To determine the expression of folate receptor β (FRβ) in synovial biopsy tissues and peripheral blood lymphocytes from rheumatoid arthritis (RA) patients and to identify novel folate antagonists that are more selective in the targeting and internalization of FRβ than methotrexate (MTX).
Immunohistochemistry and computer-assisted digital imaging analyses were used for the detection of FRβ protein expression on immunocompetent cells in synovial biopsy samples from RA patients with active disease and in noninflammatory control synovial tissues. FRβ messenger RNA (mRNA) levels were determined by reverse transcription–polymerase chain reaction analysis. Binding affinities of FRβ for folate antagonists were assessed by competition experiments for 3H-folic acid binding on FRβ-transfected cells. Efficacy of FRβ-mediated internalization of folate antagonists was evaluated by assessment of antiproliferative effects against FRβ-transfected cells.
Immunohistochemical staining of RA synovial tissue showed high expression of FRβ on macrophages in the intimal lining layer and synovial sublining, whereas no staining was observed in T cell areas or in control synovial tissue. Consistently, FRβ mRNA levels were highest in synovial tissue extracts and RA monocyte-derived macrophages, but low in peripheral blood T cells and monocytes. Screening of 10 new-generation folate antagonists revealed 4 compounds for which FRβ had a high binding affinity (20–77-fold higher than for MTX). One of these, the thymidylate synthase inhibitor BCG 945, displayed selective targeting against FRβ-transfected cells.
Abundant FRβ expression on activated macrophages in synovial tissue from RA patients deserves further exploration for selective therapeutic interventions with high-affinity–binding folate antagonists, of which BCG 945 may be a prototypical representative.