Folic acid-enhanced synergy for the combination of trimetrexate plus the glycinamide ribonucleotide formyltransferase inhibitor 4-[2-(2-amino-4-oxo-4,6,7,8-tetrahydro-3H-pyrimidino[5,4,6][1,4]thiazin -6-yl)-(S)-ethyl]-2,5-thienoylamino-L-glutamic acid (AG2034): comparison across sensitive and resistant human tumor cell lines.