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Quantitative analysis of positron emission tomography (PET) with the second generation translocator protein (TSPO) ligand [18F]GE-180
[Thesis]. Manchester, UK: The University of Manchester; 2016.
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Abstract
Background: The 18 kDa translocator protein (TSPO), expressed at a low level in the healthy human central nervous system (CNS), is upregulated in inflammatory brain diseases by activated microglia and other immune cells. Using positron emission tomography (PET) radioligands targeting TSPO, it is possible to localise this signal and map the course of microglial activation and its effects on disease progression. Here, a newly developed second generation TSPO PET ligand, [18F]GE-180, was evaluated in different models of preclinical and clinical neuroinflammatory disease.Methods: A preclinical model of low-level inflammation with lipopolysaccharide (LPS) was designed. Rats were scanned with the first generation TSPO ligand [11C]- (R)-PK11195 and either [18F]GE-180 or [18F]DPA-714, with dual scanning enabling the direct comparison of second generation tracers with [11C]-(R)-PK11195. An arterial blood sampling system for rodent imaging with [18F]DPA-714 was set up and characterised. The performance of [18F]GE-180 was assessed in a clinical study in nine relapsing-remitting multiple sclerosis patients (RRMS) and ten healthy volunteers (HV). A comparison of kinetic modelling approaches for [18F]GE-180 human brain PET data was performed, as well as a longitudinal analysis with intervention using the disease-modifying treatment, natalizumab to evaluate the potential of [18F]GE-180 as a biomarker for therapy monitoring in MS subjects. Finally, the plasma-protein binding behaviour of [18F]GE-180 was evaluated in vitro using ultrafiltration.Results: In LPS animals, [18F]GE-180 produced a significantly higher ipsi- to contralateral uptake ratio and binding potential (𝐵𝑃𝑁𝐷) than [11C]-(R)-PK11195 (p = 0.03), but [18F]DPA-714 did not. There was no significant difference between animals scanned with [18F]GE-180 and [18F]DPA-714, suggesting no overall superiority of the former. Characterisation of an arterial sampling system for rodent studies with [18F]DPA-714 allowed correction for dispersion effects. A comparison of reference regions showed that a novel externally derived tissue estimated 𝐵𝑃𝑁𝐷 with lower bias than a contralateral reference region. In human [18F]GE-180 brain PET data, the unconstrained two-tissue compartment model (2TCM) best described tracer behaviour in RRMS and HV subjects. Normal appearing white matter (NAWM) in patients was elevated over that of HVs. Standardised uptake values (SUVs) for the tracer in rodents were 0.28±0.12 and 0.84±0.31 in healthy tissue and LPS lesions respectively, and in humans were 0.36±0.04 (HV) and 0.58 (in a gadolinium- enhancing MS lesion). [18F]GE-180 uptake was also significantly reduced in the brains of RRMS subjects treated with natalizumab, correlating with clinically-identified improvement. [18F]GE-180 has a free fraction of between 1 and 8%.Conclusions: [18F]GE-180 shows good brain uptake in the rodent brain and produces superior signal to [11C]-(R)-PK11195, but not to [18F]DPA-714. The 2TCM fits human [18F]GE-180 PET data well, and the tracer is able to identify an elevated signal in RRMS patients compared to healthy subjects. [18F]GE-180 shows a large fraction of non-displaceable binding in human blood, thus further optimisation of kinetic modelling approaches is suggested.