NANOTECHNOLOGY-DERIVED PROBES FOR MOLECULAR IMAGING
Consider the images below. These were taken with a combined Positron Emission Tomography/Computed Tomography (PET/CT) molecular imaging system, and they are of a 54 year old female patient. The patient had previously had surgery and chemotheraphy treatments for ovarian cancer. As a follow-up, oncologists had been monitoring a serum protein, known as CA125, that exhibits at least some correlation with the presence of ovarian cancer. This woman’s CA125 levels were rising. However, when radiologists imaged this woman using a stand-alone CT imaging tool, they found no sign of cancer. However, the oncologists were a little hesitant to diagnose a complete remission, and so they requested the PET/CT scan you see below.
These images aren’t too difficult to read, and so here is a quick lesson: The entire PET/CT image is a three-dimensional construction, although the images shown below are two-dimensional slices. Both images are co-collected. The CT image is presented in black and white, and it reveals the internal anatomy of this patient. Note that you can resolve some of the bone structure of the hips, as well as certain of the soft tissues.
The PET image is the colored part, and what it maps out are the regions of glucose metabolism. The patient was administered a trace amount of a radiopharmaceutical molecule known as 2-deoxy-2-[F-18]fluoro-D-glucose, or FDG for short. This is simply a sugar molecule (glucose) in which one hydrogen atom has been replaced with a radiolabeled fluorine atom, 18F. When the 18F atom goes through radioactive decay, it emits two particles in opposite directions. This particle pair is known as a positron. The positron is detected using special detectors that encircle the body. The entire image collection takes just a few minutes. The body can’t really tell the difference between ordinary glucose and FDG, and so in areas where glucose metabolism rates are increased, so too are the rates of FDG metabolism. It turns out that one of the most common characteristics of cancerous tumors is that their rate of glucose metabolism is greatly increased – as much as 20-fold – as compared to normal tissues. The result is that 18F builds up within cancerous tissues as the FDG is metabolized. Positron emission thus makes those particular cancerous tissues appear bright.
Now take a look again at the images. A large, bright yellow region appears just above the crotch – this is the bladder, and the 18FDG that was given to the patient clears through the bladder. Thus, this area should be bright – it is just a sign of normal body function. However, there are also a couple of small bright yellow spots – one of them is indicated with a black horizontal line drawn through it. These are metastastic legions. Unfortunately, this patient is not in remission, her ovarian cancer has begun to metastasize.
PET scanners were invented and developed by Michael Phelps, co-Director of the NSBCC and Principal Investigator of this project. The ability of PET to detect cancers that are missed by other procedures has meant that PET scans are now FDA-approved procedures for many cancers. It is clearly a very useful tool, and, in fact, approximately 95% of all hospital beds in the U.S. are within reach of a PET scanner. However, the potential of PET still remains largely untapped, and the goal of this Project is to develop technologies that can help PET realize its full potential as a diagnostic tool for detecting cancer, as a means of following the response of cancers to therapy, and as a means of identifying whether or not a cancer drug hits its target or not. It turns out that the technology to be developed here doesn’t revolve around making better PET scanners – the figures below should provide convincing evidence that the imaging hardware of PET is already fairly advanced – and additional advancements are likely to come out of the commercial sector, such as (NSBCC partner) Siemens, GE, Hitachi – some of the companies that currently manufacture PET and PET/CT scanners (see image below). Instead, the most exciting new technology opportunities center around the molecular imaging probes.
A PET/CT Scanner
Systems biology techniques are beginning to reveal the diverse nature of cancers that were once thought to be single diseases (see Project 1 and Project 5 descriptions), and this is setting the groundwork for the emerging world of personalized, predictive, and preventive medicine. As this new world develops, therapies targeted to relatively small patient subpopulations (rather than ‘blockbuster’ drugs) will become standard. Medical diagnostics and clinical pathology must develop to the point where those patient sub-populations can be readily identified. As a result, diagnostics and therapeutics will become increasingly coupled. This places serious challenges for in vitro diagnostics, but it also brings serious challenges to the area of in vivo diagnostics in the form of molecular imaging. As an example, this means that molecules that behave similarly to 18FDG, but image a diverse array of quite different metabolic processes – processes that can be utilized to stratify patients according to the specific nature of their disease – will need to be prepared. A requirement of radiolabeled pharmaceuticals is that they must be manufactured very quickly and in high yield, since they are undergoing rapid radioactive decay throughout the entire synthesis process (18F has a half life of under 2 hours). Thus, technologies that can give chemists the freedom to synthesis a variety of molecules, but can enable those syntheses to be rapid and highly efficient, are needed. To this end, NSBCC investigators are developing a technology called chemical reaction circuits (CRC’s) that are able to produce radiolabeled molecular imaging probes in remarkably high yields and very quickly; they are developing a new class of molecular imaging probes designed specifically to probe molecular processes that are at the heart of the genetic and environmental perturbations that lead to cancer (see Project 1 description), and they are developing chips for the rapid screening of those probes within in vitro environments.
