Likelihood estimation of drug occupancy for brain PET studies

[11C]PS13 Demonstrates Pharmacologically Selective and Substantial Binding to Cyclooxygenase-1 in the Human Brain

Our laboratory recently developed [11C]PS13 as a PET radioligand to selectively measure cyclooxygenase-1 (COX-1). The cyclooxygenase enzyme family converts arachidonic acid into prostaglandins and thromboxanes, which mediate inflammation. The total brain uptake of [11C]PS13, which is composed of both specific binding and background uptake, can be accurately quantified with gold standard methods of compartmental modeling. This study sought to quantify the specific binding of [11C]PS13 to COX-1 in healthy human brain using scans performed with arterial input function at baseline and after blockade by the COX-1-selective inhibitor ketoprofen. Methods: Eight healthy volunteers underwent two 90-min [11C]PS13 PET scans with radiometabolite-corrected arterial input function, at baseline and about 2 h after oral administration of ketoprofen (75 mg). Results: Two-tissue compartment modeling effectively identified the total uptake of radioactivity in the brain (as distribution volume), showing the highest densities in the hippocampus, the occipital cortex, and the banks of the central sulcus. All brain regions exhibited displaceable and specific binding, and thus none could be used as a reference region. Ketoprofen blocked approximately 84% of the binding sites on COX-1 in the whole brain. After full occupancy was extrapolated, the average whole-brain values of [11C]PS13 were 1.6 ± 0.8 mL·cm-3 for specific uptake, 1.7 ± 0.6 mL·cm-3 for background uptake, and 1.1 ± 0.5 for the specific-to-background ratio. The hippocampus had the highest specific-to-background ratio value of 2.7 ± 0.9. Conclusion: [11C]PS13 exhibited high specific binding to COX-1 in the human brain, but its quantification requires arterial blood sampling.

Kinetic models for estimating occupancy from single-scan PET displacement studies

The traditional design of PET target engagement studies is based on a baseline scan and one or more scans after drug administration. We here evaluate an alternative design in which the drug is administered during an on-going scan (i.e., a displacement study). This approach results both in lower radiation exposure and lower costs. Existing kinetic models assume steady state. This condition is not present during a drug displacement and consequently, our aim here was to develop kinetic models for analysing PET displacement data. We modified existing compartment models to accommodate a time-variant increase in occupancy following the pharmacological in-scan intervention. Since this implies the use of differential equations that cannot be solved analytically, we developed instead one approximate and one numerical solution. Through simulations, we show that if the occupancy is relatively high, it can be estimated without bias and with good accuracy. The models were applied to PET data from six pigs where [11C]UCB-J was displaced by intravenous brivaracetam. The dose-occupancy relationship estimated from these scans showed good agreement with occupancies calculated with Lassen plot applied to baseline-block scans of two pigs. In summary, the proposed models provide a framework to determine target occupancy from a single displacement scan.

Designing drug occupancy studies with PET neuroimaging: Sample size, occupancy ranges and analytical methods

Molecular neuroimaging is today considered essential for evaluation of novel CNS drugs; it is used to quantify blood-brain barrier permeability, verify interaction with key target and determine the drug dose resulting in 50% occupancy, IC₅₀. In spite of this, there has been limited data available to inform on how to optimize study designs. Through simulations, we here evaluate how IC₅₀ estimation is affected by the (i) range of drug doses administered, (ii) number of subjects included, and (iii) level of noise in the plasma drug concentration measurements. Receptor occupancy is determined from PET distribution volumes using two different methods: the Lassen plot and Likelihood estimation of occupancy (LEO). We also introduce and evaluate a new likelihood-based estimator for direct estimation of IC₅₀ from PET distribution volumes. For estimation of IC₅₀, we find very limited added benefit in scanning individuals who are given drug doses corresponding to less than 40% receptor occupancy. In the range of typical PET sample sizes (5-20 subjects) each extra individual clearly reduces the error of the IC₅₀ estimate. In all simulations, likelihood-based methods gave more precise IC₅₀ estimates than the Lassen plot; four times the number of subjects were required for the Lassen plot to reach the same IC₅₀ precision as LEO.

Simultaneous multifactor Bayesian analysis (SiMBA) of PET time activity curve data

Positron emission tomography (PET) is an in vivo imaging method essential for studying the neurochemical pathophysiology of psychiatric and neurological disease. However, its high cost and exposure of participants to radiation make it unfeasible to employ large sample sizes. The major shortcoming of PET imaging is therefore its lack of power for studying clinically-relevant research questions. Here, we introduce a new method for performing PET quantification and analysis called SiMBA, which helps to alleviate these issues by improving the efficiency of PET analysis by exploiting similarities between both individuals and regions within individuals. In simulated [11C]WAY100635 data, SiMBA greatly improves both statistical power and the consistency of effect size estimation without affecting the false positive rate. This approach makes use of hierarchical, multifactor, multivariate Bayesian modelling to effectively borrow strength across the whole dataset to improve stability and robustness to measurement error. In so doing, parameter identifiability and estimation are improved, without sacrificing model interpretability. This comes at the cost of increased computational overhead, however this is practically negligible relative to the time taken to collect PET data. This method has the potential to make it possible to test clinically-relevant hypotheses which could never be studied before given the practical constraints. Furthermore, because this method does not require any additional information over and above that required for traditional analysis, it makes it possible to re-examine data which has already previously been collected at great expense. In the absence of dramatic advancements in PET image data quality, radiotracer development, or data sharing, PET imaging has been fundamentally limited in the scope of research hypotheses which could be studied. This method, especially combined with the recent steps taken by the PET imaging community to embrace data sharing, will make it possible to greatly improve the research possibilities and clinical relevance of PET neuroimaging.

