Daclizumab reduces CD25 levels on T cells through monocyte-mediated trogocytosis.

Daclizumab is a humanized monoclonal antibody that prevents interleukin-2 (IL-2) binding to CD25, blocking IL-2 signaling by cells that require high-affinity IL-2 receptors to mediate IL-2 signaling. The phase 2a CHOICE study evaluating daclizumab as a treatment for multiple sclerosis (MS) included longitudinal analysis of activated T cell counts. Whereas an exposure-dependent relationship was observed between daclizumab and reductions in HLA-DR(+)-activated T cells, a similar relationship was not observed for reductions in CD25 levels. The objective of this report is to determine the mechanism by which daclizumab reduces CD25 levels on peripheral blood mononuclear cells (PBMCs) using cytometric techniques. Daclizumab reduced T cell CD25 levels through a mechanism that required the daclizumab-Fc domain interaction with Fc receptors (FcR) on monocytes, but not on natural killer (NK) cells, and was unrelated to internalization or cell killing. Activated CD4(+) T cells and FoxP3(+) Treg cells showed evidence of trogocytosis of the CD25 antigen in the presence of monocytes. A daclizumab variant that retained affinity for CD25 but lacked FcR binding did not induce trogocytosis and was significantly less potent as an inhibitor of IL-2-induced proliferation of PBMCs. In conclusion, Daclizumab-induced monocyte-mediated trogocytosis of CD25 from T cells appears to be an additional mechanism contributing to daclizumab inhibition of IL-2 signaling.

Daclizumab, a humanised monoclonal antibody to the interleukin 2 receptor (CD25), for the treatment of moderately to severely active ulcerative colitis: a randomised, double blind, placebo controlled, dose ranging trial.

BACKGROUND: An uncontrolled pilot study demonstrated that daclizumab, a humanised monoclonal antibody to the interleukin 2 receptor (CD25), might be effective for the treatment of active ulcerative colitis. METHODS: A randomised, double blind, placebo controlled trial was conducted to evaluate the efficacy of daclizumab induction therapy in patients with active ulcerative colitis. A total of 159 patients with moderate ulcerative colitis were randomised to receive induction therapy with daclizumab 1 mg/kg intravenously at weeks 0 and 4, or 2 mg/kg intravenously at weeks 0, 2, 4, and 6, or placebo. The primary end point was induction of remission at week 8. Remission was defined as a Mayo score of 0 on both endoscopy and rectal bleeding components and a score of 0 or 1 on stool frequency and physician's global assessment components. Response was defined as a decrease from baseline in the Mayo score of at least 3 points. RESULTS: Two per cent of patients receiving daclizumab 1 mg/kg (p = 0.11 v placebo) and 7% of patients receiving 2 mg/kg (p = 0.73) were in remission at week 8, compared with 10% of those who received placebo. Response occurred at week 8 in 25% of patients receiving daclizumab 1 mg/kg (p = 0.04) and in 33% of patients receiving 2 mg/kg (p = 0.30) versus 44% of those receiving placebo. Daclizumab was well tolerated. The most frequently reported adverse events in daclizumab treated patients compared with placebo treated patients were nasopharyngitis (14.6%) and pyrexia (10.7%). CONCLUSION: Patients with moderate ulcerative colitis who are treated with daclizumab are not more likely to be in remission or response at eight weeks than patients treated with placebo.

L-selectin inhibition does not reduce injury in a rabbit model of transient focal cerebral ischemia.

