absorption and distribution

1. PHARMACOKINETICS
ABSORPTION AND
DISTRIBUTION
DR HARIKRISHNAN A R

2. Pharmacokinetics
 Absorption, distribution, metabolism and elimination of a drug
 Involves passage across numerous cell membranes
 Plasma membrane is the basic barrier
 Characteristics of drugs that predict the availability and movement are
1. Molecular size
2. Degree of ionisation
3. Relative lipid solubility
4. Its binding to serum and tissue proteins

3. Modes of
permeation and
transport
1. PASSIVE DIFFUSION
2. CARRIER MEDIATED
TRANSPORT
3. PINOCYTOSIS AND
PHAGOCYTOSIS
4. FILTRATION

4. Passive diffusion
 Diffusion along a concentration gradient by virtue of its solubility in the lipid
bilayer
 Non electrolyte – steady state concentration same on both sides of the membrane
 Ionic compounds – steady state concentration depends on the electrochemical
gradient and difference in pH across the membrane
 Ratio of non ionised to ionised at any pH is calculated by Henderson - Hasselbalch
equation

5. Henderson–Hasselbalch equation
𝑝𝑘𝑎 = 𝑝ℎ + 𝑙𝑜𝑔
𝑝𝑟𝑜𝑡𝑜𝑛𝑎𝑡𝑒𝑑 𝑓𝑜𝑟𝑚
𝑢𝑛𝑝𝑟𝑜𝑡𝑜𝑛𝑎𝑡𝑒𝑑 𝑓𝑜𝑟𝑚
pka – dissociation constant
Acidic drug accumulates more on the basic side of the membrane and vice versa
This phenomenon is called ion trapping

6.  𝒑𝒌𝒂 𝒂𝒄𝒊𝒅 = 𝒑𝑯 + 𝐥𝐨𝐠
𝒏𝒐𝒏𝒊𝒐𝒏𝒊𝒔𝒆𝒅 𝒂𝒄𝒊𝒅
𝒊𝒐𝒏𝒊𝒔𝒆𝒅 𝒂𝒄𝒊𝒅
 𝒑𝒌𝒂 𝒃𝒂𝒔𝒆 = 𝒑𝒉 + 𝐥𝐨𝐠
𝒊𝒐𝒏𝒊𝒔𝒆𝒅 𝒃𝒂𝒔𝒆
𝒏𝒐𝒏𝒊𝒐𝒏𝒊𝒔𝒆𝒅 𝒃𝒂𝒔𝒆
 Stronger the acid / weaker the base lower will be the pka value
 Weakly basic drugs are non ionised in basic media and weakly acidic drugs non ionised in acidic media
 If 𝑝𝑘𝑎 = 𝑝𝐻 it would remain 50% ionised and 50% non ionised
 If pH increases or decreases by 1 unit there is a 10 fold increase or decrease in the ratio of the ionised
and non ionised fraction of the drug
 Strongly acidic / basic drugs remain predominantly ionised at all pH therefore poorly absorbed

7. Carrier mediated
transport
1. FACILITATED DIFFUSION
2. ACTIVE TRANSPORT

8. Facilitated diffusion
 Carrier can move a drug substrate along its concentration gradient only
 No energy required
 Rate of diffusion depends on the binding ability of the drugs to its carrier
 Two drugs having similar physico-chemical characteristics can compete for the
same transfer mechanism thus interfere with each other
 E.g. amino acids in brain, antimetabolite anticancer drugs, antiviral drugs

9. Active transport
 Energy dependent carrier mediated transport taking place against the
electrochemical gradient
 Energy generated by membrane ATPase
 Process can be blocked by inhibiting cell metabolism or by reducing ATP levels
 Drugs having same characteristics can compete

10. Primary active transport
 Bio transportation of drugs directly coupled with ATP hydrolysis
 ATP binding cassette (ABC) carry out the process
 E.g. antibiotics, anticancer drugs

11. Secondary active transport
 One ion/solute supplies driving force for the transport of other ion/solute
 Symporter/co-transporter – in the same direction. Na+/K+/2Cl- symporter
 Antiporter/counter-transporter – in opposite direction. Na+-H+ exchanger
 Not directly linked with ATP hydrolysis

12. ABC family of super transporters
 7 subclasses (ABC A to ABC G)
 Encoded by 49 genes
 Most important example is P–glycoprotein (ABCB1 or MDR-1)

