A specific radioimmunoassay for the measurement of gadoteridol, a contrast agent for magnetic resonance imaging in biological fluids.

Gadoteridol, a nonionic gadolinium chelate, is currently being evaluated for contrast-enhanced magnetic resonance imaging. A radioimmunoassay (RIA) has been developed for the measurement of gadoteridol in biological fluids. The RIA has a range of 0 to 25 micrograms/mL and has the sensitivity to detect 0.05 microgram/mL of gadoteridol. Satisfactory zero binding and sensitivity were obtained after an overnight incubation at 4 degrees C. Separation of the antibody-bound and free radiolabel was achieved with 12.5% polyethylene glycol. A quantitative recovery of the exogenous analyte was obtained at all concentrations of gadoteridol tested. Linearity in both serum and urine was satisfactory. Intraassay coefficients of variation were 6.4 and 2.8% for the low and medium controls, respectively. Interassay coefficients of variation were 5.4, 3.8, and 12.2% for the low, medium, and high controls, respectively. Cross reactivities of the ligand 5 and the calcium salt 6 were 37 and 29%, respectively. Clinical samples from the ascending dosage studies were analyzed by the gadoteridol RIA. The results obtained from the serum specimens demonstrated an excellent linear proportionality between drug concentration in blood and administered dosage of gadoteridol. Cumulative urinary excretion data showed that 94% of the drug was excreted in the urine within 24 h.

Albumin labeled with Gd-DTPA. An intravascular contrast-enhancing agent for magnetic resonance blood pool and perfusion imaging.

Paramagnetic macromolecules like Gd-DTPA labeled albumin offer certain beneficial features that distinguish them from smaller molecular paramagnetic contrast media, such as Gd-DTPA dimeglumine, which generally distribute in the extracellular fluid space. By joining multiple paramagnets to one large carrier molecule, the molar dose of the agent necessary for image enhancement is reduced, the proton relaxation effectiveness of such macromolecular agents is increased and they may even serve as markers of perfusion and abnormal vascular permeability. The accumulated experimental results with albumin labeled with Gd-DTPA are described.

Acute myocardial ischemia and reperfusion: MR imaging with albumin-Gd-DTPA.

The utility of a macromolecular, intravascular contrast agent, albumin-gadolinium diethylenetriaminepentaacetic acid (DTPA), for the differentiation of acutely ischemic and reperfused myocardium on magnetic resonance (MR) images was investigated. Regional, reversible myocardial ischemia was produced in rats and confirmed. After reperfusion, flow to the compromised myocardial segment returned to baseline. Normal myocardium could not be differentiated from ischemic myocardium on nonenhanced MR images (n = 12). After 5 minutes of myocardial ischemia and after administration of albumin-Gd-DTPA, the ischemic zone involving the free wall of the left ventricle was characterized by the absence of significant enhancement. Normal myocardium appeared homogeneously enhanced (by 145%). This pattern persisted for up to 1 hour of myocardial ischemia. In six rats that underwent myocardial reperfusion after 5 minutes of ischemia, the normal and reperfused myocardium became isointense. Radiotracer studies with albumin-Gd-153-DTPA confirmed the decreased distribution of contrast agent to the ischemic myocardium, possibly due to decreased blood pool or a blocked primary delivery system in the ischemic myocardium.

Pyrroxamide, a nonionic nitroxyl spin label contrast agent for magnetic resonance imaging. Mutagenesis and cell survival.

Pyrroxamide [N-(1-hydroxymethyl-2,3-dihydroxypropyl)-2,2,5,5-tetramethyl pyrrolidine-1-oxyl-3-carboxyamide] is a newly tested nonionic monomeric nitroxyl compound with demonstrated effectiveness for MRI contrast enhancement at doses as low as 10(-3) M. Pyrroxamide and its hydroxylamine metabolic derivative were tested in concentrations from 10(-9) to 10(-2) M with a battery of cytotoxic and mutagenic assays using mammalian Chinese hamster ovary cells. Loci-specific mutation induction was examined at the hypoxanthine-guanine phosphoribosyltransferase (HGPRT) and the Na+/K+ ATPase loci, both in the presence and absence of a liver microsomal metabolic activating mixture (S-9 mix). Cell survival and induction of sister chromatid exchanges also were studied. All tests yielded negative results indicating that pyrroxamide and and hydroxylamine derivative were both noncytotoxic and nonmutagenic at the doses tested.

