Opiant Exhibit 2301
`Nalox-1 Pharmaceuticals, LLC v. Opiant Pharmaceuticals, Inc.
`IPR2019-00685, IPR2019-00688, IPR2019-00694
`Page 1
`
`

`

`Stability of Drugs
`and Dosage Forms
`
`Opiant Exhibit 2301
`Nalox-1 Pharmaceuticals, LLC v. Opiant Pharmaceuticals, Inc.
`IPR2019-00685, IPR2019-00688, IPR2019-00694
`Page 2
`
`

`

`Stability of Drugs
`and Dosage Forms
`
`Sumie Yoshioka
`National Institute of Health Sciences
`Tokyo, Japan
`
`and
`Valentino J. Stella
`The University of Kansas
`Lawrence, Kansas
`
`Kluwer Academic Publishers
`New York, Boston, Dordrecht, London, Moscow
`
`Opiant Exhibit 2301
`Nalox-1 Pharmaceuticals, LLC v. Opiant Pharmaceuticals, Inc.
`IPR2019-00685, IPR2019-00688, IPR2019-00694
`Page 3
`
`

`

`eBook ISBN:
`Print ISBN:
`
`0-306-46829-8
`0-306-46404-7
`
`'2002 Kluwer Academic Publishers
`New York, Boston, Dordrecht, London, Moscow
`
`All rights reserved
`
`No part of this eBook may be reproduced or transmitted in any form or by any means, electronic,
`mechanical, recording, or otherwise, without written consent from the Publisher
`
`Created in the United States of America
`
`Visit Kluwer Online at:
`and Kluwer’s eBookstore at:
`
`http://www.kluweronline.com
`http://www.ebooks.kluweronline.com
`
`Opiant Exhibit 2301
`Nalox-1 Pharmaceuticals, LLC v. Opiant Pharmaceuticals, Inc.
`IPR2019-00685, IPR2019-00688, IPR2019-00694
`Page 4
`
`

`

`Contents
`
`1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`
`2. Chemical Stability of Drug Substances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`2.1. Pathwaysof Chemical Degradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`2.1.1. Hydrolysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`2.1.1.1. Esters
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`2.1.1.2. Amides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`2.1.1.3. Barbiturates, Hydantoins, and Imides . . . . . . . . . . . . . . .
`2.1.1.4. Schiff Base and Other Reactions Involving
`Carbon-Nitrogen Bond Cleavage . . . . . . . . . . . . . . . . . . .
`2.1.1.5. OtherHydrolysisReactions . . . . . . . . . . . . . . . . . . . . . . . . .
`2.1.2. Dehydration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`2.1.3.
`IsomerizationandRacemization . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`2.1.4. Decarboxylation and Elimination . . . . . . . . . . . . . . . . . . . . . . . . . . .
`2.1.5. Oxidation
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`2.1.6.
`Photodegradation
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`2.1.7. Drug-Excipientand Drug-Drug Interactions . . . . . . . . . . . . . . . . .
`2.1.7.1. Reactions of Bisulfite, an Antioxidant
`. . . . . . . . . . . .
`2.1.7.2. Reaction of Amines with Reducing Sugars . . . . . . . . . .
`2.1.7.3. Transesterification Reactions . . . . . . . . . . . . . . . . . . . . .
`2.2. Factors Affecting Chemical Stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`2.2.1. Basic Kinetic Principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`2.2.2. The Role of MolecularStructure . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`2.2.3. Rate Equations and Kinetic Models . . . . . . . . . . . . . . . . . . . . . . . . . .
`2.2.3.1. Kinetic Models to Describe Drug Degradation
`inSolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`2.2.3.2. Kinetic Models Describing Chemical Drug
`Degradationin the Solid State
`. . . . . . . . . . . . . . . . . . . . .
`2.2.3.3. Calculationof Rate Constants byFittingtoKineticModels
`2.2.4. Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`2.2.4.1. General Principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`
`1
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`3
`4
`5
`5
`10
`12
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`15
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`28
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`34
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`52
`61
`61
`61
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`vii
`
`Opiant Exhibit 2301
`Nalox-1 Pharmaceuticals, LLC v. Opiant Pharmaceuticals, Inc.
`IPR2019-00685, IPR2019-00688, IPR2019-00694
`Page 5
`
`

