|Grade||Level of Evidence|
|A||Multiple double-blind, controlled clinical trials.|
|B||1 double-blind, controlled clinical trial.|
|C||At least 1 controlled or comparative clinical trial.|
|D||Uncontrolled, observational, animal or in-vitro studies only.|
|Grade||Effect||Size of Effect||Comments|
Reduces acne lesions, papules and pustules through a variety of mechanisms. Also effective in treating postinflammatory hyperpigmentation, a common sequelum of acne.
15% azelaic acid gel is a standard therapy for subtype 1 rosacea, and it is often combined with an oral antibiotic to treat subtype 2 rosacea.
Depigments the skin by killing abnormal melanocytes and inhibiting tyrosinase. Not effective against age spots, however.
20% azelaic acid cream lightens melasma as effectively as 4% hydroquinone.
Improves scaling and hyperkeratosis but not the itching of psoriatic plaques.
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Table of contents
Azelaic acid is a dicarboxylic acid that exists naturally in many whole grains, namely wheat, rye, barley, oat seeds and sorghum. It is also produced by Malassezia furfur (also known as Pityrosporum ovale), a yeast that lives on normal human skin as part of the commensal flora.
Azelaic acid is most commonly formulated as a 20% cream or as a 15% gel. Percutaneous absorption of azelaic acid into human skin from the 20% cream formulation is 3.6% of the dermally applied dose. The 15% gel formulation probably delivers higher amounts of azelaic acid to the skin, as studies on mice showed an 8-fold higher delivery (25.3% versus 3.4%) into viable skin for the gel than the cream, despite the better percutaneous absorption (16.3% versus 5.8%) of the cream. However, with both formulations the majority of the applied azelaic acid dose remains on the skin surface.
pH also affects the penetration and absorption of azelaic acid; a higher concentration was retained in the skin for a formulation with a pH of 4.9, compared to that with a pH of 3.9. Moreover, the flux of the ionized species of azelaic acid was 5-fold greater with the pH 4.9 formulation, suggesting that ionized azelaic acid penetrates through the skin.
The use of a co-drug of azelaic acid and hydroquinone can also improve the dermal delivery of both parent drugs, enhancing the uptake of azelaic acid by 1.4-fold and that of hydroquinone by 8.1-fold.
The addition of 1,4-cyclohexanediol, meanwhile, has been demonstrated to retard the penetration of azelaic acid without decreasing its retention in the skin, indicating that it may be useful in topical formulations to reduce potential systemic side effects.
The small proportion of azelaic acid that is systematically absorbed is mainly eliminated unchanged in the urine, though some will undergo mitochondrial β-oxidation to acetyl-CoA and malonyl-CoA.
3. Effects on the skin
3.1 Lightening effect
Azelaic acid has been evaluated as a treatment option for hyperpigmentation in skin of colour.
Azelaic acid 20% cream significantly and objectively decreased pigmentation intensity following 24 weeks of treatment in a double-blind, vehicle-controlled trial of darker-skinned patients. Azelaic acid 20% cream + glycolic acid 15% or 20% lotion has also been demonstrated to be as effective as hydroquinone 4% cream in treating hyperpigmentation in darker-skinned patients, indicating that this combination may be a viable alternative to hydroquinone, which safety has come under scrutiny.
However, 20% azelaic acid was not effective in improving solar lentigines (age spots) in Southeast Asian women after 2-3 months of twice daily applications.
Azelaic acid has several mechanisms of depigmenting the skin, including tyrosinase inhibition. Its selective action is also believed to be due to its specific cytotoxic and antiproliferative effects towards abnormal melanocytes, via inhibiting DNA synthesis and mitochondrial enzymes.
3.2 Melasma treatment
Azelaic acid has been compared to hydroquinone as a treatment for melasma, a common skin disorder that manifests as dark skin discoloration of the sun-exposed areas of the face and neck. 20% azelaic acid is superior to 2% hydroquinone and is at least as effective as 4% hydroquinone in lightening melasma, yielding good or excellent results in 65% of cases in a study on 329 women.
Tretinoin appears to enhance the efficacy of azelaic acid in treating melasma, leading to a higher proportion of excellent responders following treatment. Sequential therapy using 0.05% clobetasol propionate cream + 20% azelaic acid cream also improved melasma in 30 Indian patients more markedly than 20% azelaic acid cream alone, though at the end of 24 weeks, the vast majority of patients in both groups (96.7% for sequential therapy and 90% for azelaic acid monotherapy) had achieved good to excellent responses to treatment.
Another alternative treatment for melasma is the combination of 20% azelaic acid cream with low-fluence Q-switched Nd: YAG laser (QSNYL), which is more efficacious than low-fluence QSNYL and azelaic acid alone.
3.3 Acne vulgaris treatment
Azelaic acid targets several key pathophysiological features of acne. It is an anti-keratinizing agent that exerts cytostatic effects on keratinocytes and modulates epidermal differentiation, enabling it to inhibit comedones which result from the blockade of pores by keratin and oil. It also accumulates in follicles after a single topical application to concentrations high enough to inhibit the growth of Propionibacterium acnes and Staphylococcus epidermidis, bacteria often found in acne lesions, possibly by disrupting the transmembrane pH gradient.