Neutrophils are known to mediate injury in acute ischemic stroke especially during reperfusion. Migration of neutrophils into regions of ischemic injury involves binding to the endothelial cells via interactions with various adhesion molecules. One adhesion molecule, L-selectin, is found on the surface of leukocytes, and is shed prior to leukocyte infiltration. We studied whether a humanized antibody to L-selectin (HuDREG200) might limit ischemic injury in an experimental stroke model. New Zealand White rabbits underwent transorbital occlusion of the left middle cerebral, anterior cerebral and internal carotid arteries using aneurysm clips for 2 h followed by 6 h of reperfusion. Treatment with a saturating dose (4 mg kg-1) of HuDREG200 (n = 8) or vehicle (n = 8) was administered 20 min after occlusion and given as a single i.v. bolus. Hemispheric ischemic neuronal damage (IND) as seen on hematoxylin and eosin stained sections was no different between groups (HuDREG200, 23.3% +/- 6%; vehicle, 19.6% +/- 6%; mean +/- SEM, n.s., t-test). Immunohistochemical staining with neutrophil elastase confirmed the presence of neutrophils within regions of IND in control brains, but treatment did not alter their numbers within ischemic tissue. We conclude that antagonism of neutrophil adhesion at the level of L-selectin does not alter ischemic injury in experimental stroke.

Properties and pharmacokinetics of two humanized antibodies specific for L-selectin.

BACKGROUND: The participation of L-selectin in leukocyte recruitment during inflammation has suggested the use of L-selectin inhibitors as potential anti-inflammatory therapeutics. Blocking monoclonal antibodies could serve as such therapeutic agents, particularly if humanized to reduce their immunogenicity and improve their serum half-life. OBJECTIVES: For this purpose, two mouse monoclonal antibodies, DREG-55 and DREG-200, that block human L-selectin were humanized and characterized. STUDY DESIGN: The resulting humanized antibodies, HuDREG-55 and HuDREG-200, constructed with human IgG4 constant regions, were evaluated for their specificity, affinity and ability to block L-selectin-dependent adhesion in in vitro assays. Their pharmacokinetic behavior in rhesus monkeys was also studied. RESULTS: HuDREG-55 and HuDREG-200 were found to retain the specificity and affinity, within 2-fold, of the parent murine antibodies. HuDREG-55 and HuDREG-200 block L-selectin-dependent adhesion of human lymphocytes to high endothelial venules in frozen sections of lymph nodes. In addition, HuDREG-55 and HuDREG-200 are inhibitory in a novel L-selectin-dependent adhesion assay. This assay utilizes flow cytometry to measure binding of polymerized liposomes containing an analog of sialyl Lewis X, sialyl Lewis X glycoliposomes, to peripheral blood neutrophils and lymphocytes. Studying the pharmacokinetics of HuDREG-55 and HuDREG-200 in rhesus monkeys showed terminal elimination half-lives at 12.0 and 20.3 days, respectively. CONCLUSION: The shorter terminal elimination half-life of HuDREG-55 in rhesus monkeys may be due to the ability of HuDREG-55 but not HuDREG-200 to bind rhesus monkey L-selectin.

Humanized anti-L-selectin monoclonal antibody DREG200 therapy in acute thromboembolic stroke.

Strategies directed against activated neutrophils have reduced ischemia-induced brain injury. However, therapies targeted specially against the neutrophil adhesion protein L-selectin have not yet been examined in stroke. This study therefore examined the effects of a monoclonal antibody directed against L-selectin in a rabbit model of thromboembolic stroke with (n = 16) or without (n = 10) concomitant t-PA therapy. Rabbits received either the humanized monoclonal antibody DREG200 directed against the L-selectin receptor or humanized control monoclonal antibody HuDREG55 which does not bind to rabbit L-selectin in addition to t-PA therapy (n = 8, each group). HuDREG200 (2 mg kg-1 i.v.) was given as a bolus 3 h following clot embolization, followed immediately by a 2 h intravenous infusion of t-PA (6.3 mg kg-1. Without t-PA therapy rabbits received HuDREG200 (2 mgkg-1, i.v.; n = 5) or HuDREG55 (n = 5) 1 h following clot embolization. The group receiving HuDREG200 in addition to t-PA demonstrated a moderate improvement in brain infarct size (8.4 +/- 2.4 vs. 13.5 +/- 3.5, %hemisphere, mean +/- sem), ICP (final reading 10.0 +/- 1.6 vs. 12.4 +/- 3.0 torr) and restoration in regional cerebral blood flow (30.2 +/- 7.8 vs. 21.6 +/- 10.9 cc 100 g-1 min-1) when compared to t-PA therapy alone although statistical significance was not achieved. No efficacy was demonstrated in the group receiving HUDREG200 without concomitant t-PA therapy. The results suggest the addition of a humanized anti-L-selectin monoclonal antibody HuDREG200 in combination with t-PA may further improve outcome in acute thromboembolic stroke, although future studies are necessary to support these findings.

