Philipp Lengsfeld

Stability of Gadolinium-Based Magnetic Resonance Imaging Contrast Agents in Human Serum at 37°C

Objectives:Assessment of the complex stability and Gd3+ dissociation rate of all marketed gadolinium-based MRI contrast agents (GBCA) in human serum at pH 7.4 and 37°C. Methods and Results:The kinetic profiles of Gd3+ dissociation of GBCAs were determined by incubation for 15 days in human serum from healthy volunteers at a concentration of 1 mmol/L, pH 7.4, and 37°C. The initial rates of Gd3+ release and the amounts of Gd3+ released after 15 days were established by HPLC-ICP-MS analysis. In an attempt to simulate the situation in patients with end-stage renal disease who often have elevated serum phosphate levels, the influence of 10 mmol/L phosphate on Gd3+ dissociation was also investigated. The GBCAs were grouped and ranked in the following order according to their stabilities in native human serum at pH 7.4 and 37°C [% Gd3+ release after 15 days and initial rate (%/d) (95% confidence interval) in brackets]. Nonionic Linear GBCAs:Optimark [21 (19–22) %, 0.44 (0.40–0.51) %/d) and Omniscan [20 (17–20) %, 0.16 (0.15–0.17) %/d]. Ionic Linear GBCAs:Magnevist [1.9 (1.2–2.0) %, 0.16 (0.12–0.36) %/d], Multihance [1.9 (1.3–2.1) %, 0.18 (0.13–0.38) %/d], Vasovist [1.8 (1.4–1.9) %, 0.12 (0.11–0.18) %/d], and Primovist [1.1 (0.76–1.2) %, 0.07 (0.05–0.08) %/d]. Macrocyclic GBCAs:Gadovist, Prohance, and Dotarem (all < limit of quantification of 0.1%, <0.007%/d). In the presence of additional 10 mmol/L phosphate in serum, the initial Gd3+ release rates of the nonionic linear GBCAs, Omniscan, and Optimark increased about 100-fold, and, after 15 days, the amount of Gd3+ released from these agents was more than 75% higher than in native serum. The initial rates found for the ionic linear GBCAs increased about 12- to 30-fold, but, despite this, increase in the initial rate, the amount of Gd3+ released after 15 days was comparable to that in native serum. The elevated phosphate level did not lead to any measurable release of Gd3+ from the 3 macrocyclic GBCAs. Conclusions:The release of Gd3+ from all linear Gd complexes in human serum was several orders of magnitude greater than predicted by the conditional stability constants. After 15 days, release of Gd3+ from the nonionic linear GBCAs was about 10 times higher than from the ionic linear GBCAs. Elevated serum phosphate levels accelerated the release of Gd3+ from nonionic linear GBCAs and, to a lesser degree, from the ionic linear GBCAs. All 3 macrocyclic GBCAs remained stable in human serum at both normal and elevated phosphate levels.

Preclinical investigation to compare different gadolinium-based contrast agents regarding their propensity to release gadolinium in vivo and to trigger nephrogenic systemic fibrosis-like lesions.

Recent reports suggest that nephrogenic systemic fibrosis (NSF) is associated with the administration of gadolinium (Gd)-based contrast agents (GBCAs) and in particular with the stability of the Gd-complex. The aim of this investigation was to compare GBCAs and their potential to trigger NSF. Forty-two healthy male rats received repeated intravenous injections of six different GBCAs at high doses to simulate the exposure seen in patients with severe renal dysfunction. Histopathological and immunohistochemical analysis of the skin was performed, and the concentrations of Gd, zinc and copper were measured in several tissues by inductive coupled plasma atomic emission spectroscopy. Macroscopic and histological skin changes similar to those seen in NSF patients were only observed in rats receiving Omniscan. In addition, very high concentrations of Gd were observed in the animals treated with Omniscan, and, to a lesser extent, in animals treated with OptiMARK. Significantly lower levels of Gd were found after the treatment with ionic linear agents and even less after the treatment with macrocyclic agents. The data in this investigation strongly suggest that the stability of the Gd-complex is a key factor for the development of NSF-like symptoms in this experimental setting.

