Biomarkers are indicators of specific biological processes occurring in vivo. Basically, they are things we can measure that let us keep track of something physiologically important. For example, the size of a tumor is a simple biomarker for cancer, and capillary density of plaques in blood vessel walls are biomarkers for atherosclerosis. The advent of molecular imaging lets us study biomarkers in a mostly noninvasive fashion in living animals, and for a small number of biomarkers, even in people. Imaging comes in many modalities, depending on the application; these include magnetic resonance imaging (MRI), positron emission tomography (PET), single photon emission computed tomography (SPECT), autoradiography and optical and ultrasound imaging. Here is a look at some important uses of molecular imaging and biomarkers today—as bioresearch tools with potential clinical value.
Biomarkers and Molecular Imaging: Going Multimodal
by Caitlin Smith Biomarkers are indicators of specific biological processes occurring in vivo. Basically, they are things we can measure that let us keep track of something physiologically important. For example, the size of a tumor is a simple biomarker for cancer, and capillary density of plaques in blood vessel walls are biomarkers for atherosclerosis. The advent of molecular imaging lets us study biomarkers in a mostly noninvasive fashion in living animals, and for a small number of biomarkers, even in people. Imaging comes in many modalities, depending on the application; these include magnetic resonance imaging (MRI), positron emission tomography (PET), single photon emission computed tomography (SPECT), autoradiography and optical and ultrasound imaging. Here is a look at some important uses of molecular imaging and biomarkers today as bioresearch tools with potential clinical value.Multimodal systems
A powerful recent development in molecular imaging is the fusion of modalities for imaging biomarkers. Imaging instrument manufacturer Bruker recently bought Carestream Molecular Imaging. The company's in vivo product manager, Seth Gammon, describes the combination of modalities now available in one system. For example, his company's Xtreme and MS FX PRO in vivo imaging systems have the capability to work in any of four main modalities: fluorescence, luminescence, 2D radioisotopic for both PET and SPECT isotopes and X-ray. Xtreme is really our next-generation platform system, says Gammon. It has very high sensitivity for luminescence and fluorescence combined with rapid high-resolution X-ray and radio-isotopic imaging. For example, he says that internal R&D researchers at Bruker are using the Xtreme to look at the interaction of cell death and inflammation using fluorescence and luminescence at the same time, and in the same mouse. They go further to correlate this with X-ray images for anatomical or structural changes. Especially with the Xtreme, that's something new because the light sensitivity, the luminescent sensitivity, required for imaging luminol in vivo is pretty high, so we're very excited about that. Besides placing greater imaging power in the hands of researchers, another advantage to multimodal imaging systems is their greater accessibility. We have customers who are developing their own probes and biomarkers, so we're trying to be really vendor-agnostic, to make this as broad-spectrum as possible, says Gammon. We know that molecular imaging is still relatively new, and innovation by the academic and industrial in vivo communities of the number and types of biomarkers is still just exploding. So even though many imaging researchers rely mostly on near infra-red (IR) fluorescence, Bruker covers both visible and near IR wavelengths for the benefit of researchers who also use probes in the visible range. We have the ability to support people for imaging their visible probes in vivo while they transition over to the near IR, says Gammon. A lot of probes developed for microscopy or fluorescence-activated cell sorting (FACS), they still work just fine in the visible region. And some researchers need to use them as they transition over to the near IR.Multimodal probes
Another quickly evolving area is the development of multimodal probes, which can bridge the transitions between different imaging modalities. An example is an imaging probe labeled with both a fluorophore and a radiotracer; although not entirely a new idea, the recent developments in multimodal instrumentation have prompted renewed interest lately. They can start and validate their probes using optical imaging, which is much faster and less expensive, says Gammon. And then, without changing the chemical identity, [they can] move over to systems like our quantitative Albira PET/SPECT CT system to do quantitative 3D biodistributions. For many experiments, using a doubly labeled probe is not only more convenient but also better science. If they didn't have, for example, both the fluorophore and the radiotracer, at the same time throughout that whole process, it really wouldn't be the same chemical, says Gammon. To switch from a fluorophore at one stage to a radiotracer in the other could totally change the uptake, clearance, retention profiles and specificity, right in midstream. I don't want to do that. I want to have my biomarker moved through my evaluation continuously, without worrying that the distribution is going to change radically. Gammon also notes that multimodal probes enable you to look at data using different scales. For example, they provide the ability to measure a radiotracer in a whole animal, and then subsequently look at fluorescence signals by microscopy. It's all on the same probe, so the signals essentially correlate with each other, he says. In other words, you know the signals are coming from the same probe, so you don't have to develop a secondary probe to go after the other signal.Near IR imaging
