Thalidomide and 13-cis retinoic acid (RA) show anticancer effects as sole agents or in combination with other drugs. However, induction of homeobox (HOX) gene expression by 13-cis RA may contribute to tumor progression thereby potentially limiting its efficacy. The purpose was to test if thalidomide can inhibit 13-cis RA-induced HOXB7 expression and whether thalidomide may enhance the antiproliferative effect of 13-cis RA in U343MG glioblastoma cells. Quantitative real-time PCR showed significant inhibition of 13-cis RA-induced HOXB7 expression by thalidomide with IC(50) approximately 0.1-0.2 microg/ml when given simultaneously with 13-cis RA but not when administered 18 h later (p < 0.0001). 13-cis RA alone inhibited proliferation and colony formation in a concentration-dependent manner whereas growth inhibition by thalidomide alone at 5-100 microg/ml was constant at 80-90% of controls. At 10% serum concentration, growth inhibition by a combination of the 2 drugs was additive but at 1% serum, growth inhibition was synergistic. It is concluded that thalidomide inhibits the RA-induced HOXB7 expression in glioblastoma cells and that 13-cis RA/thalidomide combinations can in principle enhance cytotoxicity. The improved cell kill induced by thalidomide is attributed to downregulation of growth stimulatory factors induced by 13-cis RA. Implications for the modus operandi of thalidomide in embryogenesis are noted.