Influence of resistance mechanisms on cytotoxic potency of thaspine and standard topoisomerase inhibitors.

The resistance factor was defined as the IC50 value in the resistant subline divided by that in its parental cell line. The IC50 of thaspine on the parental cell lines were between 1–3 µM. Topo II, Topoisomerase II; P-gp, P-glycoprotein (ABCB1); MRP, multidrug resistance-associated protein (ABCC1). For details on cell lines and calculations of resistance factors, see [22]. CCRF-CEM are not resistant to camptothecin [21], which was therefore not included. The experiments were repeated twice.