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DESI Setup

 

Leonard Nyadong

Office:  ES&T L2-108

Office Phone:  404-385-4427

E-mail: leonard.nyadong@gatech.edu

 

Education

Aug 2005 - Present  Georgia Institute of Technology, Ph.D. Student, Analytical Chemistry

2005                      University of Toledo, Masters in Chemistry

2001                      University of Buea, Cameroon, B.Sc. Chemistry

 

Honors and Awards

2007 Runner up, Graduate Student Award Symposium, School of Chemistry and

        Biochemistry, Georgia Institute of Technology, USA

2007 Graduate Research Fellowship, United States Pharmacopiea (USP), Georgia Institute

        of Technology, USA

2007 Alternate Sponsored Fellow, Summer Research Institute, Pacific Northwest National Lab, Georgia Institute of

        Technology, USA

2006 Graduate Research Fellowship, United States Pharmacopiea (USP), Georgia Institute of Technology, USA

2006 Graduate Travel Award to Southeastern Regional Meeting of the American Chemical Society (SERMACS)

2001 Faculty of Science Award, SONARA Prize, FakoShip Prize, Professor Chumbaw Memorial Prize, Best Graduating

       Student in Chemistry, University of Buea, Cameroon

2000 Ministry of Higher Education Award, Best Science Faculty Student, University of Buea, Cameroon

 

Research Interests

Desorption Electrospray Ionization (DESI), and Direct Analysis in Real Time (DART) Mass Spectrometric Methods for Assessing the Quality of Pharmaceuticals for Drug Quality Control and Forensic Analysis.

Our research project is devoted to studying the fundamental DESI processes and associated ion generation technologies that enable the detection of molecules of pharmaceutical importance by development of qualitative and quantitative assays for antimalarials, and the extension of these methods to other pharmaceutical preparations which show potential for counterfeiting.

 

The quality of pharmaceutical formulations is usually assessed using standard drug testing methods based on high performance liquid chromatography HPLC, Nuclear magnetic resonance (NMR) spectroscopy, and other techniques. The sample preparation steps usually include crushing the sample, dissolving it in an appropriate solvent, extraction, filtration, and centrifugation. These many steps make these methods very tedious and time consuming. Recently, several high throughput method for the qualitative screening of pharmaceutical preparations based on desorption electrospray ionization (DESI) mass spectrometry have been reported in the literature. The attractive feature of these methods is that they do not require sample preparation. DESI makes use of a high-speed charged liquid spray directed at a sample held or deposited on a surface at atmospheric pressure. The DESI spray can be doped with selective reagents, in order to effect ion/molecule reactions at the interface between the charged microdroplets, and the solid surface bearing the condensed phase analyte with an enhancement in sensitivity.

 

Our research in this area is directed towards exploiting the high throughput capabilities of DESI for the direct qualitative and quantitative analysis of artemisinin based antimalarials and other drugs types in our anti counterfeiting efforts.

Reactive DESI Approach for Assessing the Quality of Artesunate Antimalarials: Qualitative Screening Assay.

Desorption electrospray ionization mass spectrometry (DESI MS) is rapidly becoming accepted as a powerful surface characterization tool for a wide variety of samples in the open air. Besides its well-established high throughput capabilities, a unique feature of DESI is that chemical reactions between the charged spray microdroplets and surface molecules can be exploited to enhance ionization. We have developed a rapid screening assay for artesunate antimalarials based on reactive DESI. The approach is based on the formation of stable non-covalent complexes between linear alkylamines dissolved in the DESI spray solution and artesunate molecules exposed on the tablet surface. We found that, depending on amine type and concentration, a sensitivity gain of up to 170x can be obtained, in comparison to reagent-less DESI. Tandem MS experiments revealed that complex formation occurred by hydrogen bonding between the amine nitrogen and the ether-like moieties within the artesunate lactone ring. After the reactive DESI assay was fully characterized, it was applied to a set of recently-collected suspicious artesunate tablets purchased in shops and pharmacies in SE Asia. Not only we found that these samples were counterfeits, but we also detected the presence of several wrong active ingredients. Of particular concern was the positive detection of artesunate traces in the surface of one of the samples, which we quantified with standard chromatographic techniques.

