Monitoring plaque inflammation in atherosclerotic rabbits with an iron oxide (P904) and (18)F-FDG using a combined PET/MR scanner.

PURPOSE: The aim of this study was to compare the ability of (18)F-FDG PET and iron contrast-enhanced MRI with a novel USPIO (P904) to assess change in plaque inflammation induced by atorvastatin and dietary change in a rabbit model of atherosclerosis using a combined PET/MR scanner. MATERIALS AND METHODS: Atherosclerotic rabbits underwent USPIO-enhanced MRI and (18)F-FDG PET in PET/MR hybrid system at baseline and were then randomly divided into a progression group (high cholesterol diet) and a regression group (chow diet and atorvastatin). Each group was scanned again 6 months after baseline imaging. R2* (i.e. 1/T2*) values were calculated pre/post P904 injection. (18)F-FDG PET data were analyzed by averaging the mean Standard Uptake Value (SUVmean) over the abdominal aorta. The in vivo imaging was then correlated with matched histological sections stained for macrophages. RESULTS: (18)F-FDG PET showed strong FDG uptake in the abdominal aorta and P904 injection revealed an increase in R2* values in the aortic wall at baseline. At 6 months, SUVmean values measured in the regression group showed a significant decrease from baseline (p = 0.015). In comparison, progression group values remained constant (p = 0.681). R2* values showed a similar decreasing trend in the regression group suggesting less USPIO uptake in the aortic wall. Correlations between SUVmean or Change in R2* value and macrophages density (RAM-11 staining) were good (R(2) = 0.778 and 0.707 respectively). CONCLUSION: This experimental study confirms the possibility to combine two functional imaging modalities to assess changes in the inflammation of atherosclerotic plaques. (18)F-FDG-PET seems to be more sensitive than USPIO P904 to detect early changes in plaque inflammation.

Detection of vascular cell adhesion molecule-1 expression with USPIO-enhanced molecular MRI in a mouse model of cerebral ischemia.

Vascular damage plays a critical role after stroke, leading notably to edema, hemorrhages and stroke recurrence. Tools to characterize the vascular lesion are thus a real medical need. In this context, the specific nanoparticular contrast agent P03011, an USPIO (ultrasmall superparamagnetic iron oxide) conjugated to a peptide that targets VCAM-1 (vascular cell adhesion molecule-1), was developed to detect this major component of the vascular inflammatory response. This study aimed to make the proof of concept of the capacity of this targeted USPIO to detect VCAM-1 with MRI after cerebral ischemia in mouse. The time course of VCAM-1 expression was first examined by immunohistochemistry in our model of cerebral ischemia-reperfusion. Secondly, P03011 or nontargeted USPIO P03007 were injected 5 h after ischemia (100 µmol iron kg⁻¹; i.v.) and in vivo and ex vivo MRI were performed 24 h after ischemia onset. Double labeling immunofluorescence was then performed on brain slices in order to detect both USPIO and VCAM-1. VCAM-1 expression was significantly up-regulated 24 h after ischemia in our model. In animals receiving P03011, both in vivo and ex vivo MRI performed 24 h after ischemia onset showed hypointense foci which could correspond to iron particles. Histological analysis showed a co-localization of the targeted USPIO and VCAM-1. This study demonstrates that VCAM-1 detection is possible with the USPIO P03011 in a model of cerebral ischemia. This kind of contrast agent could be an interesting clinical tool to characterize ischemic lesions in terms of vascular damage.

Magnetic resonance imaging of ruptured plaques in the rabbit with ultrasmall superparamagnetic particles of iron oxide.

BACKGROUND: Magnetic resonance imaging (MRI) enhanced with ultrasmall superparamagnetic particles of iron oxide (USPIO) has previously been evaluated in hyperlipidemic rabbits. The aim of this study was therefore to compare USPIO in ruptured and non-ruptured arteries in an atherosclerotic rabbit model. METHODS: Atherosclerotic-like lesions were induced by the combination of endothelial abrasion and high-cholesterol diet in iliac rabbit arteries (n = 16). Rupture of atherosclerotic lesions was realized by oversized balloon angioplasty in one iliac artery, whereas the contralateral artery was used as control. USPIO (ferumoxtran-10: 1 mmol Fe/kg) was administered immediately (n = 10) or 28 days (n = 6) after injury. MRI and histological analysis were performed 7 and 35 days after injury and in control arteries. RESULTS: In vivo MRI analysis showed extended susceptibility artifact with transluminal signal loss in all ruptured arteries 7 days after injury. In contrast, hyposignal was reduced 35 days following injury (i.e. after healing), and absent in non-ruptured arteries. Similarly, histological analysis of iron uptake was significantly increased 7 days after injury compared to healed-ruptured and control arteries. CONCLUSIONS: Accumulation ofUSPIO is significantly increased in ruptured as compared to non-ruptured arteries in the atherosclerotic rabbit model.

