Design, Synthesis And Evaluation Of Periphery-selective CB1Receptor Antagonists

Obesity has become an epidemic worldwide, which is seriously threatening thepublic health. Obesity increases the likelihood of various diseases, and becomes worldmedical society problems with AIDS, drug abuse and alcoholism. Up to date，onlythree anti-obesity drugs, Orlistat, Sibutramine and Rimonabant, were marketed forlong term treatment of obesity. Among them, Sibutramine and Rimonabant have beenwithdrawn from the market due to the serious side-effects. Although Orlistat was stillused in clinic, the more extensive use in clinic was restricted due to the limitedefficacy and the side effects on gastrointestinal tract, teeth and gums. Therefore, todevelop anti-obesity drugs with high efficiency, low toxicity and good clinicaltolerance may have great economic and social significances.To date, two GPCR superfamily receptors in the cannabinoid system have beencloned and were named CB1and CB2, respectively. The CB1receptor is mainlyexpressed in the central nervous system (CNS), which was related to decreasingappetite and food intake. The CB1receptor is also expressed in several peripheraltissues such as lungs, livers and kidneys, in which it played the role of increasingenergy metabolism. In contrast, the CB2receptor is particularly abundant in immunetissues and with lower expression in the central nervous system. It is primarilyinvolved in immune regulation and neurodegeneration. CB1as the receptor mediatedappetite and energy metabolism has been considered as the important target of curingobesity, therefore, the development of selective CB1receptor antagonist was receivedextensive attention.Rimonabant, the most representative cannabinoid CB1receptor selectiveantagonist, was approved for weight control as the first licensed cannabinoid drug. Itwas removed from the market due to the induction of CNS side-effects, such asanxiety and depression effects which are evidently linked to its central inhibition. Butit’s no doubt about the efficiency of CB1receptor as anti-obesity target. Focusing onavoiding psychiatric side-effects and safeguarding the safety of drug in clinic，it is anew tendency to develop ideal CB1antagonists which have the characteristic of peripheral selectivity.This paper analyzed the interactions of the CB1receptor antagonists with itsreceptors, and then developed the structure-activity relationships (SAR) of4-methyl-1H-diaryl-pyrazole CB1receptor antagonists. On the basis of the SAR, the structuralsimilarity of cannabinoid receptors CB1and CB2, the transmembrane ability of theantagonists, we have designed three kinds of new CB1antagonists with strongermolecule polarity and higher hydrophilicity by modified the3-position substitution ofRimonabant with quaternary ammonium salt, urea or guanidine groups, which mightminimize the CNS-related side effects noted with CB1antagonists while maintainingbeneficial effects of blocking the receptor in peripheral tissues.Some of the designed CB1receptor antagonists in this paper were synthesized asfollows:The CB1receptor antagonists of quaternary ammonium salts were designed andsynthesized.5-(4-chlorophenyl-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazol-3-carbo-xylic acid was reacted with thionyl chloride to produce its acyl chloride product,which reacted with the amine to form a amide. And finally the target compounds wereobtained by methylation of the amide.The CB1receptor antagonists involved urea groups were designed andsynthesized.5-(4-chlorophenyl-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazol-3-carboxylic acid was used the start material, which was reacted with thionyl chloride toproduce its acyl chloride product and through amino reaction with AmmoniumHydroxide, Hoffman rearrangement reaction and hydrolysis reaction to obtain anintermediate of5-(4-chlorophenyl-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazol-3-amine.The intermediate was converted to the corresponding phenyl carbamate,which was reacted with the desired amines to provide the target compounds.The CB1receptor antagonists involved guanidine groups were designed andsynthesized.5-(4-chlorophenyl-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazol-3-carbo-xylic chloride was synthesized including the method as above, and through aminoreaction with Ammonium Hydroxide, Hoffman rearrangement reaction, hydrolysisreaction and reaction with Phenyl chloroformate to obtain another intermediate phenyl(5-(4-chlorophenyl-1-(2,4-dichloroph-enyl)-4-methyl-1H-pyrazol-3-yl)carbamate.Finally through the reaction of amine-ester exchange and two substitution reactions ofphosphorus oxychloride and methylamine, the target compounds were gained.In this study, we had synthesized26target compounds via3-8steps respectively. All of these compounds had been confirmed by1H-NMR and MS.All the compounds had been evaluated on EGFP-CB1_U2OS cells model in twodifferent concentrations. The preliminary screening results indicated that, at the levelof a higher concentration,4compounds exhibited better bioactivity than that of thelead compound, Rimonabant,17compounds had equal biological activities toRimonabant, and only quaternary ammonium salts had lower biological activities;while at the level of a lower concentration, only one compound TXX-3-27had qualbiological activities to Rimonabant, most compounds had lower biological activities.