Cepharanthine (12-O-methyl cepharanoline) is a plant alkaloid and has been shown to inhibit tumour necrosis factor-alpha- or phorbol 12-myristate 13-acetate-induced HIV-1 replication in the chronically infected promonocytic cell line, U1. Its mechanism of action is considered to be the inhibition of nuclear factor kappaB, a potent inducer of HIV-1 gene expression. In this study, we have synthesized 96 derivatives of cepharanoline, including cepharanthine, and examined their inhibitory effects on HIV-1 replication in U1 cells. Among the 12-O-alkyl derivatives, cepharanthine proved to be the most active, and the activity decreased as the length of the alkyl chain increased. All of the 12-O-acyl derivatives were totally inactive, while a few 12-O-carbamoyl derivatives displayed modest activity. Since 12-O-ethyl derivatives were found to be as active as cepharanthine against HIV-1 replication, we further synthesized various 12-O-ethyl derivatives of cepharanoline. Among the derivatives, five proved to be more active inhibitors than cepharanthine, and the most active compound was 12-O-ethylpiperazinyl cepharanoline. The 50% effective concentrations of this compound and cepharanthine were 0.0041 and 0.028 microg/ml (0.0060 and 0.046 microM), respectively.