Quantification and monitoring of inflammation in murine inflammatory bowel disease with targeted contrast-enhanced US

Comparison of multiple enzyme activatable near-infrared fluorescent molecular probes for detection and quantification of inflammation in murine colitis models

BACKGROUND

Activatable near-infrared fluorescent (NIRF) probes have been used for ex vivo and in vivo detection of intestinal tumors in animal models. We hypothesized that NIRF probes activatable by cathepsins or metalloproteinases will detect and quantify dextran sulphate sodium (DSS)-induced acute colonic inflammation in wild type mice or chronic colitis in interleukin-10 (IL-10)-null mice ex vivo or in vivo.

METHODS

Wild type mice given DSS, water controls, and IL-10-null mice with chronic colitis were administered probes by retro-orbital injection. FMT2500 LX system imaged fresh and fixed intestine ex vivo and mice in vivo. Inflammation detected by probes was verified by histology and colitis scoring. NIRF signal intensity was quantified using 2-dimensional region of interest ex vivo or 3-dimensional region of interest analysis in vivo.

RESULTS

Ex vivo, 7 probes tested yielded significant higher NIRF signals in colon of DSS-treated mice versus controls. A subset of probes was tested in IL-10-null mice and yielded strong ex vivo signals. Ex vivo fluorescence signal with 680 series probes was preserved after formalin fixation. In DSS and IL-10-null models, ex vivo NIRF signal strongly and significantly correlated with colitis scores. In vivo, ProSense680, CatK680FAST, and MMPsense680 yielded significantly higher NIRF signals in DSS-treated mice than controls, but background was high in controls.

CONCLUSIONS

Both cathepsin or metalloproteinase-activated NIRF probes can detect and quantify colonic inflammation ex vivo. ProSense680 yielded the strongest signals in DSS colitis ex vivo and in vivo, but background remains a problem for in vivo quantification of colitis.

VEGFR2-targeted molecular imaging in the mouse embryo: an alternative to the tumor model

As a tumor surrogate, the mouse embryo presents as an excellent alternative for examining the binding of angiogenesis-targeting microbubbles and assessing the quantitative nature of molecular ultrasound. We establish the validity of this model by developing a robust method to study microbubble kinetic behavior and investigate the reproducibility of targeted binding in the murine embryo. Vascular endothelial growth factor receptor 2 (VEGFR2)-targeted (MBV), rat immunoglobulin G2 (IgG2) control antibody-targeted (MBC) and untargeted (MBU) microbubbles were introduced into vasculature of living mouse embryos. Non-linear contrast-specific and B-mode ultrasound imaging, performed at 21 MHz with a Vevo-2100 scanner, was used to collect basic perfusion parameters and contrast mean power ratios for all bubble types. We observed a twofold increase (p < 0.001) in contrast mean power ratios for MBV (4.14 ± 1.78) compared with those for MBC (1.95 ± 0.78) and MBU (1.79 ± 0.45). Targeted imaging of endogenous endothelial cell surface markers in mouse embryos is possible with labeled microbubbles. The mouse embryo thus presents as a versatile model for testing the performance of ultrasound molecular targeting, where further development of quantitative imaging techniques may enable rapid evaluations of biomarker expression in studies of vascular development, disease and angiogenesis.

Current status and prospects for microbubbles in ultrasound theranostics

Encapsulated microbubbles have been developed over the past two decades to provide improvements both in imaging as well as new therapeutic applications. Microbubble contrast agents are used currently for clinical imaging where increased sensitivity to blood flow is required, such as echocardiography. These compressible spheres oscillate in an acoustic field, producing nonlinear responses which can be uniquely distinguished from surrounding tissue, resulting in substantial enhancements in imaging signal-to-noise ratio. Furthermore, with sufficient acoustic energy the oscillation of microbubbles can mediate localized biological effects in tissue including the enhancement of membrane permeability or increased thermal energy deposition. Structurally, microbubbles are comprised of two principal components--an encapsulating shell and an inner gas core. This configuration enables microbubbles to be loaded with drugs or genes for additional therapeutic effect. Application of sufficient ultrasound energy can release this payload, resulting in site-specific delivery. Extensive preclinical studies illustrate that combining microbubbles and ultrasound can result in enhanced drug delivery or gene expression at spatially selective sites. Thus, microbbubles can be used for imaging, for therapy, or for both simultaneously. In this sense, microbubbles combined with acoustics may be one of the most universal theranostic tools.

