Bioavailability and Bioequivalence -2- Types, Methods, Protocol.pptx

Bioavailability and Bioequivalence

1. BIOAVAILABILITY TYPES, PARAMETERS, SIGNIFICANCE, study Protocol and Methods of assessment of bioavailability

2. Introduction Regulatory agencies such as the FDA require submission of bioavailability data in applications to market new drug products. A drug product’s bioavailability provides an estimate of the relative fraction of the administered dose that is absorbed into the systemic circulation. Determining the fraction (f) of administered dose absorbed involves comparing the drug product’s systemic exposure (represented by the concentration-versus-time or pharmacokinetic profile) with that of a suitable reference product. For systemically available drug products, bioavailability is most often assessed by determining the area under the drug plasma concentration-versus-time profile ( AUC ). The AUC is considered the most reliable measure of a drug’s bioavailability, as it is directly proportional to the total amount of unchanged drug that reaches the systemic circulation.

3. Absolute Bioavailability Absolute bioavailability compares the bioavailability of the active drug in the systemic circulation following extra-vascular administration with the bioavailability of the same drug following intravenous administration (Fig. 16-4). Intravenous drug administration is considered 100% absorbed . The route of extra-vascular administration can be inhaled, intramuscular, oral, rectal, subcutaneous, sublingual, topical, transdermal , etc. The absolute bioavailability is the dose-corrected AUC of the extra- vascularly administered drug product divided by the AUC of the drug product given intravenously . Thus, for an oral formulation, the absolute bioavailability is calculated as follows: where F abs is the fraction of the dose absorbed , expressed as a percentage; AUC po is the AUC following oral administration; D iv is the dose administered intravenously; AUC iv is the AUC following intravenous administration; and D po is the dose administered orally.

4. Absolute availability, F abs , may be expressed as a fraction or as a percent by multiplying F abs × 100. A drug given by the intravenous route will have an absolute bioavailability of 100% (f = 1). A drug given by an extra-vascular route may have an F abs = 0 (no systemic absorption) and F abs = 1.0 (100% systemic absorption).

5. Relative Bioavailability Another type of comparative bioavailability assessment is provided by a relative bioavailability study. In a relative bioavailability study, the systemic exposure of a drug in a designated formulation (generally referred to as treatment A or reference formulation) is compared with that of the same drug administered in a reference formulation (generally referred to as treatment B or test formulation). In a relative bioavailability study, the AUCs of the two formulations are compared as follows: where F rel is the relative bioavailability of treatment (formulation) A, expressed as a percentage; AUC A is the AUC following administration of treatment (formulation) A; D A is the dose of formulation A; AUC B is the AUC of formulation B; and D B is the dose of formulation B.

6. Relative bioavailability studies are frequently included in regulatory submissions. For example , the FDA recommends that new drug developers routinely use an oral solution as the reference for a new oral formulation, for the purpose of assessing how formulation impacts bioavailability. Other types of relative bioavailability studies used in drug development include studies to characterize food effects and drug–drug interactions. In a food-effect bioavailability study , oral bioavailability of the drug product given with food (usually a high-fat, high-calorie meal) is compared to oral bioavailability of the drug product given under fasting conditions. The drug product given under fasting conditions is treated as the reference treatment . The goal of a drug–drug interaction study is to determine whether there is an increase or decrease in bioavailability in the presence of the interacting drug. As such, the general drug–drug interaction study design compares drug relative bioavailability with and without (reference treatment) the interacting drug.

7. Relative bioavailability studies are used in developing new formulations of existing immediate-release drug products , such as new modified-release versions or new fixed-dose combination formulations. In the case of a new modified-release version, the reference product is the approved immediate-release product. In the case of a new fixed-dose combination , the reference product can be the single-entity drug products administered either separately ( i.e , three treatments for a fixed-dose combination doublet) or concurrently according to an approved combination regimen ( i.e , two treatments). Relative bioavailability study designs are also commonly used for bridging formulations during drug development , for example, to evaluate how drug systemic availability from a new pre-market formulation compares with that from an existing premarket formulation.

