Molecular imaging that will bring about a revolution in diagnosis and drug discovery
28 November 2008 (Volume 3 Issue 11)
Molecular imaging is a technology that helps us probe the location of target molecules in living organisms, including human beings. As the various phenomena in living beings result from interactions among molecules, molecular imaging is expected to become essential for developing a better understanding of life and its processes. Masaaki Suzuki, of the Molecular Imaging Medicinal Chemistry Laboratory, says, “Molecular imaging is the ultimate goal of life science.” Molecular imaging is expected to assist in the early detection of lifestyle-related diseases, such as cancer, dementia, and diabetes, as well as in the development of improved drugs with the fewest side-effects much more quickly. This article reports on the forefront of molecular imaging research supported by Japan’s “world’s best capabilities in chemistry.”
Observing molecules in the human body
“To begin with, please take a look at our laboratory, and you will easily understand what we are doing. No other research institute in the world has molecular imaging facilities better than ours,” says Suzuki with confidence and a smile.
The RIKEN Center for Molecular Imaging Science (CMIS), formerly the Molecular Imaging Research Program (MIRP), was established in August 2008. The research base of CMIS was moved to the Kobe Molecular Imaging Research and Development Center on Port Island in Kobe City, close to the RIKEN Center for Developmental Biology. Research and development at CMIS is actively promoted by the collaboration of the following five teams and two units:
“When you look up at the night sky, you can see the moon and stars, and distant galaxies radiating out. Scientists in the twentieth century had a dream of finding new celestial bodies. In the twenty-first century, we are trying to observe the molecules that control life functions in the living human body. This is what we call ‘molecular imaging’.”
Figure 1: How positron emission tomography (PET) works.
A tiny amount of a positron-emitting radionuclide is combined with a target molecule, and the radiolabeled chemical is then administered by intravenous injection. Carbon-11 (11C) emits a single positron upon decaying to boron-11 (11B). The positron collides with nearby electrons, emitting gamma rays in opposing directions, and the gamma rays are detected by the PET instrument. Tomographic images show the distribution and quantity of target molecules in the organism.
How, then, can we observe molecules in the human body? A powerful technique is positron emission tomography (PET). PET images are constructed based on the distribution of gamma rays produced by the collision between positrons and electrons (Fig. 1). In the PET process, a tiny amount of a positron-emitting radionuclide is combined with the molecule under investigation, and the radiolabeled product is administered to an organism. The combination of radionuclide and target molecule is called a ‘molecular probe’. Once administered, the radionuclide gradually decays to a different nuclide, emitting positrons that collide with nearby electrons. These positron-electron collisions in turn cause the emission of gamma rays, which can be readily detected. This technique therefore makes it possible to determine where, and in what quantities, the target molecules are located in the organism.
PET is frequently used in the diagnosis of cancer. The molecular probe used for cancer diagnosis is fluorodeoxyglucose (FDG), which is a combination of deoxyglucose and fluorine-18 (18F). As proliferating cancer cells take in much glucose as a source of energy, PET can be used to detect the intense gamma emissions generated by the accumulation of FDG in cancer cells. PET has thus improved the accuracy of cancer detection compared to conventional methods of cancer diagnosis based on X-ray and computed tomography scanning techniques.
However, as Suzuki points out, “The latent utility of PET is not limited to diagnosis. We have not yet exploited PET to its fullest. We may make a mistake in diagnosis because FDG accumulates not only in cancer cells but also in larger internal organs or normal actively working cells. If there were a molecular probe that interacts with molecules that are expressed only in cancer cells, a more accurate diagnosis could be obtained. Our target is therefore to develop new molecular probes that help PET work more effectively, thus contributing to the imaging of various molecules in the human body.”
Rapid C-methylation reaction that achieved the seemingly impossible
What is the process used to construct molecular probes? CMIS has two compact cyclotrons that are used for this purpose (Fig. 2-1). The radionuclide, in this case carbon-11 (11C), is formed by bombarding nitrogen gas with high-speed protons accelerated in the cyclotron. Various nuclides can be produced by changing the target being bombarded according to the specific nuclear reactions. The radionuclides thus produced are fed rapidly to a ‘hot cell’ for the synthesis of radiolabeled chemicals. The hot cell, referring to a cell used to handle radioactive materials, is completely shielded to prevent radiation leakage into the environment. An automated synthesizer installed in the hot cell (Fig. 2-2) then combines the radionuclides from the cyclotron with target molecules to afford radiolabeled chemicals. This synthesis, however, is a very difficult process.
