Mingxiang Lin - Academia.edu (original) (raw)
Papers by Mingxiang Lin
npj Vaccines
Bacterial infection remains as one of the major healthcare issues, despite significant scientific... more Bacterial infection remains as one of the major healthcare issues, despite significant scientific and medical progress in this field. Infection by Streptococcus Pneumoniae (S. Pneumoniae) can cause pneumonia and other serious infectious diseases, such as bacteremia, sinusitis and meningitis. The pneumococcal capsular polysaccharides (CPS) that constitute the outermost layer of the bacterial cell are the main immunogens and protect the pathogen from host defense mechanisms. Over 90 pneumococcal CPS serotypes have been identified, among which more than 30 can cause invasive pneumococcal diseases that could lead to morbidity and mortality. Multivalent pneumococcal vaccines have been developed to prevent diseases caused by S. Pneumoniae. These vaccines employ either purified pneumococcal CPSs or protein conjugates of these CPSs to generate antigen-specific immune responses for patient protection. Serotype-specific quantitation of these polysaccharides (Ps) antigen species are required f...
Mass spectrometry has been playing an increasingly important role in various aspects of biomarker... more Mass spectrometry has been playing an increasingly important role in various aspects of biomarker research, ranging from discovery of novel biomarkers to quantitative measurement of known biomarkers for clinical applications. Following a brief overview of multiple reaction monitoring (MRM)-based LC/MS approach, the gold standard for quantitative mass spectrometry, this chapter provides an overview of several emerging mass spectrometry techniques benefit from recent advances in mass spectrometry instrumentation and related technologies. These new techniques enable researchers to develop sensitive, specific, robust, and higher throughput biomarker assays for novel clinical applications. Multiple reaction monitoring cubed (MRM3) technique is capable of measuring analytes in complex matrices without extensive sample pretreatment. High-pressure, high-resolution separations with intelligent selection and multiplexing (PRISM) technique greatly increase the efficiency of complex sample anal...
Journal of Chromatography A, 2016
Averaging of chromatograms can lead to enhancement of signal to noise ratio (S/N) in proportion t... more Averaging of chromatograms can lead to enhancement of signal to noise ratio (S/N) in proportion to the square root of the number of measurements. Although the general principle has been known for decades, chromatogram averaging is almost never used in current pharmaceutical research. In this study we explore the utility of this approach, showing it to be a simple and easily accessible method for boosting sensitivity for quantification of minor components and trace impurities, where current techniques deliver insufficient S/N.
This thesis explores several novel applications of gas-phase ion-molecule reactions to solve anal... more This thesis explores several novel applications of gas-phase ion-molecule reactions to solve analytical problems. In all cases, the application of an ion-molecule reaction to solve a specific challenge is based on a thorough understanding of the fundamental aspects of the reaction, including kinetics, production distributions and most notably, its reaction mechanism. Ion-molecule reactions of the atomic oxygen radical anion, O •-, with selected ketones are investigated to explore a strategy to synthesize 1,3-distonic radical anions, which are precursors to 1,3-diyls. The O •-/cyclopentanone reaction is examined in detail, under the well-defined thermal conditions uniquely available in the flowing afterglow, to ascertain if cyclopentan-2-one-1,3-diyl radical anion is formed. To further the understanding of this key reaction, a series of related ketones are also examined. Rate coefficients for each ketone reaction are measured, products are identified, and the branching ratios are determined. A strategy is developed to differentiate the 1,1-and 1,3-H 2 •+ isobaric abstraction products. A total yield of 48% [M-2H] •is obtained for the O •-/cyclopentanone reaction, wherein ~15% is the absolute yield of the 1,1-and ~33% is the absolute yield of the 1,3-H 2 •+ abstraction product. A chemical reagent that specifically cleaves the peptide backbone will greatly simplify peptide sequencing as compared to nonselective energetic collisions with inert gases. In the search for peptide cleavage reagents, translationally-driven, endothermic ion-molecule reactions between peptide ions and potential cleavage reagents are investigated in a custom-built, iii
Bioanalysis, 2014
Background: Fibrinopeptide A (FPA) is a plasma peptide, formed by the action of thrombin on fibri... more Background: Fibrinopeptide A (FPA) is a plasma peptide, formed by the action of thrombin on fibrinogen during clog formation. FPA represents a direct indicator of thrombin activity and could potentially be used as a biomarker for anti-thrombotic therapy development. Results/Methodology: A LC-MS/MS assay with a high throughput solid phase extraction procedure was developed and validated to measure FPA in plasma. The lower limit-of-quantitation (LLOQ) of this assay was determined to be 0.16 nM. The inter- and intra-day%CV was <15%. Freeze–thaw stability of FPA was ±30% up to 3 cycles and linear response of FPA was observed for plasma dilution up to 16-fold. Conclusion: The assay was validated and the biological variability of FPA in plasma was established (1–30 nM).
