by Mark Lane
For most of human history, there was only one way to study the stars: with your own eyes. Now scientists rely on a variety of telescopes capturing different parts of the electromagnetic spectrum . Each tells a unique story about the universe, and those stories can be combined for greater insight.
The same is true in molecular imaging. The leading platforms used in preclinical research – SPECT, PET and CT – have their strengths and weaknesses depending on the target. At MPI Research, we use these technologies every day. We know what works best in a given study and, where appropriate, how to combine them to maximize results.
Molecular imaging visualizes, characterizes and measures biological processes at the molecular and cellular levels. This data can be captured in vivo and is critically important for understanding how a drug candidate affects the body. As new ways of using imaging systems emerge, through applications of synthetic chemistry, pharmacology and molecular biology, the demand is growing.
Let’s take a look at the three most commonly used platforms.
SPECT, or single photon emission computed tomography, captures single, high-energy photons, better known as gamma rays, that come from the decay of certain radioactive isotopes. A trace amount of a drug, labeled with a gamma-emitting isotope, allows SPECT to provide qualitative and quantitative measurement of what the drug does or doesn’t do. Since isotopes for SPECT imaging have longer half-lives, smaller quantities are needed, more data can be gathered from fewer subjects over a longer period, and multiple images can be done at the same time using different isotopes. SPECT is widely used because of its flexibility and lower cost.
PET, or positron emission tomography, is much more sensitive than SPECT imaging. It measures photons that are released when a positron (a positively charged particle) from a radioisotope annihilates an electron (a negatively charged particle) in surrounding tissue. A PET scan creates a three-dimensional image of a process in the body, like that done in a hospital on a doctor’s order. PET’s advantages include high molecular sensitivity, down to the nanomolar level, and unlimited depth penetration. It’s also more expensive, and isotope half-life isn’t as robust as those used in SPECT.
CT, better known as computed tomography, is another process commonly done in hospitals. Technically it’s not a molecular imaging tool, but it does provide a high-quality anatomical framework that complements SPECT and PET scans. CT relies on X-rays emitted from an X-ray machine that is rotated around the subject. The X-rays that are not absorbed by the body are captured by a detector to produce cross-sectional images.
These imaging processes can be used for measuring drug distribution, tumor imaging (i.e., tumor metabolism, tumor angiogenesis, tumor hypoxia), infection models, cardiac and respiratory gating, cell labeling, bone density measurements, arthritis and fetal skeletal evaluations.
Selecting the appropriate imaging tool depends on the target and the potential impact on what is being measured. Applying more than one technology in the right way can yield new insights into the form, structure and function of a drug candidate in a preclinical model.
Like the telescopes that peer at the sky, these imaging tools are opening eyes to the wonders of our molecular and biological universe.
Mark Lane is the Manager of Discovery Operations and Principal Investigator in Molecular Imaging at MPI Research. For more on our molecular imaging services, contact firstname.lastname@example.org