When we are screening extracts from natural sources, we often discard those with IC50 values higher than 20 micrograms / mL (according to Geran, R.I., Greenberg, N.H., McDonald, M.M., Schumacher, A.M., Abbott, B.J. (1972). Protocols for screening chemical agents and natural products against animal tumour and other biological systems (3rd edition). Cancer Chemother. Rep., 3, 1-103 ).

But concerning Selectivity Index, is there any guideline we can cite? obviously, if the IC50 for tumor cell line is much lower than for a "normal" control cell line, that is a good thing, but is there any guideline one can cite (ex., National Cancer Institute or some other institution)? Obviously, many papers refer "their" S.I. result as being a good one, but I would like an uncontroversial reference I could cite.