Synthesis and Characterization of Process-Related Impurities of Antihypertensive Drug OlmesartanMedoxomil

Olmesartan medoxomil (1) is the latest angiotensin receptor antagonist approved by the FDA for the treatment of hypertension. During the process development of olmesartan medoxomil, three process-related impurities were observed along with the �nal API.ese impurities were identi�ed as isopropyl olmesartan (12), dimedoxomil olmesartan (19), dibiphenyl olmesartan (17).e present work describes the synthesis and characterization of all these three impurities.


Introduction
Drugs have become an important part of human life to combat various deceases. Unlike ancient days, most of the drugs in recent years are purely synthetically made. Unambiguously, the synthetic drugs certainly contain various impurities such as either chemical or microbial. But of course most of the impurities are chemical only.
e process-related impurities in an active pharmaceutical ingredient (API) can have a signi�cant impact on the quality and safety of the drug products. e impurity levels in any drug substance are described as per its biological or toxicological data. It is quite important for "regulatory" aspect of drug approval also to provide limitation of "related impurities. " erefore, it is necessary to study the impurity pro�le of any API and control it during the manufacturing of a drug product. As per the ICH guidelines, any impurities which are forming at a level of ≥0.10% with respect to the API should be identi�ed, synthesized, and characterized thoroughly [1].
Among the various health problems, high blood pressure is one of the critical one, and of course most of the times itself does not harm much, but it leads to various chronic and panic health disorders such as cardiodeceases, cerebral hemorrhage, and others. Olmesartan medoxomil 1 (Benicar, Sankyo Pharma) is the latest angiotensin receptor antagonist [2,3] approved by the FDA for the treatment of hypertension ( Figure 1) [4]. e drug works by inhibiting the effects of angiotensin II, a potent vasoconstrictor and one of the key contributors to cardiovascular and renal disease [5,6]. During the process for the synthesis of (1), olmesartan medoxomil has been prepared by the following synthetic route [4,7] described in Scheme 1.

Experimental Section
All reagents and solvents employed were of commercial grade and were used as such, unless otherwise speci�ed. Reaction �asks were oven-dried at 200 ∘ C, �ame-dried, and �ushed with dry nitrogen prior to use. All moisture and air-sensitive reactions were carried out under an atmosphere of dry nitrogen. TLC was performed on Kieselgel 60 F254 silica-coated aluminium plates (Merck) and visualized by UV light ( 254 nm) or by spraying with a solution of KMnO 4 . Organic extracts were dried over anhydrous Na 2 SO 4 . Flash chromatography was performed using Kieselgel 60 brand silica gel (230-400 mesh). e melting points were determined in an open capillary tube using a Büchi B-540 melting point instrument and were uncorrected. e IR spectra were obtained on a Nicolet 380 FT-IR instrument (neat for liquids and as KBr pellets for solids), HPLC (Agilent Technologies, 1200 series). NMR spectra were recorded with a Varian 300 MHz Mercury Plus Spectrometer at 300 MHz ( 1 H). Chemical shis were given in ppm relative to trimethylsilane (TMS). Mass spectra were recorded on Waters quattro premier XE triple quadrupole spectrometer using either electron spray ionisation (ESI) or atmospheric pressure chemical ionization (APCI) technique.

Preparation of 1-(4-(2-Hydroxypropan
To a solution of 4 (10.0 g, 0.0139 mole) in isopropyl alcohol (50 mL) was slowly added a saturated solution of potassium hydroxide (1.0 g, 0.017 mole) at ambient temperature, and the reaction mixture was stirred at ambient temperature for 20-22 hrs. Aer completion of reaction distilled off IPA and added acetone (70 mL) followed by potassium carbonate (3.0 g, 0.021 mole) and stirred for 10 min. Added 2-bromo propane (1.8 g, 0.0146 mole) at 40-45 ∘ C, stirred for 4 hrs at 45-50 ∘ C. Aer completion of reaction, the mixture was �ltered through hy�owbed and washed with acetone (20 mL), combined the acetone layer, and the solvents were concentrated under reduced pressure, the residue partitioned between ethyl acetate (2 × 75 mL) and water (2 × 40 mL). e combined ethyl acetate layer was dried over anhydrous sodium sulfate and recovered at reduced pressure to afford crude 13, which was puri�ed by column chromatography on silica gel using 20% ethyl acetate in hexane to give 13 as viscous oil (9.0 g, 90%).
Olmesartan acid impurity

