CANADIAN RESEARCH FOCUS
October 17, 2011

Pouponneau, P., Martel, S., et al. "Co-encapsulation of magnetic nanoparticles and doxorubicin into biodegradable microcarriers for deep tissue targeting by vascular MRI navigation", Biomaterials. 32: 3481-3486, 2011

Interview with Dr. Sylvain Martel

Sylvain Martel received the Ph.D. degree in Electrical Engineering from McGill University, Institute of Biomedical Engineering, Montréal, Canada, in 1997. Following postdoctoral studies at the Massachusetts Institute of Technology (MIT), he was appointed Research Scientist at the BioInstrumentation Laboratory, Department of Mechanical Engineering at MIT. From Feb. 2001 to Sept. 2004, he had dual appointments at MIT and as Assistant Professor in the Department of Electrical and Computer Engineering, and the Institute of Biomedical Engineering at École Polytechnique de Montréal (EPM), Campus of the University of Montréal, Montréal, Canada. He is currently Professor in the Department of Computer and Software Engineering, and the Institute of Biomedical Engineering, and Director of the NanoRobotics Laboratory at EPM that he founded in 2002. Dr. Martel holds the Canada Research Chair (CRC) in Micro/Nanosystem Development, Fabrication and Validation since 2001, is a Fellow of the Canadian Academy of Engineering, and the results of his research have led to several prizes. In the medical field alone, he pioneered several innovative technologies such as the first parallel computing platform for remote surgeries, direct cardiac mapping systems designed to investigate the cause of sudden cardiac deaths, and new brain implants for decoding neuronal activities in the motor cortex. Presently, Dr. Martel is leading an interdisciplinary team involved in the development of new types of therapeutic agents and interventional platforms for cancer therapy.

Thirty years ago magnetic tumor targeting was proposed in order to increase the concentration of the cytotoxic agent in the tumor. However, despite some improvements in targeting efficiency, this approach still delivers a significant number of the targeting carriers to healthy tissues/organs after systemic administration. In addition, this targeting is only applicable for superficial cancers and is not capable of targeting deep tissues efficiently. A new approach, namely magnetic resonance navigation (MRN), has been proposed to overcome the limitations of the older systems. It has shown great potential in the targeted delivery of endovascular magnetic carriers in deep tissues to areas of interest, while minimizing systemic carrier distribution. In this paper Dr. Martel and co-authors report on a proof-of-concept preclinical study in which they consider the preparation and steering of therapeutic magnetic microcarriers (TMMCs) designed according to MRN and chemoembolization constraints. Figure 1 depicts their TMCC targeting in use in vivo.

Figure 1: Targeting of TMMC in the left lobe of a liver of a rabbit using an upgraded clinical MRI scanner (courtesy of the NanoRobotics Laboratory, École Polytechnique de Montréal)

CC-CRS:
Please explain the design requirements for the TMMC and how these will be monitored prior to use in a clinical setting? (e.g. diameter and shape, amount of nanoparticles loaded)

Dr. Martel:
Simply speaking, there are three components that must be integrated for the implementation of TMMC namely, the magnetic nanoparticles, the therapeutic load and finally, a biodegradable material capable of encapsulating both. The magnetic nanoparticles have two functions. First, they act as MRI contrast agents, allowing us to track the displacement of the TMMC and to assess the amount of TMMC and therefore the amount of therapeutic agents that reached the targeted region. Second, the same magnetic nanoparticles allow for the propulsion and steering of the TMMC along a path in the blood vessels and toward the final destination. Made of soft magnetic material, they become fully magnetized once in a high homogenous magnetic field such as when operating inside a clinical MRI scanner. By superposing 3D magnetic gradients (similar to the ones used in MRI to select an image slice of the patient) on top of this high homogenous field, we can navigate precisely these TMMC along a pre-defined path using the nanoparticles acting like "nano-motors" where the directional magnetic gradients induce a propelling force on the same nanoparticles. Since the propelling force depends on the total amount of magnetic material in each TMMC, we must have sufficient nanoparticles for effective navigation while providing space to load the therapeutic agent. As an example, approximately 30-40% for 50 micron TMMC is typically used. Finally, the envelope or body of the TMMC can be made of various materials. We already experimented with various types of materials for different tasks. Here, we used PLGA, a well known biodegradable and biocompatible polymer. The diameter of the TMMC depends on the diameter of the blood vessel that will be used for embolization where the drug will be released.

CC-CRS:
Please further explain the relationship between catheter proximity to the bifurcation and flow velocity. How is this expected to alter with vessel size and thickness?

Dr. Martel:
There are three main parameters that must be taken into account namely, the blood flow, the distance to the next bifurcation, and the response of the TMMC to the magnetic gradients (a higher percentage of nanoparticles leads to faster steering). For a given TMMC loaded with a given percentage of magnetic nanoparticles, a sufficiently long distance must be provided prior to the next bifurcation to allow enough time to steer all the TMMC toward the right branch at the next bifurcation. A higher blood flow rate would reduce the time available for steering the TMMC, then more nanoparticles in each TMMC can be used which would reduce the percentage of therapeutics in each TMMC and-or the catheter used to release the TMMC can be placed in a manner as to provide more distance and hence more time prior to the next bifurcation.

CC-CRS:
What is the desired timeline for polymer degradation? Were there any unexpected interactions between polymeric carrier and either the magnetic nanoparticles or the cytotoxic agent? Does the polymer limit the targeting and should care be taken in which type of polymer is used?

