As described in Figure 2.2, the departure of acrolein (propenal) is absolutely necessary for activation of cyclophosphamide and ifosfamide into active alkylating agents. Acrolein is the simplest unsaturated aldehyde; chemically it is very reactive and can cause hemorrhagic cystitis when it is collected in the bladder. Administration of MESNA (2-mercaptoethane sulfonate sodium [natrium, Na]) can prevent development of hemorrhagic cystitis. The sulfhydryl group in Mesna detoxifies the unsaturated vinyl group in acrolein, resulting in a water-soluble product that is excretable. Use of Mesna is a common clinical practice after administration of intermediate-dose to high-dose cyclophosphamide and ifosfamide.