On the Swell: Imaging Infection & Inflammation

Inflammation plays a major role in disease processes. For years, fluorodeoxyglucose (F-18 FDG) was the only biomarker readily available and used frequently to visualize infection and inflammation by cluing into increased metabolic activity in tissues, a telltale sign of an active immune response. In recent years, up-and-coming biomarkers both approved and still under investigation show promise for detecting the inflammation and infection involved in a range of diseases and new findings in the area of traumatic brain injury (TBI) are a growing focus for researchers.

In the kingdom of molecular imaging, F-18 FDG reigns supreme—and not just for infection imaging. The tracer is a mainstay of oncologic imaging and neurology. “FDG has been used historically in the brain for a variety of different purposes,” says James R. Stone, MD, PhD, assistant professor of radiology and medical imaging at the University of Virginia School of Medicine in Charlottesville. “It’s been helpful in the setting of Alzheimer’s disease to differentiate between the different types of entities that can cause dementia.”

Although is a widely applicable agent that can be used with both PET and SPECT and reimbursement for oncologic use was just broadened by CMS, payers are not yet on board for imaging and infection applications. To complicate matters, the modality is limited by nonspecificity.

“Although FDG-PET is already approved by the FDA and coverage was recently expanded, in certain cases, it doesn’t differentiate inflammation from cancer in particular, so there are definitely limitations,” says Xiaoyuan Shawn Chen, PhD, a senior investigator at the National Institute of Health’s National Institute of Biomedical Imaging and Bioengineering.

F-18 FDG is a valuable tool in some inflammatory processes, such as atherosclerosis and some types of arthritis, but it is limited to imaging hypermetabolic states of disease rather than elevated white blood cells and other mechanisms of immune response. Thus it yields false-positive results in some instances (Theranostics 2013; 3[7]:48-466).

“It’s extremely useful in patients with a fever of unknown origin and spinal infections,” explains Christopher Palestro, MD, chief of nuclear medicine and molecular imaging at the North Shore-LIJ System in New Hyde Park, N.Y. “Data suggest it is also useful in diagnosing large vessel vasculitis and the response to treatment and monitoring of patients with sarcoid.”

He adds that FDG also has been studied in patients with diabetic foot infections, the assessment of suspected prosthetic joint infections and in prosthetic vascular grafts, but its value for these patients is questioned. 

Novel agents for infection and inflammation

There are many more specific molecules involved in inflammation. Palestro says there are a few newer biomarkers that are the focus of much of the current research. “In vitro-labeled leukocytes that are labeled with indium-111 or technetium-99m (Tc-99m) and gallium-67 citrate (Ga-67) still represent the bulk of the work that we do,” he says. “Indium & Tc-99m labeled [white blood cells] are used primarily for bacterial infections. Ga-67 currently is used primarily for spinal osteomyelitis, sarcoid, interstitial nephritis and opportunistic infections.”
Indium-111 biotin, a bacterial growth factor, is another biomarker that Palestro says is useful for diagnosing spinal infections. However, it’s been studied almost exclusively in Italy.

Radiolabelled autologous leukocytes “have high specificity for bone infections, excluding spondylodiscitis,” notes Alberto Signore, MD, PhD, of the nuclear medicine unit at Sapienza University in Rome, Italy. F-18 FDG, he says, is highly specific for spondylodiscitis.

FDG-labelled white cells also have been studied, but Palestro explains that another limitation of this agent include variability in labeling efficiency, the quick elution of FDG from the cells and the short half-life, making it difficult to obtain delayed images if they are needed. Copper-64 (Cu-64) has a longer half-life than F-18 and Cu-64-labelled may overcome this limitation.

Another potentially useful group of molecules are the radiolabeled antimicrobial peptides, particularly a Tc-99m-labeled synthetic fragment of ubiquicidin.
“This peptide targets bacteria and appears to differentiate between infection and sterile inflammation,” Palestro explains. Research with gallium-68 has been encouraging, but, as with gallium-67, it lacks specificity. “It’s not clear  if it will be useful on its own. It may have to be complexed with another agent or compound.”

Fialuridine, a thymidine analog, is a specific substrate of bacterial thymidine kinase and has been labeled with iodine-124 in some studies. Early data in humans suggest some specificity for infection.

One recent South Korean study in mice found that imaging with I-125 2’-fluoro-2’-deoxy-5-iodo-1-beta-D-arabinofuranosyluraci (I-125 FIAU) or F-18 3’-deoxy-3’-fluorothymidine (F-18 FLT) was able to monitor a localized bacteria infection. The uptake of both molecules was high at the site of infection compared with uptake at uninfected areas (Int J Microbiol 2012; 302[2]:101-7).

Signore adds that cytokines and cytokine receptors also are used as inflammation biomarkers along with chemokine receptors, metalloprotease and macrophage antigens. Palestro says, however, that most research on cytokines comes out of the Netherlands and is only as recent as about six years ago.

Other molecules under investigation include tumor necrosis factor-alpha, a cytokine that helps bring white blood cells to areas if inflammation. The type 2 cannabinoid receptor may help detect inflammation in the brain. Interleukin-2 is a glycoprotein secreted by lymphocytes activated by inflammation. Matrix metalloproteases affect the makeup of the extracellular matrix, and more activity by these compounds may play a key role in some inflammatory conditions, such as atherosclerosis and cancer (Theranostics 2013; 3[7]:48-466).

Progress in the realm of TBI

“Detecting inflammation in the brain is different from detecting inflammation in the periphery,” says Chen. “We need small, lipophilic molecules that can cross the blood-brain barrier.”

Stone adds that while FDG can detect glucose metabolism, it is difficult to identify patterns of impaired glucose metabolism after a brain injury, so there have been efforts to identify markers that are more specific to TBI. Inflammation results after a brain injury and white blood cells migrate to the area. Researchers started tinkering with the idea of tagging certain areas of the immune system. They used an injectable imaging probe that can bind to white blood cells and infiltrate cerebrospinal fluid during brain injury. Normally white blood cells do not cross the blood-brain barrier, but when there is an injury, there may be a disruption of the barrier. Once the cells enter the brain, they attach to the sites of injury. Using a copper-based PET probe, imaging can detect where these cells end up in the brain. The peptide compound is known as cinnamoyl-Phe-(D)Leu-Phe-(D)Leu-Phe (cFLFLF). This type of imaging can help with early diagnosis of TBIs as well as help evaluate outcomes of therapy.

“What it does is help identify injuries at the cellular and subcellular level,” Stone says. “We know that in TBI, there are things occurring at these levels.” Current imaging, he explains, is geared toward visualization of large lesions visible with the naked eye. This line of research is in its early stages, but it is very promising and could proceed in the direction of regulatory approval. “We are moving toward early phase human clinical trials, but are not there yet.”