Journal of Liquid Chromatography & Related Technologies
`Copyright # Taylor & Francis Group, LLC
`ISSN 1082-6076 print/1520-572X online
`DOI: 10.1080/10826070701191151
`
`w
`
`, 30: 877–935, 2007
`
`Identification of Pharmaceutical Impurities
`
`Fenghe Qiu and Daniel L. Norwood
`Boehringer Ingelheim Pharmaceuticals Inc., Ridgefield, Connecticut, USA
`
`Abstract: Identification of pharmaceutical impurities is a critical analytical activity in
`the drug development process whose goal is to fully elucidate the chemical structures
`of unknown pharmaceutical impurities present in either drug substances or drug
`products above a particular threshold. Knowledge of the chemical structure of an
`impurity is essential to assess its toxicological implications and to gain an understand-
`ing of its formation mechanism. Knowledge of the formation mechanism is critical for
`improving the synthetic chemical processes and optimizing the drug formulation to
`reduce or eliminate the impurity. This article reviews the current regulatory require-
`ments for impurity identification and the chemical origins of various impurities, par-
`ticularly those derived from degradation of drugs. Strategies for identification of
`pharmaceutical impurities are discussed followed by an overview of the critical
`steps and the roles of various analytical techniques, such as HPLC-DAD, LC-MS,
`LC-NMR, GC-MS, and NMR, in pharmaceutical impurity identification, with an
`emphasis on applications of mass spectrometry based hyphenated techniques.
`Carefully selected examples are included to demonstrate key points in impurity
`formation and the appropriate application of various analytical techniques.
`
`Keywords: Pharmaceutical impurities, Hyphenated techniques, Impurity identification
`
`INTRODUCTION
`
`Impurity profiling and control is one of the most regulated areas in the pharma-
`ceutical industry.[1 – 7] According to ICH Q3A (R) “Impurities in the New
`Drug Substance.”[1] and ICH Q3B (R) “Impurities in the New Drug
`Product”,[2] a drug substance impurity is “any component of the new drug
`substance that
`is not
`the chemical entity defined as the new drug
`
`Address correspondence to Fenghe Qiu, Boehringer Ingelheim Pharmaceuticals Inc.,
`900 Ridgebury Rd., Ridgefield, CT 06877, USA. E-mail: fqiu@rdg.boehringer-
`ingelheim.com
`
`877
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`substance,” and a drug product impurity is “any component of the new drug
`product that is not the drug substance or an excipient in the drug product.”
`In a pharmaceutical product, an impurity is first and foremost a quality
`issue, since it could potentially compromise the efficacy of the drug
`product. Secondly, impurities also cause safety concerns.[9 – 12] Hence, any
`impurity present in a drug product must be fully understood, both qualitatively
`(chemically) and quantitatively, and qualified, if necessary, through toxico-
`logical assessment. From a chemical perspective, pharmaceutical impurities
`are inevitable because no chemical reaction has 100% selectivity and no
`chemical compound is “rock” stable. Nonetheless, it is possible to reduce
`impurities via synthetic process improvement and appropriate preformulation/
`formulation studies. Knowing the structure of an impurity is essential for
`allowing assessment of its toxicological implications and for understanding its
`formation mechanisms, which is critical knowledge for
`improving the
`synthetic chemical process and optimizing the formulation. Impurity identifi-
`cation is a specialized field in the pharmaceutical industry which requires
`specialized analytical facilities and expertise.[13 – 21] The goal of this review is
`to provide an overview of the current regulatory requirements on impurity
`identification, the chemical origins of various impurities, and the strategies
`and roles of various analytical techniques in pharmaceutical impurity identifi-
`cation, with an emphasis on mass spectrometry based hyphenated techniques.
`The examples presented in this review are used to illustrate key points in
`impurity formation chemistry and applications of various analytical techniques,
`particularly mass spectrometry based hyphenated techniques.
