Phosphoramide mustard

Antineoplastic Drugs. Cyclophosphamide (193) produces antineoplastic effects (see Chemotherapeutics, anticancer) via biochemical conversion to a highly reactive phosphoramide mustard (194) it is chiral owing to the tetrahedral phosphoms atom. The therapeutic index of the (3)-(-)-cyclophosphamide [50-18-0] (193) is twice that of the (+)-enantiomer due to increased antitumor activity the enantiomers are equally toxic (139). The effectiveness of the DNA intercalator dmgs adriamycin [57-22-7] (195) and daunomycin [20830-81-3] (196) is affected by changes in stereochemistry within the aglycon portions of these compounds. Inversion of the carbohydrate C-1 stereocenter provides compounds without activity. The carbohydrate C-4 epimer of adriamycin, epimbicin [56420-45-2] is as potent as its parent molecule, but is significandy less toxic (139). 

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. 

Cyclophosphamide s major active metabolite is phosphoramide mustard, which cross-links DNA to prevent cell replication. It suppresses T-cell and -cell function by 30-40% T-cell suppression correlates with clinical response in the rheumatic diseases. Its pharmacokinetics and toxicities are discussed in Chapter 54. 

In animal studies, NAC has been shown to prevent hemorrhagic cystitis that results from administration of cyclophosphamide or its position isomer ifosfamide. Hemorrhagic cystitis results from the toxic effect of acrolein, a metabolic product of cyclophosphamide or its position isomer ifosfamide. The mechanism whereby NAC prevents this toxicity may be prevention of the intracellular depletion of antioxidants, such as GSH, by acrolein. Concomitant administration of NAC with cyclophosphamide or ifosfamide does not impair antineoplastic activity, because both anticancer drugs are inactive until they are metabolized by the liver to their phosphoramide mustard metabolites. 

Cyclophosphamide in its parent form does not have direct cytotoxic effects, and it must be activated to cytotoxic forms by microsomal enzymes (Figure 55-5). The liver microsomal cytochrome P450 mixed-function oxidase system converts cyclophosphamide to 4-hydroxycyclophosphamide, which is in equilibrium with aldophosphamide. These active metabolites are believed to be delivered by the bloodstream to both tumor and normal tissue, where nonenzymatic cleavage of aldophosphamide to the cytotoxic forms—phosphoramide mustard and acrolein—occurs. The liver appears to be protected through the enzymatic formation of the inactive metabolites 4-ketocyclophosphamide and carboxyphosphamide. 

Cyclophosphamide (6.31) is a nitrogen mustard used for cancer treatment (Scheme 6.9). In the body, cyclophosphamide is oxidized to aminal 6.32. Compound 6.32 opens and loses acrolein to form phosphoramide mustard (6.33). Structure 6.33 is a strong bis-electrophile and reacts readily with nucleophiles. In DNA, the nucleophile tends to be N7 of guanine, which is oriented outward into the major groove (Figure 6.6). By reacting twice, 6.33 crosslinks DNA either within the same strand (intrastrand) or across the double helix (interstrand).16... 

Plowchalk DR Mattison DR (1991) Phosphoramide mustard is responsible for the ovarian toxicity of cyclophosphamide. Toxicol Appl Pharmacol, 107 472-481. 

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.]... 

Fleer R, Brendel M. 1982. Toxicity, interstrand cross-links and DNA fragmentation induced by activated cyclophosphamide in yeast Comparative studies on 4-hydroxyperoxy-cyclophosphamide, its monofunctional analogue, acrolein, phosphoramide mustard, and nor-nitrogen mustard. Chem Biol Interact 39 1-15. 

Friedman OM, Wodinsky I, Myles A. 1976. Cyclophosphamide (NSC026271) - related phosphoramide mustards Recent advances and historical perspective. Cancer Treat Rep 60 337-346. 

Flales BF. 1982. Comparison of the mutagenicity and teratogenicity of cyclophosphamide and its active metabolites, 4-hydroxycyclophosphamide, phosphoramide mustard, and acrolein. Cancer Res 42 3016-3021. 

Little SA, Mirkes PE. 1985. Induction of DNA lesions in phosphoramide mustard and acrolein treated rat embryos. Teratology 31 47A. 

Low JE, Borch RF, Sladek NE. 1982. Conversion of 4-hydroperoxycyclophosphamide to phosphoramide mustard and acrolein mediated by bifunctional catalysts. Cancer Res 42 830-837. 

Mirkes PE, Fantel AG, Greenaway JC, et al. 1981. Teratogenicity of cyclophosphamide metabolites phosphoramide mustard, acrolein, and 4-ketochlorophosphamide in rat embryos cultured in vitro Toxicol Appl Pharmacol 58 322-330. 

Slott VL, Hales BF. 1987. Enhancement of the embryotoxicity of acrolein, but not phosphoramide mustard, by glutathione depletion in rat embryos in vitro. Biochem Pharmacol 36 2019-2025. 

Spielmann H, Jacob-Mueller U. 1981. Investigations on cyclophosphamide treatment during preimplantation period. 2. In vitro sutides on effects of cyclophosphamide and its metabolites 4-OH-cyclophosphamide, phosphoramide mustard, and acrolein on blastulation of 4- cell and 8-cell mouse embryos and on their development during implantation. Teratology 23 7-13. 

Figure 32.8. Metabolic activation pathway for cyclophosphamide. Acrolein, phosphoramide mustard, and nornitrogen mustard are the immunotoxic metabolites.

Cyclophosphamide requires metabolic activation to form the teratogenic metabolites, phosphoramide mustard, and acrolein (Figure 34.5). The former metabolite produces single-stranded DNA breaks, DNA-DNA and DNA-protein crosslinking. In contrast, acrolein preferentially binds to proteins, and it concentrates in the... 

Metabolism of the cytostatic drug cyclophosphamide (CP Fig. 1) involves hydroxylation at the C-4 position by cytochrome P450. Subsequently, a number of detoxification reactions can occur (oxidation to the keto derivative, dechloroethylation, formation of a carboxylic acid). The phosphoramide mustard resulting from spontaneous decomposition of 4-hydroxy-CP is thought to be the cytotoxic chemotherapeutic species. The chiral nature of the phosphorus atom resulted in a twofold greater therapeutic index (LDjq/EDjq) for the S-(-)-enantiomer (against the ADJ/PC6 plasma cell tumor in mice) without detectable differences in metabolism... 

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