Metalloporphyrin contrast enhancement of tumors in magnetic resonance imaging. A study of human carcinoma, lymphoma, and fibrosarcoma in mice.

The diagnostic use of Mn(III)TPPS4, a paramagnetic metalloporphyrin, for MR contrast enhancement was examined in human--mouse xenograph models of carcinoma, lymphoma, and sarcoma. Spin-echo images of 15 mice, five mice for each implanted tumor type, were obtained before and at 20 minutes, and 2, 4, and 24 hours following administration of 0.09 mmol/kg of Mn(III)TPPS4. All tumors had a uniform moderate signal intensity on precontrast images. After administration of Mn(III)TPPS4, all tumors demonstrated significant enhancement of signal intensity that persisted to 24 hours. T1 relaxation times were maximally depressed at 2-4 hours and remained low to 24 hours for all three tumors. Kidney signal intensity reached a maximum at 20 minutes and remained significantly above background for 24 hours. The high relaxivity and apparent avidity of Mn(III)TPPS4 for divergent tumor histologies support the potential use of this agent for improved diagnostic specificity of MR imaging for neoplastic masses.

Magnetic resonance imaging of myocardial infarction using albumin-(Gd-DTPA), a macromolecular blood-volume contrast agent in a rat model.

Magnetic resonance (MR) contrast enhancement of acute myocardial infarction was studied in rats using albumin-(Gd-DTPA), a paramagnetic macromolecule with prolonged intravascular retention after intravenous injection. Histologic examination and distribution measurements of radiolabeled microspheres confirmed induction of regional myocardial infarction after ligation of the left coronary artery. ECG-gated spin-echo images at 2.0 Tesla, employing short, T1-weighted pulse sequence settings, demonstrated time-persistent and significant (P less than .05) enhancement of normal myocardium (66%) and an even greater enhancement of the infarcted area (100%), for as long as 60 minutes after injection of 160 mg/kg albumin-(Gd-DTPA). The contrast difference between normal and infarcted myocardium was increased significantly (P less than .05) after administration of albumin-(Gd-DTPA). The prolonged enhancing effects of albumin-(Gd-DTPA) on MR images are useful for evaluating regional differences in blood volume and capillary integrity between normal and infarcted myocardium.

Albumin labeled with Gd-DTPA. An intravascular contrast-enhancing agent for magnetic resonance blood pool imaging: preparation and characterization.

A paramagnetic-labeled macromolecule, albumin-(Gd-DTPA), was prepared for use as an intravascular contrast agent for magnetic resonance imaging. An average of 19 Gd-DTPA chelates were covalently conjugated to human serum albumin through the bifunctional anhydride of DTPA. The albumin-(Gd-DTPA) was characterized with use of high-performance liquid chromatography, sodium dodecyl sulfate gel electrophoresis, atomic absorption, biuret and Bradford protein tests, and by its effect on proton relaxation (relaxivity). The average molecular weight was 92,000 daltons, indicating the albumin conjugate was predominantly monomeric. The T1 relaxivity of albumin-(Gd-DTPA) was 273 mM-1 sec-1 relative to carrier concentration, which corresponds to a relaxivity of 14.9 mM-1 sec-1 relative to gadolinium concentration. The average conditional stability constant for albumin-bound Gd-DTPA chelate was log K = 20.0. Spin-echo images of rats demonstrated persistent enhancement of vascular tissues and slowly flowing blood. Application of albumin-(Gd-DTPA) may augment the MR assessments of blood volume, tissue perfusion, and flow characteristics.

Metabolic fate in the dog of the nitroxide moiety in a compound with potential utility as a contrast agent in MRI.

Nitroxides, paramagnetic compounds with demonstrated effectiveness as contrast agents in proton magnetic resonance imaging, shorten the relaxation times of protons and therefore cause an increase in image intensity in tissues into which they distribute. In this study, the metabolic fate of the nitroxide moiety was examined in the dog using a pyrrolidine nitroxide derivative, 2,2,5,5-tetramethylpyrrolidine-1-oxyl-3-carboxylic acid, for which contrast-enhancing properties have been previously studied in animals. After radiolabeling by microwave discharge in the presence of tritium gas, the compound was administered intravenously to a dog. Ninety-four percent of the radioactivity injected was recovered in urine within 3 days; the majority (90%) was excreted during the first 6 h. The radioactivity in the urine was identified as either the unchanged nitroxide or its corresponding hydroxylamine. Neither complete reduction of the nitroxide moiety to the amine nor any other metabolic transformation was observed.

