| Natural products have formed complex and novel structures and diverse physiological activities in natural evolution,providing a rich source for the discovery of drug lead compounds.Modular modification of natural products can modulate activity,improve the druggability,and further expand the chemical space,which is an important topic in medicinal chemistry research.Many triterpenoids have unique pharmacological activities.The pentacyclic triterpenoid betulinic acid was found to be an agonist of the bile acid membrane receptor TGR5.TGR5 is involved in various metabolic processes and the activation of TGR5 is a potentially effective treatment for metabolic diseases including type 2 diabetes and obesity.In the previous study,our group designed and synthesized a series of TGR5agonists with excellent activity by overturning the C3 hydroxyl group of betulinic acid to simulate endogenous bile acids.However,according to literature reports,systemic agonism of TGR5 can lead to adverse effects such as gallbladder filling,cholestasis,and pruritus,thus requiring the development of intestinal-restricted TGR5 agonists.The first part of the paper is to improve the physicochemical properties and reduce permeability of betulinic acid TGR5 agonists by introducing kinetophores to achieve intestinal targeting.Based on the previous work of our research group,the structure-activity relationship of betulinic acid TGR5 agonists was summarized,and different types of kinetophores with large polarity were introduced into the activity-insensitive sites to investigate the effects on activity and physicochemical properties.Compounds T8 and T25Na with low permeability and high efflux rate were obtained.Studies on the metabolic properties and tissue distribution of T8 and T25Na found that the compounds had extremely low plasma exposure and high intestinal distribution.Compound T25Na also had extremely low gallbladder exposure,successfully achieving intestinal targeting.The onset of action is prolonged after oral administration due to reduced permeability of T8 and T25Na.After adjusting the administration time,both T8 and T25Na showed hypoglycemic effect in TGR5H88Y mutant mice.In addition,in the seven-day long-term administration test,the shape and size of the gallbladder in the T8 and T25Na administration groups maintained normal,and neither caused gallbladder filling,which remarkably improved the safety.However,the agonistic activity of T8 and T25Na is weak,resulting in poor hypoglycemic activity.In order to further enhance the in vivo hypoglycemic effect of the compounds,we systematically studied the structure-activity relationship of the kinetophore side chains based on the structure of T8 and T25Na,and finally found compounds T33,T39 and T43 with significantly improved TGR5 agonist activity.In the future,we will further evaluate the newly synthesized compounds,including the physicochemical and metabolic properties,the in vivo hypoglycemic effect and the toxicity of the gallbladder.Cucurbitacin is a kind of tetracyclic triterpenoid natural product with various physiological activities such as anti-tumor and anti-inflammatory,among which cucurbitacin B is rich in sources and has been widely studied.The reported structure-activity relationship indicates that theα,β-unsaturated ketone in the side chain of cucurbitacin B is an essential structure for the activity,and the C2 and C16 hydroxyl groups also contribute to the maintenance of the activity.However,most reports on the structural modification of cucurbitacin B focued on the modification of these active sites,few studies have been done on the metabolically unstable C25 acetoxy group.By analyzing the molecular structure,a palladium-catalyzed coupling reaction suitable for cucurbitacin B was developed.The reaction condition is mild and no protective groups are required.A variety of substituted aryl and alkenyl functional groups can be introduced into the allyl acetate structure of the side chain through a one-step reaction,which expands the chemical diversity of the C25 position.Through this reaction and further derivatization,a small library of cucurbitacin B derivatives was synthesized.Most of the compounds showed antiproliferative activity against human non-small cell lung cancer A549 and human liver cancer PLC/PRF/5 cells,and the cytotoxicity of most compounds was comparable to that of cucurbitacin B,among which compounds C18,C22 and C32 showed better antiproliferative activity than cucurbitacin B.Cucurbitacin B exerts its anti-inflammatory effect by inhibiting the NF-κB signaling pathway,so the inhibitory ability of the compound on the NF-κB signaling pathway was investigated to evaluate the anti-inflammatory activity.All tested compounds showed inhibition of NF-κB at a concentration of 20μM,but some compounds showed toxicity to 293T cells at this concentration.Further research showed that compounds C11,C14,C21,and C22had stronger inhibitory activity on NF-κB than cucurbitacin B,but the cytotoxicity of these compounds should be investigated to avoid false positive.The effects of compounds on the expression of cellular inflammatory factors need further study.Studies have shown that the antitumor effect of cucurbitacin B is related to many signaling pathways and proteins,but the research on key targets is still relatively preliminary.Based on compounds C15 and C16,an aliphatic diaziridine group with less structural change was introduced,and the photoaffinity probe P1-4 with preserved antiproliferative activity was designed and synthesized.We will synthesize other types of non-photoaffinity probes and negative probes to provide powerful tools for target identification and mechanism research in future studies. |
