I. Overview of Vitamin A Vitamin A is widely used in the treatment of various skin diseases, however, the therapeutic dose is close to the toxic dose in the treatment of these diseases, which greatly limits the clinical application of vitamin A. Chemists have modified the chemical structure of vitamin A and obtained many structural analogues of vitamin A, which are collectively called “Retinoids”, because they are mainly derivatives of vitamin A acid, which are translated as “Vitamin A acid” by domestic scholars. Retinoids Retinoic acid is an intermediate metabolite of vitamin A in the body, mainly affecting bone growth and epithelial metabolism, promoting the proliferation of keratin-forming cells, differentiating keratinolysis, and maintaining the normal metabolism of cells in the skin, cornea, upper respiratory tract and genitourinary tract. Oral or topical use of retinoic acid and its derivatives can promote cell differentiation, inhibit the keratinization process, reduce sebum secretion, block the chemical carcinogenic process, enhance the function of package immunity and free macrophages, as well as anti-inflammatory, inhibit and kill acne rods. At present, retinoic acid drugs are the main drugs in the clinical treatment of skin keratosis abnormalities, especially in the treatment of psoriasis drugs have an irreplaceable position. The first generation (1969-1974) included retinoic acid, all-trans retinoic acid, isotretinoin and vincristine, which were suitable for severe nodular or aggregated acne and could also be used topically to treat ichthyosis, etc. However, there were many adverse reactions and the efficacy of treating psoriasis was not very satisfactory. The second generation (1974-1980) includes aveloxate, avelic acid, etc. It is mainly used in the treatment of Darrier’s disease (follicular keratosis), red furunculosis of hair, pustular psoriasis, erythrodermic psoriasis, ichthyosis, etc. In particular, aveloxate, which can be taken orally or applied topically, has positive efficacy in the treatment of psoriasis. Compared with aveloxate, there are relatively fewer adverse reactions, yet the efficacy is similar. Currently, there is a clinical tendency to use avobenzoic acid instead of avobenzate for the treatment of psoriasis. The third generation (1980-1996) includes aromatic retinoic acid, aromatic retinoic acid ethyl ester, and methylsulfonyl aromatic retinoic acid, etc. Other drugs introduced almost simultaneously include adapalene and tazarotene (acetyl retinoic acid). Third-generation retinoic acids are used in the treatment of psoriasis, ichthyosis, Darrier’s disease, keratoacanthoma, T-lymphocytic carcinoma, lichen planus and palmoplantar keratosis with low therapeutic concentration and few adverse effects. Adapalene and tazarotene, in particular, are currently the first-line drugs for the treatment of acne. C. Clinically used retinoic acid drugs 1. All-trans retinoic acid (All-trans retinoicacid): It mainly affects the metabolism of epithelial tissue, promotes the proliferation and differentiation of epithelial cells, and has a keratolytic effect. Topical treatment of keratosis abnormal skin disease, commonly used concentration is 0.025%, 0.05% and 0.1%, indications include psoriasis, hair red furuncle, ichthyosis, various keratosis, acne, flat moss, flat warts, common warts, hyperpigmentation, etc. 2. Isotretinoin (13-cis-retinoic acid, skin care, Isotretinoin): it is well absorbed orally and has the functions of anti-keratinization, inhibition of sebaceous gland differentiation and sebum production, anti-inflammation, inhibition of collagenase synthesis in skin tissues and immunomodulation. . Among the retinoids approved for systemic use by the FDA, only isotretinoin is effective for acne, and since its approval by the FDA in 1982, isotretinoin has become the “gold standard” drug for acne treatment. Severe acne, including nodular and cystic acne, can be treated with oral isotretinoin, which is usually effective within 8 weeks; mild and moderate acne can be treated with intermittent shock therapy, which involves 1 week of oral isotretinoin, and 1 week of oral isotretinoin per month for 6 months. Its cure rate was 88% after clinical evaluation. According to the clinical results of collaboration among four hospitals, namely Huashan Hospital, Ruijin Hospital, Huadong Hospital and Changzheng Hospital, the total effective rate of isotretinoin treatment for acne was 98.2%. The experimental results on the efficacy evaluation of isotretinoin pills combined with tanshinone capsules in the treatment of nodular cystic acne in the Dermatology Department of Zhoushan People’s Hospital in Zhejiang Province indicated that isotretinoin pills combined with tanshinone capsules can enhance the efficacy, shorten the onset of action time and course of treatment, improve the cure rate, reduce recurrence and adverse reactions, and the total effective rate reached 90%. 3, Aviolipid (etretinate): It has the ability to inhibit the abnormal proliferation and differentiation of epithelial cells, promote the normal keratinization process of epithelial tissue, and has the effect of keratolytic. Oral absorption is good, long-term use of blood accumulation of drugs can reach several months. It is mainly used for the treatment of severe psoriasis such as erythrodermic psoriasis, pustular psoriasis, congenital erythrodermic ichthyosis, Darrier’s disease, and hair red furunculosis. Adverse reactions of Avastin are more frequent and serious. Not much clinical application at present! 