The efficient delivery of nanomaterials to specific targets for in vivo biomedical imaging is hindered by rapid sequestration by the reticuloendothelial system (RES) and consequent short circulation times. To overcome these two problems, we have prepared a new stealth PEG polymer conjugate containing a terminal 1,1-bisphosphonate (BP) group for strong and stable binding to the surface of ultrasmall-superparamagnetic oxide nanomaterials (USPIOs). This polymer, PEG(5)-BP, can be used to exchange the hydrophobic surfactants commonly used in the synthesis of USPIOs very efficiently and at room temperature using a simple method in 1 h. The resulting nanoparticles, PEG(5)-BP-USPIOs are stable in water or saline for at least 7 months and display a near-zero ζ-potential at neutral pH. The longitudinal (r₁) and transverse (r₂) relaxivities were measured at a clinically relevant magnetic field (3 T), revealing a high r₁ of 9.5 mM^–1 s^–1 and low r₂/r₁ ratio of 2.97, making these USPIOs attractive as T1-weighted MRI contrast agents at high magnetic fields. The strong T1-effect was demonstrated in vivo, revealing that PEG(5)-BP-USPIOs remain in the bloodstream and enhance its signal 6-fold, allowing the visualization of blood vessels and vascular organs with high spatial definition. Furthermore, the optimal relaxivity properties allow us to inject a dose 4 times lower than with other USPIOs. PEG(5)-BP-USPIOs can also be labeled using a radiolabeled-BP for visualization with single photon emission computed tomography (SPECT), and thus affording dual-modality contrast. The SPECT studies confirmed low RES uptake and long blood circulation times (t_1/2 = 2.97 h). These results demonstrate the potential of PEG(5)-BP-USPIOs for the development of targeted multimodal imaging agents for molecular imaging.

Background—: Endothelial dysfunction promotes atherosclerosis and precedes acute cardiovascular events. We investigated whether in vivo magnetic resonance imaging with the use of an albumin-binding contrast agent, gadofosveset, could detect endothelial damage associated with atherosclerosis in apolipoprotein E–deficient (ApoE ^−/− ) mice. Furthermore, we tested whether magnetic resonance imaging could noninvasively assess endothelial function by measuring the endothelial-dependent vasodilation in response to acetylcholine. Methods and Results—: ApoE ^−/− mice were imaged at 4, 8, and 12 weeks after commencement of a high-fat diet. Statin-treated ApoE ^−/− mice were scanned after 12 weeks of a high-fat diet. Wild-type mice were imaged before and 48 hours after injection of Russell's viper venom, an endothelial toxin. Delayed enhancement magnetic resonance imaging and T1 mapping of the brachiocephalic artery, 30 minutes after injection of gadofosveset, showed increased vessel wall enhancement and relaxation rate (R ₁ ) with progression of atherosclerosis in ApoE ^−/− (R ₁ [s ⁻¹ ]: R _{4 weeks} 2.42±0.35, R _{8 weeks} 3.45±0.54, R _{12 weeks} 3.83±0.52) and Russell's viper venom–injected wild-type mice (R ₁ =4.57±0.86). Conversely, wild-type (R ₁ =2.15±0.34) and statin-treated ApoE ^−/− (R ₁ =3.0±0.65) mice showed less enhancement. Uptake of gadofosveset correlated with Evans blue staining, morphological changes of endothelial cells, and widening of the cell-cell junctions, suggesting that uptake occurs in regions of increased vascular permeability. Endothelial-dependent vasomotor responses showed vasoconstriction of the arteries of the ApoE ^−/− (−22.22±7.95%) and Russell's viper venom–injected (−10.37±17.60%) mice compared with wild-type mice (32.45±12.35%). Statin treatment improved endothelium morphology and function (−8.12±8.22%). Conclusions—: We demonstrate the noninvasive assessment of endothelial permeability and function with the use of an albumin-binding magnetic resonance contrast agent. Blood albumin leakage could be a surrogate marker for the in vivo evaluation of interventions that aim to restore the endothelium.