One of the problems encountered in the development of this procedure is the length of time needed for synthesis. The 11C radionuclide has a radioactive half-life (the period over which the number of radionuclides decreases by one-half due to radioactive decay to another nuclide) of only 20 minutes. Taking account of the time required to purify the molecular probes and transfer the radiolabeled chemical to another area for diagnostic use, a time window of only five minutes remains for combining the 11C and the target molecule. You might think that radionuclides with a longer half-life could be used. However, Suzuki asserts, “We use only radionuclides with a short half-life. If we dealt with radionuclides with a longer half-life, there would be a risk of radiation exposure. Long-half-life radionuclides cannot be used in humans. We should instead choose radionuclides that are already present in our bodies and which have as short a half-life as possible. Therefore, 11C is the best choice.”
However, there were major difficulties in applying 11C in this procedure because no method was previously known for the efficient combination of 11C atoms with carbon-containing target molecules at time scales of a few minutes. Furthermore, none of the labelling sites in the molecules were expected to be metabolized immediately. “Someone even told me that my idea was out of the question. However, the more difficult a problem seems, the more likely you are to take up the challenge.” Eventually, Suzuki succeeded in developing a ‘rapid C-methylation reaction’ that combines 11C-methyl groups (11CH3–) with carbon atoms in organic molecules in just five minutes (Fig. 3). By conventional chemical reactions, this combination reaction would take from a few hours to several dozen hours to complete. Furthermore, this highly efficient reaction can be used to combine 11C-methyl groups with almost all organic molecules. “It took five years to successfully develop a chemical reaction that takes just five minutes.” What was the key to this success? “Inspiration and perseverance,” answers Suzuki.
Figure 3: Rapid C-methylation reactions.
Carbon-11 (11C) produced in the cyclotron reacts with a tiny amount of oxygen to afford carbon dioxide (11CO2) bearing the 11C radionuclide. In the automated synthesizer, 11CO2 is converted to methanol (11CH3OH) in the presence of a reducing agent. The methanol is then reacted with iodic acid (HI) to afford methyl iodide (11CH3I). Methyl groups (CH3–) in the target molecule are then substituted with radiolabeled methyl groups by reaction with the 11C-containing methyl iodide.
Purification and enrichment of the radiolabeled chemicals are conducted automatically in an automated synthesizer, and the molecular probe bearing the radionuclide at the desired atomic sites is isolated in a glass vessel housed within a shielded lead container. The molecular probe is then transferred by the most recent linear-motor-driven conveyance system to a PET system for animals within the same laboratory (Fig. 2-3). The transfer of the molecular probe takes about one minute, and once loaded into the PET system, the molecular probe is promptly administered (Fig. 2-4). Mice are used as test subjects to monitor whether the probes move as expected, and the results are fed back to the team at the Molecular Imaging Medicinal Chemistry Laboratory for design changes as required. This process is repeated several times during the development of a new molecular probe (Fig. 4).
Straight through from basic study to human beings
At the next stage of development, molecular probes that have proved useful in animal experiments are administered to humans. CMIS has installed areas that conform to Good Maintenance Practice (GMP) standards, which are a set of manufacturing and quality control regulations established for the provision of safe and high-quality drugs and medicines, and these areas will become operational in the near future. Molecular probes created with the utmost care in the GMP synthesis area will be transferred to the Institute of Biomedical Research and Innovation, which is linked to CMIS via a connecting bridge. There, the molecular probes will be administered to humans.
Figure 4: Examples of molecular probes.
Examples of PET brain images of monkeys to which molecular probes consisting of 15R-tolylisocarbacyclin methyl ester and 11C atoms have been administered. Depending on the position of 11C on the molecules, molecular probes fail to reach the brain (center) or fail to accumulate in a specific area (right). The molecular design is indispensable in ensuring that molecular probes accumulate in a specific area.
Port Island in Kobe City hosts the Kobe Medical Industry Development Project. The RIKEN Center for Developmental Biology, the Institute of Biomedical Research and Innovation, and civilian hospitals are also located close by in the same cluster. “That is the reason why RIKEN chose this site. This location provides us with opportunities in the medical field to immediately test the most advanced results obtained through fundamental studies. We conduct basic studies at RIKEN from start to finish, but we can only contribute to society if the results can be successfully applied to humans.” The Next Generation Supercomputer being developed by RIKEN, due for completion in 2012, is also to be located on Port Island, and collaboration with this new facility is anticipated.
