MISHRA, BHAITACHARYA : BIOAVAILABILITY OF DRUGS 85 therapeutic equivalence for all drugs, they ensure and are .recognized to be t~e only certain criteria for product The bioavailability of drugs equivalence. The practical relevance of bioavailability was highlighted by the dramatic increase in the adverse effects of B. MISHRA, s. K. BHATIACHARYA phenytoin in Australia during 1968-69. The cause was finally traced to the fact that a change in the excipient of phenytoin capsules led to a marked increase in the absorption of the drug and a consequent increase in its toxicity. Bioequiva- INTRODUcrION lence problems with phenytoin manufactured by different Bioavailability describes the amount of active drug given pharmaceutical concerns are also on record. In 1969, 15 of in a pharmacological dose, which reaches the systemic circula- the 16 oxytetracycline brands available in the USA were tion after oral administration. Till fairly recently, it was discontinued because they did not meet the standard assumed that all formulations, containing similar quantities bioavailability specifications. Bioinequivalence has been of a particular drug, had the same bioavailability and were, reported for several other drugs including digoxin, therefore, therapeutically equivalent. Unfortunately, this quinidine, nalidixic acid, atenolol, propranolol, several assumption is not always true and it has become apparent antibiotics and non-steroidal anti-inflammatory agents. that different formulations of the same drug are not thera- This list is likely to be far more extensive in India and can lead pe~tically ~quivalent as their bioavailability differs. Thus, to widely different clinical responses to important and life- It IS now Important for the clinician to understand and saving drugs. It is, therefore, important that the prescriber appreciate the principles underlying the concept of bio- always poses the question 'will the drug get to its site of action . availability and its practical applications. in optimal therapeutic concentrations?' Definition ASSESSMENT OF BIOA VAILABILITY The bioavailability of a drug is thus more strictly defined as For assessing the bioavailability or clinical availability of a the amount and the rate at which the orally administered drug, it~ rate and extent of absorption and its first-pass drug reaches the biological system in an active form, capable metabolism must be evaluated. Ideally, the clinical response of exerting the desired pharmacological effect, including its o.f the pat!ent or the amount of the active drug at the target onset, intensity and duration of action. s~te of action at different time periods following administra- The bioavailability or systemic availability of an orally tion of the drug formulation, should be assessed. However, administered drug depends largely on the absorption and the these criteria are difficult, or even impossible, to quantify. extent of hepatic metabolism on its first-pass through the Hence, the methods used to assess bioavailability depend liver in addition to its action at the level of the receptors. upon the assumption that measurement of the drug con- In practice, bioavailability is determined by assessing its centration in a suitable body fluid, such as blood, plasma, plasma or. u~ine ~oncentration over a given time following serum~ urine or sometimes saliva, over a period of time oral administration, To be therapeutically equivalent, following oral administration can be correlated with its different formulations of the same drug have to be bio- clinical availability and, therefore, the clinical efficacy equival~~t, implying t~at they must have a similar systemic of the drug in a given disease. availability. Many studies have indicated that products from Bioavailability is usually determined by the following different manufacturers, and even different batches of drugs methods: from the same pharmaceutical house, are not bioequivalent 1. Determination of whole blood, plasma or serum concen- and have dissimilar systemic availability. While major differ- trati?~s of ~he unchanged drug, either after single dose ences in bioavailability are likely to be of clinical relevance, administration or III a dosage interval at the steady state small differences are also important in those drugs which have after multiple dose administration (Figs. 1 and 2). a steep dose-response curve or a narrow margin of safety. 