Novel high-throughput electrochemiluminescent assay for identification of human tyrosyl-DNA phosphodiesterase (Tdp1) inhibitors and characterization of furamidine (NSC 305831) as an inhibitor of Tdp1.

By enzymatically hydrolyzing the terminal phosphodiester bond at the 3'-ends of DNA breaks, tyrosyl-DNA phosphodiesterase (Tdp1) repairs topoisomerase-DNA covalent complexes and processes the DNA ends for DNA repair. To identify novel Tdp1 inhibitors, we developed a high-throughput assay that uses electrochemiluminescent (ECL) substrates. Subsequent to screening of 1981 compounds from the 'diversity set' of the NCI-Developmental Therapeutics Program, here we report that furamidine inhibits Tdp1 at low micromolar concentrations. Inhibition of Tdp1 by furamidine is effective both with single- and double-stranded substrates but is slightly stronger with the duplex DNA. Surface plasmon resonance studies show that furamidine binds both single- and double-stranded DNA, though more weakly with the single-stranded substrate DNA. Thus, the inhibition of Tdp1 activity could in part be due to the binding of furamidine to DNA. However, the inhibition of Tdp1 by furamidine is independent of the substrate DNA sequence. The kinetics of Tdp1 inhibition by furamidine was influenced by the drug to enzyme ratio and duration of the reaction. Comparison with related dications shows that furamidine inhibits Tdp1 more effectively than berenil, while pentamidine was inactive. Thus, furamidine represents the most potent Tdp1 inhibitor reported to date.

Synergistic inhibition of protease-inhibitor-resistant HIV type 1 by saquinavir in combination with atazanavir or lopinavir.

BACKGROUND: Double-boosted protease inhibitors (PIs) are under investigation for the treatment of patients who are unable to take nucleoside reverse transcriptase inhibitors because of cross-resistance and/or intolerance. Evidence of synergistic inhibition of wild-type HIV has been reported for saquinavir with atazanavir or lopinavir. METHODS: We investigated the activity of these two combinations against a panel of six site-directed mutant HIV-1 strains and 14 clinically derived recombinant HIV-1 strains presenting a range of PI-resistance profiles. RESULTS: No evidence of synergy was observed against wild-type virus for either combination. The combination of saquinavir and lopinavir showed evidence of synergy against four viruses displaying high-level resistance to lopinavir and low-level resistance to saquinavir. Similarly, evidence of synergy between saquinavir and atazanavir was only observed in two viruses which were more susceptible to saquinavir than to atazanavir. CONCLUSIONS: We hypothesize that differences between the PIs in intracellular protein-binding behaviour or inhibition of drug transporters (P glycoprotein, MDR1 and MDR2) could result in intracellular levels of saquinavir being increased by co-administration with lopinavir or atazanavir. The effect of this increase would be masked in cases involving viruses that were susceptible to atazanavir or lopinavir. In virus resistant to lopinavir or atazanavir but susceptible to saquinavir, the majority of the antiviral effect is due to saquinavir; thus even small increases in intracellular concentration could significantly increase virus inhibition. These results confirm that in vitro synergy can be observed between PIs and suggest that the degree of synergy observed might depend on the resistance profile of the virus.

Inhibition of human tyrosyl-DNA phosphodiesterase by aminoglycoside antibiotics and ribosome inhibitors.

DNA topoisomerase I (Top1) is the target of camptothecin, and novel Top1 inhibitors are in development as anticancer agents. Top1 inhibitors damage DNA by trapping covalent complexes between the Top1 catalytic tyrosine and the 3'-end of the broken DNA. Tyrosyl-DNA phosphodiesterase (Tdp1) can repair Top1-DNA covalent complexes by hydrolyzing the tyrosyl-DNA bond. Inhibiting Tdp1 has the potential to enhance the anticancer activity of Top1 inhibitors (http://discover.nci.nih.gov/pommier/pommier.htm) and to act as antiproliferative agents. In the present study, we report that neomycin inhibits Tdp1 more effectively than the related aminoglycosides paromomycin and lividomycin A. Inhibition of Tdp1 by neomycin is observed both with single- and double-stranded substrates but is slightly stronger with duplex DNA, which is different from aclarubicin, which only inhibits Tdp1 with the double-stranded substrate. Inhibition by neomycin can be overcome with excess Tdp1 and is greatest at low pH. To our knowledge, aminoglycoside antibiotics and the ribosome inhibitors thiostrepton, clindamycin-2-phosphate, and puromycin are the first reported pharmacological Tdp1 inhibitors.