Polymeric contrast agents for MR imaging of adrenal glands

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Abstract
A variety of adrenal imaging agents have been used in nuclear medicine, but no agent has been developed for magnetic resonance (MR) imaging. The authors have previously observed accumulation of aminated macromolecules in adrenal glands. They now report the synthesis of a model polymeric aminated contrast agent for enhanced MR imaging of the adrenal glands. The model agent consisted of a poly‐L‐lysine conjugate (molecular weight, 245kd) that had 70% free epsilon amino groups and 30% diethylenetriaminepentaacetic acid (DTPA)‐deriv‐atized amino groups to bind indium‐111 or gadolinium. One hour after intravenous administration of this compound, adrenal uptake was 10.1% ± 0.7 of injected dose per gram of tissue. When all free epsilon amino groups of the polylysine were completely substituted with DTPA, adrenal uptake was 3.4 times lower, indicating the importance of free amino groups for adrenal uptake. MR imaging in rats showed that a dose of 0.08 mmol of gadolinium per kilogram of the agent was sufficient to enhance the signal intensity of adrenal glands. Three hours after intravenous administration of the agent, signal intensity of the adrenal glands was 186% of precon‐trast values (liver, 165%; kidney, 91%). Fluorescence microscopy showed that the agent accumulated primarily in the cortical zona glomerulosa and in the adrenal medulla. These initial studies demonstrate the feasibility of designing contrast agents for MR imaging of the adrenal glands.