Mustard agents cytotoxic activity

The adventitious discovery of the antitumor action of the nitrogen mustard poison war gases led to intensive investigation of the mode of action of these compounds. In brief, it has been fairly well established that these agents owe their effect to the presence of the highly reactive bis(2-chloroethyl)amine group. The cytotoxic activity of... 

Since the formation of the ethyleniminium ion is crucial for the cytotoxic activity of the nitrogen mustards, it is not surprising that stable ethylenimine derivatives have antitumor activity. Thiophospho-ramide or thiotepa is the best known compound of this type that has been used clinically. Both thiotepa and its primary metabolite, triethylenephos-phoramide (TEPA), to which it is rapidly converted by hepatic mixed-function oxygenases form crosslinks with DNA. It is mainly used as an intravesicu-lar agent in bladder cancer. Thiotepa produces little toxicity other than myelosuppression. 

Cyclophosphamide and ifosfamide are nitrogen mustard derivatives, and are widely used alkylating agents (Table 124—14). They are closely related in structure, clinical use, and toxicity. Neither agent is active in its parent form and must be activated by mixed hepatic oxidase enzymes. The active metabolite of cyclophosphamide is phosphoramide mustard. Another metabolite, 4-hydroxycyclophos-phamide is cytotoxic, but is not an alkylating agent. Ifosfamide is hepaticaUy activated to ifosfamide mustard. Acrolein, a metabolite of both cyclophosphamide and ifosfamide, has little antitumor activity, but is responsible for some of their toxicity. ... 

Thiotepa is an alkylating agent, which is a cell-cycle nonspecific alkylating agent related to nitrogen mustard. Its radiomimetic action is believed to occur through the release of ethylenimine radicals, which disrupt the bonds of DNA. TEPA possesses cytotoxic activity. 

Cyclophosphamide (Cytoxan) is the most versatile and useful of the nitrogen mustards. Preclinical testing showed it to have a favorable therapeutic index and to possess the broadest spectrum of antitumor activity of all alkylating agents. As with the other nitrogen mustards, cyclophosphamide administration results in the formation of cross-links within DNA due to a reaction of the two chloroethyl moieties of cyclophosphamide with adjacent nucleotide bases. Cyclophosphamide must be activated metabofically by microsomal enzymes of the cytochrome P450 system before ionization of the chloride atoms and formation of the cyclic ethylenimmonium ion can occur. The metabolites phosphoramide mustard and acrolein are thought to be the ultimate active cytotoxic moiety derived from cyclophosphamide. 

Mechanism of action Cyclophosphamide [sye kloe FOSS fa mide] is the most commonly used alkylating agent. Both cyclophosphamide and ifosfamide [eye FOSS fa mide] are first biotransformed to hydroxylated intermediates by the cytochrome P-450 system (Figure 38.13). The hydroxylated intermediates undergo breakdown to form the active compounds, phospho-ramide mustard and acrolein. Reaction of the phosphoramide mustard with DNA is considered to be the cytotoxic step. [Note The therapeutic effect of these drugs is independent of the level of activity of the cytochrome P-450 system.]... 

Hepatic or renal insufficiency does not significantly alter the pharmacokinetics of CP (194). Since immunosuppressive activity resides exclusively in the metabolites of CP (i.e., phosphoromide mustard and acrolein), pharmacokinetics are not predicted by the parent compound. Correlations between CP pharmacokinetics and pharmacodynamics are difficult to demonstrate. Measuring the metabolites is technically difficult (211). Drug interaction with other cytotoxic agents may increase neutropenia. One case report found the combination of CP plus infliximab was more likely to cause T-cell lymphopenia than either agent alone (212). 

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