Studies of the retention mechanism of the brain perfusion imaging agent 99mTc-bicisate (99mTc-ECD).

The structure-activity relationship in a series of analogues of 99mTc-bicisate (99mTc-N,N'-1,2-ethylenediylbis-L-cysteine diethyl ester dihydrochloride, RP-217) is described using in vivo studies in rodent and primate models and in vitro studies in rodent and primate brain tissue. All analogues investigated were 99mTc-diamine dithiol diesters, which were neutral and lipophilic and had modified brain uptake indexes (> or = 40) suggesting adequate first-pass extraction. All analogues were poorly retained by the rodent brain. In contrast, the stereochemistry and structure of the 99mTc-complexes affected their brain retention in primates. All compounds that demonstrated selective primate brain retention were L-diesters that were metabolized in primate brain tissue to nonlypophilic complexes resulting from ester hydrolysis. Unretained complexes were not metabolized in primate brain tissue. More extensive studies were performed with 99mTc-bicisate, which demonstrated poor brain retention in several nonprimate species (i.e., dogs, ferrets, pigs, and rodents). In rodent and nonhuman primate tissue, 99mTc-bicisate was rapidly metabolized to a monoacid ester (99mTc-N,N'-1,2-ethylenediylbis-L-cysteine monoethyl ester). Therefore, brain metabolism of 99mTc-bicisate results in the formation of an acid product(s) that is selectively trapped in primate brain.

99mTc-bicisate (neurolite) SPECT brain imaging and cognitive impairment in dementia of the Alzheimer type: a blinded read of image sets from a multicenter SPECT trial.

A blinded read of images obtained with 99mTc-bicisate and single photon emission computed tomography (SPECT) was conducted to determine if a relationship exists between the severity of abnormalities on SPECT brain images and the severity of cognitive impairment in patients with dementia of the Alzheimer type (DAT) and to examine the interreader agreement for visual reading of images in a multicenter SPECT study. Images for a total of 86 subjects were available for the blinded read. The images for 28 subjects were rated as noninterpretable due to technical inadequacies. Images for 58 subjects (45 DAT patients and 13 normal volunteers) from 10 SPECT centers were selected for further analyses. The severity of abnormality was rated as mild, moderate, or severe by three readers. In DAT patients, a significant negative correlation (p < 0.05) of Mini-Mental State Examination (MMSE) score with global severity of abnormality was noted for two of the three readers. A significant correlation (p < 0.05) between MMSE score and severity of abnormality was observed for all three readers for the posterior temporoparietal region. The blinded readers rated a median of 92.3% of normal volunteers' images as normal and a median of 82.2% of DAT patients' images as abnormal. For the regional severity of abnormality, the median percentage interrater agreement across all regions ranged from 95 to 100% in normal volunteers and from 81 to 98% in DAT patients. These results suggest that SPECT brain imaging with 99mTc-bicisate provides functional information about the severity of cognitive impairment in DAT patients.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of single photon emission computed tomography brain imaging with 99mTc-bicisate in the localization and definition of mechanism of ischemic stroke.

99mTc-bicisate (99mTc-ECD) is a new brain perfusion imaging agent formulated from a radiochemically stable kit (Neurolite). A multicenter trial was conducted to determine the sensitivity and specificity of single photon emission computed tomography (SPECT) imaging with 99mTc-bicisate in the localization of ischemic stroke; 170 subjects were enrolled, 128 patients with stroke and 42 controls. Imaging results from 148 subjects (107 stroke patients and 41 controls) were considered evaluable. In the evaluable subjects, SPECT brain imaging with 99mTc-bicisate (21.0 +/- 2.5 mCi) was interpreted without clinical information and was compared with a final assessment using all clinical, diagnostic, and laboratory procedures except the 99mTc-bicisate SPECT results. 99mTc-bicisate was safe and well-tolerated. SPECT imaging with 99mTc-bicisate demonstrated a specificity of 98% and a sensitivity of 86% for localization of strokes (kappa, 0.75; 95% confidence interval, 0.64-0.86). Results were unchanged over time and were similar for all stroke mechanisms except for lacunar disease (sensitivity, 58%). In a secondary analysis, a normal image or small, deep (e.g., subcortical) perfusion defect was highly predictive of a lacunar mechanism. Defects involving the cortical surface were strongly associated with nonlacunar mechanisms. SPECT imaging with 99mTc-bicisate is a sensitive marker in the localization of perfusion defects associated with ischemic stroke and may assist in the determination of the underlying mechanism of a stroke.