Humanization and pharmacokinetics of a monoclonal antibody with specificity for both E- and P-selectin.

E- and P-selectin (CD62E and CD62P) are cell adhesion molecules that mediate leukocyte-endothelial cell and leukocyte-platelet interactions and are involved in leukocyte recruitment during inflammation. We previously developed a murine mAb, EP-5C7 (or mEP-5C7), that binds and blocks both E- and P-selectin. When used in humans, murine mAbs have short circulating half-lives and generally induce potent human anti-mouse Ab responses. We therefore engineered a humanized, complementarity determining region-grafted version of mEP-5C7 incorporating human gamma4 heavy and kappa light chain constant regions (HuEP5C7.g4). HuEP5C7.g4 retains the specificity and avidity of mEP-5C7, binding to human E- and P-selectin but not to human L-selectin, and blocking E- and P-selectin-mediated adhesion. Surprisingly, when administered to rhesus monkeys, HuEP5C7.g4 was eliminated from the circulation very rapidly, even faster than the original murine Ab. To isolate the cause of the short serum half-life of HuEP5C7.g4, several Ab variants were constructed. A chimeric IgG4 Ab was made by replacing the humanized V regions with murine V regions. A humanized IgG2 Ab, HuEP5C7.g2, was also made by replacing the human gamma4 with a gamma2 constant region. Results from pharmacokinetic studies in rhesus monkeys demonstrated that the chimeric IgG4 is also rapidly eliminated rapidly from serum, similar to the humanized IgG4 Ab, while the humanized IgG2 Ab displays a long circulation half-life, typical of human Abs.

Definition of liver tumors in the presence of diffuse liver disease: comparison of findings at MR imaging with positive and negative contrast agents.

PURPOSE: The potential to define liver tumors at magnetic resonance (MR) imaging was compared with a positive and a negative contrast agent (gadoxetic acid disodium, or gadolinium EOB-DTPA [a hepatocyte-directed agent], and ferumoxides, or superpara-magnetic iron oxide particles [a Kupffer cell-directed agent], respectively) in normal rats and in rats with induced acute hepatitis, fatty liver, or cirrhosis. MATERIALS AND METHODS: Rats with implanted liver adenocarcinomas were divided into four groups: no diffuse liver disease ("normal" [n = 6]) and diffuse liver diseases (induced acute hepatitis [n = 6], fatty liver [n = 6], or cirrhosis [n = 6]). Rats first received gadoxetic acid disodium (50 mumol/kg) and then, 45 minutes later, ferumoxides (10 mumol/kg). Liver signal intensity enhancement and tumor-to-liver contrast-to-noise ratio (C/N) were measured in each group. RESULTS: Mean liver signal intensity enhancement values with gadoxetic acid disodium and ferumoxides were excellent in the normal liver model (176% and -62%, respectively; P < .01) but were significantly reduced in the acute hepatitis model (82% and -36%, respectively). In the fatty livers compared with the normal livers, enhancement with gadoxetic acid disodium was reduced (57%) but with ferumoxides was excellent (-55%). In the cirrhotic livers compared with the normal livers, enhancement with gadoxetic acid disodium (174%) was virtually the same but was impaired with ferumoxides (-43%). CONCLUSION: Hepatic enhancement and tumor-to-liver C/N with either positive or negative liver-enhancing agents can be impaired by the presence of underlying liver disease. Prior knowledge of the type of diffuse liver disease may influence the choice of contrast agent for tumor detection.

