1. INTRODUCTION TO GREEN CHEMISTRY IN PHARMACEUTICALS

Green chemistry or sustainable chemistry is the research of designing chemical processes and products that reduce or replace the use and production of hazardous materials. Paul Anastas and John Warner coined the ideas of green chemistry in its contemporary sense in the late 1990s and early 2000s.

Green chemistry rests on twelve principles of green chemistry that aim to enhance the effectiveness of chemical processes at lower environmental and health impacts¹, commonly associated with, but not necessarily restricted to, atom economy, renewable feedstocks, energy efficiency, and inherently safer chemical design. In drug development, green chemistry encompasses the entire life of drug manufacturing, from selection of raw materials, synthesis, formulation, production, packaging, waste treatment, and end of life degradations. Green chemistry targets the whole drug development process and not only production but also involves use of green analysis, green solvents, and greener syntheses².

In pharma, green chemistry involves the application of biocatalysis, flow chemistry, supercritical fluids, microwave synthesis, and solvent free processes. Green chemistry is interested in biodegradable excipients, and green reaction conditions. Green chemistry is also important for regulatory compliance and contributes to global action aimed at attaining the United Nations Sustainable Development Goals (SDGs), with specific emphasis on those that deal with health, industry, and environmental sustainability³.

The pharmaceutical industry is an enabler of innovation and human wellness but has a large environmental footprint. A majority of operations in conventional chemical synthesis employ harmful solvents, harmful heavy metals, and high amounts of reagents with a high waste legacy. Research has shown the pharmaceutical industry produces higher levels of waste in terms of base weight per product volume than most other industries, with an average of 25 - 100 kg of waste per 1 kg of API manufactured⁴. This makes it imperative to make the change to green products. There are numerous benefits of green chemistry in the pharmaceutical industry, environmentally it is the reduction in the greenhouse gases, minimization of water and energy, and the minimized production of toxic waste. Economically it has an opportunity to reduce the cost of production through the improvement in the efficiency of the reactions, minimization of reagent usage, and ease of purification steps.

From an aspect of safety and health it provides for safer working conditions for workers through limited exposure to harmful chemicals⁵. In addition, regulatory bodies intent to enhance public health and safety, like the U.S. Environmental Protection Agency (EPA) and the European Medicines Agency (EMA) are applying greater pressure on the pharmaceuticals to implement and adopt green chemistry practices. For instance, the Green Chemistry Challenge Awards, organized by the EPA, highlights instances of effective sustainable innovation in the synthesis and production of drugs⁶. Green chemistry is not only a trend in today's pharmaceutical arena but a requirement. Green chemistry facilitates the shift toward sustainable drug development and discovery while keeping the concepts of efficacy, safety and quality essentially unchanged. The use of green chemistry in the earliest research and development processes will allow the pharma industry to meet regulatory standards, improve the effect on the environment, and cater to increasing demand for green products.

2. GREEN CHEMISTRY PRINCIPLES:

2.1 Twelve Principles:

Green chemistry is founded on a basic framework, known as the Twelve Principles of Green Chemistry, originally formulated by Paul Anastas and John Warner in 1998. The Twelve Principles are an operational plan to redesign chemical ladder-type products and processes based on a rational and systematic method for reducing or avoiding the generation of hazardous substances, maximizing efficiency, and reducing the overall environmental footprint⁷.

The twelve principles are:

1. Prevention: Waste must be averted and not treated, or cleaned up after it has happened.

2. Atom Economy: Synthetic processes must be conceived of so as to maximize use of all materials input into the process in the final product.

3. Less Hazardous Chemical Syntheses: Synthetic processes must be conceived of so as to employ and produce substances that are non-toxic to human health and the environment.

4. Designing Safer Chemicals: Chemical products must be designed to maintain effectiveness; reduce toxicity.

5. Safer solvents and auxiliaries: Use of auxiliary chemicals must be avoided or made as safe as feasible.

6. Design for Energy Efficiency: Energy demand must be kept to a bare minimum, and synthetic routes must be carried out at ambient temperature and pressure.

7. Renewable feedstocks: Raw materials must be renewable and not depleting whenever technologically and economically feasible.

8. Minimize Derivatives: Redundant derivatization (use of protective or blocking groups) should be avoided or minimized, as they consume additional reagents and thus generate waste.

