Effect of Folate-Targeted Nanoparticle Size on Their Rates of Penetration into Solid Tumors

Targeted therapies are emerging as a preferred strategy for the treatment of cancer and other diseases. To evaluate the impact of a high affinity targeting ligand on the rate and extent of tumor penetration of different sized nanomedicines, we have used intravital multiphoton microscopy to quantitate the kinetics of tumor accumulation of a homologous series of folate-PEG-rhodamine conjugates prepared with polyethylene glycols (PEG) of different molecular weights. We demonstrate that increasing the size of the folate-PEG-rhodamine conjugates results in both longer circulation times and slower tumor penetration rates. Although a “binding site barrier” is observed with the folate-linked polymers in folate receptor expressing tumors, ligand targeting eventually leads to increased tumor accumulation, with endocytosis of the targeted nanocarriers contributing to their enhanced tumor retention. Because the effects of nanocarrier size, shape, chemistry, and targeting ligand are interconnected and complex, we suggest that these parameters must be carefully optimized for each nanocarrier to ensure optimal drug delivery <i>in vivo</i>

Folate Receptor‑β in Activated Macrophages: Ligand Binding and Receptor Recycling Kinetics

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