title: Folate receptor-β in activated macrophages: ligand binding and receptor recycling kinetics.
abstract: Activated macrophages overexpress a receptor for the vitamin folic acid termed the folate receptor β (FR-β). Because conjugation of folate to low molecular weight drugs, genes, liposomes, nanoparticles, and imaging agents has minor effects on FR binding, the vitamin can be exploited to target both therapeutic and imaging agents to activated macrophages without promoting their uptake by other healthy cells. In this paper, we characterize the binding, internalization, and recycling kinetics of FR-β on activated macrophages in inflamed tissues of rats with adjuvant-induced arthritis. Our results demonstrate that saturation of macrophage FR is achieved at injection doses of ∼150-300 nmol/kg, with more rapidly perfused tissues saturating at lower doses than inflamed appendages. After binding, FR-β internalizes and recycles back to the cell surface every ∼10-20 min, providing empty receptors for additional folate conjugate uptake. Because the half-life of low molecular weight folate conjugates in the vasculature is usually <1 h, these data suggest that targeting of folate conjugates to activated macrophages in vivo can be maximized by frequent dosing at conjugate concentrations that barely saturate FR (∼150 nmol/kg), thereby minimizing nonspecific binding to receptor-negative tissues and maximizing the probability that unoccupied cell surface receptors will be exposed to folate-drug conjugate.
authors.affiliation: Department of Chemistry, Purdue University , West Lafayette, Indiana 49707, United States.
authors.forename: Bindu
authors.initials: B
authors.lastname: Varghese
authors.forename: Erina
authors.initials: E
authors.lastname: Vlashi
authors.forename: Wei
authors.initials: W
authors.lastname: Xia
authors.forename: Wilfredo
authors.initials: W
authors.lastname: Ayala Lopez
authors.forename: Chrystal M
authors.initials: CM
authors.lastname: Paulos
authors.forename: Joseph
authors.initials: J
authors.lastname: Reddy
authors.forename: Le-Cun
authors.initials: LC
authors.lastname: Xu
authors.forename: Philip S
authors.initials: PS
authors.lastname: Low
mesh_terms: D000818:Animals; D001168:Arthritis / Q000378:metabolism; D058976:Folate Receptor 2 / Q000378:metabolism*; D005492:Folic Acid / Q000378:metabolism; D006801:Humans; D007700:Kinetics; D008264:Macrophages / Q000378:metabolism*; D051381:Rats
chemical_list: D058976:Folate Receptor 2; D005492:Folic Acid
keywords: arthritis; endocytosis; folate; internalization; receptor recycling