Targeting stents with local delivery of paclitaxel-loaded magnetic nanoparticles using uniform fields

Abstract

The use of stents for vascular disease has resulted in a paradigm shift with significant improvement in therapeutic outcomes. Polymer-coated drug-eluting stents (DES) have also significantly reduced the incidence of reobstruction post stenting, a disorder termed in-stent restenosis. However, the current DESs lack the capacity for adjustment of the drug dose and release kinetics to the disease status of the treated vessel. We hypothesized that these limitations can be addressed by a strategy combining magnetic targeting via a uniform field-induced magnetization effect and a biocompatible magnetic nanoparticle (MNP) formulation designed for efficient entrapment and delivery of paclitaxel (PTX). Magnetic treatment of cultured arterial smooth muscle cells with PTX-loaded MNPs caused significant cell growth inhibition, which was not observed under nonmagnetic conditions. In agreement with the results of mathematical modeling, significantly higher localization rates of locally delivered MNPs to stented arteries were achieved with uniform-field-controlled targeting compared to nonmagnetic controls in the rat carotid stenting model. The arterial tissue levels of stent-targeted MNPs remained 4- to 10-fold higher in magnetically treated animals vs. control over 5 days post delivery. The enhanced retention of MNPs at target sites due to the uniform field-induced magnetization effect resulted in a significant inhibition of in-stent restenosis with a relatively low dose of MNP-encapsulated PTX (7.5 microg PTX/stent). Thus, this study demonstrates the feasibility of site-specific drug delivery to implanted magnetizable stents by uniform field-controlled targeting of MNPs with efficacy for in-stent restenosis.

Fig. 1.

Targeted local delivery of MNPs to a deployed 304-grade stainless steel stent mediated by the uniform-field–induced magnetization effect. The uniform field generated by paired electromagnets (A) both induces high gradients on the stent and magnetizes drug-loaded MNPs, thus creating a magnetic force driving MNPs to the stent struts and adjacent arterial tissue (B).

Fig. 2.

Physical characterization of MNPs. Transmission electron micrograph was obtained using a Tecnai G2 electron microscope (FEI) (A). Note the composite structure of MNPs and their narrow size distribution (B). MNPs exhibit a magnetic moment of 14.3 emu/g at saturation and a near-superparamagnetic behavior (C). PTX release kinetics was determined under sink conditions using a modified external sink method (D). Released drug was assayed spectrophotometrically (λ = 230 nm) in periodically replaced acceptor medium (1:1 mixture of n-heptane and 1-octanol) immiscible with an MNP aqueous suspension. Note the initial burst release followed by a sustained phase with slower kinetics.

Fig. 3.

Antiproliferative effect of PTX-loaded MNPs on A10 cells mediated by a high-gradient magnetic field (5-min exposure). Seven days post treatment A10 cells treated with MNPs at 30 ng PTX/well were observed microscopically in the bright field (A, C, E, and G) and fluorescent (B, D, F, and H) modes, and the cell viability was determined quantitatively as a function of the MNP dose (I) following staining with Calcein AM (λ_ex/λ_em = 485/535 nm). Untreated cells were used as a reference. Note the profound growth inhibition of smooth muscle cells treated with PTX-loaded MNPs under magnetic conditions (C and D), as opposed to the nonmagnetic treatment (E and F). Also note that blank MNPs have no cell growth inhibitory effect in the studied dose range after the magnetic exposure (G and H). Original magnification ×100.

Fig. 4.

In vivo targeting of MNPs to stented arteries via the uniform field-induced magnetization effect. MNPs covalently labeled with BODIPY_564/570 were delivered over 30 s under magnetic vs. nonmagnetic conditions (Upper and Lower rows, respectively) after deployment of a 304-grade stainless steel stent. The field was maintained for an additional 5 min. Stented carotid segments were excised, and the stent (A–D) and the luminal surface of the arteries (E–H) were examined by fluorescent microscopy 2 h (A, B, E, and F) and 24 h (C, D, G, and H) post treatment. Note the significantly enhanced retention of magnetically targeted MNPs at both time points. Original magnification ×200.

Fig. 5.

The effect of uniform field-controlled targeting on the biodistribution of MNPs. MNPs labeled with BODIPY_564/570 were delivered under magnetic vs. nonmagnetic conditions after placement of a 304-grade stainless steel stent, and the amount of MNPs was determined fluorimetrically in the treated arteries (A) and peripheral organs (B anf C) 5 min, 2 h, 1 d, and 5 d post surgery (n = 5). Note the significant difference in the arterial MNP levels between the animals treated under magnetic vs. nonmagnetic conditions maintained over 5 days. Data are presented as mean ± SE.

Fig. 6.

The antirestenotic effect of PTX-loaded MNPs targeted to stented carotid arteries by uniform field-induced magnetization. Animals treated with MNPs at PTX doses of 7.5 and 0.75 μg under magnetic vs. nonmagnetic conditions were killed, and the stented carotid segments were harvested 14 days post surgery. The control group was stented animals untreated with MNPs. A representative Verhoeff–van Gieson-stained section of an artery treated with 7.5 μg PTX under magnetic conditions (A) is shown in comparison with a “no treatment” control (B) (P < 0.05, Dunn’s Test Q statistic = 3.7). Original magnification 100×. Morphometric results expressed as neointima/media ratios (C) are shown as a function of the magnetic field exposure and PTX dose (n ≥ 6). Data are presented as mean ± SE.