Q. What is magnetic vectoring and why is this approach different from other delivery technologies?

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The idea of using magnetic forces to enhance treatment therapies and improve therapeutic benefit has been around for many decades, but advances in this area are only now being realised thanks to the emerging field of nanomedicine and the application of nanotools to treatment modalities.

Charles E. Seeney

Magnetic-based therapies can be traced back through several centuries and, for the interested, an excellent historical review is presented in Magnetism in Medicine (1). Recent progress, with the development of smaller and more powerful magnets leading to highly functional miniaturised devices, has seen the expansion of magnetic applications across a spectrum of treatment methodologies that are being investigated in the clinic, such as cell separation, magnetic resonance imaging, enhanced gene transfer, localised targeted delivery of therapeutics or magnetically induced hyperthermia and early-stage cancer diagnostics. Recently, magnetic nanoparticles have also been applied to applications in tissue engineering and are being used to construct multilayered cell structures. In addition, applications in areas of allergy diagnosis are under study.

One significant development is magnetically guided catheter technology, which is now used in cardiac stimulation and ablation therapy (2, 3). Although this is considered to be an invasive therapy, it offers the potential for site-specific drug delivery. In cardiac ablation therapy for the treatment of atrial fibrillation, where surgical procedures are indicated, a cardiac interventional workstation is used to navigate a catheter over short distances to the site of the arrhythmia. An extension of this procedure is the delivery of therapeutics to the site as well, being held in place using MNPs.

The advent of functionalised MNP, such as ferromagnetic materials with superparamagnetic properties, has further opened new opportunities within the field of nanomedicine for targeted therapies. Progress in the development of nanomagnetic treatment modalities can generally be described as following two paths; one is based on implanted MNPs, where the oscillation of the particles provides the benefit, while the second path uses MNPs as vehicles or carriers for targeted therapeutics.

Only a limited number of nanoscale therapeutic formulations demonstrating both enhanced solubility and drug targeting properties have received FDA approval (5). To date, however, nanomagnetic therapies have only reached limited clinical use (4). One drug targeting system for doxorubicin using micromagnetic (as opposed to nanoscale) particles, developed in the late 1990s, failed in clinical trials (6). Although presenting much potential, nanoscale biomaterials have been slow to develop into clinical applications. This can be attributed to a variety of reasons. The first is generally related to the physics of nanoparticles and the development of characterisation methods for understanding and preventing behaviour, such as particle interactions and agglomeration in the physiological environment. A second reason relates to methods for defining and understanding the biodistribution and specificity of nanoscale therapeutic formulations, which remains imperfect because of the body's complex response to nanoscale materials. Thirdly, it i difficult to find a delivery methodology for site-specific or organ-specific targeting.

Recent advances in nanomaterial preparation and characterisation indicate that it is possible to overcome these limitations by using MNPs as drug carriers to improve drug localisation and overcoming cytotoxic effects. While magnetic targeting is not likely to be effective in all situations, with further development it should provide another tool for disease treatment.

Although commercial success has been limited so far, the application of nanomagnetics to drug targeting opens a range of new opportunities for enhanced therapeutic performance, as described with magnetic vectoring (7, 8), magnetofection (9), magnetic hyperthermia (10) or with tissue vibration (11). These technologies are all at varying stages of development, but could deliver new therapeutic options for physicians and patients in the coming years.