PET as a Translational Tool in Drug Development for Neuroscience Compounds

In central nervous system drug discovery programs, early development of new chemical entities (NCEs) requires a multidisciplinary strategy and a translational approach to obtain proof of distribution, proof of occupancy, and proof of function in specific brain circuits. Positron emission tomography (PET) provides a way to assess in vivo the brain distribution of NCEs and their binding to the target of interest, provided that radiolabeling of the NCE is possible or that a suitable radioligand is available. PET is therefore a key tool for early phases of drug discovery programs. This review will summarize the main applications of PET in early drug development and discuss the usefulness of PET microdosing studies performed with direct labelling of the NCE and PET occupancy studies. The purpose of this review is also to propose an alignment of the nomenclatures used by drug metabolism and pharmacokinetic scientists and PET imaging scientists to indicate key pharmacokinetic parameters and to provide guidance in the performance and interpretation of PET studies.

Kinetic modeling and parameter estimation of TSPO PET imaging in the human brain

PURPOSE

Translocator protein 18-kDa (TSPO) imaging with positron emission tomography (PET) is widely used in research studies of brain diseases that have a neuro-immune component. Quantification of TSPO PET images, however, is associated with several challenges, such as the lack of a reference region, a genetic polymorphism affecting the affinity of the ligand for TSPO, and a strong TSPO signal in the endothelium of the brain vessels. These challenges have created an ongoing debate in the field about which type of quantification is most useful and whether there is an appropriate simplified model.

METHODS

This review focuses on the quantification of TSPO radioligands in the human brain. The various methods of quantification are summarized, including the gold standard of compartmental modeling with metabolite-corrected input function as well as various alternative models and non-invasive approaches. Their advantages and drawbacks are critically assessed.

RESULTS AND CONCLUSIONS

Researchers employing quantification methods for TSPO should understand the advantages and limitations associated with each method. Suggestions are given to help researchers choose between these viable alternative methods.

Human Positron Emission Tomography Neuroimaging

Neuroimaging with positron emission tomography (PET) is the most powerful tool for understanding pharmacology, neurochemistry, and pathology in the living human brain. This technology combines high-resolution scanners to measure radioactivity throughout the human body with specific, targeted radioactive molecules, which allow measurements of a myriad of biological processes in vivo. While PET brain imaging has been active for almost 40 years, the pace of development for neuroimaging tools, known as radiotracers, and for quantitative analytical techniques has increased dramatically over the past decade. Accordingly, the fundamental questions that can be addressed with PET have expanded in basic neurobiology, psychiatry, neurology, and related therapeutic development. In this review, we introduce the field of human PET neuroimaging, some of its conceptual underpinnings, and motivating questions. We highlight some of the more recent advances in radiotracer development, quantitative modeling, and applications of PET to the study of the human brain.

Accuracy and reliability of [11C]PBR28 specific binding estimated without the use of a reference region

[¹¹C]PBR28 is a positron emission tomography radioligand used to examine the expression of the 18 kDa translocator protein (TSPO). TSPO is located in glial cells and can function as a marker for immune activation. Since TSPO is expressed throughout the brain, no true reference region exists. For this reason, an arterial input function is required for accurate quantification of [¹¹C]PBR28 binding and the most common outcome measure is the total distribution volume (V_T). Notably, V_T reflects both specific binding and non-displaceable binding. Therefore, estimates of specific binding, such as binding potential (e.g. BP_ND) and specific distribution volume (V_S) should theoretically be more sensitive to underlying differences in TSPO expression. It is unknown, however, if unbiased and accurate estimates of these outcome measures are obtainable for [¹¹C]PBR28. The Simultaneous Estimation (SIME) method uses time-activity-curves from multiple brain regions with the aim to obtain a brain-wide estimate of the non-displaceable distribution volume (V_ND), which can subsequently be used to improve the estimation of BP_ND and V_S. In this study we evaluated the accuracy of SIME-derived V_ND, and the reliability of resulting estimates of specific binding for [¹¹C]PBR28, using a combination of simulation experiments and in vivo studies in healthy humans. The simulation experiments, based on data from 54 unique [¹¹C]PBR28 examinations, showed that V_ND values estimated using SIME were both precise and accurate. Data from a pharmacological competition challenge (n = 5) showed that SIME provided V_ND values that were on average 19% lower than those obtained using the Lassen plot, but similar to values obtained using the Likelihood-Estimation of Occupancy technique. Test-retest data (n = 11) showed that SIME-derived V_S values exhibited good reliability and precision, while larger variability was observed in SIME-derived BP_ND values. The results support the use of SIME for quantifying specific binding of [¹¹C]PBR28, and suggest that V_S can be used in complement to the conventional outcome measure V_T. Additional studies in patient cohorts are warranted.