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Omniscan (Gadodiamide) - Description and Clinical Pharmacology

 
 



DESCRIPTION

OMNISCAN (gadodiamide) Injection is the formulation of the gadolinium complex of diethylenetriamine pentaacetic acid bismethylamide, and is an injectable, nonionic extracellular enhancing agent for magnetic resonance imaging. OMNISCAN is administered by intravenous injection.

OMNISCAN is provided as a sterile, clear, colorless to slightly yellow, aqueous solution in Pharmacy Bulk Package. A Pharmacy Bulk Package is used to dispense multiple single doses, utilizing a suitable transfer device. Each 1 mL contains 287 mg gadodiamide and 12 mg caldiamide sodium in Water for Injection. The pH is adjusted between 5.5 and 7.0 with hydrochloric acid and/or sodium hydroxide. OMNISCAN contains no antimicrobial preservative. OMNISCAN is a 0.5 mol/L solution of aqua[5,8-bis(carboxymethyl)-11-[2-(methylamino)-2-oxoethyl]-3-oxo-2,5,8,11-tetraazatridecan-13-oato (3-)-N5, N8, N11, O3, O5, O8, O11, O13] gadolinium hydrate, with a molecular weight of 573.66 (anhydrous), an empirical formula of C16H28GdN5O9•xH2O, and the following structural formula:

Pertinent physicochemical data for OMNISCAN are noted below:

PARAMETER
Osmolality (mOsmol/kg water)@ 37°C789
Viscosity (cP)@ 20°C
@ 37°C
2.0
1.4
Density (g/mL)@ 25°C1.14
Specific gravity@ 25°C1.15

OMNISCAN has an osmolality approximately 2.8 times that of plasma at 37°C and is hypertonic under conditions of use.

CLINICAL PHARMACOLOGY

Pharmacodynamics

In magnetic resonance imaging, visualization of normal and pathologic tissue depends in part on variations in the radiofrequency signal intensity. These variations occur due to: changes in proton density; alteration of the spin-lattice or longitudinal relaxation time (T1); and variation of the spin-spin or transverse relaxation time (T2). OMNISCAN is a paramagnetic agent with unpaired electron spins which generate a local magnetic field. As water protons move through this local magnetic field, the changes in magnetic field experienced by the protons reorient them with the main magnetic field more quickly than in the absence of a paramagnetic agent.

By increasing the relaxation rate, OMNISCAN decreases both the T1 and T2 relaxation times in tissues where it is distributed. At clinical doses, the effect is primarily on the T1 relaxation time, and produces an increase in signal intensity. OMNISCAN does not cross the intact blood-brain barrier and, therefore, does not accumulate in normal brain or in lesions that do not have an abnormal blood-brain barrier (e.g., cysts, mature postoperative scars, etc). However, disruption of the blood-brain barrier or abnormal vascularity allows accumulation of OMNISCAN in lesions such as neoplasms, abscesses, and subacute infarcts. The pharmacokinetic parameters of OMNISCAN in various lesions are not known.

Pharmacokinetics

The pharmacokinetics of intravenously administered gadodiamide in normal subjects conforms to an open, two-compartment model with mean distribution and elimination half-lives (reported as mean ± SD) of 3.7 ± 2.7 minutes and 77.8 ± 16 minutes, respectively.

Gadodiamide is eliminated primarily in the urine with 95.4 ± 5.5% (mean ± SD) of the administered dose eliminated by 24 hours. The renal and plasma clearance rates of gadodiamide are nearly identical (1.7 and 1.8 mL/min/kg, respectively), and are similar to that of substances excreted primarily by glomerular filtration. The volume of distribution of gadodiamide (200 ± 61 mL/kg) is equivalent to that of extracellular water. Gadodiamide does not bind to human serum proteins in vitro.

Metabolism

There is no detectable biotransformation or decomposition of gadodiamide.

Special Populations

Pharmacokinetic and pharmacodynamic studies have not been systematically conducted to determine the optimal dose and imaging time in patients with abnormal renal function or renal failure, in the elderly, or in pediatric patients with immature renal function.

NONCLINICAL TOXICOLOGY

Carcinogenesis, Mutagenesis, Impairment of Fertility

Long term animal studies have not been performed to evaluate the carcinogenic potential of gadodiamide. The results of the following genotoxicity assays were negative: in vitro bacterial reverse mutation assay, in vitro CHO/HGPRT forward mutation assay, in vitro Chinese Hamster Ovary chromosome aberration assay, and the in vivo mouse micronucleus assay at intravenous doses of 27 mmol/kg (approximately 7 times the maximum human dose based on a body surface area comparison). Impairment of male or female fertility was not observed in rats after intravenous administration three times per week at the maximum dose tested of 1.0 mmol/kg (approximately 0.5 times the maximum human dose based on a body surface area comparison).

CLINICAL STUDIES

CNS (Central Nervous System)

OMNISCAN (0.1 mmol/kg) contrast enhancement in CNS MRI was evident in a study of 439 adults. In a study of sequential dosing, 57 adults received OMNISCAN 0.1 mmol/kg followed by 0.2 mmol/kg within 20 minutes (for cumulative dose of 0.3 mmol/kg). The MRIs were compared blindly. In 54/56 (96%) patients, OMNISCAN contrast enhancement was evident with both the 0.1 mmol/kg and cumulative 0.3 mmol/kg OMNISCAN doses relative to noncontrast MRI.

In comparison to the noncontrast MRI, increased numbers of brain and spine lesions were noted in 42% of patients who received OMNISCAN at any dose. In comparisons of 0.1 mmol/kg versus 0.3 mmol/kg, the results were comparable in 25/56 (45%); in 1/56 (2%) OMNISCAN 0.1 mmol/kg dose provided more diagnostic value and in 30/56 (54%) the cumulative OMNISCAN 0.3 mmol/kg dose provided more diagnostic value.

The usefulness of a single 0.3 mmol/kg bolus in comparison to the cumulative 0.3 mmol/kg (0.1 mmol/kg followed by 0.2 mmol/kg) has not been established.

OMNISCAN as a single 0.1 mmol/kg dose was evaluated in 97 pediatric patients with a mean age of 8.9 (2-18) years referred for CNS MRI. Postcontrast MRI provided added diagnostic information, diagnostic confidence, and new patient management information in 76%, 67%, and 52%, respectively, of pediatrics.

Body (Intrathoracic [noncardiac], Intra-abdominal, Pelvic and Retroperitoneal Regions)

OMNISCAN was evaluated in a controlled trial of 276 patients referred for body MRI. These patients had a mean age of 57 (9-88) years. Patients received 0.1 mmol/kg OMNISCAN for imaging the thorax (noncardiac), abdomen, and pelvic organs, or a dose of 0.05 mmol/kg for imaging the kidney. Pre- and post-OMNISCAN images were evaluated blindly for the degree of diagnostic value rated on a scale of "remarkably improved, improved, no change, worse, and cannot be determined." The postcontrast results showed "remarkably improved" or "improved" diagnostic value in 90% of the thorax, liver, and pelvis patients, and in 95% of the kidney patients.

In a dose ranging study 258 patients referred for body MRI received OMNISCAN 0.025, 0.05, 0.1 mmol/kg. The lowest effective dose of OMNISCAN for the kidney was 0.05 mmol/kg.

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