Comparative tissue distribution of 125I- and U-14C-labeled recombinant human interleukin-2 in the rat

The distribution of recombinant human interleukin-2 (rhIL-2) after intravenous administration of 125I- or U-14C-labeled rhIL-2 is reported in the major organs of the rat. Five minutes after the administration of U-14C-labeled rhIL-2, the radioactivity had been rapidly cleared from the plasma, while 38% of the dose was found in the kidneys, confirming that the major site of clearance for rhIL-2 is the kidney. After 1 h, a large fraction of the radioactivity had disappeared from the kidney and was found in the carcass. When the same experiments were carried out with 125I-labeled rhIL-2, comparable distribution results were obtained: Preferential accumulation of 125I radioactivity (37.4%) was found in the kidney at 5 min after iv administration of 125I-labeled rhIL-2. One hour after dosing 125I label was predominantly present in the carcass (46%) and skin (15%). Similar percentages of the dose of 125I or 14C radioactivity were present in other organs or tissues. The present study indicates a similar distribution of the radiolabel in selected tissue and organs regardless of whether 14C or 125I was employed.