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`viii
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`Contents
`
`2.2.4.2. Quantitation of the Temperature Dependency of
`Degradation Rate Constants . . . . . . . . . . . . . . . . . . . . . . . . . .
`2.2.4.3. Stability in Frozen Solutions . . . . . . . . . . . . . . . . . . . . . . . . . .
`pH and pH-RateProfiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`2.2.5.1. V-Type and U-Type pH-RateProfiles. . . . . . . . . . . . . . . . . .
`2.2.5.2. pH-Rate Profiles with Inflection Points Due to the
`Presence of One or More Ionized Groups
`. . . . . . . . . .
`2.2.5.3. Bell-Shaped pH-Rate Profiles Due to Ionization of
`Multiple Groups or Change in Rate-
`Determining Steps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`2.2.5.4. Miscellaneous pH-Rate Profiles
`. . . . . . . . . . . . . . . . . . . . .
`2.2.6. Buffer, General Acid-Base, andNucleophilic-Electrophilic
`97
`Catalysis
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`99
`Ionic Strength (Primary Salt Effects)
`. . . . . . . . . . . . . . . . . . . . . . . .
`2.2.7.
`2.2.8. Dielectric Constant of Solvents
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
`2.2.9. Oxygen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
`2.2.10. Light
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
`2.2.11. Crystalline State and Polymorphism in Solid Drugs . . . . . . . . . . . . . .
`107
`2.2.12. Effect of Moisture and Humidity on Solid and Semisolid Drugs . . . 108
`2.2.13. Excipients . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
`2.2.13.1. Effect of the Amount of Moisture Present in
`Excipients
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
`2.2.13.2. Effect of the Physical State of Water Molecules in
`Excipients . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
`2.2.13.3. Effect of the Mobility of Water Molecules in
`Excipients on Drug Degradation . . . . . . . . . . . . . . . . . . . . . . .
`117
`2.2.13.4. Other Properties of Excipients . . . . . . . . . . . . . . . . . . . . . .
`120
`2.2.14. Miscellaneous Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
`2.3. Stabilization of Drug Substances against Chemical Degradation
`. . . . . . . . . . 125
`2.3.1. Stabilization by Modification of Molecular Structure of
`Drug Substances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
`2.3.2. StabilizationbyComplex Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126
`2.3.3.
`Stabilization by the Formation of Inclusion Complexes
`with Cyclodextrins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128
`Stabilization by Incorporation into Liposomes, Micelles,
`or Emulsions
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
`Addition of Stabilizers Such as Antioxidants and
`Stabilization through the Use of Packaging
`. . . . . . . . . . . . . . . . . . . .
`
`2.2.5.
`
`2.3.4.
`
`2.3.5.
`
`62
`78
`80
`82
`
`84
`
`94
`96
`
`135
`
`139
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`3. Physical Stability of Drug Substances
`139
`3.1. Physical Degradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`139
`3.1.1. Crystallization of Amorphous Drugs . . . . . . . . . . . . . . . . . . . . . . . . .
`141
`3.1.2. Transitions in Crystalline States
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`3.1.3. Formation
`and Growth of Crystals
`. . . . . . . . . . . . . . . . . . . . . . . . . ..142
`3.1.4. Vapor-Phase Transfers Including Sublimation . . . . . . . . . . . . . . . . .
`143
`
`Opiant Exhibit 2301
`Nalox-1 Pharmaceuticals, LLC v. Opiant Pharmaceuticals, Inc.
`IPR2019-00685, IPR2019-00688, IPR2019-00694
`Page 6
`
`

