This guidance document is being distributed for comment purposes only.

Comments and suggestions regarding this draft document should be submitted within 60 days of publication in the Federal Register of the notice announcing the availability of the draft guidance.  Submit comments to the Division of Dockets Management (HFA-305), Food and Drug Administration, 5630 Fishers Lane, rm. 1061, Rockville, MD  20852.  All comments should be identified with the docket number listed in the notice of availability that publishes in the Federal Register.

For questions regarding this draft document contact (CDER) Shiew-Mei Huang, 301-796-1541, or (CBER) Toni Stifano, 301-827-6190.

 

U.S. Department of Health and Human Services
Food and Drug Administration
Center for Drug Evaluation and Research (CDER)
Center for Biologics Evaluation and Research (CBER)

September 2006
Clinical Pharmacology

Guidance for Industry
Drug Interaction Studies — Study Design, Data Analysis, and Implications for Dosing and Labeling

I. INTRODUCTION

This guidance provides recommendations for sponsors of new drug applications (NDAs) and biologics license applications (BLAs) for therapeutic biologics who are performing in vitro and in vivo drug metabolism, drug transport, and drug-drug interaction studies.  The guidance reflects the Agency’s current view that the metabolism of an investigational new drug should be defined during drug development and that its interactions with other drugs should be explored as part of an adequate assessment of its safety and effectiveness.  For drug-drug interactions, the approaches considered in the guidance are offered with the understanding that the relevance of a particular study depends on the characteristics and proposed indication of the drug under development.  Furthermore, not every drug-drug interaction is metabolism-based, but may arise from changes in pharmacokinetics caused by absorption, distribution, and excretion interactions.  Drug-drug interactions related to transporters are being documented with increasing frequency and are important to consider in drug development.  Although less well studied, drug-drug interactions may alter pharmacokinetic/pharmacodynamic (PK/PD) relationships.  These important areas are not considered in detail in this guidance.

Discussion of metabolic and other types of drug-drug interactions is also provided in other guidances, including the International Conference on Harmonization (ICH) E7 Studies in Support of Special Populations:  Geriatrics, and E3 Structure and Content of Clinical Study Reports, and FDA guidances for industry on Studying Drugs Likely to be Used in the Elderly and Study and Evaluation of Gender Differences in the Clinical Evaluation of Drugs.

FDA’s guidance documents, including this guidance, do not establish legally enforceable responsibilities.  Instead, guidances describe the Agency’s current thinking on a topic and should be viewed only as recommendations, unless specific regulatory or statutory requirements are cited.  The use of the word should in Agency guidances means that something is suggested or recommended, but not required.

II. BACKGROUND

A. Metabolism 

The desirable and undesirable effects of a drug arising from its concentrations at the sites of action are usually related either to the amount administered (dose) or to the resulting blood concentrations, which are affected by its absorption, distribution, metabolism, and/or excretion.  Elimination of a drug or its metabolites occurs either by metabolism, usually by the liver or gut mucosa, or by excretion, usually by the kidneys and liver.  In addition, protein therapeutics may be eliminated through a specific interaction with cell surface receptors, followed by internalization and lysosomal degradation within the target cell.  Hepatic elimination occurs primarily by the cytochrome P450 family (CYP) of enzymes located in the hepatic endoplasmic reticulum, but may also occur by non-P450 enzyme systems, such as N-acetyl and glucuronosyl transferases.  Many factors can alter hepatic and intestinal drug metabolism, including the presence or absence of disease and/or concomitant medications, or even some foods, such as grapefruit juice.  While most of these factors are usually relatively stable over time, concomitant medications can alter metabolism abruptly and are of particular concern.  The influence of concomitant medications on hepatic and intestinal metabolism becomes more complicated when a drug, including a prodrug, is metabolized to one or more active metabolites.  In this case, the safety and efficacy of the drug/prodrug are determined not only by exposure to the parent drug but by exposure to the active metabolites, which in turn is related to their formation, distribution, and elimination.  Therefore, adequate assessment of the safety and effectiveness of a drug includes a description of its metabolism and the contribution of metabolism to overall elimination.  For this reason, the development of sensitive and specific assays for a drug and its important metabolites is critical to the study of metabolism and drug-drug interactions.

B. Drug-Drug Interactions

1. Metabolism-Based Drug-Drug Interactions

Many metabolic routes of elimination, including most of those occurring through the P450 family of enzymes, can be inhibited or induced by concomitant drug treatment.  Observed changes arising from metabolic drug-drug interactions can be substantial — an order of magnitude or more decrease or increase in the blood and tissue concentrations of a drug or metabolite — and can include formation of toxic and/or active metabolites or increased exposure to a toxic parent compound.  These large changes in exposure can alter the safety and efficacy profile of a drug and/or its active metabolites in important ways.  This is most obvious and expected for a drug with a narrow therapeutic range (NTR), but is also possible for non-NTR drugs as well (e.g., HMG CoA reductase inhibitors). 

It is important that metabolic drug-drug interaction studies explore whether an investigational agent is likely to significantly affect the metabolic elimination of drugs already in the marketplace and likely in medical practice to be taken concomitantly and, conversely, whether drugs in the marketplace are likely to affect the metabolic elimination of the investigational drug.  Even drugs that are not substantially metabolized can have important effects on the metabolism of concomitant drugs.  For this reason, metabolic drug-drug interactions should be explored, even for an investigational compound that is not eliminated significantly by metabolism. 

Classical biotransformation studies are not a general requirement for the evaluation of therapeutic biologics (ICH guidance S6 Preclinical Safety Evaluation of Biotechnology-Derived Pharmaceuticals), although certain protein therapeutics modify the metabolism of drugs that are metabolized by the P450 enzymes.  Type I interferons, for example, inhibit CYP1A2 production at the transcriptional and post-translational levels, inhibiting clearance of theophylline.  The increased clinical use of therapeutic proteins may raise concerns regarding the potential for their impacts on drug metabolism.  Generally, these interactions cannot be detected by in vitro assessment.  Consultation with FDA is appropriate before initiating metabolic drug-drug interaction studies involving biologics.

