The vast majority of drugs are organic molecular entities. A clear understanding of the chemistry of drug degradation is essential to maintaining the stability, efficacy and safety of medicines throughout their shelf-life. This book examines various degradation pathways with an emphasis on the underlying chemical mechanisms. This approach is essential for degradant identification, formulation development, and manufacturing process improvement. Much of the book is devoted to relevant organic reactions which are reviewed and illustrated with examples. It finishes with a discussion of the strategies for rapid elucidation of drug degradants with regard to the current regulatory requirements and guidelines. This book will be a valuable resource for pharmaceutical and analytical scientists.
About the Author
Dr. Min Li is an Associate Director at Analytical Chemistry in Development and Supply - Supply Analytical Sciences Department of Merck & Co., Inc. He has led technical teams of senior-level scientists (Senior and Principal Scientists) for various analytical and pharmaceutical manufacturing process investigation and troubleshooting, impurity peak identification, study of drug degradation mechanisms, analytical method development, validation, specification setting, and support for new drug filing. He graduated from Fudan University and received his Ph.D. in Organic Chemistry from Johns Hopkins University, followed by a postdoctoral research at University of Illinois at Chicago in medicinal chemistry. Dr. Li was a Principal Scientist at Roche (1995 - 1998), a Scientific Fellow at Merck (1998 - 2005), and a manager at Schering-Plough (2005 - 2010). He was president of Sino-American Pharmaceutical Professionals Association (SAPA) between 2003 and 2004. Dr. Li is the first/primary author of more than 40 publications in a multi-disciplinary arena including organic, medicinal, bioconjugate, and analytical chemistry, as well as mass spectrometry. He has been invited to present in numerous international scientific meetings and conferences.
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Organic Chemistry of Drug Degradation
By Min Li
The Royal Society of ChemistryCopyright © 2012 Min Li
All rights reserved.
1.1 Drug Impurities, Degradants and the Importance of Understanding Drug Degradation Chemistry
A drug impurity is anything that is not the drug substance (or active pharmaceutical ingredient, API) or an excipient according to the definition by the US Food and Drug Administration (FDA). Impurities can be categorized into process impurities, drug degradation products (degradants or degradates), and excipient and packaging-related impurities. Process impurities are produced during the manufacture of the drug substance and drug product, while degradants are formed by chemical degradation during the storage of the drug substances or drug products. The storage conditions are typically represented by the International Conference on Harmonisation (ICH)- and World Health Organization (WHO)-recommended stability conditions which simulate different climatic zones of the world. Certain process impurities can also be degradants, if they continue to form in storage under stability conditions. Packaging-related impurities, also called leachables, are typically various plasticizers, antioxidants, UV curators, and residual monomers that leach out of the plastic or rubber components and labels of the package/container of a drug product over time.
Those process impurities that are not degradants may be controlled or eliminated by modifying or changing the process chemistry. On the other hand, control or minimization of drug degradants requires a clear understanding of the drug degradation chemistry, which is not only critically important for developing a drug candidate but also for maintaining the quality, safety, and efficacy of an approved drug product. Specifically, knowledge of drug degradation is not only vital for developing adequate dosage forms that display favorable stability behavior over the registered product shelf-life, but is also critical in assessing which impurities would be most likely to be significant or meaningful degradants so that they can be included in the specificity mixture when developing and validating stability-indicating analytical methodologies. A common problem in the development of stability-indicating HPLC methods using stress studies (or forced degradation) is a lack of proper evaluation if the stress-generated degradants would be real degradants or not. From a practical point of view, the real degradants are those that can form under long term storage conditions such as the International Conference on Harmonisation (ICH) stability conditions. On the other hand, various artificial degradants can be generated during stress studies, in particular when excessive degradation is rendered or the stress conditions are not consistent with the degradation pathways of the drug molecule under the usual stability conditions. For example, forced degradation of a ketone-containing drug, pentoxifylline, using 30% hydrogen peroxide at room temperature for eight days produced a geminal dihydroperoxide degradation product (Scheme 1.1). This compound is highly unlikely to be a real degradant of the drug product.
