Objectives: The objective of this study was to compare the hepatic uptake and biliary excretion of gadolinium-ethoxybenzyl-diethylenetriamine pentaacetic acid (Gd-EOB-DTPA) in control and multidrug resistance-associated protein 2 (Mrp2)-deficient rats by noninvasive dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) and the impact of Mrp2 deficiency on organic anion-transporting polypeptide 1 (Oatp1) transporters and liver vascularization by immunohistochemistry.
Materials and methods: Twenty rats were used in the normal control (n = 10) and Mrp2-deficient rat groups (n = 10). Dynamic contrast-enhanced magnetic resonance imaging studies were performed using Gd-EOB-DTPA (0.025 mmol Gd/kg; 0.1 mL/kg body weight) as the contrast agent. The percentages of relative enhancement were calculated at each time point after Gd-EOB-DTPA injection. In addition, relative enhancement maps were generated through pixel-by-pixel calculations before the injection and at 5, 10, 20, 30, and 40 minutes after the injection. After the DCE-MRI study, blood was sampled from all rats and 6 blood sample parameters, serum aspartate aminotransferase, alanine aminotransferase, total bilirubin, direct bilirubin, indirect bilirubin, and total bile acids, were measured. Rat livers were processed for histologic diagnosis to clarify contrast agent uptake/efflux by examining Oatp1, Mrp2, and platelet endothelial cell adhesion molecule immunohistochemical staining.
Results: The relative enhancement of the Mrp2-deficient, Eisai hyperbilirubinuria rats (EHBRs) (48.6% [3.4%]) was significantly lower than that of the control rats (64.0% [3.2%]; P < 0.001) 5 minutes after the Gd-EOB-DTPA injection. Thereafter, the relative enhancement observed in the EHBRs (10 minutes, 59.6% [5.4%]; 20 minutes, 67.8% [4.1%]; 30 minutes, 69.1% [4.2%]; 40 minutes, 71.0% [4.2%]; P < 0.0001) was significantly higher than that in the control rats at the same time points after the Gd-EOB-DTPA injection. The aspartate aminotransferase and alanine aminotransferase values were not significantly different between the 2 groups. However, total bilirubin, direct bilirubin, indirect bilirubin, and total bile acids levels in EHBRs were significantly higher than those in the control rats. The percentages of the Mrp2-positive cells in the control rats were higher compared with the EHBRs (control, 0.3% [0.1%]; EHBR, 14.1% [3.6%]; P < 0.01). However, the percentages of the Oatp1-positive cells were not different between the 2 groups. Moreover, the percentages of the platelet endothelial cell adhesion molecule-positive cells in the blood vessels of the control rat livers were higher compared with the EHBRs (control, 17.5% [3.3%]; EHBR, 9.5% [3.9%]; P < 0.01).
Conclusions: The utility of noninvasive DCE-MRI with Gd-EOB-DTPA as a tool for the assessment of Mrp2-deficient hyperbilirubinuria rats was demonstrated. We also clarified that the lower vascular density in the EHBRs may cause delayed uptake of the contrast agent compared with the control rats. In addition, the lower Mrp2 transporter expression may cause the lower efflux of the contrast agent from the Mrp2-deficient rats compared with the control rats.