A Review On Sustainable Stability-Indicating Analytical Methods For Olmesartan Medoxomil And Azilsartan Medoxomil

Hypertension continues to be a major global health issue, greatly contributing to heart-related illnesses and deaths. Among the most effective treatment options are Angiotensin II Receptor Blockers (ARBs), often referred to as "Sartans." Olmesartan Medoxomil and Azilsartan Medoxomil are key developments within this class of drugs. Olmesartan, a specific AT₁ receptor blocker, has remained a key medication in managing blood pressure, whereas Azilsartan Medoxomil, a more recent prodrug, has shown better 24-hour blood pressure control than earlier similar drugs [1]. Because these medications are frequently produced in large quantities, creating reliable analytical techniques for quality control is a major focus for pharmaceutical researchers.Traditionally, the measurement of these sartan derivatives has depended largely on Reverse-Phase High-Performance Liquid Chromatography (RP-HPLC). Although efficient, these conventional methods use large amounts of harmful organic solvents, particularly Acetonitrile (ACN) and Methanol (MeOH). Acetonitrile, which is a byproduct of acrylonitrile production, is harmful not only to lab workers but also creates a major environmental issue when disposed of. In the present age of Green Analytical Chemistry (GAC), there is an international push to substitute these "Red" solvents with "Green" alternatives like Ethanol, Acetone, or water-based micellar systems [2][3].

The difficulty in creating Stability-Indicating Methods (SIM) for Olmesartan and Azilsartan is due to their chemical sensitivity. Both medications include a medoxomil ester component, which is very prone to hydrolysis. A "Novel" stability-indicating method should be able to separate the main drug from its breakdown products (such as Olmesartan acid or Azilsartan acid) using only environmentally friendly mobile phases. This necessitates a thorough comprehension of the Analytical GREEnness (AGREE) metric to guarantee that the method is both accurate and precise in line with ICH Q2(R1) guidelines, as well as environmentally sustainable [4].

This review seeks to critically assess the shift from traditional, solvent-heavy chromatography methods to contemporary, environmentally friendly solvent approaches used for Olmesartan and Azilsartan Medoxomil. By combining existing research on forced degradation and green validation parameters, this paper offers a detailed guide to achieving sustainability in pharmaceutical analysis in the 21st century [5].

Physicochemical and Pharmacological Profiles:

A reliable analytical technique is based on the fundamental chemical characteristics of the substances being analyzed. Although Olmesartan Medoxomil and Azilsartan Medoxomil are in the same therapeutic class, they have different structural characteristics that affect their stability and performance in green solvent systems.

Olmesartan Medoxomil

is a prodrug that gets converted into its active form, Olmesartan, as it is absorbed through the gastrointestinal tract.

Molecular Formula: C {29} H {30} N {6} O {6},

Molecular Weight: 558.59 g/mol.

Structural Features: It includes a tetrazole ring and a medoxomil group, which is a (5-methyl-2-oxo-1,3-dioxol-4-yl)methyl ester. The ester bond is the main point of instability, which is why "Stability-Indicating" methods are crucial [6].

pKa: Around 4.3 (Tetrazole ring). The acidity implies that the drug's retention in RP-HPLC is strongly influenced by the pH of the green mobile phase [7].

Solubility in Green Solvents: It exhibits high solubility in Ethanol and Acetone, making these suitable alternatives to Acetonitrile.

Azilsartan Medoxomil

is a new-generation ARB, frequently given in the form of a potassium salt (Azilsartan Kamedoxomil).

Molecular Formula: C_{30}H_{24}N_{6}O_{8}.

Molecular Weight: 596.55 g/mol.

Structural Features: Like Olmesartan, it contains a medoxomil group but includes an oxadiazolone ring rather than a conventional tetrazole, which improves its binding affinity and effectiveness [8].

Log P: Around 4.7. The high lipophilicity indicates that Micellar Liquid Chromatography (MLC), employing surfactants such as Sodium Dodecyl Sulfate (SDS), could serve as a viable "Green" alternative for its separation [9].

Stability Challenge: Azilsartan is more prone to alkaline hydrolysis compared to Olmesartan. A green method should employ buffers within a pH range of 3.05.0 to avoid "on-column" degradation during analysis [10].

Comparison of Physicochemical Properties

Conventional Chromatographic Methods

The analytical background of Olmesartan and Azilsartan is based on traditional Reverse-Phase High-Performance Liquid Chromatography (RP-HPLC). These approaches were mainly created to achieve high accuracy and efficiency, frequently sacrificing environmental sustainability in the process.

RP-HPLC: The Traditional Workhorse Most established techniques for analyzing Olmesartan and Azilsartan make use of C18 (Octadecylsilane) or C8 columns as the stationary phase. Mobile Phase Composition: Common techniques often use a combination of Acetonitrile (ACN) and Phosphate Buffers (pH 3.04.5). ACN is preferred due to its low viscosity and strong eluting ability, although it is classified as a hazardous solvent that produces a significant amount of waste [11].

Detection Parameters: Both sartan derivatives show significant UV absorption. Olmesartan is usually monitored at wavelengths of 210 nm or 254 nm, whereas Azilsartan exhibits an absorption peak close to 240 nm.

Standard operating times for conventional HPLC methods for these drugs vary between 8 and 15 minutes. Although efficient, the total amount of solvent waste generated during a 24-hour Quality Control (QC) cycle can surpass 1.2 liters per instrument [12].

UPLC and UHPLC: Effectiveness versus. Environmental Cost: Ultra-Performance Liquid Chromatography (UPLC) has been used to decrease analysis time to less than 3.minutes.

