Electron microscopy data for nanoparticles formed from contrast agents incubated with 4% BSA/1X PBS. A) NPs formed from Omniscan; B) Size distribution of singular naniparticles shown in A); C) Close up of singular NP formed from Omniscan; D) NPs formed from Dotarem
×Electron microscopy data for nanoparticles formed from contrast agents incubated with 4% BSA/1X PBS. A) NPs formed from Omniscan; B) Size distribution of singular naniparticles shown in A); C) Close up of singular NP formed from Omniscan; D) NPs formed from Dotarem
Gadolinium-rich nanoparticle formation in vivo may be a key step in gadolinium-based contrast agent (GBCA) derived rare earth metallosis. Despite continuous developments aimed at GBCA safety (more stable ligands, formulation, etc.), gadolinium deposition has been observed with all major GBCA brands. While the physiological link between Gd-rich nanoparticles and disease remains unclear, we have made recent inroads into understanding the formation of nanoparticles from GBCAs.
We have found that oxalic acid readily decomposes the GBCAs Omniscan and Dotarem (among others), ultimately forming gadolinium oxalate (Gd2(CH2O4)3). Oxalates are a physiologically endogenous compound, the concentration of which may be elevated in certain disease states (oxalosis, etc.). It was found that contrast agents containing linear ligands (Omniscan, Multihance) degrade rapidly in the presence of oxalic acid, while macrocyclic agents (Dotarem, ProHance, Vueway) degrade via a two-step process, which can be measured spectrophotometrically. It was found that both step of this process were associative, meaning that oxalic acid actively degrades the contrast agent. Additionally, the effects of protein (bovine serum albumin) were studied, the results indicated that physiological concentration of protein accelerated the reaction.
While the effects of oxalic acid on GCBA is interesting, and elucidate a potential mechanistic step, questions remained, notably about what gives rise to the distinct “sea urchin” morphology of in vivo nanoparticles, which is not found in the bulk material formed by GBCA/oxalic acid interaction. Furthermore, previous studies on in vivo nanoparticles have suggested that the particles are largely composed of gadolinium phosphate, the generation of which is likewise not explained by the GBCA/oxalic acid reaction.
In order to gain a better understanding of nanoparticle formation, GBCAs were incubated with a physiological concentration of protein (BSA) in the presence of a phosphate source (1x PBS). After a 96h incubation period, it was found that Omniscan ( a linear GBCA) regularly formed nanoparticles of a similar size and shape to those formed in vivo. Dotarem (a macrocyclic GBCA) showed little propensity toward nanoparticle formation under these conditions, however, under oxidative conditions (achieved through the use of hydrogen peroxide/peroxidase), nanoparticles were readily formed from Dotarem. Additionally, it was found that the inclusion of calcium (through the addition of CaCl2) significantly altered morphology, suggesting that endogenous metals may have an effect on nanoparticle formation in vivo.
The results described herein provide insight into potential mechanisms by which nanoparticles may be formed from GBCAs in vivo. Though more study is needed to reconcile these reactions with in vivo processes, they clearly demonstrate how endogenous compounds destabilize and decompose GBCAs, thus providing a starting point for further investigation.