9. Catalysis: Catalyst reagents (or catalytically active reagents) should be used instead of stoichiometric reagents.

10. Design for Degradation: Chemical products should be designed to degrade at the end of their use into harmless degradation products.

11. Real-time monitoring to avoid pollution: Analytical methods should be created for real-time monitoring and control of toxic substances.

12. Inherently safer chemistries to avoid an accident: Chemical processes should be designed so that the possibility of accidents, such as releases, explosions, or fires, is minimized⁸.

These are not all incompatible principles and are meant to be a transition from conventional chemical production to safer, cleaner and more sustainable alternatives.

2.2 Applications of the Principles to Drug Synthesis:

In the pharmaceutical industry, the twelve principles are relevant characteristics of green chemistry due to the enormous amount of waste conventionally generated within the industry, along with the use of toxic reagents and solvents. Utilization of green chemistry during drug synthesis tends to result in cleaner processes and more secure drugs, as well as better yields.

For instance, atom economy and catalysis consideration enable production of shorter syntheses and more effective processes with fewer steps and reagents and energy used for manufacturing. The overall outcome is cost reduction of the production and general environmental footprint⁹.

Usage of safer solvents (and less toxic reagents) has the consequence of reducing byproducts that are toxic, ensuring worker and end-user safety and health. Solvents like dichloromethane and benzene are being replaced by greener alternatives (e.g, water, ethanol, and supercritical CO₂) due to their health hazards¹⁰.

Moreover, renewable feedstocks (such as biobased precursors) and biocatalysis (by enzymes) are becoming increasingly prominent. Not only do these technologies improve the selectivity of reactions, but also conform to sustainability objectives¹¹.

Analysis in real time helps monitor chemical transformations more effectively, with a focus on quality control and safety, whereas inherently safer design minimizes the likelihood of industrial accidents—a key issue in bulk pharmaceutical production¹².

The application of these concepts into drug development optimizes sustainability without sacrificing therapeutic performance, facilitating pharmaceutical firms to align with environmental policies and corporate social responsibility practices.

3. ALTERNATIVES OF GREEN SOLVENTS AND REAGENTS:

The conventional use of the pharmaceutical industry involves high amounts of volatile organic compounds (VOCs) as reagents and solvents, most of which are toxic to health and the environment.

The transition towards green chemistry has spurred the development and utilization of environmentally friendly solvents and catalysts, with a view to less toxicity, better atom economy, and waste reduction.

Green solvents like water, supercritical CO₂, ionic liquids, and deep eutectic solvents (DESs) are promising substitutes.

Similarly, greener catalysts also present better selectivity and reduced environmental influence as opposed to traditional metal catalysts.

3.1 Water and Supercritical CO₂:

Water is the most inexpensive, non-toxic, and most abundant solvent. Historically underutilized in organic synthesis because of its polarity and limited solubility of non-polar species, advances in reaction engineering have made possible aqueous-phase reactions with the use of surfactants or phase-transfer catalysts. Aqueous-phase synthesis is found to be more and more applied in processes such as amide bond formation, cycloaddition, and enzymatic catalysis¹³. Water not only minimizes dependence on toxic solvents but also simplifies product separation and purification. Supercritical carbon dioxide (scCO₂) is a similarly environmentally friendly solvent. Pressurized above the critical point of CO₂ (31.1°C and 73.8 bar), it has both gas-like and liquid-like properties, giving it high diffusivity and solvation strength.

scCO₂ is inert, non-toxic, non-flammable, and can be simply removed after reaction by depressurization.

It has been effectively employed in hydrogenation, oxidation, and polymerization reactions, and is also extensively utilized in supercritical fluid extraction of active pharmaceutical ingredients (APIs)¹⁴.

3.2 Ionic Liquids and Deep Eutectic Solvents Ionic liquids (ILs):

They are salts that are liquid at temperatures less than 100°C and are made up of only ions. Due to their tunable physicochemical properties like low vapor pressure, thermal stability, and solvation versatility, ILs are considered promising for green synthesis. ILs provide the ability to conduct multi-step reactions, electrochemical reactions, and enzymatic reactions. For example, ILs have exhibited high efficiency in catalyzing the asymmetric hydrogenation and Friedel-Crafts reaction with higher efficiency and reduced waste production¹⁵. But issues regarding the toxicity, expense, and recyclability of ionic liquids have prompted the evolution of deep eutectic solvents (DESs) as greener alternatives. DESs are created through the combination of hydrogen bond donors and acceptors (e.g., choline chloride and urea), which when combined create a liquid with a melting point below that of either of the two individual components.

