Design And Synthesis Of The Novel TSPO Ligands-2-arylimidazo[1, 2-a]pyridine-3-acetamide Derivatives

GABA is an important inhibition neurotransmitter in central nervous system. The effects between GABA and GABA receptors would be strengthened when drug ligands binding with corresponding sites in GABA-A receptor, thus, the effects play an important role in inhibiting the excitation in CNS and dealing with diseases related to mental disorders. However, GABA-A receptor is composed of many subunits, ligands binding with different subunits produce different effects such as sedation, hypnosis, anticonvulsant, antianxiety, antidepressant and muscle relaxation. The traditional drugs like diazepam and alpidem are widely used in the treatment of anxiety and depression in clinic, but they have side effects include sedation, hypnosis, etc. and tend to produce dependence and addiction. Therefore, people want to find anxiolytics with high affinity, low side effects and dependence. Researchers found that TSPO involved in the realization of various functions in vivo, among them, the function of improving neurosteroids biosynthesis may have a potential value in treating anxiety. TSPO interacting with a proper ligand can stimulate biosynthesis of neurosteroids. Subsequently, neurosteroids binding with special subunits in GABA-A receptor produce antianxiety effects without obvious side effects. Thus, it's an ideal way to cure anxiety by searching the compounds with high selectivity and affinity for TSPO.Researches on TSPO ligands are blooming by now. Many compounds show good activities in both vitro and vivo. Some of them show excellent properties in the biological evaluation. For example, AC-5216 has already been in phaseâ…¡clinical trial because of its higher selectivity and affinity, fewer side effects, less tolerance and withdrawal symptoms. However, most of them have bad pharmacokinetic properties and low bioavailabilities more or less. In this research topic, we have designed and synthesized novel TSPO ligands with high affinity and selectivity and own intellectual properties; moreover, their solubility in water is modified, pharmacokinetic properties and bioavailabilities are expected to be improved.The main research works and results are illustrated below:1. Based on the literatures about TSPO ligands, we find that the majority of the indole-3-acetamide derivatives have bad pharmacokinetic properties, and the same problem exist in 9914, an antidepression compound designed by our former topic. Although 9914 displayed definitely activities as an antidepression drug, it can't make into a medicine for its bad pharmacokinetic properties. Therefore,2-arylimidazo[1,2-a] pyridine-3-acetamide was chosen as the lead structure. After considering the structure and activity relationship (SAR) about the related structures and introducing some fragments of AC-5216, a novel series of target compounds were designed and synthesized.2.28 new compounds were synthesized, all compounds are novel and have not been reported.3. The structure of all compounds was confirmed by 1H-NMR and MS.4. All of 28 compounds were screened BZR affinity by competition-binding test in vitro with [3H]PK11195 as radioactive ligand and Ro 5-4864 as a positive control. Among them, YL-IPA01, YL-IPA08, YL-IPA10, YL-IPA11, YL-IPA13 showed as good affinity as the positive control.5. Compounds YL-IPA08, YL-IPA10, YL-IPA11 and YL-IPA13 with good affinity to BZR were selected to animal behavior experiments such as mice tail suspension test, mice elevated plus-maze test and mice open field test. Experimental results are displayed as followings:YL-IPA08 displayed an antidepression effect higher than that of the positive control in the tail suspension test, and showed significant antianxiety effects in the elevated plus-maze test but no inhibition of CNS in the open field test.YL-IPA10 also displayed the same antianxiety effects as positive control in the elevated plus-maze test and no inhibition in the open field test. Thus, YL-IPA08 and YL-IPA10 displayed definite antianxiety effect and antidepression effect.YL-IPA11 showed equivalent effects with positive control only at the single dose of 0.03 mg/kg in the elevated plus-maze test but showed inhibition in the open field test. Surprisingly, YL-IPA13 showed poor effects in elevated plus-maze test and remarkable inhibition in open field test. So, YL-IPA11 and YL-IPA13 didn't show significant antianxiety effect and antidepression effect.6. Based on the results of BZR affinity test and animal behavior test, the structure-activity relationships are as follows:1) For the affinity to BZR in vitro, the substitution effect can be concluded as follows:(1) The substituent group of 2-phenyl of the skeleton is critical to increase affinity to BZR. Hydrophobic group like 4-Cl,3,4-diCl,4-Me, etc. significantly increase the affinity while 4-OMe plays the lowest effects. This conclusions are similar to the one reported by literature, i.e., halogen plays an important performance in increasing the affinity.(2) The two substituent groups on amide are key to high BZR affinity too. On the side containing aryl group, the effect of benzyl is a bit bigger than that of the pyridylmethyl. In addition, for the pyridyl group, the effect thereof follows the sequence of 2-pyridyl>4-pyridyl>>3-pyridyl. For the alkyl group on the other side, ethyl is most suitable, beyond this, the affinity is cut down.2) As for the behavior effects in vivo, the substitution effect is different from that in vitro as bellow:(1) The substitution effect of 2-phenyl is similar to that in vitro, the best substitution is 3,4-dichloro.(2) The substitution effects of N-substituted groups of 3-acetamide are remarkably different from that in vitro. For mice tail suspension test and elevated plus-maze test, the effects of aryl groups of N-arylmethyl on one side of 3-acetamide follow the sequence of 2-pyridyl>4-pyridyl>phenyl, while for mice open field test, the sequence of their effects is phenyl>4-pyridyl>2-pyridyl. It is shown that N-benzyl substitution in this series prefers to CBR while 2-pyridylmethyl selects to TSPO. Meanwhile, ethyl is the best substitution group for TSPO among N-alkyl on the other side of 3-acetamide, 2-methoxyethyl prefers to CBR hammering spontaneous activity.In conclusion, we have designed and synthesized 28 new compounds, the results of the biological activity evaluation suggested that YL-IPA01, YL-IPA08, YL-IPA10, YL-IPA11 and YL-IPA13 have displayed a good affinity for BZR. Furthermore, YL-IPA08 and YL-IPA10 showed significant antianxiety and antidepression effects while no inhibition of spontaneous activity in animal models. These two compounds are deserving further research. The research is mostly fulfilled our expectation, and made a good basis for further research and development.