Metabolism of 99mTc-L,L-ethyl cysteinate dimer in healthy volunteers.

99mTc-L,L-Ethyl cysteinate dimer (ECD) is a brain-perfusion imaging agent, which exhibits selective retention in brain and rapid renal excretion. The pharmacokinetics and metabolism of ECD were studied in vivo in healthy humans and its metabolism in vitro was evaluated in tissue from human brain. In vitro studies showed 99mTc-L,L-ECD to be metabolized to a polar 99mTc-complex. It has been shown previously that most of the activity of 99mTc retained in the brain of the monkey in vivo is in the form of a polar 99mTc complex (Walovitch, Hill, Garrity, Cheesman, Burgess, O'Leary, Watson, Ganey, Morgan and Williams, 1989). Whole body images of the distribution of 99mTc-L,L-ECD (10 mCi i.v.) in four adult males showed good uptake in brain, with slow elimination (6.8 +/- 0.3% injected dose [mean +/- SE] at 5 min), with less than 25% decrease in activity during 4 hr of imaging. Background areas in the head and lungs washed out rapidly, providing ideal imaging conditions. Elimination of 99mTc from venous blood was biphasic, with a plateau of activity between 2-15 min (7-8% injected dose) before a terminal phase, with a t1/2 of a few hours. Organic extraction of whole venous blood showed greater than 50% of the 99mTc-L,L-ECD to be in the form of polar metabolite(s) at 5 min. They were identified in the urine as the 99mTc ethylenediylbis-L-cysteine, monoethyl ester complex (ECM) and the 99mTc-ethylenediylbis-L-cysteine complex (EC). These metabolites were excreted rapidly (75% injected dose in urine within 6 hr). The results of this study support the hypothesis that the selective retention in brain, rapid blood elimination and renal excretion of 99mTc-L,L-ECD is due to its metabolic transformation to polar end products.

Intrasubject comparison between technetium-99m-ECD and technetium-99m-HMPAO in healthy human subjects.

The pharmacokinetics and quality of planar and SPECT brain imaging of two 99mTc-labeled brain perfusion agents, d,l-hexamethyl propylene amine oxime (HMPAO) and ethyl cysteinate dimer (ECD), were compared in seven healthy, normal subjects. Both radiopharmaceuticals showed rapid brain uptake and had a net brain washout of less than 5% during the first 20 min after drug administration. However, during the same time period, 99mTc-ECD images of the head showed significantly less background facial uptake and retention when compared to 99mTc-HMPAO images. The brain-to-background contrast ratio of 99mTc-ECD (brain/neck) continued to improve over time and by 5 hr postadministration was 17 to 1 versus 2 to 1 for 99mTc-HMPAO. SPECT brain images of both agents show gray/white matter ratios that were unchanged over time and an intracerebral distribution consistent with blood flow. A blind read of these SPECT images also shows 99mTc-ECD to produce images that were "easier to interpret" with less extracerebral activity as compared to 99mTc-HMPAO. Repeat, whole-body planar spot imaging suggests that 99mTc-ECD was cleared more rapidly from the body than was 99mTc-HMPAO.

Characterization of technetium-99m-L,L-ECD for brain perfusion imaging, Part 2: Biodistribution and brain imaging in humans.

The safety, biodistribution and kinetics of a new perfusion imaging agent [99mTc-L,L]-ethyl cysteinate dimer (ECD) was evaluated in normal volunteers. Technetium-99m-L,L-ECD is a neutral, lipophilic complex, which is radiochemically pure and stable. Twelve healthy adults were injected with 25-30 mCi of 99mTc-L,L-ECD and imaged periodically for up to 24 hr. Planar imaging showed rapid brain uptake with a peak concentration of 4.9% injected dose and very slow brain washout (approximately 6% per hour during the first 6 hr). Repeat or dynamic tomographic imaging of the brain using either a rotating gamma camera or a multidetector system was performed up to 6 hr postinjection. The distribution of 99mTc-L,L-ECD in the brain did not change and was similar to the pattern seen with other perfusion agents. Background facial areas and lungs cleared rapidly. Peak blood activity was below 10% injected dose at all times and 99mTc-L,L-ECD cleared rapidly through the kidneys. Vital signs, blood and urine chemistries were normal in all volunteers and no adverse reactions were noted. These results suggest that 99mTc-L,L-ECD should be useful for routine assessment of cerebral perfusion in humans.