This book is devoted to increasing our understanding and knowledge of the organic chemistry of drug degradation. The knowledge derived from this endeavor should also be beneficial for the elucidation of drug metabolite structures and bioactivation mechanisms. Most drugs undergo at least certain level of metabolism, that is, chemical transformation catalyzed by various enzymes. Except in the case of pro-drugs, drug metabolites can be considered as drug degradants formed in vivo. Chemical degradation and drug metabolism can produce the same degradants, even though they may go through different reaction intermediates or mechanisms. In vitro chemical reactions have been used to mimic enzyme-catalyzed drug metabolism processes, in order to help elucidate the enzymatic mechanisms for the catalysis. On the other hand, understanding the mechanisms of drug metabolism may also facilitate the elucidation of drug degradation pathways in vitro.
Regardless of their origins, certain drug degradants can be toxic, which is one of the main contributors to undesirable side effects or adverse drug reactions (ADR) of drugs. In the early stage of drug development, the degradants (including metabolites) and degradation pathways (or bioactivation pathways in the case of reactive metabolites) of a drug candidate need to be elucidated, followed by toxicological evaluation of these degradants. Dependent upon the outcome of the evaluation, the structure of the drug candidate may have to be modified to avoid the formation of a particular toxicophore based on the understanding of the degradation chemistry (or bioactivation pathways) elucidated. Failure to uncover toxic degradants, usually the low level ones, in the early development stage can lead to hugely costly failure in later stage clinical studies or even withdrawal of an approved drug product from the market.
1.2 Characteristics of Drug Degradation Chemistry and the Scope of this Book
The vast majority of therapeutic drugs are either organic compounds or biological entities. The latter drugs include protein and nucleic acid (RNA and DNA)-based drugs which are biopolymers comprising small molecule building blocks. This book focuses on the organic chemistry aspect of drug degradation, in particular, the mechanisms and pathways of the chemical degradation of both small and large molecule drugs under real life degradation scenarios, as represented by the usual long term stability conditions. Stress studies or forced degradation can help elucidate the structures of real degradants and the degradation pathways of drugs. Nevertheless, caution needs to be taken in differentiating the real and artificial degradants. This subject will be discussed in detail in Chapter 8, Strategies for Elucidation of Degradant Structures and Degradation Pathways.
Drug degradation chemistry differs from typical organic chemistry in several ways. First, the yield of a drug degradation reaction is usually very low, from approximately 0.05% to a few percentage points at the most. Dependent upon the potencies and maximum daily dosages of the drugs, ICH guidelines require that the impurities and/or degradants of a drug be structurally elucidated, once they exceed certain thresholds, which are typically between 0.05% and 0.5%, relative to the drug substances. For potential genotoxic impurities, they need to be characterized and controlled at a daily maximum amount of 1.5µg for drugs intended for long term usage. Such low yields would be meaningless from the perspective of the regular organic chemistry. Second, due to the low yields and limited availability of samples, particularly stability samples of formulated drugs, the quantity of a drug degradant is usually extremely low, posing a serious challenge for its isolation and/or characterization. Despite the advent of sensitive and powerful analytical methodologies such as high resolution tandem liquid chromatography-mass spectrometry (LC-MS/MS), liquid chromatography-nuclear magnetic resonance (LC-NMR), and cryogenic micro NMR probes, the identification of drug degradants remains one of the most challenging activities in pharmaceutical development. Third, the typical conditions and "reagents" of drug degradation reactions are limited in scope. For example, the ICH long term stability conditions for different climatic zones specify the requirements for heat and moisture (relative humidity, RH), for example, 25°C/60% RH and 30°C/65% RH, while the ICH accelerated stability condition requires heating at 40°C under 75% RH. In addition to moisture, the other most important "reagent" in drug degradation reactions is molecular oxygen. Since molecular oxygen is ubiquitous and difficult to remove from drug products, oxidative degradation of drugs is one of the most common degradation pathways. Often, the impact of molecular oxygen can be indirect. For example, a number of polymeric drug excipients such as polyethylene glycol (PEG), polysorbate, and povidone, are readily susceptible to autooxidation, resulting in the formation of various peroxides including hydrogen peroxide. These peroxides can cause significant drug degradation once formulated with drug substances containing oxidizable moieties. In contrast, reductive degradation is rarely seen in drug degradation reactions owing to the lack of a reducing agent in common drug excipients that is strong enough to cause meaningful reductive degradation. Other possible "reagents" in drug degradation reactions are usually limited to drug excipients and their impurities. For example, excipients consisting of oligosaccharides and polysaccharides with reducing ends, such as lactose and starch, are frequently used in drug formulation. The aldehyde functionality of these excipients can react with the primary and secondary amine groups of drugs to undergo degradation via the Maillard reaction. This topic will be covered in Chapter 5, Drug–Excipient Interaction and Adduct Formation.