Background—: Persistent intracoronary thrombus after plaque rupture is associated with an increased risk of subsequent myocardial infarction and mortality. Coronary thrombus is usually visualized invasively by x-ray coronary angiography. Non–contrast-enhanced T1-weighted magnetic resonance (MR) imaging has been useful for direct imaging of carotid thrombus and intraplaque hemorrhage by taking advantage of the short T1 of methemoglobin present in acute thrombus and intraplaque hemorrhage. The aim of this study was to investigate the use of non–contrast–enhanced MR for direct thrombus imaging (MRDTI) in patients with acute myocardial infarction. Methods and Results—: Eighteen patients (14 men; age, 61±9 years) underwent MRDTI within 24 to 72 hours of presenting with an acute coronary syndrome before invasive x-ray coronary angiography; MRDTI was performed with a T1-weighted, 3-dimensional, inversion-recovery black-blood gradient-echo sequence without contrast administration. Ten patients were found to have intracoronary thrombus on x-ray coronary angiography (left anterior descending, 4; left circumflex, 2; right coronary artery, 4; and right coronary artery–posterior descending artery, 1), and 8 had no visible thrombus. We found that MRDTI correctly identified thrombus in 9 of 10 patients (sensitivity, 91%; posterior descending artery thrombus not detected) and correctly classified the control group in 7 of 8 patients without thrombus formation (specificity, 88%). The contrast-to-noise ratio was significantly greater in coronary segments containing thrombus (n=10) compared with those without visible thrombus (n=131; mean contrast-to-noise ratio, 15.9 versus 2.6; P <0.001). Conclusion—: Use of MRDTI allows selective visualization of coronary thrombus in a patient population with a high probability of intracoronary thrombosis.

Rationale : Abdominal aortic aneurysms constitute a degenerative process in the aortic wall. Both the miR-29 and miR-15 families have been implicated in regulating the vascular extracellular matrix. Objective : Our aim was to assess the effect of the miR-15 family on aortic aneurysm development. Methods and Results : Among the miR-15 family members, miR-195 was differentially expressed in aortas of apolipoprotein E–deficient mice on angiotensin II infusion. Proteomics analysis of the secretome of murine aortic smooth muscle cells, after miR-195 manipulation, revealed that miR-195 targets a cadre of extracellular matrix proteins, including collagens, proteoglycans, elastin, and proteins associated with elastic microfibrils, albeit miR-29b showed a stronger effect, particularly in regulating collagens. Systemic and local administration of cholesterol-conjugated antagomiRs revealed better inhibition of miR-195 compared with miR-29b in the uninjured aorta. However, in apolipoprotein E–deficient mice receiving angiotensin II, silencing of miR-29b, but not miR-195, led to an attenuation of aortic dilation. Higher aortic elastin expression was accompanied by an increase of matrix metalloproteinases 2 and 9 in mice treated with antagomiR-195. In human plasma, an inverse correlation of miR-195 was observed with the presence of abdominal aortic aneurysms and aortic diameter. Conclusions : We provide the first evidence that miR-195 may contribute to the pathogenesis of aortic aneurysmal disease. Although inhibition of miR-29b proved more effective in preventing aneurysm formation in a preclinical model, miR-195 represents a potent regulator of the aortic extracellular matrix. Notably, plasma levels of miR-195 were reduced in patients with abdominal aortic aneurysms suggesting that microRNAs might serve as a noninvasive biomarker of abdominal aortic aneurysms.

Background—: The magnetic resonance longitudinal relaxation time (T ₁ ) changes with thrombus age in humans. In this study, we investigate the possible mechanisms that give rise to the T ₁ signal in venous thrombi and whether changes in T ₁ relaxation time are informative of the susceptibility to lysis. Methods and Results—: Venous thrombosis was induced in the vena cava of BALB/C mice, and temporal changes in T ₁ relaxation time correlated with thrombus composition. The mean T ₁ relaxation time of thrombus was shortest at 7days following thrombus induction and returned to that of blood as the thrombus resolved. T ₁ relaxation time was related to thrombus methemoglobin formation and further processing. Studies in inducible nitric oxide synthase ( iNOS ^−/− )–deficient mice revealed that inducible nitric oxide synthase mediates oxidation of erythrocyte lysis–derived iron to paramagnetic Fe ³⁺ , which causes thrombus T ₁ relaxation time shortening. Studies using chemokine receptor-2–deficient mice ( Ccr2 ^−/− ) revealed that the return of the T ₁ signal to that of blood is regulated by removal of Fe ³⁺ by macrophages that accumulate in the thrombus during its resolution. Quantification of T ₁ relaxation time was a good predictor of successful thrombolysis with a cutoff point of <747 ms having a sensitivity and specificity to predict successful lysis of 83% and 94%, respectively. Conclusions—: The source of the T ₁ signal in the thrombus results from the oxidation of iron (released from the lysis of trapped erythrocytes in the thrombus) to its paramagnetic Fe ³⁺ form. Quantification of T ₁ relaxation time appears to be a good predictor of the success of thrombolysis.