PK11195 significantly increases GO cytotoxicity in HL-60-derivative cells relatively overexpressing MRP or Pgp. The cytotoxicity of 40 ng/mL GO was measured in HL-60 AML cells, in HL-60/AR cells that relatively overexpress MRP due to repeated exposures to adriamycin, and in HL-60/VCR cells that relatively overexpress Pgp due to repeated exposures to vincristine. The treated live fractions are displayed relative to untreated live cell fractions, as in Figure 2. PK11195 alone (50 μM) measurably reduced live cell fractions but also supra-additively and significantly increased GO cytotoxicity in HL-60/AR cells, relative to treatments with GO alone or PK11195 alone. Neither 10 μM MK-571 nor 2.5 μg/mL CSA significantly increased GO cytotoxicity. Neither 50 μM PK11195 nor 2.5 μg/mL CSA nor 10 μM MK-571 increased GO cytotoxicity in HL-60/VCR cells, although both PK11195 and CSA increased DOX cytotoxicity in HL-60/VCR cells (data not shown). ^*P < .05, ^**P < .01, ^***P < .0001 compared to untreated cells; ^#P < .05, ^##P < .01, ^###P < .001, ^####P < .0001 compared to cells treated with GO alone; ^@P < .0001 compared to HL-60 cells treated with GO at the corresponding dose. Data are shown as mean ± SEM from up to 5 independent experiments performed in duplicate or triplicate wells.