Quantitative DESI.

The direct quantitation of active ingredients in solid pharmaceutical tablets by desorption electrospray ionization mass spectrometry (DESI MS) is complicated by the dependence of the DESI signal on variables such as spray angles and distances, and morphological sample properties and the difficulty of properly incorporating an internal standard. We have developed a DESI MS method for the direct quantitative screening of antimalarial tablets containing artesunate. This method is based on reactive DESI, where analyte desorption and ionization occurs by the formation of non-covalent complexes between alkylamine molecules in the DESI spray solution and artesunate molecules exposed on the sample surface in the open air. For quantitation purposes, the internal standard d4-artesunic acid was synthesized by esterification of d4-succinic anhydride and dihydroartemisinin, and homogeneously dispersed on the tablet surface via a controlled deposition procedure. The analyte-to-internal standard signal intensity ratio was observed to be largely independent of all DESI variables, only showing dependence on tablet hardness. Analysis of artesunate tablet standards prepared with known amounts of the active ingredient in the 0.02-0.32 mg artesunate/mg tablet range resulted in a calibration curve with good linearity (r=0.9985). Application of this method to the direct quantitation of genuine artesunate tablets from Vietnam showed a 6% (n=4) precision and 94% accuracy after the spectral data was corrected for tablet hardness.


Current efforts are geared towards investigating the role of charge competition effects in DESI, in an effort towards quantifying multiple components without chromatographic separation.

Reactive DESI approach for Assessing the Quality of Tamiflu.

Oseltamivir, the active ingredient in Tamiflu capsules is an ester pro-drug whose in vivo hydrolysis product serves as a selective and potent inhibitor of the influenza virus. The recent outbreak of avian influenza has increased the demand for "just-in-case" Tamiflu. Reports of counterfeit Tamiflu capsules which have been shown to not contain the active ingredient have already appeared. Thus there is a growing need for rapid and sensitive Tamiflu authentication tools. In this light, we are developing a reactive DESI-MS approach, as a rapid means to qualitatively and quantitatively assess the quality of Tamiflu capsules via the competitive formation of host-guest complexes between oseltamivir and various crown ethers added to the DESI spray solution.

 

Publications

  1. J. Steeb, A. S. Galhena, Leonard Nyadong, J. Janata, F. M. Fernandez. Beta Electron-Assisted Direct Chemical Ionization (BADCI) Probe for Ambient Mass Spectrometry. Chem. Commun. 2009, In Press.

  2. Leonard Nyadong, A. Galhena, F. M. Fernandez. Desorption Electrospray/Metastable-Induced Ionization (DEMI): A Flexible Multimode Ambient Ion Generation Technique, Anal. Chem. 2009, Accepted.

  3. S. Sengaloundeth, M. D. Green, F. M. Fernandez, O. Manolin, K.g Phommavong, V. Insixiengmay, C. Y. Hampton, Leonard Nyadong, D. Mildenhall, D. Hostetler, L. Khounsaknalath, L. Syhakhang, P. N. Newton. A stratified random survey of the proportion of poor quality oral artesunate sold at medicine outlets in the Lao PDR – implications for therapeutic failure and drug resistance, Malaria J., 2009, 8, 172.

  4. Leonard Nyadong*, G. A. Harris*, S. Balayssac*, A. S. Galhena*, M. Malet-Martino, R. Martino, R. M. Parry, M. D. Wang, F. M. Fernandez, V. Gilard. Combining Two-Dimensional Diffusion-Ordered Nuclear Magnetic Resonance Spectroscopy, Imaging Desorption Electrospray Ionization Mass Spectrometry and Direct Analysis in Real Time Mass Spectrometry for the Integral Investigation of Counterfeit Pharmaceuticals. Anal. Chem., 2009, 81, 4803-4812  *These authors contributed equally

  5. Leonard Nyadong, E. G. Hohenstein, A. S. Galhena, A. L. Lane, J. Kubanek, C. D. Sherrill, and F. M. Fernandez. Desorption Electrospray Ionization Mass Spectrometry (DESI-MS) of Natural Products of a Marine Alga. Anal. Bioanal. Chem., 2009, 394, 245-254.