`

`Contents
`
`3.1.5. Moisture Adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`3.2. Factors Affecting Physical Stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`3.3. Kinetics of Solid-Phase Transitions
`
`4.
`
`Stability of Dosage Forms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`4.1. Preformulation and FormulationStabilityStudies . . . . . . . . . . . . . . . . . . . . . .
`4.1.1. Methods for Detecting Chemical and Physical Degradation . . . . .
`4.1.1.1. Thermal Analysis
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`4.1.1.2. Diffuse Reflectance Spectroscopy . . . . . . . . . . . . . . . . .
`4.1.1.3. Miscellaneous Methods . . . . . . . . . . . . . . . . . . . . . . . . . .
`Factorial Analysis
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`4.1.2.
`4.2. Functional Changes in Dosage Forms with Time . . . . . . . . . . . . . . . . . . . .
`4.2.1. Changes in Mechanical Strength . . . . . . . . . . . . . . . . . . . . . . . . . . .
`Changes in Drug Dissolution from Tablets and Capsules . . . .
`4.2.2.
`4.2.2.1
`Effectof Formulation on Changes in Dissolution . . . .
`4.2.2.2. Changes in Drug Release from Coated Dosage Forms .
`4.2.2.3. Changes in Capsule Shells with Time and
`Storage Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`4.2.2.4. Prediction of Changes in Dissolution . . . . . . . . . . . . . . . . .
`4.2.3. Changes in Melting Time of Suppositories . . . . . . . . . . . . . . . . . . . .
`Changes in Drug Release Rate from Polymeric Matrix
`4.2.4.
`Dosage Forms, Including Microspheres . . . . . . . . . . . . . . . . . . . . . .
`4.2.5. Drug Leakage from Liposomes . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`4.2.6. Aggregation inEmulsions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`4.2.7. Moisture Adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`4.2.8. Discoloration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`4.3. Effect of Packaging on Stability ofDrug Products . . . . . . . . . . . . . . . . . . . . . .
`4.3.1. MoisturePenetration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`4.3.2. Adsorption onto and Absorption into Containers and
`. . . . . . .
`Transfer of Container Components into Pharmaceuticals
`4.4. Estimation of the Shelf Life (Expiration Period) of Drug Products . . . .
`4.4.1. ExtrapolationfromReal-TimeData . . . . . . . . . . . . . . . . . . . . . . . . . . .
`4.4.2.
`Shelf-Life Estimation from Temperature-Accelerated Studies . .
`4.4.2.1. Experimental Design of Accelerated Testing . . . . . . . . .
`4.4.2.2. Estimation of Shelf Life Using Accelerated-Test
`Data at a Single Levelof Temperature . . . . . . . . . . . . . . .
`Estimation of Shelf Life under Temperature-Fluctuating
`Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`
`4.4.3.
`
`5. Stabilityof Peptide andProtein Pharmaceuticals . . . . . . . . . . . . . . . . . . . . . . . . . . .
`5.1. Degradationof Peptide and Protein Pharmaceuticals . . . . . . . . . . . . . . . . . . . . .
`5.1.1. Chemical Degradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`5.1.1.1. Deamidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`5.1.1.2.
`Isomerization and Racemization . . . . . . . . . . . . . . . . . . . . .
`
`ix
`
`144
`144
`145
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`151
`151
`151
`152
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`156
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`159
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`160
`162
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`167
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`168
`170
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`176
`178
`179
`180
`180
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`182
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`184
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`187
`187
`187
`188
`189
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`Opiant Exhibit 2301
`Nalox-1 Pharmaceuticals, LLC v. Opiant Pharmaceuticals, Inc.
`IPR2019-00685, IPR2019-00688, IPR2019-00694
`Page 7
`
`

