Guidance for Industry
`Drug Metabolism/Drug Interaction
`Studies in the Drug Development
`Process: Studies In Vitro
`
`Department of Health and Human Services
`U.S. Food and Drug Administraion
`Center for Drug Evaluation and Research
`Center For Biologics Evaluation and Research
`April 1997
`
`CLIN 3
`
`Roxane Labs., Inc.
`Exhibit 1015
`Page 001
`
`

`
`Guidance for Industry
`Drug Metabolism/Drug Interaction
`Studies in the Drug Development
`Process: Studies In Vitro
`
`Additional copies are available from:
`
`The Drug Information Branch (HFD-210)
`Division of Communications Management
`Center for Drug Evaluation and Research (CDER)
`5600 Fishers Lane, Rockville, MD 20857
`(Tel) 301-827-4573
`(Internet) http://www.fda.gov/cder/guidance.htm
`
`or
`
`Office of Communication, Training, and Manufacturers Assistance (HFM-40)
`Center for Biologics Evaluation and Research (CBER)
`1401 Rockville Pike, Rockville, MD 20852-1448
`(Internet) http://www.fda.gov/cber/cberftp.html
`(Fax) 888-CBERFAX or 301-827-3844
`(Bounce-back E-mail) cber_info@al.cber.fda.gov
`(Voice Information) 800-835-4709 or 301-827-1800
`
`Department of Health and Human Services
`U.S. Food and Drug Administraion
`Center for Drug Evaluation and Research
`Center For Biologics Evaluation and Research
`April 1997
`CLIN 3
`
`Roxane Labs., Inc.
`Exhibit 1015
`Page 002
`
`

`
`Table of Contents
`
`I.
`
`II.
`
`III.
`
`IV.
`
`V.
`
`VI.
`
`INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
`
`OBSERVATIONS AND CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
`
`TECHNIQUES AND APPROACHES FOR STUDIES IN VITRO OF DRUG
`METABOLISM AND DRUG INTERACTIONS . . . . . . . . . . . . . . . . . . . . . . . 4
`A.
`Cytochrome P-450, Microsomes, and Related Tools
`. . . . . . . . . . . . . . . . 4
`B.
`Other Hepatic Enzymes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
`C.
`Gastrointestinal Drug Metabolism . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
`D.
`Interspecies Metabolic Comparisons and Other Uses of Animal Data . . . . . . 7
`
`CORRELATION BETWEEN STUDIES IN VITRO AND IN VIVO . . . . . . . . . . . 7
`
`TIMING OF METABOLISM STUDIES . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
`
`LABELING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
`
`VII. RELATED APPLICATIONS AND CONSIDERATIONS . . . . . . . . . . . . . . . . 10
`
`i
`
`Roxane Labs., Inc.
`Exhibit 1015
`Page 003
`
`

`
`GUIDANCE FOR INDUSTRY1
`
` DRUG METABOLISM/DRUG INTERACTION STUDIES IN THE
`DRUG DEVELOPMENT PROCESS: STUDIES IN VITRO
`
`
`I.
`
`INTRODUCTION
`
`After entering the body, a drug is eliminated either by excretion or by metabolism to one or
`more active or inactive metabolites. When elimination occurs primarily by metabolism, the
`routes of metabolism can significantly affect the drug’s safety and efficacy and the directions
`for use. When elimination occurs via a single metabolic pathway, individual differences in
`metabolic rates can lead to large differences in drug and metabolite concentrations in the blood
`and tissue. In some instances, differences exhibit a bimodal distribution indicative of a genetic
`polymorphism (e.g., CYP450 2D6, CYP450 2C19, N-acetyl transferase). When a genetic
`polymorphism affects an important metabolic route of elimination, large dosing adjustments
`may be necessary to achieve the safe and effective use of the drug. Pharmacogenetics already
`has influenced therapeutics. For a drug that is primarily metabolized by CYP450 2D6,
`approximately 7 percent of Caucasians will not be able to metabolize the drug, but the
`percentage for other racial populations is generally far lower. Similar information is known
`for other pathways, prominently, CYP450 2C19 and N-acetyl-transferase. Equally important,
`if not more so, many enzymatic metabolic routes of elimination, including most of those
`occurring via the CYP450 enzymes, can be inhibited or induced by concomitant drug
`treatment. As a result, abrupt changes can occur with a co-administered agent in a single
`individual. Such interactions can lead to a substantial decrease or increase in the blood and
`tissue concentrations of a drug or metabolite or cause the accumulation of a toxic substance
`(e.g., certain antihistamine-antifungal interactions). These types of changes can alter a new
`drug’s safety and efficacy profile in important ways, particularly a drug with a narrow
`therapeutic range.