Identifying metabolic differences in patient groups based on genetic polymorphism, or on other readily identifiable factors, such as age, race, and gender, can aid in interpreting results.  The extent of interactions may be defined by these variables (e.g., CYP2D6 genotypes).  Further, in subjects who lack the major clearance pathway, remaining pathways become important and should be understood and examined.

A specific objective of metabolic drug-drug interaction studies is to determine whether the interaction is sufficiently large to necessitate a dosage adjustment of the drug itself or the drugs with which it might be used, or whether the interaction would require additional therapeutic monitoring.

In some instances, understanding how to adjust dose or dosage regimen in the presence of an interacting drug, or how to avoid interactions, may allow marketing of a drug that would otherwise have been associated with an unacceptable level of toxicity.  Sometimes a drug interaction can be used intentionally to increase levels or reduce elimination of another drug (e.g., ritonavir and lopinavir).  Rarely, the degree of interaction caused by a drug, or the degree to which other drugs alter its metabolism, can be such that it cannot be marketed safely.

            2.         Transporter-Based Drug-Drug Interactions

Transporter-based interactions have been increasingly documented.  Examples of these include the inhibition or induction of transport proteins, such as P-glycoprotein (P-gp), organic anion transporter (OAT), organic anion transporting polypeptide (OATP), organic cation transporter (OCT), multidrug resistance-associated proteins (MRP), and breast cancer resistant protein (BCRP).  Examples of transporter-based interactions include the interactions between digoxin and quinidine, fexofenadine and ketoconazole (or erythromycin), penicillin and probenecid, and dofetilide and cimetidine.  Of the various transporters, P-gp is the most well understood and may be appropriate to evaluate during drug development.  Table 1 in Appendix A lists some of the major human transporters and known substrates, inhibitors, and inducers.

 

III. GENERAL STRATEGIES

 

To the extent possible, drug development should follow a sequence in which early in vitro and in vivoinvestigations can either fully address a question of interest or provide information to guide further studies.  Optimally, a sequence of studies could be planned, moving from in vitro studies to in vivo human studies, including those employing special study designs and methodologies where appropriate.  In many cases, negative findings from early in vitro and early clinical studies can eliminate the need for later clinical investigations.  Early investigations should explore whether a drug is eliminated primarily by excretion or metabolism, with identification of the principal metabolic routes in the latter case.  Using suitable in vitro probes and careful selection of interacting drugs for early in vivo studies, the potential for drug-drug interactions can be studied early in the development process, with further study of observed interactions assessed later in the process, as needed.  These early studies can also provide information about dose, concentration, and response relationships in the general population, specific populations, and individuals, which can be useful in interpreting the consequences of a drug-drug interaction.  Once potential drug-drug interactions have been identified, based on in vitro and/or in vivo studies, sponsors are encouraged to design and examine the safety and efficacy databases of larger clinical studies, as feasible, to (1) permit confirmation/discovery of the interactions predicted from earlier studies and/or (2) verify that dosage adjustments or other prescribing modifications made in response to the potential interaction(s) have been adequate to avoid undesired consequences of the drug-drug interaction.

A. In Vitro Studies

A complete understanding of the quantitative relationship between the in vitro findings and in vivo results of metabolism/drug-drug interaction studies is still emerging.  Nonetheless, in vitro studies can frequently serve as a screening mechanism to rule out the importance of a metabolic pathway and the drug-drug interactions that occur through this pathway so that subsequent in vivo testing is unnecessary.  This opportunity should be based on appropriately validated experimental methods and rational selection of substrate/interacting drug concentrations. 

For example, if suitable in vitro studies at therapeutic concentrations indicate that CYP1A2, CYP2C8, CYP2C9, CYP2C19, CYP2D6, or CYP3A enzyme systems do not metabolize an investigational drug, then clinical studies to evaluate the effect of CYP2D6 inhibitors or CYP1A2, CYP2C8, CYP2C9, CYP2C19, or CYP3A inhibitors/inducers on the elimination of the investigational drug will not be needed. 

Similarly, if in vitro studies indicate that an investigational drug does not inhibit CYP1A2, CYP2C8, CYP2C9, CYP2C19, CYP2D6, or CYP3A metabolism, then corresponding in vivo inhibition-based interaction studies of the investigational drug and concomitant medications eliminated by these pathways are not needed.  Figure 1 in Appendix B shows a decision tree on when in vivo interaction studies are indicated based on in vitro metabolism, inhibition, and induction and in vivo metabolism data. 

The CYP2D6 enzyme has not been shown to be inducible.  Recent data have shown co-induction of CYP2C, CYP2B and ABCB1 (P-gp) transporter with CYP3A.  CYP3A appears to be sensitive to all known co-inducers. Therefore, to evaluate whether an investigational drug induces CYP1A2, CYP2C8, CYP2C9, CYP2C19, or CYP3A, the initial in vitro induction evaluation may include only CYP1A2 and CYP3A.  If in vitro studies indicate that an investigational drug does not induce CYP3A metabolism, then in vivo induction-based interaction studies of the investigational drug and concomitant medications eliminated by CYP2C/CYP2B and CYP3A may not be needed. 

Drug interactions based on CYP2B6 are emerging as important interactions.  When appropriate, in vitro evaluations based on this enzyme can be conducted.  Other CYP enzymes, including CYP2A6 and CYP2E1, are less likely to be involved in clinically important drug interactions, but should be considered when appropriate.

Appendix C describes general considerations in the in vitro evaluation of CYP-related metabolism and interactions.  Appendices C-1, C-2, and C-3 provide considerations in the experimental design, data analysis, and data interpretation in drug metabolizing enzyme identification, including CYP enzymes (new drug as a substrate), CYP inhibition (new drug as an inhibitor), and CYP induction (new drug as an inducer), respectively.   Appendix D describes general considerations in the in vitro evaluation of P-gp substrates and inhibitors.  Figures 1 and 2 in Appendix D provide decision trees on when in vivo P-gp based interaction studies are indicated based on in vitro evaluation.

