**3.2 Monomethyl ether of hydroquinone/4–0 methoxyphenol**

This compound is a phenol derivative and is also known as p-hydroxyanisole (HA) or mequinol [1].

### *3.2.1 Mechanism of action*

*Depigmentation*

*3.1.3 Precautions*

tions while using MBEH.

on the eyelids.

*3.1.4 Side effects*

adverse reaction, the patient can continue with the application of the cream on the areas of top priority and then move in stages for low priority areas. To avoid contact dermatitis, different concentrations of MBEH can be used. MBEH can be diluted to 5% for use on the neck, 10% on the face, and 20% on the arms and legs. In patients who fail to respond to 20% MBEH over a course of 3 to 4 months, the concentration of MBEH can be increased to 30% and then further to 40%. Concentrations of 30 and 40% MBEH have been used primarily on the extremities, especially the elbows

It takes anywhere between 4 and 12 months for gradual depigmentation [8]. It is to be noted that depigmentation is mostly irreversible and histologically associated

Patients should always be informed and well instructed about certain precau-

1.Application of MBEH at one site can lead to loss of pigment at distant body sites, i.e., application of MBEH to the arm may result in loss of pigment on the

2.Application of MBEH to the eyelids is not advised [8] because of risk of

ochronosis. It may lead to pigmentation of the conjunctiva if MBEH is applied

3.Avoid skin-to-skin contact on a continuous basis with another person as it can cause a decrease in pigmentation at the site of contact in the other person.

4.The use of sunscreens with a high-sun protection factor (SPF) is essential. This

also helps to prevent repigmentation as well as sunburn reactions [4].

5.Follicular repigmentation may occur spontaneously upon sun exposure [8]. This happens mainly because MBEH only destroys epidermal melanocytes

Irritant contact dermatitis and common allergic reactions can develop [9]. In which event, application of MBEH is stopped, and open wet dressings are applied to the affected area along with topical steroids. Once the dermatitis has subsided, MBEH can be restarted at a lower concentration of 5% [8]. Other side effects include exogenous ochronosis [10], unmasking of telangiectasias and phlebectasias on the lower extremities [8], pruritus, xerosis, erythema, rash, edema, conjunctival

Risk of carcinogenesis with MBEH has not been reported but cannot be ruled out, and hence it is banned from the European Union since 2001 in cosmetics [11].

All-trans retinoic acid (RA), which is a vitamin A derivative primarily employed in the treatment of acne, is shown to serve as a weak depigmenting agent when used

and knees. Concentrations greater than this are not recommended [8].

face [4]. Moreover, it can also reactivate a stable disease.

with loss of melanosomes and melanocytes [1].

keeping follicular melanocytes intact.

melanosis, and distant depigmentation [4].

*3.1.5 Combination therapy*

for several weeks.

**58**

Mequinol acts in the similar way as MBEH acts. This compound usually acts via a dose-dependent response manner. It can be used as monotherapy or in conjunction with a Q-switched ruby laser.

## *3.2.2 Administration of treatment*

The compound is used in a 20% concentration in an oil/water cream base. As with MBEH, cream is applied on an initial test patch to observe for any allergic reactions. If there are no reactions, the patient is advised to apply cream twice daily until complete depigmentation is observed [16]. The effectiveness of 4-MP has been correlated with the duration of the use of the cream; the longer the cream was used, better the results that were obtained [1].

A combination product of 2% 4-hydroxyanisole (mequinol) and 0.01% tretinoin was tested in a double-blind multicentric study and was found to significantly improve solar lentigines and related hyperpigmented lesions of the face and hands after a twice-daily application of up to 24 weeks [1].

### *3.2.3 Side effects*

Side effects include mild burning or itching, irregular leukoderma, contact dermatitis, ochronosis, and risk of carcinogenesis cannot be ruled out [11]. Protection from sunlight is necessary or repigmentation risk is high [1, 11].

## **3.3 Phenol solution (88%)**

Phenol is an inexpensive peeling agent having medium-depth capability and used for treatment of photodamage or rhytids. The toxicity of phenol toward melanocytes is well documented. Phenol has the ability to penetrate deeper into the tissue up to the upper reticular dermis.

#### *3.3.1 Mechanism of action*

Phenol is involved in melanogenesis, inducing coagulation of protein in the epidermis. The melanocytes lose their capacity to synthesize melanocytes normally. This property of phenol is different than that of MBEH and hydroquinone wherein they destroy the melanocytes [17]. Hence, 88% phenol can be used as therapeutic option to eliminate residual normally pigmented lesions in patients.

#### *3.3.2 Administration of treatment*

The area to be treated is cleaned with spirit/alcohol. Application of phenol is done with the help of a swab soaked with phenol until cutaneous frosting occurs. There might be a burning sensation experienced by the patient for approximately 60 seconds, which gradually decreases in intensity but can last from minutes to hours. In a case study reported by Zanini and Machado Filho, they reported the use of 88% phenol on a 62-year-old female patient. Post 2 sessions, with a gap of 45 days, total elimination of residual pigmentation was achieved [17].