They are biodegradable, less toxic, and easy to make.

DESs are applied in nucleophilic substitution, oxidation, and biotransformation reactions as a more environmentally friendly approach compared to conventional solvents¹⁶.

3.3 Greener Catalysts Catalysis:

It is an important function in lowering the environmental impact of pharmaceutical synthesis through enhancing reaction selectivity, lowering activation energy, and minimizing by-products. Attention has turned from stoichiometric reagents to greener catalytic systems including biocatalysts, organ catalysts, and recoverable metal catalysts. Biocatalysts such as enzymes and whole-cell systems are especially desirable because they possess substrate specificity, gentle reaction conditions, and renewability. Enzyme-catalyzed synthesis has been used effectively in chiral intermediate production, esterification, and amide synthesis with high enantioselectivity¹⁷. Organ catalysts—metal-free small organic molecules that catalyze reactions—are another green substitute. They are not only metal-free but also frequently biodegradable and can be used in crucial carbon–carbon bond-forming reactions, such as aldol, Michael, and Diels-Alder reactions¹⁸.

There has also been an attempt to immobilize metal catalysts on reusable supports (e.g., silica, polymers, or magnetic nanoparticles) such that they can easily be separated and recycled, keeping metal leaching and contamination to final products to a bare minimum.

Together, the use of green solvents and catalysts contributes significantly to achieving environmentally responsible pharmaceutical manufacturing, supporting both regulatory compliance and sustainable development.

4. ENERGY-EFFICIENT TECHNIQUES:

Contemporary pharmaceutical production is relying more and more on energy-efficient technologies in order to minimize environmental footprints, decrease reaction times, and increase safety. Traditional methods of heating are typically time-consuming and energy-hungry. Scientists have come up with alternative methods like microwave-assisted synthesis, ultrasound and photochemistry, and flow chemistry—each of which provides greener, more rapid, and more sustainable processing options.

4.1 Microwave-Assisted Organic Synthesis (MAOS):

Microwave-assisted organic synthesis (MAOS) uses microwave radiation to directly heat reactants, offering significant improvements in reaction speed and energy efficiency. Unlike traditional convection heating, which warms the reaction mixture from the outside in, microwave irradiation causes dipolar molecules and ionic species to rotate rapidly, generating uniform internal heat. This can reduce reaction times from hours to minutes and increase yields with fewer by-products¹⁹.

In the field of pharmaceutical chemistry, MAOS is extensively used in the synthesis of heterocyles, peptide coupling, and medicinal chemistry research involving the need for quick screening of analogs. Reactions which are slow under normal heating conditions find smooth execution with microwaves, at times even solvent-free. This renders MAOS cost-efficient and environmentally friendly, leading to less solvent consumption and fewer emissions²⁰.

4.2 Ultrasound and Photochemistry:

Ultrasound-promoted synthesis, or sonochemistry, utilizes the mechanical energy of ultrasonic waves (usually 20 kHz to 100 kHz) to accelerate reactions. The most important mechanism is acoustic cavitation—the rapid creation, growth, and explosive collapse of submicron-sized bubbles within liquid media. Collapse generates localized hot spots and pressures, improving mass transport and reaction kinetics²¹.

Ultrasound methods are increasingly finding applications in organic synthesis, nanomaterials, and even enzyme-catalyzed reactions. Advantages include better yields, reduced reaction times, and green solvent compatibility. For instance, esterification and oxidation reactions have been greatly enhanced when performed under ultrasonic conditions.

Photochemistry, which consists of utilizing light (usually UV or visible) to power chemical transformations, represents another highly energy-efficient strategy. Light-induced reactions tend to operate at gentler conditions and provide great selectivity. Photocatalysis, and especially that utilizing sunlight or LEDs, is under investigation for oxidations, reductions, and coupling reactions. Photocatalysis also plays a pivotal role in contemporary drug discovery for the formation of complex reactive intermediates with minimal reagents and heat²².

