The synthesis of novel 2-aminobenzothiazinone analogues and their evaluation as adenosine A1/A2A receptor antagonists

Abstract

Parkinson’s disease (PD) is a neurodegenerative disorder that is characterised by a decrease in dopamine concentration in the striatum due to the degeneration of dopaminergic neurons in the substantia nigra. PD has a distinctive symptomatic footprint including bradykinesia as the hallmark symptom paired with tremor and muscle rigidity. PD also causes non-motor symptoms including depression and cognitive dysfunction. Current treatment options provide symptomatic relief by the manipulation of dopaminergic signaling, but fails to address disease progression. A new therapy is therefore urgently required to decrease disease progression, while providing symptomatic relief. The adenosine A1 and A2A receptor subtypes have been recognised as possible drug targets for the treatment of PD. Selective adenosine A1 receptor antagonists have the potential of treating cognitive deficits such as those associated with Alzheimer's disease and PD. Selective adenosine A2A receptor antagonists on the other hand have the ability to improve motor dysfunction in PD, but they also have neuroprotective properties. Additionally, adenosine A2A receptor antagonists have been shown to exhibit antidepressant activity in animal models and may be advantageous to treat PD associated depression. Dual antagonism of adenosine A1 and A2A receptors would thus be of great benefit to potentially treat both the motor as well as the non-motor (cognitive and depressive) symptoms associated with PD. Recent research identified the benzothiazinone scaffold as a promising nonxanthine scaffold that may be used to design compounds with adenosine A1 and A2A receptor affinity. When compared to caffeine, 2-aminobenzothiazinone and benzoylaminobenzothiazinone showed a higher affinity for both the A1 and A2A adenosine receptors. Further research showed that chain elongation to phenylpropanamide-benzothiazinone lead to an increase in adenosine A2A affinity, but a decrease in A1 affinity. This higher affinity for the adenosine receptors displayed by the 2-acylaminobenzothiazinones makes it a promising scaffold for further exploration as a dual A1/A2A adenosine receptor antagonist. Furthermore, the triazolotriazine scaffold of ZM241385 has high affinity for the adenosine A2A receptor with a phenylethylamine side-chain which comfortably fits into the binding cleft of the adenosine receptor. Prompted by the above two scaffolds; an exploratory pilot study was undertaken where the N-acyl side-chain of the benzothiazinone scaffold was replaced by the flexible N-alkyl side-chain of ZM241385, thus exploring the necessity of the COgroup for adenosine affinity. In addition, different para and meta substituents on the phenyl ring in the 2-alkylamino side-chain of the 2-phenlylalkylaminobenzothiazinone scaffold was also explored, as well as different chain lengths in the phenylalkyl side-chain. A series of fourteen novel 2-phenylalkylaminobenzothiazinone derivatives were synthesised via N-alkylating using phenylhalides containing various chain lengths and para and meta phenyl substitutions. The 2-phenylalkylaminobenzothiazinones were evaluated by using a radioligand binding protocol described in literature to investigate the binding of the compounds to the adenosine A2A and A1 receptors. The tested compounds were devoid of any A1 and A2A adenosine binding affinity. The poor adenosine A1 and A2A affinity exhibited by the compounds of this study can probably be attributed to the absence of the carbonyl group in the N-alkyl side-chain of the 2-phenylalkylaminobenzothiazinones, thereby emphasising the necessity of the carbonyl group for adenosine affinity. The phenylalkyl substitution offered an attractive substitution for a hybrid non-xanthine adenosine antagonist using the 2-aminobenzothiazinone scaffold and the phenylalkyl side-chain of ZM241385, but biological evaluation proved the 2-phenylalkylaminobenzothiazinone derivatives as ineffective adenosine A1 and A2A receptor antagonists. In conclusion, this research made an important contribution showing that the carbonyl group in the 2-acylaminobenzothiazinone scaffold is a prerequisite for adequate A1 and A2A binding affinity which can be used for the designing of high affinity adenosine receptor antagonists for the treatment of PD in future