`
`REGULATORY REQUIREMENTS FOR PHARMACEUTICAL
`IMPURITY IDENTIFICATION
`
`ICH guidelines[1 – 3] classify impurities into three categories: organic impurities,
`inorganic impurities, and residual solvents. These impurities can be from a
`variety of sources, as given in Table 1. Organic impurities are derived from
`drug substance synthetic processes and degradation reactions in drug substances
`
`Impurity classification based on ICH guidelines Q3A(R),[1]
`Table 1.
`Q3B (R),[2] and Q3C[3]
`
`Organic impurities
`
`Inorganic impurities
`
`Residual solvents
`
`Starting materials
`Intermediates
`By-products
`Degradation products
`Reagents, ligands and catalysts
`Reagents, ligands and catalysts
`Heavy metals or other residual metals
`Inorganic salts
`Inorganic or organic liquids
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`Table 2.
`ICH reporting, identification and qualification threshold for organic impuri-
`ties in drug substance[1]
`
`Maximum
`daily dosea
`2 g/day
`
`.2 g/day
`
`Reporting
`threshold
`
`0.05%
`
`0.03%
`
`Identification
`threshold
`
`Qualification
`threshold
`
`0.1% or 1.0 mg TDIb,
`which ever is lower
`0.05%
`
`0.15% or 1.0 mg TDI,
`which ever is lower
`0.05%
`
`aThe amount of drug substance administered per day.
`bTDI: Total daily intake of the impurity.
`
`and drug products. Synthetic process related impurities can be derived from
`starting materials, intermediates, reagents, ligands, and catalysts used in the
`chemical synthesis, as well as by-products from the side-reactions of the
`chemical synthesis. Degradation products are derived from the chemical degra-
`dation of drug substances and drug products under storage or stress conditions.
`Control of the organic impurities in new drug substances is based on the
`Maximum Daily Dose and total daily intake (TDI) of the impurities. Table 2
`provides the ICH threshold for control of the organic impurities in new drug
`substances.[1] Depending on whether the Maximum Daily Dose higher or
`lower than 2 g, organic impurities in a new drug substance at (or greater than)
`0.05% or 0.1% require identification. Note that these thresholds do not apply
`to toxic impurities.[7,22,23] According to EMEA CHMP recommendations,[7]
`genotoxic impurities should be controlled based on compound-specific risk
`assessment. For an unstudied impurity (such risk assessment data do not
`exist), a threshold of toxicology concern (TTC) of 1.5 mg/day can be applied,
`with exceptions for aflatoxin-like, N-nitroso- and azoxy-compounds, which
`should be assessed based on compound-specific toxicity data.
`Control of organic impurities in new drug products are outlined in Table 3
`and Table 4.[2] Note that these thresholds are not the same as those for impu-
`rities in new drug substances given in Table 2. Based on the Maximum Daily
`Dose, the identification thresholds for organic impurities in new drug products
`are divided into 4 groups to give more consideration to low dose drug
`products. For most new drug products,
`the Maximum Daily Dose is
`between 10 mg – 2 g/day, therefore, any impurities at 0.2% or greater would
`have to be identified.
`
`Table 3. Reporting thresholds for impurities in new drug
`product[2]
`
`Maximum daily dose
`
`Reporting threshold
`
`1 g
`.1 g
`
`0.1%
`0.05%
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`Table 4. Identification thresholds for organic impurities in new drug
`product[2]
`
`Maximum daily dose
`
`Identification threshold
`
`,1 mg
`1 – 10 mg
`.10 mg22 g
`.2 g
`
`1.0% or 5mg TDI, which ever is lower
`0.5% or 20 mg TDI, which ever is lower
`0.2% or 2 mg TDI, which ever is lower
`0.1%
`
`Leachables are a separate class of drug product impurities, however,
`control of leachbales is not covered by ICH guidance.[2] Recently, the PQRI
`(Product Quality Research Institute) Leachables and Extractables Working
`Group proposed safety and qualification thresholds for leachables in OINDP
`(Orally Inhaled and Nasal Drug Products) to the regulatory authorities.[8]
`The proposed Safety concern threshold (SCT) is 0.15 mg/day, and the Quali-
`fication Threshold (QT) is 5 mg/day for an individual leachable in an OINDP.
