Melalong is classified therapeutically as a depigmenting agent. Tretinoin is all trans retinoic acid formed from the oxidation of the aldehyde group of retinene to a carboxyl group. Tretinoin is classified as a keratolytic agent. Hydroquinone is a melanocytotoxic chemical which is oxidized in melanocytes to produce highly toxic compounds such as quinones. These cytotoxic compounds are responsible for the destruction of pigment cells, which result in skin depigmentation. However, cells are capable of protecting themselves against cytotoxic agents by intracellular gluthathione (GSH). This protection takes place under the enzymatic action of the detoxification enzyme gluthathione S- transferase (GST), which is responsible for the conjugation of toxic species to GSH.