Bioanalysis, 2014
Since the development of monoclonal antibodies in the 1970s, antibody-based assays have been used... more Since the development of monoclonal antibodies in the 1970s, antibody-based assays have been used for the quantitation of proteins and peptides and, today, they are the most widely used technology in routine laboratory medicine and bioanalysis. However, in the last couple of decades, liquid chromatography-mass spectrometry/mass spectrometry (LC–MS/MS) techniques have been adopted in the quantitation of small molecules, and more recently have made significant contributions in the quantitation of proteins and peptides. In this article, we will review clinical MS-based assays for endogenous peptides, proteins, and therapeutic antibodies, for which validated methods exist. We will also cover the measurement of protein turnover and the unique solutions that MS can offer in this field.
Tetrahedron Letters, 2007
A variation of the Mattox rearrangement, a key degradation pathway under acidic conditions for co... more A variation of the Mattox rearrangement, a key degradation pathway under acidic conditions for corticosteroids possessing the 1,3-dihydroxyacetone side chain, has been found to occur for the 17,21-diesters of these corticosteroids but under the alkaline condition. The mechanism of this variation of the original Mattox rearrangement is proposed.
Steroids, 2009
Enol aldehydes are one type of key degradation and metabolic intermediates from a group of cortic... more Enol aldehydes are one type of key degradation and metabolic intermediates from a group of corticosteroids containing the 1,3-dihydroxyacetone side chain on their D-rings, such as betamethasone, dexamethasone, beclomethasone, and related compounds. The formation of enol aldehydes from these corticosteroids is via acid-catalyzed -elimination of water from the side chain, a process known as Mattox rearrangement. It was recently reported by our group that enol aldehydes could also be formed directly from the corresponding 17,21diesters of these corticosteroids but only under alkaline condition, which was proposed to follow a variation pathway of the original Mattox rearrangement. In this paper, we report the results of a comparative study of enol aldehyde formation from these structurally similar corticosteroids (under the original acidic Mattox condition) and their 17,21-diesters (under the alkaline Mattox variation condition), respectively. In general, enol aldehydes were found to be formed under both conditions; however, the ratios of the E-and Z-isomers of the enol aldehyde were different in each case. The only exception was beclomethasone 17,21-diester under the alkaline condition, where a competing elimination of HCl from the 9,11-positions became predominant. These results can be explained by their structural differences with regard to the Mattox mechanism and its variation pathway. Lastly, solvent effect under acidic condition was studied between an aprotic and a protic solvent and the result suggests that enol aldehyde formation is greatly favored in an aprotic environment.
