Use of an asialoglycoprotein receptor-targeted magnetic resonance contrast agent to study changes in receptor biology during liver regeneration and endotoxemia in rats.

Abstract

The hepatic asialoglycoprotein receptor is responsible for rapid clearance of desiaylated glycoproteins from the circulation by receptor-mediated endocytosis. Previous in vitro studies in hepatocyte preparations from rats subjected to partial hepatectomy (PH) of 70% to stimulate hepatic regeneration showed decreased asialoglycoprotein (ASGP) receptor binding. We used an ASGP receptor-targeted hepatic magnetic resonance imaging (MRI) agent, BMS 180550, to demonstrate similar changes in receptor biology in vivo. BMS 180550 is an arabinogalactan-coated ultrasmall superparamagnetic iron oxide preparation. Arabinogalactan is a ligand for the ASGP receptor and, thereby, targets the contrast agent exclusively to hepatocytes. Hepatic uptake of BMS 180550 was assessed by sequential precontrast and post-contrast MRI in rats subjected to PH of 70%. In regenerating liver 1 and 3 days after PH, the maximum decrease in hepatic signal intensity (62.2% +/- 6.1 and 59.4% +/- 3.8, respectively) was significantly less than the maximum decrease seen in sham-operated animals at 1 and 3 days postsurgery (39.5% +/- 2.5 and 44% +/- 1.0, respectively). The time necessary to reach 80% of the maximum decrease in hepatic signal intensity, (t80), was less than 2 minutes in control rats and increased to 7.5 +/- 1.3 min and 11.0 +/- 2.3 minutes at 1 and 3 days post-PH, respectively. By 7 days post-PH, contrast-enhanced MRI no longer detected a difference in regenerating liver. Because BMS 180550 is taken up exclusively by the liver, clearance of the agent from the blood was used as a measure of hepatic clearance. Concentration-time curves constructed by measuring changes in blood T2 after an intravenous dose of BMS 180550 were used to determine clearance of the agent. Blood clearance of BMS 180550 in normal liver (15.4 +/- 1.08 mL/min) obeyed first-order kinetics and did not vary over the dose range tested (10 to 100 micromol/L/kg iron). One, 3, 7, and 14 days post-PH, clearance varied with dose, suggesting ASGP receptor saturation. As regeneration proceeded, the dose of contrast agent needed to cause a deviation from first-order kinetics increased, suggesting gradual recovery of ASGP receptor function. Hepatic relaxation was determined by in vitro spectroscopy 60 minutes after administration of graded doses of BMS 180550 and showed dose-dependent increases in relaxation. Kinetic analysis at 1 day post-PH demonstrated a decrease in the apparent k(m) and the maximum response, R(max), suggesting a decrease in the number of functional asialoglycoprotein receptors with concomitant increase in receptor affinity. Systemic endotoxemia, which normally accompanies hepatic regeneration induced by PH, also decreased clearance of BMS 180550 and slowed hepatic uptake of the contrast agent. Altered BMS 180550 pharmacokinetics in endotoxin-treated rats was enhanced by prior administration of small doses of competing ligand. Our studies document the value of BMS 180550 in following changes in ASGP function that accompany PH or systemic endotoxemia. This agent may be useful in assessing the degree of hepatic regeneration in patients with clinical liver disease.