Synthesis, characterization, and imaging performance of a new class of macrocyclic hepatobiliary MR contrast agents.

RATIONALE AND OBJECTIVES: To investigate the effect of substituent lipophilicity, substituent position, and overall charge on the hepatobiliary clearance and tolerance of a series of aromatic ring-containing macrocyclic Gd chelates to select a candidate compound for evaluation as a hepatobiliary imaging agent. METHODS: Hepatobiliary clearance was studied in rats. Tissue distribution and tolerance were studied in mice. Imaging was performed in cats, rabbits, and Rhesus monkeys using T1-weighted pulse sequences or T1-weighted breath-hold pulse sequences. RESULTS: All the compounds were excreted bimodally. Gd-2,5-BPA-DO3A (15d) was found to have the optimal combination of hepatobiliary clearance (47% in rats, 29% in mice) and tolerance (minimum lethal dose 5.0 mmol/kg). Initial imaging studies in cats demonstrated the feasibility of Gd-2,5-BPA-DO3A for hepatic imaging. In rabbits with implanted VX-2 adenocarcinoma as a model for metastatic liver disease, Gd-2,5-BPA-DO3A provided sustained hepatic signal intensity (SI) enhancement and lesion conspicuity over a 120-minute imaging time course. In Rhesus monkeys with normal liver function, Gd-2,5-BPA-DO3A afforded sustained hepatic SI enhancement and a time-dependent increase in gallbladder SI over the entire 90-minute imaging time course. CONCLUSIONS: Gd-2,5-BPA-DO3A provides dramatic and sustained SI enhancement of hepatic tissue in cats, rabbits, and Rhesus monkeys that was superior in all respects to the extracellular space MRI agent, Gd-HP-DO3A, that was employed as a control.

Caenorhabditis elegans as an alternative animal species.

Caenorhabditis elegans has proven useful in toxicity testing of known toxicants, but its potential for assessing the toxicity of new pharmaceuticals is relatively unexplored. In this study the procedures used in aquatic testing of toxicants were modified to permit testing of small amounts (<40 mg) of gadolinium-based magnetic resonance imaging (MRI) compounds. Five blinded compounds were tested. The toxicity of these compounds determined using C. elegans was compared to existing mammalian test system data (minimum lethal dose [MLD] values for mice). Four of five compounds tested had the same relative sensitivity with C. elegans as with the mouse test system. Testing with C. elegans is efficient and could markedly reduce the cost of screening potentially useful compounds.

Utilization of the nephrectomized mouse for determining threshold effects of MRI contrast agents.

The tissue concentration of an extravascularly distributed MRI contrast agent required to achieve a 20% change in the MRI signal intensity (SI) of skeletal muscle was determined using radiolabeled gadoteridol administered to nephrectomized mice. This minimal change in the quantified SI was reliably detected qualitatively in the MR muscle images. MR images of muscle were acquired following each intravenous injection of six sequential doses of 0.8 micromol of 153Gd-labeled gadoteridol. A 2.0 T imaging spectrometer and a T1-weighted spin-echo pulse sequence were used to acquire the MR images. After imaging, the injected 153Gd in muscle was measured, and the 153Gd assay results were used to determine the gadoteridol concentration in muscle following each injection. The muscle concentrations of gadoteridol were then correlated to the quantified enhanced MR SI of muscle. Using the 20% factor, it was concluded that the amount of gadoteridol necessary to achieve a reliable change in the SI of muscle was 33+/-10 nmol/g-skeletal muscle.

A noninvasive method for monitoring renal status at bedside.