`
`An understanding of metabolic pathways and potential interactions sometimes allows the use of
`a drug that would produce an unacceptable level of toxicity if blood levels were not
`predictable. For these reasons, it is important to learn at an early stage of development
`whether a drug is eliminated primarily by excretion of unchanged drug or by one or more
`routes of metabolism. If elimination is primarily by metabolism, the principal metabolizing
`route(s) should be understood. This information will help identify the implications of
`
`This guidance has been prepared by the Drug Metabolism/Drug Interactions--In Vitro Studies Working
`1
`Group of the Clinical Pharmacology Section of the Medical Policy Coordinating Committee in the Center for Drug
`Evaluation and Research (CDER), with input from the Center for Biologics Evaluation and Research (CBER) at the
`Food and Drug Administration. This guidance represents the Agency’s current thinking on drug metabolism and
`drug interaction studies in vitro. It does not create or confer any rights for or on any person and does not operate to
`bind FDA or the public. An alternative approach may be used if such approach satisfies the requirement of the
`applicable statute, regulations, or both.
`
`Roxane Labs., Inc.
`Exhibit 1015
`Page 004
`
`

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`metabolic differences between and within individuals and the importance of certain drug-drug
`and other interactions. Having such information also will aid in determining whether the
`pharmacologic properties of certain metabolites should be explored further.
`
`This FDA guidance to industry provides suggestions on current approaches to studies in vitro
`of drug metabolism and interactions. The guidance is intended to encourage routine, thorough
`evaluation of metabolism and interactions in vitro whenever feasible and appropriate. As is
`the case for all FDA guidance documents, suggestions are not requirements, but are offered
`for consideration by drug development scientists as a means to address potentially important
`safety concerns. FDA recognizes that the importance of any approach will vary depending on
`the drug in development and its intended clinical use. The FDA also recognizes that clinical
`observations can address some of the same issues identified in this document as being
`susceptible to in vitro study. The suggested approaches delineated in this document, however,
`are efficient and inexpensive considering the breadth of information they can provide, and
`often can reduce or eliminate the need for further clinical investigations. This particular
`guidance is directed toward a broad class of drugs: molecules with a molecular weight below
`10 kiloDaltons.
`
`Although the field of in vitro assessment of drug metabolism and drug interactions has
`progressed sufficiently to allow preparation of this guidance, additional work will be required
`to allow a comprehensive characterization of drug metabolism in vitro (including induction and
`inhibition) and its implications for subsequent clinical investigations and product labeling.
`Because the assessment of drug metabolism in vitro is a rapidly evolving area of drug
`development and regulation, this guidance may require frequent revision.
`
`Review scientists at the FDA have long been interested in the impact of drug metabolism and
`drug-drug interactions on drug safety and efficacy. As a result, discussion of this topic also is
`contained in other FDA guidance documents, including General Considerations for the
`Clinical Evaluation of Drugs (FDA 77-3040), Guideline for Studying Drugs Likely to be Used
`in the Elderly (11/89), and Guideline for the Study and Evaluation of Gender Differences in the
`Clinical Evaluation of Drugs (58 FR 39406, July 22, 1993). To obtain these documents,
`contact the Drug Information Branch at the Center for Drug Evaluation and Research.
`
`II.
