Using a portable ion mobility spectrometer to screen dietary supplements for sibutramine

Abstract

In response to recent incidents of undeclared sibutramine, an appetite suppressant found in dietary supplements, we developed a method to detect sibutramine using hand-held ion mobility spectrometers with an analysis time of 15 s. Ion mobility spectrometry is a high-throughput and sensitive technique that has been used for illicit drug, explosive, volatile organic compound and chemical warfare detection. We evaluated a hand-held ion mobility spectrometer as a tool for the analysis of supplement extracts containing sibutramine. The overall instrumental limit of detection of five portable ion mobility spectrometers was 2 ng of sibutramine HCl. When sample extractions containing 30 ng/μl or greater of sibutramine were analyzed, saturation of the ionization chamber of the spectrometer occurred and the instrument required more than three cleaning cycles to remove the drug. Hence, supplement samples suspected of containing sibutramine should be prepared at concentrations of 2–20 ng/μl. To obtain this target concentration range for products containing unknown amounts of sibutramine, we provided a simple sample preparation procedure, allowing the U.S. Food and Drug Administration or other agencies to screen products using the portable ion mobility spectrometer.

Introduction

Unlike prescription and over-the-counter medications, dietary supplements do not require U.S. Food and Drug Administration (FDA) approval. Thus, supplement products are susceptible to adulteration, contamination or mislabelling. Between 2008 and 2009, undeclared drugs, including bumetanide, cetilistat, fenproporex, fluoxetine, furosemide, phenytoin, rimonabant and sibutramine, were found in over 70 weight loss supplements [1]. Adverse health events associated with those consuming these undeclared drugs can occur, especially when the dose is above that of prescription form.

Sibutramine hydrochloride monohydrate, brand name MERIDIA^®, is used to treat obesity and is legally available in the U.S. by prescription. Each MERIDIA^® capsule contains 5 mg, 10 mg or 15 mg of sibutramine hydrochloride monohydrate. Recently, sibutramine was found in counterfeit Alli, an over-the-counter product marketed for weight loss whose active ingredient is orlistat. Sibutramine has been detected in unapproved weight loss products at levels of 0.1–40 mg per capsule,¹ the latter amount being about three times more than the U.S. FDA approved dose. In the laboratory, evaluation of such products is typically performed by gas chromatography-mass spectrometry (GC/MS) or high performance liquid chromatography (HPLC) with detection by UV, mass spectrometry (MS) or tandem MS (MS/MS). Using these techniques, analysis times are on the order of 15–75 min [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], which can be costly when multiple samples are tested. Additionally, GC requires conversion of sibutramine hydrochloride to the free base form prior to analysis [2], [3], [4], which results in increased sample preparation time and possible loss of analyte. Ion mobility spectrometry (IMS) is a rapid, sensitive technique that requires minimal sample preparation, thereby making it a viable screening tool for testing pharmaceuticals [12], [13], [14], [15], [16], [17], [18], [19], [20]. Hand-held IMS instruments have been used mainly for illicit drug, explosive, volatile organic compound and chemical warfare detection [21], [22], [23], [24], [25], [26], [27]. Using a portable ion mobility spectrometer as a 15 s screening tool, we obtained a limit of detection (LOD) of 2 ng of sibutramine hydrochloride (conversion to free base before analysis by IMS was unnecessary) in dietary supplements, which was 8 times lower than that obtained in a study using HPLC-UV [7]. HPLC-MS/MS has shown to be more sensitive than HPLC-UV for the detection of sibutramine by a factor as high as 1500 [5], [7], [9]. Although the LOD of sibutramine obtained using MS/MS was lower than that determined using the portable ion mobility spectrometer, products adulterated with just 0.1 mg of sibutramine can be detected using the portable spectrometer when a solvent extraction volume between 5 and 50 ml is used.

Portable devices are not meant to replace laboratory instruments; however, they can screen samples in order to effectively prioritize collection of samples to be sent to U.S. FDA laboratories for official testing. At the same time, using portable instruments will increase the frequency of testing products with the hope of eventually testing all products entering the U.S. To address this challenge, the Center for Drug Evaluation and Research Division of Pharmaceutical Analysis (DPA) with the assistance from the Office of Compliance and the Office of Regulatory Affairs (ORA) has developed high-throughput methods for portable spectroscopic instrumentation, requiring less than a minute per analysis [28]. In addition, the U.S. FDA has recently shown that IMS is applicable to the analysis of herbal dietary supplements suspected of containing erectile dysfunction (ED) drugs such as sildenafil, tadalfil and vardenafil as well as five synthetic analogues [13]. Using a benchtop IMS instrument to evaluate methanol extractions, the analysis time was 12 s and six of the ED drugs had a limit of detection of 0.2 ng. Like a benchtop instrument, the portable spectrometer used in our studies rapidly heats and desorbs the analyte from a substrate. One shortcoming of the portable system is that the maximum operating inlet and desorber temperatures are approximately 210 °C for the positive ion mode, which generally limits the analysis to drugs that can be volatilized near or below this temperature.

The purpose of the paper is to demonstrate the application of IMS to the rapid analysis of sibutramine in dietary supplements using a portable spectrometer. We evaluated five portable SABRE 4000 ion mobility spectrometers as tools to screen herbal dietary supplement extracts containing sibutramine. Following laboratory evaluation at DPA, the instruments were sent to five U.S. FDA laboratories around the U.S. for collaborative study. From the collaborative study results, we obtained the peak amplitude (intensity), the reduced mobility (K₀) and the full width at half maximum (FWHM) of the peak due to sibutramine from each ion mobility spectrum. The peak amplitudes were used to determine the critical level (CL) threshold and the LOD for the method and these values along with the other parameters were used to set the instrument thresholds for the detection of sibutramine. The potential for interference by caffeine was also investigated since dietary supplements often contain this compound.