Non Invasive Methods Of Estimating Pharmacokinetic Parameters

1. NON INVASIVE METHODS OF
ESTIMATING PHARMACOKINETIC
PARAMETERS
By
B. Thilakchandra
M.Pharmacy
Department of pharmaceutics
Vaagdevi inistitute of pharmaceutical sciences
Bollikunta, Warangal.

2. 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

3. • Two non-invasive techniques have been
recommended as alternatives to blood
sampling, namely
1. Collection of saliva
2. Collection of urine

4. 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.

5. 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

6. Composition of saliva

7. Compasition of saliva

8. Relation between the concentrations of sodium,
potassium, chloride, and bicarbonate in the saliva and
the rate of salivary flow

9. METHODS OF STIMULATION OF SALIVA
Methods Volume
• Spitting 0.5 ml/min
• Chewing paraffin wax, parafilm®, 1 to 3 ml/min
rubber bands, pieces of Teflon
or chewing gum.
• Acid lemon drops or a few drops 5 to 10 ml/min
of 0.5 mol/l citric acid

10. TECHNIQUES FOR THE COLLECTION OF
SALIVA
• Draining method
• Spitting method
DRAINING METHOD
• Absorbent method

11. Absorbent method
The Salivette
method

12. • Salivette • OraSure • Oral-Diffusion-
Sink(ODS)
• p-cyclodextrinl
• Absorbent epichlorhydrin
• Dental cotton copolymer
roll
• 1ml
• 1.5ml • 1ml

13. FOR ACIDIC DRUGS
FOR BASIC DRUGS

14. 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

15. • 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.

16. 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

17. 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.

18. 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

19. 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

20. 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.

21. • 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.

22. 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

24. 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.

25. Determination of KE from Urinary
Excretion Data
1. Rate of Excretion Method
•
According to first-order disposition kinetics, X = X0 e-K t
e
Substituting it in above equation yields

26. Transforming To Log Form

27. 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.

28. 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:

29. • 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.

30. 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.

31. 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.