Where Do Impurities In Pharmaceutical Analysis Come From?

Part 1 Definition And Source Of Impurities

Impurities can be defined as mixtures of target components with extraneous or inherently inferior substances. It is often the final manufacturing process that has a major impact on the cost of the API. Yield, physical properties, and chemical purity are key considerations in API production, preparation formulation, and preparation production. As part of a new drug application, applicants must submit to the FDA the manufacturing and process controls for drug substances and drug products. If a production batch does not meet purity and impurity specifications, manufacturers must rework it, which not only consumes API and other resources, but also delays the production of other batches of API, which is costly. The source and type of impurities can be analyzed through the production process flow diagram of the API. The formation of impurities is interrelated at every stage of the production process.

In short, any substance that affects the purity of the drug substance or finished product can be considered an impurity. Impurities come from various sources and typically include: starting material(s), intermediates, penultimate step intermediates, by-products, transformation products, interaction products, related substances, degradation products and tautomers.

Part 2 Various Sources Of Impurities

1. Starting Materials

Regulatory agencies have long expected the control of impurities in starting materials used to manufacture APIs. The starting materials of API can be raw materials, intermediates, or APIs used in the production of APIs and as important structural parts of APIs. Starting materials for APIs usually have well-defined chemical properties and structures.

Draft FDA Guidance: Drug Substances—Chemistry and Manufacturing Control Information reflects a concern for starting materials that should be well selected and controlled, as any future changes to starting materials could compromise the safety, identity, and safety of the drug substance. performance, purity and quality.

Draft FDA guidelines and ICH guidelines provide the basis for starting material selection:

• Use an appropriate, discriminative method to test the quality of the starting material.
• Develop appropriate quality standards to ensure the quality of the API.
• The effect of starting material quality on API quality should be understood and controlled.
• The starting material should be commercially available and incorporated as an important structure into the new drug.
• The starting material can be characterized and stability is well understood.
• A starting material is a compound whose name, chemical structure, chemical and physical characteristics and properties, and impurity profile are well defined in the chemical literature.

Due to the potential impact of starting materials on the quality of the API, the closer the starting materials are to the final API in the API synthesis process, the more strictly they should be controlled.

For example, fluoronitrobenzene is a key starting material for API olanzapine. If the 2-4-difluoronitrobenzene impurity is present in the critical starting material, according to literature conditions, it will be converted to 8-fluoroolanzapine, a non-compendial impurity (United States Pharmacopeia [USP] method, relative retention time [rrt ] 1.07). 2-4-Difluoronitrobenzene enters the next stage along with fluoronitrobenzene, leading to similar compounds in the final stage.

In another example, N-[6-(4-phenylbutoxy)hexyl)]benzylamine is the starting point in the Drug Master File (DMF) for the selective long-acting 2-adrenoceptor agonist salmeterol. starting materials. The drug is clinically used as an inhaled bronchodilator for the treatment of asthma and chronic bronchitis.

In the case of salmeterol, the reaction of 4-phenylbutanol with 1,6-dibromohexane yields intermediate 1, which in turn reacts with benzyl The amine reaction yields N-[6-(4-phenylbutoxy)hexyl)]benzylamine, the starting material in salmeterol DMF. Compound 4-phenylbutanol is commercially available and prepared from benzene and succinic anhydride. If benzene contains a small amount of toluene, the toluene is converted into 4-(4-methylbenzyl)-1-butanol. The compound 4-(4-methylbenzyl)-1-butanol, present as a starting material impurity in 4-phenylbutanol, undergoes a further reaction, similar to 4-phenylbutanol, to yield methylsalmeterol Impurities. Likewise, the presence of phenylethyl alcohol in 4-phenylbutanol, 3-phenyl-1-hydroxypropane and 4-phenyl-2-hydroxybutane will produce known impurities B, C and E, respectively.

Likewise, the 6-hydroxyl and dichloro impurities, if present in the starting material of ciprofloxacin DMF, would convert to Ph. Eur. impurity F and non-compendial impurity (chlorciprofloxacin) RRT2.1.

2. Intermediates

The organic compounds formed in the synthesis of APIs are called intermediates. In the synthetic process, the compound before the final desired compound is produced is called the penultimate step intermediate.

3. Impurities Produced By Rearrangement

High-yield products are produced in shorter synthetic routes or via one- or two-pot reactions, often involving the generation of rearranged intermediates.