13. Solute linked carrier(SLC)
 Facilitated diffusion or secondary active transport
 43 SLC families encoded by different genes
 Neuronal – serotonin, NE, dopamine
 Non neuronal – GLUT, cholesterol

14. Pinocytosis
 Pinocytosis – process where a cell engulfs a fluid or a drug in solution
 This is applicable to proteins and other big molecules, and contributes little to
transport of most drugs

15. Filtration
 Free or unbound drug of small molecular size pass through pore or spaces
between cells
 E.g. urea, alcohol, glucose

17. ABSORPTION

18. From GIT
 Mainly by passive diffusion
 Sugar and other nutrients by active transport
 Gut is more permeable to non-ionised lipid soluble form of drug
Ability of a drug to be absorbed is compromised by
 P-glycoprotein efflux by enterocytes
 Metabolism in these cells and liver
 Diseases that may affect absorption

19. From mouth
 Saliva pH is slightly acidic
 When stimulated by sublingual drugs it becomes alkaline (~7.4)
 Lipid soluble non-ionised basic drugs are absorbed
 Drug goes directly into circulation bypassing the first pass metabolism
 E.g. isosorbide dinitrate

20. From stomach
 pH is acidic
 Lipid soluble acidic or neutral drugs are absorbed
 Undergoes first pass metabolism

21. From intestine
 Alkaline pH
 Lipid soluble non-ionised basic or neutral drugs are absorbed
 Chances of first pass metabolism
 Enterohepatic circulation maybe seen

22. From large intestine to colon
 Alkaline pH
 Absorption through external haemorrhoidal vein
 Minimal first pass effect

23. Absorption via parenteral sites
Intravenous – completely absorbed and rapidly distributed
reach the blood stream directly without crossing any membrane
Intramuscular and subcutaneous – absorbed mainly by passive diffusion
from the injection site to the plasma or lymph
IM absorption more rapid than SC because of high vascularity of
muscles

24. Absorption via lungs
 Lipid soluble drugs are absorbed
 Vaporised form – general anaesthetics
 Aqueous solution – salbutamol
 Spray of suspended micro fined particles – disodium cromoglycate
 Absorbed by simple diffusion
 From pulmonary epithelium and mucous membrane of trachea and lungs
 Absorption rapid – large surface area and high vascularity
 First pass metabolism avoided

25. Absorption via topical sites
 Poor absorption through intact skin
 Underlying dermis permeable to lipid soluble drugs
 Transdermal application – nitro-glycerine, scopolamine, clonidine
 Mucous membrane application - thin and highly vascular absorbing surface.
Oxytocin and vasopressin as nasal spray
 Eye drops and ointments absorbed through cornea

26. Bioavailability
 Fractional extent to which an administered dose of drug reaches its site of action or a biological
fluid (usually the systemic circulation) from which the drug has access to its site of action
 It is an absolute term which requires measurement of both true rate and total amount of drug
that reaches the general circulation from an administered dosage form
 Equivalence – comparison of two different brand products of the same drug with a set of
established standards
 Different brands of the same drug can be chemically equivalent but may not be biologically or
therapeutically equivalent

27. Bioequivalence
 Two or more similar dosage forms of the same drug reach the blood circulation at
the same relative rate and to the same relative extent
 Difference in bioavailability are primarily seen with oral dosage forms
 Differences of less than 25% will not have a significant effect on clinical outcome –
bioequivalent
 Differences of bioavailability assume much greater concern with drugs having
narrow margin of safety

28. Measurement of bioavailability
 The absorption pattern of two brand products is plotted against time
 Peak plasma concentration{C max}
 Time to attain peak plasma concentration{t max}
 Area under the curve{AUC}
 The first 2 parameters are simple indicators for the rate of absorption
 AUC reflects the extent of absorption
 For the product to be considered bio equivalent C max, t max and AUC should not
be significantly different
 AUC is a better indicator for bioavailability of drugs to be given for longer period

30. 𝑫𝒓𝒖𝒈 𝒃𝒊𝒐𝒂𝒗𝒂𝒍𝒂𝒃𝒊𝒍𝒊𝒕𝒚(%) =
𝑨𝑼𝑪 𝒐𝒓𝒂𝒍
𝑨𝑼𝑪 𝑰𝑽
× 𝟏𝟎𝟎
NB: same dose is given orally and intravenously

