ABSTRACT
The E-ring lactone is the Achilles' heel of camptothecin derivatives: although it is considered necessary for the inhibition of the enzyme topoisomerase I (topo1), the opening of the lactone into a carboxylate abolishes the generation of topo1-mediated DNA breaks. S38809 is a novel camptothecin analog with a stable 5-membered E-ring ketone; therefore, it lacks the lactone function. DNA relaxation and cleavage assays revealed that S38809 functions as a typical topo1 poison by stimulating DNA cleavage at T downward arrow G sites. The activity was strongly dependent on the stereochemistry of the C-7 carbon atom that bears the hydroxy group. S38809 proved to be a potent cytotoxic agent, with a mean IC50 of 5.4 nM versus 11.6 nM for topotecan and 3.3 nM for SN38 (the active metabolite of irinotecan) on a panel of 31 human tumor cell lines. The cytotoxicity of S38809 and its ability to stabilize cleavable complexes was considerably reduced in camptothecin-resistant cells that express a mutated topo1, confirming that topo1 is its primary target. Cell death induced by topo1 poisoning requires the conversion of DNA single-strand breaks into double-strand breaks that can be detected by the formation of phosphorylated histone H2AX. In HCT116 cells, topotecan, SN38, and S38809 induced histone H2AX phosphorylation in S phase of the cell cycle, with S38809 being as potent as SN38 and 5-fold more potent than topotecan. In vivo, S38809 showed a marked antitumor activity against HCT116 xenografts. These findings open a new route for improving the pharmacological properties of camptothecin derivatives.
Subject(s)
Antineoplastic Agents/pharmacology , Camptothecin/analogs & derivatives , Camptothecin/pharmacology , Dioxoles/pharmacology , Enzyme Inhibitors/pharmacology , Topoisomerase I Inhibitors , Apoptosis/drug effects , Caspases/physiology , Cell Division/drug effects , Cell Line, Tumor , Cell Proliferation/drug effects , G2 Phase/drug effects , Humans , PhosphorylationABSTRACT
Ten novel camptothecin (CPT) derivatives devoid of the lactone function in the E-ring were synthesized and evaluated as anticancer agents. Several of these CPT analogues bearing a five-membered E-ring are potent inhibitors of the DNA relaxation and cleavage reactions catalyzed by topoisomerase I and exhibit promising cytotoxic activities with IC(50) values in the nM range. This is the first successful design of lactone-free CPT, providing thus a new avenue to the development of topoisomerase I targeted antitumor agents.
Subject(s)
Antineoplastic Agents, Phytogenic/chemical synthesis , Camptothecin/analogs & derivatives , Topoisomerase I Inhibitors , Animals , Antineoplastic Agents, Phytogenic/pharmacology , Camptothecin/pharmacology , Cell Division/drug effects , Cell Line, Tumor/drug effects , DNA/metabolism , DNA Topoisomerases, Type I/metabolism , Drug Design , Drug Resistance , Enzyme Inhibitors/chemical synthesis , Enzyme Inhibitors/pharmacology , Humans , Lactones/chemistry , Structure-Activity RelationshipABSTRACT
Employing a two-lever, food-reinforced, Fixed Ratio 10 drug discrimination procedure, rats were trained to recognize the highly-selective serotonin (5-HT)(2A) receptor antagonist, MDL100,907 (0.16 mg/kg, i.p.). They attained criterion after a mean +/- S.E.M. of 70 +/- 11 sessions. MDL100,907 fully generalized with an Effective Dose (ED)(50) of 0.005 mg/kg, s.c. A further selective 5-HT(2A) antagonist, SR46349, similarly generalized with an ED(50) of 0.04 mg/kg, s.c. In distinction, the selective 5-HT(2B) antagonist, SB204,741 (0.63 and 10.0 mg/kg), the 5-HT(2B/2C) antagonist, SB206,553 (0.16 and 2.5 mg/kg) and the selective 5-HT(2C) antagonists, SB242,084 (2.5 and 10.0 mg/kg,) and RS102221 (2.5 and 10.0 mg/kg), did not significantly generalize. In conclusion, selective blockade of 5-HT(2A) receptors by MDL100,907 elicits a discriminative stimulus in rats which appears to be specifically mediated via 5-HT(2A) as compared with 5-HT(2B) and 5-HT(2C) receptors.
