A folate receptor-α-specific ligand that targets cancer tissue and not sites of inflammation.
Folic acid has been frequently exploited to target attached drugs to cells that overexpress a folate receptor (FR). Unfortunately, folic acid and folate-linked drugs bind equally well to both major isoforms of the FR-that is, FR-α, which is primarily expressed on malignant cells, and FR-β, which is upregulated on activated monocytes and macrophages. Because both major isoforms of FR can be expressed simultaneously in the same organism, folic acid cannot enable selective targeting of therapeutic and imaging agents to either tumor masses or sites of inflammation. In an effort to develop a targeting ligand that can selectively deliver attached imaging and therapeutic agents to tumor cells, we constructed a reduced and alkylated form of folic acid, N(5), N(10)-dimethyl tetrahydrofolate (DMTHF) that exhibits selectivity for FR-α.


METHODS
DMTHF-(99m)Tc was injected into mice bearing FR-α-expressing tumor xenografts and imaged by γ-scintigraphy. The selectivity for FR-α over FR-β in vivo was examined by γ-scintigraphic images of animal models of various inflammatory diseases such as apolipoprotein E-deficient mice with atherosclerosis, DBA/1 LacJ mice with induced arthritis, C57BL/6J mice with muscle injury, and BALB/C mice with both FR-α tumor and ulcerative colitis, by administration of equal doses of DMTHF-(99m)Tc and EC20-(99m)Tc. The uptake of radiochelates in various organs was quantified by biodistribution studies. DMTHF-near-infrared dye conjugate and DMTHF-Oregon green dye conjugates were synthesized and evaluated for FR-α selectivity over FR-β in rat peritoneal macrophages and human peripheral blood monocytes, respectively, by flow cytometry. Fluorescence-guided imaging was also performed using folate and DMTHF dye conjugates.


RESULTS
The new targeting ligand was found to bind malignant cells in mice with solid tumor xenografts but not peripheral blood monocytes or inflammatory macrophages in animal models of atherosclerosis, rheumatoid arthritis, muscle injury, or ulcerative colitis. Results from optical and radioimaging studies and biodistribution experiments confirm the differential specificity of this new ligand for malignant masses.


CONCLUSIONS
The new targeting ligand DMTHF enables selective noninvasive imaging and therapy of tumor tissues in the presence of inflammation.
Department of Chemistry, Purdue University, West Lafayette, IN 47907, USA.
Balasubramanian
B
Vaitilingam
Venkatesh
V
Chelvam
Sumith A
SA
Kularatne
Scott
S
Poh
Wilfredo
W
Ayala-Lopez
Philip S
PS
Low
D000818:Animals; D001168:Arthritis / Q000139:chemically induced / Q000000981:diagnostic imaging; D050197:Atherosclerosis / Q000000981:diagnostic imaging; D016466:CHO Cells; D045744:Cell Line, Tumor; D002614:Chelating Agents / Q000737:chemistry / Q000493:pharmacokinetics; D006224:Cricetinae; D003412:Cricetulus; D005260:Female; D005434:Flow Cytometry; D058975:Folate Receptor 1 / Q000187:drug effects*; D006801:Humans; D007249:Inflammation / Q000000981:diagnostic imaging*; D007624:KB Cells; D017737:Macrophages, Peritoneal / Q000187:drug effects; D008297:Male; D051379:Mice; D008807:Mice, Inbred BALB C; D008810:Mice, Inbred C57BL; D008819:Mice, Nude; D018482:Muscle, Skeletal / Q000000981:diagnostic imaging / Q000293:injuries; D009369:Neoplasms / Q000000981:diagnostic imaging*; D015609:Organotechnetium Compounds / Q000737:chemistry / Q000493:pharmacokinetics; D018408:Patch-Clamp Techniques; D011877:Radionuclide Imaging; D019275:Radiopharmaceuticals / Q000737:chemistry / Q000493:pharmacokinetics; D051381:Rats; D013379:Substrate Specificity; D013667:Technetium / Q000737:chemistry / Q000493:pharmacokinetics; D013763:Tetrahydrofolates / Q000737:chemistry / Q000493:pharmacokinetics; D014018:Tissue Distribution
D002614:Chelating Agents; D058975:Folate Receptor 1; D015609:Organotechnetium Compounds; D019275:Radiopharmaceuticals; D013763:Tetrahydrofolates; C574874:technetium 99m N5, N10-dimethyl tetrahydrofolate; D013667:Technetium