  6. A. Lane, Leonard Nyadong, A. Galhena, E. Stout, R. Parry, M. Kwasnik, M. Wang, M. Hay, F. M. Fernandez, and J. Kubanek.  Desorption Electrospray Ionization Mass Spectrometry Reveals Surface-mediated Antifungal Chemical Defense of a Tropical Seaweed. PNAS, 2009, 106, 7314-7319.

  7. F. M. Fernandez, C. Y. Hampton, Leonard Nyadong, A. Navare, M. Kwasnik. Liquid Chromatography and Ambient Ionization Time-of-flight Mass Spectrometry for the Analysis of Genuine and Counterfeit Pharmaceuticals in  LC/TOF-MS for Accurate Mass Analysis: Principles and Applications, ed. by I. Ferrer and E.M. Thurman, Wiley: 2009.

  8. Leonard Nyadong, E. G. Hohenstein, K. Johnson, C. D. Sherrill, M. D. Green and F. M. Fernandez, Desorption Electrospray Ionization Reactions Between Host Crown Ethers and the Influenza Neuraminidase Inhibitor Oseltamivir for the Screening of Potentially-Counterfeit Tamiflu. Analyst, 2008, 133, 1513-1522

  9. C. Ricci, Leonard Nyadong, F. Yang, F. M. Fernandez, C. D. Brown, P. N. Newton, S. G. Kazarian, Assessment of hand-held Raman instrumentation for in situ screening for potentially counterfeit artesunate antimalarial tablets by FT-Raman spectroscopy and direct ionization mass spectrometry. Anal Chim Acta, 2008, 623, 178-186

  10. Recent Developments in Ambient Ionization Techniques for High-Throughput Mass Spectrometry. Glenn A. Harris, Leonard Nyadong, and Facundo M. Fernandez, The Analyst, 2008, 133(10), 1297-1301

  11. Leonard Nyadong, Late S, Banga A, Green MD, Newton P. N, Fernández F. M, Direct Quantitation of Active Ingredients in Solid Artesunate Antimalarials by Non-covalent Complex Forming Reactive Desorption Electrospray Ionization Mass Spectrometry J. Am. Soc. Mass Spectrom. 2008; 19, 380-388 

  12. Paul N Newton, Facundo M. Fernandez, Aline Plancon-Lecadre, Dallas Mildenhall, Michael D Green, Li Ziyong, Eva Maria Christophel, Souly Phanouvong, Stephen Howells, Eric MacIntosh, Paul Laurin, Nancy Blum, Christina Y. Hampton, Kevin Faure, Leonard Nyadong, C.W.R. Soong, Budiono Santoso, Wang Zhiguang, John Newton, Kevin Palmer, A forensic epidemiological investigation into the criminal fake artesunate trade: an international collaboration between police, scientists, and health workers, PLoS Medicine, 2008, 5(2), e32

  13. Keoluangkhot V., Green M. D., Leonard Nyadong, Fernandez F. M., Mayxay M., Newton P. N., Impaired Clinical Response in a Patient with Uncomplicated Falciparum Malaria who Received Poor Quality and Underdosed Intramuscular Artemether, Am. J. Pharm. Med. Hyg. 2008, 78, 552-555

  14. Leonard Nyadong, Green M. D., De Jesus V. R., Newton P. N., Fernández F. M., Reactive desorption electrospray ionization linear ion trap mass spectrometry of latest-generation counterfeit antimalarials via noncovalent complex formation, Anal Chem. 79, 2007, 2150-2157

  15. Ricci C., Leonard Nyadong, Fernandez F. M., Newton P. N., Kazarian S. G., Combined Fourier-transform infrared imaging and desorption electrospray-ionization linear ion-trap mass spectrometry for analysis of counterfeit antimalarial tablets, Anal. Bioanal. Chem. 387, 2007, 551-559

  16. Bereman M. S., Leonard Nyadong, Fernandez F. M., Muddiman D. C., Direct high-resolution peptide and protein analysis by desorption electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry, Rapid Commun. Mass Spectrom. 20, 2006, 3409-3411