As indicated above, this book focuses on the organic chemistry of drug degradation, in particular, the mechanisms and pathways of the chemical degradation of both small and large molecule drugs under real life degradation scenarios. Owing to the variety of dosage forms of formulated drugs, degradation of drugs can occur in various states including solid (tablets, capsules, and powders), semi-solid (creams, ointments, patches, and suppositories), solution (oral, ophthalmic, and optic solutions, nasal sprays, lotions, injectables), suspension (suspension injectables), and gas phase (aerosols). Obviously, a drug molecule can exhibit different degradation pathways and kinetics in different dosage forms. Nevertheless, as the emphasis of this book is on drug degradation chemistry with regard to mechanisms and pathways in general, we will not discuss in too much detail in which state a particular degradation pathway occurs. For readers who are interested specifically in drug degradation in the solid state, the book Solid-state Chemistry of Drugs by Byrn, Pfeiffer, and Stowell is a good resource, in which an in-depth treatment of a drug's degradation behavior versus its polymorphism is presented.
Additionally, the topic of drug degradation kinetics is outside the main scope of this book, although kinetic parameters such as activation energy, Ea, reactant half-life, and reaction rate constant, are used extensively in Chapter 2, Hydrolytic Degradation, for the purpose of comparing the hydrolytic lability of various functional groups on a semi-quantitative basis. Those who are interested in drug degradation kinetics are referred to the book by Yoshioka and Stella, Stability of Drugs and Dosage Forms, in which various kinetics models of drug degradation are described. Note that the topic of process impurities of drugs is also out of the scope of this book. There are a number of publications on process chemistry development and control of process impurities. Last, this book tries to focus mainly on the major degradation pathways and mechanisms of drugs, rather than to be all-inclusive.
1.3 Brief Discussion of Topics that are Outside the Main Scope of this Book
Although there will not be a detailed discussion of topics that are outside the main scope of this book, such as those mentioned above, a brief overview of some of these topics is beneficial for a better overall understanding of drug degradation chemistry and this is given here.
1.3.1 Thermodynamics and Kinetics of Chemical Reactions
A change in Gibbs free energy, ΔG, of a chemical reaction governs the propensity of the reaction to proceed. ΔG is defined as follows:
ΔG = ΔH - TΔS (1.1)
where ΔH is the change in the reaction enthalpy, T is the reaction temperature (in Kelvin), and ΔS is the change in the reaction entropy.
For a thermodynamically favored reaction, that is, a reaction that occurs spontaneously, if allowed by the reaction kinetics, the ΔG of the reaction is negative. In other words, the free energy of the products is lower than that of the reactants in such a case. A schematic diagram of a thermodynamically favored reaction is presented in Figure 1.1. In contrast, a thermodynamically unfavorable reaction has a positive ΔG.
