Differences in neuroinflammation in people who started antiretroviral treatment during primary versus chronic HIV infection: an 18kDa Translocator protein (TSPO) positron emission tomography (PET) study

Persistent inflammation is described in people with HIV (PWH) on antiretroviral treatment (ART). Early ART initiation is associated with reduced inflammation. We aimed to evaluate neuroinflammation, using translocator protein (TSPO) [11C]PBR28 PET neuroimaging in PWH who initiated ART during acute HIV (aPWH) versus chronic HIV infection (cPWH) versus a control population. This was a cross-sectional, observational study. All participants underwent [11C]PBR28 PET-CT neuroimaging. Using a two-tissue compartment model, total volume of distribution (VT) and distribution volume ratios (DVR) using cortical grey matter as a pseudo-reference region at 20 regions of interest (ROIs) were calculated. Differences in VT and DVR were compared between groups using the Kruskall-Wallis test. Seventeen neuro-asymptomatic male PWH on ART (9 aPWH, 8 cPWH) and 8 male control participants (CPs) were included. Median (interquartile range, IQR) age was 40 (30, 46), 44 (41, 47) and 21 (20, 25) years in aPWH, cPWH and CPs, respectively. Median (IQR) CD4 (cells/µL) and CD4:CD8 were 687 (652, 1014) and 1.37 (1.24, 1.42), and 700 (500, 720) and 0.67 (0.64, 0.82) in aPWH and cPWH, respectively. Overall, no significant difference in VT and DVR were observed between the three groups at any ROIs. cPWH demonstrated a trend towards higher mean VT compared with aPWH and CPs at most ROIs. No significant differences in neuroinflammation, using [11C]PBR28 binding as a proxy, were identified between cPWH, aPWH and CPs. A trend towards lower absolute [11C]PBR28 binding was seen amongst aPWH and CPs, suggesting early ART may mitigate neuroinflammation.

Ibudilast (MN-166) in amyotrophic lateral sclerosis- an open label, safety and pharmacodynamic trial

Ibudilast (MN-166) is an inhibitor of macrophage migration inhibitory factor (MIF) and phosphodiesterases 3,4,10 and 11 (Gibson et al., 2006; Cho et al., 2010). Ibudilast attenuates CNS microglial activation and secretion of pro-inflammatory cytokines (Fujimoto et al., 1999; Cho et al., 2010). In vitro evidence suggests that ibudilast is neuroprotective by suppressing neuronal cell death induced by microglial activation. People with ALS have increased microglial activation measured by [11C]PBR28-PET in the motor cortices. The primary objective is to determine the impact of ibudilast on reducing glial activation and neuroaxonal loss in ALS, measured by PBR28-PET and serum Neurofilament light (NfL). The secondary objectives included determining safety and tolerability of ibudilast high dosage (up to 100 mg/day) over 36 weeks. In this open label trial, 35 eligible ALS participants underwent ibudilast treatment up to 100 mg/day for 36 weeks. Of these, 30 participants were enrolled in the main study cohort and were included in biomarker, safety and tolerability analyses. Five additional participants were enrolled in the expanded access arm, who did not meet imaging eligibility criteria and were included in the safety and tolerability analyses. The primary endpoints were median change from baseline in (a) PBR28-PET uptake in primary motor cortices, measured by standard uptake value ratio (SUVR) over 12-24 weeks and (b) serum NfL over 36-40 weeks. The secondary safety and tolerability endpoints were collected through Week 40. The baseline median (range) of PBR28-PET SUVR was 1.033 (0.847, 1.170) and NfL was 60.3 (33.1, 219.3) pg/ml. Participants who completed both pre and post-treatment scans had PBR28-PET SUVR median(range) change from baseline of 0.002 (-0.184, 0.156) , P = 0.5 (n = 22). The median(range) NfL change from baseline was 0.4 pg/ml (-1.8, 17.5), P = 0.2 (n = 10 participants). 30(86%) participants experienced at least one, possibly study drug related adverse event. 13(37%) participants could not tolerate 100 mg/day and underwent dose reduction to 60-80 mg/day and 11(31%) participants discontinued study drug early due to drug related adverse events. The study concludes that following treatment with ibudilast up to 100 mg/day in ALS participants, there were no significant reductions in (a) motor cortical glial activation measured by PBR28-PET SUVR over 12-24 weeks or (b) CNS neuroaxonal loss, measured by serum NfL over 36-40 weeks. Dose reductions and discontinuations due to treatment emergent adverse events were common at this dosage in ALS participants. Future pharmacokinetic and dose-finding studies of ibudilast would help better understand tolerability and target engagement in ALS.

