Synthetic macromolecular drug carriers: biodistribution of poly[(N-2-hydroxypropyl)methacrylamide] copolymers and their accumulation in solid rat tumors.

To optimize polymer design for tumor directed drug delivery, the fate and the total body distribution of soluble synthetic macromolecules, derived from copolymers of [(N-2-(hydroxypropyl)methacrylamide] (HPMA) were monitored scintigraphically after radiolabeling with 131I during a seven day time window. Equimolar concentrations of radioiodinated copolymers of HPMA with small amounts of methacryloyltyrosinamide (pHPMA) differing in molecular weight (23.4 kD, 27.3 kD, 30.5 kD, 44 kD, 58.4 kD, 60.1 kD) were injected intravenously into Copenhagen rats bearing Dunning prostate carcinomas (subline R3327-AT1). Scintigraphic data were validated by determining absolute amounts of [131I]pHPMA in both tumor tissue and normal organs after sacrificing the animals. Copolymers were cleared from blood circulation in a molecular-weight dependent manner, either via excretion or by extravasation into normal and neoplastic tissues. While distribution patterns for pHPMAs in normal organs were quite similar, absolute amounts of copolymer uptake differed. The higher the molecular weight, the more radioactivity was taken up by the organs. Highest amounts of radioactivity were seen in the lung, liver, and spleen. In solid tumors, kinetics of pHPMA accumulation was clearly dependent on molecular weight. pHPMAs below the renal threshold peaked at 24 hours p.i. and then remained constant. In contrast, copolymers above the renal clearance threshold displayed a continuous accumulation reaching a significantly higher tumor uptake, presumably due to the very small or non existent polymer release from tumor tissue. Absolute amounts of tumor uptake determined by dissection analysis were 0.5 +/- 0.1% of injected dose/g tissue for the 27.3 kD pHPMA and 1.2 +/- 0.1% for the 60.1 kD pHPMA, respectively. In conclusion, our results demonstrate the influence of the molecular weight of the synthetic polymer pHPMA on plasma circulation time, excretion and organ clearance. While pHPMAs are cleared from all normal tissues except the spleen quite effectively, these polymers accumulate in solid tumors in a size dependent manner, due to the well known "enhanced permeability and retention" (EPR) effect. These data are of fundamental interest for ongoing studies on the pharmacokinetics of synthetic polymers, especially when these molecules are conjugated with targeting moieties and therapeutic or diagnostic agents.