Pharmacokinetics and metabolism of [14C]-tozadenant (SYN-115), a novel A2a receptor antagonist ligand, in healthy volunteers

Radiosynthesis and In Vitro Evaluation of [11C]tozadenant as Adenosine A2A Receptor Radioligand

Tozadenant (4-hydroxy-N-(4-methoxy-7-morpholinobenzo[d]thiazol-2-yl)-4-methylpiperidine-1-carboxamide) is a highly selective adenosine A2A receptor (A2AR) antagonist and a promising lead structure for the development of A2AR-selective positron emission tomography (PET) probes. Although several 18F-labelled tozadenant derivatives showed favorable in vitro properties, recent in vivo PET studies observed poor brain penetration and lower specific binding than anticipated from the in vitro data. While these findings might be attributable to the structural modification associated with 18F-labelling, they could also reflect inherent properties of the parent compound. However, PET studies with radioisotopologues of tozadenant to evaluate its cerebral pharmacokinetics and brain distribution are still lacking. In the present work, we applied N-Boc-O-desmethyltozadenant as a suitable precursor for the preparation of [O-methyl-11C]tozadenant ([11C]tozadenant) by O-methylation with [11C]methyl iodide followed by acidic deprotection. This approach afforded [11C]tozadenant in radiochemical yields of 18 ± 2%, with molar activities of 50-60 GBq/µmol (1300-1600 mCi/µmol) and radiochemical purities of 95 ± 3%. In addition, in vitro autoradiography in pig and rat brain slices demonstrated the expected striatal accumulation pattern and confirmed the A2AR specificity of the radioligand, making it a promising tool for in vivo PET studies on the cerebral pharmacokinetics and brain distribution of tozadenant.

Adenosine receptor antagonists: Recent advances and therapeutic perspective

Adenosine is an endogenous purine-based nucleoside expressed nearly in all body tissues. It regulates various body functions by activating four G-protein coupled receptors, A₁, A_2A, A_2B, and A₃. These receptors are widely acknowledged as drug targets for treating different neurological, metabolic, and inflammatory diseases. Although numerous adenosine receptor inhibitors have been developed worldwide, achieving target selectivity is still a big hurdle in drug development. However, the identification of specific radioligands-based affinity assay, fluorescent ligands, and MS-based ligand assay have contributed to the development of selective and potent adenosine ligands. In recent years various small heterocyclic-based molecules have shown some promising results. Istradefylline has been approved for treating Parkinson's in Japan, while preladenant, tozadenant, CVT-6883, MRS-1523, and many more are under different phases of clinical development. The present review is focused on the quest to develop potent and selective adenosine inhibitors from 2013 to early 2021 by various research groups. The review also highlights their biological activity, selectivity, structure-activity relationship, molecular docking, and mechanistic studies. A special emphsesis on drug designing strategies has been also given the manuscript. The comprehensive compilation of research work carried out in the field will provide inevitable scope for designing and developing novel adenosine inhibitors with improved selectivity and efficacy.

Synthesis and Biological Evaluation of a Novel 18F-Labeled Radiotracer for PET Imaging of the Adenosine A2A Receptor

The adenosine A_2A receptor (A_2AR) has emerged as a potential non-dopaminergic target for the treatment of Parkinson’s disease and, thus, the non-invasive imaging with positron emission tomography (PET) is of utmost importance to monitor the receptor expression and occupancy during an A_2AR-tailored therapy. Aiming at the development of a PET radiotracer, we herein report the design of a series of novel fluorinated analogs (TOZ1-TOZ7) based on the structure of the A_2AR antagonist tozadenant, and the preclinical evaluation of [¹⁸F]TOZ1. Autoradiography proved A_2AR-specific in vitro binding of [¹⁸F]TOZ1 to striatum of mouse and pig brain. Investigations of the metabolic stability in mice revealed parent fractions of more than 76% and 92% of total activity in plasma and brain samples, respectively. Dynamic PET/magnetic resonance imaging (MRI) studies in mice revealed a brain uptake but no A_2AR-specific in vivo binding.

Quantitative Analysis of Tozadenant Using Liquid Chromatography-Mass Spectrometric Method in Rat Plasma and Its Human Pharmacokinetics Prediction Using Physiologically Based Pharmacokinetic Modeling

Tozadenant is one of the selective adenosine A2a receptor antagonists with a potential to be a new Parkinson's disease (PD) therapeutic drug. In this study, a liquid chromatography-mass spectrometry based bioanalytical method was qualified and applied for the quantitative analysis of tozadenant in rat plasma. A good calibration curve was observed in the range from 1.01 to 2200 ng/mL for tozadenant using a quadratic regression. In vitro and preclinical in vivo pharmacokinetic (PK) properties of tozadenant were studied through the developed bioanalytical methods, and human PK profiles were predicted using physiologically based pharmacokinetic (PBPK) modeling based on these values. The PBPK model was initially optimized using in vitro and in vivo PK data obtained by intravenous administration at a dose of 1 mg/kg in rats. Other in vivo PK data in rats were used to validate the PBPK model. The human PK of tozadenant after oral administration at a dose of 240 mg was simulated by using an optimized and validated PBPK model. The predicted human PK parameters and profiles were similar to the observed clinical data. As a result, optimized PBPK model could reasonably predict the PK in human.

Antiparkinsonian effects of the "Radiprodil and Tozadenant" combination in MPTP-treated marmosets

Objective

Investigate a combination of two clinically tested drugs, the NR2B antagonist Radiprodil and the A_2A antagonist Tozadenant in the MPTP-treated marmoset model of Parkinson’s Disease (PD).

Background

In PD, there remains a need for the development of non-dopaminergic drugs to effectively treat the motor symptoms without the induction of L-Dopa-induced motor complications.

Methods

Clinically relevant doses of Radiprodil and Tozadenant were given both alone and in combination without the addition of L-Dopa, and the antiparkinsonian efficacy of the treatments was assessed in a primate model of PD.

Results

When compared to the drugs tested alone, the drug combination led to a significant increase of motor activity and an improvement of motor disability in MPTP-treated marmosets. In addition, the motor restoration brought about by the combination was almost completely devoid of dyskinesia. Interestingly, treated primates were not overstimulated, but were able to move normally when motivated by the exploration of novel objects.

Conclusion

We have demonstrated in a primate model that, the “Radiprodil/Tozadenant” combination significantly improves motor activity, extending previous results obtained in unilaterally lesioned 6-OHDA-rats. The strength of the preclinical data accumulated so far suggests that the use of such an A_2A and NR2B antagonist combination could bring significant motor improvement to PD patients, without inducing the motor complications induced by L-Dopa therapy. Although encouraging, these preclinical data need to be confirmed in the clinic.