B. Specific In Vivo Clinical Investigations

In addition to in vitro metabolism and drug-drug interaction studies, appropriately designed pharmacokinetic studies, usually performed in the early phases of drug development, can provide important information about metabolic routes of elimination, their contribution to overall elimination, and metabolic drug-drug interactions.  Together with information from in vitro studies, these in vivo investigations can be a primary basis of labeling statements and can often help avoid the need for further investigations.  Further recommendations about these types of studies appear in section IV of this guidance.

C. Population Pharmacokinetic Screens

Population pharmacokinetic analyses of data obtained from large-scale clinical studies with sparse or intensive blood sampling can be valuable in characterizing the clinical impact of known or newly identified interactions, and in making recommendations for dosage modifications.  The results from such analyses can be informative and sometimes conclusive when the clinical studies are adequately designed to detect significant changes in drug exposure due to drug-drug interactions.  Simulations can provide valuable insights into optimizing the study design.  Population pharmacokinetic evaluations may detect unsuspected drug-drug interactions.  Population analysis can also provide further evidence of the absence of a drug-drug interaction when this is supported by prior evidence and mechanistic data.  However, it is unlikely that population analysis can be used to prove the absence of an interaction that is strongly suggested by information arising from in vivo studies specifically designed to assess a drug-drug interaction.  To be optimally informative, population pharmacokinetic studies should have carefully designed study procedures and sample collections.  A guidance for industry on population pharmacokinetics is available (Ref. 11).

IV. DESIGN OF IN VIVO DRUG-DRUG INTERACTION STUDIES

If in vitro studies and other information suggest that in vivo drug-drug interaction studies would be helpful (e.g., based on Figure 1 in Appendix B), the following general issues and approaches should be considered.  Consultation with FDA regarding study protocols is recommended.  In the following discussion, the term substrate (S) is used to indicate the drug studied to determine whether its exposure is changed by another drug, termed the interacting drug (I).  Depending on the study objectives, the substrate and the interacting drug can be the investigational agents or approved products.

A. Study Design

In vivo drug-drug interaction studies generally are designed to compare substrate concentrations with and without the interacting drug.  Because a specific study can consider a number of questions and clinical objectives, many study designs for studying drug-drug interactions can be considered.  A study can use a randomized crossover (e.g., S followed by S+I, S+I followed by S), a one-sequence crossover (e.g., S always followed by S+I or the reverse), or a parallel design (S in one group of subjects and S+I in another).  The following possible dosing regimen combinations for a substrate and interacting drug can also be used:  single dose/single dose, single dose/multiple dose, multiple dose/single dose, and multiple dose/multiple dose.  The selection of one of these or another study design depends on a number of factors for both the substrate and interacting drug, including (1) acute or chronic use of the substrate and/or interacting drug; (2) safety considerations, including whether a drug is likely to be an NTR (narrow therapeutic range) or non-NTR drug; (3) pharmacokinetic and pharmacodynamic characteristics of the substrate and interacting drugs; and (4) assessment of induction as well as inhibition.  The inhibiting/inducing drugs and the substrates should be dosed so that the exposures of both drugs are relevant to their clinical use, including the highest doses likely to be used.  Simulations can be helpful in selecting an appropriate study design.  The following considerations may be useful:

• When attainment of steady state is important and either the substrate or interacting drugs and/or their metabolites have long half-lives and a loading dose to reach steady state promptly cannot be used, special approaches may be needed.  These include the selection of a one-sequence crossover or a parallel design, rather than a randomized crossover study design.

• When it is important that a substrate and/or an interacting drug be studied at steady state because the effect of an interacting drug is delayed, as is the case for inducers and certain inhibitors, documentation that near steady state has been attained for the pertinent drug and metabolites of interest is critical.  This documentation can be accomplished by sampling over several days prior to the periods when test samples are collected.  This is important for both metabolites and the parent drug, particularly when the half-life of the metabolite is longer than the parent, and is especially important if both parent drug and metabolites are metabolic inhibitors or inducers.

• Studies can usually be open label (unblinded), unless pharmacodynamic endpoints (e.g., adverse events that are subject to bias) are critical to the assessment of the interaction.

• For a rapidly reversible inhibitor, administration of the interacting drug either just before or simultaneously with the substrate on the test day might increase sensitivity.  For a mechanism-based inhibitor (a drug that requires metabolism prior to its inactivation of the enzyme; examples include erythromycin), administration of the inhibitor prior to the administration of the substrate drug can maximize the effect.  If the absorption of an interacting drug (e.g., an inhibitor or an inducer) may be affected by other factors (e.g., the gastric pH), it may be appropriate to control the variables and confirm the absorption through plasma level measurements of the interacting drug.

• When the effects of two drugs on one another are of interest, the potential for interactions can be evaluated in a single study or two separate studies.  Some design options are randomized three-period crossover, parallel group, and one-sequence crossover.  

• To avoid variable study results because of uncontrolled use of dietary supplements, juices, or other foods that may affect various metabolizing enzymes and transporters during in vivo studies, it is important to exclude their use when appropriate. 

Examples of statements in a study protocol could include “Participants will be excluded for the following reasons:  Use of prescription or over-the-counter medications, including herbal products, or alcohol within two weeks prior to enrollment,” “For at least two weeks prior to the start of the study until its conclusion, volunteers will not be allowed to eat any food or drink any beverage containing alcohol, grapefruit or grapefruit juice, apple or orange juice, vegetables from the mustard green family (e.g., kale, broccoli, watercress, collard greens, kohlrabi, brussels sprouts, mustard) and charbroiled meats.” 
           

B. Study Population

Clinical drug-drug interaction studies can generally be performed using healthy volunteers.  Findings in this population should predict findings in the patient population for which the drug is intended.  Safety considerations may preclude the use of healthy subjects, however, and in certain circumstances, subjects drawn from the population of patients for whom the investigational drug is intended offer advantages, including the opportunity to study pharmacodynamic endpoints not present in healthy subjects.  Performance of phenotype or genotype determinations to identify genetically determined metabolic polymorphisms is important in evaluating effects on enzymes with polymorphisms, notably CYP2D6, CYP2C19, and CYP2C9.  The extent of drug interactions (inhibition or induction) may be different depending on the subjects’ genotype for the specific enzyme being evaluated.  Subjects lacking the major clearance pathway, for example, cannot show metabolism and remaining pathways can become important and should be understood and examined.