4.3 Flow Chemistry and Continuous Processing:

Flow chemistry, or continuous-flow processing, refers to performing chemical reactions in a continuous stream instead of batchwise. This configuration enables precise temperature, pressure, and reaction time control, which tends to result in improved reproducibility and greater efficiency. The low volumes utilized in the flow system improve safety, particularly when handling dangerous or exothermic reactions.

Pharmaceutical firms are increasingly embracing flow chemistry to make the synthesis of active pharmaceutical ingredients (APIs) and intermediates more streamlined. Its compatibility with in-line monitoring equipment enables precise real-time analysis of reactions and minimum waste generation. Additionally, continuous processing facilitates scalable manufacturing and is compatible with green chemistry goals²³.

5. BIOCATALYSIS AND ENZYME-BASED SYNTHESIS:

5.1 Benefits in Selectivity and Mild Conditions:

Biocatalysis, or the application of enzymes or microorganisms to perform chemical reactions, has emerged as the linchpin of green chemistry for pharmaceutical synthesis. Its greatest strength is its unparalleled selectivity—enzymes are evolved to be highly specific for their substrates, and thus well-suited for reactions that demand high stereoselectivity or regioselectivity.

In contrast to traditional chemical catalysts, enzymes function effectively under mild conditions—sometimes at room temperature, atmospheric pressure, and in water. This does away with the necessity of aggressive reagents, high energy requirements, or poisonous solvents, thus lowering the environmental impact of chemical reactions²⁴. Additionally, enzymatic processes tend to occur without requiring protecting groups or multiple purification steps, simplifying the synthetic route and raising atom economy.

5.2 Industrial Applications:

Biocatalysis is very popular in industrial pharma production, especially in chiral drug and active pharmaceutical ingredient (API) synthesis. A classic example is the enzymatic manufacture of the antidiabetic drug sitagliptin, where a multi-step metal-catalysed process was replaced by a single-step enzyme-catalysed amination reaction with improved yield, reduced waste, and resource usage²⁵.

Enzymes such as lipases, transaminases, and Ket reductases are increasingly utilized for the large-scale manufacture of β-blockers, statins, and antidepressants. Whole-cell biocatalysts are also being utilized in redox conversions and multi-step cascade reactions, which further simplify the synthesis process and minimize solvent consumption²⁶.

Recent technological advances in enzyme engineering and immobilization have broadened the application scope of biocatalysis, enabling designed activity, reusability, and more effective integration into continuous processing. Such developments are not only enhancing the sustainability of processes but also assisting companies in complying with regulatory requirements and lowering cost of production²⁷.

Quite simply, biocatalysis provides a clean and sustainable alternative to conventional synthetic pathways, capitalizing on high specificity coupled with minimal environmental footprints.

6. SOLVENT-FREE AND SOLID-STATE REACTIONS:

6.1 Mechanochemistry:

Mechanochemistry is a chemical change triggered by mechanical stress—i.e., grinding, shearing, or milling—instead of by heating or using solvents. This technique defies the popular assumption that solvents are necessary to facilitate molecular motions and contacts in reactions.

In mechanochemistry, the reactions are frequently conducted in ball mills in which reactants are milled together under pressure and stress. This process enables the reactions to be conducted with little or no solvent, significantly reducing waste and environmental risks²⁸. Mechanochemical synthesis is especially useful for solid-state reactions, such as cocrystallization, metal-organic frameworks (MOFs), and organic transformations such as aldol condensations and Knoevenagel reactions.

The power supplied by mechanical action ruptures and constructs chemical bonds directly, enabling rapid, clean reactions with superior control over product selectivity and yield.

6.2 Applications in Green Drug Synthesis:

The pharmaceutical sector is increasingly acknowledging the ability of mechanochemistry to enable greener synthesis of APIs and intermediates. For example, numerous active drug molecules can be prepared or tuned through mechanochemical means, such as pharmaceutical cocrystals that improve drug solubility and stability without the need for solvents²⁹.

Mechanochemistry is also helpful in synthesizing polymorphs—various solid forms of a drug that can provide enhanced bioavailability. Such reactions not only avoid solvent waste but also provide operational advantages such as reduced reaction time, improved scalability, and ease of product isolation³⁰.

