Rose bengal (4,5,6,7-tetrachloro-2',4',5',7'-tetraiodofluorescein) is a stain. Rose bengal belongs to the class of organic compounds called xanthenes. Its sodium salt is commonly used in eye drops to stain damaged conjunctival and corneal cells and thereby identify damage to the eye. The stain is also used in the preparation of Foraminifera for microscopic analysis, allowing the distinction between forms that were alive or dead at the time of collection. A form of rose bengal is also being studied as a treatment for certain cancers and skin conditions. The cancer formulation of the drug, known as PV-10, is currently undergoing clinical trials for melanoma, breast cancer. and neuroendocrine tumors. The company also has formulated a drug based on rose bengal for the treatment of eczema and psoriasis; this drug, PV-10, is currently in clinical trials as well. History and etymology Rose bengal was originally prepared in 1882 by Swiss chemist Robert Ghnem, as an analogue of fluorescein. Rudolf Nietzki at the University of Basel identified the principal constituents of rose bengal as iodine derivatives of di- and tetra-chlorofluorescein. The compound was originally used as a wool dye. Its name derives from rose (flower) and Bengal (region); it is printed as rose bengal or Rose Bengal in the scientific literature. Chemical applications Despite its complicated photochemistry involving several species, rose bengal is also used in synthetic chemistry to generate singlet oxygen from triplet oxygen. The singlet oxygen can then undergo a variety of useful reactions, particularly [2 + 2] cycloadditions with alkenes and similar systems. Derivatives and salts Rose bengal can be used to form many derivatives that have important medical functions. One such derivative was created so to be sonosensitive but photoinsensitive, so that with a high intensity focused ultrasound, it could be used in the treatment of cancer. The derivative was formed by amidation of rose bengal, which turned off the fluorescent and photosensitive properties of rose bengal, leading to a usable compound, named in the study as RB2. Salts of rose bengal include C20H4Cl4I4O5·2Na (CAS 632-69-9). This sodium salt is a dye, which has its own unique properties and uses. Biological applications PV-10 (an injectable form of rose bengal) was found to cause an observable response in 60% of tumors treated, according to researchers in a phase II melanoma study. Locoregional disease control was observed in 75% of patients. Also confirmed was a "bystander effect", previously observed in the phase