Early detection of acute tubular injury with diffusion-weighted magnetic resonance imaging in a rat model of myohemoglobinuric acute renal failure.

We evaluated the feasibility of magnetic resonance imaging (MRI) for early detection of tubular injury by monitoring changes in the apparent diffusion coefficient (ADC) of renal water in a rat model of myohemoglobinuric glycerol-induced acute renal failure (ARF). Diffusion-weighted MRI was performed concurrently with measurements of serum creatinine and blood urea nitrogen (BUN), evaluation of renal perfusion with dynamic contrast-enhanced MRI, and renal morphological examination. ADC values in the cortex and outer medulla significantly declined within minutes after the glycerol administration (70-75% of control at 4 min and 50-60% of control at 15 min). Contrast-enhanced MRI demonstrated renal hypoperfusion at 20 min after the onset of injury. Light microscopy showed normal glomeruli and edematous tubular epithelial cells at 10 and 30 min, with more severe swelling and protein casts at 30 min. No changes in serum creatinine or BUN levels were detected. We hypothesize that decrease in renal ADC may be attributed to renal ischemia and to subsequent intracellular accumulation of diffusion-restricted water. Similar imaging evaluation in other experimental models of ARF, and in patients, will define the diagnostic value of renal ADC changes in early detection of acute tubular injury.

Sensitivity of high-speed "perfusion-sensitive" magnetic resonance imaging to mild cerebral ischemia.

This study assessed the sensitivity of contrast-enhanced dynamic echo-planar imaging to subtotal stenosis of the middle cerebral artery as a model of mildly compromised cerebral blood supply. Dynamic data was analyzed in terms of the relative cerebral blood volume (rCBV) and bolus peak arrival time (BPAT), and the prognostic utility of these parameters was compared with measurements of the regional apparent diffusion coefficient of water (ADC) with the goal of identifying tissue at risk of future infarct. Dynamic echo-planar MRI in conjunction with bolus administration of a magnetic susceptibility contrast agent was used in a cat model of acute, unilateral cerebral ischemia, induced by partial occlusion (stenosis) of the right middle cerebral artery. The contrast agent transit was analyzed in terms of the regional time of arrival of the peak bolus-induced signal loss as well as the time integral of agent concentration. Pixel-by-pixel maps of cerebrovascular parameters (rCBV, BPAT) were constructed along with spatial maps of the ADC, derived from diffusion-weighted MR images at the same anatomical level. Arterial stenosis was maintained for a 6 h period, after which histological determination of tissue viability was obtained. Maps of BPAT showed sensitivity to mild flow perturbations not detectable from cerebral blood volume estimations from the same bolus injection or from determinations of the apparent diffusion coefficient of water. Of nine animals subjected to subtotal stenosis, BPAT identified compromised tissue in all nine after 1 h of stenosis. No animals had differences in rCBV or ADC at this point. Stenosis was maintained for 6 h in 7 of the cats. After 6 h, two cats had developed identifiable injury on ADC and rCBV maps. Of the remaining five, where rCBV and ADC appeared normal even after 6 h, three exhibited abnormal histological staining, whereas two indeed appeared normal. In the other two cats where initial subtotal stenosis was later made total, the anatomical region identified as "compromised" during stenosis, by the appearance of delayed bolus peak arrival, matched the area of subsequent infarct after total occlusion of the same vessel. Echo planar imaging in conjunction with bolus administration of a magnetic susceptibility contrast agent appears sensitive to mild perturbations to blood supply. These perturbations may not be resolved on synthesized maps of relative cerebral blood volume or apparent diffusion coefficient. Although "compromised" blood supply does not necessarily lead to infarct (over the 6-h course of this study), it may represent tissue particularly at risk of infarct in the event of further insult.

Contrast-enhanced magnetic resonance imaging estimation of altered capillary permeability in experimental mammary carcinomas after X-irradiation.

