The problem of specificity and selectivity of alkylating cytostatics; studies on N-2-chloroethylamido-oxazaphosphorines

Abstract

a) N-2-chloroethylamido-oxazaphosphorines such as cyclophosphamide, ifosfamide and trofosfamide differ from “non-oxazaphosphorine-substituted” (alkylating) N-mustards by the selectivity of their cancerotoxic action in vivo (measured by their therapeutic index) and by the fact that for exertion of their alkylating and cytotoxic properties they must first be transformed in the body. This bio-transformation is a multi-step process which commences with enzymatic hydroxylation at carbon 4 of the oxazaphosphorine ring (activation). b) Cancerotoxic selectivity is closely bound to the oxazaphosphorine ring. Transfer of a chloroethyl group from the extracyclic amide-nitrogen (cyclophosphamide) to the cyclic amide-nitrogen (ifosfamide) or replacement of a 2-chloroethyl group by a 2-mesyloxyethyl group leaves the essential basic properties of the N-2-chloroethylamido-oxazaphosphorines largely unchanged. In spite of this, some compounds show significant pharmacotherapeutic peculiarities regarding organotropic effects and cumulative properties. c) The cancerotoxic selectivity of the N-2-chloroethylamido-oxazaphosphorines in vivo is closely related to the cytotoxic specificity of their activated primary metabolites against cancer cells in vitro. In the case of 4-hydroxycyclophosphamide, these two properties are abolished by toxification and liberation of the strongly alkylating nitrogen mustard phosphorid acid diamide. d) The toxification is the rate-determining step of the spontaneous breakdown of 4-hydroxy-cyclophosphamide. It is pH-dependent and can be indirectly controlled by competitive deactivation reactions. e) Apart from enzymatic dehydrogenation of the primary activated metabolites of cyclophosphamide into non-toxic metabolites, a reversible nonenzymatically catalysed deactivation by thiol compounds is described. This does not involve the alkylating moiety of N-2-chloroethylamido-oxazaphosphorines, but the cyclic cryptoaldehyde group of activated oxazaphosphorines, leading to the formation of 4-(SR)-mercapto-oxazaphosphorines. Since free sulfhydryl groups or proteins were found to react according to the same mechanism, activated N-2-chloroethylamido-oxazaphosphorines can thus be bound to cellular structures and enzymes. f) The significance of the various deactivation reactions for cancerotoxic selectivity and cytotoxic specificity of N-2-chloroethylamido-oxazaphosphorines is discussed with particular regard to the importance of thiol deactivation for cytotoxic specifivity and cancerotoxic selectivity against tumor cells.