The possible explanations for the CA's effect could be at least 1) CA has 6 structure-similar metabolites in vivo which might also be active against cancer 84, and 2) CA is a substrate of P-glycoprotein (P-gp) 85, 86; thus, the in vivo metabolites of CA might increase CA intracellular concentration by blocking P-gp efflux competitively, forming a synergistic action against glioma. Here, PGP is linked to central nervous system cancer.