Gabriele Schuhmann-Giampieri

In vivo and in vitro evaluation of Gd-DTPA-polylysine as a macromolecular contrast agent for magnetic resonance imaging

Polylysine covalently linked to moieties of gadopentetate (Gd-DTPA), for use as a macromolecular blood pool marker for contrast material-enhanced magnetic resonance imaging (MRI), was characterized by means of physicochemical measurements and pharmacokinetics in rats and rabbits and compared with Gd-DTPA. Gd-DTPA-polylysine was composed of a series of polymers of different molecular sizes that on average were labeled with 60 to 70 Gd-DTPA moieties (average molecular weight, 48,700 daltons [D]). For the macromolecular compound Gd-DTPA-polylysine, relaxivity was three times higher than that of Gd-DTPA. The LD50 value of 17 mmol/kg reflects a fairly high acute intravenous tolerance of the macromolecular compound in mice. Even though the volume of distribution of Gd-DTPA-polylysine in rabbits approached the extracellular fluid space (indicating that the macromolecular compound was also leaking slowly into the interstitial space), the half-life of distribution of the macromolecular compound in the extracellular fluid space was significantly prolonged, thus making the compound suitable as a blood pool marker for MRI. In rats the elimination of Gd-DTPA-polylysine occurred predominantly via the renal route. High-pressure liquid chromatography-size-exclusion chromatography of the fractionated urine samples revealed that the renal clearance must be the integral sum of the separate clearances of each molecular weight species. No biodegradation of the polypeptide was observed, and biodistribution studies revealed only minimal retention of Gd in the body of the rat.

PHARMACOKINETICS, DOSE PROPORTIONALITY, AND TOLERABILITY OF GADOBUTROL AFTER SINGLE INTRAVENOUS INJECTION IN HEALTHY VOLUNTEERS

RATIONALE AND OBJECTIVES.Gadobutrol is a new gadolinium-based hydrophilic and neutral macrocyclic contrast medium for magnetic resonance imaging. In this article, the authors report on the first application of gadobutrol in humans, up to a dose of 0.5 mmol/kg. METHODS.Gadobutrol was investigated after single intravenous administration in two phase-1 studies testing low (0.5 mol/L) and high concentrations (1 mol/L) in healthy, male volunteers using a double-blind, randomized, placebo-controlled study with n = 55 for the low concentration (0.04, 0.1, 0.2, 0.3, and 0.4 mmol/kg body weight), followed by n = 36 for the high concentration (0.3, 0.4, and 0.5 mmol/kg body weight). Vital signs and laboratory parameters were measured for all dose groups investigated, whereas for the calculation of the pharmacokinetic parameters, the dose groups 0.04, 0.1, and 0.4 mmol/kg body weight were selected. RESULTS.Gadobutrol was well tolerated up to doses of 0.5 mmol/kg, and no relevant changes in vital signs and laboratory parameters occurred. The terminal disposition half-life of gadobutrol in plasma was approximately 1.5 hours. Total clearance approximated renal clearance and approximated the value of 120 mL/min, indicating glomerular filtration as the main pathway of elimination. The steady-state volume of distribution indicated predominantly extracellular distribution of gadobutrol. No metabolites were detected. The renal excretion rate was linear over the large dose range tested, indicating dose-proportionate, first-order kinetics of gadobutrol. CONCLUSION.Single intravenous administration of gadobutrol was well tolerated up to the dose level of 0.5 mmol/kg body weight. These factors suggest that gadobutrol will be a safe magnetic resonance imaging contrast agent.

PRE-CLINICAL EVALUATION OF GADOBUTROL: A NEW, NEUTRAL, EXTRACELLULAR CONTRAST AGENT FOR MAGNETIC RESONANCE IMAGING

The Gd(3+)-complex of 10-(2,3-dihydroxy-1-hydroxymethylpropyl)-1,4,7,10-tetraazacyclo dodecane-1,4,7-triacetic acid(gadobutrol) is a new, neutral Gd-chelate for use as an extracellular contrast agent in magnetic resonance imaging (MRI). The blood level in dogs after intravenous (i.v.) injection decreased with a terminal half-life of about 45 min, the clearance was about 3.75 ml/min per kg and the distribution volume of 0.23 l/kg suggested an extracellular distribution. Biodistribution experiments in rats revealed that only a very small amount (0.16%) of the dose was left in the body 7 days after i.v. injection. Measurable amounts of Gd could be detected only in the liver, kidneys and bones. The osmolality (0.57 osmol/kg at 0.5 mol/l and 1.39 osmol/kg at 1 mol/l) is in the range of other low osmolality contrast media for MRI. Only very little interaction with biologically relevant molecules was suggested by a histamine release test and a lysozyme inhibition test. An i.v.-LD50 of 23 mmol/kg in mice combined with a comparatively high T1-relaxivity (5.6 l/mmol per s at 0.47 T and 6.1 l/mmol per s at 2 T) in plasma promises a high margin of safety. In preliminary imaging experiments, gadobutrol caused high enhancement in different lesions (cerebral infarct, brain tumor) of the rat. Tripling of the typical clinical dose of 0.1 mmol/kg was shown to provide additional diagnostic gain in lesions of this type.

