Aniline mustard analogues of the DNA-intercalating agent amsacrine: DNA interaction and biological activity.

Two series of analogues of the clinical antileukemic drug and DNA-intercalating ligand amsacrine have been prepared, containing aniline mustard sidechains of varying reactivity, linked either at the 4-position of the intercalating acridine chromophore (type A) or at the 1'-position of the 9-anilino group (type B). DNase I footprinting assays showed that compounds of type B had stronger reversible binding to DNA than did compounds of type A. Compounds of each type showed similar patterns of alkylation-induced cleavage of DNA, and alkylate at the N7 of guanines in runs of guanines (similar to the pattern for untargeted mustards) as well as some adenines. Both classes of compounds crosslinked DNA, although those bearing relatively inactive mustards did so only at high drug/base pair ratios. However, while the patterns of DNA alkylation were broadly similar, the compounds were considerably more cytotoxic than analogous untargeted mustards. Comparison of their cytotoxicities in wild-type and DNA repair-deficient lines indicated this toxicity was due to DNA crosslinks (except for the least reactive SO2-linked mustards). The 4-linked analogues showed slightly higher in vivo antileukemic activity than the corresponding 1'-linked analogues.

Synthesis and anti-tumour activity of the spatially-separated mustard bis-N,N'-[3-(N-(2-chloroethyl)-N-ethyl)amino-5-[N,N-dimethylamino)methy l)-aminophenyl]-1,4-benzenedicarboxamide, which alkylates DNA exclusively at adenines in the minor groove.

The mustard derivative, bis-N,N'-[3-(N-(2-chloroethyl)-N-ethyl)amino-5- [N,N-dimethylamino)methyl)aminophenyl]-1,4-benzenedicarboxamide has been synthesized from 3-acetamido-5-nitrobenzoic acid in a 6-step procedure. This compound alkylates exclusively in the minor groove of DNA, at the N3 site of adenines occurring in sequences of runs of adenines and (to a small extent) at 5'-TA and 5'-AT sites. Gel electrophoresis studies and in vitro cytotoxicity assays against repair-deficient AA8 mutant cell lines show it has a high degree of DNA interstrand cross-linking ability.

DNA-directed alkylating agents. 3. Structure-activity relationships for acridine-linked aniline mustards: consequences of varying the length of the linker chain.

Four series of acridine-linked aniline mustards have been prepared and evaluated for in vitro cytotoxicity, in vivo antitumor activity, and DNA cross-linking ability. The anilines were attached to the DNA-intercalating acridine chromophores by link groups (-O-, -CH2-, -S-, and -SO2-) of widely varying electronic properties, providing four series of widely differing mustard reactivity where the alkyl chain linking the acridine and mustard moieties was varied from two to five carbons. Relationships were sought between chain length and biological properties. Within each series, increasing the chain length did not alter the reactivity of the alkylating moiety but did appear to position it differently on the DNA, since cross-linking ability (measured by agarose gel assay) altered with chain length, being maximal with the C4 analogue. The in vivo antitumor activities of the compounds depended to some extent on the reactivity of the mustard, with the least reactive SO2 compounds being inactive. However, DNA-targeting did appear to allow the use of less reactive mustards, since the S-linked acridine mustards showed significant activity whereas the parent S-mustard did not. Within each active series, the most active compound was the C4 homologue, suggesting some relationship between activity and extent of DNA alkylation.

DNA-directed alkylating ligands as potential antitumor agents: sequence specificity of alkylation by intercalating aniline mustards.

The sequence preferences for alkylation of a series of novel parasubstituted aniline mustards linked to the DNA-intercalating chromophore 9-aminoacridine by an alkyl chain of variable length were studied by using procedures analogous to Maxam-Gilbert reactions. The compounds alkylate DNA at both guanine and adenine sites. For mustards linked to the acridine by a short alkyl chain through a para O- or S-link group, 5'-GT sequences are the most preferred sites at which N7-guanine alkylation occurs. For analogues with longer chain lengths, the preference of 5'-GT sequences diminishes in favor of N7-adenine alkylation at the complementary 5'-AC sequence. Magnesium ions are shown to selectively inhibit alkylation at the N7 of adenine (in the major groove) by these compounds but not the alkylation at the N3 of adenine (in the minor groove) by the antitumor antibiotic CC-1065. Effects of chromophore variation were also studied by using aniline mustards linked to quinazoline and sterically hindered tert-butyl-9-aminoacridine chromophores. The results demonstrate that in this series of DNA-directed mustards the noncovalent interactions of the carrier chromophores with DNA significantly modify the sequence selectivity of alkylation by the mustard. Relationships between the DNA alkylation patterns of these compounds and their biological activities are discussed.