Tools for imaging biomarkers also are available from LI-COR Biosciences, which offers the Pearl Impulse, a near IR fluorescent optical imaging system for use with mice. In addition, LI-COR provides a range of near IR dyes for molecular imaging for use in conjunction with the Pearl Impulse or other near IR imaging systems. These dyes have been developed and optimized for molecular imaging and have excellent clearance properties, says Jeffrey Harford, LI-COR's senior product marketing manager, who notes that the company's IRDye 800CW dye, in particular, is a popular example because of its excitation and emission characteristics, clearance profile and brightness. Indeed, Harford believes the most exciting development in fluorescent optical imaging has been the emergence of 800 nm dyes, such as IRDye 800CW. Prior to imaging at 800 nm, sensitivity was so poor it did not really offer the researcher the sensitivity needed for detecting targets earlier and deeper in the animal, he says. Bioluminescence thus remained the primary optical imaging modality. The newer 800 nm dyes give a greater sensitivity using fluorescently labeled molecules. This advancement has created a lot of interest in the use of optical imaging in the clinic, which seemed unreasonable five years ago, says Harford. The main challenge for researchers using fluorescent optical imaging is translating the agent into the clinic.Moving into the clinic
Although there is strong interest in moving fluorescent imaging agents into clinical settings to evaluate biomarkers important in human diseases, the pathway from the lab to the clinic is not well delineated. Researchers are really crying out for a well-defined path by regulatory agencies to bring biomarkers all the way from the preclinical to the clinical side, says Gammon. There's a lot of confusion about how to go about doing that effectively. Validating the imaging agents pre-clinically, and doing the right controls, scientists generally understand. But the challenge comes whenever they want to figure out a way to get these things into patients, actually to allow them to be used clinically. Specifically, he says, some researchers he talks to seem confused about the relevant regulatory agency, the proper types of toxicity studies and what results the regulatory agency might be looking (or not looking) for.Theranostics
Another clinical area to watch for in the emergence of imaging and biomarker technology is theranostics, which combines molecular agents with drugs that are targeted to treat particular diseases [1]. Theranostics was first born to treat cancer patients with therapeutic drug delivery. Since then, it has expanded to deliver genes or photosensitizing agents via tiny nanoscaffold carriers. Theranostic nanomedicine is growing quickly to include applications for other diseases. Although today theranostics may be found more commonly in cancer treatment, there is some potential for this idea in atherosclerosis, according to atherosclerosis researcher William Kerwin, associate professor in the department of radiology at the University of Washington.Biomarkers in atherosclerosis
Theranostics is compatible with Kerwin's atherosclerosis research, because theranostics imaging targets are nearly identical to atherosclerotic drug targets. This fact that has not gone unnoticed by drug developers. Kerwin's research uses MRI to study blood vessel walls to detect dangerous plaques before heart attacks or strokes are triggered. Thus our imaging techniques can be used to detect positive changes in the plaques in response to experimental drug therapy, says Kerwin. Kerwin's group uses dynamic contrast enhanced (DCE)-MRI, in which images are taken in rapid succession to observe how an imaging agent is taken up into plaques over time. From this, we can compute microscopic characteristics of the plaque, such as capillary density and permeability, says Kerwin. These features are associated with plaque inflammation, a key target in current drug development. The major advantage of this approach is that it is currently available for human studies and has been successfully used to track changes in plaques with therapy in humans. Kerwin expresses an interest in another new take on molecular imaging of atherosclerosis: targeted nanoparticles, which allow researchers to pinpoint imaging to specific molecular targets [2]. For example, in our DCE-MRI technique, we can detect capillary density, says Kerwin. But with a targeted nanoparticle, we could determine whether or not these capillaries are expressing alpha-v-beta-3 integrin, for example, which is associated with angiogenesis and active inflammation. Another important advantage of using nanoparticles is the ability to attach a lot of the imaging agent to the particle. This is important because the sensitivity of the imaging technique to detect the agent generally requires a large amount of the agent, but the target is often relatively sparse, says Kerwin. Although key areas in the molecular imaging of biomarkers are evolving quickly, others are following more slowly. However, as the development of multimodal imaging and probes continues to speed along in the research lab, their wider clinical applications are only a matter of time.References
- Prabhu, P, Patravale V, The upcoming field of theranostic nanomedicine: an overview, J Biomed Nanotechnol, 8(6): 859-882, 2012.
- Lowell, AN, Qiao, H, Liu, T, Ishikawa, T, Zhang, H, Oriana, S, Wang, M, Ricciotti, E, Fitzgerald, GA, Zhou, R, Yamakoshi, Y, Functionalized low-density lipoprotein nanoparticles for in vivo enhancement of atherosclerosis on magnetic resonance images, Bioconjug Chem, 23(11):2313-9, 2012.