Acute neuroimmune stimulation impairs verbal memory in adults: A PET brain imaging study

Psychiatric and neurologic disorders are often characterized by both neuroinflammation and cognitive dysfunction. To date, however, the relationship between neuroinflammation and cognitive dysfunction remains understudied in humans. Preclinical research indicates that experimental induction of neuroinflammation reliably impairs memory processes. In this paradigm development study, we translated those robust preclinical findings to humans using positron emission tomography (PET) imaging with [¹¹C]PBR28, a marker of microglia, and lipopolysaccharide (LPS), a potent neuroimmune stimulus. In a sample of 18 healthy adults, we extended our previous findings that LPS administration increased whole-brain [¹¹C]PBR28 availability by 31-50%, demonstrating a robust neuroimmune response (Cohen's ds > 1.6). We now show that LPS specifically impaired verbal learning and recall, hippocampal memory processes, by 11% and 22%, respectively (Cohen's ds > 0.9), but did not alter attention, motor, or executive processes. The LPS-induced increase in [¹¹C]PBR28 binding was correlated with significantly greater decrements in verbal learning performance in the hippocampus (r = -0.52, p = .028), putamen (r = -0.50, p = .04), and thalamus (r = -0.55, p = .02). This experimental paradigm may be useful in investigating mechanistic relationships between neuroinflammatory signaling and cognitive dysfunction in psychiatric and neurologic disorders. It may also provide a direct approach to evaluate medications designed to rescue cognitive deficits associated with neuroinflammatory dysfunction.

Detecting neuroinflammation in the brain following chronic alcohol exposure in rats: A comparison between in vivo and in vitro TSPO radioligand binding

Excessive alcohol consumption is associated with neuroinflammation, which likely contributes to alcohol-related pathology. However, positron emission tomography (PET) studies using radioligands for the 18-kDa translocator protein (TSPO), which is considered a biomarker of neuroinflammation, reported decreased binding in alcohol use disorder (AUD) participants compared to controls. In contrast, autoradiographic findings in alcohol exposed rats reported increases in TSPO radioligand binding. To assess if these discrepancies reflected differences between in vitro and in vivo methodologies, we compared in vitro autoradiography (using [3 H]PBR28 and [3 H]PK11195) with in vivo PET (using [11 C]PBR28) in male, Wistar rats exposed to chronic alcohol-vapor (dependent n = 10) and in rats exposed to air-vapor (nondependent n = 10). PET scans were obtained with [11 C]PBR28, after which rats were euthanized and the brains were harvested for autoradiography with [3 H]PBR28 and [3 H]PK11195 (n = 7 dependent and n = 7 nondependent), and binding quantified in hippocampus, thalamus, and parietal cortex. Autoradiography revealed significantly higher binding in alcohol-dependent rats for both radioligands in thalamus and hippocampus (trend level for [3 H]PBR28) compared to nondependent rats, and these group differences were stronger for [3 H]PK11195 than [3 H]PBR28. In contrast, PET measures obtained in the same rats showed no group difference in [11 C]PBR28 binding. Our in vitro data are consistent with neuroinflammation associated with chronic alcohol exposure. Failure to observe similar increases in [11 C]PBR28 binding in vivo suggests the possibility that a mechanism mediated by chronic alcohol exposure interferes with [11 C]PBR28 binding to TSPO in vivo. These data question the sensitivity of PBR28 PET as a methodology to assess neuroinflammation in AUD.