C. Choice of Substrate and Interacting Drugs
 

1. Investigational Drug as an Inhibitor or an Inducer of CYP Enzymes

In contrast to earlier approaches that focused mainly on a specific group of approved drugs (digoxin, hydrochlorothiazide) where co-administration was likely or the clinical consequences of an interaction were of concern, improved understanding of the mechanistic basis of metabolic drug-drug interactions enables more general approaches to and conclusions from specific drug-drug interaction studies.  In studying an investigational drug as the interacting drug, the choice of substrates (approved drugs) for initial in vivo studies depends on the P450 enzymes affected by the interacting drug.  In testing inhibition, the substrate selected should generally be one whose pharmacokinetics are markedly altered by co-administration of known specific inhibitors of the enzyme systems to assess the impact of the interacting investigational drug.  Examples of substrates include (1) midazolam for CYP3A; (2) theophylline for CYP1A2; (3) repaglinide for CYP2C8; (4) warfarin for CYP2C9 (with the evaluation of S-warfarin); (5) omeprazole for CYP2C19; and (6) desipramine for CYP2D6.  Additional examples of substrates, along with inhibitors and inducers of specific CYP enzymes, are listed in Table 2 in Appendix A.  If the initial study determines an investigation drug either inhibit or induce metabolism, further studies using other substrates, representing a range of substrates, based on the likelihood of co-administration, may be useful.  If the initial study is negative with the most sensitive substrates (for sensitive substrates, see Tables 3 and 4 in Appendix A), it can be presumed that less sensitive substrates will also be unaffected.

CYP3A inhibitors can be classified based on their in vivo fold-change in the plasma AUC of oral midazolam or other CYP3A substrate, when given concomitantly.  For example, if an investigational drug increases the AUC of oral midazolam or other CYP3A substrates by 5-fold or higher (> 5-fold), it can be labeled as a strong CYP3A inhibitor.  If an investigational drug, when given at the highest dose and shortest dosing interval, increases the AUC of oral midazolam or other sensitive CYP3A substrates by between 2- and 5-fold ( > 2- and <5-fold) when given together, it can be labeled as a moderate CYP3A inhibitor.  Similarly, if an investigational drug, when given at the highest dose and shortest dosing interval, increases the AUC of oral midazolam or other sensitive CYP3A substrates by between 1.25- and 2-fold ( > 1.25- and  < 2-fold), it can be labeled as a weak CYP3A inhibitor.  When an investigational drug is determined to be an inhibitor of CYP3A, its interaction with sensitive CYP3A substrates or CYP3A substrates with narrow therapeutic range (see Table 3 in Appendix A for a list) can be described in various sections of the labeling, as appropriate.  Similar classifications of inhibitors of other CYP enzymes are discussed in section V.

When an in vitro evaluation cannot rule out the possibility that an investigational drug is an inducer of CYP3A (see Appendix C-3), an in vivo evaluation can be conducted using the most sensitive substrate (e.g., oral midazolam, see Table 3 in Appendix A).  When midazolam has been co-administered orally following administration of multiple doses of the investigational drug, as may have been done as part of an in vivo inhibition evaluation, and the results are negative, it can be concluded that the investigational drug is not an inducer of CYP3A (in addition to the conclusion that it is not an inhibitor of CYP3A).  In vivo induction evaluation has often been conducted with oral contraceptives.  However, as they are not the most sensitive substrates, negative data may not exclude the possibility that the investigational drug may be an inducer of CYP3A.

Simultaneous administration of a mixture of substrates of CYP enzymes in one study (i.e., a “cocktail approach”) in human volunteers is another way to evaluate a drug’s inhibition or induction potential, provided that the study is designed properly and the following factors are present:  (1) the substrates are specific for individual CYP enzymes; (2) there are no interactions among these substrates; and (3) the study is conducted in a sufficient number of subjects (see section IV.G).  Negative results from a cocktail study can eliminate the need for further evaluation of particular CYP enzymes.  However, positive results can indicate the need for further in vivo evaluation to provide quantitative exposure changes (such as AUC, Cmax), if the initial evaluation only assessed the changes in the urinary parent to metabolite ratios.  The data generated from a cocktail study can supplement data from other in vitro and in vivo studies in assessing a drug’s potential to inhibit or induce CYP enzymes.

2.         Investigational Drug as a Substrate of CYP Enzymes

In testing an investigational drug for the possibility that its metabolism is inhibited or induced (i.e., as a substrate), selection of the interacting drugs should be based on in vitroorin vivo studies identifying the enzyme systems that metabolize the drug.  The choice of interacting drug can then be based on known, important inhibitors of the pathway under investigation.  For example, if the investigational drug is shown to be metabolized by CYP3A and the contribution of this enzyme to the overall elimination of this drug is either substantial (> 25% of the clearance pathway) or unknown, the choice of inhibitor and inducer could be ketoconazole and rifampin, respectively, because they are the most sensitive in identifying an effect of interest.  If the study results are negative, then absence of a clinically important drug-drug interaction for the metabolic pathway would have been demonstrated.  If the clinical study of the strong, specific inhibitor/inducer is positive and the sponsor wished to determine whether there is an interaction between the test drug and other less potent specific inhibitors or inducers, or to give advice on dosage adjustment, further clinical studies would generally be needed (see Table 2, Appendix A, for a list of CYP inhibitors and inducers; see Table 5, Appendix A, for additional 3A inhibitors).  If a drug is metabolized by CYP3A and its plasma AUC is increased 5-fold or higher by CYP3A inhibitors, it is considered a sensitive substrate of CYP3A.  The labeling can indicate that it is a “sensitive CYP3A substrate” and its use with strong or moderate inhibitors may call for caution, depending on the drug’s exposure-response relationship.  If a drug is metabolized by CYP3A and its exposure-response relationship indicates that increases in the exposure levels by the concomitant use of CYP3A inhibitors may lead to serious safety concerns (e.g., Torsades de Pointes), it is considered as a “CYP3A substrate with narrow therapeutic range” (see Table 3 of Appendix A for a list).  Similar classifications of substrates of other CYP enzymes are discussed in section V and listed in Table 6, Appendix A.

