Novel sulfa Schiff bases were synthesized and characterized by a reaction between aromatic sulfonamides and aromatic aldehydes or heterocyclic ketones in equimolar ratios. Their cytotoxicity was evaluated by the resazurin assay towards human sensitive CCRF-CEM and multidrug-resistant CEM/ADR5000 leukemia cells. Three of the tested compounds viz., 4-(anthracen-9-ylmethyleneamino)-N-(pyrimidin-2-yl)benzenesulfonamide (4), 4-(anthracen-9- ylmethyleneamino)benzenesulfonamide, (5) and 4-((3-phenylallylidene)amino)benzene-sulfonamide, (6) were cytotoxic (IC50 values: 5.38-19.96 µM). CEM/ADR5000 cells were not cross-resistant to these compounds, indicating activity against otherwise drug-resistant tumors. Compound 6 inhibited P-glycoprotein by increasing doxorubicin accumulation and reducing expression of P-glycoprotein in CEM/ADR5000 cells. A human P-glycoprotein homology model was used for molecular docking studies. Compound 6 and verapamil (a well-known P-glycoprotein inhibitor) docked with similar binding energies to the same binding pocket.