NON INVASIVE METHODS OF ESTIMATING PHARMACOKINETIC PARAMETERS
By B. Thilakchandra M.Pharmacy Department of pharmaceutics Vaagdevi inistitute of pharmaceutical sciences Bollikunta, Warangal.
INTRODUCTION • Most pharmacokinetic studies involve the measurement of drug concentrations in plasma. However, the collection of blood is not entirely without risk and subjects the patients or healthy volunteers to inconvenience and discomfort . • In addition, the subject; may become apprehensive about the repeated sampling of blood, resulting in a slowing of the rate of absorption from the gastrointestinal tract and, possibly other changes in pharmacokinetic parameters
• Two non-invasive techniques have been recommended as alternatives to blood sampling, namely 1. Collection of saliva 2. Collection of urine
COLLECTION FROM SALIVA •Drug concentarations are usually measured in mixed saliva. •This contain secretions from parotid, submandular, Sublingual and minor glands. •Many of the advantages of measuring drugs in saliva relate to the noninvasive nature of the easy collection procedure. •For the collection of samples on a patient basis, mixed whole saliva is the only practical alternative. • Therefore, if the measurement of a drug level in saliva is to be of general clinical value it will need to be done on mixed (whole) saliva.
MECHANISM OF DRUG TRANSPORT BETWEEN PLASMA AND SALIVA
a- active transport b- passive transpor . -t c- simple filtration e- duct cells pump Na to blood f- cell membrane g- pore h- intracellular spaces i- acinar cell
Composition of saliva
Compasition of saliva
Relation between the concentrations of sodium, potassium, chloride, and bicarbonate in the saliva and the rate of salivary flow
METHODS OF STIMULATION OF SALIVA Methods • Spitting
Volume 0.5 ml/min
• Chewing paraffin wax, parafilm®, rubber bands, pieces of Teflon or chewing gum.
1 to 3 ml/min
• Acid lemon drops or a few drops of 0.5 mol/l citric acid
5 to 10 ml/min
TECHNIQUES FOR THE COLLECTION OF SALIVA • Draining method • Spitting method DRAINING METHOD
• Absorbent method
Absorbent method The Salivette method
• Salivette
• Dental cotton roll
• 1.5ml
• OraSure
• Absorbent
• 1ml
• Oral-DiffusionSink(ODS) • p-cyclodextrinl epichlorhydrin copolymer
• 1ml
FOR ACIDIC DRUGS
FOR BASIC DRUGS
Factors influencing passive diffusion of a drug from blood to saliva •Relating to drug 1. Acidic or basic, and the pKa 2. Lipid-solubility • Relating to the circulating drug level in the free (nonprotein-bound) form 1. Nonprotein-bound blood level 2. Dose and clearance of drug • Relating to saliva 1. Saliva flow-rate 2. Saliva Ph 3. Saliva binding proteins - usually minimal
• For those acidic drugs with pKa > 8.5 and those basic drugs with pKa < 5.5, the S/P ratio is independent of pKa. • This ratio must therefore equal the ratio of fp to fs and, since fs can usually be considered as unity, the S/P ratio is equal to the fraction of unbound drug in plasma.
SALIVARY CLEARANCE • According to mass-balance relationships Rate of salivary secretion = Qs . CS
• From definition of clearance rate of salivary secretion= CLs . CP
• From above equations Cls = Qs . Cs / Cp
Neutral and acidic drugs • Similar half lives in saliva and plasma • For acidic eg: salicylates, sulphapyridine and its acetyl metabolites, sulphamethaxazole • For neutral eg: alcohol, phenytoin, theophylline, carbamazepine.