Current status and prospects for microbubbles in ultrasound theranostics

Encapsulated microbubbles have been developed over the past two decades to provide improvements both in imaging as well as new therapeutic applications. Microbubble contrast agents are used currently for clinical imaging where increased sensitivity to blood flow is required, such as echocardiography. These compressible spheres oscillate in an acoustic field, producing nonlinear responses which can be uniquely distinguished from surrounding tissue, resulting in substantial enhancements in imaging signal-to-noise ratio. Furthermore, with sufficient acoustic energy the oscillation of microbubbles can mediate localized biological effects in tissue including the enhancement of membrane permeability or increased thermal energy deposition. Structurally, microbubbles are comprised of two principal components--an encapsulating shell and an inner gas core. This configuration enables microbubbles to be loaded with drugs or genes for additional therapeutic effect. Application of sufficient ultrasound energy can release this payload, resulting in site-specific delivery. Extensive preclinical studies illustrate that combining microbubbles and ultrasound can result in enhanced drug delivery or gene expression at spatially selective sites. Thus, microbbubles can be used for imaging, for therapy, or for both simultaneously. In this sense, microbubbles combined with acoustics may be one of the most universal theranostic tools.

Molecular imaging of inflammation in inflammatory bowel disease with a clinically translatable dual-selectin-targeted US contrast agent: comparison with FDG PET/CT in a mouse model

PURPOSE

To develop and test a molecular imaging approach that uses ultrasonography (US) and a clinically translatable dual-targeted (P- and E-selectin) contrast agent (MBSelectin) in the quantification of inflammation at the molecular level and to quantitatively correlate selectin-targeted US with fluorodeoxyglucose (FDG) combined positron emission tomography (PET) and computed tomography (CT) in terms of visualization and quantification of different levels of inflammation in a murine acute colitis model.

MATERIALS AND METHODS

Animal studies were approved by the Institutional Administrative Panel on Laboratory Animal Care at Stanford University. MBSelectin was developed by covalently binding an analog of the naturally occurring binding ligand P-selectin glycoprotein ligand 1 fused to a human fragment crystallizable(or Fc) domain onto the lipid shell of perfluorobutane and nitrogen-containing MBs. Binding specificity of MBSelectin was assessed in vitro with a flow chamber assay and in vivo with a chemically induced acute colitis murine model. US signal was quantitatively correlated with FDG uptake at PET/CT and histologic grade. Statistical analysis was performed with the Student t test, analysis of variance, and Pearson correlation analysis.

RESULTS

MBSelectin showed strong attachment to both human and mouse P- and E-selectin compared with MBControl in vitro (P ≤ .002). In vivo, US signal was significantly increased (P < .001) in mice with acute colitis (173.8 arbitrary units [au] ± 134.8 [standard deviation]) compared with control mice (5.0 au ± 4.5). US imaging signal strongly correlated with FDG uptake on PET/CT images (ρ = 0.89, P < .001). Ex vivo analysis enabled confirmation of inflammation in mice with acute colitis and high expression levels of P- and E-selectin in mucosal capillaries (P = .014).

CONCLUSION

US with MBSelectin specifically enables detection and quantification of inflammation in a murine acute colitis model, leveraging the natural pathway of leukocyte recruitment in inflammatory tissue. US imaging with MBSelectin correlates well with FDG uptake at PET/CT imaging.

Earlier detection of breast cancer with ultrasound molecular imaging in a transgenic mouse model

While there is an increasing role of ultrasound for breast cancer screening in patients with dense breast, conventional anatomical ultrasound lacks sensitivity and specificity for early breast cancer detection. In this study, we assessed the potential of ultrasound molecular imaging using clinically translatable vascular endothelial growth factor receptor type 2 (VEGFR2)-targeted microbubbles (MB(VEGFR2)) to improve the diagnostic accuracy of ultrasound in earlier detection of breast cancer and ductal carcinoma in situ (DCIS) in a transgenic mouse model [FVB/N-Tg(MMTV-PyMT)634Mul]. In vivo binding specificity studies (n = 26 tumors) showed that ultrasound imaging signal was significantly higher (P < 0.001) using MB(VEGFR2) than nontargeted microbubbles and imaging signal significantly decreased (P < 0.001) by blocking antibodies. Ultrasound molecular imaging signal significantly increased (P < 0.001) when breast tissue (n = 315 glands) progressed from normal [1.65 ± 0.17 arbitrary units (a.u.)] to hyperplasia (4.21 ± 1.16), DCIS (15.95 ± 1.31), and invasive cancer (78.1 ± 6.31) and highly correlated with ex vivo VEGFR2 expression [R(2) = 0.84; 95% confidence interval (CI), 0.72-0.91; P < 0.001]. At an imaging signal threshold of 4.6 a.u., ultrasound molecular imaging differentiated benign from malignant entities with a sensitivity of 84% (95% CI, 78-88) and specificity of 89% (95% CI, 81-94). In a prospective screening trail (n = 63 glands), diagnostic performance of detecting DCIS and breast cancer was assessed and two independent readers correctly diagnosed malignant disease in more than 95% of cases and highly agreed between each other [intraclass correlation coefficient (ICC) = 0.98; 95% CI, 97-99]. These results suggest that VEGFR2-targeted ultrasound molecular imaging allows highly accurate detection of DCIS and breast cancer in transgenic mice and may be a promising approach for early breast cancer detection in women.