Neuroinflammation and Cytokines in Myalgic Encephalomyelitis/Chronic Fatigue Syndrome (ME/CFS): A Critical Review of Research Methods

Myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) is the label given to a syndrome that can include long-term flu-like symptoms, profound fatigue, trouble concentrating, and autonomic problems, all of which worsen after exertion. It is unclear how many individuals with this diagnosis are suffering from the same condition or have the same underlying pathophysiology, and the discovery of biomarkers would be clarifying. The name "myalgic encephalomyelitis" essentially means "muscle pain related to central nervous system inflammation" and many efforts to find diagnostic biomarkers have focused on one or more aspects of neuroinflammation, from periphery to brain. As the field uncovers the relationship between the symptoms of this condition and neuroinflammation, attention must be paid to the biological mechanisms of neuroinflammation and issues with its potential measurement. The current review focuses on three methods used to study putative neuroinflammation in ME/CFS: (1) positron emission tomography (PET) neuroimaging using translocator protein (TSPO) binding radioligand (2) magnetic resonance spectroscopy (MRS) neuroimaging and (3) assays of cytokines circulating in blood and cerebrospinal fluid. PET scanning using TSPO-binding radioligand is a promising option for studies of neuroinflammation. However, methodological difficulties that exist both in this particular technique and across the ME/CFS neuroimaging literature must be addressed for any results to be interpretable. We argue that the vast majority of ME/CFS neuroimaging has failed to use optimal techniques for studying brainstem, despite its probable centrality to any neuroinflammatory causes or autonomic effects. MRS is discussed as a less informative but more widely available, less invasive, and less expensive option for imaging neuroinflammation, and existing studies using MRS neuroimaging are reviewed. Studies seeking to find a peripheral circulating cytokine "profile" for ME/CFS are reviewed, with attention paid to the biological and methodological reasons for lack of replication among these studies. We argue that both the biological mechanisms of cytokines and the innumerable sources of potential variance in their measurement make it unlikely that a consistent and replicable diagnostic cytokine profile will ever be discovered.

A pilot trial of RNS60 in amyotrophic lateral sclerosis

INTRODUCTION

RNS60 is a novel immune-modulatory agent that has shown neuroprotective effects in amytrophic lateral sclerosis (ALS) preclinical models. RNS60 is administered by weekly intravenous infusion and daily nebulization. The objective of this pilot open-label trial was to test the feasibility, safety, and tolerability of long-term RNS60 administration in ALS patients.

METHODS

The planned treatment duration was 23 weeks and the primary outcomes were safety and tolerability. Secondary outcomes included PBR28 positron emission tomography (PET) imaging and plasma biomarkers of inflammation.

RESULTS

Sixteen participants with ALS received RNS60 and 13 (81%) completed 23 weeks of RNS60 treatment. There were no serious adverse events and no participants withdrew from the trial due to drug-related adverse events. There were no significant changes in the biomarkers.

DISCUSSION

Long-term RNS60 administration was safe and well-tolerated. A large, multicenter, phase II trial of RNS60 is currently enrolling participants to test the effects of RNS60 on ALS biomarkers and disease progression. Muscle Nerve 59:303-308, 2019.

