Structural elucidation of a process-related impurity in ezetimibe by LC/MS/MS and NMR

Ezetimibe: A Review of Analytical Methods for the Drug Substance, Pharmaceutical Formulations and Biological Matrices

Ezetimibe (EZM) is a selective inhibitor of the sterol transporter Niemann-Pick C1-Like 1 in the small intestine used as an adjunctive therapy to lower cholesterol levels in cases of hyperlipidemia. The goal of this work was to summarize the main physical-chemical, pharmacological and pharmacokinetic characteristics of EZM, as well as to describe the main analytical methodologies for the quantification of the drug. Methods described in the United States Pharmacopeia for EZM raw material and tablets were also presented. The drug has a large number of process-related impurities and degradation products and needs strict quality control of its impurities. Specific chiral methods for the evaluation of its chiral impurities are also a need for EZM. The main advantages and disadvantages of the compiled analytical methods were presented, as well as the limits of detection and quantitation. The fastest and most efficient methods were highlighted. Most methods for analyzing EZM used C₈ or C₁₈ stationary phases in gradient mode with binary mobile phases containing acetonitrile and an acidic buffer solution with ultraviolet detection. For analysis of EZM in biological matrices, liquid chromatography-tandem mass spectrometry is generally employed using electron spray ionization in negative ionization mode using multiple reaction monitoring. Different methods in the literature evaluate a large number of impurities for EZM, however new stability-indicating high-performance liquid chromatography methods for the drug are still needed.

Development of an LC-MS Method for 4-Fluoroaniline Determination in Ezetimibe

A rapid and sensitive high-performance liquid chromatography-mass spectrometry method was developed and validated to determine 4-fluoroaniline concentration in ezetimibe. Chromatographic separation was achieved on a Phenomenex Gemini-NX C18 column (150 × 4.6 mm, 3 μm) maintained at 30°C. The liquid chromatography system was operated in gradient mode with an injection volume of 20 μL at a flow rate of 1 mL/min. Mobile phase A was water and mobile phase B consisted of acetonitrile with 0.05% acetic acid. The detection was performed using a single quadrupole mass spectrometer in single ion monitoring mode by using positive ionization. An m/z value of 112 was selected for monitoring 4-fluoroaniline. The method showed good linearity over the concentration range of 0.94-30.26 ng/mL. The limit of quantification and limit of detection were 0.19 and 0.94 ng/mL, respectively. The precision relative standard deviations were less than 8.7% (n = 12), and the accuracy values were within 92-99%. A standard solution of 4-fluoroaniline was stable for at least 24 h at 25°C. Small changes in the organic phase acidity of the mobile phase, flow rate, column temperature, and the instrument parameters had no significant effect on the results for 4-fluoroaniline.

Identification, synthesis and characterization of process related desfluoro impurity of ezetimibe and HPLC method validations

Ezetimibe, which selectively inhibits cholesterol absorption across the intestinal wall and is used as an antihyperlipidemic agent, is synthesized for commercial use as a drug substance in highly pure form. During the synthetic process development studies of ezetimibe, an impurity was detected in the final product at levels ranging from 0.05% to 0.15% in reverse phase gradient high performance liquid chromatography (HPLC) method and its molecular weight was determined by LC-MS analysis. The impurity was identified as (3R,4S)-3-((S)-3-(4-fluorophenyl)-3-hydroxypropyl)-4-(4-hydroxyphenyl)-1-phenylazetidin-2-one which is called desfluoro ezetimibe (lactam-related) impurity, synthesized and characterized, the mechanism of its formation was discussed in detail. After all standardization procedures, it was used as a reference standard during validation of HPLC method and routine analyses. In addition, content of Eze-1 desfluoro impurity in Eze-1 intermediates was specified as 0.10% to keep the formation of desfluoro ezetimibe impurity under control and the related substances HPLC method was validated accordingly.