8. STUDY PROTOCOL

9. Reference Listed Drug (RLD) For bioequivalence studies of generic products , one formulation of the drug is chosen as a reference standard against which all other formulations of the drug are compared. The FDA designates a single reference listed drug as the standard drug product to which all generic versions must be shown to be bioequivalent . The FDA hopes to avoid possible significant variations among generic drugs and their brand-name counterparts . Such variations could result if generic drugs were compared to different reference listed drugs. The reference drug product should be administered by the same route as the comparison formulations unless an alternative route or additional route is needed to answer specific pharmacokinetic questions . For example , if an active drug is poorly bioavailable after oral administration , the drug may be compared to an oral solution or an intravenous injection . For bioequivalence studies on a proposed generic drug product, the reference standard is the reference listed drug (RLD), which is listed in the FDA’s Approved Drug Products with Therapeutic Equivalence Evaluations—the Orange Book and the proposed generic drug product is often referred to as the “ test” drug product.

10. The RLD is generally a formulation currently marketed with a fully approved NDA for which there are valid scientific safety and efficacy data. The RLD is usually the innovator’s or original manufacturer’s brand-name product and is administered according to the dosage recommendations in the labeling. Before beginning an in vivo bioequivalence study , the total content of the active drug substance in the test product (generally the generic product) must be within 5% of that of the reference product . Moreover, in vitro comparative dissolution or drug-release studies under various specified conditions are usually performed for both test and reference products before performing the in vivo bioequivalence study.

11. Methods For Assessing Bioavailability And Bioequivalence

12. Methods For Assessing Bioavailability And Bioequivalence Direct and indirect methods may be used to assess drug bioavailability. Bioequivalence of a drug product is demonstrated by the rate and extent of drug absorption, as determined by comparison of measured parameters . The FDA’s regulations (US-FDA, CDER, 2014a) list the following approaches to determining bioequivalence, in descending order of accuracy, sensitivity, and reproducibility : In vivo measurement of active moiety or moieties in biological fluid ( i.e , a pharmacokinetic study) In vivo pharmacodynamic (PD) comparison In vivo limited clinical comparison In vitro comparison Any other approach deemed acceptable (by the FDA)

13. A. IN VIVO MEASUREMENT OF ACTIVE MOIETY OR MOIETIES IN BIOLOGICAL FLUIDS 1. Plasma Drug Concentration Measurement of drug concentrations in blood, plasma, or serum after drug administration is the most direct and objective way to determine systemic drug bioavailability. By appropriate blood sampling , an accurate description of the plasma drug concentration–time profile of the therapeutically active drug substance(s) can be obtained using a validated drug assay. t max : The time of peak plasma concentration , t max , corresponds to the time required to reach maximum drug concentration after drug administration . At t max , peak drug absorption occurs and the rate of drug absorption exactly equals the rate of drug elimination (Fig. 16-3).

14. Drug absorption still continues after t max is reached, but at a slower rate . When comparing drug products , t max can be used as an approximate indication of drug absorption rate. The value for t max will become smaller (indicating less time required to reach peak plasma concentration ) as the absorption rate for the drug becomes more rapid. Units for t max are units of time ( eg , hours, minutes). For many systemically absorbed drugs , small differences in t max may have little clinical effect on overall drug product performance . However, for some drugs, such as delayed action drug products, large differences in t max may have clinical impact.

15. C max : The peak plasma drug concentration , C max , represents the maximum plasma drug concentration obtained after oral administration of drug . For many drugs, a relationship is found between the pharmacodynamic drug effect and the plasma drug concentration . C max provides indications that the drug is sufficiently systemically absorbed to provide a therapeutic response . In addition, C max provides warning of possibly toxic levels of drug. The units of C max are concentration units ( e.g , mg/ mL , ng / mL ). Although not a unit for rate , C max is often used in bioequivalence studies as a surrogate measure for the rate of drug bioavailability. So, the expectation is that as the rate of drug absorption goes up , the peak or C max will also be larger . If the rate of drug absorption goes down, then the peak or C max is smaller.