`
`OBSERVATIONS AND CONCLUSIONS
`
`The following general observations and conclusions underlie the suggestions found in this
`guidance:
`
`The concentrations of the parent drug and/or its active metabolite(s) circulating in the
`body are the effectors of desirable and/or adverse drug actions.
`
`A principal regulator of drug concentration in the body is clearance. Metabolism can
`be a major determinant of clearance.
`
`2
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`Roxane Labs., Inc.
`Exhibit 1015
`Page 005
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`

`
`Even for drugs that are not substantially metabolized, the potential effect of that drug
`on the metabolism of concomitant drugs could be important.
`
`Large differences in blood levels can occur because of individual differences in
`metabolism. Some drugs, such as tricyclic antidepressants, exhibit order of magnitude
`differences in blood concentrations depending on the enzyme status of patients. Drug-
`drug interactions can have similarly large effects when one drug inhibits the metabolism
`of another. For example, ketoconazole greatly increases concentrations of parent
`terfenadine, leading to QT prolongation and torsades de pointes.
`
`Major advances have been made recently in the availability and use of human tissue and
`recombinant enzymes for studies in vitro of drug metabolism and drug-drug
`interactions.
`
`The development of sensitive and specific assays for a drug and its metabolites is
`critical to the study of drug metabolism and interactions. Development of such assay
`methods has long been a high priority for drug development programs, and such assays
`are increasingly available early in development. Once reliable assays are available, the
`techniques are available for readily assessing drug metabolism and drug-drug
`interactions in vitro and interpreting the results.
`
`The studies in vitro described in this document are one set of approaches to developing
`information about drug metabolism and drug-drug interactions. Mechanistic and empirical
`clinical study approaches are available as well to provide further information. As always, a
`carefully designed mix of approaches is likely to yield optimal results in the shortest time and
`at the least cost. Metabolic effects and drug-drug interactions should be considered as early as
`possible as well as later in the drug development process. Appropriately designed
`pharmacokinetic/phase 1 studies could provide important information about drug metabolism,
`relevant metabolites, and actual or potential drug interactions. Blood level data obtained
`during phase 2 and 3 clinical trials, for example, via a pharmacokinetic screen, also could
`reveal interactions or marked inter-individual differences. Because clinical trial protocols
`sometimes limit concomitant drug use, some later studies may not be optimally informative
`about possible drug interactions. Decreasing exclusions of concomitant drug treatment and
`measurement of blood levels before and after treatment with a test drug (interaction screen), as
`well as testing drug blood levels more frequently, could make later phase clinical studies more
`useful. All of these studies could be more informative if significant metabolites and prodrugs
`could be identified and their pharmacological properties described.
`
`Identifying metabolic differences in patient groups based on genetic polymorphisms, or on
`other readily identifiable factors such as age, race, and gender, could help guide the design of
`dosimetry studies for such populations groups. This kind of information also will provide
`improved dosing recommendations in product labeling, facilitating the safe and effective use of
`a drug by allowing prescribers to anticipate necessary dose adjustments. Indeed, in some
`cases, understanding how to adjust dose to avoid toxicity may allow the marketing of a drug
`
`3
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`Roxane Labs., Inc.
`Exhibit 1015
`Page 006
`
`

`
`that would have an unacceptable level of toxicity were its toxicity unpredictable and
`unpreventable.
`
`III.
`
`TECHNIQUES AND APPROACHES FOR STUDIES IN VITRO OF DRUG
`METABOLISM AND DRUG INTERACTIONS
`
`The goals in evaluating in vitro drug metabolism are: (1) to identify all of the major metabolic
`pathways that affect the test drug and its metabolites, including the specific enzymes
`responsible for elimination and the intermediates formed; and (2) to explore and anticipate the
`effects of the test drug on the metabolism of other drugs and the effects of other drugs on its
`metabolism. Pharmacologic effects of the test drug and its major metabolites also should be
`studied, if feasible. Knowledge that a particular drug is not a substrate for certain metabolic
`pathways is helpful. For example, if it is learned early in drug development that a molecule is
`not a substrate for CYP450 3A4 or that this pathway represents only a minor contribution to
`overall metabolism, then concern is lessened or eliminated for possible inhibition of 3A4
`metabolism by drugs such as ketoconazole and erythromycin or possible induction of
`metabolism by drugs such as rifampin and anticonvulsants. Studies in vitro also could indicate
`whether a drug itself is or is not an inhibitor of common metabolic pathways. The potential
`for a drug inhibiting the metabolism of other drugs is almost always present for drugs
`metabolized by the same pathway, but can also be present for entirely separate pathways,
`including the principal metabolic route for a compound. This potential was first appreciated
`for quinidine, which is a substrate for metabolism by CYP450 3A4 and is also a very potent
`inhibitor of CYP450 2D6.
