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Gadolinium-based contrast agents (GBCA) are intravenously-administered drugs used in diagnostic imaging procedures to enhance the quality of magnetic resonance imaging (MRI) and are commonly used for enhancement of vessels as in MR angiography (MRA) or for solid tumor enhancement, including brain tumors associated with the degradation of the blood-brain barrier.
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Gadolinium (Gd)-based contrast agents have not proved safer than the iodinated hydrophilic radiocontrast agents used in X-ray radiography or computed tomography; and because these gadolinium contrast agents pass the blood-brain barrier, increased awareness has focused on their toxicity. According to the FDA, patients who have received four or more MRIs involving Gd-based contrast agents showed traces of Gd in brain tissue. Consequently, the FDA revised its class warnings for all gadolinium-based contrast media and advised that the use of gadolinium-contrast agents must be based on careful consideration. As of now, extra care must be taken with patients requiring multiple doses, including pregnant and pediatric patients, and those with inflammatory conditions.
The FDA (Food and Drug Administration) updated its Medication Guide, May 16, 2018, stating that all MRI centers should provide a Medication Guide the first time an outpatient receives a GBCA injection or when the information is substantially changed. However, hospital inpatients are not required to receive a Medication Guide unless the patient or caregiver requests it. A health care professional who determines that it is not in a patient's best interest to receive a Medication Guide because of significant concerns about its effects may direct that it not be provided to that patient; however, the Medication Guide should be provided to any patient who requests the information. FDA also states:
Health care professionals should consider the retention characteristics of each agent when choosing a GBCA for patients who may be at higher risk for gadolinium retention. These patients include those requiring multiple lifetime doses, pregnant women, children, and patients with inflammatory conditions. Minimize repeated GBCA imaging studies when possible, particularly closely spaced MRI studies. However, do not avoid or defer necessary GBCA MRI scans.
Chemical Structure and Use of Gd-Contrast Agents
According to their chemical structure, Gd-based contrast agents are subdivided into ionic and nonionic, macrocyclic and linear contrast agents (see Table 1). The cyclic structure creates a stronger bond to gadolinium. Macrocyclic agents have a cage-like structure that is less likely to release the Gd(III) ion. In contrast, linear agents have an elongated molecular structure, making them more likely to release gadolinium. Linear contrast agents are so-called Gd chelates with open, mobile chains that have no strong binding to the toxic Gd3+ ion. (Hemsen 2012, Marckmann 2006).
Table 1: Linear vs Macrocyclic Agents
Source: Rogosnitzky M, Branch S. Gadolinium-based contrast agent toxicity: a review of known and proposed mechanisms. Biometals.2016; 29:365-376
Risks and Side Effects
According to information from the European Medicines Agency (EMA) and the Federal Institute for Drugs and Medical Devices (January 2018), the long-term risks of gadolinium contrast agent administration are still unknown. In the EU, the intravenous use of specific linear gadolinium-containing contrast agents has been suspended (see Table 2). The linear agents in question are gadobenic acid, gadodiamide, gadopentetic acid and gadoversetamide. In lieu of the information available, it would seem reasonable to delay the use of any linear GBCA. However, after gadolinium producers and imaging societies raised objections, the EMA will re-examine its decision regarding these agents.
Table 2: Overview of Gadolinium-based Contrast Agents
"While there is clear evidence that all types of [gadolinium contrast agents], both linear and macrocyclic, may result in trace amounts of gadolinium in the brain, there exists no clinical evidence that this leads to an increase in risk or harm to patients," a press release from GE Healthcare defended the company's MRI contrast agent Omniscan; and continued stating that "Omniscan has a specific cardiac indication in several European Member States; removing it would limit clinical choice."
In patients with impaired renal function (i.e. an impaired elimination of the drug), gadolinium-based contrast agents (GBCAs) may increase the risk for nephrogenic systemic fibrosis (NSF), a rare and serious syndrome involving fibrosis of skin, joints, eyes and internal organs. In these patients, the use of GBCAs must be avoided unless the diagnostic information is essential and not available with non-contrasted MRI or other modalities. The risk for NSF appears highest among patients with chronic, severe kidney disease (GFR <30 mL/min/1.73m2) or acute kidney injury. Before GBCAs are administered to patients with chronically reduced renal function, including diabetics, hypertensive and older patients (>60 years), the estimation of the glomerular filtration rate (GFR) is essential.
GBCAs were first considered as a cause of NSF as early as 2006 (Agarwal 2009, Grobner 2006). Reports indicated that NSF developed within days or months after administration in patients with renal insufficiency (Dawson 2008).
Prof. Detlef Moka, MD, and Chief Executive Officer of BDN stated in interview: "If gadolinium stays in the body longer in patient with renal insufficiency, Gd can accumulate in the skin and organs and cause the severe connective tissue disease NSF."
