The aim of the study was to determine the elimination course and a potential long-term retention of Gd in the skin of rats after administration of different marketed GBCAs.
Regarding the elimination of Gd in the rat skin, in the immediate phase (wash-out phase), starting after the injection and lasting for several days (macrocyclic GBCAs) to weeks (linear GBCAs), the Gd concentration in the skin decreased considerably. This suggests that the Gd eliminated in this phase is readily eliminable, i.e. most likely water soluble and intact GBCA.
In the second phase (plateau phase), a steady-state level of Gd was observed, which decreased only minimally over time. For the linear GBCAs we observed Gd levels well above the level observed in control animals for up to a year after the last Gd injection (most pronounced in animals treated with Omniscan). For the macrocyclic GBCAs we observed the same low Gd levels in the skin as in control animals.
It has to be noted that we detected minor amounts of Gd in skin tissue of control animals, especially in set 1. This might be attributed to a potential Gd contamination in our laboratory. The level of contamination was in the same range as observed after administration of the macrocyclics, but well below the amount of Gd observed after administration of linear GBCAs (Table 2). Those differences in background values most likely also explain the minor differences in the Gd concentration and exposure of the skin to Gd observed after treatment with the different macrocyclic GBCAs.
The fact that the observed Gd was retained for such a long period in the skin of treated animals leads us to hypothesize that the Gd may exist in a water-insoluble state, which would make it less subject to elimination. But, as noted before, the methods used in this experiment to measure Gd levels could not distinguish between the various forms of Gd. Long-term retention of Gd in tissues has also been reported from NSF patients after multiple Gd-enhanced MRI investigations .
The time-course of the Gd concentrations in the skin (Fig. 1) and the exposure of the skin to Gd correlate with the stability of the GBCAs.
Based on kinetic considerations, the macrocyclic GBCAs are the most stable. In-vitro serum stability experiments could not detect any Gd release from macrocyclic agents within 15 days . In the present experiment, all three macrocyclic GBCAs showed no long-term Gd retention in the skin tissue of treated rats that exceeded the level observed in control animals. All three macrocyclic agents showed virtually identical patterns of Gd elimination from the skin tissue of the study animals, reinforcing the notion that the minute in-vitro stability differences between the macrocyclic GBCAs are extremely unlikely to result in relevant differences in-vivo .
The ionic linear GBCAs are more stable than the non-ionic linear GBCAs , but both may release Gd in-vivo under certain circumstances. However, based on the results of in-vitro stability experiments in human serum  and buffered saline solution , any such release is expected to be significantly less with ionic linear agents than with non-ionic linear agents. Likewise, in our experiment, the Gd values observed on day 364 after the last injection of the ionic linear compound Magnevist were about nine-times lower compared with the treatment with the non-ionic linear compound Omniscan.
The non-ionic linear GBCAs have the highest level of Gd in the skin tissue of study animals in the plateau phase. Still, only a minute proportion of the injected Gd actually is retained in the body. The Gd values observed in the skin on day 364 p.i. of the Omniscan-treated animals correspond to about 0.081% of the total dose of Omniscan administered. This small fraction of Gd may explain why earlier pharmacokinetic studies failed to detect any Gd retention . The difference in retained Gd between Omniscan and OptiMARK treatment may be related to the difference in the amount of excess ligand in the formulation of these two GBCAs .
While the difference in Gd retention is qualitatively correlated to the stability of the respective GBCA, no such correlation exists with the selectivity of the respective ligand to Gd over other metal ions or the acute toxicity of GBCAs, as indicated by LD50 values (Table 1). Such a hypothesis has been discussed previously [8, 9]. In contrast, our study suggests the importance of the stability of GBCAs to minimize Gd retention and to prevent the potential release of Gd.
This study has certain limitations. The experimental setting can be used to evaluate the concentration of Gd retained in rats after administration of different GBCAs using the dosing regimen employed in this study. However, due to significant differences in the pharmacokinetic properties of MultiHance in rats and humans, it is not possible to make valid, clinically relevant comparisons between MultiHance and other GBCAs using this model. Furthermore, as with all animal studies, caution must be exercised when extrapolating results to humans.
As the employed method for measuring Gd cannot distinguish between chelated and unchelated Gd, it is not possible to determine whether the retained Gd observed in the plateau phases after application of linear agents is chelated or unchelated. Furthermore, the study cannot answer whether the long-term retention of small amounts of the originally applied Gd doses plays any role in or contributes to the fibrosis observed in skin samples of NSF patients.
However, if Gd release plays a role in the onset of NSF, the data from this preclinical study suggest that there are potential differences in the risk profile for the different marketed GBCAs, with non-ionic linear agents possibly presenting a higher risk than the macrocyclic agents.