Preclinical comparison study between [18F]fluoromethyl-PBR28 and its deuterated analog in a rat model of neuroinflammation

We designed and synthesized deuterium-substituted [¹⁸F]fluoromethyl-PBR28 ([¹⁸F]1-d₂) as a novel translocator protein 18 kDa (TSPO)-targeted radioligand with enhanced in vivo stability. The comparison studies between [¹⁸F]fluoromethyl-PBR28 ([¹⁸F]1) and its deuterate analog ([¹⁸F]1-d₂) were investigated in terms of in vitro binding affinity, lipophilicity and in vivo stability. In addition, the accuracies of both radioligands were determined by comparing the PET imaging data in the same LPS-induced neuroinflammation rat model. Both aryloxyanilide analogs showed similar lipophilicity and in vitro affinity for TSPO. However, [¹⁸F]1-d₂ provided significantly lower femur uptake than [¹⁸F]1 (1.5 ± 1.2 vs. 4.1 ± 1.7%ID/g at 2 h post-injection) in an ex vivo biodistribution study. [¹⁸F]1-d₂ was also selectively accumulated in the inflammatory lesion with the binding potential of the specifically bound radioligand relative to the non-displaceable radioligand in tissue (BP_ND = 3.17 ± 0.48), in a LPS-induced acute neuroinflammation rat model, comparable to that of [¹⁸F]1 (BP_ND = 2.13 ± 0.51). These results indicate that [¹⁸F]1-d₂ had higher in vivo stability, which resulted in an enhanced target-to-background ratio compared to that induced by [¹⁸F]1.

Translocator protein (TSPO) ligands for the diagnosis or treatment of neurodegenerative diseases: a patent review (2010-2015; part 1)

INTRODUCTION

The translocator protein (TSPO) is an emerging target in diverse neurodegenerative diseases. Up-regulated TSPO in the central nervous system (CNS) appears to be involved in neuroinflammatory processes; therefore, the development of potent TSPO ligands is a promising method for alleviating or imaging patients with neurodegenerative diseases. Areas covered: This review will provide an overview of recently developed TSPO ligands patented from 2010 to 2015. Part 1 will present a summary focusing on TSPO ligands other than indole-based or cholesterol-like compounds, which will be discussed in part 2. Part 1 covers diverse benzodiazepine-derived analogues such as isoquinoline carboxamides and aryloxyanilides. Moreover, bicyclic ring structures such as imidazopyridine, pyrazolopyrimidine, and phenylpurine will be highlighted as promising scaffolds for TSPO ligands. A brief analysis of currently reported TSPO structures will also be covered in part 1. Expert opinion: Although the underlying pharmacological mechanism of TSPO remains to be elucidated, several TSPO ligands have shown therapeutic efficacy in experimental animal models of neurodegenerative diseases. In addition, radioactive TSPO ligands have been extensively studied for the diagnosis of neurodegenerative processes. Thus, further studies on both the basic and applied mechanisms of TSPO are warranted in the pursuit of successful pharmacological applications of TSPO ligands.

Imaging robust microglial activation after lipopolysaccharide administration in humans with PET

Neuroinflammation is associated with a broad spectrum of neurodegenerative and psychiatric diseases. The core process in neuroinflammation is activation of microglia, the innate immune cells of the brain. We measured the neuroinflammatory response produced by a systemic administration of the Escherichia coli lipopolysaccharide (LPS; also called endotoxin) in humans with the positron emission tomography (PET) radiotracer [11C]PBR28, which binds to translocator protein, a molecular marker that is up-regulated by microglial activation. In addition, inflammatory cytokines in serum and sickness behavior profiles were measured before and after LPS administration to relate brain microglial activation with systemic inflammation and behavior. Eight healthy male subjects each had two 120-min [11C]PBR28 PET scans in 1 d, before and after an LPS challenge. LPS (1.0 ng/kg, i.v.) was administered 180 min before the second [11C]PBR28 scan. LPS administration significantly increased [11C]PBR28 binding 30-60%, demonstrating microglial activation throughout the brain. This increase was accompanied by an increase in blood levels of inflammatory cytokines, vital sign changes, and sickness symptoms, well-established consequences of LPS administration. To our knowledge, this is the first demonstration in humans that a systemic LPS challenge induces robust increases in microglial activation in the brain. This imaging paradigm to measure brain microglial activation with [11C]PBR28 PET provides an approach to test new medications in humans for their putative antiinflammatory effects.

