Kinetic analysis of drug-target interactions with PET for characterization of pharmacological hysteresis

Drug characteristics derived from kinetic modeling: combined 11C-UCB-J human PET imaging with levetiracetam and brivaracetam occupancy of SV2A

BACKGROUND

Antiepileptic drugs, levetiracetam (LEV) and brivaracetam (BRV), bind to synaptic vesicle glycoprotein 2A (SV2A). In their anti-seizure activity, speed of brain entry may be an important factor. BRV showed faster entry into the human and non-human primate brain, based on more rapid displacement of SV2A tracer ¹¹C-UCB-J. To extract additional information from previous human studies, we developed a nonlinear model that accounted for drug entry into the brain and binding to SV2A using brain ¹¹C-UCB-J positron emission tomography (PET) data and the time-varying plasma drug concentration, to assess the kinetic parameter K₁ (brain entry rate) of the drugs.

METHOD

Displacement (LEV or BRV p.i. 60 min post-tracer injection) and post-dose scans were conducted in five healthy subjects. Blood samples were collected for measurement of drug concentration and the tracer arterial input function. Fitting of nonlinear differential equations was applied simultaneously to time-activity curves (TACs) from displacement and post-dose scans to estimate 5 parameters: K₁ (drug), K₁(¹¹C-UCB-J, displacement), K₁(¹¹C-UCB-J, post-dose), free fraction of ¹¹C-UCB-J in brain (f_ND(¹¹C-UCB-J)), and distribution volume of ¹¹C-UCB-J (V_T(UCB-J)). Other parameters (K_D(drug), K_D(¹¹C-UCB-J), f_P(drug), f_P(¹¹C-UCB-J, displacement), f_P(¹¹C-UCB-J, post-dose), f_ND(drug), k_off(drug), k_off(¹¹C-UCB-J)) were fixed to literature or measured values.

RESULTS

The proposed model described well the TACs in all subjects; however, estimates of drug K₁ were unstable in comparison with ¹¹C-UCB-J K₁ estimation. To provide a conservative estimate of the relative speed of brain entry for BRV vs. LEV, we determined a lower bound on the ratio BRV K₁/LEV K₁, by finding the lowest BRV K₁ or highest LEV K₁ that were statistically consistent with the data. Specifically, we used the F test to compare the residual sum of squares with fixed BRV K₁ to that with floating BRV K₁ to obtain the lowest possible BRV K₁; the same analysis was performed to find the highest LEV K₁. The lower bound of the ratio BRV K₁/LEV K₁ was ~ 7.

CONCLUSIONS

Under appropriate conditions, this advanced nonlinear model can directly estimate entry rates of drugs into tissue by analysis of PET TACs. Using a conservative statistical cutoff, BRV enters the brain at least sevenfold faster than LEV.

Mechanistic Multilayer Quantitative Model for Nonlinear Pharmacokinetics, Target Occupancy and Pharmacodynamics (PK/TO/PD) Relationship of D-Amino Acid Oxidase Inhibitor, TAK-831 in Mice

Purpose

TAK-831 is a highly selective and potent inhibitor of D-amino acid oxidase (DAAO) currently under clinical development for schizophrenia. In this study, a mechanistic multilayer quantitative model that parsimoniously connects pharmacokinetics (PK), target occupancy (TO) and D-serine concentrations as a pharmacodynamic (PD) readout was established in mice.

Methods

PK, TO and PD time-profiles were obtained in mice and analyzed by mechanistic binding kinetics model connected with an indirect response model in a step wise fashion. Brain distribution was investigated to elucidate a possible mechanism driving the hysteresis between PK and TO.

Results

The observed nonlinear PK/TO/PD relationship was well captured by mechanistic modeling framework within a wide dose range of TAK-831 in mice. Remarkably different brain distribution was observed between target and reference regions, suggesting that the target-mediated slow binding kinetics rather than slow penetration through the blood brain barrier caused the observed distinct kinetics between PK and TO.

Conclusion

A quantitative mechanistic model for concentration- and time-dependent nonlinear PK/TO/PD relationship was established for TAK-831 in mice with accounting for possible rate-determining process. The established mechanistic modeling framework will provide a quantitative means for multilayer biomarker-assisted clinical development in multiple central nervous system indications.

Electronic supplementary material

The online version of this article (10.1007/s11095-020-02893-x) contains supplementary material, which is available to authorized users.

