Pharmacokinetics, Overview

Pharmacokinetics: the study of the movement of drugs in the body, including the processes of absorption, distribution, localization in tissues, biotransformation and excretion

Learning pharmacokinetics is of great practical importance in the choice and administration of a particular drug for a particular patient, e.g., one with impaired renal function

Drugs need to achieve an adequate concentration in their target tissues. The two fundamental processes that determine the concentration of a drug at any

moment and in any region of the body are:– translocation of drug molecules– chemical transformation by drug metabolism and other processes involved in drug elimination

These are critically important for choosing appropriate routes of administration

Pharmacokinetics, Introduction

Translocation of drug molecules: drug molecules move around the body in two ways:

bulk flow transfer (i.e. in the bloodstream) The chemical nature of a drug makes no difference to its transfer by bulk flow.

diffusional transfer (i.e. molecule by molecule, over short distances)

Diffusional transfer (transmembrane movement of the drugs): ability to cross hydrophobic diffusion barriers is strongly influenced by lipid solubility. delivering drug molecules to and from the non-aqueous barriers is influenced by water solubility

The Movement of Drug Molecules Across Cell Barriers

Gaps between endothelial cells are packed with a loose matrix of proteins that act as filters, retaining large molecules and letting smaller ones through.

In some organs (e.g. the liver and spleen) endothelium is discontinuous, allowing free passage between cells.

In other organs, especially in the CNS (blood brain barrier) and the placenta (placental barrier),

There are tight junctions between the cells the endothelium is enclosed in an impermeable layer of periendothelial cells (pericytes). These features prevent potentially harmful molecules from leaking from the blood into these

organs and have major pharmacokinetic consequences for drug distribution.

Aqueous Diffusion: It occurs within the larger aqueous compartments of the body (interstitial space, cytosol, etc) and across epithelial membrane tight junctions and the endothelial lining of blood vessels through aqueous pores.

It is probably important in the transfer of gases such as carbon dioxide

The Movement of Drug Molecules Across Cell Barriers

Passage of drugs across cell membranes

1) Passive transfer: a. Simple diffusion: The vast majority of drugs gain access to the body through this mechanism.

Drugs must be first in aqueous solution to gain access to the lipid membrane Drugs pass along concentration gradient No energy or carrier is required It is not inhibited by metabolic inhibitors It is not saturable. It depends on:

concentration gradient lipid solubility degree of ionization, thickness of membrane molecular size.

Concentration gradient is maintained by removal of the drug from other side of the membrane. Lipid solubility is measured by lipid/water partition coefficient (ratio of drug concentration in lipid phase and water phase when shaken in one immiscible

lipid/water system). Ionized drugs generally have low lipid/water coefficient.

The Movement of Drug Molecules Across Cell Barriers, Lipid solubility: weak acids and weak bases/Clinical Significance

Ka Ka

HA <==> H+ + A- BH+ <==> B + H+ [UI] [I] [I] [UI]

pKa=pH+log(HA/A-) pKa=pH+log(BH+/B)

ASPIRIN pKa = 3.5 (weak acid)100mg orally

99.9 = [ UI ] [ UI ]

Stomach pH = 2

BloodpH = 7.4

0.1 = [ I ]

Aspirin is reasonably absorbed Strychnine is not absorbed until from stomach (fast action) enters duodenum

0.1 = [ UI ] [ UI ]

BloodpH = 7.4

99.9 = [ I ]

STRYCHNINE pKa = 8.0 (weak base)100mg orally

Stomach pH = 2

In drug poisoning, renal elimination of drugs can be enhanced by changing urinary pH to increase drug ionization and inhibits tubular re-absorption. Alkalinization of urine by NaHCO3 increases excretion of acidic drugs e.g. aspirin.

Acidification of urine by vitamin C or NH4Cl increases excretion of weak base drugs e.g. amphetamine.

The Movement of Drug Molecules Across Cell Barriers, Lipid solubility: weak acids and weak bases/Clinical Significance, contd.

The Movement of Drug Molecules Across Cell Barriers, contd.

b. Filtration: In capillaries, pores have large size and so nearly all free drugs in plasma can be filtered. It depends on hydrostatic and osmotic pressure, so it is limited by blood flow but not by lipid solubility and it is not saturable

2) Specialized transport: - Substances that are too large or poorly lipid soluble as amino acids and glucose are carried by specialized

carriers.

