Molecular imaging plays a key role in pharmacology evaluation and is expected to grow significantly in the future. Even though preclinical imaging has proven its worth, it is still a relatively new technology. Consistent paradigms have yet to be developed and applied to drug discovery and development.
As instrumentation and software continue to improve, new tracers develop, and multi-modality approaches refine, proof-of-concept in the preclinical setting will likely translate into valuable clinical diagnostic tools.
Novel advances in molecular imaging for preclinical applications were discussed at the recent “WorldPharma Congress” conference.
Quantitative whole-body autoradiography (QWBA) is widely used by the majority of the pharmaceutical industry to provide quantitative tissue distribution data as part of a preclinical ADME program.
Conventionally, QWBA is used to support clinical study safety by ensuring that drug and associated radioactivity exposures, based on a predicted dose level, are appropriate for human testing.
“Being able to quantitate how much drug or drug equivalent you have, based on a radioactive tag in a particular tissue three-cell-layers thick, has increased the drug safety and tissue distribution aspect 10-fold,” commented Stefan Linehan, manager, preclinical services, XenoBiotic Laboratories, and president, Society of Whole Body Autoradiography.
If the radioactive concentration is low or at a similar concentration in adjacent tissues, as displayed on a standard gray-scale autoradioluminogram, the mapping out and outlining of the regions of interest can be best-guess determined, if at all. Linehan addresses these resolution issues with a technology he terms cryo-imaging and quantitative autoradiography (CIQA).
CIQA allows for a series of optical images, captured every 25 µm, throughout the whole body of a frozen rodent carcass embedded in a solid block of carboxymethylcellulose. Periodic consecutive sections are acquired and processed to generate informative images produced by autoradiography, histology, fluorescence, and immunohistochemistry.
Using a customized software program, the images are registered and reconstructed in 3D, allowing entire organs to be viewed in high resolution, 25–50 µm, with all the informative sections interlaced.
In addition, the long half-life of the radioisotopes used, such as 14C, allows for pharmacokinetic and pharmacodynamic models within different tissues over an extended timeframe.
“CIQA is meant to be a complementary technique, to help add to the knowledge gained from other imaging modalities. The recent advancements can be used to provide useful information in many other fields such as toxicology, pharmacology, neurology, and oncology,” concluded Linehan.
Maximizing Information Extraction
Extracting as much encoded information as possible from an image, including deriving kinetic parameters, is crucial in molecular image analysis.
In pharmacokinetic groups, a common way to analyze data is measurements of signal changes over some period of time, the area under the curve (AUC). However, that analysis does not capture the kinetics.
“We use DCE-MRI to investigate anti-angiogenic and antivascular therapies for tumors because it allows us to measure tissue perfusion and permeability,” discussed Matt Silva, Ph.D., director, research imaging sciences, Amgen.
“The problem with dynamic-measuring methods is that in order to fit to a compartmental model, we need to know the input function to the system, the injected contrast agent, and how it transverses through the body. There is no easy way to do that.”
Current input method estimations—location of a vessel, population-based averages, or the use of reference regions—have flaws. They do not capture the kinetics, and this is why researchers using DCE-MRI extended the analysis beyond AUC into the Ktrans value.
“Utilizing the target tissue as an estimate for the input function for modeling may be a more sensitive way to detect changes and has the added advantage of enhancing the performance of the software. This methodology is not specific to DCE-MRI and can be used in other modalities, like FDG PET. You can measure AUC, but if you want to extract the metabolic rate of glucose you need to do compartmental analysis.
“There are additional challenges in imaging analysis. We spend weeks doing whole-body analyses. Half our work is data collection and half analysis. Looking at specific regions, or organs, during data analysis can be very time consuming,” continued Dr. Silva.
“So we initiated a digital mouse atlas project. The concept is to be able to quickly screen with the digital atlas and rank order on semi-automated analyses. Then our scientists would step in and evaluate the most critical datasets.”
In collaboration with inviCRO, the digital mouse atlas project is still in early phases, but the proof-of-concept is promising.