Gadolinium(III) ions occurring in water-soluble salts are toxic to mammals. Chelated gadolinium(III) compounds (i.e. gadolinium bound to a chelate) are far less toxic because the chelate carries the tightly bound gadolinium ions through the kidneys and out of the body before free ions can be released. Because of its paramagnetic properties, solutions of chelated gadolinium complexes are used intravenously in magnetic resonance imaging.
Theory and Practice
With healthy kidney function, Gd contrast agents should be excreted within a short time. Gadodiamite, for example, is a linear and thus less stable Gd chelate. One milliliter of gadodiamide contains 287 mg (0.5 mmol) of the Gd drug. The pharmaceutical manufacturer GE Healthcare states, "the recommended dose is usually 0.1 mmol / kg BW (equivalent to 0.2 ml / kg BW) up to a body weight (BW) of 100 kg. If the body weight is more than 100 kg, then 20 ml is usually sufficient to obtain a desired contrast for diagnosis."
Consider the theoretical reduction of contrast agent after administration of 20 ml gadodiamide as shown in Table 3. About 32.5 hours after the intravenous injection of 20 ml Omniscan, only 0.2 µg gadodiamide should remain in the system. After three days, no gadolinium should be detected in urine.
Table 3: Theoretical Reduction after Administration of 20 ml Omniscan (Gadodiamide)
Data from 550 randomized urinary specimens before chelation showed a mean Gd concentration of 5.76 μg/l with a standard deviation of 128 μg/l. The maximum value was 2990 μg/l (Source: Micro Trace Minerals Laboratory (MTM 2006). The detection limit for gadolinium in urine is currently 0.05 µg/l .
Further surveillance carried out in 2011, 2017, and 2018, showed a similar Gd mean concentration, and again a high standard deviation. In 2018, another statistical evaluation of more than 12,000 baseline urine measurements showed a mean gadolinium concentration below the detection limit with a high standard deviation of 2605 μg/l, indicating the presence of some urine samples with very high gadolinium concentrations. Of the 12,000 baseline urines tested, 80 urine samples showed a Gd concentration of more than 100 μgGd/l. In 11 of these samples, Gd values greater than 1000 μg/l were detected.
The highest gadolinium concentration was 290,000 µg/l (two-hundred and ninety-thousand). The urine creatinine value of this sample was inconspicuous, reflecting normal renal function. The second-highest Gd urine concentration was 57,000 µg /l with a urine creatinine value of 2.56 g/l, indicating renal stress. In both cases, available patient information did not provide information regarding the time the contrast agent was administered, nor did we receive the contrast agent's product name.
It is of specific interest that none of the extreme values outlined above came from a urine sample following chelation. This demonstrates that the renal clearance of the above-mentioned urine tests is due to the body's own excretion mechanism. It shows that gadolinium is continuously eliminated over a given time without the use of chelating agents.
Chelation therapists debate if chelating agents such as DMPS, DTPA, EDTA, or DMSA are useful in de-chelating GBCAs. Our evaluation of data suggests otherwise. The problem we noticed is that provocation urine test results were not compared with urine test results of unprovoked (baseline) urines, leading to misinterpretation.
The contrast agent's molecular structure determines its stability and if it can be re-chelated by a given chelating agent. The molecular formula of GdDTPA and ZnDTPA are similar, as is the Log K (thermodynamic stability constant). For ZnDTPA the stability constant is 18.40 and for GdDTPA it is 18.25 (at pH 7.4), which indicates that binding ability and stability constant of DTPA with Zn and Gd are similar. Whether ZnDTPA is a suitable chelating agent for GBCAs is to be determined.
We compared the gadolinium concentration of urine samples before and after chelation. Selected pairs came from the same patient and had been taken at the same day. Samples were submitted by various clinics and the results, as shown in Table 4, seem to indicate that chelation is not as successful as proclaimed.
Table 4: Gadolinium in Urine Before and After Chelation with DMPS iv, 250 mg (1 Ampule)
Most importantly, if the gadolinium excretion value before and after provocation are not compared, post chelation results are most likely misinterpreted.
We evaluated data received from chelation with Dimaval (DMPS, Sodium 2,3-dimercaptopropane-l-sulfonate), the chelator of choice for many European physicians. Dimaval is produced by Heyl, Berlin, and in talking with the responsible scientist Dr. J. Ruprecht, we learned that this chelator is not likely to bind gadolinium. Our evaluation of available data confirmed his statement (see Table 4).
Some physicians claim treatment success by using combination treatments such as DMPS+CaEDTA or DMPS+ZnDTPA. In our database, we could find few pairs of pre and post chelation samples involving such pre and post test results. Those located did not prove that gadolinium binding happened with chelation (Table 5 and 6).
Table 5: Gd in Urine Before and After Chelation with DMPS+CaEDTA
Table 6: Gd in Urine Before and After Chelation with DMPS+ZnDTPA
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