Gadolinium-based contrast agents and their potential role in the pathogenesis of nephrogenic systemic fibrosis: the role of excess ligand.

To investigate the role of excess ligand present in gadolinium (Gd) ‐based contrast agents in the development of nephrogenic systemic fibrosis (NSF). Using a dosing regimen to simulate the exposure seen in patients with severe renal impairment, we investigated the effect of excess ligand on Gd‐deposition and the depletion of endogenous ions.

Impact of renal impairment on long-term retention of gadolinium in the rodent skin following the administration of gadolinium-based contrast agents.

Objective:Several publications have suggested a possible association between Gd-based contrast agents (GBCAs) and the development of nephrogenic systemic fibrosis, a rare but serious disease. To date, nephrogenic systemic fibrosis has been observed only in patients with severe renal insufficiency.The aim of this study was to determine the impact of a prolonged circulation time of GBCAs caused by reduced renal clearance on the long-term retention of Gd in the skin of rats after administration of different GBCAs. Material and Methods:Renally impaired Han Wistar Rats (5/6-nephrectomized rats) were injected with Omniscan, OptiMARK, Magnevist, or Gadovist. The contrast agents were administered once daily for 5 consecutive days into the tail vein at a dose of 2.5 mmol Gd/kg b.w. Skin biopsies were taken at various time points, and the gadolinium (Gd) concentration was determined by inductive coupled plasma mass spectrometry (ICP-MS) over an observation period of 168 days post injection (p.i.). Results:Differences in the skin Gd concentrations were observed between the 4 investigated GBCAs. For the nonionic linear compounds, Omniscan and OptiMARK, high Gd concentrations were maintained in the skin over the observation period of up to 168 days p.i. For the ionic linear compound, Magnevist, comparatively lower Gd retention in the skin was observed over time. For the macrocyclic compound, Gadovist, the Gd values in the skin were even lower, and significantly lower than Gd values in the skin in Omniscan and OptiMARK treated animals. Conclusion:The results of this preclinical study support the use of 5/6-nephrectomized rats as a model for prolonged circulation time of GBCAs as seen in patients with severe renal impairment. Surgically induced severe renal impairment resulted in delayed clearance of the administered GBCAs in the study animals. The highest amount of Gd was observed in the skin after treatment with the nonionic linear GBCAs, whereas the lowest Gd values were observed after treatment with the macrocyclic agent. This suggests that the difference in the Gd values observed in rat skin tissue after treatment with the different GBCAs is caused of a different propensity of the different GBCAs to release Gd in vivo. However, the analytical method used does not distinguish between chelated and unchelated Gd.

Retention of iodine and expression of biomarkers for renal damage in the kidney after application of iodinated contrast media in rats.

Objective:Commercially available iodinated contrast media (CM) show significantly different physico-chemical properties. The relevance of the viscosity of CM may be underestimated as a contributing factor for clinically relevant renal failure as suggested by a large registry data analysis (Swedish registry study). The objective of this preclinical study is to assess differences of a low and high-viscous CM regarding their retention time in the kidney. Furthermore, we investigated the expression of marker genes for renal damage and hypoxia to evaluate a potential renal damage and hypoxia after application of iodinated CM. Material and Methods:After application of Iopromide 300 and Iodixanol 320 CM, the iodine concentration over time was determined using computed tomography and x-ray fluorescence analysis in healthy Han Wistar and renally impaired ZSF1 rats. The latter served as a model for age and diabetes-related renal impairment. X-ray attenuation (Hounsfield units) in the renal cortex was analyzed by 2 independent blinded readers. Furthermore, the expression of kidney injury molecule 1 (Kim-1/Havcr1) and heme oxygenase I (HO-1/HMOX1) was measured by quantitative reverse transcription-polymerase chain-reaction. Results:Computed tomography and x-ray fluorescence analysis in the kidneys of animals treated with Iodixanol revealed significantly prolonged retention of iodine in the kidney as compared with animals treated with Iopromide. This difference was even more pronounced in renally impaired rats. Twenty-four hours after Iodixanol treatment, significantly increased levels of Kim-1/Havcr1 and HO-1/HMOX1 transcript levels were observed compared with the saline and Iopromide treatment. Conclusions:A prolonged retention of contrast media in the kidney was observed after administration of dimeric CM (Iodixanol 320). One possible explanation for this effect could be the high viscosity of the dimeric CM (Iodixanol 320) and the lack of dilution by osmotic diuresis. This prolonged exposure is possibly associated with higher renal toxicity as indicated by the elevated expression of biomarkers for hypoxia and renal injury.

