AbstractContezolid acefosamil (CZA) is an intravenous prodrug of oxazolidinone antibiotic contezolid (CZD). It is being developed to treat infections due to Gram‐positive bacteria including multidrug‐resistant pathogens, while addressing myelosuppression and neurotoxicity limitations associated with long‐term use of this class of antibiotics. In vivo, CZA is rapidly deacylated into its first metabolite MRX‐1352, which is then dephosphorylated to release active drug CZD. Four‐week repeat‐dose toxicity studies of intravenous CZA were conducted in Sprague–Dawley rats (40, 80, and 160/120 mg/kg/dose twice a day [BID]) and beagle dogs (25, 50, and 100/75 mg/kg/dose BID). The high doses administered to both rats and dogs were adjusted due to adverse effects including decreased body weight and food consumption. Additionally, a dose‐dependent transient reduction in erythrocyte levels was recorded at the end of dosing phase. Importantly, no myelosuppressive reduction in platelet counts was observed, in contrast to the myelosuppression documented for standard‐of‐care oxazolidinone linezolid. The no‐observed‐adverse‐effect level (NOAEL) of CZA was 80 and 25 mg/kg/dose BID in rats and dogs, respectively. Separately, 3‐month neuropathological evaluation in Long–Evans rats (25, 37.5, and 50 mg/kg/dose, oral CZA, BID) demonstrated no neurotoxicity in the central, peripheral, and optical neurological systems. Toxicokinetic data from these studies revealed that CZD exposures at NOAELs were higher than or comparable with that for the intended clinical dose. These results confirm the favorable safety profile for CZA and support its clinical evaluation for long‐term therapy of persistent Gram‐positive infections, beyond the application for earlier oxazolidinones.