ABSTRACT
Keywords: medical imaging, contrast agents, micelles, liposomes, dendrim-ers, radioopaque elements, radioopaque nanoparticles, ultrasound contrast agents, core-shell nanoparticles, tissue targeting
other tissues to achieve contrast, and should be non-toxic and non-immunogenic. In this chapter, we discuss how the use of biomaterials has advanced the development of novel nanoparticles as contrast agents for computed tomography (CT) and ultrasound (US).An essential role of medical imaging is the ability to discriminate between normal and diseased tissue. This discrimination can be achieved by using contrast agents that selectively accumulate in the diseased tissue, increasing the signal from that tissue, thus distinguishing it from adjacent, normal, tissue. The increasing ability of therapies to address specific subtypes of disease as well as the associated increases in the cost of these therapies demands a corresponding improvement in the ability to define disease, preferably non-invasively and to assess response to therapy in near real time. Medical imaging has the ability to answer these demands through increasingly specific contrast agents. In the spectrum of available molecular imaging technologies, nuclear medicine, using either single-photon or positron-emitting tracers, provides the greatest sensitivity, with the ability to detect targets that are present at the nanomolar level. The trade-off for this high sensitivity is relatively lower resolution (5-10 mm) than is available with magnetic resonance imaging (MRI), computed tomography (CT), or ultrasound (US), all of which can achieve anatomic resolution of 1 mm or less. However, MRI, CT, and US require a much greater mass of contrast agent at the imaging site to provide adequate differentiation between normal and diseased tissue. An obvious way to provide this increase in mass is through the use of targeted nanoparticles, which by their very nature deliver a large mass of material to the target, assuming that the technical challenges associated with delivering such a massive particle to an in vivo target can be overcome. These challenges include evading the body’s defenses against particulate contaminants (including the reticuloendothelial system), developing a particle that is small enough not to be trapped in the capillary bed while still large enough to provide an adequate signal once it reaches the target, and overcoming the toxicological challenge inherent in administering a significant mass of material intravenously. The development of novel biomaterials offers a great opportunity to overcome
these challenges. The aim of this review is to discuss the role of biomaterials in development of contrast agents in two fields, CT and US, their similarities and differences, and how the novel properties of nanoparticles have facilitated this development. Owing to the breadth of this subject and space limitations, this review is not intended to be comprehensive, but rather to provide an introduction to the requirements of the field, as well as an overview of how advances in nanotechnology have exploited the unique properties of biomaterials to address the challenges outlined above. 15.2 Contrast Media for Computed
Tomography Computed tomography (CT) is an imaging modality in which x-rays are projected through the subject at different angles and the resulting data are then reconstructed into three-dimensional images. X-rays are able to pass through the different types of tissue to varying degrees, with bone being relatively radiodense, air being essentially radiotransparent, and water lying between these two extremes. The attenuation of the x-rays is measured in Hounsfield Units (HU), with air and water arbitrarily set to 0 and 1000, respectively. As soft tissues tend to have very similar radiodensities, CT imaging can be significantly improved by injecting radioopaque compounds intravenously, improving the definition of vascular structures and providing some degree of soft tissue targeting and differentiation. There is both a need and an opportunity for new CT contrast agents that target specific tissues or provide prolonged imaging times. Magnetic resonance imaging (MRI) can also provide high-resolution anatomic images as well as quantitative information on blood flow, but CT offers both technical and pragmatic advantages compared with MRI; there is a direct proportionality between x-ray absorption and contrast agent concentration, simplifying quantitative studies, and CT is less expensive and more widely available than MRI. The primary disadvantage of CT compared with MRI is that CT scans involve exposure to ionizing radiation, but the radiation dose can be minimized by only imaging the region of
interest and by careful selection of imaging parameters (i.e., power (kvp) and x-ray current). CT contrast media must be very water soluble because of the large quantities that are injected. The solubility requirement results in rapid clearance from the blood, which in turn limits the available imaging time. Prolonging the clearance time, by means such as increasing the molecular weight of the agent, has, therefore, been an important research focus. A second research focus is the development of CT contrast agents that target specific tissues. This objective has proved difficult to achieve because a significant mass of the contrast agent must accumulate in the target tissue before the tissue density increases enough to improve the contrast of the image. This requirement, in turn, increases the risk of toxicity. While iodine is the primary element used in CT contrast agents, primarily because of its low cost and minimal toxicity, a third research focus is the evaluation of radiodense elements other than iodine (e.g., barium). 15.2.1 Macromolecular Contrast AgentsTo address the problem of increasing the time available for imaging, the most straightforward approach is to increase the size of the contrast agent to the nanoparticle range (typically 1-100 nM), which slows its clearance from the blood (for more detailed discussion, see Hallouard et al. [1]). If one adopts this approach, the choices that must be made are: (1) What type of nanoparticle to use and how this choice affects the particle’s in vivo behavior, (2) What radioopaque element will be used and how will it be attached to the nanoparticle, and (3) What other steps are required for in vivo use? In terms of which nanoparticle is optimal, there has been a progression in the field from use of the relatively simple micelles, to more complex but also more efficacious liposomes, to dendrimers, and finally to the use of microcrystalline or particulate forms of radioopaque elements. We will consider each of these in turn. 15.2.1.1 Micelles Micelles are single-layer vesicles that form spontaneously in surfactant solutions when the surfactant concentration exceeds
the critical micelle concentration (CMC). The CMC varies with the surfactant, so the materials used in the manufacture of intravenously administered micellar drugs must be chosen so that the CMC is low enough that the micelle does not collapse when the drug is diluted upon injection, achieving the desired goal of remaining intact in the circulatory system for a prolonged period of time. Micelles can be used as contrast agents in two ways: The micelle can be used as a carrier, with the radioopaque element transported in the hydrophobic core of the micelle, or the radioopaque element can be covalently attached to the polymer constituents of the micelle, to either the hydrophilic outer “head” or the hydrophobic inner “tail.” Covalent attachment of the radioopaque element to the constituents of the micelle is ideal as it improves retention of the element by the micelle, increasing specificity, and also reducing toxicity. For a micellar CT contrast agent to be effective, it must deliver sufficient mass of the radioopaque element to the tissue of interest to increase the tissue density versus that of adjacent tissue. Improved carrying capacity can be achieved through using larger hydrophobic molecules, but with the downside that this increases the CMC, which in turn means that a higher micelle concentration is required to achieve adequate in vivo stability. Therefore, the use of micelles in CT imaging is inherently limited. In early studies of the use of micelles as CT contrast agents, micelles were used as carriers for iodinated molecules. However, a significant weakness of this approach was the loss of the iodinated molecules from the core of the micelle. In a more recent study of micelles as CT contrast agents, Torchilin et al. [2] used amphipathic block-polymers containing iodine. These compounds form micelles with an average diameter of 80 nm and an iodine content of 33.8%. Detectable organ opacification was observed from 5 min to 3 h post-injection in rats; attenuation increased from 85 HU to 253 HU in the aorta and from 92 HU to 156 HU in the liver post-micelle injection over this 3 h window. The authors note that while this was a significant achievement in the use of micelles as CT contrast agents, much more work must be carried out before these materials are ready for clinical use. For recent reviews, see Ref. [3].