The impact of the viscosity and osmolality of iodine contrast agents on renal elimination.

Objective:The iodinated contrast agents (CAs) that are currently used in radiographic procedures possess special physicochemical properties and a high safety profile; however, according to a large retrospective study (Swedish registry), the viscosity of CAs may have an underestimated impact on renal failure. The aim of our study was to investigate the possible consequences of CA viscosity differences, such as CA retention in the kidney. Material and Methods:Five Göttingen minipigs were each intravenously injected in a crossover setting at intervals of at least 7 days with monomeric (Iopromide) and dimeric (Iodixanol) CAs at 2 doses (1 and 2 g iodine/kg bodyweight), and the retention of the CA in the kidneys was determined during the first 6 hours postinjection using a 64-slice computed tomography scanner. Additionally we performed in vitro dialysis of the monomeric and dimeric CAs across the various physiological osmolalities of the renal tubulus (300, 600, 800, and 1200 mOsm/kg H2O) to estimate CA viscosity in vivo. Following the dialyzes, iodine concentrations and CA viscosities were determined. Results:A different exposure of the kidneys to iodine and a different elimination kinetics from the kidneys was observed after the administration of monomeric and dimeric CAs. The monomeric agent was observed to clear from the kidney immediately after administration. In contrast, after administration of the dimeric CA an increase in iodine concentration in the kidney was observed up to 180 minutes postinjection, before the CA was observed to begin clearing; however, no difference was observed between the plasma half-lives of the 2 investigated CAs. In vitro dialysis of the dimeric CA increased iodine concentrations and strongly increased viscosity at all of the tested osmolalities. In contrast, the monomeric agent only demonstrated increases in iodine concentration and viscosity at 800 and 1200 mOsm/kg, and these changes were smaller than those observed for the dimeric CA. In summary, dialysis strongly enhanced the viscosity differences between the 2 investigated CAs. Conclusion:The viscosity differences between the investigated monomeric and dimeric CAs are strongly enhanced by concentration processes, such as the process taking place in the tubular system. These viscosity differences may be the cause of the prolonged retention and the different elimination kinetics from the kidney observed after application of the dimeric CA relative to the monomeric CA.

The influence of x-ray contrast agents in computed tomography on the induction of dicentrics and γ-H2AX foci in lymphocytes of human blood samples

The aim of this study was to investigate and quantify two biomarkers for radiation exposure (dicentrics and gamma-H2AX foci) in human lymphocytes after CT scans in the presence of an iodinated contrast agent. Blood samples from a healthy donor were exposed to CT scans in the absence or presence of iotrolan 300 at iodine concentrations of 5 or 50 mg ml(-1) blood. The samples were exposed to 0.025, 0.05, 0.1 and 1 Gy in a tissue equivalent body phantom. Chromosome aberration scoring and automated microscopic analysis of gamma-H2AX foci were performed in parts of the same samples. The theoretical physical dose enhancement factor (DEF) was calculated on the basis of the mass energy-absorption coefficients of iodine and blood and the photon energy spectrum of the CT tube. No significant differences in the yields of dicentrics and gamma-H2AX foci were observed in the absence or presence of 5 mg iodine ml(-1) blood up to 0.1 Gy, whereas at 1 Gy the yields were elevated for both biomarkers. At an iodine concentration of 50 mg ml(-1) serving as a positive control, a biological DEF of 9.5 +/- 1.4 and 2.3 +/- 0.5 was determined for dicentrics and gamma-H2AX foci, respectively. A physical DEF of 1.56 and 6.30 was calculated for 5 and 50 mg iodine ml(-1), respectively. Thus, it can be concluded that in the diagnostic dose range (radiation and contrast dose), no relevant biological dose-enhancing effect could be detected, whereas a clear biological dose-enhancing effect could be found for a contrast dose well outside the diagnostic CT range for the complete radiation dose range with both methods.

