An automated pulse labelling method for structure-activity relationship studies with antibacterial oxazolidinones

The 3-aryl-2-oxooxazolidinones are a new class of synthetic antibacterial agents that potently inhibit protein synthesis. An automated pulse labelling method with [3H]-lysine was developed with Bacillus subtilis to obtain additional quantitative activity data for structure-activity relationship studies with the oxazolidinones. Inhibition constants were calculated after a Logit fit of the data into the formula: % of control = 100/(1 + e[-B(X - A)]), where B is the slope of the model, X is the natural log of the inhibitor concentration and A is the natural log of the inhibitor concentration required to inhibit protein synthesis by 50% (ln IC50). When substituents at the 5-methyl position of the heterocyclic ring (B-substituent) were NHCOCH3, OH or Cl, the correlation coefficient was 0.87 between the MIC and IC50 values (for all compounds with MICs less than or equal to 16 micrograms/ml). The D-isomers of DuP 721 (A-substituent = CH3CO) and DuP 105 (A-substituent = CH3SO) gave MICs of 128 micrograms/ml and IC50s of greater than or equal to 50 micrograms/ml for protein synthesis, showing that only the L-isomers were active. By MIC testing, oxazolidinones with the B-substituent of NHCOCH3 and the A-substituent of CH3CO, NO2, CH3S, CH3SO2 or (CH3)2CH had comparable antibacterial potency; however, pulse labelling analysis showed that compounds with an A-substituent of CH3CO or NO2 were more potent inhibitors of protein synthesis.