ABSTRACT
The influence of structure on DT-diaphorase substrate activity, topoisomerase II inhibition activity, and DNA reductive alkylation was studied for the 6-aziridinylpyrrolo[1,2-alpha]benzimidazolequinones (PBIs) and the 6-acetamidopyrrolo[1,2-alpha]benzimidazolequinones (APBIs). The PBIs are reductively activated by DT-diaphorase and alkylate the phosphate backbone of DNA via major groove interactions, while the APBIs are reductively inactivated by this enzyme since only the quinone form inhibits topoisomerase II. Bulk at the 7-position (butyl instead of methyl) significantly decreases k(cat)/K(m) for DT-diaphorase reductase activity for both PBIs and APBIs. As a result, a 7-butyl PBI has little cytotoxicity while the 7-butyl APBI has enhanced cytotoxicity. The type of 3-substituent and the configuration of the 3-position of the PBIs and APBIs influence DT-diaphorase substrate activity to a lesser degree. Bulk at the 7-position (butyl instead of methyl) had an adverse effect on APBI inhibition of topoisomerase II while the configuration of the 3-position had either an adverse or positive effect on inhibition of this enzyme. The configuration of the 3-position, when substituted with a hydrogen bond donor, influences the PBI reductive alkylation of DNA homopolymers. The rationale for this observation is that the R or S stereoisomers will determine if the 3-substituent points in the 3' or 5' direction and thereby influence the hydrogen-bonding interactions. The above findings were used to rationalize the relative cytotoxicity of various PBI and APBI derivatives.
Subject(s)
Antineoplastic Agents, Alkylating/pharmacology , Benzimidazoles/pharmacology , DNA/metabolism , Enzyme Inhibitors/pharmacology , NAD(P)H Dehydrogenase (Quinone)/metabolism , Pyrroles/pharmacology , Quinones/pharmacology , Topoisomerase II Inhibitors , Alkylation , Animals , Antineoplastic Agents, Alkylating/chemistry , Antineoplastic Agents, Alkylating/metabolism , Benzimidazoles/chemistry , Benzimidazoles/metabolism , Drug Screening Assays, Antitumor , Electrophoresis, Agar Gel , Enzyme Inhibitors/chemistry , Enzyme Inhibitors/metabolism , Kinetics , Liver/enzymology , Molecular Structure , Oxidation-Reduction , Pyrroles/chemistry , Pyrroles/metabolism , Quinones/chemistry , Quinones/metabolism , Rats , Rats, Inbred Strains , Structure-Activity Relationship , Substrate Specificity , Tumor Cells, CulturedABSTRACT
The synthesis and cytotoxic properties of benzimidazole-based DNA-cleaving agents are presented herein. These agents include pyrrolo[1,2-a]benzimidazole (PBI), benzimidazole (BI), and tetrahydropyrido[1,2-a]benzimidazole (TPBI) analogues. As a result of these studies, it is concluded that the pyrrolo ring is not necessary for cytotoxicity (PBI is only slightly more cytotoxic than BI) but that homologation of the pyrrolo ring by one carbon results in a system, TPBI, prone to decomposition. Another conclusion is that the 6-aziridinyl derivative of the PBI system is more potent than the 7-aziridinyl derivative. Comparative studies with known antitumor agents revealed that the benzimidazole-based DNA-cleaving agents possess a unique spectrum of activity. Noteworthy observations are the high level of cytotoxicity against melanoma cell lines and the complete absence of activity against leukemia cell lines. The reductive activation and DNA-cleavage properties of the most active analogue (BI-A) are also presented. Reduction of the quinone ring to the hydroquinone results in nucleophile and proton trapping by the aziridinyl group. Documented nucleophiles include water and the oxygen anion of 5'-dAMP. In addition, reduced BI-A reacts with DNA to form a stable adduct, which cleaves at G+A bases upon heating in basic gel-loading solution.