`Note that proposed safety thresholds only apply to OINDP and not other drug
`product type (e.g., injectables, ophthalmic, etc.)
`Inorganic impurities are, in most cases, introduced from the synthetic
`process of the drug substance (e.g., catalyst), or as impurities present in exci-
`pients. Analysis and control of inorganic impurities usually follows pharma-
`copoeial monographs or other appropriate procedures and will not be
`discussed further in this review.
`Residual solvents are defined as organic volatile chemicals that are used
`or produced in the manufacture of drug substances or excipients, or in the
`preparation of drug products.[3] Residual solvents are divided by a risk assess-
`ment approach into three classes.
`Class 1 solvents are known human carcinogens, strongly suspected human
`carcinogens, and environmental hazards; therefore, these solvents should be
`avoided in the production of drug substance, excipients, or drug products,
`unless their use can be strongly justified in a risk-benefit assessment. If unavoid-
`able, the level of an individual Class 1 residual solvent should be strictly
`controlled below the concentration limits (for example the limit for benzene is
`2 ppm).[3] Class 2 solvents are non-genotoxic animal carcinogens or possible
`causative agents of other irreversible toxicity such as neurotoxicity or teratogeni-
`city. Class 2 solvents are controlled according to the PDEs (Permitted Daily
`Exposure) and Maximum Daily Dose (Option 1 and Option 2). ICH Q3C[3]
`provides PDEs of all Class 2 solvents. Class 3 solvents are solvents with low
`toxic potential to man. It is recommended that amounts of these residual
`solvents of 50 mg per day or less would be acceptable without justification.
`For solvents for which no adequate toxicological data are available, man-
`ufacturers should supply justification for residual levels of these solvents in
`pharmaceutical products.
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`ORIGINS OF PHARMACEUTICAL IMPURITIES
`
`Organic impurities can be originated from a variety of sources in both drug
`substances and drug products. Generally, the origins of impurities can be
`categorized into the following sources: synthetic process of
`the drug
`substance, degradation of the drug substance, container/closer and packing
`materials, and extraneous contaminants.
`
`Impurities Originating from Drug Substance Synthetic Processes
`
`Most small molecule drug substances are chemically synthesized. Chemical
`entities, other than the drug substance, that are involved or produced in the
`synthetic process can be carried over to the final drug substance as trace
`level impurities. These chemical entities include raw materials, intermediates,
`solvents, chemical reagents, catalysts, by-products, impurities present in the
`starting materials, and chemical entities formed from those starting material
`impurities (particularly those involved in the last steps of the synthesis).
`These impurities are usually referred to as process impurities. The goal of
`process impurity identification is to determine the structures and origins of
`these impurities. This knowledge is critical for improving the synthetic
`chemical process, in order to eliminate or minimize process impurities.
`
`Process Impurities Originating from Starting Materials and
`Intermediates
`
`Starting materials and intermediates are the chemical building blocks used to
`construct the final form of a drug substance molecule. Unreacted starting
`materials and intermediates, particularly those involved in the last a few
`steps of the synthesis, can potentially survive the synthetic and purification
`process and appear in the final product as impurities.[23] For example, in the
`synthesis of tipranavir drug substance, the “aniline” is the intermediate in
`the last step of the synthesis.[24] Because the similarity between the structures
`of the “aniline” and the final product, it is difficult to totally eliminate it in the
`subsequent purification step. Consequently, it appears in the drug substance at
`around 0.1%.
