Bioavailability &

Bioequivalence

byMr. Sachin Jadhav

UDPS, Utkal university.

Drug Product

Drug in Blood

Distribution toTissue and Receptor sites

MetabolismExcretion

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CONTENTS Definitions Objectives of Bioavailability studies Methods of Bioavailability measurement --Pharmacokinetic methods: 1. Plasma level time studies 2. Urinary excretion studies --Pharmacodynamic methods: 1. Acute pharmacological response 2. Therapeutic response In vitro dissolution studies and bioavailability IVIVC Correlation Bioequivalence experimental study designs 1. Completely randomized designs 2. Randomized block designs 3. Repeated measures, cross over, carry-over designs 4. Latin square designs Statistical interpretation of bioequivalence data 1.Analysis of variance (ANOVA) 2.Confidence interval approach

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Bioavailability: It is rate and extent of absorption of unchanged drug from its dosage form.Rate- acute conditions- asthma, pain etcExtent( amount ) – chronic conditions- hypertension.Influence of route of administrationPARENTRAL> ORAL> RECTAL>TOPICAL

Absolute bioavailability:When systemic availability of a drug administered orally is determined in comparison to its I.V. administration, denoted by F.

Relative bioavailability:When systemic availability of a drug after oral administration is Compared with that of oral standard of the same drug ( Solution or suspension )and denoted by Fr.

Definitions

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Chemical equivalence: When two or more drug products contain the same chemical substance as an active ingredient in the same amount it is called chemical equivalence.

Bioequivalence: It is relative term that denotes drug substance in two or more identical dosage forms reaches the systemic circulation at the same relative rate to the same relative extent. i.e. plasma concentration-time profiles will be identical without significant statistical differences.

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Objectives of Bioavailability studies

It is important in thePrimary stages of development of dosage form of new drug entity to find its therapeutic utility.

Determination of influence of excipients on absorption.

Development of new formulations of existing drugs.

Control of quality of drug products and influence of processing factors , storage and stability on absorption.

Comparison of drug in different dosage forms or same dosage form of different manufacturer.

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Single dose versus multiple dose studies: Single dose bioavailability studies are very common, easy, less exposure to drugs, less tedious. However it is difficult to predict the steady state characteristics and inter subject variability with these studies. Advantages of multiple dose study: -Accurately reflects manner in which drug will be used clinically. -Requires collection of few blood samples. -Drug levels are higher due to cumulative effect and useful for less sensitive analytical methods -Better evaluation of controlled released formulation. -Nonlinearity if present , can be easily determined. -No need of long wash out periods. Healthy subjects versus Patients: Generally bioavailability study should be carried out in patients, as patient get benefited from the study, reflects better therapeutic efficacy, drug absorption pattern in disease state can be studied, avoids ethical quandary of administering drug to healthy subjects. but there are also various drawbacks like diseases, other drugs, physiological changes, fasting state is difficult etc. Hence usually these studies performed on young (20-40yrs) healthy male adult volunteers( body weight ±10%) under restricted dietary and fixed activity conditions.

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Methods of Bioavailability measurementPharmacokinetic methods

1. Plasma level time studies: most reliable method of choice comparison to urine data

methodSingle dose: serial blood samples collection – 2-3 half

lifes Plot concentration vs timeFor I.V. Sampling started within 5 min and subsequent

samples at 15 min intervalsFor oral dose at least 3 points taken on absorption curve

( ascending part)Parameters considered important in plasma level time

studies1. Cmax : It is peak plasma concentration. It increases

with dose as well as increase in rate of absorption.2. Tmax: The peak time at which Cmax atended.3. AUC: Area under curve explains about amount of drug.

oral

ravenous

ravenous

oral

DoseDose

AUCAUCF int

int

TEST

STD

STD

TESTrel Dose

DoseAUCAUCF

stdTEST

testSTD

MAX

MAXrel tDose

tDoseCSSCSSF

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2. Urinary excretion studies:This method is based on the principle that the urinary

excretion of unchanged drug is directly proportional to the plasma concentration of drug.

It can be performed if-At least 20% of administered dose is excreted unchanged

in urine.The study is useful for- Drugs that extensively excreted unchanged in urine eg.

Thiazide diuetics- Drugs that have urine as site of action eg. Urinary

antiseptics like nitrofurontoin. Steps involved:-collection of urine at regular intervals for 7 half

lifes.- Analysis of unchanged drug in collected sample.- Determination of amount of drug at each interval and

cumulative as well.Criteria's must be followed- At each sample collection total emptying of bladder is

necessary.- Frequent sampling is essential in the beginning to

compute correct rate of absorption.- The fraction excreted unchanged in urine must remain

constant.Parameters considered important in Urinary excretion

studies1. (Dx/dt)max: Maximun urinary excretion rate2. (tu)max: Time for maximum excretion rate3. Xu∞: Cumulative amount of drug excreted in the urine.

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1. Acute pharmacological response: When bioavailability measurement by pharmacokinetic

methods is difficult, inaccurate or non reproducible this method is used. Such as ECG, EEG, Pupil diameter etc.

It can be determined by dose response graphs. Responses measure for at least 3 half lifes.