RATIONALE AND OBJECTIVES: Dynamic magnetic resonance imaging (MRI) enhanced with a macromolecular contrast medium, albumin-(Gd-DTPA)35, was used to detect changes in microvascular characteristics in R3230 mammary adenocarcinomas induced by x-irradiation. METHODS: Tumors were implanted in either flank in nine rats. One of the tumors was exposed to single-dose x-irradiation (30 Gy) 3 days before MRI. The contralateral control tumor was shielded from irradiation. RESULTS: Capillary permeability to macromolecular contrast medium in irradiated tumors was elevated significantly (P < .05) compared to the control nonirradiated tumors. The mean estimated permeability surface area product for the irradiated tumors increased more than three-fold; 0.511 +/- .046 mL hr-1 cm-3 compared with 0.121 +/- .011 mL hr-1 cm-3 for the nonirradiated tumors. This radiation-induced increase in permeability was corroborated using a macromolecular Evans blue-protein complex measured in the same tumors using an invasive spectrophotometric technique. CONCLUSIONS: Dynamic MRI-enhanced with macromolecular contrast medium permits noninvasive quantitative estimates of capillary permeability in tumors, with and without x-irradiation. Because the transendothelial permeability for macromolecular solutes likely influences tumoral accumulation of macromolecular chemotherapeutic agents, this noninvasive technique may prove to be clinically useful in tailoring tumor treatment programs which combine radiation and chemotherapy.

Effect of varying the molecular weight of the MR contrast agent Gd-DTPA-polylysine on blood pharmacokinetics and enhancement patterns.

The effects of varying the molecular weight of gadolinium-DTPA (diethylenetriaminepentaacetic acid)-polylysine, a macromolecular magnetic resonance (MR) imaging contrast agent, on blood pharmacokinetics and dynamic tissue MR imaging signal enhancement characteristics were studied in normal rats. Blood elimination half-life, total blood clearance, volume of the central compartment (Vcc) and the steady-state distribution volume (Vssd) were calculated for four Gd-DTPA-polylysine polymers with average molecular weights of 36, 43.9, 139, and 480 kd and compared with corresponding values for Gd-DTPA (0.57 kd) and Gd-DTPA-albumin (92 kd). Blood elimination half-life increased seven-fold with an increase in molecular weight from 36 to 480 kd. The Vcc values for all polylysine polymers did not differ significantly from the Vcc value for Gd-DTPA-albumin but were significantly smaller than the Vcc value for Gd-DTPA. The Vssd value for Gd-DTPA did not differ significantly from the Vssd value for the 36- and 43.9-kd polymers but was significantly larger than the Vssd values for the 139- and 480-kd polymers and for Gd-DTPA-albumin. On T1-weighted coronal spin-echo MR images, dynamic signal enhancement profiles in liver and kidney for the 36-, 43.9-, and 480-kd Gd-DTPA-polylysine chelates corresponded to the blood pharmacokinetic data. Increasing molecular weight of Gd-DTPA-polylysine formulations substantially slows blood clearance and produces a prolonged, almost constant tissue signal enhancement for the 60-minute observation period.

Quantification of tissue plasma volume in the rat by contrast-enhanced magnetic resonance imaging.

Magnetic resonance imaging enhanced with a macromolecular contrast medium (MMCM), albumin-Gd-DTPA, was used to estimate the plasma volume in vivo in the myocardium, lung, liver, and skeletal muscle of 10 normal rats. The plasma volumes of the same tissues in a parallel group of six rats were estimated in vitro by a conventional radioisotopic technique (111In-transferrin). Plasma volumes of myocardium, lung, liver, and skeletal muscle estimated by the MR technique (microliter plasma cc-1 of tissue) were 101, 109, 163, and 11.0, respectively, while plasma volumes measured by the 111In-transferrin radioisotope technique (mg plasma g-1 of tissue) were 78.6, 215, 143, and 11.2, respectively. Assuming a ratio of densities of aerated lung to blood of 0.45 and of other tissues to blood of 1.0, correlation between the methods was excellent (R2 = 0.99) indicating that MR imaging enhanced with MMCM permits reliable in vivo estimation of tissue plasma volume in the rat.