16. AUC : The area under the plasma level–time curve, AUC, is a measurement of the extent of drug bioavailability (see Fig. 16-3). The AUC reflects the total amount of active drug that reaches the systemic circulation . The AUC is the area under the drug plasma level–time curve from t = 0 to t = ∞, and is equal to the amount of unchanged drug reaching the general circulation divided by the clearance. where F = fraction of dose absorbed, D = dose, k= elimination rate constant, and V D = volume of distribution. The AUC is independent of the route of administration and processes of drug elimination as long as the elimination processes do not change. The AUC can be determined by a numerical integration procedure, such as the trapezoidal rule method . The units for AUC are concentration × time ( eg , mg·h / mL ).

17. For many drugs, the AUC is directly proportional to dose . For example, if a single dose of a drug is increased from 250 to 1000 mg, the AUC will also show a fourfold increase (Figs. 16-5 and 16-6). In some cases, the AUC is not directly proportional to the administered dose for all dosage levels. For example , as the dosage of drug is increased, one of the pathways for drug elimination may become saturated (Fig. 16-7). Drug elimination includes the processes of metabolism and excretion . Drug metabolism is an enzyme-dependent process . For drugs such as salicylate and phenytoin , continued increase of the dose causes saturation of one of the enzyme pathways for drug metabolism and consequent prolongation of the elimination half-life . The AUC thus increases disproportionally to the increase in dose, because a smaller amount of drug is being eliminated ( i.e , more drug is retained). When the AUC is not directly proportional to the dose, bioavailability of the drug is difficult to evaluate because drug kinetics may be dose dependent. Conversely, absorption may also become saturated resulting in lower-than-expected changes in AUC .

18. 2. Urinary Drug Excretion Data Urinary drug excretion data is an indirect method for estimating bioavailability. The drug must be excreted in significant quantities as unchanged drug in the urine. In addition, timely urine samples must be collected and the total amount of urinary drug excretion must be obtained. The cumulative amount of drug excreted in the urine, D u , is related directly to the total amount of drug absorbed . Experimentally, urine samples are collected periodically after administration of a drug product. Each urine specimen is analyzed for free drug using a specific assay . A graph is constructed that relates the cumulative drug excreted to the collection-time interval (Fig. 16-8).

19. The relationship between the cumulative amount of drug excreted in the urine and the plasma level– time curve is shown in Fig. 16-8. When the drug is almost completely eliminated (point C), the plasma concentration approaches zero and the maximum amount of drug excreted in the urine, Du, is obtained. dD u / dt : The rate of drug excretion. Because most drugs are eliminated by a first-order rate process , the rate of drug excretion is dependent on the first-order elimination rate constant , k, and the concentration of drug in the plasma, C p . In Fig. 16-9, the maximum rate of drug excretion, ( dD u / dt ) max , is at point B, whereas the minimum rate of drug excretion is at points A and C. Thus, a graph comparing the rate of drug excretion with respect to time should be similar in shape to the plasma level–time curve for that drug (Fig. 16-10). t∞ : The total time for the drug to be excreted. In Figs. 16-9 and 16-10, the slope of the curve segment A–B is related to the rate of drug absorption, whereas point C is related to the total time required after drug administration for the drug to be absorbed and completely excreted, t = ∞. The t∞ is a useful parameter in bioequivalence studies that compare several drug products .

20. B. Bioequivalence Studies Based On Pharmacodynamic End points— in Vivo Pharmacodynamic (PD) Comparison In some cases, the quantitative measurement of a drug in plasma is not available or in vitro approaches are not applicable. The following criteria for a PD endpoint study are important: A dose–response relationship is demonstrated. The PD effect of the selected dose should be at the rising phase of the dose–response curve, as shown in Fig. 16-11. Sufficient measurements should be taken to assure an appropriate PD response profile . All PD measurement assays should be validated for specificity, accuracy, sensitivity, and precision. For locally acting , non-systemically absorbed drug products, such as topical corticosteroids , plasma drug concentrations may not reflect the bioavailability of the drug at the site of action. An acute pharmacodynamic effect , such as an effect on forced expiratory volume, FEV1 (inhaled bronchodilators), or skin blanching (topical corticosteroids) can be used as an index of drug bioavailability.

