Retinoids definition

Retinoid-A term undergoing nearly continual modification in order to accurately describe the characteristics of a large family derived from the carotenes and xanthophylls. The family exhibits specific properties required in vision, metabolism and reproduction. Many of these properties are dependent on the stereo-chemistry of the materials. Only some of the stereo-chemical features may be required in a given application. The definition of Spom, et. al. in 1994 is the most widely used. However, it does not stress the visual properties required to form a chromophore in vision. 

These definitions illustrates the fact that the roles of retinol as a vitamin and/or hormone have little connection to its role as a vitamin. A more current description of the role of the retinoids considered part of the Vitamin A group is to... 

Spom, Roberts and Goodman81 have recently provided a broad overview of the field of the retinoids, based on the IUPAC-IUB Joint commission definition that Retinoids are a class of compounds consisting of four isoprenoid units joined in a head-to-tail manner. (The work was updated in 1994) Whereas there is only a handful of naturally occurring retinoids using this definition, more than a thousand man-made retinoids are known. The IUPAC-IUB bulletin recommended that the term, retinal, not be used as a chemical designation but be reserved as an adjective referring to the retina. They suggested that retinaldehyde be used instead in scientific literature as the name of chemical related to vision. 

Retinoids have been defined as a class of compounds consisting of four isoprenoid units (H2C=C(CH3)-CH=CH2) joined in a head-to-tail manner. The retinoid molecule can be divided into three parts a trimethylated cyclohexene ring, a conjugated tetraene side chain, and a polar carbon-oxygen functional group. Retinol (I), retinaldehyde (II), and retinoic acid (III), as well as their derivatives whose structures are shown in Structure 1 (/), are included by this definition. 

The effects of retinoids on differentiation of epithelia in organ culture undoubtedly result from a combination of complex cellular responses and interactions of different cell types in the explant. These systems therefore have definite limitations for analysis of the molecular mechanism of action of the retinoids. The introduction of cell culture methodology to such studies was therefore of great importance and now is allowing molecular investigation of the role of retinoids in proliferation and differentiation. 

B. Clinical Cancer Studies This section examines the clinical use of retinoids for the treatment of established cancer. While results of these studies are quite preliminary, initial data suggest definite antitumor activity in certain disorders. 

---