Detection of levamisole exposure in cocaine users by liquid chromatography-tandem mass spectrometry

Levamisole, a veterinary antihelminthic, was recently recognized as an adulterant in cocaine and is known to cause severe adverse reactions in some cocaine users. Because of the health concerns involving levamisole-adulterated cocaine, we developed a liquid chromatography-tandem mass spectrometry (LC-MS-MS) method for the detection of levamisole in urine. This method was used to determine the prevalence of levamisole in cocaine-positive patient samples. All cocaine-positive urine samples that were sent to the San Francisco General Hospital Clinical Laboratory were tested for levamisole for one month. For LC, an Agilent 1200 series was used with a C(18) column and a gradient of mobile phase A (0.05% formic acid) and B (acetonitrile/methanol). Detection was carried out with an Applied Biosystems QTRAP(®) LC-MS-MS. The levamisole LC-MS-MS method was linear over the range of 5-2500 ng/mL (r > 0.996). Interassay and intraassay CVs were < 6%. The lower limit of detection for levamisole was 0.5 ng/mL. Out of 949 total urine drug screens, 20% were positive for benzoylecgonine, and of those, 88% were positive for levamisole. The high prevalence of levamisole-adulterated cocaine and potential toxicity in cocaine users is a serious public health concern. These findings validate the utility of an LC-MS-MS method for the detection of levamisole.