Use of Oppositely Polarized External Magnets To Improve the Accumulation and Penetration of Magnetic Nanocarriers into Solid Tumors

Abstract

Drug delivery to solid tumors is hindered by hydrostatic and physical barriers that limit the penetration of nanocarriers into tumor tissue. When exploiting the enhanced permeability and retention (EPR) effect for passive targeting of nanocarriers, the increased interstitial fluid pressure and dense extracellular matrix in tumors limits the distribution of the nanocarriers to perivascular regions. Previous strategies have shown that magnetophoresis enhances accumulation and penetration of nanoparticles into solid tumors. However, because magnetic fields fall off rapidly with distance from the magnet, these methods have been limited to use in superficial tumors. To overcome this problem, we have developed a system comprising two oppositely polarized magnets that enables the penetration of magnetic nanocarriers into more deeply seeded tumors. Using this method, we demonstrate a 5-fold increase in the penetration and a 3-fold increase in the accumulation of magnetic nanoparticles within solid tumors compared to EPR.

Keywords: magnetic; magnetophoresis; nanoparticles; penetration; tumor.

Figure 1.

A magnetic device, comprising two oppositely-polarized magnets, enhances magnetic drug targeting in deep tissues. (a) Magnetic targeting strategies that use a single external magnet are limited to use in surface regions. In contrast, this device can be used to encourage nanoparticle penetration into both deep and surface tissues. (b) While the gradient from a single magnet falls off rapidly with distance, the magnetic device generates a constant gradient throughout the tissue. (c) This constant gradient translates into a constant outward radial force on magnetic nanocarriers, encouraging nanocarrier penetration into tissues. In contrast, a single external magnet only pulls nanocarriers towards the direction of the magnet, while diffusion results in very little particle penetration into tissue compared to either magnetic strategy.

Figure 2.

SPION micelles are monodisperse and magnetic. (a) TEM image of SPION micelles shows a core size of ~85 nm, with each micelle core containing ~50 SPIONs. (b) In water, the micelles have a hydrodynamic diameter of ~100 nm (DLS by intensity). (c) SPION micelles have an r2 value of 544 ± 5 mM⁻¹s⁻¹.

Figure 3.

The magnetic device contains a sharp zero point surrounded by constant field gradients. (a) The magnetic device comprises two oppositely-polarized static magnets aligned within an aluminum tube. Large steel threaded rods can be used to control the inter-magnet distance, while aluminum plates and steel rods mechanically stabilize the system. The magnet in the device’s active region generates an avoidance region (outset) from which particles disperse. Measurements of the field within the active region show that this configuration results in a zero point at the center of the device in both the radial (b) and axial (c) directions, consistent with COMSOL simulations (d, e). The field gradient in the device is constant and directed radially outward (f). Inter-magnet distance for simulations and measurements = 20 mm.

Figure 4.

The velocity of nanoparticles in the system depends on their magnetic moment (a) and the gradient of the magnetic field. In a single magnet system, the particle velocity (b) is high close to the magnet surface, but rapidly decreases to < 70% of the maximum velocity within 6 mm of the magnet surface. However, in our two-magnet system, all particles outside of a ~3 mm radius are expected to travel at > 70% of the maximum velocity achievable by a single magnet of the same strength, as calculated using the magnetic saturation curve (as shown in Fig. 4a) and the measured magnetic field (as shown in Fig. 3f) (b).

Figure 5.

Ferrofluids demonstrate the magnetic field and gradients within the device. In the absence of a magnetic field (diffusion only) the ferrofluid in the acrylic well plate does not respond (a, d). In the presence of a single magnet, the ferrofluid in the wells closest to the magnet respond, while the ferrofluid in further wells does not (b, e). In the magnetic device, ferrofluid in all wells move outward (c, f). Overlay: field lines (a-c), magnitude of magnetic field (d-f).

Figure 6.

The magnetic device radially disperses SPION micelles through a 0.4% agarose tissue phantom. There is minimal diffusion of SPION micelles through the tissue phantom over 24 hours (a: top, b). When placed next to a single magnet, SPION micelles move slightly through the gel toward the direction of the magnet (a: middle, c). However, in the magnetic device, particles disperse radially away from the zero point of the device (a: bottom, d). Magnet positions are indicated by dark gray bars.

Figure 7.

The magnetic device significantly reduces T2 signal (hypointensity) from SPION micelles in tumors. (a) Pre- and post-contrast images have similar signal distributions in the control. However, mice that have been exposed to the device show much less T2 signal (hypointensity) post-contrast. Water control labeled “w”. Scale bar = 5 mm. (b) There is a significant reduction in average relative signal intensity (rSI) pre- and post-contrast in control (p < 0.001), 1-magnet (p < 0.01) and device-exposed (p < 0.001) animals, consistent with EPR. However, device animals also show a significant reduction in rSI compared to controls (p = 0.037) and 1-magnet-exposed (p = 0.026) animals post-contrast despite no difference in pre-contrast. There is no difference between control and 1-magnet rSI post-contrast. (c) T2 maps show the change in distribution of T2 times pre- and post-contrast for control and device animals. Tumors in device-exposed animals show more regions with shortened T2 times (indicating more accumulation of magnetic particles) compared to controls. (d) The change in the weighted average T2 time for each tumor slice was calculated. The shift in T2 time was significantly greater in device-exposed animals compared to control (p = 0.015) and 1-magnet-exposed (p = 0.046) animals. There is no difference between control and 1-magnet-exposed animals. N = 3 animals × 3 slices. Statistical testing used ANOVA to establish a difference between distributions, followed by pairwise t-tests.

Figure 8.

Histology images show further tumor penetration by SPION micelles in device-exposed animals compared to controls. (a) Tumors harvested from device-exposed animals are grossly darker than those harvested from control animals. (b) Most nanoparticles, identified by Prussian blue staining, are located near vascular areas with red blood cells in controls (c: detail of gray box). (d) However, SPION micelles have dispersed from the vasculature and traversed through the tumor interstitium in device-exposed animals (e: detail of gray box). (f) SPION micelles are able to disperse significantly further from blood vessels in device-exposed animals compared to control animals (p < 0.001) and 1-magnet-exposed animals (p < 0.001). There is no difference between control and 1-magnet-exposed animals (p = 0.36). (g) Furthermore, there is more SPION micelle accumulation in device-exposed animals compared to control animals (p < 0.001) and 1-magnet-exposed animals (p = 0.0048). There is no difference between control and 1-magnet-exposed animals (p = 0.81) Scale bar in panels (b) – (e): 100 μm. N = 3 animals × 15 slices.