The detrimental impact of brain disorders can be life changing. To better understand their causes, scientists are pioneering a new molecular imaging process, which is both economic and accessible. Using this method to visualise the operations of a glycoprotein present in the brain, they ultimately aim to design benign treatments, which modulate its potentially harmful functions

There are as many neurons in the human brain as there are stars in the Milky Way. It’s not in the least bit surprising, then, that within the inner space of our cerebra, mysterious biological processes that science cannot yet define occur. Although their physical symptoms are often discomfortingly visible, the limitations of investigative technology often means that triggers for brain disorders, at a molecular level, are difficult to discern. Prising secrets from this delicate organ requires highresolution examinations of live subjects, which has stimulated a need for new monitoring tools.

“PET (Positron Emission Tomography) is the current, ‘state of the art’ system used for analysing sensitive molecular activity within the brain. It uses scanner systems, which monitor radioactive tracers injected into a subject, to generate images of biological processes in vivo – that is, within the body,” explains Dr. Gert Luurtsema. A specialist from the University of Groningen, University Medical Center Groningen (UMCG), Luurtsema currently heads a STW project seeking to better understand and control the role of P-glycoprotein (P-gp) in the brain. P-gp is an efflux transporter, a protein which ejects harmful and toxic compounds from brain cells. Located at the protective blood brain barrier (BBB) and several peripheral organs, P-gp acts like a microscopic doorman, granting entry to benign objects and molecules whilst shielding the brain from perceived threats. Whilst widely acknowledged as playing a critical role in developing the body’s immunity to certain types of medicines, it is also now becoming recognised as a potential catalyst of brain sickness.

Funded by a €500,000 budget across its four year duration, the venture, launched in 2012, unites a team of multinational specialists at a European centre renowned for its expertise in PET technology. “It’s a multidisciplinary group, which includes radiochemists, pharmacologists, biologists, physicists, and physicians,” says Luurtsema. “We’re collaborating with Dr. Bert Windhorst from the VU University Medical Center of Amsterdam and Prof. Dr. Nicola Antonio Colabufo from the University in Bari, Italy, and have also forged relationships with partners in Brazil.” A comprehensive infrastructure supports their activities at the department of Nuclear Medicine and Molecular Imaging of the UMCG, including the latest PET-CT scanners, in-house GMP- facilities for synthesising tracers and a recently refurbished set of cyclotron units (particle accelerators used to produce radionuclides for PET diagnostics).

“We strongly suspect that the functionality of the BBB is related to brain disorders,” says Luurtsema. “If its operations are disturbed, we believe that they can become a causal factor in conditions like Alzheimer’s and Parkinson’s disease. Inadequate clearance of peptides [an operation undertaken by the P-gp] could lead them to accumulate in the brain. This could, we propose, lead to toxic phenomena when they penetrate the BBB.” But, he admits, the researchers don’t entirely know how these mechanisms work in vivo. If these functions can be better understood, improved diagnostics and perhaps therapeutic treatments could be devised to address them.

However, current imaging tools used to examine brain functions are inadequate for the task. Carbon-11, a commonly used radionuclide for synthesizing a PET tracer, has a short half life of twenty minutes, which curtails the duration of monitoring cycles. Moreover, this carbon-11 labelled tracer can only be used by PET facilities which possess an on-site cyclotron, as well as GMP compliant radiopharmaceutical production configuration. “Developing very specific fluorine-18 labelled tracers that are specifically carried by P-gp, and no other transporters, has been absolutely critical for us and is therefore the focus of this project,” says Luurtsema. “These fluorine-18 labelled tracers must also be stable once in vivo, and remain so throughout an entire PET scan.

Their radiochemistry must be reliable, and the radioactive yields sufficient to facilitate robust PET scanning. Simultaneously, they must also exhibit fast radiochemistry, which decays during a brief time span, without posing a hazard to the subject.” By using an alternative radionuclide, fluorine-18, the team has been able to synthesise a novel tracer which exhibits these attributes. Its properties will be augmented further by the researchers, who seek to design new tracers to measure an increase or reduce P-gp activity, thereby accentuating their profiles once scanned. “Measuring the transporters is paramount, but understanding if their functionality can be controlled by modulating or inducing P-gp responses may also have important medicinal applications.”

Another, wider scientific benefit of the group’s pioneering tracers is that they can be utilised in PET facilities without a cyclotron. “Once our compounds are refined, industrialised and patented, we hope to use spin-off companies to sell them to other molecular imaging centres,” says Luurtsema. “Their dissemination will also create more opportunities for advanced research throughout the field, since they require less costly equipment to conduct scans”. Commercially, this is a significant European market – with the current radiopharmaceutical sector in the Netherlands alone worth around €12.5m per year.

“We’ve been successful in locating new lead compounds, which are not only highly specific, but also very stable in vivo. A lot of in vitro data exists which concerns transporters within the brain, but this is inapplicable in vivo. This is quite a new innovative science, and the most unique aspect of our work,” says Luurtsema. The Netherlandsbased researchers have initiated their first radiochemistry experiments, and evaluated a first test on genetically modified mice. “Our immediate goals are to undertake pre-clinical assessments, but the next step will be for us to internally produce these compounds in our facilities at Groningen. Subsequently, we’ll scrutinise the tracers in healthy, volunteer subjects, in conjunction with specialists in neurology and psychiatry,” says Luurtsema. “Of course, this is dependent on the success of pre-clinical trials, and obtaining proof of concept in the tests we’re conducting on mice, rats and through full tracer kinetic modelling.”