Novel stable camptothecin derivatives replacing the E-ring lactone by a ketone function are potent inhibitors of topoisomerase I and promising antitumor drugs.

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.

Synthesis and pharmacological evaluation of novel non-lactone analogues of camptothecin.

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.

The selective serotonin(2A) receptor antagonist, MDL100,907, elicits a specific interoceptive cue in rats.

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.