Disadvantages: - Pharmacological response is variable and

accurate correlation drug and formulation is difficult. -Observed response may be due to active

metabolite. 2. Therapeutic response: This method is based on observing clinical response

in patients. Drawbacks: - Quantitation of observed response is too improper. -The physiological status of subject assumed that does

not change significantly over duration of study. -If multiple dose protocols are not involved. Patient

receive only single dose for few days or a week -The patient s receiving more than one drug treatment

may be compromised due to drug-drug interaction.

Pharmacodynamic methods

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In vitro dissolution studies and bioavailability:The physicochemical property of most drugs that has greatest influence on absorption from GIT is dissolution rate. However in vitro dissolution is good substitute for in vivo study in terms of saving cost and time. The best available tool today which can at least quantitatively assure about the bioavailability of drug from its formulation is in vitro dissolution test. In vitro- in vivo correlation ( IVIVC):It is defined as the predictive mathematical model that describes the relationship between in vitro property ( rate & extent of dissolution) and in vivo response ( plasma drug concentration). The main objective of developing and evaluating IVIVC is to use dissolution test to serve as alternate for in vivo study in human beings.IVIVC Levels:Level A: The highest category of correlation. It represents point to point correlation between in vitro dissolution and in vivo rate of absorption.Advantages: serves as alternate for in vivo study, change in manf. Procedure or formula can be justified without human studies.Level B: The mean in vitro dissolution time is compare with mean in vivo residence time. It is not point to point correlation . Data can be used for quality control standards.Level C: It is single point correlation. e.g. t50%, Tmax, Cmax. This level is only useful as guide for formulation development or quality control.

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Types of bioequivalence studiesIn vivo bioequivalence studies: when needed, 1. Oral immediate release product with

systemic action -Indicated for serious conditions requiring

assured response. -Narrow therapeutic window. - complicated pharmacokinetic, absorption <70%,

presystemic elimination>70%, nonlinear kinetics. 2. Non-oral immediate release products 3. Modified release products with systemic

action.In vitro bioequivalence studies: If none of the

above criteria is applicable comparative in vitro dissolution studies can be done.

Biowaivers: In vivo studies can be exempted under certain conditions.

1. Drug product only differ in strength of drug provided, - Their pharmacokinetics are linear, Drug &

excipient ratio is same, - both products manufactured by same manuf. at

same site. - BA/BE study done for original product, disso. rate

same under same conditions. 2. The method of production slightly modified in a way

that not affect bioavailability 3. The drug product meet following requirements: The

product is in solubilised form, no excipients affecting absorption, Topical use, Oral

but not absorbed, inhalation as gas or vapour.

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Bioequivalence experimental study designs1.Completely randomized designs: All treatments are randomly allocated among all experimental subjects.e.g. If there are 20 subjects, number the from 1 to 20. randomly select non repeating numbers among these labels for the first treatment. And then repeat for all other treatments .Advantages: Easy to construct, can accommodate any number of treatment and subjects, Simple to analyze.Disadvantages:Although can be used for number of treatments, but suited for few treatments.All subjects must be homogenous or random error will occur.2.Randomized block designs:First subjects are sorted in homogenous groups, called blocks and then treatments are assigned at random within blocks.Advantages:Systematic grouping gives more precise results.No need o equal sample size, any number of treatments can be followed, statistical analysis is simple, block can be dropped , variability can be introduced.Disadvantages: Missing observations in a block require more complex analysis.Degree f freedom is less.

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3.Repeated measures, cross over designs: It is a kind of randomized block design where same subject serves as a block. Same subject utilized repeatedly so called as repeated measure design. The administration of two or more treatments one after the other in a specified or random order to the same group of patients is called cross-over designs. Advantages: Good precision, Economic, can be performed with few subjects, useful in observing effects of treatment over time in the same subject. Disadvantages: Order effect due to position in treatment order. Cary over effect due to preceding treatment. Wash out period necessary – 10 elimination half lifes.4.Latin square designs: All other above designs are continuous trial. However in Latin square design each subject receives each treatment during the experiment. It is a two factor design ( Rows=Subjects and Columns=Treatments ). Carry –over effects are balanced. Advantages: minimize variability of plasma profiles and carry-over effects. Small scale experiments can be carried out for pilot studies. Possible to focus on formulation variables. Disadvantages: Less degree of freedom, randomization is complex, long time study, more formulations more complex study, subject dropout rates are high.

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After the data has been collected , statistical methods must be applied to determine the level of significance or any observed difference in rate and /or extent of absorptionto establish bioequivalence between two or more drug products.1. Analysis of varience ( ANOVA): It is statistical

procedure use to test data for differences within and between treatment and control groups. A statistical difference between the pharmacokinetic parameters obtained from two or more drug products is considered statistically significant if there is probability of less than 1 in 20 or 0.05 (p≤0.05) . The value of p indicates the level of statistical significance.

2. Confidence interval approach: It is also called as two one-sided procedure and used to demonstrate if bioavailability of test product is too low or too high in comparison to reference product. 90% confidence interval of two drug products must be within ±20% for bioavailability parameters such as AUC or Cmax. ( i.e. between 80 to 102 %). For log transformed data 90% confidence interval is set at 80-125%.

Statistical interpretation of bioequivalence data

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References Biopharmaceutics and pharmacokinetics – A Treatise , D. M. Brahmankar, Sunil B.Jaiswal. Vallabh prakashan IInd edition, pp- 315-366. Basics of Pharmaokinetics, Leon Shargel, fifth edition, willey publications, pp- 453-490. Internet sources.

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