The need for beneficial use of sedatives in oncologic patients is increasing. Therefore, in this study, antiproliferative characteristics of herbal and synthetic sedatives were examined in vitro in SNU-C4 human colorectal adenocarcinoma cells. Apigenin (50% inhibition concentration, IC(50) = 1.8 +/- 0.5 microM) and diazepam (IC(50) = 7.0 +/- 0.5 microM) showed concentration-dependent inhibition of SNU-C4 cancer cell survival. Efficacy of cancer cell survival inhibition by apigenin and diazepam was much lower than that of 5-fluorouracil (5-FU), a known chemotherapeutic drug. However, 10(-6) M concentration of apigenin and diazepam potentiated 5-FU-induced cytotoxicity. In SNU-C4 cells, 10(-6) M concentrations of diazepam, flumazenil (Ro15-1788), Ro5-4864, or PK11195, all ligands for central- or peripheral-type benzodiazepine (BZD) receptors, inhibited cell survival like the flavonoid apigenin (4',5,7-trihydroxyflavone) and fisetin (3,7,3',4'-tetrahydroxyflavone). Also like the plant flavonoids, treatment with 10(-6) M concentration of diazepam for 3 days hardly affect the peripheral-type BZD receptor (PBR) messenger RNA (mRNA) expression and inhibited glucose utilization of SNU-C4 cells. Treatment with flavonoids or diazepam for 6 days upregulated PBR mRNA expression and cell cytotoxicity of SNU-C4 cells. Furthermore, treatment with 10(-6) M concentration of apigenin, a natural sedative material originating from traditional herbs, positively modulated BZD-induced antiproliferative cytotoxicity in SNU-C4 cells. Overall, the in vitro antiproliferative activity on SNU-C4 cancer cells of herbal sedatives, such as apigenin, plus additive enhancement of synthetic BZD- and 5-FU-induced antiproliferative activities, were shown. In conclusion, this study provides experimental basis for advanced trial in the future.