Bases • Most are strong bases with pKa values in excess of 8.0 • Saliva concentration is greater than plasma • Cs / Cp of basic show intrasubject and intersubject variation • For some basic drugs – procainamide, diazepam and nitrazepam changes in salivary concentration lag behind changes in plasma concentration is indication of slow transfer between plasma and saliva
URINARY ECRETION OF DRUGS • The pharmacokinetic and biopharmaceutical properties of drugs and drug products can frequently be studied from rate of cumulative excretion of drugs and metabolites in urine. • This method allows to study kinetic studies to be conducted before more definitive studies involving the measurement of plasma drug concentrations
CRITERIA FOR OBTAINING VALID URINARY EXCRETION DATA • Amount of unchanged drug excreted in the urine (at least 10%). • The analytical method must be specific for the unchanged drug; metabolites should not interfere. • Water-loading should be done by taking 400 ml of water after fasting overnight, to promote diuresis and enable collection of sufficient urine samples. • Before administration of drug, the bladder must be emptied completely after 1 hour from water-loading and the urine sample taken as blank; the drug should then be administered with 200 ml of water and should be followed by 200 ml given at hourly intervals for the next 4 hours.
• Volunteers must be instructed to completely empty their bladder while collecting urine samples. • Frequent sampling should be done in order to obtain a good curve. During sampling, the exact time and volume of urine excretedshould be noted. • An individual collection period should not exceed one biologic half-life of the drug and ideally should be considerably less. • Urine samples must be collected for at least 7 biological half-lives in order to ensure collection of more than 99% of excreted drug. • Changes in urine pH and urine volume may alter the urinary excretion rate.
Determination of KE from Urinary Excretion Data THE FIRST ORDER ELIMINATION RATE CONSTANT CAN BE COMPUTED FROM URINE DATA BY TWO METHODS 1. Rate of excretion method 2. Sigma-minus method
ADVANTAGES OF URINARY EXCRETION DATA • lack of sufficiently sensitive analytic techniques to measure concentration of drugs in plasma with accuracy. • noninvasive and therefore better subject compliance is assured. • less sensitive analytic method is required for determining urine drug concentration • When coupled with plasma level-time data, it can also be used to estimate renal clearance of unchanged drug according to following equation: Total amount of drug excreted unchanged • C1R = Area under the plasma level-time curve • If Vd is known, total systemic clearance and nonrenal clearance can also be calculated.
Determination of KE from Urinary Excretion Data 1. Rate of Excretion Method •
According to first-order disposition kinetics, X = X0 e-K t Substituting it in above equation yields e
Transforming To Log Form
ADVANTAGES • An advantage is for drugs having long half-lives, urine may be collected for only 3 to 4 half-lives. • no need to collect all urine samples since collection of any two consecutive urine samples yield points on the rate plot from which a straight line can be constructed. DISADVANTAGE A disadvantage of rate of excretion method in estimating KE is that fluctuations in the rate of drug elimination are observed to a high degree and in most instances, the data are so scattered that an estimate of half-life is. difficult.
SIGMA-MINUS METHOD
Integration
•
As time approaches infinity i.e. after 6 to 7 half-lives, the value e-KE∞ becomes zero and therefore the cumulative amount excreted at infinite time Xu∞ can be given by equation:
• Substituting this in above equation
• Converting to log
• Disadvantage Total urine collection has to be carried out until no unchanged drug can be detected in the urine i.e upto 7 half-lives, which may be tedious for drugs having long t/2.
CONCLUSION • For the measurement of drugs, saliva was suggested as early as the 1970's as an alternative medium. Since these years, saliva has been used for therapeutic and toxicological drug monitoring of a variety of drugs. The easy noninvasive, stress-free nature of saliva and urine collection makes it one of the most accessible body fluids to obtain. The major disadvantage of saliva is that many drugs are retained for a shorter period of time than they are in urine. New collecting devices should make physicians more comfortable with using saliva as an alternative to blood or urine. • Measurements of saliva and urine drug concentrations will usually be of value, only if they accurately reflect the plasma level., but in future research the mechanisms by which drugs enter the saliva and urine have to be clarified more adequately.
References • Non- invasive methods of estimating pharmacokinetic parameters. G.G.GRAHAM • Salivary Diagnostics. DAVID T.WONG • SALIVA AS AN ANALYTICAL TOOL IN TOXICOLOGY. Karin M. Höld, B.S.; Douwe de Boer, Ph.D.; Jan Zuidema, Ph.D.; Robert A.A. Maes, Ph.D.