Technology Shift: UPLC offers sharper peaks and increased sensitivity by utilizing columns with a particle size below 2 \mum.

Critical Review:-Although UPLC decreases the amount of solvent used per injection, it still depends significantly on Acetonitrile. In a "Green" review, UPLC is considered a temporary improvement, although it does not completely remove the chemical risks linked to ACN and Methanol [13].    

Comparison of Reported Chromatographic Parameters

Limitations of Conventional Methods

• High Toxicity: Contact with Acetonitrile can pose health risks, including respiratory and skin-related issues, for laboratory staff.
• Solvent Waste Management: The expense of burning halogenated or dangerous organic waste through incineration represents a significant operational cost for pharmaceutical companies.
• Global Shortages: When global supply chains experience disruptions, the availability of HPLC-grade ACN may vary, posing a risk to ongoing manufacturing and quality control [14].

Green Solvent Strategies in Chromatography Adopting

Green Analytical Chemistry (GAC) for Olmesartan and Azilsartan requires a thoughtful reconfiguration of the mobile phase. The aim is to choose solvents that have minimal environmental toxicity, high biodegradability, and lower life-cycle assessment (LCA) scores.

Ethanol: The Renewable Powerhouse Ethanol (EtOH) stands out as the primary eco-friendly substitute for Acetonitrile. While ACN is a byproduct of plastic production, ethanol is generated through the fermentation of biomass.

Eluting Power: Although Ethanol has a greater viscosity than ACN (resulting in increased backpressure), its ability to elute Olmesartan is similar.

Overcoming Viscosity: Contemporary eco-friendly techniques make use of Higher Column Temperatures (45\text{}60^{\circ}\text{C}$). This decreases the viscosity of the mobile phase, enhances the sharpness of the Azilsartan peak, and reduces the pressure on the HPLC pump [15].

Selectivity: Ethanol-water mixtures frequently offer a different selectivity compared to acetonitrile-water, which is beneficial for separating the "Medoxomil" prodrug from its polar "Acid" degradation products.

Micellar Liquid Chromatography (MLC)

MLC embodies a "Super-Green" method in which the mobile phase is composed of an aqueous surfactant solution (e.g., Sodium Dodecyl Sulfate (SDS) at a level exceeding its critical micelle concentration.

• Mechanism: The micelles function as a pseudo-stationary phase. For highly lipophilic drugs such as Azilsartan (Log P ~4.7), MLC enables separation using less than 10% organic solvent, significantly cutting down on chemical waste [16].
• Safety: Since the mobile phase is mainly water-based, it is non-flammable and poses no risk to laboratory personnel.

Acetone and Ethyl Acetate as Eco-Friendly Solvents

Although less frequently used than Ethanol, Acetone is listed as a "Recommended" environmentally friendly solvent in the GSK and Pfizer solvent selection guidelines.

• UV Cut-off Challenge: Acetone exhibits a significant UV cut-off at 230 nm. For Olmesartan, typically detected at 210 nm, researchers need to employ Refractive Index (RI) or Evaporative Light Scattering Detectors (ELSD) when working with Acetone-based mobile phases [17].
• In Green HPTLC, Ethyl Acetate serves as a safer substitute for Dichloromethane in the separation of Azilsartan impurities on silica gel plates [18].

Comparison of Solvent "Greenness" Scores

Stability-Indicating Studies & Forced Degradation: A Stability-Indicating Method (SIM) is an analytical technique that precisely measures the active pharmaceutical ingredient (API) without being affected by degradation by-products, impurities, or excipients. According to the ICH Q1A (R2) guidelines, stress testing is required for Olmesartan and Azilsartan Medoxomil to determine degradation pathways and assess the inherent stability of the molecules [19].

Hydrolytic Degradation: The Medoxomil Vulnerability Both medications are prodrugs that include a medoxomil ester group, which is intended to enhance oral absorption. However, this ester bond is very prone to hydrolysis.

• Acid Hydrolysis: When exposed to 0.1 N HCl at 60^{\circ}\text{C}, it generally leads to the production of Olmesartan acid and Azilsartan acid. Green methods employing ethanol-water mobile phases must achieve a resolution (R_s) greater than 2.0 between these polar acids and the less polar parent medoxomil esters [20].
• Alkaline hydrolysis: This represents the fastest degradation pathway. In 0.1 N NaOH, Azilsartan Medoxomil may break down by over 20% within 30 minutes. The difficulty with a green method lies in ensuring the stability of the silica-based C18 column during the analysis of these basic stress samples [21]

Oxidative Stress: Oxidation tests typically use hydrogen peroxide concentrations ranging from 3% to 30%.

Olmesartan: It exhibits reasonable stability, although it may produce N-oxide derivatives at the tetrazole or imidazole ring.

Azilsartan: The oxadiazolone ring represents a possible location for oxidative splitting.

Green Analytical Note: Conventional approaches rely on high levels of Acetonitrile to wash out these oxidative breakdown products. Current green approaches employ Micellar Liquid Chromatography (MLC), in which surfactant monomers safeguard the drug against additional "on-column" oxidation during the analysis [22].

Photolytic and Thermal Degradation Photolysis:

• When exposed to UV light (ICH Q1B), Olmesartan undergoes "Photo-isomerization." Green HPTLC techniques are especially beneficial in this context, as they enable the simultaneous analysis of several photostress samples on one plate while using very little solvent [23].
• Thermal Stress: Dry heat (60^{\circ}\text{C} to 105^{\circ}\text{C}) is applied. Both medications remain fairly stable in their solid form but break down considerably when dissolved in solution.

Summary of Degradation Behavior