**2.3 Washout period**

In a Latin square cross over design, each subject receives each formulation, and even in BIBD, each subject receives two formulations at different occasions. The time interval between the two treatments is called "washout period." Washout period is required for the elimination of the administered dose of a drug so as to avoid the carryover. For most of drugs in crossover design, at least 10 half-lives should be allowed between treatments. This should ensure an elimination of 99.9% of the administered dose and a maximum carryover of less than 0.1% from first treatment. The number of washout period is a function of the half-life and the dose of the drug administered. The number of washout periods in a study depends upon the type of crossover design used and the number of formulations to be evaluated.

*Pharmaceutical Formulation Design - Recent Practices*


#### **Table 2.**

*Balanced incomplete block design (BIBD) for four formulations.*

In case of digitoxin, which has a half-life of 6–9 days, the total study period exceeds 1 year if four formulations have to be evaluated using Latin square design. Because a very large number of drugs have been found to have half-lives between 1 and 10 hours, a washout period of 1 week was usually found suitable in most of the reported studies. It should be noted that the metabolites of the drug should also be eliminated from the body before the commencement of next treatment.

#### **2.4 Drug product and reference standard**

Test product may be new drug formulations developed by pharmaceutical technologists or new dosage forms of an existing drug. A test product may be compared to a reference standard recognized by the Food and Drug Administration for getting approval for marketing the drug product. Test product are generally evaluated to select best dosage form of a new drug or existing drug among different dosage forms, to select the best formulation of a new drug or existing drug among different formulations that have shown equal performance *in vitro* tests and to compare biological performance of a test product to that of a recognized standard [17, 18].

A generic product has to compare with some standard dosage form to verify it's *in vivo* performance. In general, Food and Drug Administration (FDA) accepts any innovator's drug product as a reference standard. The innovator is the one who originally received approval from the FDA to market the product in the country. Sometime, several manufactures may hold approval for certain drugs. Therefore, any one of the permitted drug products can be used as a reference standard. In many of these instances, the FDA would request that only of these products be used as a reference product in order to obtain a more easily comparable data.

Most of the times orally administered dosage forms are subjected for bioavailability studies. However, dosage forms administered by other routes such as buccal, transdermal, and intramuscular should also be evaluated for their biological performance. The therapeutic utility of these dosage forms depends on the rate and extent of absorption of the drug from these dosage forms. Orally administered dosage forms show a much variation in their performance because of intersubject and intrasubject variations.

**71**

*Bioavailability and Bioequivalence Studies DOI: http://dx.doi.org/10.5772/intechopen.85145*

need a multiple dose study.

**2.5 Single versus multiple dose study design**

**2.6 Administration of drug products and sampling**

administered to the subjects in a sequential manner [20, 21].

until 1/10 or 1/20 of the peak levels are reached.

unchanged drug in the urine sample.

than the half-life of the study drug.

If the dosage forms are to be evaluated only for bioequivalence purposes, single dose studies are sufficient. This is because the relative bioavailability of most tablets and capsules can be determined on a single dose basis and usually this is predictive of multiple dose levels. Dosage forms determined for a single dose administration for a therapeutic benefit such as analgesic for the relief of head ache needs only single dose studies [19]. However, certain dosage forms designed to achieve special release profiles of drugs may require multiple dose studies like time release products, enteric-coated preparations, and some intramuscular injections. Even the drugs that undergo the first pass metabolism do

Administration of drug products or formulations to the subjects should be based on randomization. After the administration, blood samples are withdrawn from the subjects at fixed time intervals. Some time is taken to withdraw a sample from each subject, and the total time difference between first subject and the last subject may range from 10 to 20 minutes depending upon the number of subjects and technicians involved in the study. If the sampling schedule is not followed rigorously in the same sequential manner, significant differences can conceivably exist in the actual duration of the drug in the body and the stated sampling time given for each subject. This 10 to 20 min difference in sample withdrawal from each subject during the study would represent a substantial change in the drug concentrations observed in the blood if under these conditions treatments are

If the bioavailability of a given dosage form is to be evaluated by a blood level study, some estimate of the area under the serum concentration versus time curve, peak plasma concentration (Cmax), and time of peak plasma concentration (Tmax) must be obtained from the study. Therefore, the frequency of sampling and the duration of sampling are very important for study. It will vary with the drug. There must be sufficient sampling points to allow for proper evaluation of the area under the blood level curve. A blood sampling done up to three to five half-lives of the drug, and if the half-life of the drug is not known, blood sampling should proceed

Urinary excretion studies are used when it is either not possible to measure a given drug in the blood, plasma or serum or when ethical considerations do not allow the collection of samples over a period of time. The advantage of this method is it involves noninvasive method of sampling, concentration of the drug in the urine is often greater than serum and the amount of the drug excreted in urine is obtained directly. But it is not useful in estimating the absorption rate of rapidly absorbed drugs and sometimes metabolites may interfere with the estimation of the

Sampling must be continued for a sufficient time period to ensure that the area

extrapolated from the time of the last measured concentration to infinite time should be less than 20% of the total AUC. AUC calculations are not useful in case of enterohepatic recycling where the terminal elimination rate constant cannot be calculated accurately. In such case, at least three sampling points from absorption phase, three to four points from Tmax and four points during the elimination phase has been taken. Intervals between successive sampling points in terminal elimination phase are used to calculate the elimination rate constant. It should not be longer *Pharmaceutical Formulation Design - Recent Practices*

**Subject Treatment for period No.**

1 A B 2 B A 3 A C 4 C A 5 A D 6 D A 7 B C 8 C B 9 B D 10 D B 11 C D 12 D C

I II

In case of digitoxin, which has a half-life of 6–9 days, the total study period exceeds 1 year if four formulations have to be evaluated using Latin square design. Because a very large number of drugs have been found to have half-lives between 1 and 10 hours, a washout period of 1 week was usually found suitable in most of the reported studies. It should be noted that the metabolites of the drug should also be

Test product may be new drug formulations developed by pharmaceutical technologists or new dosage forms of an existing drug. A test product may be compared to a reference standard recognized by the Food and Drug Administration for getting approval for marketing the drug product. Test product are generally evaluated to select best dosage form of a new drug or existing drug among different dosage forms, to select the best formulation of a new drug or existing drug among different formulations that have shown equal performance *in vitro* tests and to compare biological performance of a test product to that of a recognized standard [17, 18]. A generic product has to compare with some standard dosage form to verify it's *in vivo* performance. In general, Food and Drug Administration (FDA) accepts any innovator's drug product as a reference standard. The innovator is the one who originally received approval from the FDA to market the product in the country. Sometime, several manufactures may hold approval for certain drugs. Therefore, any one of the permitted drug products can be used as a reference standard. In many of these instances, the FDA would request that only of these products be used

eliminated from the body before the commencement of next treatment.

as a reference product in order to obtain a more easily comparable data.

Most of the times orally administered dosage forms are subjected for bioavailability studies. However, dosage forms administered by other routes such as buccal, transdermal, and intramuscular should also be evaluated for their biological performance. The therapeutic utility of these dosage forms depends on the rate and extent of absorption of the drug from these dosage forms. Orally administered dosage forms show a much variation in their performance because of intersubject

**2.4 Drug product and reference standard**

*Balanced incomplete block design (BIBD) for four formulations.*

**70**

**Table 2.**

and intrasubject variations.
