In vitro activity: SN-38, a biological active metabolite of irinotecan hydrochloride (CPT-11). SN-38 causes the strongest inhibition of DNA topoisomerase I, followed by CPT and then CPT-11. CPT-11 dose dependently shifts the position of relaxed DNA in the direction of nicked DNA, but SN-38 and CPT shows no effect on the position of relaxed DNA. SN-38 dose-dependently and time-dependently inhibit DNA synthesis. Respective IC50 values of SN-38, in DNA synthesis is 0.077 μM. The inhibitory effect of SN-38 on RNA synthesis is less than that on DNA synthesis and it does not inhibit protein synthesis. SN-38 caused frequent DNA single-strand breaks in P388 cells.

Kinase Assay: One unit (the minimum amount for full relaxation of 0.5 μg SV40 DNA under the conditions of this study) of topoisomerase I, 0.5 μL of the test compounds, and 0.5μg SV40 DNA are added sequentially to the reaction buffer, which is composed of 25 mM Tris-HCl (pH 7.5), 50 mM KC1, 5 mM MgCl2, 0.25 mM EDTA disodium salt, 0.25 mM dithiothreitol, 15μg /mL bovine serum albumin, and 5% glycerol. Then, the reaction mixture (50 μL) is incubated for 10 min at 37 °C, and the reaction is terminated by treatment with 7.5 μL of a solution consisting of 1% sodium dodecyl sulfate, 20 mM EDTA disodium salt, and 0.5 mg/mL proteinase K for an additional 30 min at 37°C. The samples are mixed with 5 μL of the loading buffer containing 10 mM Na2HPO4, 31.3% sucrose, and 0.3% bromophenol blue. Relaxed (form Ir) DNA is separated from supercoiled (form I) and nicked (form II) DNA by electrophoresis on 0.8% agarose gel at 50 mA and 20 V for 17 h in the presence of 2 μg/mL chloroquine, 10 mM EDTA, 30 mM NaH2PO4, and 36 Mm Tris-HCl (pH 7.8). After electrophoresis, the gel is stained with 0.05% ethidium bromide and photographed with UV light (302 nm). The amount of DNA is quantified using a densitometer.

Cell Assay: MTT assay

Cancer Res. 1991 Aug 15;51(16):4187-91; Cancer Chemother Pharmacol. 1997;40(3):259-65.