Distribution of gadomelitol in a human breast tumor model in mice.

The study evaluates the tumor distribution of the rapid clearance blood pool agent (RCBPA) gadomelitol, in a breast tumor model. Different techniques were used : (1) tissue gadolinium concentrations measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES), (2) whole body quantitative autoradiography using radiolabeled [153Gd] gadomelitol and (3) dynamic contrast-enhanced MRI with compartmental analysis. An accumulation of gadomelitol in tumors compared to muscle was observed 30 min and 3 h post injection (p.i.). Thirty minutes p.i., the gadomelitol tumor distribution evaluated by autoradiography showed a marked difference between the rim and the center, whereas both areas showed comparable concentrations after 3 h. Using dynamic contrast-enhanced MRI, three phases could be observed during the 1 hour observation period: (1) rapid tumor uptake within the first few minutes post-injection (2) a progressive increase in tumor signal enhancement over 10 min and (3) a steady-state phase. Average +/- SD (n=5) transendothelial permeability K(PS) and the fractional blood volume fBV were 12.2+/-1.6 microl/min(-1)/g and 5.4+/-0.2% respectively. Due to its slow extravasation and high tumor residence time, gadomelitol may potentially be useful to improve characterization between benign versus malignant tumors using dynamic MRI.

Physicochemical and biological evaluation of P792, a rapid-clearance blood-pool agent for magnetic resonance imaging.

RATIONALE AND OBJECTIVES: To summarize the physicochemical characterization, pharmacokinetic behavior, and biological evaluation of P792, a new monogadolinated MRI blood-pool agent. METHODS: The molecular modeling of P792 was described. The r1 relaxivity properties of P792 were measured in water and 4% human serum albumin at different magnetic fields (20, 40, 60 MHz). The stability of the gadolinium complex was assessed. The pharmacokinetic and biodistribution profiles were studied in rabbits. Renal tolerance in dehydrated rats undergoing selective intrarenal injection was evaluated. Hemodynamic safety in rats and in vitro histamine and leukotriene B4 release were also tested. RESULTS: The mean diameter of P792 is 50.5 A and the r1 relaxivity of this monogadolinium contrast agent is 29 L x mmol(-1) x s(-1) at 60 MHz. The stability of the gadolinium complex in transmetallation is excellent. The pharmacokinetic and biodistribution profiles are consistent with that of a rapid-clearance blood-pool agent: P792 is mainly excreted by glomerular filtration, and its diffusion across normal endothelium is limited. Renal and hemodynamic safety is comparable to that of the nonspecific agent gadolinium-tetraazacyclododecane tetraacetic acid. No histamine or leukotriene B4 release was found in RBL-2H3 isolated mastocytes. CONCLUSIONS: The relaxivity of P792 at clinical field is very high for a monogadolinium complex without protein binding. The pharmacokinetic and biodistribution profiles are consistent with those of a rapid-clearance blood-pool agent. Its initial safety profile is satisfactory. Experimental and clinical studies are underway to confirm the potential of P792 in MRI.

Inhibition of neuronal nitric oxide synthase protects against MPTP toxicity.

Previous work showed that several relatively specific inhibitors of neuronal nitric oxide synthase (nNOS) produce protection against MPTP induced dopaminergic toxicity. We examined whether a highly specific novel inhibitor of nNOS, ARRI 7338, could also protect against MPTP toxicity. ARR17338 produced dose-dependent significant protection against MPTP induced depletion of dopamine and protected against MPTP induced depletions of tyrosine hydroxylase immunostained neurons in the substantia nigra. These results provide further evidence that inhibitors of nNOS may be useful for the treatment of Parkinson's disease.

Effects of non-ionic monomeric and dimeric iodinated contrast media on renal and systemic haemodynamics in rats.