`

`x
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`Contents
`
`5.1.1.3. Hydrolysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`5.1.1.4. Cross-Linking through Disulfide Bond Formation
`and Other Covalent Interactions . . . . . . . . . . . . . . . . . . . .
`5.1.1.5. Oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`5.1.2. Physical Degradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`5.1.3. Degradation inPeptide andProteinFormulations . . . . . . . . . . . . . . .
`5.2. Factors AffectingtheDegradationof Peptide andProteinDrugs . . . . . . . . .
`5.2.1. Moisture Content and Molecular Mobility . . . . . . . . . . . . . . . . . . . . .
`5.2.2. The Role of Excipients . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`5.3. Degradation Kinetics of Peptide andProteinPharmaceuticals . . . . . . . . . . .
`5.3.1. Quantitative Description of Peptide and Protein Degradation . . . .
`5.3.2. Temperature Dependence of the Degradation Rate of Peptide
`and Protein Drugs
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`
`190
`
`190
`192
`193
`194
`194
`194
`196
`197
`197
`
`199
`
`205
`
`6 . Regulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`6.1. ICH Harmonised Tripartite Guideline for Stability Testing of New Drug
`Substances and Products
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`ICH Harmonised Tripartite Guideline for Photostability Testing of
`New Drug Substances and Products
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`6.3 Major Concerns Raised by the EU, the United States, and Japan at the
`International Conference on Harmonisation of Technical Requirements
`223
`for Registration of Pharmaceuticals for Human Use . . . . . . . . . . . . . . . . . .
`223
`6.3.1. Storage Conditions for Stability Testing . . . . . . . . . . . . . . . . . . . . . .
`6.3.2. Photostability Testing
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..224
`225
`6.3.3. Bracketing and Matrixing
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`
`6.2
`
`205
`
`217
`
`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`References
`Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
`
`227
`263
`
`Opiant Exhibit 2301
`Nalox-1 Pharmaceuticals, LLC v. Opiant Pharmaceuticals, Inc.
`IPR2019-00685, IPR2019-00688, IPR2019-00694
`Page 8
`
`

`

`Chapter 2
`
`Chemical Stability of Drug Substances
`
`The most easily understood and most studied form of drug instability is the loss of drug
`through a chemical reaction resulting in a reduction of potency. Loss of potency is a
`well-recognized cause of poor product quality.
`In this chapter, the quantitation of chemical drug loss is discussed and analyzed.
`However, loss of drug potency per se by various pathways is only one of many possible
`reasons for quantitating drug loss. Identification of the product(s) formed provides a better
`understanding of the mechanism(s) of these chemical reactions as well as other valuable
`information. Other reasons for quantitating drug loss include the following.
`
`1. The drug may degrade to a toxic substance. Therefore, it is important to determine
`not only how much drug is lost with time but also what are its degradants. In some cases,
`the degradants may be of known toxicity. For example, the drug pralidoxime degrades via
`two parallel, pH-sensitive pathways. Under basic pH conditions, the toxic product cyanide
`is formed (Scheme 1).1 For other drugs, the toxicity of degradants is initially unknown. For
`example, a degradant of tetracycline is epianhydrotetracycline, known to cause Fanconi
`syndrome (Scheme 2). 2,3
`Sometimes, reactive intermediates are formed that are known or suspected to be toxic.
`For example, penicillins rearrange under acidic pH conditions to penicillenic acids, which
`are suspected to contribute to the allergenicity of penicillins (Scheme 3).4 Gosselin et al.
`
`Scheme 1. Parallel degradation pathways for pralidoxime leading to cyanide formation under basic pH conditions.
`(Reproduced from Ref. 1 with permission.)
`
`3
`
`Opiant Exhibit 2301
`Nalox-1 Pharmaceuticals, LLC v. Opiant Pharmaceuticals, Inc.
`IPR2019-00685, IPR2019-00688, IPR2019-00694
`Page 9
`
`