31. Factors influencing
absorption and
bioavailability

32. Pharmaceutical factors
 Particle size – dissolves more rapidly when surface area is increased by decreasing particle
size. e.g. micro-fined aspirin, spironolactone
 Salt form – salt dissolves better than their parent compounds. E.g. phenytoin sodium
 Crystal form – e.g. amorphous novobiocin have better bioavailability compared to their
crystalline form
 Water of hydration – many drugs associates with water to form hydrates. E.g. anhydrous
forms of ampicillin have better bioavailability than their hydrous forms
 Nature of excipients and adjuvants – these are pharmacologically inert substances used as
filling material or binding agent. With a drug which obeys zero or mixed order kinetics
change from one brand to other may lead to therapeutic failure or drug intoxication due to
different excipients/adjuvants
 Degree of ionisation – non ionised lipid soluble drugs are better absorbed

33. Pharmacological factors
 Gastric emptying and gastrointestinal motility – gastric emptying increases absorption as it reaches
larger surface area. Promoted by fasting, anxiety, lying on right side, hypothyroidism and gastro-
kinetic drugs
 Gastro-intestinal disease – in coeliac disease amoxicillin shows decreased absorption while
cephalexin shows increased absorption ampicillin shows no change.
 Food and other substances – empty stomach increases absorption
 First-pass effect – drug degradation occurring before drug entering the systemic circulation for drugs
taken orally therefore there is decreased bioavailability and diminished therapeutic response

34. Cont.
 Drug-drug interactions – e.g. paraffin decreases bioavailability of fat soluble
vitamins as it emulsifies fats
 Pharmacogenetic factors – e.g. slow acetylators of isoniazid show increased
bioavailability in American whites leading to isoniazid toxicity
 Miscellaneous factors – route of administration, area of absorbing surface, state
of circulation at the site of absorption

35. Drug distribution
 Pattern of scattering of the specific amount of drug among various locations
within the body
 Distributed into all organs including non relevant to its pharmacological effect
 After absorption the drugs may not only get reversibly associated with its site of
action but may bind to plasma proteins and may accumulate in various storage
sites
 Distribution, metabolism and excretion termed as drug disposition

36. Factors affecting drug distribution
 Lipid solubility
 Ionisation at physiological pH ( a function of pKa)
 Extent of binding to plasma and tissue proteins
 Presence of tissue specific transporters
 Differences in regional blood flow

38. Volume of distribution
 Volume of the plasma that would accommodate all the drug in the body, if the
concentration throughout was the same as in plasma
𝒂𝑽𝒅 =
𝒕𝒐𝒕𝒂𝒍 𝒂𝒎𝒐𝒖𝒏𝒕 𝒐𝒇 𝒅𝒓𝒖𝒈 𝒊𝒏 𝒕𝒉𝒆 𝒃𝒐𝒅𝒚
𝒎𝒈
𝒌𝒈
𝒄𝒐𝒏𝒄 𝒐𝒇 𝒕𝒉𝒆 𝒅𝒓𝒖𝒈 𝒊𝒏 𝒕𝒉𝒆 𝒑𝒍𝒂𝒔𝒎𝒂(
𝒎𝒈
𝑳
)
aVd = apparent volume of distribution
Expressed in L/kg

39.  If the drug does not cross capillary walls aVd = plasma water i.e. 3L. Seen in
drugs with high molecular weight or lesser lipid soluble drugs
 Drugs highly bound to plasma proteins have a low aVd value and vice versa
 aVd for many drugs may be much more than the actual body volume which
means that they are widely distributed and are difficult to be removed by
haemodialysis.
 If a drug is having smaller aVd have less protein binding also , it is easier to be
removed by haemodialysis
 aVd<5L – drug is retained within the vascular compartment. e.g.
heparin, insulin
 aVd~15L – drug is restricted to the extracellular fluid. e.g. aspirin, amoxycillin
 aVd>20l – indicates distribution throughout the total body water.
e.g. phenytoin, methyldopa

40. Redistribution
 Highly lipid soluble drugs get initially distributed to organs with high blood flow
 Then to less vascular but more bulky tissues
 Plasma concentration falls and the drug is withdrawn from the highly perfused
sites
 If the site of action was in one of the highly perfused organs , redistribution results
in termination of drug action
 E.g. anaesthetic action of thiopentone sodium injected IV is terminated in few
minutes due to redistribution