Rapid Communications in Mass Spectrometry, 2010
Protonated benzene, C6H, has been studied extensively to understand the structure and energy of a... more Protonated benzene, C6H, has been studied extensively to understand the structure and energy of a protonated organic molecule in the gas phase. The formation of C6H is either through direct protonation of benzene, i.e., chemical ionization, or through fragmentation of certain radical cations produced from electron ionization or photon ionization. We report a novel observation of C6H as a product ion formed in the collision‐induced dissociation (CID) of protonated benzamide and related molecules produced via electrospray ionization (ESI). The formation of C6H from these even‐electron precursor ions during the CID process, which has not been previously reported, is proposed to occur from the protonated molecules via a proton migration in a five‐membered ring intermediate followed by the cleavage of the mono‐substituent CC bond and concurrent formation of an ion‐molecule complex. This unique mechanism has been scrutinized by examining some deuterated molecules and a series of structur...
Journal of the American Society for Mass Spectrometry, 2007
Trp-cage is a synthetic 20-residue miniprotein that uses tertiary contacts to stabilize its nativ... more Trp-cage is a synthetic 20-residue miniprotein that uses tertiary contacts to stabilize its native conformation. NMR, circular dichroism (CD), and UV-resonance Raman spectroscopy were used to probe its energy landscape. In this quadrupole/time-of-flight study, electrospray ionization charge state distribution (CSD) and solution-phase H/D exchange are used to probe Trp-cage's tertiary structure. The CSDs of Trp-cage and its mutant provide spectra showing a pH-dependent conformation change. Solution-phase H/D exchange in 30% deuterated trifluoroethanol solution of the wild type shows increased protection of one labile hydrogen in the native state. Together, CSDs and solution-phase H/D exchange are demonstrated to constitute a simple but effective means to follow conformation changes in a small tertiary protein.
Journal of Pharmaceutical and Biomedical Analysis, 2009
Betamethasone dipropionate is an active pharmaceutical ingredient (API) that is used in various d... more Betamethasone dipropionate is an active pharmaceutical ingredient (API) that is used in various dosage forms of finished products for the treatment of inflammatory disorders. An unknown degradant was observed during a solution stability study of betamethasone dipropionate. An approach that combines LC-MS(n), mechanism-based stress studies, semi-preparative HPLC purification and structure elucidation by NMR spectroscopy was used to identify the unknown species. The key step of this approach is the design of relevant stress studies based on the plausible degradation mechanism that is revealed by the informative LC-MS(n) analysis. The appropriately designed mechanism-based stress studies not only verify the degradation mechanism but also produce enough quantities of the unknown species for further structure elucidation/confirmation by NMR spectroscopy. With this strategy, the unknown degradant was rapidly identified as lumibetametasone dipropionate, a photodegradation product of betamethasone dipropionate.
Journal of Pharmaceutical and Biomedical Analysis, 2008
Through a case study, the use of LC-MS(n) technique in conjunction with a mechanism-based stress ... more Through a case study, the use of LC-MS(n) technique in conjunction with a mechanism-based stress study is shown to be a very effective way in the rapid elucidation of unknown drug impurities. In this case, the drug substance sample was first analyzed using LC-MS(n) through which the unknown species was found to be a valeryl-containing, isomeric impurity of the active pharmaceutical ingredient (API), betamethasone 17-valerate, based on its molecular ion and major fragments. Since a substantial knowledge regarding a large number of isomeric impurities of betamethasone has been accumulated in the literature as well as in our laboratory, a hydrolytic stress study (forced degradation) of the isolated unknown species was then designed and carried out accordingly in order to remove the valeryl group from the unknown species. During the stress study, a betamethasone isomer was generated as expected. However, a new unknown species isomeric to betamethasone 17-valerate was also formed unexpectedly. By comparing the UV spectra and more importantly MS(n) fragmentation patterns of the two newly formed species with those of betamethasone, dexamethasone, betamethasone 17-valerate, and betamethasone 21-valerate, these two unknown species generated in the stress study were identified as dexamethasone and dexamethasone 21-valerate, respectively. Based on the plausible reaction mechanism of the forced degradation, the original impurity present in betamethasone 17-valerate drug substance was then identified as dexamethasone 17-valerate; the structure assignment was later confirmed by various 1D and 2D NMR experiments. The efficient conversion from dexamethasone 17-valerate to dexamethasone 21-valerate was also observed during a 2D NMR acquisition of the isolated dexamethasone 17-valerate sample.