For example, the cyclization of bromonitrostyrene in the API ropinirole involves a rearrangement of the intermediate ring ion to generate the indole ring and form the hydroxamate and chloroxime acetate impurities.

4. Impurities Produced By In Situ Reaction

Advances in synthetic chemistry have allowed multistep reactions to be performed in one or two steps without isolation of intermediates. A disadvantage of such reactions is the formation of substantial quantities of unanticipated impurities due to failure to isolate intermediates and reagents.

For example, the alkylation of (S)-2-aminobutanamide, the key starting material of the API levetiracetam, with chlorobutyryl chloride using potassium hydroxide in the presence of tetrabutylammonium bromide yields the intermediate and Final cyclization to levetiracetam. However, this intermediate is present in the final product as USP Impurity A.

5. Non-Reactive Intermediates

A non-reactive intermediate is an intermediate stage impurity formed due to residual reactions with reagents used in subsequent stages. Such impurities remain unreactive in subsequent stages.

For example, 4-phenylbutanol is a key starting material for the synthesis of salmeterol intermediates 1 and 2. Intermediate 1 is reacted with 4-phenylbutanol in the presence of sodium hydride and toluene to yield compound 1, which is an unreactive impurity in subsequent steps. Intermediate 2 reacts with a small amount of intermediate 1 to form compound 2 under the same conditions.

6. Reactive Intermediates

Reactive intermediates, as the name implies, are by-products or impurities resulting from intermediate steps of a reaction that have the potential to react with reagents or catalysts used in subsequent stages. They enter every stage as reactive intermediates up to the final API.

During salmeterol process development, an unknown impurity was detected at 2.08 RRT at a concentration of 0.11%, which was identified as compound 3 after separation. The impurity formed in the final API due to the presence of N-benzyl-6-(4-cyclohexylbutoxy)hexan-1-amine in intermediate 2 produced the salmeterol cyclohexyl impurity.

The reactive intermediate, N-benzyl-4-phenylbutan-1-amine, is present in intermediate 2 (see Figure 2). Formed by reaction of 4-phenylbutanol with benzylamine and competes with intermediate 2 to form compound 4 in all reaction steps.

The main challenge in developing the olefination route of API aprepitant is the subsequent reaction of the vinyl ether intermediate with biscyclopentadienyldimethyltitanium to form ethyl impurities.

7. Double Compound Impurities

Even if successful at a smaller scale, new or unknown impurities may form when the process is scaled up. Examination of the molecular weight of such impurities often reveals that the compound is exactly twice the weight of the compound formed in this reaction step. Such dimeric derivatives are known as bicompound impurities.

8. By-Products

In synthetic organic chemistry, it is rare to obtain a single end product in 100% purity due to conversion to by-products, which can be formed through various side reactions such as incomplete reactions, overreactions, isomerizations, or starting materials , intermediates, chemical reagents or catalysts for unwanted reactions. For example, during mass production of acetaminophen, a by-product diacetoacetaminophen can be formed.

The Claisen rearrangement of the aryne ether in diethylaniline at high temperature leads to the formation of the expected chroman product accompanied by the continuous formation of more and more furan by-products.

In the ropinirole synthesis, a somewhat similar situation was observed in the final step. The reaction between ropinirole precursor 4 – (2-bromoethyl)-13-dihydro-2H-indol-2-one and aqueous solution of dipropylamine produces ropinirole in a typical yield of 57%, And styrene as a by-product is 38%.

In another example, thiophene is an important heterocyclic compound widely used as a building block for many agrochemicals and pharmaceuticals. The synthesis of 2-amino-5-methylthiophene-3-carbonitrile was achieved by using a mixture of triethylamine with sulfur, propionaldehyde, malononitrile, dimethylformamide.

The reaction of propionaldehyde with malononitrile and sulfur leads to the formation of two unknown impurities, up to 7%, separated and confirmed by 1 H NMR (nuclear magnetic resonance spectroscopy), correlation spectroscopy, nuclear Overhauser effect spectroscopy and single crystal X-ray crystallization as impurity 1. It is found that these impurities further react with 2-fluoronitrobenzene to generate impurities in the next step, which are controlled through the purification of each step.

