Co-encapsulation of magnetic nanoparticles and doxorubicin into biodegradable microcarriers for deep tissue targeting by vascular MRI navigation

Abstract

Magnetic tumor targeting with external magnets is a promising method to increase the delivery of cytotoxic agents to tumor cells while reducing side effects. However, this approach suffers from intrinsic limitations, such as the inability to target areas within deep tissues, due mainly to a strong decrease of the magnetic field magnitude away from the magnets. Magnetic resonance navigation (MRN) involving the endovascular steering of therapeutic magnetic microcarriers (TMMC) represents a clinically viable alternative to reach deep tissues. MRN is achieved with an upgraded magnetic resonance imaging (MRI) scanner. In this proof-of-concept preclinical study, the preparation and steering of TMMC which were designed by taking into consideration the constraints of MRN and liver chemoembolization are reported. TMMC were biodegradable microparticles loaded with iron-cobalt nanoparticles and doxorubicin (DOX). These particles displayed high saturation magnetization (Ms = 72 emu g⁻¹), MRI tracking compatibility (strong contrast on T2∗-weighted images), appropriate size for the blood vessel embolization (∼50 μm), and sustained release of DOX (over several days). The TMMC were successfully steered in vitro and in vivo in the rabbit model. In vivo targeting of the right or left liver lobes was achieved by MRN through the hepatic artery located 4 cm beneath the skin. Parameters such as flow velocity, TMMC release site in the artery, magnetic gradient and TMMC properties, affected the steering efficiency. These data illustrate the potential of MRN to improve drug targeting in deep tissues.

Introduction

Magnetic targeting was proposed 30 years ago as a means to increase cytotoxic agent concentration in tumors [1], [2], [3], [4]. This approach consists in applying an external magnetic field to trap drug-loaded carriers in a targeted site [5]. Magnetic carriers, generally made with iron oxide, have been developed for the treatment of tumors located in organs such as brain [6], [7], lungs [2], and liver [8], [9]. Despite an increase in the amount of particles reaching the targeted area [2], [6], [10], such carriers are also delivered to a significant extent to healthy tissues/organs after systemic administration. This moderately controlled process may result in side effects thereby limiting the maximal tolerated dose [11]. Furthermore, since the targeting efficiency depends on the distance between the magnet and the tumor [12], [13], this approach is only applicable for the treatment of superficial cancers, or tumors implanted in small animals. Indeed, the magnetic targeting of deep tissues is highly challenging [2], and is not used in clinical practice [14].

Recently, a new approach referred to as magnetic resonance navigation (MRN) has been proposed to steer and track in real time endovascular magnetic carriers in deep tissues to target areas of interest [15], [16], [17], [18], and restrain the systemic carrier distribution. MRN is achieved with a clinical magnetic resonance imaging (MRI) scanner upgraded with an insert of steering coils [18], [19]. The scanner allows tracking the carrier during MRN along a pre-planned trajectory. The magnetic field (1.5 T or higher) of the system enables saturation magnetization (Ms) of ferromagnetic materials throughout the body [15], [17]. Hence, the problem of weaker magnetic field in deep tissues observed with external magnet can be overcome. A magnetic gradient is generated to steer the carrier in a particular direction. Since a clinical MRI scanner generates a 40-mT m⁻¹ gradient, only millimeter-scale carriers can be piloted [15], [19]. To achieve MRN of micrometer-scale carriers for therapeutic purposes, steering coils generating a gradient up to 400-mT m⁻¹ are needed [16], [18], [19]. Moreover, the successful application of MRN is relying on several medical [16] and MRN parameters such as magnitude of the steering magnetic gradient [18], Ms and diameter of the carrier [17], blood vessel properties (flow velocity and diameter) [16], [20] and injection site position (Fig. 1A).

In this manuscript, the conception and in vivo steering of therapeutic magnetic microcarriers (TMMC) designed for the treatment of hepatocellular carcinoma (HCC) via trans-arterial chemoembolization (TACE) in the hepatic artery (Fig. 1) are reported. HCC remains the third cause of death related to cancer [21]. Improvements in current therapeutic modalities are critically required since treatment of unresectable HCC is associated with a low survival rate [22]. By controlling chemoembolic material distribution, MRN could improve embolization and drug concentration in the tumor area while limiting chemoembolization of healthy blood vessels and the hepatic complications [23], [24], [25] (Fig. 1). The TMMC characteristics such as diameter and drug release profile were based on drug eluting beads (DEB) design. DEB are recognized as promising chemoembolic systems by inducing tumor anoxia due to physical obstruction of blood flow and to the sustained release of a cytotoxic agent (e.g. doxorubicin, DOX) [26], [27], [28], [29]. The TMMC consisted of biodegradable poly(D,L-lactic-co-glycolic acid) (PLGA) microparticles loaded with the antitumor drug and magnetic nanoparticles for steering and tracking (Fig. 1B). In this feasibility study, physiological parameters (of the rabbit) were taken into consideration for designing the TMMC. Our previously published in vitro data [16] indicated that the TMMC mean diameter should be 50 μm to get a distal embolization without extravasation in liver parenchyma and that the microparticles should contain 30% FeCo nanoparticles (w/w) to give adequate Ms for steering (Fig. 1C). FeCo nanoparticles were preferred over iron oxide nanoparticles because of their higher Ms [30] which reduces the magnetic material loading in TMMC [16].