A Folate Receptor-α–Specific Ligand That Targets Cancer Tissue and Not Sites of Inflammation

Abstract

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-a, which is primarily expressed on malignant cells, and FR-b, 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-a. Methods: DMTHF-99m Tc was injected into mice bearing FR-a-expressing tumor xenografts and imaged by g-scintigraphy. The selectivity for FR-a over FR-b in vivo was examined by g-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-a 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 FRa selectivity over FR-b 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. Conclusion: The new targeting ligand DMTHF enables selective noninvasive imaging and therapy of tumor tissues in the presence of inflammation.

Predicting Response to Therapy for Autoimmune and Inflammatory Diseases Using a Folate Receptor-Targeted Near-Infrared Fluorescent Imaging Agent

“…Due to the enhanced accumulation of FR + macrophages in inflamed lesions from patients with inflammatory/ autoimmune diseases [20][21][22][23][24], we decided to evaluate the ability of OTL0038 (a FR-targeted NIR dye) to accurately predict patient response to therapy. For this purpose, mice were induced to develop collagen-induced arthritis (CIA) and subjected 28 days later to treatment with dexamethasone, abatacept, or saline (disease control).…”

“…These FR-β + macrophages have been shown to constitute key players in the development and progression of a number of inflammatory and autoimmune diseases, and FR-targeted imaging agents have been exploited to visualize sites of activated macrophage accumulation in both animals and humans [20][21][22][23][24]. Importantly, uptake of FR-targeted imaging agents by activated macrophages has been found to correlate directly with severity of disease symptoms, indicating the potential use of FR-targeted imaging agents for monitoring autoimmune disease progression and response to therapy [25].…”

Novel functions of folate receptor alpha in CNS development and diseases

“…In subsequent elegant experiments, Low and colleagues [60] constructed a reduced and alkylated form of folic acid, N 5 , N 10 -dimethyl tetrahydrofolate (DMTHF) that exhibits selectivity for FRα. DMTHF99m Tc was injected into mice bearing FRα-expressing tumor xenografts and imaged by γ-scintigraphy.…”

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“…In an experiment a reduced and alkylated form of folic acid, N5, N10-dimethyl tetrahydrofolate (DMTHF) that exhibits selectivity for FR-α was injected into mouse bearing FR-α-expressing tumor xenografts and imaged by g-scintigraphy. It was seen that 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 91 .…”