`
`A.
`
`Cytochrome P-450, Microsomes, and Related Tools
`
`1.
`
`Assessing the metabolism of a test drug
`
`The most mature technology for the study in vitro of drug metabolism (enzymes
`involved, metabolites formed, and potential inhibitors) is associated with the set
`of enzymes contained in the cytochrome CYP450 superfamily. These enzymes
`are responsible for the metabolism of the majority of drugs given to humans.
`Metabolism usually occurs in the liver, but the enzymes (especially CYP450
`3A4) also are important in gut metabolism. Human liver microsomes provide
`the most convenient way to study CYP450 metabolism. Microsomes are a
`subcellular fraction of tissue obtained by differential high-speed centrifugation.
`All of the CYP450 enzymes are collected in the microsomal fraction. The
`CYP450 enzymes retain their activity for many years in microsomes or whole
`o
`liver stored at low temperature (e.g., -70 C). Cofactor requirements for
`CYP450-mediated reactions are well characterized, consisting primarily of a
`redox sustaining system such as NADPH. Hepatic microsomes can be obtained
`commercially, with or without prior phenotyping, for most important drug-
`
`4
`
`Roxane Labs., Inc.
`Exhibit 1015
`Page 007
`
`

`
`metabolizing enzymes.
`
`During studies to identify metabolic routes of elimination for an investigational
`new drug, microsomes from several donors should be used, either individually
`or pooled, to avoid reliance on microsomes that are deficient in one or more
`metabolic pathways, unless this is a specific objective of the study. With the
`use of selective chemical inhibitors for each major pathway, the metabolic
`pathways for a new drug can be readily demonstrated or ruled out. Careful
`consideration of incubating concentrations of both inhibitor and substrate is
`essential to maintain a selective approach. For example, quinidine and
`ketoconazole are relatively selective inhibitors of 2D6 and 3A4, respectively, at
`concentrations below 1 micromolar, but both will also inhibit other CYP450
`enzymes at higher concentrations, an inhibition that is not known to be clinically
`pertinent. Antibodies to specific CYP450 enzymes also can be used to attempt
`selective inhibition of metabolic pathways, but at present this approach is
`limited by lack of wide commercial availability of the antibodies, incomplete
`characterization of their selectivity, and high laboratory-to-laboratory variation
`for antibody inhibition results in comparison to chemical inhibitors.
`
`The cDNAs for the common CYP450s have been cloned, and the recombinant
`human enzymatic proteins have been expressed in a variety of cells. After the
`apparent metabolic pathway has been determined using microsomes, use of
`these recombinant enzymes provides an excellent way to confirm results
`identified in microsomes.
`
`The most complete picture for hepatic metabolism can be obtained with intact
`liver systems, in which the cofactors are self-sufficient and the natural
`orientation for linked enzymes is preserved. Isolated hepatocytes and precision-
`cut slices have these desirable features. Radiolabeled drugs are very helpful at
`this stage. A major logistic problem with these preparations, however, is that
`enzymatic activities are not stable for much longer than 24 hours. Overcoming
`that limitation will be valuable for investigating induction of enzyme activity.
`
`Studies in vitro can identify critical metabolic pathways for a new drug and
`metabolites that are formed by these pathways. The clinical significance of this
`information should generally be confirmed via studies in the clinic. Absence of
`a finding that certain metabolic pathways are important via in vitro studies may
`obviate the need for further clinical investigations or at least help focus the
`design of these studies.