`

`4
`
`Chapter 2 (cid:149) Chemical Stability of Drug Substances
`
`Scheme 2. Dehydration and epimerization of tetracycline, leading to formation of epianhydrotetracycline, known
`to be associated with Fanconi syndrome. (Reproduced from Refs. 2 and 3 with permission.)
`
`proposed a protecting group for phosphates that produces episulfide, a sulfur analog of
`ethylene oxide of unknown toxicity.5
`2. Degradation of the drug may make the product esthetically unacceptable. Products
`are presumed to be adulterated if significant changes in, for instance, color or odor have
`occurred with time. For example, epinephirine is oxidized to adrenochrome (Scheme 4), a
`highly colored red material. Any epinephrine-containing product that develops a significant
`pink tinge is usually considered adulterated.
`Recently, one of the authors was asked to comment on the acceptability of a drug
`substance that degraded to volatile, odor-producing, sulfur-containing degradant. Even
`minor degradation of the drug produced an unacceptable odor. This was of specific concern
`because one intended route of drug administration was via a nasal spray.
`3. Even though a drug may be stabilized in its intended formulation, the formulator
`must show that the drug is also stable under the pH conditions found in the gastrointestinal
`tract, if the drug is intended for oral use. Most drug substances are fairly stable at the neutral
`pH values found in the small intestine (disregarding enzymatic degradation) but can be
`unstable at pH values found in the stomach. Examples of drugs that are very acid-labile are
`various penicillins,4,6 erythromycin and some of its analogs,7 and the 2·,3·-dideoxypurine
`nucleoside anti-AIDS drugs.8 Knowledge of the stability of a drug in the pH range of 1-2
`at 37(cid:176)C is important in the design of potentially acid-labile drugs and their dosage forms.
`
`2. 1. Pathways of Chemical Degradation
`Drug substances used as pharmaceuticals have diverse molecular structures and are,
`therefore, susceptible to many and variable degradation pathways. Possible degradation
`pathways include hydrolysis, dehydration, isomerization and racemization, elimination,
`oxidation, photodegradation, and complex interactions with excipients and other drugs. It
`would be very useful if we could predict the chemical instability of a drug based on its
`molecular structure. This would help both in the design of stability studies and, at the earliest
`
`Scheme 3. Representative example of the rearrangement of penicillins to their penicillenic acids under acidic pH
`conditions. (Reproduced from Ref. 4 with permission.)
`
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`IPR2019-00685, IPR2019-00688, IPR2019-00694
`Page 10
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`2.1.
`
`(cid:149) Pathways of Chemical Degradation
`
`5
`
`Scheme 4. Oxidation of epinephrine to the highly colored adrenochrome.
`
`stages of drug development, in identifying ways in which problematic drugs could be
`formulated to minimize chemical degradation. The immense chemical and pharmaceutical
`literature is probably underutilized as a source of such information. Expert systems are also
`being developed for predicting stability.
`Below, the major-degradation pathways in relation to molecular structure are discussed
`and examples provided.
`
`2.1.1. Hydrolysis
`For most parenteral products, the drug comes into contact with water and, even in solid
`dosage forms, moisture is often present, albeit in low amounts. Accordingly, hydrolysis is
`one of the most common reactions seen with pharmaceuticals. Many researchers have
`reported extensively on the hydrolysis of drug substances. In the 1950s, elegant studies,
`especially considering the lack of high-throughput analytical techniques, concerning the
`aspirin,11,12
`chloramphenicol,13-15
`atropine,16-18
`and methyl-
`hydrolysis of procaine,9,10
