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Published:
Journal of Analytical Toxicology,
ISSN 0146-4760,
Volume 26, Number 4, May/June
pp. 236-238
Technical Note: Long-Term Stability of Methadone in Clinical
Plasma Samples Stored at –20°C
M.L. López[1], M.D. Baño[1], and J.L.
Guillén[2]
[1]Programa Municipal de Drogas de Majadahonda, C/Dr. Calero Nº 20 3ª planta,
28220 Majadahonda, Spain and
[2]Escuela de Análisis Clínicos, Inmunología, Facultad
de Farmacia, Universidad Complutense, Avda. Complutense s/n, 28040 Madrid, Spain
Methadone is a synthetic opioid
analgesic with actions qualitatively similar to morphine that affects the central
nervous system and the smooth muscle. There is crossed tolerance between opiates,
and it has a half-life of 24 h, making methadone to be the suitable candidate
used in heroin detoxification (1). Because of interindividual differences in
the methadone pharmacokinetics, it is necessary to control the levels of methadone
in biological fluids in order to obtain maximum treatment efficacy and to prevent
toxicity.
The studies of stability of a drug over short periods of time are easily realized.
The problems occur in therapeutic drug monitoring in which it is necessary to
determine kinetics in patients, for example, when the patients in methadone
maintenance programs are in treatment with nevirapine (2–5), efavirenz,
riphampin (6), or fenitoine (7) in which a decrease of methadone level is observed
and a new adjustment in dosage regimen is difficult. In these cases the ideal
is to have an aliquot of samples stored in the same conditions in order to be
able to parallel process them, thus avoiding the intertest variation.
The change of the methadone level in a stored sample can be caused by diverse
mechanisms (e.g., absorption on the plastic walls, desiccation [8], etc.); therefore,
it is important to make studies of the stability of methadone in plasma to be
able to determine the reliability of the results based on the analysis method.
In this work, the stability of methadone in stored plasma samples at –20°C
in both short- (365 days) and long-term (730 and 1095 days) storage is tested.
We intended to possibly use the pool of validated plasma obtained from patients
in methadone maintenance program as an internal control assay. Reproduction
of editorial content of this journal is prohibited without publisher’s
permission.
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