Tumor Targeting with Novel Pyridyl 6-Substituted Pyrrolo[2,3-d]Pyrimidine Antifolates via Cellular Uptake by Folate Receptor α and the Proton-Coupled Folate Transporter and Inhibition of De Novo Purine Nucleotide Biosynthesis

Tumor-targeted specificities of 6-substituted pyrrolo-[2,3-d]pyrimidine analogues of 1, where the phenyl side-chain is replaced by 3′,6′ (5, 8), 2′,5′ (6, 9), and 2′,6′ (7, 10) pyridyls, were analyzed. Proliferation inhibition of isogenic Chinese hamster ovary (CHO) cells expressing folate receptors (FRs) α and β were in rank order, 6 > 9 > 5 > 7 > 8, with 10 showing no activity, and 6 > 9 > 5 > 8, with 10 and 7 being inactive, respectively. Antiproliferative effects toward FRα- and FRβ-expressing cells were reflected in competitive binding with [(3)H]folic acid. Only compound 6 was active against proton-coupled folate receptor (PCFT)-expressing CHO cells (~4-fold more potent than 1) and inhibited [(3)H]methotrexate uptake by PCFT. In KB and IGROV1 tumor cells, 6 showed