Journal of Pharmaceutical and Biomedical Analysis, 2007
During the content uniformity test of a drug product (tablet formulation), an unknown peak was ob... more During the content uniformity test of a drug product (tablet formulation), an unknown peak was observed in the HPLC chromatograms. Upon further investigation, it was determined that the unknown peak was originated from an external source and, therefore, the drug product is free of this unknown peak. The next step was to identify the structure of this unknown peak in order to determine the source of this contaminant species. In this paper, we wish to present the strategy and the results of the experiments that led to the identification of this unknown peak. LC-PDA/UV and LC-MS n analyses were conducted to obtain the UV spectrum, molecular weight and MS n fragmentation pathways of the unknown peak. The MS analysis revealed certain structural features of the unknown species and a number of model compounds that contain such features were then examined for their UV absorbance profiles in an attempt to establish the functional group connectivity within the unknown species. A careful examination of these results in conjunction with the determination of the high-resolution molecular weight led to a short list of potential candidates for the unknown species, among which the most likely one was 1,3-diphenylguanidine. The identification of the unknown contaminant was confirmed by spiking experiments using the authentic compound. The potential source of this contaminant was also identified as derived from the safety filler of the pipette bulb used to prepare the sample solutions during the drug analysis.
Bioorganic & Medicinal Chemistry Letters, 2014
Herein we report the design and synthesis of a series of novel bicyclic DGAT1 inhibitors with a c... more Herein we report the design and synthesis of a series of novel bicyclic DGAT1 inhibitors with a carboxylic acid moiety. The optimization of the initial lead compound 7 based on in vitro and in vivo activity led to the discovery of potent indoline and quinoline classes of DGAT1 inhibitors. The structure-activity relationship studies of these novel series of bicyclic carboxylic acid derivatives as DGAT1 inhibitors are described.
Bioorganic & Medicinal Chemistry Letters, 2013
ACS Medicinal Chemistry Letters, 2014
Diacylglycerol acyltransferase 1 (DGAT1) presents itself as a potential therapeutic target for ob... more Diacylglycerol acyltransferase 1 (DGAT1) presents itself as a potential therapeutic target for obesity and diabetes for its important role in triglyceride biosynthesis. Herein we report the rational design of a novel class of DGAT1 inhibitors featuring a benzomorpholine core (23n). SAR exploration yielded compounds with good potency and selectivity as well as reasonable physical and pharmacokinetic properties. This class of DGAT1 inhibitors was tested in rodent models to evaluate DGAT1 inhibition as a novel approach for the treatment of metabolic diseases. Compound 23n conferred weight loss and a reduction in liver triglycerides when dosed chronically in mice with diet-induced obesity and depleted serum triglycerides following a lipid challenge.
International Journal of Mass Spectrometry, 2004
Rapid Communications in Mass Spectrometry, 2009
Use of liquid chromatography/tandem mass spectrometric (LC/MS(n)) molecular fingerprinting is sys... more Use of liquid chromatography/tandem mass spectrometric (LC/MS(n)) molecular fingerprinting is systematically demonstrated as a very effective tool for rapid structural elucidation of pharmaceutical impurities through a case study in which three isomers of betamethasone sodium phosphate (BSP) were rapidly identified as degradants formed due to the D-homoannular ring expansion of the steroid core structure of BSP in the solid state. The rapid structural elucidation of these degradants was achieved by matching or closely matching the UV profiles, molecular weights, and more importantly the fragmentation patterns obtained from the LC/MS(n) (n = 1 to 3) analysis of their enzyme-catalyzed hydrolytic products, respectively, with those of a D-homoannular isomer of betamethasone available in our LC/MS(n) molecular fingerprint database. This strategy of using LC/MS(n) molecular fingerprinting to obtain high-confidence structures of unknown species is then validated by structure verification through one- (1D) and two-dimensional (2D) nuclear magnetic resonance (NMR) experiments.
