Conclusions: Combined with our earlier study on a TNBC (triple-negative breast cancer) mouse model (MB-MDA-231 with a medium FcRn expression level), SPEDOX-6’s antitumor efficacy displays an inverse relationship with the FcRn expression level in three different tumor mouse models, thereby providing a potential mechanism for SPEDOX-6 to effectively target tumors with low FcRn expression levels. This evidence concerns the gene FCGRT and triple-negative breast carcinoma.