Impurity 1 is a novel tricyanobicyclic compound, unknown in the literature as of this writing. Determined by the OSIRISProperty Explorer software used to calculate the correlation of various drugs to the chemical structure, the predicted LogP is 0.65, the drug-likeness is 4.04, and the drug classification is 0.45. Structure-activity relationship, quantitative structure-activity relationship, and other modified organic/inorganic heterocyclic groups to design drugs may produce certain biological activities.

Molecular design of impurities 1 for specific and non-specific purposes (e.g. DNA binding, enzyme inhibition, anticancer efficacy) is based on knowledge of molecular properties such as functional group activity, molecular geometry, electronic structure, and classification information based on similar molecules . The compound 2,6-diamino-7-ethyl-8-methylbicyclo[2.2.2]-2,5-octadiene-1,3,5-tricyano can be coupled to active or inactive peptides to examine biological activity as prodrugs or drugs. Potential therapeutic and prophylactic activity of antimalarials, antimitotics and antineoplastics may also be pursued. After in vivo and in vitro tests, the bicyclic compound can be used alone as a single drug, or used together with any organic or inorganic salt in chemotherapy, or used together with other chemotherapy drugs.

9. Transformation Product

Transformation products refer to theoretical and non-theoretical products produced in the reaction. Probably a synthetic derivative of by-products, closely related to by-products.

A reaction in which a conversion product is present is the formation of a salicylaldehyde chloroacetyl derivative in the acylation of salicylaldehyde with bromoacetyl bromide using methylene chloride (MDC) and aluminum chloride (AlCl3). Mechanistically, formation of chloroacetyl derivatives may not be expected using bromoacetyl bromide, but it is assumed that a conversion reaction may occur due to halogen exchange. In the Friedel–Craft acylation reaction using the Lewis acid AlCl3 in dichloromethane, the Lewis acid forms an ionic complex [Cl–AlCl2–Br]–, which eventually undergoes ion exchange between a halogen and a bromoacyl group to produce a chloroacetyl derivative. The formation of this impurity in the reaction is as high as 7–20%, and it is an impurity that is not controlled in the production process. However, this impurity does not affect the purity of the final drug substance.

10. Interaction Products

Interaction products involve the intentional or unintentional interaction of two or more intermediates/compounds with various chemicals. Interaction products are slightly more complex than by-products and transformation products. Two types of interaction products that are common are drug substance-excipient interaction products and drug substance-container/closure interaction products.

11. Related Substances

Related substances refer to impurities that have a similar structure to the drug substance and may exhibit similar biological activities. However, this structural similarity does not in itself provide any guarantee of similar activity. An example of a related substance is 8-fluoroolanzapine.

12. Degradation Products

In the manufacture of APIs, impurities formed through the decomposition or degradation of the final product are called degradation products. The term also includes degradation products resulting from storage, formulation or aging.

13. Tautomeric Impurities

Tautomers are structural isomers that are readily interconvertible and can coexist in equilibrium. For APIs or drug molecules with tautomerism, it is easy to be confused when identifying the two tautomeric forms. If one tautomer is thermodynamically stable and is the predominant form, the other tautomer should be considered as an impurity or called a tautomer of the API or drug molecule. To the authors’ knowledge, the literature has not addressed the isolation, synthesis, or characterization of the final API tautomeric impurity.

Linezolid is a drug used to treat Gram-positive nosocomial infections. Oxazolidinones have a unique inhibitory mechanism for bacterial protein synthesis. Linezolid has an N-acetyl group (–NH–CO–CH3) due to this lactam-lactam tautomerism, which may have occurred during the synthesis, and may remain stable. An efficient analytical method needs to be developed to identify the two tautomers.

The key starting material of pemetrexed disodium 2,4-diamino-6-hydroxypyrimidine shows that the keto-enol form exists in different ratios, which can be converted to the final drug using known synthetic routes.

Different tautomers have different kinetic and thermodynamic stabilities, making it difficult to determine whether they can be separated or analyzed. The use of the term “tautomeric impurity” in API/drugs will be an important discussion point in the near future.

Part 3 Conclusion

This article highlights the source and classification of impurities, providing a perspective on impurities in drug substances and drug products. The impurity profile of the drug substance is becoming increasingly important to ensure the quality of the drug product. Regardless of the type of impurity, identifying and adequately controlling it is a huge challenge for process development chemists. Because no two drugs are the same, no two development paths are the same. Each drug candidate poses different challenges with respect to impurities, and establishing an effective method to isolate and control impurities is a critical task.