group): placebo; lurbinectedin at MTD (0.180mg/kg), 0.75MTD, 0.5MTD and 0.25MTD; gemcitabine at MTD (180mg/kg), 0.75MTD, 0.5MTD and 0.25MTD; and, lurbinectedin + gemcitabine at (1+1), (0.75+0.75), (0.50+0.50) and (0.25+0.25) MTDs. Treatments were given intravenously once per week during the placebo-treated survival time. The combination index (CI) was determined by the CI-isobol method. At 12 h post-treatment, placebo as well as lurbinectedin, gemcitabine and lurbinectedin + gemcitabine highly-dosed tumors were assayed for Hoescht and the number of stained cells in 5 high-power fields (×400) counted by 2 independent blinded counters. Results: The in vivo antitumor effect recorded in the experimental groups indicated synergism (CI