`
`Process Impurities Originating from Impurities in the Starting Materials
`
`Impurities present in the staring materials[25 – 27] could follow the same
`reaction pathways as the starting material itself, and the reaction products
`could carry over to the final product as process impurities. Knowledge of
`the impurities in starting materials helps to identify related impurities in
`the final product, and to understand the formation mechanisms of these
`related process impurities. An often encountered scenario involves starting
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`materials containing structural analogs, positional isomers, etc., as trace
`level impurities. These impurities are difficult to separate and remove from
`the starting material during its manufacturing process. When the starting
`material is used for synthesis of a drug substance, these impurities will
`likely form analogs or isomers of the desired drug substance. Some real
`world examples of process impurities derived from impurities in starting
`materials were reported by Go¨ro¨g et al.[28 – 30] One such example is the
`presence of a 4-trifluoromethyl positional isomer in 3-trifluoromethyl-a-ethyl-
`benzhydrol (flumecinol), due to the presence of 4-trifluoromethylbenzene
`impurity in the starting material, 3-trifluoromethylbenzene. A second
`example involves a 2-methyl analogue present as a trace impurity in tolperi-
`sone, due to the presence of 2-methylpropiophenone in the starting material,
`4-methylpropiophenone.
`
`Process Impurities Originating from Chemical Reagents, Ligands and
`Catalysts
`
`Chemical reagents, ligands, and catalysts used in the synthesis of a drug
`substance can be carried over to the final products as trace level impurities.
`For example, carbonic acid chloromethyl
`tetrahydro-pyran-4-yl ester
`(CCMTHP), which is used as an alkylating agent in the synthesis of a ß-
`lactam drug substance, was observed in the final product as an impurity.[31]
`Many chemical reactions are promoted by metal based catalysts. For
`instance, a Ziegler-Natta catalyst contains
`titanium, Grubb’s catalyst
`contains ruthenium, and Adam’s catalyst contains platinum, just to name a
`few. It is expected that these heavy metals will appear in drug substances at
`trace levels when such catalysts are employed. Most heavy metals have
`safety concerns; therefore, specifications and analytical methods should be
`in place to monitor the heavy metals involved in the process.[32,33] Garrett
`et al.[34] recently reviewed the practices for meeting palladium specifications
`in drug substances. In some cases, reagents or catalysts may react with
`intermediates or final products to form by-products. Go¨ro¨g et al.[35] reported
`that pyridine, a catalyst used in the course of synthesis of mazipredone,
`reacts with an intermediate to form a pyridinium impurity. Figure 1 shows
`a process where t-BuOK (potassium t-butyloxide) is used to extract a
`proton from an intermediate and promote the coupling of two intermediates
`to form a drug substance. During this process, t-butyloxyl also attacks
`the cyclopentanol ester moiety of one intermediate to form an impurity
`with a molecular weight of M-12 (M being molecular weight of the
`intermediate).
`Another example of reagent related impurities is the observation of
`elemental sulfur (S8) in a batch of bulk drug. In this case, the last step of
`the synthesis involves Na2S2O3 as a reducing reagent
`to scavenge the
`residual oxidant. When Na2S2O3 is used in excessive amount, elemental
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`Figure 1. Formation of a process impurity (M-12) due to the side reaction between an
`intermediate and a reagent.
`
`sulfur is formed, according to the proposed mechanism:
`
`þ 1=8S8 ðsolidÞ
`
` 3
`
`
`
`þ Hþ!HSO
`
`S2O2
`
`3
`
`Elemental sulfur could not be eliminated by the subsequent cleaning steps; as
`a consequence, it appeared in the final product as process impurities.
`
`Process Impurities Originating from By-Products of the Synthesis
`
`The selectivity of a chemical reaction is rarely 100%, and side-reactions are
`common during the synthesis of drug substances. By-products from the side-
`reactions are among the most common process impurities in drugs. By-pro-
`ducts can be formed through a variety of side reactions, such as incomplete
`reaction, overreaction,
`isomerization, dimerization,
`rearrangement, or
`unwanted reactions between starting materials or
`intermediates with
`chemical reagents (e.g., as shown in Figure 1) or catalysts.[35] There are
`numerous case studies in identification and control of by-product related
`process impurities in the literature.[35 – 43] The following three examples
`demonstrate typical formation pathways of by-products of drug substances.