If an orally administered drug is a substrate of CYP3A and has low oral bioavailability because of extensive presystemic extraction contributed by enteric CYP3A, grapefruit juice may have a significant effect on its systemic exposure.  Use of the drug with grapefruit juice may call for caution, depending on the drug’s exposure-response relationship (see section V for labeling implications).

If a drug is a substrate of CYP3A or P-gp and co-administration with St. John’s wort can decrease the systemic exposure and effectiveness, St John’s wort may be listed in the labeling along with other known inducers, such as rifampin, rifabutin, rifapentin, dexamethasone, phenytoin, carbamazepine, or phenobarbital, as possibly decreasing plasma levels.

If a drug is metabolized by a polymorphic enzyme (such as CYP2D6, CYP2C9, or CYP2C19), the comparison of pharmacokinetic parameters of this drug in poor metabolizers versus extensive metabolizers may indicate the extent of interaction of this drug with strong inhibitors of these enzymes, and make interaction studies with such inhibitors unnecessary.  When the above study shows significant interaction, further evaluation with weaker inhibitors may be necessary.    

There may be situations when an evaluation of the effect of multiple CYP inhibitors on the drug can be informative.  For example, it may be appropriate to conduct an interaction study with more that one inhibitor if all of the following conditions are met: (1) the drug exhibits blood concentration-dependent safety concerns; (2) multiple CYP enzymes are responsible for the metabolic clearance of the drug; (3) the residual or non-inhibitable drug clearance is low.  Under these conditions, the effect of multiple, CYP-selective inhibitors on the blood AUC of a drug may be much greater than the product of the fold AUC changes observed when the inhibitors are given individually with the drug.  The degree of uncertainty will depend on the residual fractional clearance (the smaller the fraction, the greater the concern) and the relative fractional clearances of the inhibited pathway.  However, if results from a study with a single inhibitor trigger a safety concern (i.e., contraindication), no multiple inhibitor studies will be necessary.  Additional considerations may include the likelihood of co-administration of the drug with multiple inhibitors. Before investigating the impact of multiple inhibitors on drug exposure, it is important to first characterize the individual effects of the CYP inhibitors and to estimate the combined effect of the inhibitors based on computer simulation. For safety concerns, lower doses of the investigational drug may be appropriate for evaluating the fold increase in systemic exposure when combined with multiple inhibitors.

The implications of simultaneous inhibition of a dominant CYP enzyme(s) and an uptake or efflux transporter that controls the availability of the drug to CYP enzymes can be just as profound as that of multiple CYP inhibitors.  For example, the large effect of co-administration of itraconazole and gemfibrozil on the systemic exposure (AUC) of repaglinide may be attributed to collective effects on both enzyme and transporters.  Unfortunately, current knowledge does not permit the presentation of specific guidance.  The sponsor will need to use appropriate judgement when considering this situation.

3.         Investigational Drug as an Inhibitor or an Inducer of P-gp Transporter

In testing an investigational drug for the possibility that it may be an inhibitor/inducer of P-gp, selection of digoxin or other known substrates of P-gp may be appropriate.

4.         Investigational Drug as a Substrate of  P-gp Transporter

In testing an investigational drug for the possibility that its transport may be inhibited or induced (as a substrate of P-gp), an inhibitor of P-gp, such as ritonavir, cyclosporine, or verapamil, or an inducer, such as rifampin should be studied.  In cases where the drug is also a CYP3A substrate, inhibition should be studied by using a strong inhibitor of both P-gp and CYP3A, such as ritonavir.

5.         Investigational Drug as a Substrate of other Transporters

In testing an investigational drug for the possibility that its disposition may be inhibited or induced (i.e., as a substrate of transporters other than or in addition to P-gp), it may be appropriate to use an inhibitor of many transporters (e.g., P-gp, OATP), such as cyclosporine.  Recent interactions involving drugs that are substrates for transporters other than or in addition to P-gp include some HMG Co-A reductase inhibitors, rosuvastatin, and pravastatin.

D. Route of Administration

The route of administration chosen for a metabolic drug-drug interaction study is important.  For an investigational agent, the route of administration should generally be the one planned for clinical use.  When multiple routes are being developed, the need for metabolic drug-drug interaction studies by multiple routes depends on the expected mechanism of interaction and the similarity of corresponding concentration-time profiles for parent and metabolites.  If only oral dosage forms will be marketed, studies with an intravenous formulation are not usually needed, although information from oral and intravenous dosings may be useful in discerning the relative contributions of alterations in absorption and/or presystemic clearance to the overall effect observed for a drug interaction.  Sometimes certain routes of administration can reduce the utility of information from a study.  For example, intravenous administration of a substrate drug may not reveal an interaction for substrate drugs where intestinal CYP3A activity markedly alters bioavailability.  For an approved agent used either as a substrate or interacting drug, the route of administration will depend on available marketed formulations.

E. Dose Selection

For both a substrate (investigational drug or approved drug) and interacting drug (investigational drug or approved drug), testing should maximize the possibility of finding an interaction.  For this reason, we recommend that the maximum planned or approved dose and shortest dosing interval of the interacting drug (as inhibitors or inducers) be used.  For example, when using ketoconazole as an inhibitor of CYP3A, dosing at 400 mg QD for multiple days would be preferable to lower doses.  When using rifampin as an inducer, dosing at 600 mg QD for multiple days would be preferable to lower doses.  In some instances, doses smaller than those to be used clinically may be recommended for substrates on safety grounds.  In such instances, any limitations of the sensitivity of the study to detect the drug-drug interaction due to the use of lower doses should be discussed by the sponsor in the protocol and study report.

F. Endpoints

Changes in pharmacokinetic parameters can be used to assess the clinical importance of drug-drug interactions.  Interpretation of findings from these studies will be aided by a good understanding of dose/concentration and concentration/response relationships for both desirable and undesirable drug effects in the general population or in specific populations.  A CDER/CBER guidance for industry on Exposure-Response Relationships — Study Design, Data Analysis, and Regulatory Applications provides considerations in the evaluation of exposure-response relationships.  In certain instances, reliance on endpoints in addition to pharmacokinetic measures/parameters may be useful.  Examples include INR measurement (when studying warfarin interactions) or QT interval measurements.