DNA-directed alkylating agents. 1. Structure-activity relationships for acridine-linked aniline mustards: consequences of varying the reactivity of the mustard.

A series of DNA-targeted aniline mustards have been prepared, and their chemical reactivity and in vitro and in vivo cytotoxicity have been evaluated and compared with that of the corresponding simple aniline mustards. The alkylating groups were anchored to the DNA-intercalating 9-aminoacridine chromophore by an alkyl chain of fixed length attached at the mustard 4-position through a link group X, while the corresponding simple mustards possessed an electronically identical small group at this position. The link group was varied to provide a series of compounds of similar geometry but widely differing mustard reactivity. Variation in biological activity should then largely be a consequence of this varying reactivity. Rates of mustard hydrolysis in the two series related only to the electronic properties of the link group, with attachment of the intercalating chromophore having no effect. The cytotoxicities of the simple mustards correlated well with group electronic properties (with a 200-300-fold range in IC50S). The corresponding DNA-targeted mustards were much more potent (up to 100-fold), but their IC50 values varied much less with linker group electronic properties. Most of the DNA-targeted mustards showed in vivo antitumor activity, being both more active and more dose-potent than either the corresponding untargeted mustards and chlorambucil. These results show that targeting alkylating agents to DNA by attachment to DNA-affinic units may be a useful strategy.

'Petite' mutagenesis and mitotic crossing-over in yeast by DNA-targeted alkylating agents.

Although the biological properties (cytotoxicity, mutagenicity and carcinogenicity) of alkylating agents result from their bonding interactions with DNA, such compounds generally do not show any special binding affinity for DNA. A series of acridine-linked aniline mustards of widely-varying alkylator reactivity have been designed as DNA-directed alkylating agents. We have considered whether such DNA targeting has an effect on mutagenic properties by evaluating this series of drugs in comparison with their untargeted counterparts for toxic, recombinogenic and mutagenic properties in Saccharomyces cerevisiae strain D5. The simple untargeted aniline mustards are effective inducers of mitotic crossing-over in this strain, but resemble other reported alkylators in being rather inefficient inducers of the "petite" or mitochondrial mutation in yeast. However, the majority of the DNA-targeted mustards were very efficient petite mutagens, while showing little evidence of mitotic crossing-over or other nuclear events. The 100% conversion of cells into petites and the lack of a differential between growing and non-growing cells are similar to the effects of the well characterised mitochondrial mutagen ethidium bromide. These data suggest very different modes of action between the DNA-targeted alkylators and their non-targeted counterparts.

Bacterial mutagenicity studies of DNA-intercalating aniline mustards: an insight into the mode of action of a novel class of anti-tumour drugs.

Bifunctional alkylating agents are reactive compounds which work by crosslinking DNA, but which have no special affinity for it. A series of acridine-linked aniline mustards of widely-varying alkylator reactivity (5-11), designed as DNA-directed alkylating agents, have been evaluated in various strains of Salmonella typhimurium with differing DNA repair capabilities to obtain information about their mechanism of action. The compounds showed greatly increased potency (determined as D37 values) compared with the corresponding untargeted mustards, in the repair-proficient strain TA1978+. All but the most unreactive mustards were considerably more potent (greater than 10-fold) in the corresponding repair-deficient strain TA98, indicating that DNA-crosslinking is the major cytotoxic mechanism. However, the mutagenic profile of the compounds in four Salmonella strains, particularly in the excision repair-deficient strains TA98 and TA100, suggest the compounds also form substantial levels of mutagenic monoadducts. The possibility that intercalative binding by the acridine chromophore provides an additional geometrical restraint on cross-linking by the mustard is an important facet of design which needs further study.