A good example is the mechanochemical synthesis of ibuprofen derivatives, for which grinding of reactants in the presence of catalytic quantities of acid leads to the product with high purity without any solvent and heat³¹.

In addition, the integration of mechanochemistry with green catalysts or enzyme co-crystals is becoming a breakthrough area in solid-state green chemistry. The hybrid strategy is promising to catalyze biotransformations without the use of any solvent, paving ways for greener enzymatic processing³².

Overall, solvent-free mechanochemical protocols agree well with the fundamental principles of green chemistry, and they provide effective, sustainable, and safe protocols for contemporary pharmaceutical synthesis.

7. GREEN METRICS AND REGULATORY PERSPECTIVES:

7.1 E-Factor, PMI, and Eco Scale:

While the pharmaceutical industry is adopting green chemistry, measurement and assessment of how environmentally benign a process is becomes inevitable. For this reason, green metrics have been devised to allow quantitative evaluation of chemical processes. Some of the most popular are the E-Factor (Environmental Factor), Process Mass Intensity (PMI), and Eco Scale.

Roger Sheldon developed the E-Factor, the ratio of waste per kilogram of product. It's a simple measure: the smaller the E-Factor, the more environmentally friendly the process. Bulk chemical production typically has E-Factors in the range 1 to 5, but pharmaceutical processes have much higher values—sometimes more than 25—because of intricate synthesis and purification procedures³³.

PMI builds upon this concept by including all the materials consumed in the process, such as solvents, reagents, and even water. It is a more general view of material use and is particularly useful in process development and scale-up. PMI is utilized as a sustainability measure by most pharmaceutical companies, such as Pfizer and GSK³⁴.

The Eco Scale is a semi-quantitative measure that takes into account multiple parameters including yield, safety, cost, environmental effect, and energy consumption. The ideal value is 100, with deductions for penalties. It is especially applied in the comparison of alternative synthetic pathways at the early stages of research³⁵.

These instruments collectively allow chemists to make sound judgments and process-improve to be safer, more sustainable, and cost-efficient.

7.2 ICH and FDA Guidelines' Role:

The regulatory agencies play an important role in advancing green chemistry. Although the International Council for Harmonisation (ICH) and the U.S. Food and Drug Administration (FDA) do not mandate green chemistry practices directly, they promote significantly the underlying principles thereof.

ICH guidelines, especially Q8 (Pharmaceutical Development), Q9 (Quality Risk Management), and Q11 (Development and Manufacture of Drug Substances), all place a strong focus on a Quality by Design (QbD) philosophy. This means understanding the manufacturing process profoundly, reducing variability, and designing in quality initially—steps that inherently support green chemistry ideals³⁶.

The FDA, on its side, promotes sustainable manufacturing with its Emerging Technology Program and Green Chemistry Awards. These efforts identify and facilitate the implementation of new paradigms such as continuous manufacturing, biocatalysis, and minimizing solvents³⁷.

Additionally, environmental considerations are becoming a part of drug approval discussions. Agencies are increasingly looking at pharmaceutical residue in water, the utilization of toxic solvents, and waste disposal procedures. Pharmaceutical firms are therefore embracing green chemistry not only for sustainability, but also to meet changing regulatory standards³⁸.

Generally, green metrics and regulatory inducement are building towards a future where compliance and sustainability happen hand in hand.

8. CHALLENGES AND FUTURE DIRECTIONS:

8.1 Industrial Implementation Barriers:

Although green chemistry holds great promise, its adoption throughout pharmaceutical production is still hindered by a number of factors. The most frequent hurdle is the expense and process difficulty of abandoning tried and true synthetic pathways for more environmentally friendly alternatives. Much current practice is optimized for yield rather than sustainability, and refurbishing greener solutions can be costly and time-consuming³⁹.

Another is limited greener reagent and solvent availability on industrial scales. Even though laboratory-scale experiments can utilize novel bio-based solvents or designer catalysts, purchasing such materials in bulk quantities is still a challenge. Furthermore, the prolonged drug development timelines of 10 years or more render it challenging to integrate new green innovations downstream without jeopardizing regulatory delays⁴⁰.

Another limitation is the absence of training and awareness among process chemists and formulators. Although green chemistry is increasingly popular among academics, most practitioners within industry have limited exposure to principles of sustainable design. A solution to this involves specific training, academic-industry collaboration, and robust internal sustainability policy⁴¹.

