Method for Enhancing the Safety of Metal-Ligand Chelates as Magnetic Resonance Imaging Agents by Addition of Calcium Ions

The intravenous LD.sub.50 for calcium chloride (CaCl.sub.2) in the mouse is reported to be 42 mg/kg (RTECS). This calculates to about 15 mg/kg of calcium or about 0.3 mg of calcium for a 20 gram mouse. When calcium chloride was added to 0.68M disodium gadolinium diethyenetriaminepentaacetic acid (Na.sub.2 GdDTPA) at 130 mg/kg (6.5 mg/ml) or 260 mg/kg (13.0 mg/ml) the lethal effects of the Na.sub.2 GdDTPA were greatly diminished even though these levels of added calcium would provide 47 and 94 mg/kg, i.e. 0.94 and 1.88 mg respectively to a 20 gram mouse. These values are 3.1 and 6.2 times higher than the i.v. LD.sub.50 of calcium administered as CaCl.sub.2. Whereas 4 of 4 mice given 13.6 mMol/kg of Na.sub.2 GdDTPA alone died, only 1 of 4 mice died at those doses of Na.sub.2 GdDTPA with 2.34 mg/ml of added calcium and 2 of 4 died at the 4.68 mg/ml level of added calcium. Clearly then the 0.68 m Na.sub.2 GdDTPA solution must complex a substantial amount of the added calcium in a way to block the calcium's in-vivo toxicity.

Conversely, the calcium added to the Na.sub.2 GdDTPA formulation blocks the in vivo calcium complexation by Na.sub.2 GdDTPA and thereby reduces its toxicity, i.e. prevents tetanic convulsions and death. Clearly this protective effect of added calcium must be balanced to the calcium complexing potential of the paramagnetic contrast agent.

Addition of calcium chloride at 430 (21.5 mg/ml and 860 mg/kg (43 mg/ml), i.e. at 155 and 310 mg of calcium /kg, results in doses of 3.1 and 6.2 mg of calcium per 20 gram mouse. These doses of calcium added to 0.68 m Na.sub.2 GdDTPA were not protective and did not enhance the safety of the Na.sub.2 GdDTPA formulation. All mice injected with 0.65 m NAGdDTPA at those two dose levels of calcium died. It may be inferred that these levels of added calcium were excessive and exceeded the calcium binding optimum of the solution and that death from calcium toxicity ensued.

In these examples 1 and 2 (see Table 1) it is shown that calcium added to 0.68 m Na.sub.2 GdDTPA as calcium chloride at concentrations of 6.5, 13.0, 21.5 and 43 mg/ml of solution and which would result in concentrations of calcium of 2.34 mg, 4.68 mg, 7.74 mg and 15.48 mg per milliliter respectively, provided different levels of protection against the toxicity of Na.sub.2 GdDTPA. On a stoichiometric basis the four added calcium levels approximate 9%, 17%, 29% and 57% respectively of the 0.68 m concentration of Na.sub.2 GdDTPA. Based on this data, concentrations of added calcium of 30-60% stoichiometric to that of the subject formulation are excessive and do not enhance the safety. However, concentrations of added calcium of 9-17% stoichiometry to the subject formulation were protective based on acute toxicity determinations.

Clearly the optimum amount of calcium to be added will vary based on the ligand chosen and its concentration in the forumulation.