DNA-binding Products of Nornitrogen Mustard, a Metabolite of Cyclophosphamide

Nornitrogen mustard was shown to bind to the N-7 position of guanine in DNA and guanosine. The initial binding product was NOR-G. N-(2-chloroethyl)-N-[2-(7-guaninyl)ethyl]amine but it was converted with time to a hydroxylated derivative. NOR-OH-G and to a cross-link, G-NOR-G. The NOR-OH-G adduct was also seen in the reactions of aziridineethanol. The cross-link appeared to depurinate slower than the two other adducts so that it was the main adduct after 72 h of incubation. The rate of reaction was higher at pH 6.0 than at pH 7.4 but even at pH 7.4 adducts were formed. When isolated NOR-G was incubated it was shown to be converted exclusively to NOR-OH-G at pH 5 but at pH 7.4 the main product was a dimer. The half-life of NOR-OH-G formation at pH 7.4 and 37 degrees C was 4 h. The mustard arms of NOR-G and NOR-OH-G had pKa values of about 7.5 and 10, respectively. This was probably related to the somewhat higher rate of imidazole ring-opening of NOR-OH-gua as compared to NOR-gua. The lability of the imidazole ring of the guanosine adducts of nornitrogen mustard is intermediate to those of phosphoramide mustard and nitrogen mustard.