BACKGROUND Loxoprofen tromethamine is a novel structural compound related to loxoprofen. It has been used for the treatment of pain and inflammation. However, the embryo-fetal developmental toxicity (EFDT) of loxoprofen tromethamine has not been evaluated in detail in vivo. This study investigated the EFDT and toxicokinetics of loxoprofen tromethamine in rats. METHODS The aim of this study was to investigate the potential reproductive toxicity on embryo-fetal development of loxoprofen tromethamine (0, 1, 3, and 10 mg/kg/day) and sodium cyclophosphamide (CP) (2.8 mg/kg/day) administered by intravenous injection to pregnant rats during gestation days (GDs) 6-15. Pregnant rats were euthanized on GD20. The numbers of live/dead fetuses, resorptions, implantations, and corpora lutea, gravid uterus mass, placenta mass, fetal gender ratios, body weight, and skeletal development were evaluated. In a concomitant toxicokinetic (TK) study (10 pregnant rats per group), plasma TK parameters and the tissue distribution of loxoprofen tromethamine were tested. RESULTS On GD20, rats were anesthetized and dissected by caesarean section. The appearance, internal organs, gravid uterus weight, embryo implantation number, and implantation loss rate in maternal rats of each group did not reveal any lesions. In fetuses, there were no significant differences in the fetus weight, embryo resorption number, stillbirth number, or fetal visceral examination in all test groups compared to the negative control group. However, in the high-dose group, the fetuses showed significant differences in the anomalies of the bones compared to the negative control group. The TK study showed that in the dose range of 1-10 mg/kg, the Cmax and AUC(0-t) of loxoprofen tromethamine in animals after the first administration increased proportionally to the dose, showing linear kinetic characteristics; after the last administration, the Cmax and AUC(0-t) increased disproportionately to the dose, showing nonlinear kinetic characteristics. The results of tissue distribution show that loxoprofen tromethamine was mainly distributed in the placenta and lung after the intravenous administration to pregnant rats; the content in the liver was lower and increased sharply in the heart with increasing doses; the content in all tissues was lower than that in the plasma. Loxoprofen tromethamine in fetal tissues and organs was mainly distributed in fetal lungs, liver and heart, and the lowest content was in amniotic fluid. CONCLUSIONS In conclusion, the no-observed-adverse-effect level (NOAEL) and lowest-observed-adverse-effect level (LOAEL) of loxoprofen tromethamine were considered to be 1 and 10 mg/kg/day, respectively.