RATIONALE AND OBJECTIVES: The authors demonstrate the feasibility of monitoring renal status continuously and noninvasively at a patient's bedside, avoiding both radioactivity and blood and urine samples. METHODS: Gadolinium-153-labeled ProHance and a glomerular filtration rate (GFR) standard technetium-99m-DTPA were coadministered to anesthetized normal and nephrectomized rats with their tails hanging in a PC 20 spin analyzer. Blood samples and T1 measurements were collected and analyzed. RESULTS: Log time plots of 153Gd, 99mTc (from blood samples) and T1 of the rat tails were all linear and parallel. Halftimes were 32 +/- 2, 32 +/- 6, and 32 +/- 6 minutes for the decay of the T1, 153Gd and 99mTc, respectively. The halftime of the nephrectomized animal was 2000 +/- 4000 minutes. CONCLUSIONS: T1 of an appendage remote from the kidneys reflects the concentration of gadolinium in the blood, which is in rapid equilibrium with tissue interstitial space gadolinium. The decay in T1 of the appendage reflects glomerular filtration. Thus, it is feasible to detect changes in renal status at a patient's bedside by monitoring T1 of a finger or wrist using a small, inexpensive magnet.

Biodistribution of radiolabeled, formulated gadopentetate, gadoteridol, gadoterate, and gadodiamide in mice and rats.

RATIONALE AND OBJECTIVES: The authors studied the long-term distribution of gadolinium (Gd) in mice and rats after the administration of four commercially available magnetic resonance imaging contrast media. The goals were to determine any possible product dissociation in vivo and to evaluate the effects that product excipients had on the tissue distributions. METHODS: Gadolinium-153 (153Gd)-labeled gadopentetate (Magnevist), gadoteridol (ProHance), gadoterate (Dotarem), and gadodiamide (Omniscan) were administered intravenously to mice (0.48 mmol/kg) and rats (0.1 mmol/kg). At various times up to 14 days posttreatment, the residual 153Gd was measured in selected tissues. The tissue distributions obtained were used to make intra- and interchelate distribution evaluations and comparisons regarding tissue clearance and any possible in vivo dissociation of the Gd chelates. RESULTS: Differences were found among the chelates studied relative to the amounts of residual 153Gd present in tissues known to sequester free Gd, particularly in liver and femur at 7 and 14 days after administration, in both mice and rats. The pattern of the 153Gd distribution suggested that the linear chelates, gadopentetate and gadodiamide, dissociated in vivo resulting in more 153Gd present in bone and liver at the longer residence times than in the subjects injected with the macrocyclic chelates, gadoteridol and gadoterate. The only excipient found to affect the distribution profile was calcium(DTPA-BMA); this excipient in formulated gadodiamide decreased the amounts of residual Gd measured in whole body, bone, and liver in mice compared with levels obtained when gadodiamide was injected alone. CONCLUSIONS: The molecular feature found to be most important in differentiating the four chelates evaluated is the presence or absence of a macrocyclic structure. The Gd chelates containing this structure, gadoteridol and gadoterate, have the lowest residual Gd at long residence times in both mice and rats. The order of residual whole body Gd at 14 days (lowest to highest) was: gadoteridol integral of gadoterate < or = gadopentetate << gadodiamide. The only excipient that affected the biodistribution was found in the gadodiamide formulation where the addition of 5% calcium (DTPA-BMA) reduced residual Gd to just less than 10 times greater than that found for the macrocyclic chelates with the lowest residual Gd, gadoteridol and gadoterate.

MRI evaluation of potential gastrointestinal contrast media.

Diluted ProHance [Gd(HP-DO3A), Squibb Diagnostics, Princeton, NJ], Sustacal (Meadjohnson, Evansville, IN), a nutritional drink, and a ProHance/Sustacal mixture have been investigated as potential oral contrast agents. At 2 T, T1-weighted (SE 500/20) images demonstrated hyperintense (positive) signal enhancement of rat GI tracts within 10 min after the ingestion of 2.0 mM Gd(HP-DO3A) or 2.0 mM ProHance/Sustacal. T2-weighted (SE 3000/80) images demonstrated hypointense (negative) signal intensity within 10 min after ingestion of 10 mM ProHance. Medical imaging applications of these oral contrast media are feasible.

Sources of heterogeneous contrast enhancement in the gastrointestinal tract.