Non-ionic dimeric contrast media (CM) are a new class of CM which are iso-osmolar with plasma. The aim of this study was to investigate their effects on systemic and renal haemodynamics. The non-ionic dimeric CM iodixanol and the non-ionic monomeric agent iobitridol (both at a dose of 1,600 mgI/kg) were compared in terms of their effects on systemic blood pressure (BP) and renal blood flow (RBF) in two strains of rats (Wistar and Sprague Dawley). Iodixanol significantly lowered BP in Wistar rats (-33 +/- 9% of baseline, 10 min post-injection, P < 0.001 vs. saline and iobitridol). Iobitridol had virtually no effect on BP. Iobitridol and iodixanol significantly decreased RBF. This effect was more marked following injection of the dimer rather than the monomer (iodixanol: -32 +/- 13% iobitridol: -20 +/- 4 of baseline at 16 min, P < 0.05). For both agents, RBF was still decreased 50 min following injection (iodixanol: -30 +/- 11%, and iobitridol: -20 +/- 5% of baseline). Iodixanol also decreased RBF in Sprague Dawley rats, while BP remained unchanged. This suggests that changes in BP/RBF autoregulation do not account for the renal haemodynamic effects of this agent. When measured 2 h following injection, the iodixanol-induced renal hypoperfusion was still detectable (-29% vs. saline-treated rats), although not significant (P = 0.06). This effect was no longer observed 4 h following injection. Increasing the saline infusion rate (18 mL/h vs. 2 mL/h) during the experiment did not significantly decrease the effects of iodixanol on BP and RBF in Wistar rats. In spite of its iso-osmolality, iodixanol, a non-ionic dimeric CM, depressed RBF and BP significantly more than iobitridol, a monomeric non-ionic agent, in Wistar rats. This effect was long-lasting and was not alleviated by increasing the hydration rate.

Mice deficient in cellular glutathione peroxidase show increased vulnerability to malonate, 3-nitropropionic acid, and 1-methyl-4-phenyl-1,2,5,6-tetrahydropyridine.

Glutathione peroxidase (GSHPx) is a critical intracellular enzyme involved in detoxification of hydrogen peroxide (H(2)O(2)) to water. In the present study we examined the susceptibility of mice with a disruption of the glutathione peroxidase gene to the neurotoxic effects of malonate, 3-nitropropionic acid (3-NP), and 1-methyl-4-phenyl-1,2,5,6-tetrahydropyridine (MPTP). Glutathione peroxidase knock-out mice showed no evidence of neuropathological or behavioral abnormalities at 2-3 months of age. Intrastriatal injections of malonate resulted in a significant twofold increase in lesion volume in homozygote GSHPx knock-out mice as compared to both heterozygote GSHPx knock-out and wild-type control mice. Malonate-induced increases in conversion of salicylate to 2,3- and 2, 5-dihydroxybenzoic acid, an index of hydroxyl radical generation, were greater in homozygote GSHPx knock-out mice as compared with both heterozygote GSHPx knock-out and wild-type control mice. Administration of MPTP resulted in significantly greater depletions of dopamine, 3,4-dihydroxybenzoic acid, and homovanillic acid in GSHPx knock-out mice than those seen in wild-type control mice. Striatal 3-nitrotyrosine (3-NT) concentrations after MPTP were significantly increased in GSHPx knock-out mice as compared with wild-type control mice. Systemic 3-NP administration resulted in significantly greater striatal damage and increases in 3-NT in GSHPx knock-out mice as compared to wild-type control mice. The present results indicate that a knock-out of GSHPx may be adequately compensated under nonstressed conditions, but that after administration of mitochondrial toxins GSHPx plays an important role in detoxifying increases in oxygen radicals.

Influence of the viscosity of iodixanol on medullary and cortical blood flow in the rat kidney: a potential cause of Nephrotoxicity.

The aim of the present study was to investigate the effects of four iodinated contrast media on cortical, inner medullary and outer medullary blood flow in the rat kidney by using laser-Doppler flowmetry. The high-osmolar contrast medium diatrizoate did not significantly modify medullary perfusion but moderately decreased the cortical blood flow when injected at a dose of 1600 mg iodine kg(-1). Similar effects were obtained with the low-osmolar contrast media ioxaglate and iobitridol. In contrast, the new iso-osmolar contrast medium iodixanol induced a dose-dependent reduction of perfusion in all regions tested. This effect was accompanied by concomitant hypotension. The reduction of inner medullary and cortical blood flow induced by iodixanol was partially alleviated by heating the solution prior to injection and subsequently reducing its viscosity. In the outer medulla, however, this procedure did not improve blood flow. These results suggest that lowering the viscosity may palliate the harmful effects of iodixanol on the inner medulla and cortex, but may not protect the outer medulla from hypoxic injury.

A microdialysis study investigating the mechanisms of hydroxyl radical formation in rat striatum exposed to glutamate.