`
`2.
`
`Assessing effects on other drugs
`
`Human microsomes are also the most useful tool for screening for the effects of
`a new drug on common CYP450 pathways and for providing rapid initial
`
`5
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`Roxane Labs., Inc.
`Exhibit 1015
`Page 008
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`

`
`information on potential drug-drug interactions. A general assessment of effects
`on major metabolic pathways can be obtained by simultaneous incubation of the
`investigational new drug with standard probe compounds, which are available
`for many CYP450 pathways. The experiments are exceptionally rapid and
`straightforward, requiring no special equipment. In general, if appropriate
`concentrations of the test drug are used with established probes, a negative
`result in vitro (no interaction identified) is reassuring and can generally
`eliminate the need for further clinical evaluation. Positive results suggest the
`need for further clinical evaluation.
`
`B.
`
`Other Hepatic Enzymes
`
`Although the CYP450 superfamily is the dominant group of metabolizing enzymes,
`other classes of important enzymes for drug metabolism are present in humans,
`including enzymes responsible for acetylation, methylation, glucuronidation, sulfation,
`and de-esterification (esterases). Approaches in vitro are not as widely applied for
`these enzymes as to the CYP450s, but considerable progress has been made, and
`further important efforts are underway.
`
`In addition to the CYP450 enzymes, microsomes contain other enzymes, including a
`variety of transferases. For conjugating reaction pathways, supplementation of
`microsomal preparations with conjugating moieties as added cofactors has been
`successful. Cytosolic (soluble) enzymes are not contained in the microsomal fraction,
`but may be readily investigated using other subcellular fractions (e.g., S9).
`
`C.
`
`Gastrointestinal Drug Metabolism
`
`Much emphasis in metabolic research and development has focused on the liver,
`because this organ has always been regarded as the principal site of drug metabolism.
`For particular drugs, however, other tissues may predominate (e.g., the kidney or
`gastrointestinal mucosa). Because most drugs are given orally, interest has been
`increasing in the effect of gastrointestinal mucosal enzymes on drug entry to the
`systemic circulation. Drugs susceptible to metabolism via CYP450 3A4 may exhibit
`low and/or variable bioavailability. Thus, determining the susceptibility of a drug to
`metabolism by CYP450 3A4 may be important not only in identifying routes of
`elimination but also in predicting the likelihood of significant first-pass metabolism.
`
`D.
`
`Interspecies Metabolic Comparisons and Other Uses of Animal Data
`
`Animal toxicology studies are an important component of assessing safety for
`subsequent human exposure. Although comparative metabolism has long been of
`interest, this emphasis has grown in recent years, and many drug development
`
`6
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`Roxane Labs., Inc.
`Exhibit 1015
`Page 009
`
`

`
`programs now produce extensive characterization of metabolites in animals. This work
`has not regularly been linked to parallel findings in humans, but the availability of tools
`for the study of human metabolism in vitro provides an opportunity to refocus and
`enhance the goals of pharmacokinetic and metabolic studies in animals.
`
`Animal studies provide the means to determine whether new chemical species generated
`by human metabolic studies in vitro are active pharmacologically (toxicologically) and
`how they compare to the parent compound, often a critical determinant of the effect of
`drug-drug interactions or genetic diversity. Early identification of human metabolic
`routes of elimination and metabolites by studies in vitro can provide clear direction for
`preclinical studies in animals.
`
`An especially valid application of in vitro and appropriate clinical follow-up studies is
`to compare drug and metabolite exposure in humans and animals. Reasonably similar
`exposure supports the relevance of a particular animal species to the assessment of a
`potential human risk, and knowledge of differences (e.g., a toxic metabolite in animals,
`but not in humans) could aid in interpretation of clinical data. The earlier this is done,
`the easier it will be to use the information in planning and interpreting clinical studies.