Measurement of capillary permeability to macromolecules by dynamic magnetic resonance imaging: a quantitative noninvasive technique.

A simple, linear kinetic model has been developed for the noninvasive assessment of capillary permeability to macromolecules in the rat by dynamic magnetic resonance imaging using albumin-Gd-DTPA. Data required by the model are signal intensity responses from a target tissue and a venous structure such as inferior vena cava before and after bolus intravenous injection of albumin-Gd-DTPA. Additional requirements include an early temporal resolution of approximately one image/min and a blood sample for hematocrit. The model does not require measurement of albumin-Gd-DTPA concentration in either arterial or venous blood. Pilot experiments suggest that this technique is adequate for estimation of the fractional leak rate of macromolecules from plasma to interstitial water as well as tissue plasma volume, the product of which yields a measure of the permeability surface area product of the tissue if the extraction fraction is modest (< 0.2). The technique may be generally applicable to the study of abnormal capillary permeability in humans as well as animals.

MR imaging of blood oxygenation-dependent changes in focal renal ischemia and transplanted liver tumor in rat.

The potential of using fast magnetic resonance (MR) imaging in conjunction with apnea-induced blood deoxygenation for the noninvasive monitoring of relative perfusion in the rat abdomen has been studied with two experimental models: glycerol-induced focal renal ischemia and transplanted liver tumor. Gradient-echo echo-planar imaging (GRE-EPI) (TE of 20 msec at 2T) of liver and kidney was performed before, during, and after a 60-second apnea episode and then was followed in the same rat by contrast-enhanced (a) GRE-EPI and (b) T1-weighted spin-echo imaging (TR msec/TE msec = 200/6) with polylysine-(gadolinium-DTPA [diethylenetriaminepentaacetic acid]). The results indicate that a noninvasive vascular challenge due to apnea can be used for the detection of focal tissue perfusion abnormalities in rat kidney and liver tumor.

Pulmonary oxygen toxicity: demonstration of abnormal capillary permeability using contrast-enhanced MRI.

An animal model of oxygen-induced pulmonary injury was used to assess the potential of contrast-enhanced MRI to identify and quantify abnormal capillary permeability. Sprague-Dawley rats were exposed to 100% oxygen for 48 h (n = 5) or 60 h (n = 9). Axial spin-echo MR images were acquired in intubated, anesthetized rats with ECG-gating (TR 400; TE 6) immediately or 7 days after the cessation of oxygen exposure. Polylysine-Gd-DTPA, a macromolecular paramagnetic blood-pool marker, was then given intravenously and the lungs were serially imaged for 42 to 47 min to monitor changes in signal intensity. Pulmonary enhancement was stable in rats exposed to 48 h of oxygen, and in rats exposed to 60 h of oxygen and given 7 days to recover. However, animals exposed to 100% oxygen for 60 h without a period of recovery showed a progressive increase in lung signal intensity for 15 min after polylysine-Gd-DTPA. Pleural effusions also showed progressively increasing signal, reflecting a capillary endothelial leak. A two compartment model describing the kinetics of polylysine-Gd-DTPA in the plasma and interstitial water of the lung was consistent with the dynamic MRI data and allowed estimation of the fractional leak rate (0.235 min-1) of the contrast agent from plasma to interstitial water. Given the assumption of our kinetic model, MRI following intravenous administration of polylysine-Gd-DTPA can be used to quantitate changes in capillary integrity induced by hyperoxia, including acute capillary leakiness and return to normal endothelial integrity with recovery from hyperoxic injury.

Comparison of Gd-EOB-DTPA and Gd-DTPA for contrast-enhanced MR imaging of liver tumors.