Considerable evidence has linked hydroxyl radicals (.OH) to excitotoxicity. Glutamate infused through a microdialysis probe into rat striatum induced a massive .OH production, which was completely blocked by PBN and attenuated by dizocilpine, 2-amino-5-phosphonopentanoic acid (AP-5), NG-nitro-L-arginine methyl ester (L-NAME) and mepacrine. Thus, we suggest that the neurotoxic effects of glutamate in vivo may derive from an increased formation of .OH resulting from excessive activation of NMDA receptors and downstream enzymes such as NOS and PLA2.

Glutamate induces hydroxyl radical formation in vivo via activation of nitric oxide synthase in Sprague-Dawley rats.

It was recently reported that neuronal nitric oxide synthase (NOS) generates oxygen-derived free radicals in vitro at low concentrations of L-arginine. Using the microdialysis technique, we monitored both hydroxyl radical (.OH) and nitric oxide (.NO) formation in rat striatum perfused with glutamate (500 mM). .OH and .NO were quantitated in microdialysates by measuring the amounts of the non-enzymatic hydroxylation product of salicylate (2,3-dihydroxybenzoic acid) and the metabolites of .NO (nitrite + nitrate), respectively. .OH levels were dramatically increased during glutamate perfusion, while .NO generation was virtually abolished. .OH production was inhibited by the specific NOS blocker, NG-nitro-L-arginine methyl ester. This effect was not reversed but potentiated by L-arginine. Thus, it is likely that NOS generates oxygen-derived free radicals instead of .NO in brain subjected to highly excitotoxic conditions.

alpha-Phenyl-N-tert-butylnitrone attenuates excitotoxicity in rat striatum by preventing hydroxyl radical accumulation.

Various in vitro experiments have indicated that oxygen-derived free radicals may contribute to excitotoxic neuronal death. In the present study we induced excitotoxicity in rat striatum by perfusing glutamate at a high concentration through a microdialysis probe. We observed an increased formation of hydroxyl radicals (.OH) during the perfusion of the excitotoxin and an extensive striatal lesion 24 h after the insult. The spin trap, alpha-phenyl-N-tert-butylnitrone (PBN), attenuated both hydroxyl radical levels and the volume of the lesion. This result suggests that the neuroprotection may be due to a free radical scavenging mechanism. It also implies that PBN may be used in pathological situations involving excitotoxicity such as stroke, brain trauma, and chronic neurologic diseases.

Role of the L-arginine-nitric oxide pathway in the basal hydroxyl radical production in the striatum of awake rats as measured by brain microdialysis.

In the present study, using the microdialysis technique, we provided evidence of the existence of hydroxyl radicals (.OH) in the striatum of awake rats under physiological conditions. This .OH generation was virtually abolished by the N-methyl-D-aspartate (NMDA) receptor antagonist dizocilpine. On the contrary, it was significantly enhanced by the .NO synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME). The effect of L-NAME was completely reversed by L-arginine. These results suggest that the basal .OH production is largely the consequence of an NMDA receptor-mediated glutamatergic tone. Moreover, it is likely that endogenous .NO exerts an antioxidant activity in brain by preventing the rise in .OH levels.

Detection of hydroxyl radicals in rat striatum during transient focal cerebral ischemia: possible implication in tissue damage.

As increasing arguments suggest that the reperfusion phase following an ischemic insult may aggravate tissue injury by yielding hydroxyl radicals ('OH), we examined whether these oxyradicals are generated in rat striatum during transient focal cerebral ischemia. .OH were detected in dialysate samples by intrastriatal microdialysis coupled with the technique of salicylate hydroxylation. Ischemia was achieved by tandem occlusion of the left middle cerebral artery and common carotid arteries (45 min) followed by reperfusion. An .OH formation occurred both during ischemia and early reperfusion. Additionally, the volume of the striatal infarct induced by ischemia correlated positively with the amount of .OH produced during ischemia and reperfusion. Taken together, these results provide evidence of the formation of cytotoxic .OH in rat striatum which might participate in the ischemic injury of this structure.

Striatal dopamine participates in glutamate-induced hydroxyl radical generation.

Using a microdialysis technique we showed that the exposure of the rat striatum to glutamate yields hydroxyl radicals and results in striatal damage. We postulated that dopamine release is enhanced by glutamate perfusion and that the enzymatic metabolism of dopamine may account for this hydroxyl radical formation. The inhibition of monoamine oxidases by i.p. co-administration of clorgy-line and deprenyl reduced hydroxyl radical production induced by glutamate perfusion, but significantly increased the striatal damage. Our results suggest that the enzymatic metabolism of dopamine participates in glutamate-induced hydroxyl radical generation but that other by-products of dopamine may be responsible for the aggravation of the striatal injury.