The evolution of ultrasound in rheumatology

Musculoskeletal ultrasound is a powerful tool not only for evaluating joint and related structures but also for assessing disease activity. Ultrasound in rheumatology has rapidly evolved and been incorporated into routine clinical practice over the past decade. Moreover, technological development of equipment has made it more accessible for rheumatologists. We present a review of advances in ultrasound in rheumatology, focusing on major chronological developments.

Molecular body imaging: MR imaging, CT, and US. Part II. Applications

Molecular imaging is expected to have a major impact on the early diagnosis of diseases and disease monitoring in the next decade. Traditionally, nuclear imaging techniques have been the mainstay of molecular imaging in the clinical arena. However, with continued development of molecularly targeted contrast agents for nonnuclear imaging techniques such as magnetic resonance (MR), computed tomography (CT), and ultrasonography (US), the spectrum of clinical molecular imaging applications is expanding. In the second part of this review series, an overview of applications of molecular MR imaging-, CT-, and US-based imaging strategies that show promise for clinical translation is presented, and key challenges that need to be addressed to successfully translate these promising techniques in the future are discussed. © RSNA, 2012.

Microbubbles as ultrasound contrast agents for molecular imaging: preparation and application

OBJECTIVE

The purpose of this review is to describe trends in microbubble application in molecular imaging.

CONCLUSION

Microbubbles are used for contrast ultrasound imaging as blood-pool agents in cardiology and radiology. Their promise as targeted agents for molecular imaging is now being recognized. Microbubbles can be functionalized with ligand molecules that bind to molecular markers of disease. Potential clinical applications of molecular imaging with microbubble-based ultrasound contrast agents are in the monitoring of the biomarker status of vascular endothelium, visualizing tumor vasculature, and imaging inflammation and ischemia-reperfusion injury zones and thrombi.

Fast microbubble dwell-time based ultrasonic molecular imaging approach for quantification and monitoring of angiogenesis in cancer

PURPOSE: To develop and test a fast ultrasonic molecular imaging technique for quantification and monitoring of angiogenesis in cancer. MATERIALS AND METHODS: A new software algorithm measuring the dwell time of contrast microbubbles in near real-time (henceforth, fast method) was developed and integrated in a clinical ultrasound system. In vivo quantification and monitoring of tumor angiogenesis during anti-VEGF antibody therapy was performed in human colon cancer xenografts in mice (n=20) using the new fast method following administration of vascular endothelial growth factor receptor 2 (VEGFR2)-targeted contrast microbubbles. Imaging results were compared with a traditional destruction/replenishment approach (henceforth, traditional method) in an intra-animal comparison. RESULTS: There was excellent correlation (R(2)=0.93; P<0.001) between the fast method and the traditional method in terms of VEGFR2-targeted in vivo ultrasonic molecular imaging with significantly higher (P=0.002) imaging signal in colon cancer xenografts using VEGFR2-targeted compared to control non-targeted contrast microbubbles. The new fast method was highly reproducible (ICC=0.87). Following anti-angiogenic therapy, ultrasonic molecular imaging signal decreased by an average of 41±10%, whereas imaging signal increased by an average of 54±8% in non-treated tumors over a 72-hour period. Decreased VEGFR2 expression levels following anti-VEGF therapy were confirmed on ex vivo immunofluorescent staining. CONCLUSIONS: Fast ultrasonic molecular imaging based on dwell time microbubble signal measurements correlates well with the traditional measurement method, and allows reliable in vivo monitoring of anti-angiogenic therapy in human colon cancer xenografts. The improved work-flow afforded by the new quantification approach may facilitate clinical translation of ultrasonic molecular imaging.