How will molecular imaging change the medical field? “In 10 years, we expect PET to be used regularly for diagnosis and treatment. It will allow the early detection of various diseases, including lifestyle-related diseases such as dementia, cancer, and diabetes,” says Suzuki.
To that end, researchers need to develop a library of disease-specific molecular probes. “If candidate molecules for diagnosis can be found, it remains only for chemists to process these into molecular probes. This is not a problem for us,” says Suzuki confidently. Recent advances in genome analysis also back up his confidence because it has become easier to find candidate molecules. “Japan is aging at a rapid pace,” says Suzuki. “We will surely face health problems with the elderly, such as dementia. We want to expand the molecular probe library so that many more patients can be diagnosed.”
Another item that is high on the list of aims is the contribution to drug discovery. “A drug discovery revolution will occur,” asserts Suzuki. At present, candidate drugs are administered to mice, and only when proved to be very efficient are the drugs finally administered to humans in clinical trials. Many candidate drugs are eliminated at this stage because of side effects or ineffectiveness in the human body, even if they have proved effective in mice. The development of even a single new drug is thus very expensive and laborious, and is a task that takes many years. “Molecular imaging will be the solution to this problem,” says Suzuki.
By preparing molecular probes from candidate drugs, researchers can confirm the effects of the drug on the human body before starting clinical trials. Such a molecular probe will interact with target receptor molecules, but also possibly with unexpected molecules. If the unexpected interactions prove to induce side effects, the developer has an opportunity to change the structure of the candidate drug such that it interacts more specifically with the target receptor molecules. In this manner it is possible to considerably reduce the time and cost incurred in the development of new drugs.
On 28 December 2007, the Japanese Ministry of Health, Labour and Welfare released a draft version of Guidelines for the Conduct of Microdose Clinical Trials, and also a revised draft version of the Building and Facility Standards for Manufacturing Facilities for Investigational Medical Products (Investigational Medical Product GMP). A microdose clinical trial is a clinical study conducted in the early stages of drug development, and involves the administering of a single ‘microdose’ of one or more test substances to humans in order to examine its intended and side effects. The conventional Investigational Medical Product GMP targets only the bulk production of drugs, whereas the revised version permits more flexible operations, such as microdose clinical trials. The formal version of the guidelines was released on 3 June 2008. With these developments, molecular imaging is becoming a reality.
Japan has the world’s best capabilities in chemistry
Many international scientists visit CMIS. “They are all astounded at the high level of our advanced studies,” says Suzuki. Why is CMIS taking the lead in molecular imaging research? “The reason is that Japan has the world’s best capabilities in chemistry. Successful research on molecular imaging requires the fusion of all fields of sciences, including chemistry, medicine, pharmacy, biology and engineering. Among these, chemistry is the most important,” says Suzuki. The combination of radionuclides with molecules and the alteration of molecular structures are both based on chemical reactions. “Japan has the world’s best capabilities in chemistry; Japanese scientists have won the Nobel Prize in Chemistry for three consecutive years. In fact, the president of RIKEN, Ryoji Noyori, is himself a Nobel Laureate in Chemistry,” says Suzuki.
Noyori presented PET images of human brains during his Nobel lecture in 2001. For these images, Suzuki himself served as the human subject. “Dr Noyori listed molecular imaging as one of the most important technologies that should be promoted in chemistry. Dr Noyori was my teacher. I want to use the advantages of chemistry to promote molecular imaging so that it can yield significant results.”
About the Researcher
Masaaki Suzuki
Masaaki Suzuki was born in Gifu, Japan in 1947. He graduated from Nagoya University in 1970 and obtained his PhD in 1975 (Prof. Y. Hirata) from the same university. After postdoctoral training at Harvard University (Prof. R.B. Woodward) until 1977, he returned to Nagoya University and served as assistant professor with Prof. R. Noyori until 1983. He then worked as associate professor at the same university before attaining full professorship at Gifu University, Japan, in 1993. He served as a professor there until 2007, when he was appointed vice program director of the RIKEN Molecular Imaging Research Program. He is now vice director of RIKEN CMIS in Kobe, Japan.
