An LC–MS/MS method was developed and validated to quantify the tyrosine kinase inhibitor erlotinib in human scalp hair, as alternative matrix to monitor long-term erlotinib exposure. Hair samples from 10 lung cancer patients were measured and correlated with plasma concentrations. Hair segments of 1 ± 0.1 cm each were pulverized and for at least 18 h incubated in methanol at ambient temperature. A liquid-liquid extraction purified the extracts and they were analyzed with LC–MS/MS, using erlotinib-d6 as internal standard. The procedure method was validated for selectivity, sensitivity, precision, lower limit of detection, linearity and accuracy. The within and between run precisions including the lower limit of quantification did not exceed 12.5%, while the accuracy ranged from 103 to 106%. A weak correlation between hair and plasma concentration was found (R2 = 0.48). Furthermore, a large inter-individual variability was noted in the disposition of both plasma and hair samples. The highest hair concentrations were observed in black hair compared with other (grey and brown) hair colors. Generally, a linear reduction in hair concentration was found from proximal to distal hair segments. Additional in vitro experiments suggest an accelerated degradation of erlotinib in hair by artificial UV light and also wash-out by shampoo mixtures pretreatment compared with control samples. In conclusion, a reliable and robust LC–MS/MS method was developed to quantify erlotinib in hair. However, clinical and in vitro evaluations showed that the method is not suitable for monitoring long-term erlotinib exposure. The pitfalls of this application outweigh the current benefits.

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Journal Journal of Pharmaceutical and Biomedical Analysis
Braal, C.L., Veerman, GDM, Peric, R., Aerts, J.G.J.V, Mathijssen, A.H.J, Koolen, S.L.W, & de Bruijn, P.J. (2019). Quantification of the tyrosine kinase inhibitor erlotinib in human scalp hair by liquid chromatography-tandem mass spectrometry: Pitfalls for clinical application. Journal of Pharmaceutical and Biomedical Analysis, 172, 175–182. doi:10.1016/j.jpba.2019.04.031