`Proudfoot et al.[43] reported that isomeric by-products are formed in the
`course of the synthesis of nevirapine ring systems. The mechanism of
`formation of these isomeric impurities is through a so called Smiles
`rearrangement. Horvath et al.[44,45] reported a case where an over-reaction
`product, an epimeric impurity and a dimeric impurity were formed
`17a-ethinyl-17-hydroxy
`during
`the
`synthesis
`of
`steroids. During
`preparation of LY297802 tartrate, Olsen and Baertschi[46] observed two
`by-products in the step from intermediate I to intermediate II due to
`elimination of HCl and hydroxylation, as shown in Figure 2. These
`by-products were able to undergo the next reaction in the synthesis in the
`same way as
`the desired intermediate,
`to form process
`impurities,
`Impurity 1 and Impurity 2.
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`F. Qiu and D. L. Norwood
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`Figure 2. By-product impurities formed through side reactions during preparation of
`the drug substance LY297802 (adapted from ref. 46).
`
`Process Impurities Originating from Solvents
`
`Residual solvents are organic volatile impurities in drug substances and drug
`products which are used or produced in the manufacturing process of the drug
`substance (e.g., solvents for chemical reaction, separation, and crystalliza-
`tion), excipients, and drug products (e,g., solvents for wet granulation and
`coating). Residual solvents are expected impurities; therefore, identification
`of residual solvents is relatively straightforward. Recent progress in analysis
`of residual solvents was reviewed by Camarasu et al.[15]
`
`Impurities Originating from Degradation of the Drug Substance
`
`Degradation of the drug substance is one of the main sources of impurities in
`both bulk drug and formulated product. Degradation of the drug substance is
`caused by chemical instability of the drug substance under the conditions (e.g.,
`heat, humidity, solvent, pH, light, etc.) of manufacturing, isolation, purifi-
`cation, drying, storage, transportation, and interactions with other chemical
`entities in the formulation (e.g., excipients and coating materials). Chemical
`stability is an inherent property of a drug substance and is a reflection of
`the chemical properties of all functional groups in the drug molecule. In the
`following sections, various impurities originating from degradations of drug
`substances are discussed based on their formation mechanisms.
`
`Impurities Formed from Hydrolysis of the Drug Substance
`
`Hydrolysis of the drug substance is a chemical reaction between the drug
`substance and water. Obviously, the presence of water is a prerequisite for
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`the hydrolysis of the drug substance. In a drug substance or drug product,
`water can be introduced as residual water from the manufacturing process
`of the drug or excipient, or absorbed from the environment. Most hydrolytic
`degradations are catalyzed by both acid and base. In some cases, it can
`follow other mechanisms as well. For example, it was reported that metal
`ions can catalyze the hydrolysis of acrylate esters and amides.[47]
`A variety of functional groups are subject to hydrolysis under acidic or
`basic conditions.[50] Common functional groups that are susceptible to
`hydrolysis and some real world examples are summarized in Table 5.
`Depending on the structure of the drug substance, hydrolysis can either
`break the drug molecule into two pieces or break a cyclic structure (e.g., in
`lactones and lactams) into a linear structure. Esters and amides (including
`cyclic esters and amides) are the most common functional groups in drugs
`that are susceptible to hydrolysis. The hydrolysis of esters and amides is
`mostly catalyzed by acid and base. In most cases, drugs with these functional
`groups give the same hydrolytic degradation products under acidic or basic
`conditions. Sometimes, different hydrolytic degradation pathways can be
`observed. For example, penicillin undergoes different hydrolytic pathways
`under acidic and basic conditions to give two different primary hydrolytic
`degradation products, i.e., penicillenic acid under acidic condition, and peni-
`cilloic acid under basic condition, respectively.[58] Hydrolysis of carbonyl-
`containing functional groups, such as esters, amides, and carbamates, etc.,
`usually takes place at the acyl-O(N) bond. Exceptions can be found where
`the alkyl-O(N) bond cleaves.[78] In this hydrolytic pathway, the alkyl groups
`are usually those functional groups that can stabilize the carbonium ion
`formed by alkyl-O(N) cleavage, such as a tertiary carbon or a benzyl group.