1.         Pharmacokinetic Endpoints

The following measures and parameters of substrate PK should be obtained in every study:  (1) exposure measures such as AUC, Cmax, time to Cmax (Tmax), and others as appropriate; and (2) pharmacokinetic parameters such as clearance, volumes of distribution, and half-lives.  In some cases, these measures may be of interest for the inhibitor or inducer as well, notably where the study is assessing possible effects on both study drugs.  Additional measures may help in steady state studies (e.g., trough concentration) to demonstrate that dosing strategies were adequate to achieve near steady state before and during the interaction.  In certain instances, an understanding of the relationship between dose, blood concentrations, and response may lead to a special interest in certain pharmacokinetic measures and/or parameters.  For example, if a clinical outcome is most closely related to peak concentration (e.g., tachycardia with sympathomimetics), Cmax or another early exposure measure might be most appropriate.  Conversely, if the clinical outcome is related more to extent of absorption, AUC would be preferred.  The frequency of sampling should be adequate to allow accurate determination of the relevant measures and/or parameters for the parent and metabolites.  For the substrate, whether the investigational drug or the approved drug, determination of the pharmacokinetics of important active metabolites is important.

2.         Pharmacodynamic Endpoints

Pharmacokinetic measures are usually sufficient for drug-drug interaction studies, although pharmacodynamic measures can sometimes provide additional useful information.  Pharmacodynamic measures may be indicated when a pharmacokinetic/pharmacodynamic relationship for the substrate endpoints of interest is not established or when pharmacodynamic changes do not result solely from pharmacokinetic interactions (e.g., additive effect of quinidine and tricyclic antidepressants on QT interval).  In most cases, when an approved drug is studied as a substrate, the pharmacodynamic impact of a given change in blood level (Cmax, AUC) caused by an investigational interaction should be known from other data.  If a PK/PD study is needed, it will generally need to be larger than the typical PK study (e.g., a study of QT interval effects).

G. Sample Size and Statistical Considerations

The goal of the interaction study is to determine whether there is any increase or decrease in exposure to the substrate in the presence of the interacting drug.  If there is, its implications must be assessed by an understanding of PK/PD relations both for Cmax and AUC.

Results of drug-drug interaction studies should be reported as 90% confidence intervals about the geometric mean ratio of the observed pharmacokinetic measures with (S+I) and without the interacting drug (S alone).  Confidence intervals provide an estimate of the distribution of the observed systemic exposure measure ratio of (S+I) versus (S alone) and convey a probability of the magnitude of the interaction.  In contrast, tests of significance are not appropriate because small, consistent systemic exposure differences can be statistically significant (p < 0.05) but not clinically relevant.

When a drug-drug interaction of potential importance is clearly present (e.g., comparisons indicate twofold (or lower for certain NTR drugs) or greater increments in systemic exposure measures for (S+I)), the sponsor should provide specific recommendations regarding the clinical significance of the interaction based on what is known about the dose-response and/or PK/PD relationship for either the investigational agent or the approved drugs used in the study.  For a new drug, the more difficult issue is the impact on the investigational drug as substrate.  For inhibition or induction by the investigational drug, the main consequence of a finding will be to add the drug to the list of inhibitors or inducers likely already present in labeling of the older drug.  This information can form the basis for reporting study results and for making recommendations in the package insert with respect to either the dose, dosing regimen adjustments, precautions, warnings, or contraindications of the investigational drug or the approved drug.  FDA recognizes that dose-response and/or PK/PD information can sometimes be incomplete or unavailable, especially for an older approved drug used as S.

The sponsor may wish to make specific claims in the package insert that no drug-drug interaction of clinical significance occurs.  In these instances, it would be helpful for the sponsor to recommend specific no effect boundaries, or clinical equivalence intervals, for a drug-drug interaction.  No effect boundaries represent the interval within which a change in a systemic exposure measure is considered not clinically meaningful. 

There are two approaches to defining no effect boundaries:

Approach 1:  No effect boundaries can be based on population (group) average dose and/or concentration-response relationships, PK/PD models, and other available information for the substrate drug to define a degree of difference caused by the interaction that is of no clinical consequence.  If the 90% confidence interval for the systemic exposure measurement in the drug-drug interaction study falls completely within the no effect boundaries, the sponsor can conclude that no clinically significant drug-drug interaction was present. 

Approach 2:  In the absence of no effect boundaries defined in Approach 1, a sponsor can use a default no effect boundary of 80-125% for both the investigational drug and the approved drugs used in the study.  When the 90% confidence intervals for systemic exposure ratios fall entirely within the equivalence range of 80-125%, standard Agency practice is to conclude that no clinically significant differences are present.  This is, however, a very conservative standard and a substantial sample would need to be studied to meet it.

The selection of the number of subjects for a given drug-drug interaction study will depend on how small an effect is clinically important to detect or rule out, the inter- and intra-subject variability in pharmacokinetic measurements, and possibly other factors or sources of variability not well recognized.

V. LABELING IMPLICATIONS

It is important that all relevant information on the metabolic pathways and metabolites and pharmacokinetic interactions be included in the PHARMACOKINETICS subsection of the CLINICAL PHARMACOLOGY section of the labeling.  The clinical consequences of metabolism and interactions should be placed in DRUG INTERACTIONS, WARNINGS AND PRECAUTIONS, BOXED WARNINGS, CONTRAINDICATIONS, or DOSAGE AND ADMINISTRATION sections, as appropriate.  Information related to clinical consequences should not be included in detail in more than one section, but rather referenced from one section to other sections, as appropriate.  When the metabolic pathway or interaction data results in recommendations for dosage adjustments, contraindications, or warnings (e.g., co-administration should be avoided) that are included in the BOXED WARNINGS, CONTRAINDICATIONS, WARNINGS AND PRECAUTIONS, or DOSAGE AND ADMINISTRATION sections, these recommendations should also be included in HIGHLIGHTS.  Refer to the guidance for industry on Labeling for Human Prescription Drug and Biological Products – Implementing the New Content and Format Requirements, and Clinical Pharmacology and Drug Interaction Labeling for more information on presenting drug interaction information in labeling. 