Characterization of technetium-99m-L,L-ECD for brain perfusion imaging, Part 1: Pharmacology of technetium-99m ECD in nonhuman primates.

Technetium-99m ethyl cysteinate dimer ([99mTc]ECD) is a neutral, lipophilic complex which rapidly crosses the blood-brain barrier. Brain retention and tissue metabolism of [99mTc]ECD is dependent upon the stereochemical configuration of the complex. While both L,L and D,D enantiomers are extracted by the brain, only the L,L but not the D,D form, is metabolized and retained in the monkey brain (4.7% injected dose initially, T 1/2 greater than 24 hr). Dynamic single photon emission computed tomography imaging studies in one monkey indicates 99mTc-L,L-ECD to be distributed in a pattern consistent with regional cerebral blood flow for up to 16 hr postinjection. Dual-labeled 99mTc-L,L-ECD and [14C]iodoantipyrine autoradiography studies performed 1 hr after administration show cortical gray to white matter ratios of both isotopes to be equivalent (approximately 4-5:1). These data suggest that 99mTc-L,L-ECD will be useful for the scintigraphic assessment of cerebral perfusion in humans.

Preclinical evaluation of the pharmacokinetics, biodistribution, and elimination of MS-325, a blood pool agent for magnetic resonance imaging.

RATIONALE AND OBJECTIVES: The authors evaluate MS-325, a new albumin-targeted magnetic resonance imaging (MRI) contrast agent, for its pharmacokinetics, biodistribution, and elimination characteristics in multiple animal species. METHODS: Studies were performed in rats, rabbits, and nonhuman primates at intravenous doses ranging from 0.025 to 0.20 mmol/kg. Concentrations of MS-325 in blood, urine, feces, and organs were determined using gadolinium-153-labeled MS-325 and gamma counting or by using non-labeled MS-325 and inductively coupled plasma atomic emission spectrometry. RESULTS: In rabbits and nonhuman primates, MS-325 is approximately 85% to 95% bound to serum proteins and, as a result, exhibits low volume of distribution (Vd) values, 0.11 to 0.14 L/kg, and a long elimination half-life (Te1/2), 2 to 3 hours. Some dose-dependence in the parameters is apparent in rabbits. MS-325 is eliminated primarily through the renal system in non-human primates. In contrast, the behavior of MS-325 in rats is different, exhibiting increased biliary excretion, a larger Vd value, and a shorter Te1/2. CONCLUSIONS: The pharmacokinetics and elimination profile of MS-325, including vascular retention and renal excretion, are favorable for use in humans as an intravascular contrast agent for MRI.

Effects of morphine on rat kidney glomerular podocytes: a scanning electron microscopic study.

Degenerative kidney changes are associated with heroin use in human addicts, but it is not known whether these changes result from exposure to the opioid or from contaminants in street heroin. In the present study, 4-6-month-old rats each received 1 subcutaneous pellet containing 75 mg of morphine or placebo, followed 3 days later by implantation of 2 additional morphine or placebo pellets. Seven days after implantation of the first pellet, the rats were killed by aldehyde perfusion. The right kidney was excised, and coronal slices were prepared for scanning electron microscopy. Micrographs were taken at 5000X and were scored on the presence of short or long microprojections (a score of '1' indicating few and a score of '4' indicating many). Morphine significantly altered the frequencies of scores for long microprojections, suggesting that morphine treatment increased the number of microprojections on glomerular podocytes. No changes in filtration slits, pedicels, or blebbing (foval enlargements) were noted. The data support the view that kidney degeneration associated with opioid abuse reflects effects of opioids per se, and they are consistent with microprojection changes as a function of altered intracellular cyclic AMP levels.

Co-dergocrine, cerebral glucose utilization and maze performance in middle-aged rats.