Liver Contrast Media for Magnetic Resonance Imaging: Interrelations between Pharmacokinetics and Imaging

In the development of liver CM for MRI, three mainstream approaches have been undertaken: targeting of water-soluble MRI-CM to the hepatocytes, targeting of particles to the Kupffer cells of the liver, and application of macromolecular CM to tumorous tissue. As with the biliary iodinated CM, the physiological function of the liver has been used to target paramagnetic chelates (T1 agents) to the hepatocytes. Gd-EOB-DTPA and Gd-BOPTA are taken up mainly by hepatocytes and excreted into the bile by organic anion transporter (bilirubin transporter), whereas MnDPDP also uses the ability of hepatocytes to excrete metal ions, such as manganese. However, unlike the biliary iodinated CM, besides the specific accumulation in the hepatocytes, the low binding to plasma proteins and the high sensitivity of MRI, combined with the strong increase in relaxivity inside the hepatocellular environment, make the paramagnetic chelates very effective in the detection of liver lesions. Targeting of T2 agents (e.g. SPIO) to the Kupffer cells of the liver also has proved to be very effective in liver lesion detection. However, limited information is available regarding the pharmacokinetics of these particles in man and other problems, such as cardiovascular tolerance and manufacturing, must be overcome before widespread use of particulate CM can be implemented. The third approach is based on the differences in the vessel permeability, the vessel density, and functional lymphatics between normal and tumorous liver tissue when macromolecular CM are administered. This approach, however, is at an early research stage.

CHARACTERIZATION OF IOPROMIDE LIPOSOMES

RATIONALE AND OBJECTIVES.Iopromide-carrying liposomes were prepared and were characterized pharmaceutically and biologically. METHODS.The liposomes were prepared by the ethanol evaporation method and were characterized by quasi-elastic light scattering (size) and equilibrium dialysis (encapsulation efficiency and stability). Acute and subchronic toxicity was tested in mice and/or rats and cardiovascular tolerance in rabbits. Pharmacokinetic parameters were determined in rats. Computed tomography (CT) imaging efficiency was obtained from rat and rabbit studies. RESULTS.The mean diameter was 0.5 ± 0.1 μm and the encapsulation efficiency ranged between 30% and 40%. The liposomes were stable in human and rabbit plasma for approximately 24 hours. The LD50 in mouse and rat was approximately 3 g iodine/kg. In a subchronic toxicity study in rats with six doses of 1 g iodine/kg given every three days, no adverse effects were observed. The pharmacokinetics in rats were dose-dependent. Increasing the dose resulted in lower total clearance, and longer terminal half-life. Elimination of iodine was complete and the main route of excretion was via the kidneys. A clinically relevant computed tomography enhancement of the liver was reached after approximately 200 mg iodine/kg in rats and 150 mg iodine/kg in rabbits. CONCLUSIONS.The iopromide-carrying liposomes were well tolerated in animal studies and seemed to be suitable for the imaging of the liver.

EVALUATION OF GADOBUTROL IN A RABBIT MODEL AS A NEW LANTHANIDE CONTRAST AGENT FOR COMPUTED TOMOGRAPHY

RATIONALE AND OBJECTIVES.The efficacy of the neutral lanthanide contrast agent gadobutrol was compared to that of the iodinated contrast agent iopromide in rabbits. METHODS.The computed tomography (CT) attenuation of increasing concentrations of gadolinium (Gd) (gadobutrol) and iodine (I) (iopromide) was measured in Hounsfield units (HU) in aqueous solution at 80,120, and 137 kV. The peak enhancement (net increase in CT attenuation compared with baseline) and the time-enhancement product in the aorta and in the renal parenchyma of the outer and inner cortex were measured in rabbits over a 5-minute period after the animals were given single intravenous injections of 0.7,1.0, and 1.5 mmol Gd/kg of gadobutrol and 1.0 and 2.4 mmol I/kg of iopromide. RESULTS.In vitro, the CT attenuation of gadolinium was 40% higher than that of iodine at equivalent mass concentrations (120 kV). The mean peak enhancements in the aorta after the injections of 0.7,1.0, and 1.5 mmol Gd/kg and 1.0 and 2.4 mmol I/kg were 216,313,591, 224, and 498 HU, respectively. In addition, a 30-second injection of the high dose of gadobutrol resulted in an attenuation profile that was suitable for a three-dimensional reconstruction of the aorta and the renal vasculature. CONCLUSIONS.Because of the higher CT attenuation of gadolinium compared with that of iodine, the neutral macrocyclic chelate gadobutrol is a more effective contrast agent than iopromide for CT at lower doses of the imaging atom.