Water soluble gadolinium chelates demonstrate heterogeneous enhancement of the gastrointestinal tract (GI) when administered orally. To investigate the causes, ProHance (2.0 mM) was administered orally to rats. There was a dramatic enhancement of rat GI lumen signal intensity in T1-weighted MR images which provided increased contrast relative to the adjacent abdominal tissues. Heterogeneity of MRI signal enhancement along the rat GI tract was investigated by sampling rat GI fluid at various times post-ingestion and at different locations along the GI tract. The corresponding T1 and T2 relaxation times, Gd concentrations, and viscosities of each GI fluid sample revealed that changes in each of these parameters contribute to the observed heterogeneity of MRI signal enhancement.

Dose-dependent biodistribution of [153Gd]Gd(acetate)n in mice.

[153Gd]Gd(acetate)n was administered i.v. to mice to study the effect of dose on the distribution of free Gd. Distribution from blood was slow with the majority of the Gd distributing in the liver. Gd saturated in bone. Heart, lungs, kidneys, brain and skeletal muscle exhibited time-dependent decreases in Gd concentration. Gd that washed out of heart, lungs, kidneys and/or muscle redistributed in liver, spleen and femur. These results indicate a complex dose- and time-dependent tissue distribution for Gd and emphasize the importance of eliminating unchelated free Gd as a contaminant in Gd-chelates before testing in biodistribution experiments. The long-term residual accumulation of Gd suggests the need to minimize Gd-chelate dissociation in vivo.

A multi-organ, axially distributed model of capillary permeability for a magnetic resonance imaging contrast agent.

Capillary permeability was resolved from uptake data for eight rat organs with gadoteridol, which is a stable, well-tolerated, nonionic, highly water soluble, gadolinium-containing, magnetic resonance imaging contrast agent. The extracellular kinetics were elucidated with an axially distributed, plasma interstitial fluid model and measured plasma flow, organ plasma volume, and interstitial fluid volume. The molecular and biological properties of gadoteridol and this kinetic model provide magnetic resonance imaging with a tool to begin measuring physiologic processes.

Dissociation of gadolinium chelates in mice: relationship to chemical characteristics.

Tissue distributions of seven 153Gd-labeled Gd chelates were determined at five residence intervals (5 min to 14 days) following intravenous administration of 0.4 mmol/kg to mice. Relationships were sought among physicochemical parameters: thermodynamic and conditional (pH 7.4) equilibrium stability constants (log K and log K'), acid dissociation rate constants (k(obs)), lipophilicity (log P), overall charge, and size (molecular weight). Size and lipophilicity did not correlate with tissue distributions. There were possible correlations between anionic charge and rapid, early renal excretion and between stability constants and long-term residual Gd deposition. Strong correlations (r greater than 0.99) were found between acid dissociation rates and long-term deposition of Gd in the whole body, liver, and femur. This is attributed to dissociation of Gd from the chelates in vivo. Acid dissociation rates may be useful in predicting dissociation of Gd from chelates in vivo.

Quantitative dependence of MR signal intensity on tissue concentration of Gd(HP-DO3A) in the nephrectomized rat.

Cardiac-gated SE 20/224 +/- 20 MR images were obtained from nephrectomized rats before and after intravenously administering 153Gd-Gd(HP-DO3A). The concentration of Gd, [Gd], was linear in dose in myocardium, skeletal muscle, and blood. Under steady-state conditions, where d[Gd]/dt = 0, image intensities (IIN) in regions of interest were compared with the measured [Gd]. IIN was linear in myocardium at less than or equal to 0.61 mumol/g-myocardium (less than or equal to 0.5 mmol/kg dose) and in skeletal muscle at less than or equal to 0.63 mumol/g-muscle (less than or equal to 0.75 mmol/kg). Above 0.6 mumol Gd/g-tissue, IIN did not increase further. The in vivo data were consistent with measured ex vivo and in vivo relaxivities. A 29% greater slope for IIN versus [Gd] in myocardium [14,439 +/- 4350 IIN (mumol/g)] than in muscle [10,258 +/- 5,296 IIN/(mumol/g)] was attributed to a significant difference in blood content: 25% versus 2% weight blood in myocardium and skeletal muscle, respectively. Two components were apparent from plots of ex vivo 1/T1 versus [Gd] in myocardium and muscle, and only one for blood.

