Mechanism of action of the nitrosoureas-III. Reaction of bis-chloroethyl nitrosourea and bis-fluoroethyl nitrosourea with adenosine

Previous papers in this series have provided evidence for the formation of haloethyl nucleoside derivatives from the interaction of the therapeutic nitrosoureas with cytidine and guanosine. Such derivatives could be important in explaining the cytotoxic action of bis-chloroethyl nitrosourea (BCNU), bis-fluoroethyl nitrosourea (BFNU), and related therapeutic agents. We now report the formation of 1-haloethyl adenosines from the reaction of BCNU and BFNU with adenosine. These 1-substituted haloethyl adenosines cyclize to form 1,N⁶-ethanoadenosine: 1-chloroethyladenosine with a half-life of 20 min in neutral aqueous solution at 37°, and 1-fluoroethyladenosine with a half-life of 20 hr under the same conditions. 1-Hydroxyethyladenosine is also a major product of the reaction of either BCNU or BFNU with adenosine, but it is not formed from the hydrolysis of either 1-haloethyladenosine. Accordingly, a reaction mechanism involving a cyclized nitrosourea derivative is proposed to explain the formation of this and other hydroxyethyl nucleosides.