Dr. Martel:
The drug (here DOX) release profile were based on drug eluting beads (DEB) design with degradation over several days. But other types of release are also possible but need further investigation such as using different synthesis processes and in some other cases, using external triggering methods. We observed no unexpected interactions between the polymer, the nanoparticles, and the cytotoxic agent. The polymer controlling release through biodegradation did not limit the targeting. Indeed, the choice of the polymer is a critical issue in the synthesis of TMMC. In this study, we used biodegradable poly(D,L-lactic-co-glycolic acid) (PLGA).

CC-CRS:
What happens to the magnetic nanoparticles once they have been released from the polymeric carrier? Will their release hinder the targeting of 'fresh' carriers to the area of interest?

Dr. Martel:
This is a good question. There are presently two main types of nanoparticles that are being considered by our group. One is iron-cobalt nanoparticles and the other is iron-oxide nanoparticles. In this study, iron-cobalt nanoparticles have been used. Iron-cobalt nanoparticles have a much (almost three times) higher magnetization saturation level than iron-oxide nanoparticles of the same dimensions. This higher saturation level translates in an engineering point-of-view to a higher performance induced propelling force and hence faster and more efficient steering of TMMC to the targeted branch at vessel bifurcation. To maintain saturation magnetization level of the iron-cobalt nanoparticles, a thin layer of graphite only a few nanometers in thickness was synthesized to prevent oxidation. Because of this new type of nanoparticles, we do not know yet what happen to these nanoparticles once in the body and further investigations on this matter would be necessary. On the other hand, iron-oxide nanoparticles can also be used (but leading to less steering performance) and since these nanoparticles have been intensively used as contrast agents for MRI, it is probably for now, a safer choice until we know more about iron-cobalt nanoparticles.

CC-CRS:
How do you explain the higher steering efficiency for left steering compared to the right steering? (i.e. that MRN was more efficient in preserving right liver lobe from the chemoembolization than left lobe)

Dr. Martel:
To keep it short and without going in too many technical details, I would say that since the TMMC characteristics were the same, the gradients applied were the same and the catheter release sites were the same, then this can be explained by differences in vascular-related physiological conditions between the two lobes affecting the steering of the TMMC.

CC-CRS:
As the steering efficiency did not reach 100% with these carriers, do you have any plans to further optimize the design to achieve greater effect? And, in considering the variation in animal steering efficiency, do you expect the design of the TMMCs to be altered in any form for clinical application in humans? If so please explain how.

Dr. Martel:
Yes, we have plans to increase the steering efficiency and we are presently investigating several of these to enhance such targeting. To answer the second question, we perform the tests on rabbits using the same gradient magnitude that would be technologically and physiologically possible on humans. I must say that such steering is somewhat more difficult in rabbit models considering the blood flows, shorter distance before the next bifurcation, and the use of smaller TMMC and hence, smaller amount of magnetic material. For humans, we typically have a longer distance prior to the next bifurcation and larger arteries than rabbits which suggest the use sensibly larger TMMC with more magnetic nanoparticles. Then combining longer distances and more steering responsive TMMC to magnetic gradients suggest indeed better targeting for humans compared to rabbits. But this hypothesis still needs to be validated at some point in the future.

CC-CRS:
What major hurdles remain to be overcome in this technology before patients can benefit from its application? In pursuing the application of MRN in a vascular network with several bifurcations, or for targeting actual tumors, do you foresee any significant challenges?

Dr. Martel:
Indeed, the two major challenges are to increase the number of bifurcations that such TMMC can navigate effectively and second, to reach the tumor in some other types of interventions instead of chemoembolization used here for the liver. We have shown so far that we can not only encapsulate a drug in a MRI-navigable (referred to here as Magnetic Resonance Navigation - MRN) microcarriers (the TMMC) as small as 50 micrometers in diameter and to release it at a targeted site, but that we can generate sufficient magnetic gradient for future operation in humans for effective targeting. But presently, the technology is relatively slow at changing the direction of the TMMC and considering that the distance between successive bifurcations decreases as we go deeper in the vascular network, accessing some targeted sites located deeper in the vascular network becomes very challenging. As such we are presently investigating techniques and new complementary approaches to allow navigation of TMMC across several bifurcations. For targeting inside the tumors, our previous experimental data and our theoretical models show that this approach would be limited to TMMC larger than of approximately 50 micrometers, which is too large to target inside the tumors. Ideally, a microcarriers of 2 micrometers in diameters with sufficient thrust propelling force which is impossible to induce for a TMMC at such a scale, would be necessary. As such, we are using a special bacterium (2 micrometer is diameter) propelled by flagellated motors and fully controllable by computer that proved to be very effective in the microvasculature and the angiogenesis network that provides an entrance to the tumoral site. We are presently developing within a pharmaceutical consortium a new therapeutic agent based on these bacteria to treat colorectal cancer. Unfortunately, these bacteria are less effective in larger blood vessels due to larger blood flows and a too long distance to travel. We are presently investigating the encapsulation of these bacteria in special microcarriers (with characteristics similar to the TMMC) acting as "submarines" being steered (navigated) using the same MRI navigation technique to transport and to release these bacteria carrying the therapeutic load closer to the microvasculature to deliver their loads deep inside tumors.