42. Physiological
barriers to drug
distribution
1. BLOOD-BRAIN BARRIER
2. BLOOD-CSF BARRIER
3. CSF-BRAIN BARRIER
4. PLACENTAL BARRIER

43. Blood-brain barrier(BBB)
 Endothelial cells of brain capillaries are tightly joined and lack intercellular pores
 Glial cells envelope these capillaries which are less permeable
 Anatomically there exists a dual barrier in CNS ; blood-brain barrier and the blood- CSF
barrier(located in the choroid plexus)
 Efflux transporters – molecular barrier. P-glycoprotein and ABCC1
 Protects brain tissue from toxic substances and some neurotransmitters
 Only lipid soluble non-ionised form of drugs penetrate
 Inflammatory conditions like meningitis increases penetration to penicillins

45. Regions of the brain which are relatively
permeable
1. Pituitary gland
2. Pineal body
3. Median eminence
4. Choroid plexus
5. Area postrema near floor of fourth ventricle

46. Blood-CSF and CSF-brain barrier
 CSF is secreted by epithelial cells of choroid plexus which are lined by occluding
zonulae
 CSF-brain barrier there is no occluding zonulae. CSF brain barrier is extremely
permeable toto drug molecules.
 In cases of brain abscess drugs can be given intrathecally which easily cross CSF-
brain barrier

47. Placental barrier
 Placental membrane lipid in nature – passive diffusion of non-ionised lipid soluble
substances
 Active transport and pinocytosis are also operative
 Drug administration during pregnancy severely restricted
 Hypoxia increases permeability of drugs

48. Reservoirs
 Cellular reservoir - tissue affinity could be due to binding to tissue proteins or
nucleoproteins. E.g. digoxin and emetine in skeletal muscles, heart, kidney
 Fat as reservoir – highly lipid soluble drugs get selectively accumulated in fat and
adipose tissue. E.g. thiopentone
 Transcellular reservoir – aqueous humour, CSF, endolymph, joint fluid, pleural,
pericardial and peritoneal sacs
 Bones and connective tissue as reservoir – e.g. tetracyclines, cisplatin, lead

49. Important proteins that contribute to drug
binding
1. Plasma proteins – albumin. Most of the acidic drugs bind. E.g. warfarin
2. α1-Acid glycoprotein - lipophilic basic drugs bind. E.g. quinidine . It is an acute
phase reactant protein, its plasma concentration increases in physiological and
pathological stress
3. Tissue proteins and nucleoproteins – drugs having high apparent volume of
distribution bind. E.g. digoxin
4. Miscellaneous binding proteins – steroids bind to corticosteroid binding
globulin, thyroxine to α-globulins, antigens to gamma globulins

50. Plasma protein binding
 Drugs usually bind to plasma and cellular proteins in a reversible manner and in
dynamic equilibrium
 𝑭𝒓𝒆𝒆 𝒅𝒓𝒖𝒈 + 𝑷𝒓𝒐𝒕𝒆𝒊𝒏 ↔ 𝑫𝒓𝒖𝒈𝑷𝒓𝒐𝒕𝒆𝒊𝒏 𝒄𝒐𝒎𝒑𝒍𝒆𝒙
 As the free or unbound drug is eliminated from the body more drug dissociates
from the drug plasma protein complex
 Extensive protein binding does not prevent the drug from reaching its site of
action but only prolongs the drug availability and duration of action

51. Clinically important aspects of plasma
protein binding
 High plasma protein bound drugs remain largely restricted to the vascular
compartment and have lower volumes of distribution
 Bound fraction is not available for action
 High degree of binding makes the drug long action
 High protein bound drugs are difficult to be removed by dialysis
 Binding of drugs to plasma proteins is a capacity-limited and saturable process
 More than one drug can bind to the same site of albumin leads to displacement
interaction. Drug with high affinity displaces that with low affinity. If just 1% of a drug
which is 99% bound is displaced the concentration of free form will be doubled

52. Displacement reactions
 Phenylbutazone displaces tolbutamide from its binding sites leading to
hypoglycaemia
 Tolbutamide displaces warfarin resulting in increased risk of haemorrhage
 Sulphonamides displaces bilirubin leading to kernicterus in neonates
 Salicylates displaces methotrexate

53. THANK YOU