Pharmaceutical impurities and degradation products: uses and applications of NMR techniques

Current standards and regulations demand the pharmaceutical industry not only to produce highly pure drug substances, but to achieve a thorough understanding of the impurities accompanying their manufactured drug substances and products. These challenges have become important goals of process chemistry and have steadily stimulated the search of impurities after accelerated or forced degradation procedures. As a result, impurity profiling is one of the most attractive, active and relevant fields of modern pharmaceutical analysis. This activity includes the identification, structural elucidation and quantitative determination of impurities and degradation products in bulk drugs and their pharmaceutical formulations. Nuclear magnetic resonance (NMR) spectroscopy has evolved into an irreplaceable approach for pharmaceutical quality assessment, currently playing a critical role in unequivocal structure identification as well as structural confirmation (qualitative detection), enabling the understanding of the underlying mechanisms of the formation of process and/or degradation impurities. NMR is able to provide qualitative information without the need of standards of the unknown compounds and multiple components can be quantified in a complex sample without previous separation. When coupled to separative techniques, the resulting hyphenated methodologies enhance the analytical power of this spectroscopy to previously unknown levels. As a result, and by enabling the implementation of rational decisions regarding the identity and level of impurities, NMR contributes to the goal of making better and safer medicines. Herein are discussed the applications of NMR spectroscopy and its hyphenated derivate techniques to the study of a wide range pharmaceutical impurities. Details on the advantages and disadvantages of the methodology and well as specific challenges with regards to the different analytical problems are also presented.

Identification, isolation and characterization of process related impurities in ezetimibe

During the synthesis of ezetimibe, two process related impurities were detected were HPLC analysis at levels ranging from 0.05 to 0.8%. These two impurities were isolated by column chromatography and co-injected with ezetimibe sample to confirm the retention times in HPLC. These two impurities were characterized as 2-(4-hydroxybenzyl)-N,5-bis(4-fluorophenyl) pentanamide (impurity-I) and 1-(4-fluorophenyl)-3(3-(4-fluorophenyl)propyl)-4-(4-hydroxyphenyl)azetidin-2-one (impurity-II). Isolation, structural elucidation of these impurities by spectral data ((1)H NMR, (13)C NMR, MS and IR) and probable mechanism of their formation have been discussed.

Structure of the major degradant of ezetimibe

In a recent contribution to this Journal Gajjar and Shah described the isolation and structure elucidation of the major alkaline degradant of ezetimibe, a lipid lowering agent [A.K. Gajjar, V.D. Shah, J. Pharm. Biomed. Anal. 55 (2011) 225-229]. Based on (1)H NMR, (13)C NMR and mass spectrometric studies the authors concluded that the structure of the degradant is 5-(4-fluorophenyl)-2-[(4-fluorophenylamino)-(4-hydroxyphenyl) methyl]-pent-4-enoic acid. In a subsequent "Letter to the Editor" submitted to the Journal, Barhate and Mohanra pointed out that the aforementioned structure is inconsistent with the spectroscopic data reported by Gajjar and Shah, consequently it must be wrong [Ch.R. Barhate, K. Mohanra, J. Pharm. Biomed. Anal. 55 (2011) 1237-1238]. However, Barhate and Mohanra did not offer a correct structure in their critical letter. Based on the cited NMR data we realised that previously we had had the same degradant in hand and had unambiguously determined its structure from detailed (1)H, (13)C, COSY, H-C HSQC, H-C HMBC and 1D-NOESY NMR investigations. Herein we report the correct structure to be (2R,3R,6S)-N,6-bis(4-fluorophenyl)-2-(4-hydroxyphenyl)-3,4,5,6-tetrahydro-2H-pyran-3-carboxamide. However, the structure is not new and was described earlier [G.Y.S.K. Swamy et al., Acta Cryst. E 61 (2005) o3608-o3610; K. Filip et al., J. Mol. Struct. 991 (2011) 162-170]. The aim of our present communication is to bring together the various threads of analytical effort involving this degradant into a compact and hopefully instructive conclusion on the pages of this Journal. For the sake of completeness and clarity we also list the correct NMR spectral assignments for ezetimibe which was also given partly erroneously in the earlier literature, and we propose a mechanism for the formation of the degradant.

Ezetimibe

Ezetimibe is a drug substance which can be used for lowering low-density lipoprotein cholesterol. Its crystal form and polymorphism have been determined using X-ray diffraction and thermal methods. Quantitative and qualitative analysis of Ezetimibe as well as study of its impurities and degradations were summarized in this chapter.