New PET scan tracer allows first imaging of the epigenetics of the human brain

A novel PET radiotracer developed at the Martinos Center for Biomedical Imaging at Massachusetts General Hospital (MGH) is able for the first time to reveal epigenetic activity – the process that determines whether or not genes are expressed – within the human brain. In their report published in Science Translational Medicine, a team of MGH/Martinos Center investigators reports how their radiochemical – called Martinostat – shows the expression levels of important epigenetics-regulating enzymes in the brains of healthy volunteers.

(Image caption: Martinostat imaging of the human brain reveals levels of the epigenetic enzymes called HDACs, with red reflecting the highest and blue the lowest levels. Credit: H.-Y. Wey et al., Science Translational Medicine (2016))

“The ability to image the epigenetic machinery in the human brain can provide a way to begin understanding interactions between genes and the environment,” says Jacob Hooker, PhD, of the Martinos Center, senior author of the report. “This could allow us to investigate questions such as why some people genetically predisposed to a disease are protected from it? Why events during early life and adolescence have such a lasting impact on brain health? Is it possible to ‘reset’ gene expression in the human brain?”

A key epigenetic mechanism is the packaging of DNA into chromosomes, in which it wraps around proteins called histones forming a structure called chromatin. Modification of histones by the addition or removal of molecules called epigenetic factors can regulate whether or not an adjacent gene is expressed. One of the most important of these factors is the acetyl molecule, addition of which allows a gene to be transcribed and removal of which – called deacetylation – prevents transcription.

Enzymes called histone deacetylases (HDAC) are important regulators of gene transcription, and one group of HDACs has been linked to important brain disorders. Several established neuropsychiatric drugs are HDAC inhibitors, and others are currently being studied as potential treatment for Alzheimer’s disease and Huntington’s disease. Martinostat was developed in Hooker’s laboratory and is patterned after known HDAC inhibitors in order to tightly bind to HDAC molecules in the brain.

PET scans with Martinostat of the brains of eight healthy human volunteers revealed characteristic patterns of uptake – reflecting HDAC expression levels – that were consistent among all participants. HDAC expression was almost twice as high in gray matter as in white matter; and within gray matter structures, uptake was highest in the hippocampus and amygdala and lowest in the putamen and cerebellum. Experiments with brain tissues from humans and baboons confirmed Martinostat’s binding to HDAC, and studies with neural progenitor stem cells revealed specific genes regulated by this group of HDACs, many of which are known to be important in brain health and disease.

“HDAC dysregulation has been implicated in a growing number of brain diseases, so being able to study HDAC regulation both in the normal brain and through the progression of disease should help us better understand disease processes,” says Hooker, who is an associate professor of Radiology at Harvard Medical School. “We’ve now started studies of patients with several neurologic or psychiatric disorders, and I believe Martinostat will help us understand the different ways these conditions are manifested and provide new insights into potential therapies.”

Days 23-26: July 5-8: Nuclear Medicine Week

Nuclear Medicine Week. 75% of the students in this program will shudder at the very mention of this name. But, because I cater to those who want to learn and share in my experiences, I will recount my survival of Medicine Week for those readers who might be interested in the material. Who I commend for their bravery and dedication to such a ridiculous (but very important) science.

Tuesday: Dr. Cathy Cutler from Washington University in Missouri and Dr. Henry Van Brocklin from University of California-San Francisco were our guest lecturers for the week. Dr. Cutler also works at the University of Missouri Research Reactor (MURR.) I had the hardest time staying awake for all 6 hours of lecture this day. But I took some notes on some interesting things so bear with me:

