RPS6KB1 and cancer: However, it was reported that wortmannin, which is primarily known as a PI3K inhibitor, also inhibits mTOR/p70S6K signaling in ZR-75.1 cells, possibly inhibiting p-p70S6K by blocking mTOR and thus abrogating the PI3K negative feedback loop (17–19), causing mitogen-activated protein kinase (MAPK) to be constitutively activated in human cancer.