DNA-binding properties and antitumour activity of monofunctional alkylating groups attached to the DNA-intercalating chromophore phenanthridine: n-bromoalkylphenanthridinium bromides.

We have synthesised an homologous series of n-bromoalkylphenanthridinium bromides and studied their DNA-binding and antitumour properties. Each of these compounds has the capacity both to intercalate and alkylate DNA. Dialysis measurements reveal a relatively high affinity for calf thymus DNA, being about 10(5) M-1 at ionic strength 0.01. Incubating calf thymus DNA-ligand complexes having a ligand-to-basepair ratio of 0.4 at 37 degrees C for 18 h leads to maximum alkylation levels of about one ligand molecule bound irreversibly per 40 basepairs. The reactivity of these compounds towards DNA is chain-length dependent, the n-decyl compound, for example, requiring about 10-times the ligand-to-basepair input ratio of the n-hexyl derivative to reach the same level of alkylation. The limited degree of alkylation is a consequence of conversion of the alkylbromides to the less reactive alkylchlorides in the buffer medium. The results of DNA sequencing experiments indicate that the n-hexyl derivative alkylates at guanines occurring in 5'-GT-3' sequences and in runs of guanines [(Gp)n]. The corresponding n-decyl compound, on the other hand, is highly selective for guanines in 5'-GT-3' sequences only and also reacts weakly with some adenines. None of the phenanthridinium compounds showed significant antitumour activity in the P388 murine leukaemia test system.