Design,Synthesis And Antibacterial Evaluation Of Dihydroquinazolin And Phenanthridium Derivatives Based On FtsZ

The increasing number of drug-resistant bacteria has set off the alarm for public health,especially for patients undergoing cancer chemotherapy,surgery and organ transplants.Therefore,it is imminent to develop antibacterial drugs with novel mechanisms of action and new targets to effectively control the generation and spread of multidrug-resistant bacteria.FtsZ,a key protein of bacterial cell division,has attracted more and more researchers’ attention as a novel antimicrobial target that has not been widely developed clinically.In this study,the GTP-binding region and cleft region between H7 helix and C-terminal of FtsZ were selected as the two target binding sites.Based on the structures of the found GTP analogs and information from the co-crystallized structure of GTP and FtsZ of Methanococcus jannaschii,a total of 16 novel GTP analogs of A-D series were designed and synthesized hrough using a dihydroquinazoline group to substitut the guanine of GTP,introducing a straight side chain at the 7 or 8 position of dihydroquinazoline to replace the five-membered sugar chain of GTP,and substituting a carboxyl group for the phosphate group of GTP at the end of side chain.The results of antibacterial activity showed that the A-D series had poor antibacterial activity(MICs>128 mu g/mL).This indicated that the design ideas of substituting the dihydroquinazoline for the parent nucleus of GTP and changing the side chain of GTP at the same time to simulate the structure of GTP was not feasible.On the basis of the above study,a total of 24 phenanthridium derivativesof E-G serieswere designed and synthesized through simplifying the structure of the natural product sanguinarine according to the characteristics of the cleft region formed by the H7 helix,T7-loop and C-terminal of FtsZ.The related bioactivity results indicated that the 5-methyl-2-phenylphenanthridium derivatives showed significantly antimicrobial activity and good inhibition activity of FtsZ polymerization,better than the positive controls.The results were summarized as follows:the 5-methyl-2-phenylphenanthridium derivatives showed good activity against eight the tested Gram-positive bacteria strains,but had poor antibacterial activity against two the tested Gram-negative bacteria strains.Among the three series of E-G,E series of compounds exhibited remarkably activity against sensitive bacteria comparable to or slightly better than the positive controls,but poor activity against resistant bacteria less than sanguinarine.In general,G series of compounds possessed more potent activity than E series of compounds.The most active F series of compounds showed particularly prominent activity against the tested Gram-positive bacteria strains,in which compounds F4,F5 and F6 were the representative compounds.For example,compound F4 was found to display the most effective activity against five sensitive bacteria strains of S.aureus ATCC25923,S.epidermidis,S.pyogenes PS,B.subtilis ATCC9372 and B.pumilus ATCC63202,and three resistant bacteria strains of MRSA,S.aureus PR and S.pyogenes PR,the MIC values of which were 0.06,0.125,1,0.06,0.06,0.25,0.25 and 2 μg/mL,respectively,and which were 133 and 67,512 and 512,33 and 67,133 and 533,8 and 1,64 and 256,64 and 256,32 and 32-fold more powerful than ciprofloxacin and oxacillin sodium,respectively.In addition,compounds F4,F5 and F6 showed excellent activity against MRSA and S.aureus PR with MIC values of 0.25-1 μg/mL,which is much stronger than ciprofloxacin and oxacillin sodium(MICs>64 μg/mL).The time-killing curve revealed the kinetic bactericidal activity of 5-methyl-2-phenylphenanthridium derivatives,demonstrating that they were bactericidal in a concentration-dependent manner.Morphological observation of the cells initially identified that 5-methyl-2-phenylphenanthridium derivatives could target FtsZ.The light scattering experiments further confirmed that these compounds couldtarget FtsZ by inhibitingFtsZ polymerization in a concentration-dependent manner.Based on the structural characteristics of the target compounds and their antibacterial activity data,the preliminary structure-activity relationships(SARs)were summarized as follows.(1)Introduction of a small group such as methyl group or methoxy group at the 4-position of the 5-methyl-2-phenylphenanthridium structure could significantly increase the antibacterial activity.Their antibacterial order was methyl group>methoxy group>hydrogen.That is F series(MIC = 0.06-4 μg/mL)>G series(MIC = 0.5-16 μg/mL)>E series(MIC = 2-64 μg/mL).(2)Introduction of an electron-withdrawing group or an electron-donating group on the 2-phenyl group of the 5-methyl-2-phenylphenanthridium structure also remarkably enhanced the anti-sensitive(MIC = 0.06-16 μg/mL)and-resistant(MIC = 0.25-64 pg/mL)bacteria activity.In particular,introduction of an electron-withdrawing group on the 2-phenyl group(MIC = 0.06-16 μg/mL).(3)The position of the substituent group on the 2-phenyl group also influenced the antibacterial activity.Generally,introduction of an electron-withdrawing group at the 4’-position of the 2-phenyl group was the most active in the antibacterial activity.In particular,introduction of methyl group at the 4-position and introduction of an electron-withdrawing group such as trifluoromethyl group,4’-phenyl group or trifluoromethoxy group,etc.at the 4’-position of the 5-methyl-2-phenylphenanthridium structure displayed extremely excellent anti-sensitive and-resistant bacteria activity.In summary,we successfully designed and synthesized A-F series a total of 40 novel antibacterial compounds targeting the FtsZ.Based on the analysis of phenanthridiums such as MIC,MBC,time-killing curve,cell morphology,protein polymerization test:We found that F series compounds showed remarkable antibacterial activity.In particular,the compounds F4,F5 and F6 against drug-resistant bacteria and antibiotic-resistant bacteria with MIC valves were as low as 0.06 μg/mL and 0.25 μg/mL.It is believe that 5-methyl-2-phenylphenanthridium can be used as a precursor structure to target FtsZ protein for further study,which will provides a new ideas and methods for the research and development of FtsZ inhibitors.