`phenidate19 were reported. Hydrolysis is often the main degradation pathway for drug
`substances having ester and amide functional groups within their structure.
`
`2.1.1.1. Esters
`Many drug substances contain an ester bond. Traditional esters are those formed
`between a carboxylic acid and various alcohols. Other esters, however, include those formed
`between carbamic, sulfonic, and sulfamic acids and various alcohols. These ester compounds
`are primarily hydrolyzed through nucleophilic attack of hydroxide ion or water at the ester,
`as shown in Scheme 5 for the case of a carboxylic acid ester.
`The degradation rate depends on the substituents R1 and R2, in that electron-withdraw-
`ing groups enhance hydrolysis whereas electron-donating groups inhibit hydrolysis. As
`shown in Table 1, substituted benzoates having an electron-withdrawing group, such as a
`nitro group, in the para position of the phenyl ring (R1) exhibit higher decomposition rates
`than the unsubstituted benzoate. On the other hand, the decomposition rate decreases with
`increasing electron-donating effect of the alkyl group (in the alcohol portion of the ester
`(R2)) (e.g., it decreases in the order methyl > ethyl > n-propyl). Replacing a hydrogen atom
`
`Scheme 5. Hydrolysis of a carboxylic acid ester.
`
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`6
`
`Chapter 2 (cid:149) Chemical Stability of Drug Substances
`
`Table 1. Second-Order Rate Constants for the Hydrolysis of Various Benzoic Acid Esters
`through Nucleophilic Attack of Hydroxide Ion,
`in Accordance with Scheme 5 (R1 = R·
`Second-order rate constant
`K(cid:150) OH
`(x 10-4 M-1 s-1)a
`6.08
`1.98
`1.67
`0.319
`33.6
`12.4
`2.65
`12.1
`19.1
`6.51
`5.11
`1.21
`103
`276
`98.8
`76.0
`19.6
`1140
`
`R’
`H
`H
`H
`H
`H
`H
`CH3
`F
`C1
`C1
`C1
`C1
`C1
`NO2
`NO2
`NO2
`NO2
`NO2
`
`R2
`
`CH3
`C2H5
`n-C3H7
`iso-C3H7
`Phenyl
`C2H4C1
`CH3
`CH3
`CH3
`C2H5
`n-C3H7
`iso-C3H7
`Phenyl
`CH3
`C2H5
`n-C3H7
`iso-C3H7
`Phenyl
`
`aIn 50% acetonitrile-0.02M phosphate buffer solution; 25(cid:176)C.
`
`with an electron-withdrawing halogen such as chlorine, e.g., -C2H5 versus -C2H4C1, also
`increases the rate of decomposition.20
`Another way of viewing this reaction is by considering leaving-group ability. The
`mechanism of ester hydrolysis can be considered an addition/elimination reaction, the
`leaving group being R2OH. The rate of the elimination step will be determined in part by
`the ability of the leaving alcohol to sustain the buildup of negative charge on the oxygen
`atom. This will also be reflected in the pKa of the alcohol. For example, hydrolysis of phenyl
`benzoate is much faster than that of ethyl benzoate (Table 1) because the pKa values of
`ethanol and phenol are 18 and 10, respectively.
`Steric factors also play a role. Bulky groups on either R1 or R2 decrease the decompo-
`sition rate. For example, when an iso-propyl group is substituted for an n-propyl group on
`R2, the decomposition is five times slower (Table 1).
`Attack of hydroxide ion on an ester bond is also affected by the presence of neighboring
`charges. For example, the hydrolysis rates of all ester bonds within poly(butylene tartrate)
`are not equal; the ester bonds close to the negatively charged, terminal carboxylate group
`are less reactive toward hydroxide-ion attack than are the ester groups removed from the
`negatively charged carboxylate group.21
`
`2.1.1.1.a. Carboxylic Acid Esters of Pharmaceutical Relevance. Representative exam-
`ples of carboxylic acid esters that are susceptible to hydrolysis are shown in Fig. 1. These
`include ethylparaben,22
`benzocaine,10,23,24
`procaine?9-10
`oxathiin carboxanilide
`(NSC-
`
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`2.1.
`
`(cid:149) Pathways of Chemical Degradation
`
`7
`
`Figure 1. Representative examples of carboxylic acid esters of pharmaceutical interest, susceptible to hydrolysis.
`