npj Vaccines
Bacterial infection remains as one of the major healthcare issues, despite significant scientific... more Bacterial infection remains as one of the major healthcare issues, despite significant scientific and medical progress in this field. Infection by Streptococcus Pneumoniae (S. Pneumoniae) can cause pneumonia and other serious infectious diseases, such as bacteremia, sinusitis and meningitis. The pneumococcal capsular polysaccharides (CPS) that constitute the outermost layer of the bacterial cell are the main immunogens and protect the pathogen from host defense mechanisms. Over 90 pneumococcal CPS serotypes have been identified, among which more than 30 can cause invasive pneumococcal diseases that could lead to morbidity and mortality. Multivalent pneumococcal vaccines have been developed to prevent diseases caused by S. Pneumoniae. These vaccines employ either purified pneumococcal CPSs or protein conjugates of these CPSs to generate antigen-specific immune responses for patient protection. Serotype-specific quantitation of these polysaccharides (Ps) antigen species are required f...
Mass spectrometry has been playing an increasingly important role in various aspects of biomarker... more Mass spectrometry has been playing an increasingly important role in various aspects of biomarker research, ranging from discovery of novel biomarkers to quantitative measurement of known biomarkers for clinical applications. Following a brief overview of multiple reaction monitoring (MRM)-based LC/MS approach, the gold standard for quantitative mass spectrometry, this chapter provides an overview of several emerging mass spectrometry techniques benefit from recent advances in mass spectrometry instrumentation and related technologies. These new techniques enable researchers to develop sensitive, specific, robust, and higher throughput biomarker assays for novel clinical applications. Multiple reaction monitoring cubed (MRM3) technique is capable of measuring analytes in complex matrices without extensive sample pretreatment. High-pressure, high-resolution separations with intelligent selection and multiplexing (PRISM) technique greatly increase the efficiency of complex sample anal...
Journal of Chromatography A, 2016
Averaging of chromatograms can lead to enhancement of signal to noise ratio (S/N) in proportion t... more Averaging of chromatograms can lead to enhancement of signal to noise ratio (S/N) in proportion to the square root of the number of measurements. Although the general principle has been known for decades, chromatogram averaging is almost never used in current pharmaceutical research. In this study we explore the utility of this approach, showing it to be a simple and easily accessible method for boosting sensitivity for quantification of minor components and trace impurities, where current techniques deliver insufficient S/N.
This thesis explores several novel applications of gas-phase ion-molecule reactions to solve anal... more This thesis explores several novel applications of gas-phase ion-molecule reactions to solve analytical problems. In all cases, the application of an ion-molecule reaction to solve a specific challenge is based on a thorough understanding of the fundamental aspects of the reaction, including kinetics, production distributions and most notably, its reaction mechanism. Ion-molecule reactions of the atomic oxygen radical anion, O •-, with selected ketones are investigated to explore a strategy to synthesize 1,3-distonic radical anions, which are precursors to 1,3-diyls. The O •-/cyclopentanone reaction is examined in detail, under the well-defined thermal conditions uniquely available in the flowing afterglow, to ascertain if cyclopentan-2-one-1,3-diyl radical anion is formed. To further the understanding of this key reaction, a series of related ketones are also examined. Rate coefficients for each ketone reaction are measured, products are identified, and the branching ratios are determined. A strategy is developed to differentiate the 1,1-and 1,3-H 2 •+ isobaric abstraction products. A total yield of 48% [M-2H] •is obtained for the O •-/cyclopentanone reaction, wherein ~15% is the absolute yield of the 1,1-and ~33% is the absolute yield of the 1,3-H 2 •+ abstraction product. A chemical reagent that specifically cleaves the peptide backbone will greatly simplify peptide sequencing as compared to nonselective energetic collisions with inert gases. In the search for peptide cleavage reagents, translationally-driven, endothermic ion-molecule reactions between peptide ions and potential cleavage reagents are investigated in a custom-built, iii
Bioanalysis, 2014
Background: Fibrinopeptide A (FPA) is a plasma peptide, formed by the action of thrombin on fibri... more Background: Fibrinopeptide A (FPA) is a plasma peptide, formed by the action of thrombin on fibrinogen during clog formation. FPA represents a direct indicator of thrombin activity and could potentially be used as a biomarker for anti-thrombotic therapy development. Results/Methodology: A LC-MS/MS assay with a high throughput solid phase extraction procedure was developed and validated to measure FPA in plasma. The lower limit-of-quantitation (LLOQ) of this assay was determined to be 0.16 nM. The inter- and intra-day%CV was <15%. Freeze–thaw stability of FPA was ±30% up to 3 cycles and linear response of FPA was observed for plasma dilution up to 16-fold. Conclusion: The assay was validated and the biological variability of FPA in plasma was established (1–30 nM).