`Although the use of in vitro techniques to determine the most metabolically relevant
`species for nonclinical testing may enhance the value of these studies, selecting
`appropriate species or strains is not a simple matter. The need for historical control
`data and prior experience in toxicology studies for a particular species and strain could
`limit the ability to select species and strains based on similarities of metabolic pathways
`to humans. Nonetheless, major metabolic dissimilarities between the test species and
`humans reduce the confidence in these studies as predictors of safety in humans.
`
`IV.
`
`CORRELATION BETWEEN STUDIES IN VITRO AND IN VIVO
`
`A complete understanding of the relationship between metabolism in vitro and in vivo is still
`emerging. Strong correlations have been documented between well-conducted studies in vitro
`and in vivo, but considerable effort is necessary before complete validation of these
`correlations is obtained, including an appreciation for whatever limits may exist for the
`correlations. When a difference arises between findings in vitro and in vivo, the results in
`vivo should always take precedence over studies in vitro. In many cases, however, studies in
`vitro, which are inexpensive and readily carried out, will serve as an adequate screening
`mechanism that can rule out the importance of a metabolic pathway and make in vivo testing
`unnecessary. If investigations in vitro suggest that the answer to the question "Does CYP450
`2D6 metabolize this drug?" is "no,” clinical studies to identify the impact of the slow
`metabolizer phenotype or to study the effect of CYP450 2D6 inhibitors will not be needed.
`Because studies in vitro, however, cannot adequately define the importance of a metabolic
`pathway, if the in vitro study answer is “yes,” additional clinical studies will be important to
`answer whether CYP450 2D6 is clinically important to the elimination of the drug.
`
`7
`
`Roxane Labs., Inc.
`Exhibit 1015
`Page 010
`
`

`
`Additional information also may be necessary to identify which inhibitors, if any, affect
`metabolism in vivo significantly. For example, although a drug may be extensively
`metabolized in vitro, a mass balance study in vivo may demonstrate that metabolism is less
`important than urinary or biliary excretion. In addition, inhibition studies often will not be
`definitive in vitro unless only a relatively low degree of inhibition is present, with intermediate
`to high degrees of inhibition needing subsequent clinical confirmation. In this setting,
`inhibition of metabolic pathways will not have a clinical impact except for patients with severe
`impairment of excretory function, and the effect of induction on elimination will be limited.
`In general, if there is some, but not a large, effect in vitro, predicting the effect in vivo will be
`difficult. Experiments in vitro should be conducted at concentrations similar to the relevant
`concentration in vivo. As previously noted for chemical inhibition studies, different pathways
`may be affected at various concentrations. This may be difficult to determine, however, when
`the interaction occurs in the gut.
`
`V.
`
`TIMING OF METABOLISM STUDIES
`
`A question frequently raised is when during drug development should studies in vitro be
`conducted. Sponsors are reluctant to allocate resources during the investigation of a drug that
`has not yet demonstrated a suggestion of clinical activity. This is a reasonable concern, and
`where possible, it is reasonable to identify some useful activity in short-term clinical studies
`before embarking on a major metabolic evaluation. Nevertheless, an early understanding of
`how a compound is metabolized could influence selection among several pharmacologically
`similar agents and could lead to dose regimens that would be more likely to detect a positive
`clinical effect. When attempting to determine the most appropriate time to conduct
`metabolism studies in vitro, it is helpful to reconsider the reasons for conducting such studies.
`Two of the major clinical reasons, as previously mentioned, are (1) to identify all of the major
`metabolic pathways that affect the drug and its metabolites and (2) to anticipate the effects of
`the drug on the metabolism of other drugs. With these objectives in mind, an understanding of
`the metabolic profile of a drug in vitro would be useful prior to the initiation of phase 2 studies
`and is especially important before phase 3 trials, when a broader population will be studied.
`This knowledge would permit the efficient design of clinical dose/response, interaction, and
`special population studies and also would enable adequate attention to be given to patient
`variability and potential interactions in phase 2 and 3 studies. Of course, drugs have been
`developed successfully even when the evaluation of metabolic routes of elimination occurred
`during the later phases of drug development or were not explored at all. Today, however, it is
`difficult to justify marketing a drug without knowing how it is metabolized or how it could
`influence, or be influenced by, the drugs being taken with it. Therefore, sponsors are
`encouraged to conduct appropriate metabolic studies prior to commencement of phase 3 trials.