The objective of this study was to determine the effect of co-dergocrine in rats on local cerebral glucose utilization and performance in a complex T-maze. Middle-aged (12-16 months) male Fischer-344 rats were given injections of co-dergocrine (3 or 10 mg/kg, IP) 35 min before behavioral testing or the administration of 2-deoxy-D-[1-14C]glucose ([14C]DG), a radiotracer for local cerebral glucose utilization (LCGU). Both doses stimulated LCGU in the locus ceruleus and median raphe nucleus and in subcortical structures associated with learning and memory (hippocampus and subiculum). The higher dose also stimulated LCGU in motor areas (caudate-putamen, globus pallidus, internal capsule). In contrast, co-dergocrine decreased LCGU in the frontal cortex. Poorer performance in a complex maze (increased shocks, errors and run time) was observed in middle-aged as compared with younger animals (3 months). Acute co-dergocrine treatment did not improve performance of middle-aged rats in this task. Thus, in the present experimental paradigm employing single dose administration, co-dergocrine's stimulation of LCGU was not associated with an alteration of maze performance in age-matched animals.

Naloxone fails to alter local cerebral glucose utilization in the rat.

The autoradiographic, 2-deoxy-D-[1-14C]glucose ([14C]DG) method was used to map the effects of intravenous (IV) naloxone (1.0, 10.0, and 20.0 mg/kg) on local cerebral glucose utilization (LCGU), an index of local brain function. Naloxone injected 5 min before [14C]DG did not alter LCGU in any of the fifty-six brain regions examined. Our findings suggest that acute naloxone at these doses does not significantly affect cerebral metabolism.

Cerebral glucose metabolism in aging rodents: effects of co-dergocrine.

Because glucose is the primary substrate for oxidative metabolism of the adult brain, measurements of glucose utilization provide information on cerebral energy metabolism, which ultimately is linked to neuronal activity. Early studies utilized in vitro techniques to measure carbohydrate oxidation as well as glycolytic and citric acid cycle enzymatic activities as a function of age. Although the results of these studies are somewhat equivocal, they generally indicate a decline in cerebral carbohydrate metabolism in senescent rodents. More recently, local cerebral glucose utilization (LCGU) has been measured in awake, resting rats with the 2-deoxy-D[1-14C]glucose metabolic mapping technique. Using this technique, decrements in LCGU have been observed in rats by midlife. Co-dergocrine, an ergot alkaloid used extensively in geriatric psychopharmacology, reversed subcortical LCGU decrements in brain areas associated with motor function, motivation, and learning. In contrast, the drug decreased frontal cortical LCGU in middle-aged rats. Stimulatory effects of co-dergocrine on LCGU in brain areas associated with motivation and learning support the view that the drug may be useful against age-associated disorders of cognition. Furthermore, the co-dergocrine-induced decrease in cortical glucose utilization coupled with the drug's cerebral vasoconstrictive action may render cortical cells less susceptible to the sequelae of ischemia.

Effects of hyperprolactinemia on plasma prolactin and glucose and on local cerebral glucose utilization.

Elevated blood levels of prolactin increase the synthesis, turnover, and release of 3,4-dihydroxyphenylethylamine (dopamine) from the tuberoinfundibular dopaminergic neurons, which project to the median eminence. The present study examined whether hyperprolactinemia also increases local cerebral glucose utilization, as determined by the 2-deoxy-D-[1-14C]glucose method, in the median eminence and other brain structures. Adult male rats were given ovine prolactin (4 mg/kg) subcutaneously every 8 h for 48 h. This treatment exerted an autoregulatory feedback effect on endogenous rat prolactin secretion, as evidenced by decreased circulating levels of rat prolactin. Ovine prolactin treatment also decreased plasma glucose concentrations. However, in both partially immobilized and free-ranging rats, glucose utilization in brain structures containing tuberoinfundibular dopaminergic cell bodies (the arcuate nucleus) and terminals (the median eminence) was not affected by ovine prolactin treatment. Hyperprolactinemia was, however, associated with decreased glucose utilization in the medial forebrain bundle and the CA subfield of the dorsal hippocampus. The lack of a significant effect of prolactin treatment on glucose utilization in the median eminence indicates that the resolution of the deoxyglucose technique, as used here, is not adequate to detect the ovine prolactin-induced increase in tuberoinfundibular dopaminergic neuronal activity, that the median eminence does not utilize glucose as its primary energy substrate, or that ovine prolactin treatment causes a counterbalancing decrease in the activity of other neurons projecting to the median eminence.

Three-dimensional MRI of coronary arteries using an intravascular contrast agent.