Bioanalysis, 2014
Since the development of monoclonal antibodies in the 1970s, antibody-based assays have been used... more Since the development of monoclonal antibodies in the 1970s, antibody-based assays have been used for the quantitation of proteins and peptides and, today, they are the most widely used technology in routine laboratory medicine and bioanalysis. However, in the last couple of decades, liquid chromatography-mass spectrometry/mass spectrometry (LC–MS/MS) techniques have been adopted in the quantitation of small molecules, and more recently have made significant contributions in the quantitation of proteins and peptides. In this article, we will review clinical MS-based assays for endogenous peptides, proteins, and therapeutic antibodies, for which validated methods exist. We will also cover the measurement of protein turnover and the unique solutions that MS can offer in this field.
Tetrahedron Letters, 2007
A variation of the Mattox rearrangement, a key degradation pathway under acidic conditions for co... more A variation of the Mattox rearrangement, a key degradation pathway under acidic conditions for corticosteroids possessing the 1,3-dihydroxyacetone side chain, has been found to occur for the 17,21-diesters of these corticosteroids but under the alkaline condition. The mechanism of this variation of the original Mattox rearrangement is proposed.
Steroids, 2009
Enol aldehydes are one type of key degradation and metabolic intermediates from a group of cortic... more Enol aldehydes are one type of key degradation and metabolic intermediates from a group of corticosteroids containing the 1,3-dihydroxyacetone side chain on their D-rings, such as betamethasone, dexamethasone, beclomethasone, and related compounds. The formation of enol aldehydes from these corticosteroids is via acid-catalyzed -elimination of water from the side chain, a process known as Mattox rearrangement. It was recently reported by our group that enol aldehydes could also be formed directly from the corresponding 17,21diesters of these corticosteroids but only under alkaline condition, which was proposed to follow a variation pathway of the original Mattox rearrangement. In this paper, we report the results of a comparative study of enol aldehyde formation from these structurally similar corticosteroids (under the original acidic Mattox condition) and their 17,21-diesters (under the alkaline Mattox variation condition), respectively. In general, enol aldehydes were found to be formed under both conditions; however, the ratios of the E-and Z-isomers of the enol aldehyde were different in each case. The only exception was beclomethasone 17,21-diester under the alkaline condition, where a competing elimination of HCl from the 9,11-positions became predominant. These results can be explained by their structural differences with regard to the Mattox mechanism and its variation pathway. Lastly, solvent effect under acidic condition was studied between an aprotic and a protic solvent and the result suggests that enol aldehyde formation is greatly favored in an aprotic environment.
Rapid Communications in Mass Spectrometry, 2010
Protonated benzene, C6H, has been studied extensively to understand the structure and energy of a... more Protonated benzene, C6H, has been studied extensively to understand the structure and energy of a protonated organic molecule in the gas phase. The formation of C6H is either through direct protonation of benzene, i.e., chemical ionization, or through fragmentation of certain radical cations produced from electron ionization or photon ionization. We report a novel observation of C6H as a product ion formed in the collision‐induced dissociation (CID) of protonated benzamide and related molecules produced via electrospray ionization (ESI). The formation of C6H from these even‐electron precursor ions during the CID process, which has not been previously reported, is proposed to occur from the protonated molecules via a proton migration in a five‐membered ring intermediate followed by the cleavage of the mono‐substituent CC bond and concurrent formation of an ion‐molecule complex. This unique mechanism has been scrutinized by examining some deuterated molecules and a series of structur...