  • Radiotracers are very small amounts of a radioactive isotope that are injected into the body that participate in biological processes but do not perturb the processes. It can be used either for treatment or diagnosis of diseases. 
  • The Magic Bullet was Paul Ehrlich’s idea to use a drug that would seek out and attack the ‘parasitic invader’ but not harm the host. This resulted in the cure for syphilis in 1909. He also attached toxins to antibodies so that the body could naturally transport the drug to the diseased site. This paved the way for the development of immunotoxins.
  • Technetium-99m is a very important isotope that is used extensively in radiopharmacy. We’ll come back to all the cool things it does.
  • Imaging! I’m sure most of you have heard of one of the following: MRI (Magnetic Resonance Imaging), SPECT (Single-Photon Emission Computed Tomography), or PET (Positron-Emission Tomography) scans. I’ll go into detail about them later.
  • Radiolabeled Probes/Drugs: there were a couple stories in here about how iodine-123 is used for treatment of the thyroid (maybe for cancer?? I don’t know, my notes started to suck at this point) and strontium-90 is used to treat bone cancer. Also, it is difficult to get the isotope to go exactly where you want it to in the body, so that is a current challenge. Other challenges to consider when using radioisotopes in medicine are production capacity (can we make enough of it for efficient use?) and the economy (can we afford to regularly make the drug with this isotope?) Something to keep in mind is that every radioisotope used in medicine has to have a fairly short half life so that it decays away quickly and the patient doesn’t receive an excess dose of radiation. That means the production of the drug needs to be fast, and the process of administration to the patient needs to be fast. 
  • George de Hevesy did research with radioactive isotopes of lead-210 (which he thought were forms of radium, called “radium-D”) in Ernest Rutherford’s lab in his earlier career, won the 1943 Nobel Prize in chemistry for his studies of radiotracers in plants, animals, and humans, developed the technique of neutron activation analysis (NAA - you’ll hear about this technique later) and discovered element # 72, Hafnium. He was an accomplished man, but throughout the course of Medicine Week we heard this famous landlady story about his first radiotracer 'investigation’ a billion times: While at boarding school in Manchester, England, he suspected that the landlady was recycling their food scraps from each meal after he began having chronic upset stomach. To test his hypothesis, he placed trace amounts of radioactive material in the Sunday meal, then tested the meal served a few days later, confirming the presence of radioactivity in the food and confronting a bewildered landlady. Cheeky Bastard.

Wednesday: I came to class prepared with an energy drink the size of my quad that day. I definitely enjoyed today’s lecture the best (maybe because I was awake for all of it??) Dr. Cutler talked about radionuclide generators because we did a lab that afternoon using one. Basically a generator is a device that produces a useful short-lived supply of a medical radionuclide (called the “daughter” in nuke chem) from a non-medical long-lived radionuclide (called the “parent.”) [Note: the reaction is the parent with the long half-life decays into the daughter with the short half-life.] Here’s a summary of the lab we did, which includes the concepts that complemented it from lecture:

  1. Prepared Technetium-99m PnAO (the “m” means “meta-stable,” so a very short half-life) by directly combining 99mTcO4-, PnAO, Sn2+ and NaHCO3.
  2. Prepared it again using a Glucoscan kit as an exchange ligand. The kit contains glucoheptonate, Sn2+ and some other stabilizers.
  3. Executed paper chromatography using the Technetium-99m PnAO produced using both the above methods with ether, acetone, and saline as the chromatography solvents.
  4. Executed a solvent extraction on both types of Technetium-99m PnAO solutions, separating the 99mTcPnAO from the pertechnetate anion and the 99mTc-GH in ether and saline. The former compound partitioned into the organic ether layer, while the latter two sank into the aqueous saline layer.
  5. Both the Chromatography and Solvent Extraction methods were used to illustrate the concept of Quality Control. QC just checks on the purity of the compounds used in the generator, to make sure you are synthesizing exactly what you need and the amount of that product you need without the presence of by-products (radiochemical purity) or other unwanted isotopes (radionuclidic purity.)

I’m convinced that the write-up instructions for this lab were sent to us directly from Hell. I even received an email from Dr. Van Brocklin asking if I sent him the entire report or if the last few pages got lost in cyberspace… I had to assure him that what I sent him was my report in its entirety :P But in happier news, all the suffering I endured last Fall semester in Biochemistry I paid off… I understood quite a bit of the chemistry between the radionuclides and the body! Yay for retention! Also, for all you biochem kids out there, when Dr. Cutler starting talking about 'current Good Manufacturing Practice,’ or 'cGMP,’ all I could think about was cyclic guanosine monophosphate! Hahaha

Thursday: Dr. Van Brocklin lectured for a RIDICULOUS AMOUNT OF TIME about PET scans. No one should know that much about PET, let alone choose to teach it to a group of nuclear chemistry students who would rather play with accelerator beams and fission reactions than even utter the words “molecular imaging.” However, we sally forth. Dr. Cutler talked about MURR and the research she does over there with medical radioisotope production. The MURR, as the name suggests, is home to a nuclear reactor that uses a flux of neutrons generated in the cyclotron to produce molybdenum-99 (which decays to the famous technetium-99 isotope) as well as many other medical radionuclides.