`Hydrolysis of the drug substance is often followed by further degradation.
`Primary hydrolytic degradation products can further undergo dehydration,
`decarboxylation, cyclization, or rearrangement, etc., to form the final degra-
`dation products.[57,78]
`
`Impurities Formed from Oxidative Degradation of the Drug Substance
`
`In organic chemistry, oxidation is a class of chemical reactions that leads to an
`increase in oxidation state of an element. In the pharmaceutical industry,
`oxidative degradation is confined to chemical reactions of drug substances
`that increase the oxidation state of C and, in some cases, N, S, P, etc., via
`addition of oxygen or abstraction of hydrogen. Autoxidation is the reaction
`of organic compounds with elemental oxygen under mild conditions (e.g.,
`drug storage conditions). Autoxidation involves ground state elemental
`oxygen; thus, under normal storage conditions, it is the most important
`oxidative degradation pathway of drugs. Autoxidation is a free radical
`reaction;
`typically it requires a free radical
`initiator to start
`the chain
`reaction. Autoxidation initiators are often trace level impurities in the drug
`substance or excipients, such as peroxide or metal ions. These impurities
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`Table 5. Functional groups susceptible to hydrolysis in drugs
`
`Functional group
`
`Basic structure
`
`Example drugs
`
`Ester
`
`Lactone
`
`Amide
`
`Lactam
`
`Imide
`
`Imine
`
`Carbamic ester
`
`Phosphate
`
`Ether
`Thioether
`Nitrile
`
`Acetal/ketal
`
`Halides
`Sulfonamide
`
`Aspirin,[48] cisatracurium besylate,[51]
`nicergolin,[52] lovastatin,[53] and
`atropine[68]
`
`Lovastatin[53] and warfarin[69]
`
`Acetaminophen,[49] indinavir,[54] and
`indomethacin[55]
`
`Amoxicillin,[56,57] penicillin,[58] cepha-
`losporin,[58] bromazepam,[67]
`diazepam[70]
`
`Phenobarbital[59]
`
`Adinazolam,[60] famotidine,[61]
`diazepam,[70]
`
`Benzimidazole anthelmintics,[62] zol-
`mitriptan,[63] loratadine[77]
`
`Rivastigmine,[64] triamcinolone aceto-
`nide 21-phosphate[74]
`
`Diphenhydramine hydrochloride,[65]
`duloxetine[79]
`Penicillin[58]
`Danazol,[66] cimetidine[71]
`Erythromycin,[72] ECyd acetal deriva-
`tives,[73] triamcinolone[75]
`
`Chlorambucil[76]
`Sulfamethazine[78]
`
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`can be brought into the drug product from the manufacturing process or
`formed from degradation of formulation components (e.g., PEG and PVP)
`in the drug product. Autoxidation can be self initiated because the ground
`state oxygen molecule is a biradical; however, this reaction is expected to
`be slow because the ground state of the majority of organic molecules is
`singlet; thus the reaction is spin-forbidden. A typical autoxidation mechanism
`is shown below:
`
`†
`
`R
`
`ROO
`
`2ROO
`
`RH þ ln
`† !R† þ lnHðIntiation StepÞ
`
`† þ O2!ROOH
`† ðPropagation StepÞ
`† þ RH!ROOH þ R
`†!Non-radical moleculeðTermination StepÞ
`† þ R
`†!Non-radical moleculeðTermination StepÞ
`† þ R
`†!Non-radical moleculeðTermination StepÞ
`
`ROO
`
`R
`
`The non-radical molecules formed in the propagation and termination steps are
`likely to appear in the drug substance or drug product as impurities.
`Autoxidation can be catalyzed by heavy metals. These metals usually
`have two readily accessible oxidation states differing by one unit (e.g.,
`þ
`Cu2þ/Cu
`and Fe3þ/Fe2þ
`). Peroxides react with both the upper and lower
`oxidation states of these metals to generate more free radicals.[80] As a
`result, metal ions can greatly enhance the effectiveness of peroxides as
`initiators.
