"Large IC50=non-cytotoxic"? - (Mar/08/2006 )

If a plant extract tested for cytotoxic activity has the IC50 value of more than 60ug/ml considered to be non-active? I do understand that NCI do have a certain cut off point in determining whether or not a substance is considered active. But, i was told by somebody that having a large IC50 value does not mean that the extract is not cytotoxic. The cytotoxic compound may be minute in amount and may exhibit cytotoxic effect if the extract or the compound is concentrated. Any cases on plants having large IC50 and yet there had been cytotoxic compound successfully separated? or any reading materials? thanks.

any opinion?