Binding of the D3-preferring antipsychotic candidate F17464 to dopamine D3 and D2 receptors: a PET study in healthy subjects with [11C]-(+)-PHNO

RATIONALE

F17464, a dopamine D3 receptor antagonist with relatively high D3 selectivity (70 fold vs D2 in vitro), exhibits an antipsychotic profile in preclinical studies, and therapeutic efficacy was demonstrated in a randomized placebo-controlled clinical trial in patients with schizophrenia (Bitter et al. Neuropsychopharmacology 44(11):1917-1924, 2019).

OBJECTIVE

This open-label study in healthy male subjects aimed at characterizing F17464 binding to D3/D2 receptors and the time course of receptor occupancy using positron emission tomography (PET) imaging with a D3-preferring tracer, [¹¹C]-(+)-PHNO.

METHODS

PET scans were performed at baseline and following a single 30 mg or 15 mg dose of F17464 (3 subjects/dose), and blood samples were collected for pharmacokinetic analysis. Receptor occupancy was calculated based upon reduction in binding potential of the tracer following F17464 administration. The relationship between plasma F17464 concentration and D3/D2 receptor occupancy was modeled and the plasma concentration corresponding to 50% receptor occupancy (EC₅₀) calculated.

RESULTS

Both doses of F17464 robustly blocked [¹¹C]-(+)-PHNO D3 receptor binding, with substantial occupancy from 1 h post-administration, which increased at 6-9 h (89-98% and 79-87% for the 30 mg and 15 mg groups, respectively) and remained detectable at 22 h. In contrast, D2 binding was only modestly blocked at all time points (< 18%). F17464 exhibited a combination of rapid peripheral kinetics and hysteresis (persistence of binding 22 h post-dose despite low plasma concentration). The best estimate of the EC₅₀ was 19 ng ml^-1 (~ 40 nM).

CONCLUSION

Overall, F17464 was strongly D3-selective in healthy volunteers, a unique profile for an antipsychotic candidate drug.

Quantifying cancer cell receptors with paired-agent fluorescent imaging: a novel method to account for tissue optical property effects

Dynamic fluorescence imaging approaches can be used to estimate the concentration of cell surface receptors in vivo. Kinetic models are used to generate the final estimation by taking the targeted imaging agent concentration as a function of time. However, tissue absorption and scattering properties cause the final readout signal to be on a different scale than the real fluorescent agent concentration. In paired-agent imaging approaches, simultaneous injection of a suitable control imaging agent with a targeted one can account for non-specific uptake and retention of the targeted agent. Additionally, the signal from the control agent can be a normalizing factor to correct for tissue optical property differences. In this study, the kinetic model used for paired-agent imaging analysis (i.e., simplified reference tissue model) is modified and tested in simulation and experimental data in a way that accounts for the scaling correction within the kinetic model fit to the data to ultimately extract an estimate of the targeted biomarker concentration.

Quantitative PET Imaging in Drug Development: Estimation of Target Occupancy

Positron emission tomography, an imaging tool using radiolabeled tracers in humans and preclinical species, has been widely used in recent years in drug development, particularly in the central nervous system. One important goal of PET in drug development is assessing the occupancy of various molecular targets (e.g., receptors, transporters, enzymes) by exogenous drugs. The current linear mathematical approaches used to determine occupancy using PET imaging experiments are presented. These algorithms use results from multiple regions with different target content in two scans, a baseline (pre-drug) scan and a post-drug scan. New mathematical estimation approaches to determine target occupancy, using maximum likelihood, are presented. A major challenge in these methods is the proper definition of the covariance matrix of the regional binding measures, accounting for different variance of the individual regional measures and their nonzero covariance, factors that have been ignored by conventional methods. The novel methods are compared to standard methods using simulation and real human occupancy data. The simulation data showed the expected reduction in variance and bias using the proper maximum likelihood methods, when the assumptions of the estimation method matched those in simulation. Between-method differences for data from human occupancy studies were less obvious, in part due to small dataset sizes. These maximum likelihood methods form the basis for development of improved PET covariance models, in order to minimize bias and variance in PET occupancy studies.

