Weak Binding of Levamisole by the Cocaine-Binding Aptamer Does Not Interfere with an Aptamer-Based Detection Assay.

Levamisole is a common and harmful adulterant of street samples of cocaine and can cause electrochemical tests for cocaine to give false negative results. To see if levamisole would interfere with aptamer-based bioassays, we analyzed the binding of levamisole to the cocaine-binding DNA aptamer. At low aptamer concentrations (0.5 to 20 μM) using isothermal titration calorimetry methods and thermal stability measurements, no binding of levamisole to the cocaine-binding aptamer was observed. At higher levamisole concentrations (500 μM), weak binding to the cocaine-binding aptamer was detected using nuclear magnetic resonance (NMR) spectroscopy chemical shift perturbations. NMR-detected titrations show that levamisole binding is competitive with cocaine binding, indicating that both ligands share a common binding site. Finally, we show that the presence of levamisole does not interfere with the photochrome aptamer switch binding assay for cocaine. We conclude that assays using low concentrations of cocaine, and consequently low concentration of levamisole as an adulterant, should be unaffected by the weak binding of levamisole.