Salicylic acid against skin inflammations.

Theskin inflammation is a sign of an immune response following a stimulus or trigger. When this occurs, many types of immune system cells get involved and start to release various substances that can dilate blood vessels and make them more permeable. This increases blood flow and thus allows immune cells to more easily reach the affected area.

This phenomenon then leads to numerous symptoms associated with inflammation, including redness, skin rashes, a sensation of heat, pain, itching, and swelling. The triggering of the inflammatory process has many possible causes which can be acute or chronic:

• the allergic reactions following the consumption of a specific food, the intake of a certain medication, or direct contact with a particular object;

• the photosensitivity, that is to say, an immune response to direct sunlight;

• the infections bacterial, viral or fungal, such as impetigo, cellulitis, ringworm, etc. ;

• the autoimmune diseases such as psoriasis, vitiligo, lupus erythematosus, scleroderma, etc..

From toothaches to fever and pain associated with childbirth, salicylates (salicin, salicylic acid, acetylsalicylic acid, methyl salicylate, etc.) have been used for centuries to treat symptoms of inflammation in various parts of the body. However, its anti-inflammatory properties can also work when salicylic acid is applied topically, and it is currently frequently used for its keratolytic properties, antibacterial and photoprotective, thus targeting irritated and inflamed skin.

Indeed, the topical application of salicylic acid is believed to help soothe skin inflammation, alleviate irritation from skin eruptions, and calm symptoms of redness, swelling, and pain. However, the specific mechanisms and signalling pathways used by salicylic acid remain somewhat unclear, and studies are still limited. Nevertheless, it has been suggested that this anti-inflammatory action is mediated by two main mechanisms of action.

• Dynamic control of prostaglandin production: WU K. K. and his colleagues have demonstrated that salicylates work by suppressing the transcriptional activation of the gene coding for cyclooxygenase-2 (COX-2) induced by inflammatory mediators, a key enzyme in the biosynthesis of pro-inflammatory prostaglandins. This then allows for the reduction of prostaglandin synthesis at inflammatory sites.

• Inhibition of the expression of various inflammatory genes: research has indicated that salicylic acid is capable of inhibiting the inflammatory response in cells through the inhibition of the NF-κB signalling pathway, a nuclear factor involved in controlling the transcription of many genes involved in the immune response. Thus, the expression of several pro-inflammatory cytokines (IL-1β, IL-6, TNF-α) has been regulated in a dose-dependent manner.

However, studies on salicylic acid have primarily focused on its peeling properties and its comedolytic mechanism. Regarding its anti-inflammatory effect, most of the bibliographic data comes from studies on its oral form. Very few studies have been conducted on the effects of its topical use. Therefore, additional research is necessary.

• WU K. K. & al. Suppression of inducible cyclooxygenase 2 gene transcription by aspirin and sodium salicylate. PNAS (1999).

• ARIF T. Salicylic acid as a peeling agent: a comprehensive review. Clinical, Cosmetic and

  Investigational Dermatology (2015).

• RANDJELOVIĆ N. & al. The beneficial biological properties of salicylic acid. Acta Facultatis Medicae Naissensis (2015).

• JIANG X. & al. Salicylic acid treats acne vulgaris by suppressing AMPK/SREBP1 pathway in sebocytes. Experimental Dermatology (2019).