In order for drugs1 to become incorporated into the hair, they must enter the follicle2. This occurs by two routes. One route of entry is from the capillaries surrounding the follicle. The other route of entry is through the sebum3 secreted by the sebaceous glands connected to the follicle (drugs can enter the follicle from sweat as well). Most drugs will have little trouble leaving the capillaries since there are many pores or fenestrations that allow their passage. In order to get into the hair follicles and hair cells, to some extent, the drug must exist in a non-polar form to be able to diffuse across the follicle and hair cell membranes. For example, drugs that are weak bases4 tend to exist in both polar (or ionized) and non-polar (or unionized) forms, depending on the pH of their environment. At body pH (7.4), there is enough non-polar forms of these drugs to penetrate the hair follicles and cells. But, once inside the keratinocytes, the environment is now acidic (from the melanin), so drugs that are weak bases tend to exist in a predominantly ionized or charged form (they accept a H+ from the donor) (see Module 1). This is the case for drugs such as nicotine, morphine, cocaine, and amphetamine. And once these drugs get into the follicles and the hair cells, they will bind to the acidic melanin.
The nature of the bond between oppositely charged atoms of the weak bases and the melanin is electrostatic. [In other modules, electrostatic bonding between drugs and proteins (either enzymes6 or receptors7) is discussed.] The problem is that the charged form of these drugs (i.e., weak bases) becomes trapped inside the hair cells. As charged molecules, they have difficulty diffusing through the lipid membranes of the hair cells to reach the capillaries. The more melanin present in the hair, the more binding of weak base drugs. So, for the same dose of drugs such as cocaine and morphine, higher levels of these drugs are present in black and brown hair compared to blond hair. Non-Caucasian races with black hair have higher concentrations of these drugs in their hair compared to Caucasians. After a single dose, the drugs remain in the hair from 2-8 months (see Figure 7), depending on the individual. Drugs such as cocaine can even be detected in the hair of newborn babies after use of cocaine by the mothers during the 2nd and 3rd trimesters of pregnancy. Washing of the hair does not eliminate the drug, although anything that destroys the pigment will decrease the binding of the drug.
1 a substance that affects the structure or function of a cell or organism.
2 a small sac; hair follicles are internalized structures of epithelial cells in which the hair is synthesized and grows
3 a fatty substance made in and secreted by sebaceous glands attached to hair follicles
4 a compound that tends to accept a H+ when placed in an acidic solution
5 the pigment found in hair, skin, feathers, etc.. It is a biopolymer found in granules within melanocytes and transferred to keratinocytes to provide hair color.
6 a protein that catalyzes the rate at which a reaction occurs. It binds to one of the reactants (a substrate) to cause a change in the reactant’s structure, facilitating the reaction.
7 a protein to which hormones, neurotransmitters and drugs bind. They are usually located on cell membranes and elicit a function once bound.