Retinyl palmitate is a natural ester form of vitamin A .¹ It is a major storage form of vitamin A in vivo.² Retinyl palmitate (1-100 µM) is cytotoxic to HeLa cells both alone and to a greater effect when combined with photodynamic therapy.³ It 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 at a dose of 2,500 IU/animal.⁴