In certain cases, information based on clinical studies not using the labeled drug can be described, with an explanation that similar results may be expected for that drug.  For example, if a drug has been determined to be a strong inhibitor of CYP3A, it does not need to be tested with all CYP3A substrates to warn about an interaction with sensitive CYP3A substrates and CYP3A substrates with narrow therapeutic range.  An actual test involving a single substrate would lead to labeling concerning use with all sensitive and NTR substrates. 
Table 3 in Appendix A lists examples of sensitive CYP3A substrates and CYP3A substrates with narrow therapeutic range.

Table 5 in Appendix A lists examples of strong, moderate, and weak CYP3A inhibitors.  If a drug has been determined to be a sensitive CYP3A substrate or a CYP3A substrate with a narrow therapeutic range, it does not need to be tested with all strong or moderate inhibitors of CYP3A to warn about an interaction with strong or moderate CYP3A inhibitors, and it might be labeled in the absence of any actual study if its metabolism is predominantly by the CYP3A route.  Similarly, if a drug has been determined to be a sensitive CYP3A substrate or a CYP3A substrate with a narrow therapeutic range, it does not need to be tested with all CYP3A inducers to warn about an interaction with CYP3A inducers.  Examples of CYP3A inducers include rifampin, rifabutin, rifapentin, dexamethasone, phenytoin, carbamazepine, phenobarbital, and St. John's wort.

A similar classification system can be used for inhibitors of other CYP enzymes (Table 6 in Appendix A).

 

APPENDIX A- Tables

Table 1. Major human transporters (1,2)

(1) Note that this is not an exhaustive list.  For an updated list, see the following link http://www.fda.gov/cder/drug/drugInteractions/default.htm

(2)   ABC:ATP-binding cassette transporter superfamily; SLC: solute-linked carrier transporter family; SLCO: solute-linked carrier organic anion transporter family; MDR1: multi-drug resistance; MRP: multi-drug resistance related protein; BSEP:bile salt export pump; BCRP: breast cancer resistance protein; OAT: organic anion transporter; OCT: organic cation transporter; NTCP: sodium taurocholate co-transporting polypeptide; ASBT: apical sodium-dependent bile salt transporter.

Table 2. Examples of in vivo substrate, inhibitor, and inducer for specific CYP enzymes recommended for study (oral administration) (1,2)

(1) Substrates for any particular CYP enzyme listed in this table are those with plasma AUC values increased by 2-fold or higher when co-administered with inhibitors of that CYP enzyme; for CYP3A, only those with plasma AUC increased by 5-fold or higher are listed.  Inhibitors listed are those that increase plasma AUC values of substrates for that CYP enzyme by 2-fold or higher.  For CYP3A inhibitors, only those that increase AUC of CYP3A substrates by 5-fold or higher are listed.  Inducers listed are those that decrease plasma AUC values of substrates for that CYP enzyme by 30% or higher.
(2) Note that this is not an exhaustive list.  For an updated list, see the following link
http://www.fda.gov/cder/drug/drugInteractions/default.htm
(3) A clinical study can be conducted in smokers as compared to non-smokers (in lieu of an interaction study with an inducer), when appropriate.
(4) A clinical study can be conducted in poor metabolizers (PM) as compared to extensive metabolizers (EM) for the specific CYP enzyme (in lieu of an interaction study with an inhibitor), when appropriate.

Table 3.  Examples(1) of sensitive CYP3A substrates or CYP3A substrates with narrow therapeutic range

Sensitive CYP3A substrates(2)	CYP3A Substrates with Narrow therapeutic range(3)
budesonide, buspirone, eplerenone, eletriptan, felodipine, fluticasone, lovastatin, midazolam, saquinavir, sildenafil, simvastatin,  triazolam, vardenafil	alfentanil, astemizole(a), cisapride(a), cyclosporine, diergotamine, ergotamine, fentanyl, pimozide, quinidine, sirolimus, tacrolimus, terfenadine(a)

(1)  Note that this is not an exhaustive list.  For an updated list, see the following link http://www.fda.gov/cder/drug/drugInteractions/default.htm
(2)  Sensitive CYP3A substrates refers to drugs whose plasma AUC values have been shown to increase 5-fold or higher when co-administered with a known CYP3A inhibitor.
(3)  CYP3A substrates with narrow therapeutic range refers to drugs whose exposure-response indicates that increases in their exposure levels by the concomitant use of CYP3A inhibitors may lead to serious safety concerns (e.g., Torsades de Pointes).
(a) Not available in the United States.

Table 4.  Examples(1) of sensitive CYP substrates or CYP substrates with narrow therapeutic range

Sensitive  CYP1A2 substrates(2)	CYP1A2 substrates with narrow therapeutic range(3)
duloxetine, alosetron	theophylline, tizanidine
Sensitive CYP2C8 substrates(2)	CYP2C8 substrates with narrow therapeutic range(3)
repaglinide	paclitaxel
Sensitive CYP2C9 substrates(2)	CYP2C9 substrates with narrow therapeutic range(3)
	warfarin, phenytoin
Sensitive CYP2C19 substrates(2)	CYP2C19 substrates with narrow therapeutic range(3)
omeprazole	s-mephenytoin
Sensitive CYP2D6 substrates(2)	CYP2D6 substrates with narrow therapeutic range(3)
desipramine	thioridazine

 (1)  Note that this is not an exhaustive list.  For an updated list, see the following link http://www.fda.gov/cder/drug/drugInteractions/default.htm
(2)  Sensitive CYP substrates refers to drugs whose plasma AUC values have been shown to increase 5-fold or higher when co-administered with a known CYP inhibitor.
(3)  CYP substrates with narrow therapeutic range refers to drugs whose exposure-response indicates that increases in their exposure levels by the concomitant use of CYP inhibitors may lead to serious safety concerns (e.g., Torsades de Pointes).

