Hydroquinone

What is hydroquinone?

Hydroquinone occurs in some plants as free hydroquinone or as arbutin, also known as glycosylated hydroquinone. Food sources containing arbutin include wheat products, pears, coffee and tea, while free hydroquinone is found in small amounts in coffee, red wine, wheat cereals and broccoli (R). Cigarette smoke also contains free hydroquinone (R).

Does hydroquinone get absorbed?

Due to its lipophilicity, hydroquinone readily penetrates human skin following topical application (R, R). In one study, percutaneous absorption with human skin in vitro gave a bioavailability of 43.3% of the applied dose, while the in vivo bioavailability was 45.3% of the dose from a 2% cream formulation (R).

Penetration enhancers such as azone, dimethyl isosorbide and diethylene glycol monoethyl ether did not increase the skin uptake of hydroquinone (R, R). Fractional laser irradiation did not enhance the permeation of hydroquinone either (R). The addition of the sunscreen Escalol 507 on the other hand decreased the penetration of hydroquinone, though the reason for this is unclear (R).

However, emulsions containing liquid crystalline phases were found to aid the penetration of hydroquinone in experiments on human abdominal skin. This was attributed to an increased partitioning of hydroquinone into the skin (R). The use of a co-drug bound covalently to hydroquinone also seems to improve dermal delivery of the parent drugs, which includes hydroquinone. Furthermore, the co-drug strategy minimizes penetration of hydroquinone through the skin, which is important as systemic exposure to hydroquinone can cause serious side effects (R, R).

After hydroquinone reaches the bloodstream, 35% is present as free hydroquinone, another 35% is reversibly bound to protein, and the remainder is irreversibly bound to protein (R). Because its excretion via urine is slower than its uptake through the skin, it is reasonable to assume that daily topical application of hydroquinone leads to accumulation (R).