inhibitors (L01CB and L01XX) Topoisomerases  are essential  enzymes  that maintain the  topology  of DNA. Inhibition of type I or type II topoisomerases interferes with both  transcription  and  replication  of DNA by upsetting proper DNA supercoiling . Some type I topoisomerase inhibitors include  camptothecins :  irinotecan  and  topotecan . Examples of type