Pharmacokinetic analysis of blood distribution of intravenously administered 153Gd-labeled Gd(DTPA)2- and 99mTc(DTPA) in rats.

Rat plasma distribution data obtained following IV administration of 99mTc(DTPA) alone or after co-administration of 99mTc(DTPA) and 153Gd-labeled Gd(DTPA)2- at 0.001, 0.1, and 1.0 mmol Gd/kg were evaluated using compartmental modeling techniques. A three-compartment open model was found to fit the data significantly better (P less than 0.01) than a two- or four-compartment open model. This model incorporates and links the plasma and urine data and includes a delay to account for the transit time through the kidneys/ureters. The two nonplasma compartments of the model were assumed to be related to rapidly and slowly equilibrating tissues. Tc(DTPA) and Gd(DTPA)2- had nearly identical pharmacokinetic profiles in plasma and the rate constants were essentially the same. No significant dose dependent pharmacokinetic differences were found for the range of Gd(DTPA)2- doses tested. Simulations of the proposed three-compartment model were used to generate concentration-time curves for each of the three compartments.

A neutral technetium-99m complex for myocardial imaging.

Technetium-99m-CDO-MeB [Bis[1,2-cyclohexanedione-dioximato(1-)- O]-[1,2-cyclohexanedione dioximato(2-)-O]methyl-borato(2-)- N,N',N'',N''',N'''',N''''')-chlorotechnetium) belongs to a family of compounds generally known as boronic acid adducts of technetium dioxime complexes (BATOs). It has an intrinsic affinity for the myocardium, with negligible lung activity and rapid blood clearance. The uptake of 3.44% ID in rat heart at 1 min postinjection for [99mTc]CDO-MeB versus 3.03% for 201TI indicates high extraction of [99mTc]CDO-MeB by the myocardium. In dogs an ischemic defect is clearly seen in SPECT images obtained 10 min after injection of [99mTc]CDO-MeB. Tissue distribution data in rats show that [99mTc]CDO-MeB is excreted primarily in the feces and to a lesser extent in the urine. Approximately 80% of the activity is excreted within 24 hr after injection.

Comparative chemical structure and pharmacokinetics of MRI contrast agents.

The blood clearance kinetics of five gadolinium complexes, Gd(L), were determined in rats and the results interpreted in terms of an open two-compartment pharmacokinetic model. The complexes were tested in vitro for stability in serum and in aqueous solutions of ions that they might encounter in vivo and that might be expected to react with the Gd(L) complexes to produce uncomplexed gadolinium. Reaction with serum was observed in two instances. Chemical structural differences among the chelating ligands appear to govern the overall reactivity of their Gd(L) complexes. It may be inferred from the results that a preferred structural feature of the ligand is the presence of a 12-membered 1,4,7,10-tetraaza macrocycle.

Comparison of the biodistribution of 153Gd-labeled Gd(DTPA)2-, Gd(DOTA)-, and Gd(acetate)n in mice.

The biodistributions of 153Gd-labeled Gd(DTPA)2-, Gd(DOTA)-, and Gd(acetate)n were determined in mice at five residence times. Gd(DTPA)2- and Gd(DOTA)- had similar distributions with greater than 89% renally excreted in 1 h. At 7 days and longer after Gd(DTPA)2- administration, 153Gd bone levels were higher than could be attributed to free 153Gd3+, suggesting that Gd(DTPA)2- dechelates in vivo. Gd(DOTA)- did not appear to dechelate. Gd(acetate)n did not readily clear and 153Gd levels remained high in liver, lungs, and spleen for 28 days.