Preparation of Dibiphenyl Impurity (17)
To a solution of 4 (10.0 g, 0.0139 mole) in isopropyl alcohol (50 mL) was slowly added a saturated solution of potassium hydroxide (1.0 g, 0.017 mole) at ambient temperature, and the reaction mixture was stirred at ambient temperature for 20-22 hrs. Aer completion of reaction distilled off IPA and added acetone (70 mL) followed by potassium carbonate (3.0 g, 0.021 mole) and stirred for 10 min. Added 5-(4 � -(bromomethyl) biphenyl-2-yl)-1-trityl-1H-tetrazole 3 (7.8 g, 0.0139 mole) at 40-45 ∘ C, stirred for 4 hrs at 45-50 ∘ C. Aer completion of reaction, the mixture was �ltered through hy�owbed and washed with acetone (20 mL), combined the acetone layer and the solvents were concentrated under reduced pressure, the residue partitioned between ethyl acetate (2 × 1 mL) and water (2 × 5 mL). e combined ethyl acetate layer was dried over anhydrous sodium sulfate and recovered at reduced pressure to afford crude 18, which was puri�ed by column chromatography on silica gel using 10% methanol in ethyl acetate to give 18 as a brown colored powder (15.0 g, 92%).

Preparation of (2
Formic acid (50 mL) was added to a solution of 18 (10.0 g, 0.0085 mole) in methanol-acetonitrile (1 : 1, 200 mL) and heated to 50 ∘ C for 7 hours. e solution was cooled to room temperature, and the solvents were removed under reduced pressure. e residue was taken in water (150 mL) and extracted with ethyl acetate (2 × 2 mL). e combined organic extracts were washed with 15% NaHCO 3 solution (60 mL) and dried over anhydrous sodium sulfate. Evaporation of the solvents under reduced pressure at 55 ∘ C afforded crude 17 which on puri�cation by column chromatography on silica gel using 10% methanol in ethyl acetate afforded solid which on recrystallisation in acetone afforded pure 17 as a white solid (4.0 g, 68%).

Preparation of 1-((2 � -(1H-Tetrazol-5-yl) biphenyl-4-yl) methyl)-4-(2-hydroxy propan-2-yl)-2-propyl-1H-imidazole-5carboxylic acid (8).
To a solution of 4 (10.0 g, 0.0139 mole) in isopropyl alcohol (50 mL) was slowly added a saturated solution of potassium hydroxide (1.0 g, 0.017 mole) at ambient temperature, and the reaction mixture was stirred at ambient temperature for 20-22 hrs. Aer completion of reaction distilled regular one and the crude mass dissolve in dichloromethane and methanol (1 : 1, 100 mL). Formic acid (50 mL) was added at 0-5 ∘ C and heated to 50 ∘ C for 7 hours. e solution was cooled to room temperature, and the solvents were removed under reduced pressure. e residue was taken in water (150 mL) and extracted with ethyl acetate (2 × 150 mL). e combined organic extracts were washed with 15% NaHCO 3 solution (80 mL) and dried over anhydrous sodium sulfate. Evaporation of the solvents under reduced pressure at 55 ∘ C afforded crude 8 which on puri�cation by column chromatography on silica gel using 10% methanol in dichloromethane afforded solid which on recrystallisation in acetone afforded pure 8 as a white solid (5.0 g, 83%).

