Design,Synthesis And Antibacterial Activity Of Pleuromutilin Derivatives

ObjectiveThe repeated use and abuse of antibiotics lead to the emergence and spread of drug-resistant bacteria,causing a serious threat to human health and life.Pleuromutilin and its derivatives are extremely valuable chemical entities because of their unique mechanism and no significant cross-resistance.Therefore,it is great significance for the treatment of infectious diseases to search for pleuromutilin derivatives with significant anti-drug-resistant bacterial activity.MethodsA series of novel pleuromutilin derivatives were designed.The synthetic route of the target compounds was determined:using pleuromutilin as the starting material,the target compounds were synthesized by sulfonation,substitution and oxidation reaction.The antibacterial activity was evaluated in vitro against five Gram-positive bacteria,including methicillin-sensitive Staphylococcus epidermidis（MSEE）,methicillin-resistant Staphylococcus epidermidis（MRSE）,methicillin-sensitive Staphylococcus aureus（MSSA）,methicillin-resistant Staphylococcus aureus（MRSA）and vancomycin-intermediate Staphylococcus aureus（VISA）,by agar dilution method,using retapamulin and azamulin as positive control.ResultsIn this paper,thirty-seven pleuromutilin derivatives were designed and synthesized.The chemical structures of target compounds and its intermediates were confirmed by HRMS,¹HNMR and ¹³CNMR.The results of anti-Gram-positive bacteria activity test showed that:（1）Ten target compounds（3B,3D,3E,4B,4C,4F,5A,5C,5D and 5E）were more active than positive control against most of the tested Gram-positive bacteria.Compounds 3D,3E,4B,and 4D display the best antibacterial activity with MIC values<0.06μg/mL.（2）Compared with sulfoxide derivatives,the corresponding thioether pleuromutilin derivatives exert a more potency antibacterial activity.The thioether structure was critical for the antibacterial activity of pleuromutilin derivatives.（3）The derivatives 3B,3D and 3E showed the superior or comparable activities to positive control against drug-resistant bacteria（MRSE,MRSA and VISA）.The antibacterial activity of compounds 3B and 3C with hydroxyl group on pyrimidine ring was significantly reduced.Furthermore,the degree of reduction is more pronounced as the number of hydroxyl groups increases.When the amino group was introduced,the antibacterial activity of compound3D was unchanged comparing with compound 3A without substituents on pyrimidine ring.Compound 3D was more effective than azamulin and was almost equivalent to retapamulin.The introduction of methoxyl into pyrimidine ring could keep its antibacterial activity unchanged.（4）When the number of nitrogen atoms on the five-membered heterocycle increased,the antibacterial activity of the target compounds decreased;However,the introduction of the amino group on the five-membered heterocycle,the antibacterial activity could be increased.（5）When the C22 position was substituted by benzothiazole,both sulfoxide derivatives and thioether derivatives displayed superior antibacterial activity to the the corresponding compounds substituted by benzoxazole or benzimidazole,especially against MSSA,MRSA and VISA.The thioether derivative 5A which was substituted by benzothiazole exhibited a strong inhibitory effect with MIC value≤0.06μg/mL.Nevertheless,the introduction of chlorine atom on the benzothiazole ring was detrimental to the activty.When sulfur atom was replaced by its bioisosterism oxygen and imine,the antibacterial activity against MSEE and MRSE was maintained,but the inhibition of MSSA,MRSA and VISA was significantly reduced.The introduction of a methoxy group on benzimidazole ring could achieve an antibacterial activity comparable to or better than that of compound 5A.ConclusionsIn this paper,the introduction of aromatic heterocycles at the end of the C14 side chain not only enriched the structural diversity of the pleuromutilin derivatives,but also contributed to broaden the design concept of this class of antibiotics.The structure-activity relationship summarized in this work can provide valuable clues for the future study of pleuromutilin derivatives.