Effects of folinic acid on hepatoma cells containing methotrexate polyglutamates

The effects of folinic acid on a toxic pulse exposure of cultured hepatoma cells to methotrexate (4-amino-10-methylpteroylglutamic acid) is reported. Inclusion of folinic acid (5-formyl-5,6,7,8-tetrahydropteroylglutamic acid) (10 micro M) with the 2-hr pulse of methotrexate (10 micro M) nearly completely prevents the uptake and gamma-glutamylation of methotrexate and prevents toxicity. Addition of folinic acid after methotrexate results in a partial rescue that is time and concentration dependent. Restoration of cell growth in the presence of increasing amounts of folinic acid is accompanied by a concentration-dependent elevation in tritium release from [5-3H]deoxyuridine. In the absence of folinic acid, the release of tritium from [5-3H]deoxyuridine remains inhibited for three days after exposure to methotrexate, which can be related to the cellular formation and retention of methotrexate polyglutamates. Following the 2-hr pulse of methotrexate, the cellular pool consists of 70% polyglutamates of which the predominant species has three glutamate residues (4-NH2-10-CH3PteGlu3). When methotrexate is removed from medium, following the pulse, unmetabolized methotrexate rapidly leaves the cells, and 4-NH2-10-CH3PteGlu3 is converted to methotrexate polyglutamates containing four to six glutamate residues. Addition of folinic acid after the methotrexate pulse prevents the conversion of 4-NH2-10-CH3PteGlu3 to the higher-chain-length derivatives and causes a reduction in the total methotrexate cell pools over the next 48 hr. These results suggest that the effects of folinic acid on methotrexate polyglutamates may play a role in the rescue of cells containing these derivatives.