Test-retest reproducibility of [(11)C]PBR28 binding to TSPO in healthy control subjects

PURPOSE

The PET radioligand [(11)C]PBR28 binds to the translocator protein (TSPO), a marker of brain immune activation. We examined the reproducibility of [(11)C]PBR28 binding in healthy subjects with quantification on a regional and voxel-by-voxel basis. In addition, we performed a preliminary analysis of diurnal changes in TSPO availability.

METHODS

Twelve subjects were examined using a high-resolution research tomograph and [(11)C]PBR28, six in the morning and afternoon of the same day, and six in the morning on two separate days. Regional volumes of distribution (V T) were derived using a region-of-interest based two-tissue compartmental analysis (2TCM), as well as a parametric approach. Metabolite-corrected arterial plasma was used as input function.

RESULTS

For the whole sample, the mean absolute variability in V T in the grey matter (GM) was 18.3 ± 12.7 %. Intraclass correlation coefficients in GM regions ranged from 0.90 to 0.94. Reducing the time of analysis from 91 to 63 min yielded a variability of 16.9 ± 14.9 %. There was a strong correlation between the parametric and 2TCM-derived GM values (r = 0.99). A significant increase in GM V T was observed between the morning and afternoon examinations when using secondary methods of quantification (p = 0.028). In the subjects examined at the same time of the day, the absolute variability was 15.9 ± 12.2 % for the 91-min 2TCM data.

CONCLUSION

V T of [(11)C]PBR28 binding showed medium reproducibility and high reliability in GM regions. Our findings support the use of parametric approaches for determining [(11)C]PBR28 V T values, and indicate that the acquisition time could be shortened. Diurnal changes in TSPO binding in the brain may be a potential confounder in clinical studies and should be investigated further.

Comparison of standardized uptake values with volume of distribution for quantitation of [(11)C]PBR28 brain uptake

INTRODUCTION

[(11)C]PBR28 is a high-affinity ligand for the Translocator Protein 18 kDa (TSPO), which is considered to be a marker for microglial activation. Volume of distribution (VT) estimated with an arterial plasma input function is the gold standard for quantitation of [(11)C]PBR28 binding. However, arterial sampling is impractical at many PET sites for multiple reasons. Reference region modeling approaches are not ideal for TSPO tracers, as the existence of a true reference region cannot be assumed. Given that it would be desirable to have a non-invasive index of [(11)C]PBR28 binding, we elected to study the utility of the semi-quantitative metric, standardized uptake value (SUV) for use in brain [(11)C]PBR PET studies. The primary goal of this study was to determine the relationship between SUV and VT.

METHODS

We performed a retrospective analysis of data from sixteen [(11)C]PBR28 PET scans acquired in baboons at baseline and at multiple time points after IV injection of lipopolysaccharide, an endotoxin that transiently induces neuroinflammation. For each scan, data from 14 brain regions of interest were studied. VT was estimated with the Logan plot, using metabolite-corrected input functions. SUV was calculated with data from 30 to 60 minutes after [(11)C]PBR28 injection.

RESULTS

Within individual PET studies, SUV tended to correlate well with VT. Across studies, the relationship between SUV and VT was variable.

CONCLUSIONS

From study to study, there was variability in the degree of correlation between [(11)C]PBR28 VT and SUV. There are multiple physiological factors that may contribute to this variance.

ADVANCES IN KNOWLEDGE

As currently applied, the non-invasive measurement of SUV does not appear to be a reliable outcome variable for [(11)C]PBR28. Additional work is needed to discover the source of the discrepancy in SUV between [(11)C]PBR28 scans.

IMPLICATIONS FOR PATIENT CARE

There is a need to develop alternatives to arterial plasma input functions for TSPO ligands in order to facilitate multi-center trials.