To assess the effectiveness of an intravascular contrast agent, MS-325, for enhancing the vascular signal in coronary MR angiograms, six minipigs were studied using a three-dimensional, gradient-echo sequence with retrospective respiratory gating. To suppress the myocardial signal, preparatory RF pulses were applied before data acquisition. With the administration of MS-325, the blood signal-to-noise ratio increased by 97-276%, depending on the region of interest in which the blood signal was measured and the precontrast imaging sequence structures. The blood/myocardium contrast-to-noise ratio also significantly increased. High-resolution images (0.58 x 0.58 x 1 mm3) obtained from postmortem pig hearts demonstrated the potential delineation of coronary arteries with MS-325. In conclusion, this study supports further evaluation of the utility of MS-325 in improving coronary MR angiography in humans.

Regional cerebral blood flow and distribution of [99mTc]ethyl cysteinate dimer in nonhuman primates.

Increases in regional cerebral blood flow have been described in a variety of cerebral pathologic states, including stroke and seizure disorders. The usefulness of technetium-99m-labeled cysteinate dimer as a marker in the measurement of regional cerebral blood flow was tested in five cynomolgus monkeys. To expand the range of blood flow to beyond the normal limits, 40 mg/kg i.v. of the carbonic anhydrase inhibitor acetazolamide was administered. Regional cerebral blood flow in all five monkeys was measured using radiolabeled microspheres (before and after acetazolamide) and the marker (after acetazolamide) in 60-70 samples from 12 brain regions. Acetazolamide significantly increased the mean +/- SEM regional cerebral blood flow measured using microspheres from 0.56 +/- 0.21 to 1.71 +/- 0.9 ml/min/g (p less than 0.01 for each region). A significant positive correlation was found between regional cerebral blood flow values calculated using microspheres and the marker after normalizing the values to those in the cerebellum (r = 0.773, p less than 0.0001). The mean +/- SEM regional cerebral blood flow determined using the marker in a single monkey (1.21 +/- 0.04 ml/min/g) did not differ significantly from that determined in the same monkey using microspheres (1.13 +/- 0.04 ml/min/g). These data support the potential use of this new brain perfusion imaging agent to assess regional cerebral blood flow over a clinically relevant range of blood flows.

Diurnal rhythmicity and hypothalamic deficits in glucose utilization in aged ovariectomized rats.

Aging is associated with a loss of cyclic gonadotropin release in female animals. This deficit may reflect dampened circadian rhythmicity of neuroendocrine events and/or altered function in hypothalamic nuclei important to regulation of cyclic female reproduction. The purpose of this study was to determine if diurnal periodicity and glucose metabolism in the hypothalamus are altered with age and whether such changes could help to explain the age-related deficits in gonadotropin release. Young (3-4-month-old) and old (18-21-month-old) rats were ovariectomized and subjected to the 2-deoxy-D-1-14C-glucose technique to measure rates of cerebral glucose utilization (GU), an index of neural function (Sokoloff et al., 1977) in various brain areas and in the pineal gland. We measured GU during the light (1400 hours) and the dark (2200 hours) in 17 anatomical regions including the following hypothalamic areas: medial preoptic nucleus, suprachiasmatic preoptic nucleus, suprachiasmatic nucleus, paraventricular nucleus, arcuate nucleus, and median eminence. Serum concentrations of luteinizing hormone (LH) and prolactin were measured in the same rats to determine the effect of age on both of these hormones. Diurnal periodicity of GU was observed in the suprachiasmatic nucleus and the pineal gland in young and old rats. Although there was no age difference in GU of the pineal gland, GU was reduced during the light and dark in the suprachiasmatic nucleus and all other hypothalamic areas examined except the suprachiasmatic preoptic nucleus and the median eminence. Ovariectomy induced an attenuated increase in concentrations of LH in old, compared to young rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Radio-frequency-induced coagulation necrosis in rabbits: immediate detection at US with a synthetic microsphere contrast agent.

