On the basis of harmine and 1-methoxy-canthin-6-one chemical structures, a series of novel 1,4-disubstituted and 1,4,9-trisubstituted β-carbolines and tetracyclic derivatives were designed and synthesized. Cytotoxic activities of these compounds in vitro were investigated in a human tumor cell line panel. Almost all compounds demonstrated interesting cytotoxic activities in particular against prostate cancer cells PC-3 with IC50 in the low micromolar range. Compound X was found to be the most potent one with IC50 value of 8.0 µM; this suggests further studies with models of prostate cancer.