Comparison of initial biodistribution patterns of Gd-DTPA and albumin-(Gd-DTPA) using rapid spin echo MR imaging.

The initial biodistribution patterns of gadolinium-diethylenetriaminepentaacetic acid (Gd-DTPA), an extracellular fluid contrast agent, and human serum albumin, paramagnetically labeled with 19 Gd-DTPA groups and used as an intravascular agent, were compared in the brain, heart, liver, and major mediastinal vessels of rats. Repeated 4 s spin echo images acquired after injection of 0.2 mmol/kg Gd-DTPA demonstrated a maximum enhancement between 15 and 25 s of 57% in brain, 307% in heart, 220% in liver, 83% in subcutaneous tissue, and 380% in slowly flowing blood in mediastinal vascular structures. In the following 55 s there was a continuous decrease (average 45%) in signal intensity in each tissue except brain. Albumin-(Gd-DTPA), injected at a four times lower molar dose (0.045 mmol/kg) with respect to Gd-DTPA, demonstrated maximal enhancement of brain by 34%, heart by 237%, liver by 186%, and blood in mediastinal vessels by 325%. Gadolinium-DTPA, which rapidly diffuses from the small vessels into the interstitial space, was noted to accumulate in solid tissues and subsequently to be partially eliminated within 70 s of administration. Signal enhancement achieved with albumin-(Gd-DTPA) remained at a constant level over the 70 s observation period. These data further support the notion that albumin-(Gd-DTPA), due to its predominantly intravascular distribution, might be applied advantageously for the assessment of perfusion and blood-volume disorders.

Contrast-enhancing properties of Gd-DTPA at 2.0 Tesla.

The contrast enhancing properties of Gd-DTPA in various tissues, including myocardium, liver, brain, skeletal muscle, and subcutis were investigated in vivo at 2.0 Tesla. Employing T1-weighted short pulse sequences (TR = 300 msec, TE = 14 msec) signal intensity of myocardium increased by 88%, brain by 19%, liver by 61%, skeletal muscle by 50%, and subcutis by 52% at 2 min after intravenous injection of 0.2 mmol/kg Gd-DTPA in rats. Signal intensity was highest at 2 min after injection and was relatively decreased at 20 min. Tissue intensities had returned to baseline at 60 min. In a separate experiment, fast (4 sec), repeated spin-echo image acquisitions allowed for assessment of the initial, dynamic distribution pattern of Gd-DTPA between 4 sec and 72 sec after intravenous injection. Tissue signal intensities peaked between eight and 16 seconds after injection and slowly decreased thereafter. This study shows that the contrast-enhancing properties of Gd-DTPA can be effectively assessed using T1-weighted short pulse sequences at 2.0 Tesla.

Comparison of the contrast-enhancing properties of albumin-(Gd-DTPA) and Gd-DTPA at 2.0 T: and experimental study in rats.

Albumin-(gadolinium-diethylenetriaminepentaacetic acid), albumin-(Gd-DTPA), a macromolecular MR contrast agent designed for intravascular distribution, was compared with gadolinium-diethylenetriaminepentaacetic acid (Gd-DTPA), an extracellular fluid agent, for imaging characteristics in normal rats at 2.0 T. Albumin-(Gd-DTPA) produced larger-intensity increases in myocardium (125%), liver (114%), and brain (21%), at a dose of 0.062 mmol Gd/kg than did Gd-DTPA at a dose of 0.2 mmol/kg. The duration and pattern of enhancement differed between the two compounds; the enhancement with albumin-(Gd-DTPA) persisted at relatively constant levels from 2 min to 1 hr. The Gd-DTPA tissue enhancement peaked at 2 min and had virtually disappeared at 60 min. Gd-DTPA better enhanced subcutaneous tissues, presumably because of capillary permeability and enhancement of the abundant extracellular fluid space. When albumin-(Gd-DTPA) is used, the strong persistent enhancement of the microvascular compartment is well demonstrated on subtracted images and supports potential application of albumin-(Gd-DTPA) for blood-volume and perfusion-dependent contrast enhancement of myocardium, liver, and brain.