teniposide and etoposide were found by stähelin to have a new mechanism of action: they blocked the entry of cells into mitosis (with arrest in late s or g2 phase), in contrast to sp-g and sp-i, which were spindle poisons. his subsequent investigations showed that the early biochemical effect of the two epipodophyllotoxins on proliferating cells "in vitro" differed from that of the alkylating agents, antimetabolites and vinca alkaloids. after a landmark clinical comparative analysis by f. muggia and m. rozencweig, etoposide and teniposide were licensed to bristol-myers in 1978 for further clinical development. a few years later, in november 1983, etoposide was approved by the u.s. food and drug administration.