4.Avea (etretinate, avea acid, Acitretin): It is the active metabolite of avea ester in the body, with strong biological activity, comparable efficacy with avea ester, good oral absorption, and less chance of blood accumulation by long-term use. It is widely used in the clinical treatment of psoriasis, and is used to replace avobenzone as the first-line drug in the clinical treatment of psoriasis because of its rapid efficacy, significant effect and small relative side effects. According to the clinical experimental results of Huang Chaowei, Liu Xinting and Liu Xiaoming, the dose of Avia for the treatment of psoriasis vulgaris was 20-60mg/d for 12 weeks, and the effective rate was 84.2%, with significant clinical effect. 5, Viaminate (Viaminati): for China’s own development of a kind of vitamin A ester drugs, chemical structure similar to the all-trans vitamin A ester, has to promote epithelial differentiation and growth, inhibit the role of keratinization process. It is mainly used for the treatment of acne and other abnormal keratinization skin diseases. 6.Tazarotene (acetyl retinoic acid, Tazarotene): It is a new type of receptor-selective retinoic acid, mainly used for the treatment of plaque psoriasis. It is commonly used as a topical dosage form (gel) with a concentration of 0.05%-0.1%, with fast onset of action, better therapeutic effect, low relapse rate and extended relapse time. Tazarotene is the first receptor-selective, third-generation aromatic retinoid that selectively binds mainly to two retinoic acid receptors (RAR-β; RAR-γ), but not to retinoic acid X receptor (RXR). It is clinically safe and effective in the treatment of psoriasis and acne, and is used in keratosis pilaris, follicular sebaceous gland disease, and pre-cancerous skin lesions. According to Liu Xiaohong on the clinical efficacy of tazarotene cream for the treatment of flat warts 120 cases of experimental results show that the total effective rate of 8 weeks of treatment reached 98.3%, the efficacy of the exact, safe and reliable, worthy of clinical promotion. 7, Adapalene (Adapatene): topical mainly, chemically stable, good permeability, topical irritation of the skin is small, with strong acne dissolving activity and anti-inflammatory effect, commonly used formulations for the gel, can be used to treat acne, the usual concentration of 0.1%. According to the experimental results of Liu Lei and Zhang Yuping on the efficacy of adapalene in the treatment of mild and moderate acne: Adapalene gel was applied thinly to the beneficent area and gently massaged twice a day for four weeks, with an efficiency of 91.91%. It can be seen that the clinical effect of adapalene in the treatment of mild and moderate acne is satisfactory and worth promoting. Adverse reactions and precautions for the use of retinoic acid 1. The common adverse reactions of retinoic acid drugs include: dry skin mucous membrane, urticaria, rash, petechiae, palmoplantar peeling, skin pruritus, fixed drug rash, labyrinthitis, conjunctivitis, etc. The incidence is up to 90%. 2, the current clinical use of internal retinoids can lead to congenital malformations in infants, so women taking retinoids should avoid pregnancy before treatment, during the treatment period and for a longer period of time after treatment. 3, long-term use of large amounts of retinoic acid drugs, prone to adverse reactions of the central nervous system, such as headache (incidence of about 46%), dizziness and other reactions, should control the dose or at the same time take glutamate, vitamin B1, vitamin B6 and other drugs, can make the headache and other reactions to reduce or disappear. Retinoic acid drugs should not be used for acute dermatitis, eczema-like diseases; digestive tract and liver and kidney insufficiency caution. 4, the topical use of retinoic acid drugs should be avoided for thin skin folds, pay attention to the concentration should not be too high (0.3% or less is more appropriate), so as not to cause erythema, peeling, burning sensation and slight pain and other local irritation. If these reactions are mild, treatment can be continued. If the reaction is serious, the drug should be discontinued. 5, Avastin before use, 1 month after the drug and every 3 months after the liver function must be checked, once the liver function abnormalities, it must be checked once a week. If liver function does not return to normal in the short term or continues to deteriorate, the drug should be discontinued. The blood triglyceride level should also be monitored during the use of drugs, high blood lipids are prohibited. 6, taking retinoic acid drugs, digestive system symptoms such as nausea, vomiting, loss of appetite, abdominal pain, etc. are also relatively common, most of the symptoms gradually alleviated after discontinuation of the drug. Retinoic acid drugs should not be taken at the same time with tetracycline, minomycin, vitamin A, in order to prevent the metabolic process of the drug caused by vitamin A accumulation poisoning. 7, retinoic acid drugs affect the growth and development of children, so children in long-term high doses of oral retinoic acid, a year should be done once a year radiological monitoring.