`scopolamine,27 methylphenidate,19 meperidine,28
`atropine,16-18,26
`aspirin,11,12
`615985),25
`steroid esters such as hydrocortisone sodium succinate29,30 and methylprednisolone sodium
`succinate,31 and succinylcholine chloride.32,33 Cocaine has two ester bonds that hydrolyze
`to produce benzoylecgonine or ecgonine methyl ester, as shown in Scheme 6.34,35 It is said
`to undergo parallel pathways of degradation. Shown in all future reaction schemes are the
`primary reaction pathways. As such, these are not meant to be complete; that is, some
`compounds undergo other competing reactions.
`Based on the structures of these various esters, it can be readily seen that having
`information on the reactivity of one ester should provide valuable insight into that of a second
`
`Scheme 6. Parallel hydrolysis pathways for cocaine.
`
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`8
`
`Chapter 2 (cid:149) Chemical Stability of Drug Substances
`
`ester. For example, ethylparaben and benzocaine are very similar in structure; both have a
`para electron-donating group and both are ethyl esters. Therefore, information about the
`reactivity of one of them could be the basis for predicting the stability of the other. Similarly,
`ester group hydrolysis in atropine should be similar in rate and pH dependency to that in
`scopolamine. Is it not reasonable to expect the hydrolysis of methylprednisolone sodium
`succinate to be similar to that of hydrocortisone sodium succinate? Therefore, if one is
`presented with a new drug substance containing a hydrolyzable ester moiety, it should be
`possible, using appropriate literature examples of similar drugs, to make a good estimate of
`the sensitivity of the ester group to hydrolysis.
`Lactones, or cyclic esters, also undergo hydrolysis. As shown in Fig. 2, pilocarpine,36-38
`dalvastatin,39 warfarin,40,41 and camptothecin42 exhibit ring opening due to hydrolysis. Note
`that, unlike linear esters, lactones often exist in dynamic equilibrium with their carboxylic
`acid/carboxylate forms.
`Apparent rate constants for the hydrolysis of various carboxylic acid esters are shown
`in Table 2 for the comparison of their reactivities. As these values were obtained under
`different conditions of temperature, pH, ionic strength, and buffer species, they are for rough
`comparison only. Nevertheless, they do point out the role that structure plays in the relative
`reactivity of the ester bond.
`
`Figure 2. Representative lactones of pharmaceutical interest susceptible to hydrolysis. Note that, unlike esters,
`lactones often exist in dynamic equilibrium (pH dependent) with their carboxylate forms.
`
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`2.1.
`
`(cid:149) Pathways of Chemical Degradation
`
`9
`
`Table 2. Apparent Rate Constants for the Hydrolysis of Various Carboxylic Acid Esters
`pH
`Reference
`k (s-1)
`6.0 x l0-5 (25(cid:176)C)
`7.13
`42
`Camptothecin
`Aspirin
`3.7 x 10-6 (25(cid:176)C)
`6.90
`12
`Methylprednisolone sodium succinate 2.5 x l0-7 (25(cid:176)C)
`7.30
`31
`1.8 x l0-7 (25”C)
`6.92
`25
`Oxathiin carboxanilide
`5.7 x 10-8 (25(cid:176)C)
`9.2
`4
`Benzocaine
`4.2 x 10-8 (25(cid:176)C)
`9.16
`22
`Ethylparaben
`4.97 x 10-6 (30(cid:176)C)
`7.25
`34
`Cocaine
`5.0 x 10-5(400”C)
`8.00
`32
`Succinylcholine
`Procaine
`6 x 10-6 (40(cid:176)C)a
`8
`9
`Pilocarpine
`1.7 x 10-6 (40”C)a
`8
`36
`Atropine
`1.8 x 10-7 (40oC)
`7.01
`17
`3.2 x 10-6 (50”C)
`6.07
`19
`Methylphenidate
`9.0 x 10-6 (65.2”C)
`7.0
`29
`Hydrocortisone sodium succinate
`1 x 10-7 (25”C)b
`29
`1.8 x 10-7 (89.7”C)
`28
`
`Meperidine
`
`6.192
`
`aValue of k estimated from plots in the reference.
`bValue of k estimated using the reported value of the activation energy ( Ea ).
`
`2.1.1.1.b. Other Esters. Carbamic acid esters such as chlorphenesin carbamate43 and
`carmethizole,44 shown in Scheme 7, are known to undergo hydrolysis in strongly acidic and