Journal of the American Society for Mass Spectrometry, 2007
Trp-cage is a synthetic 20-residue miniprotein that uses tertiary contacts to stabilize its nativ... more Trp-cage is a synthetic 20-residue miniprotein that uses tertiary contacts to stabilize its native conformation. NMR, circular dichroism (CD), and UV-resonance Raman spectroscopy were used to probe its energy landscape. In this quadrupole/time-of-flight study, electrospray ionization charge state distribution (CSD) and solution-phase H/D exchange are used to probe Trp-cage's tertiary structure. The CSDs of Trp-cage and its mutant provide spectra showing a pH-dependent conformation change. Solution-phase H/D exchange in 30% deuterated trifluoroethanol solution of the wild type shows increased protection of one labile hydrogen in the native state. Together, CSDs and solution-phase H/D exchange are demonstrated to constitute a simple but effective means to follow conformation changes in a small tertiary protein.
Journal of Pharmaceutical and Biomedical Analysis, 2009
Betamethasone dipropionate is an active pharmaceutical ingredient (API) that is used in various d... more Betamethasone dipropionate is an active pharmaceutical ingredient (API) that is used in various dosage forms of finished products for the treatment of inflammatory disorders. An unknown degradant was observed during a solution stability study of betamethasone dipropionate. An approach that combines LC-MS(n), mechanism-based stress studies, semi-preparative HPLC purification and structure elucidation by NMR spectroscopy was used to identify the unknown species. The key step of this approach is the design of relevant stress studies based on the plausible degradation mechanism that is revealed by the informative LC-MS(n) analysis. The appropriately designed mechanism-based stress studies not only verify the degradation mechanism but also produce enough quantities of the unknown species for further structure elucidation/confirmation by NMR spectroscopy. With this strategy, the unknown degradant was rapidly identified as lumibetametasone dipropionate, a photodegradation product of betamethasone dipropionate.
Journal of Pharmaceutical and Biomedical Analysis, 2008
Through a case study, the use of LC-MS(n) technique in conjunction with a mechanism-based stress ... more Through a case study, the use of LC-MS(n) technique in conjunction with a mechanism-based stress study is shown to be a very effective way in the rapid elucidation of unknown drug impurities. In this case, the drug substance sample was first analyzed using LC-MS(n) through which the unknown species was found to be a valeryl-containing, isomeric impurity of the active pharmaceutical ingredient (API), betamethasone 17-valerate, based on its molecular ion and major fragments. Since a substantial knowledge regarding a large number of isomeric impurities of betamethasone has been accumulated in the literature as well as in our laboratory, a hydrolytic stress study (forced degradation) of the isolated unknown species was then designed and carried out accordingly in order to remove the valeryl group from the unknown species. During the stress study, a betamethasone isomer was generated as expected. However, a new unknown species isomeric to betamethasone 17-valerate was also formed unexpectedly. By comparing the UV spectra and more importantly MS(n) fragmentation patterns of the two newly formed species with those of betamethasone, dexamethasone, betamethasone 17-valerate, and betamethasone 21-valerate, these two unknown species generated in the stress study were identified as dexamethasone and dexamethasone 21-valerate, respectively. Based on the plausible reaction mechanism of the forced degradation, the original impurity present in betamethasone 17-valerate drug substance was then identified as dexamethasone 17-valerate; the structure assignment was later confirmed by various 1D and 2D NMR experiments. The efficient conversion from dexamethasone 17-valerate to dexamethasone 21-valerate was also observed during a 2D NMR acquisition of the isolated dexamethasone 17-valerate sample.