PURPOSE: To determine whether a synthetic ultrasonographic (US) contrast agent can be used to differentiate coagulation necrosis from untreated tumor immediately after radio-frequency ablative therapy. MATERIALS AND METHODS: VX2 (adenocarcinoma) tumors (0.8-1.5-cm diameter) were implanted into 12 rabbits. Gray-scale and color Doppler US were performed with or without intravenous injection of a US contrast agent composed of poly-lactide-co-glycolic acid polymeric (PLGA) microspheres (2-micron diameter) filled with perfluorocarbon gas. Radio frequency was applied to each nodule for 6 minutes at 127 mA +/- 33 (mean +/- SD) (tip temperature, 92 degrees C +/- 2). Repeat US with a second dose of the contrast agent was performed immediately after ablation. In four animals, a third dose was administered 30-120 minutes after ablation. Radiologic-histopathologic correlation was performed and included in vivo staining and studies of mitochondrial function. RESULTS: Intense contrast agent enhancement was seen throughout the tumor prior to ablation. At gray-scale US, ablation produced hyperechoic foci, which were within 1 mm of the foci identified at histopathologic examination in seven of 12 animals (58%). After the administration of contrast material, foci devoid of previously visualized enhancement, which measured 7.3-15.0 mm, were identified. These were within 1 mm of the size of the foci identified at histopathologic examination in 11 of 12 animals (92%, P < .01). In two animals, enhancement depicted viable tumor, which appeared hyperechoic, on nonenhanced images. On delayed images, hyperechoic areas decreased in size, whereas the nonenhanced region remained unchanged. CONCLUSION: A PLGA microspherical US contrast agent enabled the immediate detection of coagulation necrosis as a region devoid of contrast enhancement after radio-frequency ablation in rabbit hepatic tumors. Therefore, this agent could provide real-time guidance during complex ablative procedures and may provide an efficient technique for postprocedural assessment.

Intravascular contrast agent improves magnetic resonance angiography of carotid arteries in minipigs.

This study was designed to optimize three-dimensional (3D) time-of-flight (TOF) magnetic resonance angiography (MRA) sequences and to determine whether contrast-enhanced MRA could improve the accuracy of lumen definition in stenosed carotid arteries of minipigs. 3D TOF MRA was acquired with use of either an intravascular (n = 13) and/or an extravascular contrast agent (n = 5) administrated at 2 to 4 weeks after balloon-induced injury to a carotid artery in 16 minipigs. Vascular contrast, defined as signal intensity differences between blood vessels and muscle normalized to the signal intensity of muscle, was compared before and after the injection of each contrast agent and between the two agents. Different vascular patencies were observed among the animals, including completely occluded vessels (n = 5), stenotic vessels (n = 3), and vessels with no visible stenosis (n = 8). Superior vascular contrast improvement was observed for small arteries and veins and for large veins with the intravascular contrast agent when compared with the extravascular contrast agent. In addition, preliminary studies in two of the animals showed a good correlation for the extent of luminal stenosis defined by digital subtraction angiography compared with MRA obtained after administration of the intravascular contrast agent (R2 = .71, with a slope of .96 +/- .04 by a linear regression analysis). We concluded that use of an intravascular contrast agent optimizes 3D TOF MRA and may improve its accuracy compared with digital subtraction angiography.

MS-325: albumin-targeted contrast agent for MR angiography.

PURPOSE: To evaluate the protein-binding and signal enhancement characteristics of MS-325, a gadolinium-based magnetic resonance (MR) imaging blood pool agent that binds to albumin, and compare results with those obtained with existing gadolinium- and iron oxide-based agents. MATERIALS AND METHODS: Protein binding in human plasma was measured by means of ultrafiltration. T1 relaxation times (20 MHz) were measured in human plasma or ex vivo samples from rabbits and monkeys injected with 0.1 mmol of MS-325 per kilogram of body weight. Imaging (three-dimensional fast imaging with steady-state precession, or FISP) was performed at 1.0 T in phantoms, which contained varying concentrations of different agents, or rabbits after injection of 0.015-0.100 mmol/kg MS-325. RESULTS: MS-325 is 80%-96% bound in human plasma and exhibits a relaxivity approximately six to 10 times that of gadolinium diethylenetriaminepentaacetic acid (DTPA). Images of phantoms containing MS-325 were significantly brighter than those containing existing gadolinium chelates or iron particles (monocrystalline iron oxide nanoparticle, or MION) at equivalent concentrations. Findings of in vivo studies indicated strong, persistent plasma T1 reduction with MS-325 for 1 hour (T1 of MS-325, 50-100 msec; T1 of Gd-DTPA, 200-400 msec) and strong vascular enhancement on MR images. CONCLUSION: MS-325 is highly protein bound after injection and provides vascular signal enhancement superior to that provided with other agents. As the first gadolinium-based blood pool agent in human trials, MS-325 has the potential to enhance both dynamic and steady-state MR angiograms.