To judge the selectivity of this series for MRSA vs. other cells, toxicity of key compounds was evaluated against MRC-5SV2 (human lung fibroblast cell line) and PMM (primary mouse macrophages) and the selectivity index (SI) calculated (where SI = ratio of cytotoxic TC50 and MRSA MIC; additional data can be found in Table in Supplementary Information -Biology). Most of the inactive compounds exhibited low toxicity to either cell line. The imidazopyridine 983 showed a good selectivity index (SI = 10) for the MRC5 cell line, but this was accompanied by poor ADME properties. The best overall balance of parameters in these systems was found with 975, a potent compound with reasonable solubility and low intrinsic clearance in rat hepatocytes, with low toxicity against MRC5 cells and no toxicity against PMM.