Ytterbium- and dysprosium-EOB-DTPA. A new prototype of liver-specific contrast agents for computed tomography.

RATIONALE AND OBJECTIVES A series of studies was conducted to determine whether metal complexes of the EOB-DTPA type are useful as contrast agents for computed tomography (CT). METHODS Metal complexes using EOB-DTPA as ligand were synthesized with lanthanide metal ions (lanthanum [La], cerium [Ce], praseodyme [Pr], gadolinium [Gd], dysprosium [Dy], ytterbium [Yb], and lutetium [Lu]) and with nonlanthanides (lead [Pb] and bismuth [Bi]). Complex stability was assessed by measuring binding to bone meal. The physicochemical parameters partition coefficient, osmolality, viscosity, and protein binding were determined in vitro. Tolerability was tested both in vitro (thromboplastin time, effect on erythrocytes) and in vivo (acute, neural, and cardiovascular toxicities). Biliary excretion and tissue distribution, especially liver, kidney, and bone concentrations, were measured in rats after intravenous doses of 0.5 mmol/kg. Imaging performance using CT was investigated in vitro in a phantom model and, for Gd-EOB-DTPA, in vivo by injecting doses of 0.5 mmol/kg into healthy or tumor-bearing rats and rabbits. RESULTS The kinetic stability of M-EOB-DTPA complexes differed widely. Nonlanthanide metals, especially Pb-EOB-DTPA, provided less stable complexes than lanthanides with an optimum of stability for the metals Gd, Dy, Yb, and Lu. Tolerability was good for all compounds, best results were obtained for Gd and Yb. Concentrations in rat liver after administration of Gd-EOB-DTPA, 0.5 mmol/kg intravenous, were approximately 1 mumol/g, resulting in CT enhancement of 16 Hounsfield units (HU). Tumor tissue was not enhanced. In rabbits, at the same dose level 30 HU was found. CONCLUSIONS Metal complexes of the EOB-DTPA type, especially those of Gd and Yb seem to be useful as iodine-free liver-specific contrast agents for CT.

Dose proportionality of iopromide pharmacokinetics and tolerability after IV injection in healthy volunteers

Twelve healthy male volunteers participated in a single-blind, randomised, placebo-controlled cross-over study of IV iopromide in doses of 15 g iodine or 80 g iodine infused over a period of 15 min. The volunteers were observed for three days during which time blood samples, urine and faeces were collected.The terminal disposition phase half-life of iopromide was 2 h and 1.9 h, and the total clearance was 110 and 103 ml·min-1 at the lower and at the higher dose levels, respectively. The steady state volume of distribution was 16 and 17 l, indicating predominantly extracellular distribution of iopromide.Statistical analysis (one-sided t-test) showed that all the target parameters (AUC, half-life and urinary excretion) were equivalent at both dose levels, indicating dose proportionate, first order kinetics of iopromide over the large dose range tested. Iopromide was well tolerated after both doses.

Characterization of Iopromide-Carrying Liposomes

AbstractIopromide-carrying liposomes were prepared by the ethanol evaporation method and pharmacokinetic parameters and CT imaging efficiency were determined in rats and rabbits. The mean diameter of the liposomes was 0.5±0.1 urn and the encapsulation efficiency was between 30 and 40%. The liposomes were stable in human, bovine, dog, pig, rat and rabbit plasma for more than 6 h. The pharmacokinetics in rats and rabbits were dose-dependent. Increasing the dose resulted in lower total clearance, and longer terminal half-life. Elimination of iodine was complete and the main route of excretion was via the kidneys. A clinically relevant CT enhancement of the liver was reached after 200 mg iodine/kg in rat and 150 mg iodine/kg in rabbit.

Pharmaceutical properties, biodistribution, and imaging characteristics of manganese-mesoporphyrin. A potential hepatobiliary contrast agent for magnetic resonance imaging.

OBJECTIVES.Manganese (III) mesoporphyrin (Mn-mesoporphyrin) was investigated for its pharmaceutical properties and magnetic resonance imaging characteristics as a potential hepatobiliary contrast agent. METHODS.Solubility, partition coefficient, plasma binding, proton relaxation enhancement, biodistribution, biliary excretion, liver extraction ratio, and liver enhancement were measured in various in-vitro and in-vivo systems. RESULTS.Mn-mesoporphyrin was soluble and stable at moderate alkaline pH in phosphate buffer. The octanol/water coefficient was 25.98, and the compound was highly protein bound. R1 for water and plasma were 1.94 and 2.35 L/mmol sec, respectively. R1 in liver was calculated to be 15.72 L/mmol sec. Biodistribution studies in rats and mice confirmed hepatotrophic properties and biliary excretion was 65% over 24 hours. First pass liver uptake was 15%. Magnetic resonance imaging studies showed persistent liver enhancement at 0.05 mmol/kg. CONCLUSION.Mn-mesoporphyrin is a lipophilic compound that shows potential as a hepatobiliary magnetic resonance contrast agent.