- a. Facilitated diffusion: is similar to simple diffusion but requires a carrier and it is saturable.

- A carrier molecule is a transmembrane protein which binds one or more molecules or ions, changes conformation and releases them on the other side of the membrane.

- Carrier molecules facilitate entry and exit of physiologically important molecules, such as sugars, amino acids, neurotransmitters and metal in the direction of their electrochemical gradient

The Movement of Drug Molecules Across Cell Barriers, contd.

b. Active transport: where drugs pass against concentration gradient, so it requires: energy, carrier (thus it is saturable) Example: many drugs, especially weak acids (e.g., penicillin, uric acid) and weak bases (e.g., histamine), are actively secreted into the renal tubule, and thus more rapidly excreted

Mechanism Direction Energy required Carrier Saturable

Passive diffusion Along gradient No No No

Facilitated diffusion Along gradient No Yes Yes

Active transport Against gradient Yes Yes Yes

The Movement of Drug Molecules Across Cell Barriers, contd.

c. Pinocytosis: It involves invagination of part of the cell membrane and the trapping of a small vesicle containing

extracellular constituents within the cell

The vesicle contents can then be released within the cell, or extruded from its other side

Examples: Pinocytosis of vitamin B12 (complexed with intrinsic factor).

It is important for the transport of some macromolecules (e.g. insulin, which crosses the blood–brain barrier by this process)

Plasma level curve

Cmax = maximal drug level obtained with the dose.

tmax = time at which Cmax occurs. Lag time = time from administration to appearance in blood. Onset of activity = time from administration to blood level reaching minimal effective concentration

(MEC). Duration of action = time plasma concentration remains greater than MEC. Time to peak = time from administration to Cmax.

1- Absorption It is the process of entry of drug from site of administration into systemic circulation.

Factors influencing absorption A- Factors related to drug

a) Physicochemical properties:

1-Degree of ionization: highly ionized drugs are poorly absorbed.

2-Degree of solubility: High lipid/water partition coefficient increases absorption.

3-Chemical nature: inorganic iron is better absorbed than organic iron.

4-Valency: ferrous salts are more absorbed than ferric,

-so vitamin C increases absorption of iron.

b) Pharmaceutical form of drug:

Absorption of solutions is better than suspensions or tablets.

1- Absorption, Factors Influencing Absorption, contd

B- Factors related to the patient:1-Route of administration:

absorption is faster from i.v. > inhaled > i.m. > oral > dermal administration

2-Area and vascularity of absorbing surface: absorption is directly proportional to both area and vascularity. Thus absorption of the drug across the intestine is more efficient than across the stomach, as intestine has more blood flow and much bigger surface area than those of the stomach

3-State of absorbing surface: e.g. atrophic gastritis and mal-absorption syndrome decrease rate of absorption of drugs.

4-Rate of general circulation: e.g., in shock, peripheral circulation is reduced and I.V. route is used.

5-Specific factors and presence of other drugs: e.g. intrinsic factor of the stomach is essential for vitamin B12 absorption from lower ileum and adrenaline induces vasoconstriction so delay absorption of local anesthetics.

seconds minutes hours

Bioavailability It is the fraction of drug that reaches systemic circulation in an unchanged form and

becomes available for biological effect following administration by any route. It is 100% after IV administration.

It is calculated by comparison of the area under the plasma concentration time curve (AUC) after IV dose of a drug with that observed when the same dose is given by another route e.g. oral.

Area under the curve (AUC) oral x 100 Oral bioavailability =

Area under the curve (AUC) I.V.

Oral bioavailability depends on amount absorbed and amount metabolized before reaching systemic circulation (first pass metabolism)

Bioequivalence:Bioequivalence occurs when two formulations of the

same compound have the same bioavailability and the same rate of absorption

2-Distribution

Distribution of a drug from systemic circulation to tissues is dependent on lipid solubility , ionization, molecular size , binding to plasma proteins , rate of blood flow and special barriers

The body compartments include extracellular (plasma, interstitial) and intracellular which are separated by capillary wall and cell membrane

Intracellular compartment

Interstitial compartment

Intravascular (Plasma) compartment

Extracellular

Cell membrane

Endothelium of capillary wall

The major compartments are:

—plasma (5% of body weight)

—interstitial fluid (16%)

—intracellular fluid (35%)

—transcellular fluid (2%)

—fat (20%)

Distribution: Movement of drug from the central compartment (blood) to peripheral compartments (tissues) where the drug is present.