Journal of Pharmaceutical and Biomedical Analysis, 2007
During the content uniformity test of a drug product (tablet formulation), an unknown peak was ob... more During the content uniformity test of a drug product (tablet formulation), an unknown peak was observed in the HPLC chromatograms. Upon further investigation, it was determined that the unknown peak was originated from an external source and, therefore, the drug product is free of this unknown peak. The next step was to identify the structure of this unknown peak in order to determine the source of this contaminant species. In this paper, we wish to present the strategy and the results of the experiments that led to the identification of this unknown peak. LC-PDA/UV and LC-MS n analyses were conducted to obtain the UV spectrum, molecular weight and MS n fragmentation pathways of the unknown peak. The MS analysis revealed certain structural features of the unknown species and a number of model compounds that contain such features were then examined for their UV absorbance profiles in an attempt to establish the functional group connectivity within the unknown species. A careful examination of these results in conjunction with the determination of the high-resolution molecular weight led to a short list of potential candidates for the unknown species, among which the most likely one was 1,3-diphenylguanidine. The identification of the unknown contaminant was confirmed by spiking experiments using the authentic compound. The potential source of this contaminant was also identified as derived from the safety filler of the pipette bulb used to prepare the sample solutions during the drug analysis.
Bioorganic & Medicinal Chemistry Letters, 2014
Herein we report the design and synthesis of a series of novel bicyclic DGAT1 inhibitors with a c... more Herein we report the design and synthesis of a series of novel bicyclic DGAT1 inhibitors with a carboxylic acid moiety. The optimization of the initial lead compound 7 based on in vitro and in vivo activity led to the discovery of potent indoline and quinoline classes of DGAT1 inhibitors. The structure-activity relationship studies of these novel series of bicyclic carboxylic acid derivatives as DGAT1 inhibitors are described.
Bioorganic & Medicinal Chemistry Letters, 2013
ACS Medicinal Chemistry Letters, 2014
Diacylglycerol acyltransferase 1 (DGAT1) presents itself as a potential therapeutic target for ob... more Diacylglycerol acyltransferase 1 (DGAT1) presents itself as a potential therapeutic target for obesity and diabetes for its important role in triglyceride biosynthesis. Herein we report the rational design of a novel class of DGAT1 inhibitors featuring a benzomorpholine core (23n). SAR exploration yielded compounds with good potency and selectivity as well as reasonable physical and pharmacokinetic properties. This class of DGAT1 inhibitors was tested in rodent models to evaluate DGAT1 inhibition as a novel approach for the treatment of metabolic diseases. Compound 23n conferred weight loss and a reduction in liver triglycerides when dosed chronically in mice with diet-induced obesity and depleted serum triglycerides following a lipid challenge.
International Journal of Mass Spectrometry, 2004
Rapid Communications in Mass Spectrometry, 2009
Use of liquid chromatography/tandem mass spectrometric (LC/MS(n)) molecular fingerprinting is sys... more Use of liquid chromatography/tandem mass spectrometric (LC/MS(n)) molecular fingerprinting is systematically demonstrated as a very effective tool for rapid structural elucidation of pharmaceutical impurities through a case study in which three isomers of betamethasone sodium phosphate (BSP) were rapidly identified as degradants formed due to the D-homoannular ring expansion of the steroid core structure of BSP in the solid state. The rapid structural elucidation of these degradants was achieved by matching or closely matching the UV profiles, molecular weights, and more importantly the fragmentation patterns obtained from the LC/MS(n) (n = 1 to 3) analysis of their enzyme-catalyzed hydrolytic products, respectively, with those of a D-homoannular isomer of betamethasone available in our LC/MS(n) molecular fingerprint database. This strategy of using LC/MS(n) molecular fingerprinting to obtain high-confidence structures of unknown species is then validated by structure verification through one- (1D) and two-dimensional (2D) nuclear magnetic resonance (NMR) experiments.