ΔG determines if the reaction of A+B[right arrow]C+D is favored or not, but it does not determine how fast the reaction, whether thermodynamically favored or not, would take place. The rate of the reaction or its kinetics is governed by the energy that is necessary to activate the reactants to a certain state so that they can convert to their products. There are two theories describing this process: collision theory and transition state theory. Collision theory is embodied in the well-known Arrhenius equation (equation (1.2)), which was first proposed by van't Hoff in 1884 and later justified and interpreted by Arrhenius in 1889:
k = Ae-Ea/RT (1.2)
where k is the reaction rate constant, A is the pre-exponential (or frequency) factor which can generally be approximated as a temperature-independent constant, Ea is the activation energy which is defined as the minimum energy the reactants must acquire through collision in order for the reaction to occur, R is the gas constant, and T is the reaction temperature (in Kelvin).
According to the Arrhenius equation, the rate constant of a reaction is temperature dependent and by taking the natural logarithm of equation (1.2), the Arrhenius equation takes the following format (equation (1.3)):
ln k = -Ea/R 1/T + ln A (1.3)
This expression shows that the higher the temperature, the faster the reaction rate. Additionally, if one measures the reaction rate constants (k) at different temperatures (T), one should get a linear relationship by plotting ln k versus 1/T. Hence, the activation energy, Ea, can be obtained from the slope (–Ea/R) of the linear plot and ln A from the y-intercept.
Despite its widespread use, the Arrhenius equation and its underlying collision theory have been challenged over time. The major competing theory appears to be transition state theory which was developed independently by Eyring, and Evans and Polanyi in 1935. The equation derived according to transition state theory is the Eyring equation, also called the Eyring–Polanyi equation (1.4):
k = kBT/h e-ΔG*/RT (1.4)
where ΔG* is Gibbs free energy of activation, kB is the Boltzmann constant, and h is Planck's constant.
This equation bears some resemblance to the Arrhenius equation in that the kBT/h item corresponds to the pre-exponential factor, A, and ΔG* corresponds to the activation energy, Ea. Nevertheless, in the Eyring equation, ΔG*, in addition to kBT/h, is temperature dependent, as ΔG*=ΔH*-TΔS*. Hence, the Erying equation can be written as equation (1.5) after taking natural logarithm and rearrangement:
1n k/T = (-ΔH*/R)(1/T) + 1n kB/h + ΔS*/R (1.5)
where ΔH* is enthalpy of activation and ΔS* is entropy of activation.
Hence, ΔH* can be obtained from the slope (- ΔH*/R) of a linear plot of lnk/T versus 1/T, while ΔS* can be obtained from the y-intercept (lnkB/h+ΔS*/R). Therefore, one can obtain both Ea(from equation (1.3)) and ΔH* and ΔS*(from equation (1.5)) from a single dataset of reaction rate constant, k, versus reaction temperature, T. Although application of the Eyring equation enables one to obtain both ΔH* and ΔS* values and the ΔS* value should help elucidate the reaction mechanism, it appears that the use of Arrhenius equation exceeds the use of Eyring equation, at least in the hydrolytic stability studies of drugs. With respect to the numeric difference between the values of Ea and ΔH*, we can again rearrange the Eyring equation (1.5) into the following format (equation (1.6)):
ln k = (-ΔH*/R)(1/T) + ΔS*/R + 1n kB/h + ln T (1.6)
Among the last three items of the equation, only ln T is a variable of reaction temperature, while the other two are constants. However, for reactions that are studied within a relatively narrow window of temperature, say no greater than 100 K above room temperature (298 K), a temperature change of 100 K with regard to ln T does not appear to have too much impact on the overall value of the summation for the last three items. Hence, the Arrhenius equation may be considered a simplified version of the Eyring equation when reactions are studied within a relatively narrow range of temperature; the vast majority of the degradation reactions of drugs fall into this category. Therefore, numerically the value of Ea would not be too much different from that of ΔH*. Indeed, in a hydrolysis study of a group of sulfamides, the difference between the two values is no more than 1 kcal mol-1.
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Table of Contents
Introduction; Hydrolytic Degradation; Oxidative Degradation; Various Types and Mechanisms of Degradation Reactions; Drug-Excipient Interactions and Adduct Formation; Photochemical Degradation; Chemical Degradation of Biological Drugs; Strategies for Elucidation of Degradant Structures and Degradation Pathways; Control of Drug Degradation; Subject index