2-Distribution

Selective distribution: Some drugs have special affinity for specific tissue. e.g. calcium in bones, iodide in thyroid gland and tetracycline in bone and teeth.

Volume of Distribution: The apparent volume of distribution, Vd, is defined as the volume of fluid required to contain the total amount, Q, of drug in the body at the same concentration as that present in the plasma, Cp.

Vd is not a real volume, small volume indicates extensive plasma protein binding, but large volume indicates extensive tissue binding. Vd is increased by increased tissue binding, decreased plasma binding and increased lipid solubility. N.B. in average 70 kg adult, the total body water is 42 liter, extracelllular volume is 12-14 liter and plasma volume is 4 liter, so: -Drugs concentrated in plasma have Vd 3-4 L e.g. heparin. -Drugs distributed extracellularly have Vd 12-14 L e.g. aspirin. -Drugs distributed to all body fluids have Vd 42 L e.g. phenytoin and alcohol. -Drugs distributed intracellularly e.g. digoxin has Vd 500 L, imipramine 1600 L.

Drugs in vascular spaceDrugs are present in blood in :

1-Free form: active, diffusible, available for biotransformation and excretion.

2-Bound form (mainly to albumin): inert, non-diffusible, not available for metabolism and excretion. It acts as a reservoir for drug.Binding to plasma proteins is reversibleSignificance of binding to plasma proteins:

* Two drugs may have affinity for plasma protein binding sites, thus compete with each other leading to drug interactions.

* An example: Phenylbutazone and salicylates can displace warfarin (oral anticoagulant) and oral hypoglycemics from plasma proteins.

* Drugs highly bound to plasma proteins are in general expected to persist in body longer than those less bound and are expected to have lower therapeutic activity, less efficient distribution and less available for dialysis in poisoning.

3-Biotransformation (Metabolism) The conversion of a substance from one form to another by the actions of organisms or enzymes. Phases of biotransformation:

Phase I (Non-synthetic) reactions: introduction or unmasking of functional group by oxidation, reduction or hydrolysis. These reactions may result in :

1-Drug inactivation (most of drugs)

2-Conversion of inactive drug into active metabolite (cortisone→ cortisol)

3- Conversion of active drug into active metabolite (phenacetin→ paracetamol)

4-Conversion to toxic metabolite (methanol → formaldehyde)

Phase II (Synthetic) reactions: Functional group or metabolite formed by phase I is masked by conjugation with natural endogenous constituent as glucuronic acid, glutathione, sulphate, acetic acid, glycine or methyl group. These reactions usually result in drug inactivation with few exceptions e.g. morphine-6- conjugate is active Most of drugs pass through phase I only or phase II only or phase I then phase II. Some drugs as isoniazid passes first through phase II then phase I (acetylated then hydrolyzed to isonicotinic acid).

First-pass metabolism

Prodrug Active drugor

Active drug Inactive metaboliteor

Lipid soluble drug Water soluble drug

3-Biotransformation (Metabolism) Sites of biotransformation and types of enzymes

1- Microsomal enzymes: they are present in smooth endoplasmic reticulum of cells especially liver Microsomal enzymes catalyze:

-Glucuronide conjugation.

-Oxidation by microsomal cytochrome P450 enzymes (CYP450)

-Hydroxylation.

-Dealkylation.

-Reduction.

-Hydrolysis. They are affected by drugs and age

2- Non-microsomal enzymes: present in liver, kidney, plasma, skin and GIT…etc They catalyze:

-Conjugations rather than glucuronic acid.

-Oxidation by soluble enzymes in cytosol or mitochondria of cells e.g. MAO (monoamine oxidase) and alcohol dehydrogenase.

-Reduction.

-Hydrolysis. Their activity is stable throughout life.