`neutral-to-alkaline solutions, respectively. The two carbamate ester groups in carmethizole
`undergo hydrolysis at significantly different rates owing in large part to completely different
`mechanisms.44 The first carbamate group is cleaved by more of an elimination reaction via
`carbonium formation whereas the second carbamate linkage appears to hydrolyze via a
`normal hydrolysis mechanism.
`Cyclodisone,45 a sulfonic acid ester, and sulfamic acid 1,7-heptanediyl ester (NSC-
`329680),46 a sulfamic acid ester, have been reported to hydrolyze in the neutral-to-alkaline
`pH range (Scheme 8). Both hydrolyze via carbon-oxygen bond cleavage rather than
`sulfur(cid:150)oxygen bond cleavage.45,46
`
`Scheme 7. Representative carbamic acid esters of pharmaceutical relevance susceptible to hydrolysis.
`
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`10
`
`Chapter 2 (cid:149) Chemical Stability of Drug Substances
`
`Scheme 8. Representative sulfonic esters and sulfamic esters susceptible to hydrolysis.
`
`Phosphoric acid esters such as hydrocortisone disodium phosphate47,48 and echothio-
`phate iodide49 are known to hydrolyze (Scheme 9). Although nitric esters such as nitroglyc-
`erin50 and nicorandil51 undergo hydrolysis, nitroglycerin is relatively stable (Scheme 9).
`Phosphatidylcholine and phosphatidylethanolamine in intravenous lipid emulsion and aque-
`ous liposome dispersions have been reported to hydrolyze in the neutral pH range.52,53
`
`2.1.1.2. Amides
`Amide bonds are commonly found in drug molecules. Amide bonds are less susceptible
`to hydrolysis than ester bonds because the carbonyl carbon of the amide bond is less
`electrophilic (the carbon-to-nitrogen bond has considerable double bond character) and the
`leaving group, an amine, is a poorer leaving group (Scheme 10). Figure 3 shows the structure
`
`Scheme 9. Other esters of pharmaceutical relevance susceptible to hydrolysis.
`
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`2.1.
`
`(cid:149) Pathways of Chemical Degradation
`
`11
`
`Scheme 10. Hydrolysis of amides.
`
`sul-
`and
`indomethacin,56-59
`lincomycin,55
`chloramphenicol,13-15
`acetaminophen,54
`of
`facetamide,60 all of which are known to produce an amine and an acid through hydrolysis
`of their amide bonds; moricizine, a derivative of phenothiazine, which undergoes hydrolysis
`of its amide bonds followed by oxidation61; and HI-6, a bis(pyridimium)aldoxime having
`an amide bond, which exhibits fast hydrolysis in concentrated aqueous solutions owing to
`the acidifying effect of a strongly acidic oxime group.62
`β-Lactam antibiotics such as penicillins and cephalosporins, which are cyclic amides
`or lactams, undergo rapid ring opening due to hydrolysis. Ring opening of the β-lactam
`group has been
`reported
`for penams,
`such as, benzylpenicillin,63-64
`ampicillin,65
`amoxicillin,66 carbenicillin,67 phenethicillin,68 and methicillin69
`(Scheme 11), and for
`cephems, such as cephalothin70 cefadroxil,71-72 cephradine,70 and cefotaxime73-75
`(Scheme
`12). These drug substances have both a lactam and an amide bond in their molecular
`structure, the former being considerably more susceptible to hydrolysis. Cephalothin and
`cefotaxime are also acetoxy esters, and opening of their lactam ring competes with hydroly-
`sis of the ester bond. Decomposition products produced by hydrolysis of penam and cephem
`β-lactams are still reactive and undergo various side reactions. For example, condensation
`products were formed upon hydrolysis of cefaclor,76 and dimeric products were detected
`upon hydrolysis of loracarbef,77 as shown in Scheme 13, as well as of ampicillin.78
`Cycloserine, which can be considered a cyclic amide, undergoes opening of its isoxazolidone
`ring due to hydrolysis in acidic media,79 as shown in Scheme 14. Like loracarbef and
`ampicilllin, it also undergoes self-condensation.
`The reactivity of these amides tow