The osmolality of nonionic, iodinated contrast agents as an important factor for renal safety.

ObjectiveNonionic iodinated contrast agents (CAs) can be divided into monomeric, low-osmolar, and dimeric, iso-osmolar classes. In clinical practice, renal tolerance of CAs is a concern, especially in patients with impaired renal function. With regard to renal safety, we wanted to evaluate the role of osmolality and viscosity in renal tolerance. Material and MethodsWe generated a formulation (iodixanol/mannitol) consisting of the dimeric iodixanol with an osmolality of the monomeric iopromide. Male Han-Wistar rats were intravenously injected with low-osmolar iopromide 300, iso-osmolar iodixanol 320, and iodixanol/mannitol. Saline and diatrizoate were used as controls. A total number of 227 rats were used in the following experiments. We compared the impact of osmolality on renal iodine retention using computed tomography 2 and 24 hours postinjection (p.i.). The animals were killed 2, 24, and 72 hours after injection, and the kidneys were excised for further investigations. Changes in renal cell proliferation were analyzed by 5-bromo-2′-deoxyuridine incorporation 48 hours p.i. as a degree of tissue regeneration after induced injury. To specify potential renal injury, we quantified the expression of acute kidney injury (AKI) markers (kidney injury marker-1 [KIM-1], neutrophil gelatinase–associated lipocalin [NGAL], and plasminogen activator inhibitor-1 [PAI-1]) by quantitative real-time polymerase chain reaction. Furthermore, the kidneys were analyzed histologically, including immunofluorescence analysis. ResultsAfter intravenous application of the CAs into Han-Wistar rats, renal iodine concentration was increased (3-fold) for iodixanol 2 hours p.i. and iodine retention was detected to be prolonged 24 hours p.i. compared with iopromide injection (iodixanol, 520 ± 50 Hounsfield Units [HU] vs iopromide, 42 ± 5 HU). The higher iodine concentration 2 hours p.i. upon iodixanol injection was reduced almost to the level of iopromide when injecting iodixanol/mannitol (iopromide: 289 ± 68 HU vs iodixanol/mannitol: 343 ± 68 HU). In addition, iodixanol application induced increased renal cell proliferation (2.7-fold vs saline), indicating renal injury, which was significantly lower in iopromide-treated animals (1.6-fold vs saline). More detailed analysis of markers for AKI revealed that iodixanol significantly induced the expression of PAI-1 (7.7-fold at 2 hours) as well as KIM-1 (2.1-fold) and NGAL (3.2-fold) at 2 and 24 hours when compared with saline treatment. In contrast, the expression of markers for AKI was low after iopromide (1.4-fold NGAL, 1.7-fold PAI-1, KIM-1 not significant) and iodixanol/mannitol (1.6-fold NGAL, 2.6-fold PAI-1, KIM-1 not significant) injection. ConclusionThe present results clearly show that prolonged iodine retention and the enhanced expression of kidney injury markers are caused mainly by the explicitly higher urine viscosity induced by iodixanol. We conclude that the osmolality of low-osmolar CAs such as iopromide induces a positive diuretic effect that is responsible for rapid iodine clearance and prevents increased expression of acute injury markers in the kidney.