`
`VI.
`
`LABELING
`
`8
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`Roxane Labs., Inc.
`Exhibit 1015
`Page 011
`
`

`
`Each year, large numbers of new drug-drug interactions are discovered, precluding the
`possibility that any prescriber could memorize them all. Based on the increasing amount of
`valuable information that is available, it is now possible to label for class effects for various
`enzymes, and the ability to extrapolate from partial data is growing. Standardized approaches
`to labeling are likely to emerge and be helpful, in a manner analogous to the class labeling
`used for certain categories of drugs. For example, certain powerful inhibitors (quinidine for
`CYP450 2D6, ketoconazole for CYP450 3A4) are likely to affect all drugs metabolized by
`these pathways. For this reason, if a new drug is found to be a substrate for certain CYP450
`enzymes, then certain interactions may be anticipated, even though specific data are lacking.
`This understanding relies on knowledge about the activity of the drug and its metabolites.
`Similarly, it would be helpful to know what metabolic pathways are not involved in the
`elimination of a drug. When generalizations are made from studies in vitro, the conditions of
`extrapolation should be explicitly stated. Thus, conclusions based on data gained from in vitro
`studies that are extrapolated to the clinical situation should be identified and distinguished from
`conclusions based on clinical observations in vivo. Under these circumstances, the best advice
`available at any given time may be provided, and class effects may be updated as new
`information is obtained. The following text is an example of class labeling based on studies in
`vitro:
`
`Although clinical studies have not been conducted, on the basis of this drug's
`metabolism by CYP450 3A4, ketoconazole, itraconazole, erythromycin, and grapefruit
`juice are likely to inhibit its metabolism. Furthermore, rifampin, dexamethasone, and
`certain anticonvulsants (phenytoin, phenobarbital, carbamazepine) may induce this
`drug's metabolism. Thus, if a patient has been titrated to a stable dosage on this drug,
`and then begins a course of treatment with one of these inducers or inhibitors, it's
`reasonable to expect that a dose adjustment may be necessary to prevent toxicity or
`therapeutic failure.
`
`The example below demonstrates where the class effects would be inserted and also where
`information on the drug's inhibitory effects would be stated:
`
`This drug is metabolized by CYP450 3A4 <insert current statement>. At clinical
`doses, the drug itself does not inhibit the metabolism of other 3A4 substrates, but does
`inhibit the metabolism of substrates metabolized via the CYP450 2D6 pathway.
`
`Given the tendency to include many potential interactions, it is sometimes unclear if anything
`is noninteracting. In such a circumstance, labeling statements that denote both positive and
`negative expectations may be helpful. For example:
`
`This drug is a substrate for CYP450 1A2. Although inhibition of its metabolism by
`ciprofloxacin is observed, quinidine, erythromycin, ketoconazole, and itraconazole are
`not inhibitors.
`
`9
`
`Roxane Labs., Inc.
`Exhibit 1015
`Page 012
`
`

`
`VII. RELATED APPLICATIONS AND CONSIDERATIONS
`
`The same techniques for evaluating potential drug-drug interactions can also provide
`information related to the influence of social (smoking, alcohol), environmental (diet, e.g.,
`grapefruit juice), and genetic factors upon therapeutics. For example, several studies have
`demonstrated that tobacco smoking is a strong inducer of the CYP450 1A2 enzymes. As a
`result, larger doses of theophylline are recommended for patients who smoke and receive this
`drug. In the future, evidence of induction as well as inhibition may also be developed via in
`vitro studies. Metabolic characterization of racemic drugs should be conducted in accordance
`with previously expressed guidances and guidelines on the development of stereoisomers. In
`particular, if the development of a single enantiomer is to be pursued, the preclinical
`metabolism studies in vitro should be conducted with the relevant enantiomer, rather than the
`racemic mixture.
`
`10
`
`Roxane Labs., Inc.
`Exhibit 1015
`Page 013