stähelin quickly assumed the leading role in the discovery and development of "podophyllum" compounds; four of them, including etoposide (vepesid), were later marketed. he was able to detect in a partially purified, chemically modified extract of the "podophyllum" plant the presence of a then-unknown agent with interesting properties, which had at first been considered by the chemists to be "dirt". the chemists, under the direction of a. von wartburg, analysed this impurity. guided by stähelin's "in-vitro" and "in-vivo" assays, they found the active compound responsible for the good antitumor activity of sp-g (proresid oral) and sp-i (proresid intravenous; discovered in april 1959, commercialized 1963). further chemical modifications led to the well-known thenylidene derivative vm-26 (teniposide, vumon, discovered in october 1965, commercialized by sandoz in 1976) and to the ethylidene derivative vp-16-213 (etoposide, vepesid, discovered on october 21, 1966), which remains very clinically successful.