Relationship of effective molecular size to systemic clearance in rats of recombinant interleukin-2 chemically modified with water-soluble polymers.

We have examined the effects of a variety of chemical modifications to recombinant human interleukin-2 (rIL-2) on its pharmacokinetic behavior in rats. Unmodified rIL-2 is cleared from plasma with half-lives of 3 and 44 min for the alpha and beta phases. Modification of rIL-2 with monomethoxy polyethylene glycol or polyoxyethylated glycerol increased the half-lives as much as 20-fold, although the volume of distribution remained unchanged at 88 +/- 13 ml/kg. The clearance rates correlated with the effective molecular size of the modified protein determined by size exclusion chromatography. Clearance decreased rapidly as the effective molecular size increased from 19.5 to 70 kDa, whereas above 70 kDa the clearance decreased more slowly. This abrupt change at 70 kDa may be related to the permeability threshold of the kidney glomerular membrane which retains proteins larger than albumin in the plasma. Using the relationship between clearance and effective molecular size, the clearance rates of mixtures of modified rIL-2 could be predicted based on their average effective molecular size. Since the effectiveness of rIL-2 therapy is likely to be related to its pharmacokinetic behavior, the ability to design a molecule with a predictable time course in plasma provides a means to study this relationship.