A new hepatobiliary contrast agent for magnetic resonance (MR) imaging, gadolinium-ethoxybenzyl (EOB)-diethylemetriaminepentaacetic acid (DTPA), was compared with Gd-DTPA to define the potential for improving tumor-liver contrast in a rodent liver adenocarcinoma model. With a T1-weighted spin-echo sequence, the contrast-to-noise ratio (C/N) for tumor before contrast agent administration was 5 (arbitrary units), the tumor appearing slightly hypo-intense with respect to liver parenchyma. After Gd-DTPA injection (0.1 mmol/kg), tumor enhanced more strongly than liver, resulting in an equalization of tumor and liver signal intensities and a decline in C/N to zero at 3 minutes after injection. After Gd-EOB-DTPA injection (0.1 mmol/kg), liver enhanced more strongly than tumor. Five minutes after injection, C/N increased from 5 to 25 and remained above 17 for 50 minutes. The data indicate that Gd-EOB-DTPA yields higher and more prolonged tumor-liver contrast than Gd-DTPA on T1-weighted spin-echo images. The high liver-tumor contrast after Gd-EOB-DTPA administration should prove clinically advantageous for MR imaging detection of focal hepatic masses.

Acute liver rejection: evaluation with cell-directed MR contrast agents in a rat transplantation model.

Enhancement patterns with gadolinium (4s)-4-(4-ethoxybenzyl)-3,6,9-tris (carboxylatomethyl)-3,6,9-triazaundecandioic acid (EOB) diethylenetriaminepentaacetic acid (DTPA), a hepatocyte-directed magnetic resonance (MR) contrast agent, and superparamagnetic iron oxide particles (SPIO), a Kupffer cell-directed contrast agent, were compared at MR imaging in rat models of allogeneic and syngeneic liver transplantation. Light and electron microscopy were performed to monitor the morphologic changes in these livers and revealed typical findings in acute rejecting liver (allogeneic transplants) and normal architecture in syngeneic transplants. Liver enhancement with Gd-EOB-DTPA at MR imaging was delayed and prolonged with transplantation, and even more with rejection, but the level of enhancement when compared with that of control rats remained unaffected. Uptake of SPIO was decreased in the acutely rejecting livers but was normal in the syngeneic transplantation model. Enhanced MR imaging with cell-directed contrast agents depicted decreased phagocytotic activity in acute transplant rejection and enabled the excretory function of hepatocytes to be monitored. Additionally, Gd-EOB-DTPA may be applied advantageously for detection of focal lesions in transplanted livers.

Magnetic resonance imaging demonstration of pharmacologic-induced myocardial vasodilatation using a macromolecular gadolinium contrast agent.

RATIONALE AND OBJECTIVES: Adenosine is a potent vasodilator used clinically in nuclear scintigraphy to assess coronary artery reserves. The potential to identify this vasodilating effect of adenosine using magnetic resonance imaging (MRI), which is superior in spatial resolution to nuclear scintigraphy, combined with a blood-pool MRI contrast agent, was investigated in normal rats. METHODS: Groups of Sprague-Dawley rats received successive infusions of either adenosine (3 mg/kg/minute; n = 7) or dipyridamole (negative control; up to 1.0 mg/kg/minute; n = 9), both before and after contrast enhancement, with a macromolecular blood-pool MRI contrast agent, albumin-gadolinium-DTPA35 (Gd-DTPA35) (4.0 mumol Gd per kilogram). Electrocardiographically (ECG) gated MRIs (2.0 Tesla), acquired serially before and after contrast enhancement, and with and without either adenosine or dipyridamole infusions, to monitor potential pharmacologic responses. RESULTS: During repeated infusions of adenosine, the postcontrast myocardial enhancement, reflecting blood volume, increased significantly (P < .05), up to 150%, compared with pre-adenosine enhancement. Infusions of dipyridamole, pharmacologically inactive in rats, produced no change in myocardial enhancement. CONCLUSIONS: The increased myocardial signal intensity observed during adenosine infusions after enhancement of the blood pool can be attributed to increased blood volume accompanying coronary vasodilatation. The method, which does not require a continuous infusion of contrast agent, has potential for the clinical evaluations of coronary artery reserves.