The pharmacodynamics of polylysine-(Gd-DTPA) (Schering, Berlin, Germany), a new blood pooling contrast agent for MRI, were studied in the rabbit and the rat. Polylysine-(Gd-DTPA) is a compound with high LD50. Due to its high molecular weight (50.000) and physico-chemical properties, it remains in the vascular system; during the first hour, the plasma level is three times higher than for Gd-DTPA. MRI was performed at 1.5 T using a SE sequence with TR/TE = 300/15 or 20 msec. Signal intensities of muscle, liver and kidney were measured before and after intravenous injection of the contrast agent (0.1 mmol/kg) during 8 hours in the rat (n = 3) and up to 2 wk in the rabbit (n = 3). A dose response study in three additional rabbits confirmed that the 0.1 mmol/kg dose was optimal. The pharmacodynamics results show that the effects of polylysine-(Gd-DTPA) are similar in both the rabbit and the rat. The liver signal is enhanced by about 60% immediately after injection in both species. This enhanced signal decays to half its maximal value in about one hour, which makes the contrast agent useful for clinical applications at a dose of 0.1 mmol/kg. In the kidney medulla and cortex the signals are enhanced by much larger factors (about 3 to 4); it takes at least one day for the kidney to clear the contrast agent in both species.
