Investigation of the toxic functional group of cephalosporins by zebrafish embryo toxicity test.

2-mercapto-5-methyl-1,3,4-thiadiazole (MMTD) is the 3'-side chain of cephalosporin including cefazolin sodium (CFZL) and cefazedone (CFZD). It is not only present in finished products as the residual precursor, but also produced through drug degradation. Performing the zebrafish embryo toxicity test, we evaluated the toxicity effects of cefazolin sodium, cefazedone, their synthetic precursors and intermediates. Our results suggest that the teratogenic effect of cefazedone and cefazolin sodium on zebrafish embryonic development is associated with the structure of MMTD. They mainly interfere with the development of tissues and organs derived from embryonic ectoderm and mesoderm. We further consider the rationality of the quality control limit of MMTD (1.0%) in the specification. As the acceptable daily intakes (ADIs) of cefazolin is 10 µg/kg per day 16 and the minimum teratogenic concentration of MMTD is tenfold lower than that of cefazolin sodium, we recommend that the acceptable daily intakes of MMTD should be 1 µg/(kg day). In general, the therapeutic dose of cefazolin sodium is 2-4 g/day. Based upon the calculation of MMTD quality control limits (1.0%), MMTD intake can be 20-40 mg/day, which will be much more than the acceptable daily intake value of 1 µg/(kg day). Thus, MMTD should be recommended as a specified impurity and qualified as serious again.