Cyclophosphamide Endoxana

The design of this mustard prodrug, first synthesized in 1958, was based on the concept that the P=O group should decrease the availability of the nitrogen lone pair in an analogous manner to the phenyl ring of the aromatic mustards, thus reducing the electrophilicity of the molecule (Structure 3.8). Furthermore, it was postulated that the P=O group might be cleaved by phosphoramidase enzymes that were thought to be overexpressed by some tumor cells, thus releasing the nitrogen lone pair and restoring the electrophilicity of the molecule selectively at the tumor site.

Cyclophosphamide (Endoxana™) Mesna (Uromitexan™)

HS-CH2CH2-SO3Na

STRUCTURE 3.8 Structures of cyclophosphamide (Endoxana™) and Mesna (Uromitexan™).

Although at the time this appeared to be a rational design concept, it was later shown that activation in vivo is not due to enzyme-catalyzed hydrolysis of the P=O group but rather to initial oxidation at the C4-position of the oxazaphosphorine ring by liver microsomal enzymes. After 4-hydroxylation, the molecule then fragments to give phosphoramide mustard, which is thought to be the biologically active species, along with the highly electrophilic acrolein, which is toxic and can cause hemorrhagic cystitis. The further breakdown of phosphoramide mustard to normus-tine [HN(CH2CH2Cl)2] has also been observed (Scheme 3.9).

"Cl "Cl

Cyclophosphamide (Endoxana™) NH2

Cl Cl

Acrolein (Can be neutralized with Mesna)

Phosphoramide Mustard

Bio-oxidation Liver

Cl Cl

Cl Cl

Cl Cl

SCHEME 3.9 Mechanism of activation of cyclophosphamide (Endoxana™) to give the DNA cross-linking species phosphoramide mustard and the toxic by-product acrolein.

Cyclophosphamide has a broad spectrum of clinical activity and is widely used in the treatment of solid tumors, including carcinomas of the bronchus, breast, and ovary, and various sarcomas. It is also used to treat CLL and lymphomas. This agent is administered intravenously or orally but is not active until metabolized by the liver. The by-product acrolein is excreted in the urine and is a potent electrophile, reacting with nucleophiles on the surfaces of cells that line the bladder and causing the very serious but fortunately rare complication of hemorrhagic cystitis in susceptible patients. An increased fluid intake after administration of cyclophosphamide can help to avoid this problem. However, after high-dose therapy (more than 2 g intravenously), the adjuvant agent mesna (Uromitexan™) is routinely coadministered (and also given afterwards) to neutralize the effects of acrolein. Mesna, which is sodium 2-mercaptoethanesulfonate, acts as a "sacrificial" nucleophile, reacting with acrolein via a Michael addition to form a water-soluble biologically inactive adduct that is safely eliminated in urine. Mesna is also given before and after oral therapy with cyclophosphamide (see Scheme 3.10).

In addition to the usual side effects associated with mustard agents, cyclophos-phamide also suppresses B-cell activity and antibody formation.

Acrolein And Mesna
SCHEME 3.10 Mechanism of detoxification of acrolein by covalent interaction with Mesna (Uromitexan™) through a Michael addition reaction.

The slow rate of in vivo hydroxylation of cyclophosphamide in humans has led to the synthesis of a number of experimental 4-hydroxy derivatives (e.g., a 4-hydro-peroxy analog) designed to spontaneously cleave in vivo without the necessity for bioactivation in the liver. However, none of these derivatives has shown any advantage over cyclophosphamide, which remains an important drug in clinical practice today.

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