Folate receptor beta as a potential delivery route for novel folate antagonists to macrophages in the synovial tissue of rheumatoid arthritis patients.
OBJECTIVE
To determine the expression of folate receptor beta (FRbeta) 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 FRbeta than methotrexate (MTX).


METHODS
Immunohistochemistry and computer-assisted digital imaging analyses were used for the detection of FRbeta protein expression on immunocompetent cells in synovial biopsy samples from RA patients with active disease and in noninflammatory control synovial tissues. FRbeta messenger RNA (mRNA) levels were determined by reverse transcription-polymerase chain reaction analysis. Binding affinities of FRbeta for folate antagonists were assessed by competition experiments for 3H-folic acid binding on FRbeta-transfected cells. Efficacy of FRbeta-mediated internalization of folate antagonists was evaluated by assessment of antiproliferative effects against FRbeta-transfected cells.


RESULTS
Immunohistochemical staining of RA synovial tissue showed high expression of FRbeta 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, FRbeta 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 FRbeta 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 FRbeta-transfected cells.


CONCLUSIONS
Abundant FRbeta 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.
van der Heijden
Joost W
JW
VU University Medical Center Amsterdam, Amsterdam, The Netherlands.
Oerlemans
Ruud
R
Dijkmans
Ben A C
BA
Qi
Huiling
H
van der Laken
Conny J
CJ
Lems
Willem F
WF
Jackman
Ann L
AL
Kraan
Maarten C
MC
Tak
Paul P
PP
Ratnam
Manohar
M
Jansen
Gerrit
G
D000818:Animals; D001172:Arthritis, Rheumatoid / Q000188:drug therapy* / Q000276:immunology / Q000473:pathology; D001706:Biopsy; D016466:CHO Cells; D002352:Carrier Proteins / Q000235:genetics* / Q000378:metabolism*; D002455:Cell Division / Q000187:drug effects; D006224:Cricetinae; D003412:Cricetulus; D058973:Folate Receptors, GPI-Anchored; D005493:Folic Acid Antagonists / Q000493:pharmacokinetics*; D006801:Humans; D007150:Immunohistochemistry; D008264:Macrophages / Q000276:immunology / Q000378:metabolism* / Q000473:pathology; D012333:RNA, Messenger / Q000378:metabolism; D011956:Receptors, Cell Surface / Q000235:genetics* / Q000378:metabolism*; D013583:Synovial Membrane / Q000276:immunology / Q000473:pathology; D014162:Transfection; D014316:Tritium
D002352:Carrier Proteins; D058973:Folate Receptors, GPI-Anchored; D005493:Folic Acid Antagonists; D012333:RNA, Messenger; D011956:Receptors, Cell Surface; D014316:Tritium