Chemical Reaction Circuits Chemical reaction circuits (CRCs) take advantage of the versatility of integrated, elastomeric multilayer elastomeric microfluidics systems. Those systems were developed at Caltech by NSBCC consultant Professor Steve Quake (now at Stanford) and Caltech Physics Professor Axel Scherer. They differ from more conventional glass microfluidics systems in that pressure-actuated valves and pumps are constructed directly onto the chip. This is done through the use of multiple layers of the elastomeric material from which the chip is constructed: one layer contains the the chambers and channels that are used for the reaction, and the second layer contains pressure actuated valves and pumps that can serve to isolate one chamber from another, mix fluids, meter out small fluidic volumes of reagents, etc. Basically, the various chemical processes involved in synthesizing a radiopharmaceutical, such as 18FDG, are laid out in microfluidics space, and then the reaction is carried out under automated control. Processes such as ion exchange, the loading of 18F [fluoride], solvent exchange, acid hydrolysis, etc., are all possible within this environment. A key advantage is that the process illustrated by the figure below – the CAD-assisted design of a CRC to the construction of an operating chip, takes only a couple of days, and a different chip potentially equates to a different radiolabeled PET imaging probe.
The CRC chip shown in the above two figures was a first generation chip designed to synthesize 18FDG, but it was also designed to execute approximately a half-dozen very different chemical processes sequentially for that synthesis. These process included ion exchange, anhydrous chemical reactions (in acetonitrile solvent), aqueous based chemical reactions, acid hydrolysis, solvent exchange, etc. A second generation chip was developed and that chip produced sufficient 18FDG to image to mouse models of cancer. A paper describing those two chips was recently published in Science. This multidisciplinary effort involved several NSBCC faculty, including Hsian-Rong Tseng (corresponding author), Steve Quake, Michael Phelps, Owen Witte, Hartmuth Kolb, and Jim Heath as well as the companies Siemens Biomarker Solutions and Fluigidm, Inc. NSBCC researchers Guodong You (Tseng group), Frank Lee (Quake group), and Dr. Arkadij Elizarov (Heath group) were co-first authors. As of this writing (Jan. 2006) 6th generation chips have been designed and are currently undergoing testing, as part of a project that involves Caltech (Heath group), Siemens (Hartmuth Kolb), Materia (founded by NSBCC faculty and Nobel Laureate Robert Grubbs), Liquidia (founded by CCNE (North Carolina Center) investigator Professor Joseph DeSimone) and Fluidigm. These advanced chips are designed to produce 5 human doses per synthetic run, within a timeframe of approximately 5 minutes. We expect that CRCs for the preparation of radiolabeled PET imaging probes will enter FDA trials during calendar year 2006 and be commercialized shortly thereafter.
Other technologies being developed by NSBCC researchers that directly impact the goals of Project 3 include in situ click chemistry approaches towards the fabrication of high-affinity radiolabeled molecular imaging probes against specific cancer targets (Kolb), CRCs screening large libraries of compounds that are used in the in situ click chemistry approaches (Kolb and Tseng), a novel in vitro chip for screening PET compounds, with PET-on-a-chip capabilities (Tseng), and new solvent and chemically resistant materials for elastomeric microfluidics (Heath, Kolb, Materia and Liquidia). For Dr. Kolb’s papers describing in situ click chemistry, click here. Several of these projects are highly leveraged through other funds, including the Institute for Molecular Medicine at UCLA (IMED), and a couple of grants and contracts from Siemens to the Tseng and Heath groups.