Table 5.  Classification of CYP3A inhibitors(1) 

Strong CYP3A inhibitors	Moderate CYP3A inhibitors	Weak CYP3A inhibitors
> 5-fold increase in AUC	> 2 but <5-fold increase in AUC	> 1.25 but <2-fold increase in AUC
atazanavir,  clarithromycin, indinavir,  itraconazole, ketoconazole, nefazodone, nelfinavir, ritonavir, saquinavir, telithromycin	amprenavir, aprepitant, diltiazem, erythromycin, fluconazole, fosamprenavir, grapefruit juice(a), verapamil	cimetidine

(1)  Please note the following:

• A strong inhibitor is one that caused a > 5-fold increase in the plasma AUC values or more than 80% decrease in clearance of CYP3A substrates (not limited to midazolam, a sensitive CYP3A substrate) in clinical evaluations
• A moderate inhibitor is one that caused a > 2- but < 5-fold increase in the AUC values or 50-80% decrease in clearance of sensitive CYP3A substrates when the inhibitor was given at the highest approved dose and the shortest dosing interval in clinical evaluations.
• A weak inhibitor is one that caused a > 1.25 - but < 2-fold increase in the AUC values or 20-50% decrease in clearance of sensitive CYP3A substrates when the inhibitor was given at the highest approved dose and the shortest dosing interval in clinical evaluations
• This is not an exhaustive list.  For an updated list, see the following link http://www.fda.gov/cder/drug/drugInteractions/default.htm
  (a)  The effect of grapefruit juice varies widely.
  

Table 6.  Classification of inhibitors of other CYP enzymes(1) 

Strong CYP1A2 inhibitors	Moderate CYP1A2 inhibitors	Weak CYP1A2 inhibitors
fluvoxamine	ciprofloxacin, mexiletine, propafenone, zileuton	acyclovir, cimetidine, famotidine, norfloxacin, verapamil
Strong CYP2C8 inhibitors	Moderate CYP2C8 inhibitors	Weak CYP2C8 inhibitors
gemfibrozil		trimethoprim
Strong CYP2C9 inhibitors	Moderate CYP2C9 inhibitors	Weak CYP2C9 inhibitors
	amiodarone, fluconazole, oxandrolone	sulfinpyrazone
Strong CYP2C19 inhibitors	Moderate CYP2C19 inhibitors	Weak CYP2C19 inhibitors
omeprazole
Strong CYP2D6 inhibitors	Moderate CYP2D6 inhibitors	Weak CYP2D6 inhibitors
fluoxetine, paroxetine, quinidine	duloxetine, terbinafine	amiodarone, sertraline

 (1)  Please note the following:

• A strong inhibitor is one that caused a > 5-fold increase in the plasma AUC values or more than 80% decrease in clearance of CYP substrates (not limited to  sensitive CYP substrate) in clinical evaluations
• A moderate inhibitor is one that caused a > 2- but < 5-fold increase in the AUC values or 50-80% decrease in clearance of sensitive CYP substrates when the inhibitor was given at the highest approved dose and the shortest dosing interval in clinical evaluations.
• A weak inhibitor is one that caused a > 1.25 - but < 2-fold increase in the AUC values or 20-50% decrease in clearance of sensitive CYP substrates when the inhibitor was given at the highest approved dose and the shortest dosing interval in clinical evaluations
• This is not an exhaustive list.  For an updated list, see the following link http://www.fda.gov/cder/drug/drugInteractions/default.htm

  

Appendix B- Figures

Figure 1.   CYP-Based Drug-Drug Interaction Studies — Decision Tree

NME:  New molecular entity
* Additional population pharmacokinetic analysis may assist the overall evaluation.
+ See Appendix C for criteria to determine whether an NME is an inhibitor (Appendix C-2) or an inducer (Appendix C-3) of a specific CYP enzyme; negative results from a cocktail study would preclude further evaluation to determine whether an NME is an inhibitor or an inducer of a particular CYP enzyme (see IV.C.1).  (Reference: Journal of Clinical Pharmacology, 39:1006-1014, 1999.)

APPENDIX C-1

In Vitro Drug Metabolizing Enzyme Identification

Drug metabolizing enzyme identification studies, often referred to as reaction phenotyping studies, are a set of experiments that identify the specific enzymes responsible for metabolism of a drug.  Oxidative and hydrolytic reactions involve cytochrome P450 (CYP) and non-CYP enzymes.  For many drugs, transferase reactions are preceded by oxidation or hydrolysis of the drug.  However, direct transferase reactions may represent a major metabolic pathway for compounds containing polar functional groups. 

An efficient approach is to determine the metabolic profile (identify metabolites formed and their quantitative importance) of a drug and estimate the relative contribution of CYP enzymes to clearance before initiating studies to identify specific CYP enzymes that metabolize the drug.  Identification of CYP enzymes is warranted if CYP enzymes contribute > 25% of a drug’s total clearance.  In vitro identification of drug metabolizingCYPenzymes helps predict the potential for in vivo drug-drug interactions, the impact of polymorphic enzyme activity on drug disposition, and the formation of toxic or active metabolites.  There are few documented cases of clinically significant drug-drug interactions related to non-CYP enzymes, but the identification of drug metabolizing enzymes of this kind (i.e., glucuronosyltransferases, sulfotransferases, and N-acetyl transferases) is encouraged.  Although classical biotransformation studies are not a general requirement for the evaluation of therapeutic biologics, certain protein therapeutics modify the metabolism of drugs that are metabolized by CYP enzymes.  Given their unique nature, consultation with FDA is appropriate before initiating drug-drug interaction studies involving biologics.

1. Metabolic Pathway Identification Experiments (Determination of Metabolic Profile

(a) Rationale and Goals

Data obtained from in vitro drug metabolic pathway identification experiments help determine whether experiments to identify drug metabolizing enzymes are warranted, and guide the appropriate design of any such experiments.  The metabolic pathway identification experiments should identify the number and classes of metabolites produced by a drug and whether the metabolic pathways are parallel or sequential.

(b)  Tissue Selection for Metabolic Pathway Identification Experiments

Human