8.2 Innovations and Emerging Technologies:

In spite of these challenges, a number of new technologies are forging the path to a greener future for pharmaceuticals. Among the most promising is continuous flow chemistry, which enables safer, scalable, and more resource-effective production. Novartis and Vertex have already incorporated flow reactors into commercial processes, cutting waste and energy consumption dramatically⁴².

Biocatalysis and enzyme engineering are increasingly coming into vogue, facilitating highly selective conversions under benign conditions. These methodologies not only minimize chemical usage but also create access to complex chiral molecules that are otherwise recalcitrant in synthesis⁴³.

AI and machine learning are also being utilized to create greener processes. Predictive software assists in reaction conditions optimization, minimizing solvent use, and even detecting less toxic pathways at the early stages of development.

Another promising development is the use of 3D printing technology in pharmaceutical formulation, which allows for individualized medicine dosing and minimization of overproduction, resulting in less waste material⁴⁴.

In the future, blending green metrics, life cycle assessments, and regulatory adaptability will be crucial to mass adoption of green chemistry. Industry leaders, academic scientists, and regulatory agencies must work together to promote innovation while maintaining safety, efficacy, and environmental responsibility⁴⁵.

9. CONCLUSION:

Green chemistry has become an effective and imperative transformation of how drugs are conceptualized, synthesized, and produced. It is a transition away from conventional chemical methods that rely extensively on harmful reagents, unwanted waste production, and energy-consuming processes. Green chemistry, on the other hand, aims to develop safer, more environmentally friendly, and efficient procedures that not only enhance environmental but also human health, operational security, and economical feasibility. Throughout the pipeline of pharmaceutical development—from drug discovery and laboratory-scale synthesis to commercial-scale production—there are plenty of places to implement the 12 principles of green chemistry. Whether it's enhancing atom economy, minimizing the use of toxic solvents, choosing renewable feedstocks, or designing safer molecules in the first place, each principle promotes innovation while lowering the industry's footprint. Solvents and catalysts, once thought necessities of chemical synthesis, have been dramatically changed. Greener solutions like water, supercritical CO₂, ionic liquids, and deep eutectic solvents are becoming increasingly used in place of volatile organic compounds. Similarly, applications involving biocatalysts and recyclable metal complexes illustrate the ways enzymatic and catalytic methods can lead to efficiency while also meeting sustainability objectives. These shifts are not hypothetical—they are already boosting product yields, lowering waste, and facilitating cleaner reactions in the lab and in industry. Energy-efficient methods such as microwave-assisted synthesis, ultrasound, photochemistry, and flow chemistry have helped accelerate the shift toward greener processes further. Such instruments reduce reaction times, minimize energy input, and provide improved control over the process, which in turn help reduce costs and environmental footprint. Green chemistry also is facilitated by quantitative measures like the E-Factor, PMI, and Eco Scale, which enable chemists to quantitatively assess and compare synthetic methods in terms of waste, material usage, and the environment. At the same time, regulatory guidelines—those of ICH and FDA, for instance—are progressively facilitating sustainable practices via Quality by Design (QbD) and adaptable, innovation-based guidelines. With all the advances, there are still roadblocks. Very high development expenses, unavailability of scalable green reagents, limited knowledge, and change resistance can decelerate industrial uptake. Yet, these hurdles are being overcome by new technologies like AI-driven process design, enzyme engineering, continuous flow technology, and even 3D printing of customized medicine—all of which hold out a cleaner, smarter future for pharma manufacturing.

Finally, green chemistry is no longer merely an environmental imperative—it is a scientific, strategic, and ethical necessity. It allows for companies to meet increasing regulatory requirements, ensure public health, enhance operational efficiency, and support international sustainability objectives. As its awareness and innovation expand, green chemistry will certainly be the standard rather than the exception in pharmaceutical science.

By incorporating green principles early on in the drug development process and encouraging cooperation among scientists, industries, and regulators, the world of pharma can really change—one cleaner, safer, and wiser molecule at a time.

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Received on 17.07.2025 Revised on 20.09.2025

Accepted on 14.10.2025 Published on 06.11.2025

Available online from November 11, 2025

Asian J. Research Chem.2025; 18(6):401-408.

DOI: 10.52711/0974-4150.2025.00061

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