21. In this case, the acute pharmacodynamic effect is measured over a period of time after administration of the drug product. Measurements of the pharmacodynamic effect should be made with sufficient frequency to permit a reasonable estimate for a time period at least three times the half-life of the drug (Gardner, 1977). The use of an acute pharmacodynamic effect to determine bioavailability generally requires demonstration of a dose–response curve (Fig. 16-11 and Chapter 21). Bioavailability is determined by characterization of the dose–response curve . For bioequivalence determination, pharmacodynamic parameters including the total area under the acute pharmacodynamic effect–time curve, peak pharmacodynamic effect , and time for peak pharmacodynamic effect are obtained from the pharmacodynamic effect–time curve (Fig. 16-12). The onset time and duration of the pharmacokinetic effect may also be included in the analysis of the data.

22. The use of pharmacodynamic endpoints for the determination of bioavailability and bioequivalence is much more variable than the measurement of plasma or urine drug concentrations. Some examples of drug products for which bioequivalence PD endpoints are recommended are listed on Table 16-2.

23. C. Bioequivalence Studies Based On Clinical Endpoints—clinical Endpoint Study The clinical endpoint study is the least accurate , least sensitive to bioavailability differences, and most variable . A predetermined clinical endpoint is used to evaluate comparative clinical effect in the chosen patient population . Highly variable clinical responses require the use of a large number of patient study subjects, which increases study costs and requires a longer time to complete compared to the other approaches for determination of bioequivalence. A placebo arm is usually included to demonstrate that the study is sufficiently sensitive to identify the clinical effect in the patient population enrolled in the study. The FDA considers this approach only when analytical methods and pharmacodynamic methods are not available to permit use of one of the approaches described above. The clinical study is usually a limited, comparative, parallel clinical study using predetermined clinical endpoint(s).

24. Clinical endpoint BE studies are recommended for those products that have negligible systemic uptake , for which there is no identified PD measure, and for which the site of action is local . Comparative clinical studies have been used to establish bioequivalence for topical antifungal drug products ( eg , ketoconazole ) and for topical acne preparations. For dosage forms intended to deliver the active moiety to the bloodstream for systemic distribution , this approach may be considered acceptable only when analytical methods cannot be developed to permit use of one of the other approaches. Some examples of drug products where a clinical endpoint bioequivalence study is recommended (Davit and Conner, 2015) are listed in Table 16-3.

25. D. In Vitro Studies Comparative drug release/dissolution studies under certain conditions may give an indication of drug bioavailability and bioequivalence . Ideally, the in vitro drug dissolution rate should correlate with in vivo drug bioavailability (see Chapter 15 on in vivo–in vitro correlation, IVIVC ). The test and reference products for which in vitro release rates form the basis of the bioequivalence usually demonstrate Q 1 /Q 2 sameness (qualitatively same inactive ingredients in the quantitative same amounts). Comparative dissolution studies are often performed on several test formulations of the same drug during drug development. Comparative dissolution profiles may be considered similar if the similarity factor (f2) is greater than 50 (see Chapter 15). For drugs whose dissolution rate is related to the rate of systemic absorption , the test formulation that demonstrates the most rapid rate of drug dissolution in vitro will generally have the most rapid rate of drug bioavailability in vivo . Under certain conditions, comparative dissolution profiles of higher and lower dose strengths of a solid oral drug product such as an immediate-release tablet are used to obtain a waiver ( biowaiver ) of performing additional in vivo bioequivalence studies (see section on biowaivers ).

26. E. Other Approaches Deemed Acceptable (By The FDA) The FDA may also use in vitro approaches other than comparative dissolution for establishing bioequivalence. The use of in vitro biomarkers and in vitro binding studies has been proposed to establish bioequivalence. For example, cholestyramine resin is a basic quaternary ammonium anion-exchange resin that is hydrophilic , insoluble in water, and not absorbed in the gastrointestinal tract. The bioequivalence of cholestyramine resin is performed by equilibrium and kinetic binding studies of the resin to bile acid salts (US-FDA, CDER, 2012a).

27. 7. Bioequivalence Studies Based On Multiple Endpoints The FDA may recommend two or more bioequivalence studies, each based on a different approach, for some drug products with complex delivery systems or mechanisms of action. Some examples of drug products that FDA requires multiple bioequivalence studies (Davit and Conner, 2015) are listed in Table 16-4.

28. Reference & Keys Applied Biopharmaceutics & pharmacokinetics by LEON SHARGEL Sixth Edition, Page # 406-413.