3-Biotransformation (Metabolism)

Factors affecting drug metabolism1-Drugs: They can stimulate (induce) or inhibit microsomal metabolizing enzymes.

* Enzyme induction: Some drugs increase the synthesis or decrease degradation of enzymes.

Examples: phenobarbitone, phenytoin, carbamazepine, rifampicin, griseofulvin, testesterone, some glucocorticoids, tobacco smoking, ethyl alcohol (chronic).

Importance of enzyme induction:a) It decreases effect of other drugs.

b) Tolerance is sometimes explained by a drug inducing its own metabolism, e.g. ethyl alcohol, phenobarbitone.

c) It is a mechanism of adaptation to environmental pollutants (pollutants induce their own metabolism reducing their toxic effects).

* Enzyme inhibition (drugs that inhibit drug metabolism): it occurs faster than enzyme induction and causes serious drug interactions.

Examples: cimetidine, chloramphenicol, erythromycin, oestrogen, progesterone, sodium valproate, cotrimoxazole, isoniazid, MAOIs, ketoconazole and ciprofloxacin

3-Biotransformation (Metabolism)

2-Genetics variation: The most important factor is genetically determined polymorphisms. Example: Isoniazid is metabolized in the liver via acetylation. There are two forms (slow and fast) of the enzyme responsible for acetylation (N-acetyl transferase ), thus some patients metabolize the drug quicker than others. Slow acetylators are prone to peripheral neuritis while fast acetylators are prone to hepatic toxicity.

3-Nutritional state: Conjugating agents are sensitive to body nutrient level. For example, low protein diet can decrease glycine.

4-Dosage: High dose can saturate metabolic enzyme leading to drug accumulation. If metabolic pathway is saturated due to high dose or depletion of endogenous conjugate, an alternative pathway may appear e.g. paracetamol may undergo N-hydroxylation to hepatotoxic metabolite.

5-Age: Drug metabolism is reduced in extremes of age (old patients and infants).

6-Gender: androgen, estrogen and glucocorticoids can affect CYP450 enzyme. Diazepam, caffeine and paracetamol metabolism is faster in women while propranolol and lidocaine metabolism is faster in men.

3-Biotransformation (Metabolism)

7-Disease state:

-Liver disease decreases the ability to metabolize drugs.

-In cases of heart failure and shock reduced hepatic flow will increase the effect of rapidly metabolized drugs whose hepatic clearance is blood flow dependent e.g. lidocaine, morphine, propranolol, verapamil….

-Kidney disease reduces the excretion of drugs.

8-Circadian rhythm. In rats and mice, the rate of hepatic metabolism of some drugs follows a diurnal rhythm. This may be true in humans as well.

9-Route of administration: 1st pass effect occurs for drugs administered orally

4- Excretion of drugs

It is the process by which a drug or metabolite is eliminated from the body

Routes of excretion1- Renal Excretion: It is the result of three processes:

Passive glomerular filtration, active tubular secretion in proximal tubules and passive tubular re-absorption.

Factors affecting renal excretion:

1-Glomerular filtration rate. Only free unbound water soluble drugs with low molecular weight are filtered.

2-Change in urinary pH affect excretion of weak acid and base drugs. Thus: -Alkalinization of urine by NaHCO3 increases excretion of acidic drugs e.g. aspirin.

-Acidification of urine by NH4CL or vitamin C increases excretion of base drugs e.g., amphetamine.

3-Active tubular secretion e.g., probenecid, penicillin, uric acid …...

2-Gastrointestinal Tract:

a.Salivary glands: e.g., iodides, rifampicin and acidic drugs as salicylates.

b.Stomach: e.g., morphine (free and conjugated).

c. Large intestine: e.g., tetracycline, streptomycin.

d.Liver through bile, e.g.

-Ampicillin and rifampicin are excreted in active form so can be used in biliary infection and ampicillin in typhoid carriers.

3-Sweat: e.g., rifampicin, vitamin B1.

4-Lungs: e.g., gases and volatile anesthetics.

5-Milk: basic drugs are trapped and excreted in acidic milk, e.g., morphine, amphetamine. Also chloramphenicol and oral anticoagulants can be excreted in milk.

4- Excretion of drugs