Macroautophagy (hereafter autophagy) is a lysosomal catabolic pathway that controls cellular homeostasis and survival. It has recently emerged as an attractive target for the treatment of a variety of degenerative diseases and cancer. The targeting of autophagy has, however, been hampered by the lack of specific small molecule inhibitors. Thus, we screened two small molecule kinase inhibitor libraries for inhibitors of rapamycin-induced autophagic flux. The three most potent inhibitors identified conferred profound inhibition of autophagic flux by inhibiting the formation of autophagosomes. Notably, the autophagy inhibitory effects of all three compounds were independent of their established kinase targets, i.e. ataxia telangiectasia mutated for KU55933, protein kinase C for Gö6976, and Janus kinase 3 for Jak3 inhibitor VI. Instead, we identified phosphatidylinositol 3-kinase (PtdIns3K) as a direct target of KU55933 and Gö6976. Importantly, and in contrast to the currently available inhibitors of autophagosome formation (e.g. 3-methyladenine), none of the three compounds inhibited the cell survival promoting class I phosphoinositide 3-kinase-Akt signaling at the concentrations required for effective autophagy inhibition. Accordingly, they proved to be valuable tools for investigations of autophagy-associated cell death and survival. Employing KU55399, we demonstrated that autophagy protects amino acid-starved cells against both apoptosis and necroptosis. Taken together, our data introduce new possibilities for the experimental study of autophagy and can form a basis for the development of clinically relevant autophagy inhibitors.Autophagy is an intracellular degradative process by which cells recycle macromolecules and organelles (1-4). In this process, cellular material is sequestered in double membrane vesicles termed autophagosomes that fuse with lysosomes to form autolysosomes, in which the cargo is exposed to acidic hydrolases. Autophagy is essential for energy homeostasis and removal of damaged organelles and protein complexes during various kinds of stresses, such as starvation, growth factor deprivation, hypoxia, and DNA damage. It is also involved in physiological processes like development, immunity, and aging as well as in various diseases including neurodegenerative disorders and cancer. Whereas autophagy clearly has a beneficial effect in preventing many degenerative disorders, its role in cancer is more complex. It can function as a tumor suppressor mechanism, and yet it can also promote tumor growth by protecting cancer cells against the hostile tumor environment and antineoplastic drugs (5, 6).The mammalian target of rapamycin complex 1 (mTORC1) 3 serine/threonine kinase integrates information on cell metabolic, growth, and stress status to regulate biosynthetic pathways and autophagy (7,8). It activates biosynthetic pathways and inhibits autophagy in response to various growth factors via MAPK/ERK and class I phosphoinositide 3-kinase (PI3K)/Aktdependent pathways. On the other hand, when the ene...