Interrelationship between affinity for DNA, cytotoxicity and induction of DNA-breaks in cultured L1210 cells for two series of tricyclic intercalators: Simplified analogues of ellipticine derivatives

Abstract

The interrelationship between affinity for DNA, cytotoxicity and induction of single-strand DNA breaks in cultured L1210 cells was studied for 21 compounds belonging to two series of tricyclic intercalators: 1-amino-substituted 4-methyl-5 H-pyrido[4,3- b]indoles (γCARB) and 1-amino-substituted 4-methyl-5 H-pyrido[3',4': 4,5]pyrrolo[2,3-c]pyridines (PPP), which are simplified analogues of Ellipticine derivatives obtained by deletion of one cycle. Adriamycin ®, m-AMSA (4'-(9-acridinylamino) methanesulfon- m-anisidide), PZE (10-[diethylaminopropyl amino]-6-methyl-5 H-pyrido[3',4': 4,5]-pyrrolo [2,3-g] isoquinoline and RTE ([1-(3-diethylaminopropylamino)-9-methoxy ellipticine, bimaleate) are used as reference compounds. The intercalation of these compounds into DNA was strongly suggested by three experimental observations: (i) the competitive inhibition of ethidium bromide intercalation, (ii) bathochromic and hypochromic effects on absorption spectra induced by DNA, and (iii) drug-induced increase of the DNA length, measured by viscosimetry. PPP derivatives are generally less cytotoxic and induce DNA breaks less efficiently than the γCARB ones, both in terms of maximum breakage frequencies and required drug concentrations. The most active compounds induced SSB in the DNA of L1210 cells, in a bell-shaped manner: the SSB frequency increased, rose to a maximum and then decreased as the drug concentrations increased. The maximum SSB frequencies induced by the most active compounds are of the same order as those of reference compounds Adriamycin ® and PZE. The structurally important requirements are essentially the same for both DNA breakage activity and cytotoxicity: (i) a N-CH 3 in the 5-position, (ii) a CH 3 in the 4-position, (iii) a hydroxy in the 8-position and (iv) the presence of an (aminoalkyl)amino side chain with preferentially a 3 carbon unit. There is no direct relationship between DNA affinity in vitro and induction of DNA breaks in cells, although a relatively high affinity seemed to be a necessary condition, since the most active compounds have the highest affinities and compounds having a very low affinity are totally inactive. The close correlation between cytotoxicity and extent of induction of DNA breaks suggests that these breaks may be in fact the lethal lesions responsible for cell death and thereby for the antitumor properties of these tricyclic intercalators.