Vol. 26, Issue 7, 689-700, July 1998

Fentanyl in Hair
Chemical Factors Involved in Accumulation and Retention of Fentanyl in Hair after External Exposure or In Vivo Deposition

Peter R. Stout, David J. Claffey, and James A. Ruth

Department of Molecular Toxicology and Environmental Health Sciences, University of Colorado Health Sciences Center

The levels of fentanyl extractable from mouse hair after chronic systemic administration and the suitability of externally loaded hair samples for establishing control and comparison samples were determined. Additionally, the effects of chemical modification of specific polar functionalities within the hair protein matrix on the deposition and recovery of fentanyl in hair subjected to external loading were determined. BALB/c mice entering a second phase of synchronized hair growth were treated ip with fentanyl (0.02, 0.05, or 0.10 mg/kg) on Monday, Wednesday, and Friday for 3 weeks. At that time, fentanyl concentrations in hair, as determined by GC/MS, were 0.025-0.050 ng/mg of hair. Hair samples exposed to fentanyl in phosphate buffer (ionized drug) showed no significant accumulation of drug into the hair, as determined by loss of fentanyl from the loading solution or by extraction of the hair. Hair samples exposed to nonionized fentanyl in methanolic solution (10, 50, and 100 ng/ml) showed significant accumulation of drug in the hair and significant removal of drug from the incubation solution. Fentanyl removal from solution plateaued after 24 hr, suggesting equilibration between fentanyl in solution and fentanyl in the hair. A mass balance between drug lost from the incubation solution and drug recovered from hair samples suggests that 94% of accumulated fentanyl is tightly bound to the hair matrix or resides in water-inaccessible compartments within the hair. These results suggest that fentanyl accumulation after in vivo administration differs, in the nature of storage, from fentanyl accumulation from external solutions and that external spiking of hair may not provide suitable control samples. Chemical modification of hair protein functionalities (reaction with diazomethane to esterify carboxylic acid groups or with acetic anhydride and pyridine to acetylate amine and hydroxyl functionalities) led to reproducible protein structure modification, as demonstrated by Fourier transform-IR and by pH titration. Hair from BALB/c mice was used. The accumulation of fentanyl was examined in hair samples exposed to fentanyl in methanol or methylene chloride solutions (10 ng/ml, 24 hr). Fentanyl was recovered from hair by 24-hr extraction in phosphate buffer, pH 6. Esterification of hair resulted in significantly less uptake of nonionized fentanyl from a methanolic solution and significantly lower recovery of drug from hair, relative to untreated hair, suggesting that carboxylic acid functionalities are necessary for the incorporation of drug. Acetylation of hair resulted in increased removal of fentanyl from methylene chloride solutions and increased recovery of fentanyl. This is consistent with the creation or expansion of a less polar compartment. Fentanyl uptake from a methanolic solution was also greater in acetylated hair. These results demonstrate that solution-accessible ionizable functionalities of hair play a significant role in the accumulation and retention of nonionized fentanyl from organic solutions.