Absolute Quantification of Regional Renal Blood Flow in Swine by Dynamic Contrast-Enhanced Magnetic Resonance Imaging Using a Blood Pool Contrast Agent

Aim:To evaluate for the first time in an animal model the possibility of absolute regional quantification of renal medullary and cortical perfusion by dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) using a blood pool contrast agent. Material and Methods:A total of 18 adult female pigs (age, 16–22 weeks; body weight, 45–65 kg; no dietary restrictions) were investigated by DCE-MRI. Absolute renal blood flow (RBF) measured by an ultrasound transit time flow probe around the renal vein was used as the standard of reference. An inflatable stainless cuff placed around the renal artery near its origin from the abdominal aorta was used to reduce RBF to 60%, 40%, and 20% of the baseline flow. The last measurement was performed with the cuff fully reopened. Absolute RBF values during these 4 perfusion states were compared with the results of DCE-MRI performed on a 1.5-T scanner with an 8-channel phased-array surface coil. All scans were acquired in breath-hold technique in the coronal plane using a field of view of 460 mm.Each dynamic scan commenced with a set of five 3D T1-weighted gradient echo sequences with different flip angles (&agr; = 2°, 5°, 10°, 20°, 30°): TE, 0.88 milliseconds; TR, 2.65 milliseconds; slice thickness, 8.8 mm for 4 slices; acquisition matrix, 128 × 128; and acquisitions, 4. These data served to calculate 3D intrinsic longitudinal relaxation rate maps (R10) and magnetization (M0). Immediately after these images, the dynamic 3D T1-weighted gradient echo images were acquired with the same parameters and a constant &agr; = 30°, half Fourier, 1 acquisition, 64 frames, a time interval of 1.65 seconds between each frame, and a total duration of 105.6. Three milliliters of an albumin-binding blood pool contrast agent (0.25 mmol/mL gadofosveset trisodium, Vasovist, Bayer Schering Pharma AG, Berlin, Germany) was injected at a rate of 3 mL/s. Perfusion was calculated using the arterial input function from the aorta, which was extracted from the dynamic relaxation rate change maps and perfusion images were calculated on a voxel-by-voxel basis using a singular value decomposition. Results:In 11 pigs, 4 different perfusion states were investigated sequentially. The reduced kidney perfusion measured by ultrasound highly correlated with total renal blood flow determined by DCE-MRI, P < 0.001. The correlation coefficient between both measurements was 0.843. Regional cortical and medullary renal flow was also highly correlated (r = 0.77/0.78, P < 0.001) with the degree of flow reduction. Perfusion values smaller than 50 mL/min/100 cm3 were overestimated by MRI, high perfusion values slightly underestimated. Conclusion:DCE-MRI using a blood pool contrast agent allows absolute quantification of total kidney perfusion as well as separate determination of cortical and medullary flow. The results show that our technique has sufficient accuracy and reproducibility to be transferred to the clinical setting.

Impact of iso- and low-osmolar iodinated contrast agents on BOLD and diffusion MRI in swine kidneys.

Purpose:To assess whether functional MR imaging using blood-oxygenation level-dependent (BOLD) imaging and diffusion-weighted imaging demonstrate changes in renal oxygenation and apparent diffusion coefficient (ADC) in a pig model. Materials and Methods:After administration of either 1-g iodine/kg body weight low-osmolar iopromide or iso-osmolar iodixanol, 8 mini pigs underwent a series of repeated BOLD measurements (TR/TE 106/5.9–48.7 ms, slice thickness 4 mm) and diffusion-weighted imaging measurements (TR/TE 3900/79 ms, slice thickness 4 mm) for 1 hour at 1.5 T. In this intraindividual cross over study, the second contrast agent injection with the other iodinated contrast agent was performed at least 24 hours after the initial contrast agent injection. BOLD-based R2* values as indirect measures of the renal oxygenation were determined for the cortex, the inner medulla, and the outer medulla. ADC values were measured for the cortex and the whole kidney. Results:For both contrast agents, a drop in R2* was found in the cortex, which normalized after 55 minutes. In the outer medulla and particularly in the inner medulla, a decreased initial drop of R2* was encountered with both contrast agents, with a slow increase toward the baseline R2*. In the inner medulla, elevated R2* values were found with the iso-osmolar contrast agent only. The ADC revealed an initial increase, which slowly decayed over the measurement period. This finding was more pronounced for the cortex compared with the whole-kidney analysis. Conclusion:Functional MR imaging of the kidneys demonstrates increased R2* in the inner medulla only after the administration of the iodixanol potentially indicating hypoxia and thus a pathomechanism of contrast-induced nephropathy.