Tailored magnetic nanoparticles for optimizing magnetic fluid hyperthermia

Abstract

Magnetic Fluid Hyperthermia (MFH) is a promising approach towards adjuvant cancer therapy that is based on the localized heating of tumors using the relaxation losses of iron oxide magnetic nanoparticles (MNPs) in alternating magnetic fields (AMF). In this study, we demonstrate optimization of MFH by tailoring MNP size to an applied AMF frequency. Unlike conventional aqueous synthesis routes, we use organic synthesis routes that offer precise control over MNP size (diameter ∼10 to 25 nm), size distribution, and phase purity. Furthermore, the particles are successfully transferred to the aqueous phase using a biocompatible amphiphilic polymer, and demonstrate long-term shelf life. A rigorous characterization protocol ensures that the water-stable MNPs meet all the critical requirements: (1) uniform shape and monodispersity, (2) phase purity, (3) stable magnetic properties approaching that of the bulk, (4) colloidal stability, (5) substantial shelf life, and (6) pose no significant in vitro toxicity. Using a dedicated hyperthermia system, we then identified that 16 nm monodisperse MNPs (σ-0.175) respond optimally to our chosen AMF conditions (f = 373 kHz, H₀ = 14 kA/m); however, with a broader size distribution (σ-0.284) the Specific Loss Power (SLP) decreases by 30%. Finally, we show that these tailored MNPs demonstrate maximum hyperthermia efficiency by reducing viability of Jurkat cells in vitro, suggesting our optimization translates truthfully to cell populations. In summary, we present a way to intrinsically optimize MFH by tailoring the MNPs to any applied AMF, a required precursor to optimize dose and time of treatment.

Figure 1

Typical heating curves of MNPs with various median diameters, obtained using a dedicated hyperthermia instrument and a fiber optic thermocouple. The temperature increase is normalized for iron oxide concentration.

Figure 2

(A) TEM image of a monodisperse 16 nm sample. Because of very high monodispersity MNPs form close-packed 2D and 3D hexagonal self-assembled arrays. (B) DLS measurement of MNPs before (10 nm) and after (20 nm) phase transfer using PMAO-PEG. (C) Magnetization curves of 13, 15 and 16 nm as synthesized MNPs. (D) Magnetic properties of 12 nm particles are retained even after phase transfer, for up to 5 months.

Figure 3

Powder X-ray diffraction, θ-2θ scans, of magnetite nanoparticles. Sizes indicated in legends were determined by Scherrer’s formula using the peak at 2θ=35.4°. The magnetite reference (bottom) was obtained from the International Centre for Diffraction Data (PDF# 019–0629).

Figure 4

(A) MNPs preferentially disperse in the hydrophobic chloroform phase before phase transfer (left), while preferring the aqueous phase after phase transfer (right). (B) Zeta potential of MNP@PMAO-PEG as a function of pH. (B) Hydrodynamic size measurements of MNP@PMAO-PEG in RPMI 1640+10% FBS cell culture medium as a function of time.

Figure 5

In vitro cytotoxicity of MNP@PMAO-PEG in Jurkat cells. Viability measured via Luciferase assay and toxicity measured via LDH assay. MNPs were incubated for 24 hours in physiological conditions (37°C and 5% CO₂)

Figure 6

Bright field images of Jurkat cells after 24 hours incubation with MNP@PMAO-PEG. Images A–D were taken at 20X and E–H at 60X magnification, respectively.

Figure 7

Specific loss power (W/g Fe₃O₄) as a function of size and size distribution. Frequency (f) = 373 kHz and H_o = 14 kA/m

Figure 8

In vitro heating of Jurkat cells using MNPs of median diameters (A) 12 nm, (B) 13 nm and (C) 16 nm. AMF was applied for 15 minutes

Figure 9

Cell viability relative to control calculated as AC OFF_avg. − AC ON_avg.