`
`Fe2 þ ROOH!FeðOHÞ2þ þ OR
`FeðOHÞ2þ þ ROOH!Fe2þ þ HOH þ ROO
`
`†
`
`†
`
`Drug substance oxidative degradation can also occur through an electron
`transfer mechanism to form reactive anions or cations. Amines, sulfides, and
`phenols are susceptible for electron transfer oxidation to give N-oxides,
`hydroxylamine, sulfones, sulfoxides, etc.[84] C55C double bonds may react
`with hydrogen peroxide and peroxy acid to form epoxides via a similar
`ionic mechanism.[81] In addition, heavy metals such as Fe3þ
`and Cu2þ
`can
`also be directly involved in the oxidation of drug substances through an
`electron transfer mechanism.
`To some extent, the susceptibility of a drug substance to oxidative degra-
`dation can be predicted from the functional groups in its chemical structure.
`Functional groups with a labile hydrogen, such as benzylic carbon, allylic
`carbon, tertiary carbon, or a-positions of a hetero atom[100] are often suscep-
`tible to oxidation to form hydroperoxide, hydroxide, or ketone.[84] Functional
`groups without a labile hydrogen, such as a tertiary amine or thioether, can
`undergo oxidative degradation to form N-oxide[86] or sulfoxide.[88] Common
`functional groups in drugs that are sensitive to oxidation and examples of
`drugs that contain these functional groups are summarized in Table 6.
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`Table 6. Functional groups susceptible to oxidative degradation in drugs
`
`Functional group
`
`Basic structure
`
`Example drugs
`
`Benzyl
`
`Ph-CH2-
`
`-CH55CH-CH2-
`
`C55C
`Ar-OH
`R-OH
`0
`R-O-R
`0
`R-S-R
`
`Allylic
`
`Tertiary C
`
`Olefins
`Phenol
`Alcohol
`Ether
`Thioether
`
`Tertiary Amine
`
`Primary/secondary
`amine
`
`Tipranavir[83] methoxamine
`hydrochloride,[89] impipramine
`hydrochloride[90]
`Tetrazepam,[82] Reserpine,[85]
`
`Tipranavir[83]
`
`L-tryptophan[87]
`Epinephrine[91]
`Lovastatin[92]
`Ragaglitazar[98]
`Pergolidemesylate,[93] fluphena-
`zine enanthate,[94] tipredane[99]
`
`Pipamperone[86], dibucaine
`hydrochloride,[96] raloxifene
`hydrochloride[97]
`Brinzolamide[95]
`
`Abstraction of protons from functional groups, such as CH2-CH2,
`CH-OH, or CH-NH in the drug molecule, to form unsaturated bonds C55C,
`C55O or C55N increases the oxidation state of C, and N;
`therefore,
`dehydrogenation is also an oxidative degradation. Dehydrogenation can
`occur in both solid state and in liquid phase. Typical types of dehydrogenation
`include aromatization, conversion of alcohols to ketones or aldehydes,
`conversion of amine to imine or nitriles, etc. For example, L-1,4-cyclohexa-
`diene-1-alanine hydrate degrades to L-phenylalanine in solid state through
`dehydrogenation;[101] nifedipine can convert to dehydrogenated degradation
`products in solution within a few hours under photo irradiation.[102]
`
`Impurities Formed from Photolytic Degradation of the Drug Substance
`
`When exposed to light, many types of organic molecules can undergo photo-
`chemical reactions and form photolytic degradation products.[103] There are
`two main types of photochemical reactions that are relevant to degradations
`of drugs. The first type is a non-oxidative photochemical reaction, which
`includes light induced isomerization, cyclization, dimerization, rearrange-
`ment, hydrolysis, decarboxylation, and homolytic cleavage of X-C hetero
`bonds, such as halogen bond, ether bond, N-alkyl bond in amine (dealkylation
`irradiation, cis-/trans
`or deamination), SO2-C bond, etc. Under
`light
`
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`Identification of Pharmaceutical Impurities
`
`889
`
`isomerization and dimerization are very common in drug molecules that
`contain a C55C double bond.[104 – 106,108] Other types of photochemical degra-
`dations of drugs, such as light induced hydrolysis of halogen,[107] rearrange-
`ment,[109,111] dealkylation,[110] and cyclization[112] were also reported.
