Model for specific rescue of normal hepatocytes during methotrexate treatment of hepatic malignancy

We utilized the presence of hepatic receptors for galactose-terminal glycoproteins to achieve specific rescue of differentiated hepatocytes from methotrexate toxicity. This was accomplished by administration of a conjugate formed by covalent coupling of the methotrexate antagonist, folinic acid (leucovorin), to galactose-terminating (desialylated) bovine fetuin. Two cultured hepatocyte-derived cell lines were chosen to test rescue from methotrexate toxicity: Hep G2 cells are capable of receptor-mediated endocytosis of galactose-terminating glycoproteins; PLC/PRF/5 cells are not. In the presence of methotrexate alone, both cells lines were reduced in number to 20% of control populations by day 5. Growth rates of both lines returned to normal when free folinic acid was added to the medium containing methotrexate. When asialofetuin-folinic acid conjugate was given to both cell lines in the presence of methotrexate, Hep G2 cells doubled their numbers by day 4 and reached control (without methotrexate) populations by day 8. However, PLC/PRF/5 cells failed to respond to administered asialofetuin-folinic acid conjugate under identical conditions, and the growth curve was the same as that for cells which received methotrexate alone. Conjugates of asialofetuin bound to folic acid (folate derivative requiring dihydrofolate reductase activity for conversion to an active metabolite) and fetuin (non-galactose-terminal glycoprotein) linked to folinic acid also were made for control studies. These conjugates failed to rescue either cell line.