These explain the observed clinical efficacy of azelaic acid in clearing acne in numerous clinical trials. Preliminary studies as early as the 1980s demonstrated that 20% azelaic acid cream was an effective therapy for papulopustular and comedonal acne. 20% azelaic acid cream also compares well with 0.05% tretinoin, 5% benzoyl peroxide, 2% topical erythromycin and oral tetracycline, being of similar efficacy but lacking the systemic side effects of oral antibiotics and causing less irritation than benzoyl peroxide and tretinoin.
Later studies on 15% azelaic acid gel have also confirmed its effectiveness, reducing lesion count by 60.6% in one study and facial papules and pustules by 70-71% in another. It is also effective in treating postinflammatory hyperpigmentation, a common sequelum of acne, due to its anti-tyrosinase activity, as well as adult-onset acne.
Combining azelaic acid with other topical or oral anti-acne agents also seems to be beneficial; when combined with 4% benzoyl peroxide gel, 1% clindamycin gel, 0.025% tretinoin cream or 3% erythromycin/5% benzoyl peroxide gel, the efficacy of azelaic acid cream is enhanced. Other studies have also shown that 20% azelaic acid cream + a glycolic acid lotion was as effective and better-tolerated than 0.025% tretinoin therapy, and that 20% azelaic acid cream + oral minocycline is highly effective in treating severe forms of acne, making it a suitable alternative to oral isotretinoin, especially in female acne patients of child bearing potential.
In addition, the combination of 5% azelaic acid and 2% erythromycin or clindamycin is also more effective than 2% erythromycin alone or 20% azelaic acid cream alone, as well as individual treatment with 5% azelaic acid or 2% clindamycin, respectively.
3.4 Rosacea treatment
Rosacea is a chronic inflammatory disease of the skin that commonly presents as facial redness, papules, pustules or telangiectasias. Azelaic acid is approved by the US FDA as an intervention for rosacea and has been shown to maintain control over rosacea over 6 months, significantly decreasing mean inflammatory lesion count and erythema severity compared to vehicle in a review of randomized controlled trials.
As a single treatment, once-daily dosing with 15% azelaic acid gel is safe, effective and economical. It is more effective than 0.75% metronidazole gel, probably more effective than 5% permethrin cream, and as effective as once-daily 1% metronidazole gel when used on a twice daily basis. In 2013, a new foam formulation of 15% azelaic acid was also established to be efficacious and well-tolerated.
While topical azelaic acid is one of the standard therapies for erythematotelangiectatic rosacea (subtype 1), it is usually combined with an oral antibiotic for managing papulopustular rosacea (subtype 2). Oral doxycycline + azelaic acid 15% gel in particular has been shown to be appropriate for initial therapy of mild to severe papulopustular rosacea, with subantimicrobial dosing helping to avoid the risk of antibiotic resistance.
Azelaic acid's mechanism of action in rosacea has not been clarified, but has been linked to its antimicrobial, anti-inflammatory and antioxidant effects. It inhibits protein synthesis by Staphylococcus epidermidis and Propionibacterium acnes and decreases the free fatty acid content of skin surface lipids, making the skin environment less hospitable to microbes. It also reduces the generation and action of reactive oxygen species in vitro, including those produced by neutrophils. Additionally, azelaic acid seems to modulate the inflammatory response in human keratinocytes through the activation of PPAR-γ and directly inhibiting the expression of kallikrein 5, a serine protease that is thought to initiate the heightened inflammatory response in rosacea.
Apart from topical medication, skin moisturization is important as well since epidermal barrier dysfunction and transepidermal water loss play a pathophysiologic role in rosacea. Hence, the use of a mild cleanser and moisturizer before the application of azelaic acid can help relieve symptoms and maintain the integrity of the skin barrier.
3.5 Psoriasis treatment
Psoriasis is a chronic inflammatory skin disorder characterized by the occurrence of red and silver scaly plaques. A single-blind, controlled clinical trial of 31 patients suffering from psoriasis found that 15% azelaic acid applied twice daily for 1 month effectively reduced the scaling and hyperkeratosis, but not pruritus (itching) of psoriatic plaques. More and longer studies are needed to confirm the efficacy however.
4. Side Effects
Azelaic acid is not toxic, teratogenic, associated with systemic adverse events or photodynamic reactions.  The most frequent adverse events that occur during administration of topical azelaic acid are stinging/burning and pruritus (itching) sensations that are mostly transient and of mild to moderate intensity.
Also, the gel formulation has a lower lipid concentration than the cream formulation and hence requires less emulsifiers. Because emulsifiers tend to function as surfactants or detergents that can irritate the skin, the result is that the gel formulation causes less skin irritation. Azelaic acid entrapped in nanovesicles are also suitable for topical use, as it did not cause toxicity to normal cell lines or allergies on rabbit skin in a safety assessment.
Despite the adverse skin reactions, topical azelaic acid does not actually disrupt or damage the skin barrier, as evidenced by tests on transepidermal water loss and corneometry. It does not enhance the erythema and therefore increased sunburn risk induced by UVB exposure, either.
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