Small Animal Imaging in the Era of Molecular Medicine

An Industry Insight by Bioscan

New technologies for imaging molecules are increasingly being used to understand the complexity, diversity and in-vivo behavior of diseases. By Staf C. Van Cauter, Bioscan

While various genomic and proteomic approaches are providing comprehensive ‘snapshots’ of biological indicators, or biomarkers, of many disease states, non-invasive imaging can take this information a step further, showing the activity of these markers in-vivo and how their location and activity changes over time. Advances in experimental and clinical imaging are therefore likely to improve our understanding of diseases at the systems level which will ultimately lead to efficiency improvements in the development of new therapies. In this respect, in-vivo imaging is unique in that it is able to track and monitor biological processes of an animal over a period of time and throughout the body, thus aiding in the analysis of drug distribution, side effects and the effects of therapy.

Over the past few decades, multiple new technologies have been introduced into the field of non-invasive imaging. These have included anatomical techniques such as X-ray CT (x-ray computed tomography), US (ultrasound) and MRI (magnetic resonance imaging) imaging, and molecular or functional imaging techniques such as SPECT (single photon emission computed tomography), PET (positron emission tomography) and non-invasive Optical imaging. Some of these imaging technologies have seen clinical use for decades. Indeed, anatomical technologies such as CT, US and MRI have become diagnostic staples of current clinical practice. Likewise, the functional imaging modalities PET and SPECT have found broad clinical use in cardiac function testing and in oncologic disease diagnosis and staging. These decades of experience have progressed an understanding of the utility of these methodologies to clinical assessment, disease progression monitoring and treatment response. Advances in clinical understanding have been accompanied by technological advances in instrument performance and image reconstruction software, resulting in improved image quality, visibility and interpretation. Human non-invasive imaging is now a mature technology with proven clinical applicability. However, what has been a challenge in the molecular imaging field is overcoming the limitation of human PET and SPECT scanners for use with small species. Indeed, the main challenge in PET and SPECT scanner development for small animals is the design of systems with high spatial resolution (in the nanoliter range) and high sensitivity (in the picomolar range) at the same time. Optical imaging technologies on the other hand have the required sensitivity, but limited penetrability into living organisms. While this favors small animal work, it complicates the translation of results to humans. This in turn is a significant disadvantage of the use of optical imaging for the development of new therapies.

Translational challenge
Functional imaging modalities include optical (bioluminescence and fluorescence) and PET and SPECT. Optical imaging represents the low cost option and is capable of efficient, high throughput functional assays in rodents. However, it has limited spatial resolution and 3D imaging capabilities and suffers from a severe depth of penetration limitation, making the technology unsuited for translational studies. As for imaging rodents, most optical techniques are limited to imaging xenografts. The obvious advantage of the flank model is the convenience of measurement, but this convenience comes at the cost. It has been well documented that many xenograft models lose their tissue-specific characteristics and can be considered primarily a general model of cancer, most useful for identifying broad-spectrum cytotoxic drugs aimed at widely-shared cellular processes such as proliferation or basic metabolism. In the emerging age of targeted therapeutics, this is increasingly problematic.

Unlike optical imaging techniques, PET and SPECT can probe subtle molecular signals deep within tissue, making these technologies suited for use with both xenografts and spontaneous cancer models, and applicable to both small and large subjects. The ability of PET and SPECT to penetrate large subjects has lead to the establishment of these modalities as clinical standards of care and therefore, pre-clinical discoveries and developments using these technologies are more likely to translate into the clinic. Although the theoretical capabilities of SPECT and PET are ideal for translational imaging purposes, the practical use of SPECT and PET for imaging very small animals such as mice presents several challenges beyond those faced in clinical faculties. First, because mice are 1500 times smaller than humans, small-animal imaging must achieve sub-millimeter resolution so that images in mice can be obtained with the same visual acuity as in humans. Second, a high level of detection sensitivity is needed to minimize the amount of radioactive probe to such an amount that it can be injected, and at the same time, to allow procedures to be completed within a time period of maximum 30-45 min during which small animals can be anesthetized safely.

With the recent introduction of new ‘nano-technology’ based nuclear imagers, the NanoSPECT/ CT and NanoPET/ CT systems, it has become possible to obtain images from subjects such as mice with virtually the same detail as can be obtained from human scanners used in the clinic (see images below). These modern nuclear imagers meet these challenges by combining nanoliter volumetric resolution with picomolar detection sensitivity, and by automatically fusing high-contrast PET and SPECT images with the anatomical details of X-ray CT.

Molecular imaging technology
While optical imaging represents the low cost option and is capable of efficient, high throughput functional assays in rodents, it’s application is limited by depth of penetration, 2D rather than real 3D tomographic images, limitations in labeling strategies and a lack of applicability in translational studies. The new nuclear technologies embedded in the NanoPET and NanoSPECT systems on the other hand, represent high resolution, high sensitivity functional methodologies well suited for translational applications from mouse to man. PET and SPECT each possess relative strengths and weaknesses. PET isotope and tracer availability is more limited than for SPECT, but PET is essential for imaging small molecule drugs, due to the characteristics of its available isotopes. SPECT, on the other hand, offers easier access to longer-lived isotopes which are well suited for labeling biologics (peptides and antibodies) and for use with biomarkers.

For an introduction to the power of functional imaging, optical imaging is hard to argue against, except that their suitability for cancer models other than flank models, remains largely unproven. For the most general range of applications, X-ray CT combined with PET or SPECT are the most appropriate technologies, with PET offering small molecule studies and SPECT more applicable in a biologics oriented laboratory, or for monitoring the therapeutic effect on biomarker response. In this respect, a second important consideration is the kinetics of the drugs under study.

For drug candidates with slow kinetics, the best approach is to use SPECT tracers because of the longer half-live of these radioisotopes. These drugs fall usually in the category of biopharmaceuticals; larger molecules for which the addition of a SPECT radioisotope does not impair the action of the drug or biomarker. For small molecules on the other hand, the PET isotope 11-C is the ideal radioisotope since it can be synthesized in the candidate drug. Unfortunately, the very short half-live and the low specific activity of this tracer restricts the use of C-11 tracers to molecules that can be relatively quickly synthesized and for imaging studies of short duration. For other small molecule applications, radiohalogens for PET and SPECT may have to be used instead. In this respect, one of the advantages of using radiohalogens for SPECT is the ability to use I-125 and I-131 tracers used commonly for in-vitro and clinical in-vivo studies respectively. Since NanoSPECT /CT has the unique ability to handle both these isotopes equally well, the system can be used to bridge both the in-vitro to in-vivo gap and the translational mouse-to-human gap. Indeed, maintaining a consistency in the imaging protocols and the biomarkers used at the bench and in the clinic is a prerequisite for speeding-up translational research.

Staf C. Van Cauter is Executive Vice-President of Bioscan Inc, Washington, DC, US. Prior to joining Bioscan, Mr. Van Cauter was Corporate Vice-President and Chief Technology Officer of Packard BioScience Company until its acquisition by PerkinElmer, Inc. He served as a strategic consultant to PerkinElmer from 2001 until 2003. He holds a Masters Degree of Science in Industrial Engineering from the Higher Institute for Technology in Mechelen-Leuven, Belgium and completed post-graduate studies in business administration at the University of London.