PET Reveals Inflammatory Cycle in the Brain

Neuroinflammation caused by a reactive immune system could be tripping off the neurodegeneration seen in certain dementias, multiple sclerosis, and other deadly diseases of the nervous system. A novel molecular imaging technique could be the key to understanding how best to treat these and other devastating diseases, according to a recent study presented at the 2015 Annual Meeting of the Society of Nuclear Medicine and Molecular Imaging (SNMMI).

At the heart of this maladaptive immune response are microglia, immune cells in the central nervous system that can be activated to trigger neuroinflammation. For this study, researchers used positron emission tomography (PET) to measure activation of microglia by employing a molecule from E. coli bacteria called lipopolysaccharide (LPS), or endotoxin. LPS stimulates the immune system and is accompanied by a radiotracer called carbon-11 PBR28 (C-11 PBR28). This form of molecular imaging allows the minimally invasive visualization of neuroinflammation. C-11 PBR28, is injected and binds to translocator proteins expressed on activated microglia. A PET scanner can then detect the radioactive particles emitted from inside the brain, representing areas of increased microglial activation before and after immune stimulation with LPS.

“The imaging technique could shed light on the immune dysfunction that underpins a broad range of neuroinflammatory diseases, such as Alzheimer’s disease, depression, post-traumatic stress disorder and addiction,” said Christine Sandiego, PhD, lead author of the study and a researcher from the department of psychiatry at the Yale School of Medicine in New Haven, Conn. “This is the first human study that accurately measures this immune response in the brain. The discoveries made with this technique could contribute to promising new drug treatments.”

The PET radiotracer C-11 PBR28 was administered to eight healthy men around the age of 25, give or take six years, followed by two separate PET scans on the same day for each subject before and after injection with LPS. Adverse symptoms were self-reported and blood samples were taken to assess levels of peripheral inflammation. Results of the study showed that administering LPS led to a substantial spike in the systemic inflammatory response and levels of reported sickness, and activated microglia in the central nervous system.

With further research, eventual drug therapies could potentially cut the activation of neurodegenerative microglia and encourage neuroprotective processes in the brain.

Amyloid PET May Lead to Better Treatment for Alzheimer’s Patients

New research presented during the 2015 Annual Meeting of the Society of Nuclear Medicine and Molecular Imaging (SNMMI) demonstrates that amyloid positron emission tomography (amyloid PET) scans of the brain provide clearer diagnosis and earlier, more effective treatment for Alzheimer’s patients, when results of a more conventional PET scan remain ambiguous.

Researchers reviewed the clinical outcomes of two kinds of PET scans: a preliminary scan with a common radiotracer called fluorodeoxyglucose (FDG), which acts like glucose in the brain to capture images of metabolic activity, and amyloid PET, which involves a different kind of imaging agent that binds to deposits of naturally occurring beta-amyloid proteins in the brain. Results of the study showed that for FDG-PET scans that did not provide a definitive diagnosis, an additional amyloid PET scan contributed to an accurate prediction of cognitive decline.

“By acquiring an amyloid imaging scan within a month of the indeterminate FDG-PET, managing physicians can treat patients with an Alzheimer’s-like pattern of reduced brain metabolism with a better chance of preserving cognitive function,” said Erica Parker, lead author of the study and a research coordinator at the University of California, Los Angeles.

For this study, FDG and amyloid PET data from 100 study participants with mild cognitive impairment were evaluated independently in two blinded readings conducted by a radiologist and a nuclear medicine physician. Readings were documented as positive, negative or indeterminate for neurodegenerative disease.

Results showed 82 percent of patients whose readings were initially indeterminate experienced subsequent cognitive decline. Amyloid PET scans were found to be positive in half (50 percent) of cases that were indeterminate using FDG-PET alone. These results were then integrated with results of another study, which found that Alzheimer’s treatment was administered earlier for 40 percent of patients with positive FDG-PET scans. Additionally, the subjects who began the Alzheimer’s treatment earlier had better cognitive outcomes. Combining these analyses, the researchers found that up to 17 percent of those who had an amyloid scan after an ambiguous FDG-PET scan can expect to experience statistically significant improvements in cognition by the two-year follow-up.

More than 44.3 million people worldwide are estimated to have Alzheimer’s disease. These numbers are expected to increase to more than 135 million by 2050, according to 2013 data from Alzheimer’s Disease International (ADI).