Mechanisms of drug incorporation into hair
It is generally accepted that drugs can enter into hair by two processes: adsorption from the external environment and incorporation into the growing hair shaft from blood that supplies the hair follicle. Drugs can enter the hair from exposure to chemicals in aerosols, smoke or secretions from sweat and sebaceous glands. Sweat is known to contain drugs present in blood. As hair is very porous and can increase its mass up to 18% by absorbing liquids, drugs may be transferred easily into hair via sweat. Chemicals present in air (smoke, vapours, etc.) can be deposited onto hair (Fig. 1). Drugs appear to be incorporated into the hair by at least three mechanisms:
Figure 1 Possible model of drug incorporation into hair.
• from the blood during hair formation
• from sweat and sebum
• from the external environment.
This model is more able than a passive model (transfer from the blood into the growing cells of the hair follicle) to explain several experimental findings such as:
• drug and metabolite ratios in blood are quite different from those found in hair
• drug and metabolite concentrations in hair differ markedly in individuals who receive the same dose.
Evidence for the transfer of the drug via sweat and sebum is that drugs and metabolites are present in sweat and sebum at high concentrations and persist in these secretions for longer than they do in blood. The parent drug can be found in sweat long after it has disappeared from the blood (Henderson 1993; Cone 1996).
The exact mechanism by which chemicals are bound into hair is not known. It has been suggested that passive diffusion may be augmented by drug binding to intracellular components of the hair cells, such as the hair pigment melanin. For example, codeine concentrations in hair after oral administration are dependent on melanin content (Kronstrand et al. 1999). However, this is probably not the only mechanism, since drugs are trapped in the hair of albino animals which lack melanin. Another proposed mechanism is the binding of drugs with sulfhydryl-containing amino acids present in hair. There is an abundance of amino acids, such as cystine, in hair; these form cross-linking S1S bonds to stabilise the protein fibre network. Drugs that diffuse into hair cells could be bound in this way.
The course of the appearance of drugs in hair has been evaluated in beard hair. Variable time lags between the administration and appearance in hair were observed in all cases: 1 day for codeine to 8 days for morphine and codeine (Mangin 1996). This time lag probably results from the growth time necessary for the hair shaft to emerge from the bulb area in the follicle to a sufficient height above the skin surface for collection. Various studies have demon strated that, after the same dosage, black hair incorporates much more drug than blond hair (Henderson et al. 1998; Höld et al. 1999). This has resulted in discussions about a possible genetic variability of drug deposition in hair, and is still under evaluation.
