Retinoid

Overview
The retinoids are a class of chemical compounds that are related chemically to vitamin A. Retinoids are used in medicine, primarily due to the way they regulate epithelial cell growth.

Retinoids have many important and diverse functions throughout the body including roles in vision, regulation of cell proliferation and differentiation, growth of bone tissue, immune function, and activation of tumor suppressor genes.

Research is also being done into their ability to treat skin cancers. Currently 9-cis retinoic acid may be used topically to help treat skin lesions from Kaposi's sarcoma.

Types
There are three generations of Retinoids:


 * First generation retinoids: which include retinol, retinal, tretinoin (Retin-A), isotretinoin and alitretinoin.
 * Second generation retinoids: which include etretinate and its metabolite acitretin.
 * Third generation retinoids: which include tazarotene and bexarotene.

Structure
The basic structure of the retinoid molecule consist of a cyclic end group, a polyene side chain and a polar end group. The conjugated system formed by alternating C=C double bonds in the polyene side chain are responsible for the color of retinoids (typically yellow, orange, or red). Hence, many retinoids are chromophores. Alternation of side chains and end groups creates the various classes of retinoids.

First and Second generation retinoids are able to bind with several retinoid receptors due to the flexibility imparted by their alternating single and double bonds.

Third generation retinoids are less flexible than First and Second generation retinoids and therefore, interact with fewer retinoid receptors.

Uses
Retinoids are used in the treatment of many diverse diseases and are effective in the treatment of a number of dermatological conditions such as inflammatory skin disorders, skin cancers, disorders of increased cell turnover(e.g. psoriasis), and photoaging.

Common skin conditions treated by retinoids include acne and psoriasis.

Toxicity
Toxic effects occur with prolonged high intake (in children 25,000-500,000 IU daily). A medical sign of chronic poisoning is the presence of painful tender swellings on the long bones. Anorexia, skin lesions, hair loss, hepatosplenomegaly, papilloedema, bleeding, general malaise, pseudotumor cerebri, and death may also occur.

Chronic overdose also causes an increased liability of biological membranes and of the outer layer of the skin to peel.

Recent research has suggested a role for retinoids in cutaneous adverse effects for a variety of drugs including the Antimalarial drug proguanil. It is proposed that drugs such as proguanil act to disrupt retinoid homeostasis.