Imaging in Central Nervous System Drug Discovery

The discovery and development of central nervous system (CNS) drugs is an extremely challenging process requiring large resources, timelines, and associated costs. The high risk of failure leads to high levels of risk. Over the past couple of decades PET imaging has become a central component of the CNS drug-development process, enabling decision-making in phase I studies, where early discharge of risk provides increased confidence to progress a candidate to more costly later phase testing at the right dose level or alternatively to kill a compound through failure to meet key criteria. The so called "3 pillars" of drug survival, namely; tissue exposure, target engagement, and pharmacologic activity, are particularly well suited for evaluation by PET imaging. This review introduces the process of CNS drug development before considering how PET imaging of the "3 pillars" has advanced to provide valuable tools for decision-making on the critical path of CNS drug development. Finally, we review the advances in PET science of biomarker development and analysis that enable sophisticated drug-development studies in man.

Investigation of Saliva as an Alternative to Plasma Monitoring of Voriconazole

BACKGROUND AND OBJECTIVES

Therapeutic drug monitoring (TDM) of voriconazole is increasingly being implemented in clinical practice. However, as blood sampling can be difficult in paediatric and ambulatory patients, a non-invasive technique for TDM is desirable. The aim of this study was to compare the pharmacokinetics of voriconazole in saliva with the pharmacokinetics of unbound and total voriconazole in plasma in order to clinically validate saliva as an alternative to plasma in voriconazole TDM.

METHODS

In this pharmacokinetic study, paired plasma and saliva samples were taken at steady state in adult haematology and pneumology patients treated with voriconazole. Unbound and bound plasma voriconazole concentrations were separated using high-throughput equilibrium dialysis. Voriconazole concentrations were determined with liquid chromatography-tandem mass spectrometry. Pharmacokinetic parameters were calculated using log-linear regression.

RESULTS

Sixty-three paired samples were obtained from ten patients (seven haematology and three pneumology patients). Pearson's correlation coefficients (R values) for saliva versus unbound and total plasma voriconazole concentrations showed a very strong correlation, with values of 0.970 (p < 0.001) and 0.891 (p < 0.001), respectively. Linear mixed modelling revealed strong agreement between voriconazole concentrations in saliva and unbound plasma voriconazole concentrations, with a mean bias of -0.03 (95 % confidence interval -0.14 to 0.09; p = 0.60). For total concentrations below 10 mg/L, the mean ratio of saliva to total plasma voriconazole concentrations was 0.51 ± 0.08 (n = 63), which did not differ significantly (p = 0.76) from the unbound fraction of voriconazole in plasma of 0.49 ± 0.03 (n = 36).

CONCLUSIONS

Saliva can serve as a reliable alternative to plasma in voriconazole TDM, and it can easily be implemented in clinical practice.

Measuring blood-brain barrier penetration using the NeuroCart, a CNS test battery

To systematically study the pharmacodynamics of a CNS drug early in the development process, we developed and validated a battery of drug-sensitive CNS tests, which we call NeuroCart. Using this test battery, data-intensive phase 1 studies in healthy subjects can be performed to demonstrate the specific, time- and dose-dependent, neurophysiological and/or neuropsychological effects of a compound, thereby confirming whether the test compound reaches its intended target in the CNS - or does not reach its intended target. We use this test battery to demonstrate that a compound passes the blood-brain barrier.

Receptor Occupancy of the κ-Opioid Antagonist LY2456302 Measured with Positron Emission Tomography and the Novel Radiotracer 11C-LY2795050

The κ-opioid receptor (KOR) is thought to play an important therapeutic role in a wide range of neuropsychiatric and substance abuse disorders, including alcohol dependence. LY2456302 is a recently developed KOR antagonist with high affinity and selectivity and showed efficacy in the suppression of ethanol consumption in rats. This study investigated brain penetration and KOR target engagement after single oral doses (0.5-25 mg) of LY2456302 in 13 healthy human subjects. Three positron emission tomography scans with the KOR antagonist radiotracer (11)C-LY2795050 were conducted at baseline, 2.5 hours postdose, and 24 hours postdose. LY2456302 was well tolerated in all subjects without serious adverse events. Distribution volume was estimated using the multilinear analysis 1 method for each scan. Receptor occupancy (RO) was derived from a graphical occupancy plot and related to LY2456302 plasma concentration to determine maximum occupancy (rmax) and IC50. LY2456302 dose dependently blocked the binding of (11)C-LY2795050 and nearly saturated the receptors at 10 mg, 2.5 hours postdose. Thus, a dose of 10 mg of LY2456302 appears well suited for further clinical testing. Based on the pharmacokinetic (PK)-RO model, the rmax and IC50 of LY2456302 were estimated as 93% and 0.58 ng/ml to 0.65 ng/ml, respectively. Assuming that rmax is 100%, IC50 was estimated as 0.83 ng/ml.