and potential causes for design failure are discussed and highlighted with selected examples.

Development of 68Ga-labelled ultrasound microbubbles for whole-body PET imaging

Microbubble (MB) contrast agents have revolutionalised the way ultrasound (US) imaging can be used clinically and pre-clinically. Contrast-enhanced US offers improvements in soft-tissue contrast, as well as the ability to visualise disease processes at the molecular level. However, its inability to provide in vivo whole-body imaging can hamper the development of new MB formulations. Herein, we describe a fast and efficient method for achieving 68Ga-labelling of MBs after a direct comparison of two different strategies. The optimised approach produces 68Ga-labelled MBs in good yields through the bioorthogonal inverse-electron-demand Diel-Alder reaction between a trans-cyclooctene-modified phospholipid and a new tetrazine-bearing HBED-CC chelator. The ability to noninvasively study the whole-body distribution of 68Ga-labelled MBs was demonstrated in vivo using positron emission tomography (PET). This method could be broadly applicable to other phospholipid-based formulations, providing accessible solutions for in vivo tracking of MBs.

Recent advances in molecular, multimodal and theranostic ultrasound imaging

Ultrasound (US) imaging is an exquisite tool for the non-invasive and real-time diagnosis of many different diseases. In this context, US contrast agents can improve lesion delineation, characterization and therapy response evaluation. US contrast agents are usually micrometer-sized gas bubbles, stabilized with soft or hard shells. By conjugating antibodies to the microbubble (MB) surface, and by incorporating diagnostic agents, drugs or nucleic acids into or onto the MB shell, molecular, multimodal and theranostic MBs can be generated. We here summarize recent advances in molecular, multimodal and theranostic US imaging, and introduce concepts how such advanced MB can be generated, applied and imaged. Examples are given for their use to image and treat oncological, cardiovascular and neurological diseases. Furthermore, we discuss for which therapeutic entities incorporation into (or conjugation to) MB is meaningful, and how US-mediated MB destruction can increase their extravasation, penetration, internalization and efficacy.

Biodistribution of P-selectin targeted microbubbles

PURPOSE

To evaluate binding of P-selectin targeted microbubbles (MB) in tumor vasculature; a whole-body imaging and biodistribution study was performed in a tumor bearing mouse model.

METHODS

Antibodies were radiolabeled with Tc-99 m using the HYNIC method. Tc-99 m labeled anti-P-selectin antibodies were avidin-bound to lipid-shelled, perfluorocarbon gas-filled MB and intravenously injected into mice bearing MDA-MB-231 breast tumors. Whole-body biodistribution was performed at 5 min (n = 12) and 60 min (n = 4) using a gamma counter. Tc-99 m-labeled IgG bound IgG-control-MB group (n = 12 at 5 min; n = 4 at 60 min), Tc-99 m-labeled IgG-control-Ab group (n = 5 at 5 min; n = 3 at 60 min) and Tc-99 m-labeled anti P-selectin-Ab group (n = 5 at 5 min; n = 3 at 60 min) were also evaluated. Planar gamma camera imaging was also performed at each time point.

RESULTS

Targeted-MB retention in tumor (60 min: 1.8 ± 0.3% ID/g) was significantly greater (p = 0.01) than targeted-MB levels in adjacent skeletal muscle at both time points (5 min: 0.7 ± 0.2% ID/g; 60 min: 0.2 ± 0.1% ID/g) while there was no significant difference (p = 0.17) between muscle and tumor retention for the IgG-control-MB group at 5 min.

CONCLUSIONS

P-selectin targeted MBs were significantly higher in tumor tissue, as compared with adjacent skeletal tissue or tumor retention of IgG-control-MB.

Aggregate enhanced trimodal porphyrin shell microbubbles for ultrasound, photoacoustic, and fluorescence imaging

Microbubbles (MBs) are currently used as ultrasound (US) contrast agents and as delivery vehicles for site-specific US-triggered drug and gene delivery. Multimodal US-based imaging methods have been applied preclinically to assess and validate the effectiveness and fate of MBs in imaging and therapy. Here we present the first intrinsically trimodal MBs by incorporating a dense concentration of porphyrin molecules within a MB shell, enabled by the use of a single porphyrin-lipid component. These MBs possess US, photoacoustic, and fluorescence properties that are demonstrated in solution and in a mouse tumor xenograft model. They also have potential to be extended to other imaging modalities such as magnetic resonance imaging and nuclear imaging.

Multimodality imaging using SPECT/CT and MRI and ligand functionalized 99mTc-labeled magnetic microbubbles

Background

In the present study, we used multimodal imaging to investigate biodistribution in rats after intravenous administration of a new ^99mTc-labeled delivery system consisting of polymer-shelled microbubbles (MBs) functionalized with diethylenetriaminepentaacetic acid (DTPA), thiolated poly(methacrylic acid) (PMAA), chitosan, 1,4,7-triacyclononane-1,4,7-triacetic acid (NOTA), NOTA-super paramagnetic iron oxide nanoparticles (SPION), or DTPA-SPION.

Methods

Examinations utilizing planar dynamic scintigraphy and hybrid imaging were performed using a commercially available single-photon emission computed tomography (SPECT)/computed tomography (CT) system. For SPION containing MBs, the biodistribution pattern of ^99mTc-labeled NOTA-SPION and DTPA-SPION MBs was investigated and co-registered using fusion SPECT/CT and magnetic resonance imaging (MRI). Moreover, to evaluate the biodistribution, organs were removed and radioactivity was measured and calculated as percentage of injected dose.

Results

SPECT/CT and MRI showed that the distribution of ^99mTc-labeled ligand-functionalized MBs varied with the type of ligand as well as with the presence of SPION. The highest uptake was observed in the lungs 1 h post injection of ^99mTc-labeled DTPA and chitosan MBs, while a similar distribution to the lungs and the liver was seen after the administration of PMAA MBs. The highest counts of ^99mTc-labeled NOTA-SPION and DTPA-SPION MBs were observed in the lungs, liver, and kidneys 1 h post injection. The highest counts were observed in the liver, spleen, and kidneys as confirmed by MRI 24 h post injection. Furthermore, the results obtained from organ measurements were in good agreement with those obtained from SPECT/CT.

Conclusions

In conclusion, microbubbles functionalized by different ligands can be labeled with radiotracers and utilized for SPECT/CT imaging, while the incorporation of SPION in MB shells enables imaging using MR. Our investigation revealed that biodistribution may be modified using different ligands. Furthermore, using a single contrast agent with fusion SPECT/CT/MR multimodal imaging enables visualization of functional and anatomical information in one image, thus improving the diagnostic benefit for patients.