Hydroquinone

Mechanism of Action

Hydroquinone acts as a skin depigmentation agent by inhibiting melanin synthesis. It inhibits the conversion of L-3,4- dihydroxyphenylalanine (L-DOPA) to melanin by inhibiting tyrosinase due to its structural similarity to an analog of melanin precursors.[6]

The Pathway of Melanin Synthesis[9]

1. Hydroxylation of L-phenylalanine to L-tyrosine
2. Tyrosinase hydroxylates L-tyrosine to 3,4 L-dihydroxyphenylalanine (L-DOPA)
3. L-DOPA oxidized to dopaquinone
4. Production of eumelanin and pheomelanin resulting in black to brown and yellow to red color of skin

Administration

Hydroquinone is only used topically as a depigmentation agent. A thin layer is applied with fingertips and rubbed into the face (or other affected areas) 1 to 2 times a day for 3 to 6 months. If there are no results after 2 to 3 months, hydroquinone should be discontinued. It is essential to evenly apply hydroquinone over the entire face to prevent uneven pigmentation and use concurrently with sunscreen to protect from damaging UV light, which increases pigmentation.  Physicians recommended stopping treatment after this time for a few months before restarting to decrease the risk of side effects. It can also be applied during weekends only or three times a week for more extended maintenance therapy with minimal complications.[6]

Hydroquinone is available in 2% over the counter or as 4% prescribed.[2] It comes in the following dose forms: cream, emulsion, gel, or solution.

Multiple studies have shown that maximum results occur when using hydroquinone as a combination therapy with a retinoid and corticosteroid. The most widely used triple combination cream is composed of hydroquinone 4%, tretinoin 0.05%, and fluocinolone acetonide 0.01%.[6] Use in combination with other agents requires a prescription by a dermatologist.

Results vary among each patient.

About 35% to 45% of hydroquinone is absorbed systemically after topical use.[10]

Adverse Effects

Adverse Effects Include[11][6][2]

• Irritation
• Allergic contact dermatitis
• Erythema
• Inflammation
• Xeroderma
• Stinging
• Rarely ochronosis - ochronosis is a blue-black or gray-blue discoloration; it is rare and more commonly seen in patients that use a high concentration of hydroquinone for a long period on large areas of the body.

Studies also suggest that hydroquinone can falsely elevate capillary glucose when measured with a glucometer.[12]

There has been no confirmation regarding concerns that hydroquinone cream is carcinogenic either in clinical practice or human research.[6]

Contraindications

• Allergic reaction or hypersensitivity hydroquinone 
• Sun exposure
• The use of this drug requires caution with other medications that cause photosensitivity.

Pregnancy Category C

About 35 to 45% of the topical hydroquinone dose is absorbed systemically. Current studies have not shown an increased risk of malformations or adverse effects in pregnant women, but it is recommended to minimize exposure due to the substantial absorption.[10] The safety of breastfeeding mothers and children remains unestablished, and as such, breastfeeding females should avoid this drug.

Monitoring

Monitor for any hypersensitivity or long-term irritation, in which case the medication should be discontinued. Also, in rare cases, ochronosis develops; the patient should stop using hydroquinone and consult a clinician.

Toxicity

No significant toxicity has been found with the topical use of the hydroquinone cream.

Some studies report malignancies in animals treated for an extended period with large oral doses.[2]

Hydroquinone in the Environment

In the environment, hydroquinone is a chemical and can be toxic in human and industrial activities by promoting the generation of reactive oxygen species, oxidative stress, and hence the potential for DNA damage. It is a major benzene metabolite and is known to be hepatotoxic and carcinogenic in these settings. Some studies suggest it can promote tumor cell growth and suppress the immune response. It is used in photography and is present in dyes, paints, varnishes oils, and motor fuels. In its oxidized form, it is more toxic and less degradable. It demonstrates high toxicity to aquatic organisms and rodents and may induce leukemia, renal tubular cell tumor, and liver cancer. It has also been found to influence immune cell responses and causes an increase in an allergic reaction by increasing interleukin-4 production and immunoglobulin E levels.[13]

Enhancing Healthcare Team Outcomes

The interprofessional health care team, such as physicians, nurses, and pharmacists, play an essential role in monitoring patients on hydroquinone. It is important to monitor patients to ensure that they only use the medication as prescribed for no more than 5 to 6 months to limit side effects. Nursing staff can play a significant role in monitoring the patient and evaluating compliance, as it is important to follow instructions for the medication application, its frequency, keep a watchful eye for any adverse effect, which should be clearly explained to the patient. Patients should be instructed to cease using the medication if any irritation, hypersensitivity, or allergic reaction occurs. Pharmacists can review the use and adverse events when dispensing the drug, and monitor when the patient comes in for refills, alerting the prescribing clinician regarding any concerns. The rare, yet most potentially harmful, side effect of ochronosis needs to be explained to patients using hydroquinone, and they should be instructed to discontinue this medication immediately if this occurs. A collaborative interprofessional team approach to hydroquinone therapy will guide outcomes most effectively. [Level 5]