2. Determination of the total amount of unchanged drug Fortunately most drugs have relatively flat dose-response excreted in the urine either after single dose administra- c~rves, so tha.t only marked differences in bioequivalence tion or after multiple dose administration in the steady WIllproduce differences in therapeutic equivalence. state. The currently available compendial standards laid down 3. Determination of the total amount of a major metabolite for bioequivalence, including the testing of the finished of the drug excreted in the urine either after a single dose product or the specifications for the raw materials and or at the steady state after multiple dose administration. manufacturing process, leave much to be desired. The 4. Methods based on clinical trials in which the effect of the problem is particularly acute in India where such specifica- drug can be quantified. tions either do not exist or are largely ignored, since bioavailability studies are rarely done or compared with The first three pharmacokinetic methods are more practical acceptable standards which meet compendial criteria. While and discriminative than the fourth which is based on a clinical bioequivalence studies are not absolute guarantees of outcome. It has been estimated that, while it is possible to assess a 25% difference in the bioavailability of a drug from two formulations by a suitable pharmacokinetic method Institute o~ Medical Sciences, Banaras Hindu University, using only 12 subj~cts, the clinical method may require as Varanasi 221005, Uttar Pradesh, India many as 1000 subjects. Thus, if a clinician has estimated B. MISHRA, S. K. BHAITACHARYA bioavailability using, say, 50 patients and not found a per- Department of Pharmacology ceptible difference between two drug formulations, it does Correspondence to S. K. BHA IT ACHARYA not mean that the two drug dosages are bioequivalent but © The National Medical Journal of India 1993 86 THE NATIONAL MEDICAL JOURNAL OF INDIA VOL. 6, NO.2 ~ 75 Steady-state achived ~ PQok g concentration c ~ .Q 50 o\... C ~ u c o Steady state-~ u concentration Ol \...:J o a 345678 Time (mult iples oflt-2) Steady state is achieved after approximately Tlrnethoursl five doses. FIG 1. Plasma concentration-time curve following FIG2. Plasma concentration profile of a drug given chronically administration of a single oral dose at half-life intervals may indicate that the number of patients has been too small. curve. Thus, the early decline may reflect the net result of The most common and widely used methods, involving the absorption, distribution and elimination. Any decline in the determination of plasma drug concentrations after a single curve indicates that the rate of elimination of the drug dose administration of a drug are as follows: exceeds its absorption. Eventually, drug absorption ceases with the bioavailable quantum of the drug having been Plasma concentration-time curves absorbed and the plasma drug concentration is now controlled A typical plasma concentration-time curve, following a by the rate of its elimination by metabolism and/or excretion single oral dose administration of a drug formulation is (elimination phase of the curve). Thus drug distribution to depicted in Fig. 1. The plasma drug concentration at the the tissues and extracellular fluid, as well as absorption and time of the drug administration (zero time) should be nil. As elimination, will affect the shape of the plasma concentra- the drug passes into the stomach and/or the intestine, the tion-time curve (Fig. 3). product disintegrates releasing the drug, which dissolves and Based on the plasma concentration-time curve, the absorption ensues. Initially the plasma concentration of the following measurements important for bioavailability studies drug rises (denoted by the ascending 'absorption phase' of can be assessed (Fig. 4). the curve) as the rate of absorption exceeds the rate at which Peak concentration (Cmax) represents the highest the drug is removed from the plasma by distribution and concentration attained by the drug in the plasma. elimination. The plasma level continues to rise until a Time of peak concentration (Tmax) is the time required to maximum or peak concentration is attained. However, the achieve the peak plasma concentration after single dose elimination of the drug starts with its appearance in the administration of the drug, and can be used to assess the rate plasma and continues until the total amount has been of absorption. eliminated. Similarly, absorption of the drug does not stop Minimum effective plasma concentration: The minimum abruptly with the attainment of the peak plasma concentra- plasma concentration of the drug required to achieve a given tion but may continue into the declining portion of the pharmacological or therapeutic response. This value varies Increased risk OT toxicity Maximum safe c t a c Concentra t ion Maximum safe .9 o -L- Concentration o -L. Peak Concentration C -01 -C 1 V (II ----~ c V 1 c 8 a I LTherapeu t ic Ol U I'1tensity ;:) I Range L- 1 Minimum Effective C1l I o ;:) I Concentration L. I o a E I III a o E Time of a.. III , I