Results and Discussion
During the API process development of olmesartan medoxomil (1), various process-related impurities have been identi-�ed. Initially, two known impurities (7 and 8) were detected in HPLC of compound 1 (Figure 2), and besides, of these, three unknown impurities were also observed during the process development exercise for the preparation of olmesartan medoxomil following Scheme 1. e two known impurities were prepared by Schemes 2 and 3 and characterized and conformed. e structural data of the known impurities were con�rmed with literature reported values [4,[8][9][10].
A comprehensive study was undertaken to identify the unknown impurities by LC-MS followed by con�rming through synthesis of respective unknown impurities, followed by characterization based on spectroscopic techniques such as 1 H NMR and IR mass spectroscopy.
Presence of these unknown impurities was detected by HPLC in synthesized olmesartan medoxomil (1) and studied the mechanistic aspect behind the formation of these and control the mentioned new impurities by improving reaction conditions and developed recrystallization techniques (to remove even if they formed). However, to the best of our knowledge, neither identi�cation nor characterization nor synthesis of these three new process-related impurities was not reported so far. e identi�ed impurities present at nonpolar end of the chromatogram compared to required product. e RRT of unknown impurities 12, 17, and 19 are 1.09, 1.40, 1.63, respectively, each of the impurities contaminating the product with 0.2 to 0.5% (area by HPLC). Being researchers of organic chemistry, we intended to identify the possible structures by considering the molecular weights (by LC-MS), followed by their synthesis and con�rming through correlating these with the impurities forming in the reaction (by HPLC) (Figures 3, 4 and 5). e LC-MS details of unknown impurities were given in Table 1.
e relevant chemical structures of the impurities were predicted by considering the mass details (m/z value) by LC-MS. While reviewing the mechanistic aspects of the possibilities of these impurities formation, the most probable structures were identi�ed �rst and started synthesizing the same to con�rm through structural elucidation followed by employing the chromatographic techniques (HPLC). e unknown impurity 12 has lower m/z value than that of olmesartan medoxomil and as per analysis of mass spectral data of LC-MS, presence of two extra methyl groups was suspected. Since mechanistic aspect of the 9, 10, 11 does not support much, usage of isopropanol may be causing trans ester�cation and ending up with the product 12 having isopropyl ester. It was intended to synthesize the same thorough following the Scheme 4, and all the spectral data con�rm the structure. Chromatographic studies (HPLC) with varying the concentration of impurity are also conducted and concluded that the same impurity existed in targeted entity Olmesartan medoxomil.
e unknown impurity 17 has higher m/z value than that of olmesartan medoxomil, and as per analysis of mass spectral data of LC-MS, presence of extra biphenyl methyl containing tetrazole was suspected. Initially, suspected either of the �rst three structures (14, 15, and 16) but if it is so, one should be able to recognize these prior to the hydrolysis stage only, that is, �rst step of the Scheme 1, as per LC-MS analysis the relevant impurity was not detected, then the attention was shied to the remaining predicted structure (17). e possibility could be remnant bromo compound 3 reactions with carboxylic acid 5 lead to the generation of the biphenyl methyl ester impurity. Aer progressing various transformation of Scheme 1, �nally the olmesartan medoxomil is le with contamination of relevant impurity.
e proposed impurity was synthesized by following Scheme 5, and all the spectral data con�rm the structure. Chromatographic studies (HPLC) with varying the concentration of impurity are also conducted and concluded that the same impurity existed in targeted entity olmesartan medoxomil.       Since the impurity is identi�ed and mechanistic possibility of expected, the formation of the respective contaminant (impurity 17) was subsidized by assuring the absence of the bromo compound 3 in �rst reaction of Scheme 1.

Unknown Impurity (19) (M/Z: 670.67)
e unknown impurity 19 has higher m/z value than that of olmesartan medoxomil and as per analysis of mass spectral data of LC-MS, presence of extra medoxomil moiety intacts with target chemical entity 1. e possibility could be the reaction of known impurity 19 with medoxomil and leads to both O and N alkylation. It was prepared by employing the same strategy (mechanistic reason), and all the spectral data con�rm the structure. Chromatographic studies (HPLC) with varying the concentration of impurity are also conducted and concluded that the same impurity existed in targeted entity olmesartan medoxomil.

Proposed Structures of "Unknown Impurity
(19) (M/Z: 670.76)" Since the impurity is identi�ed and mechanistic possibility of expected, the formation of the respective contaminant (Impurity 19) was minimized by assuring the no excess presence of medoxomil in �nal reaction.
Chromatographic studies (HPLC) with varying the concentration and coinjection of impurity were also conducted and concluded that the same impurities were existed in targeted entity olmesartan medoxomil. e reason for formation is also discussed, and preventive studies were also carried out and process has been modi�ed such that the formation of mentioned impurities is subsidized.

Conclusion
e possible process-related impurities of olmesartan medoxomil are identi�ed by LC-MS data followed by con�rmation by chemical synthesis and characterization using analytical tools such as HPLC, 1 HNMR, IR, mass, and melting point.