The use of a binary chelate formulation: Could gadolinium based linear contrast agents be rescued by the addition of zinc selective chelates?

Tissue and bone retention of gadolinium based contrast agents (GBCAs) has become a clinical concern because of the potential short and long term toxic effects of free gadolinium. This is a critical problem for most open-chain agents that more readily transmetallate in vivo, in comparison to macrocyclic compounds. Gadolinium diethylene tri-aminepentaacetic acid bis-glucosamide (Gd-DTPA-BIGA) is an experimental, open-chain contrast agent which has a significantly increased relaxivity coefficient in comparison to other GBCAs. This results in greater signal intensity and improved contrast enhancement. These superior imaging qualities initiated a search for a solution to the transmetallation of this agent. Plasma zinc is a well-known GBCA transmettalation agent. Since the base chelate of Gadodiamide (Gd-DPTA-Bis-Methylamide or Omniscan), DTPA-Bis-Methylamide (DTPA-BMA), readily transmettalates with and binds serum zinc, we hypothesized that a plasma "zinc sink," may significantly reduce transmetallation of linear agents. 5% DTPA-BMA was added to a formulation of Gd-DTPA-BIGA, which was tested against the original formulation of Gd-DTPA-BIGA with 0.2% of the base chelate DTPA-BIGA. These formulations, including gadodiamide, were labeled with ¹⁵³GdCl₃ followed by infusion into cohorts of Sprague Dawley rats which were sacrificed at 1, 30 and 60 days. Internal organs were harvested, along with blood, skin and femur, and analyzed for residual gadolinium. A subset of tissues were also interrogated with ICP-MS. Labeled Gadodiamide and saline where used as controls. Conclusion: The addition of 5% DTPA-BMA, as a zinc binding agent, reduced the transmetallation of the linear agent Gd-DTPA-BIGA, in comparison to its original formulation supplemented with 0.2% BIGA. This result indicates that supplementing linear GBCAs with ancillary chelates may hold promise for reducing, or eliminating the biological archiving of gadolinium in tissues. In addition, this paper provides valuable animal data on the long term retention of gadolinium from linear based contrast agents.