A natural form of vitamin A; a major storage form of vitamin A; cytotoxic to HeLa cells alone and when combined with photodynamic therapy when used at concentrations of 1-100 μM; decreases tumor volume by 50% in a human gallbladder cancer mouse xenograft model when administered at doses of 1,000 or 2,500 IU/animal per day for three weeks and when administered prior to tumor implantation