`Photo-oxidative degradation is the other type of photolytic degradation of
`drugs. Photo-oxidative degradation products can be formed from either a
`triplet oxygen (3O2) or a singlet oxygen (1O2) mechanism, depending on the
`electronic state of the oxygen molecule. The former requires sensitization
`and formation of a free radical of the drug molecule, which then reacts with
`a triplet oxygen molecule to form a peroxide. Singlet oxygen molecules
`react with unsaturated bonds such as alkenes, dienes, polynuclear aromatic
`hydrocarbons,
`etc.,
`to
`form photo-oxidative
`degradation
`products.
`Depending on the structure of the drug molecule, singlet oxygen oxidation
`
`can take place through three pathways, i.e., [2 þ 2] cycloaddition, [4 þ 2]
`
`cycloaddition, and “ene” reaction, as illustrated in Figure 3. For example,
`the C55C double bond in thiothixene undergoes photo-oxidative degradation
`through a [2 þ 2] cycloaddition mechanism.[113] Heterocyclic unsaturated
`
`rings (e.g., thiazole, oxazole, and pyrazole, etc.) are known to be sensitive
`toward light irradiation. For instance, an oxazole containing drug undergoes
`photolytic
`degradation
`through
`a
`proposed
`cycloaddition
`
`[4 þ 2]
`
`Figure 3. Singlet O2 oxidative degradation pathways of drugs that contain unsatu-
`rated bonds.
`
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`890
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`F. Qiu and D. L. Norwood
`
`mechanism.[114] An example of an “ene” reaction was discussed by
`Boccardi.[115] Usually, the peroxides formed through these reactions are
`not stable and are just intermediates to the final photolytic degradation
`products.
`Although rare, photolytic degradation of drugs can also take place
`through a reduction mechanism. A typical example of such degradation is
`the reduction of the nitro group to a nitroso group in nifedipine.[102]
`
`Impurities Formed through Isomerization and Oligomerization of the
`Drug Substance
`
`Isomerization and oligomerization are common degradation pathways
`of drugs. Formation of
`isomers and oligomers of a drug substance
`can occur through a variety of mechanisms; therefore, it is worthwhile
`to have a separate overview of the isomerization and oligomerization of
`drugs.
`Commonly seen isomerizations include photo-induced cis/trans isomer-
`ization of drugs with a C55C double bond,[104,105] other asymmetric double
`bonds (e.g., C55N-OH and C55N-NH2)[116 – 118] and racemization or epimer-
`ization of drug substance with chiral centers. Drug molecules with chiral
`light,[119]
`centers can undergo racemization and epimerization under
`heat,[119] acidic,[120] and basic[121] conditions. Aso et al.[122] found that
`gamma cyclodextrins can facilitate the epimerization and racemization of car-
`benicillin. Rearrangement
`is also a source of isomeric impurities. For
`example,
`fenoprofen calcium can degrade to a mixture of
`isomeric
`biphenyls via a photo-Fries rearrangement.[111] In the presence of water,
`FK506 can readily epimerize to form tautomeric impurities.[123]
`In addition to the photo-cycloaddition of alkenes discussed above,
`other
`types of dimerization or oligomerization can also occur. For
`example,
`thiols can dimerize under oxidative conditions to form disul-
`fides;[124] indoles can dimerize under acidic conditions;[125] nalidixic acid
`dimerizes through a thermo-decarboxylation pathway,[126] and losartan
`dimerization is induced by moisture and acid.[127] It was reported that for-
`maldehyde, which is a potential impurity in some excipients (e.g., PEG
`and PVP), can react with primary or secondary amin