MMP9 and breast cancer: In 2022, Bhadwal et al. demonstrated SPHK1 is increased in tumor tissues in 31 breast cancer patients; in addition, invasion-related MMP9 and MMP2 expression and drug-resistance-related ABCC1 and ABCG2 expression are correlated with SPHK1 expression [38] and Zhang et al. demonstrated the suppressive effect of SPHK1 inhibitors such as compound “28” and PF-543 on lung metastasis of triple-negative breast cancer [39].