PHARMACOKINETICS- II

Dr. POOJA. M

Pharmacokinetics

Includes:

•Absorption

•Distribution

•Metabolism (Biotransformation)

•Excretion.

BIOTRANSFORMATION

•Involves enzymic conversion of one chemical entity to

another within the body.

• Occurs between absorption of the drug into the

circulation and its elimination.

• Renders non polar (lipid soluble) compounds polar

(lipid insoluble).

• Sites- liver, GIT, lungs, kidneys, brain, skin.

Consequences in a biotransformation reaction:

Formation of an inactive metabolite from a pharmacologically active drug.Eg: 6- Mercaptopurine 6- Mercapturic acid

(Active drug) (Inactive metabolite)

Formation of an active metabolite from an inactive or a lesser active drug.Eg: L- dopa Dopamine in basal ganglia

(Inactive) (Active)

Formation of an active metabolite from an equally active drug.

Eg: Diazepam Oxazepam(Active) (Active metabolite)

Formation of a toxic metabolite from an active drug.

Eg: Paracetamol N- acetyl- p- benzoquinoneimine(Active) (Toxic metabolite)

MICROSOMAL ENZYMES

• Drug metabolizing enzymes associated with smooth endoplasmic reticulum of the liver.

•Principal enzymes involved: - Mixed Function Oxidase

- Cytochrome P450

•Non specific in action.

•Can be induced, activated. Can metabolize only lipid soluble drugs.

•Primarily concerned with phase I oxidation and reduction.

The activity of MFO’s require a reducing agent (nicotinamide adenine dinucleotide phosphate [NADPH]) and molecular oxygen.

In a typical reaction, one molecule of oxygen is consumed (reduced) per substrate molecule, with one oxygen atom appearing in the product and the other in the form of water.

Drug + O2 + NADPH + H+ Drug metabolite + H2O + NADP+

Cytochrome P450 abbreviated as P450 or CYP- a haemoprotein.

Classified into families designated as 1,2,3,4 and subfamilies by letters A, B, C, D.

Another number is added to indicate specific isoenzyme. Eg: CYP2A6.

These enzymes differ from one another in:

Amino acid sequence.

Sensitivity to inhibitors and inducing agents.

Specificity of the reactions they catalyse.

Biotransformation reactions- 2 types:

Phase I/ Non synthetic reactions

Phase II/ Synthetic reactions

PHASE I REACTIONS

Functions to convert lipophilic molecules into polar

molecules by introducing or unmasking a polar

functional group like –OH or –NH2 .

Involves Oxidation, Reduction and Hydrolysis.

OXIDATION:

Microsomal oxidation causes aromatic or aliphatic hydroxylation, deamination, dealkylation or S-oxidation.

These reactions involve reduced nicotinamide adenine dinucleotide phosphate(NADP), molecular O2 and one or more group of CYP450.

Drug + O2 + NADPH + H+ Drug- OH + H2O + NADP+

Can also involve other MFO’s like flavin containing monooxygenases or epoxide hydrolases.

REDUCTION:

Reduction requires reduced NADP-cytochrome-c

reductase or reduced NAD-cytochrome b5 reductase.

HYDROLYSIS:

These reactions do not involve hepatic microsomal enzymes.

Occur in plasma and other tissues.

Both ester and amide bonds are susceptible to hydrolysis.

PHASE II REACTIONS

Consists of conjugation reactions.

Drugs already possessing an –OH, -NH2 , -COOH

group may enter phase II directly without prior phase I

metabolism.

Involves acetylation, methylation, glucuronidation,

sulphation, mercaptopuric acid formation, glutathione

conjugation.

AMINO ACID REACTIONS: Glycine and glutamine are chiefly involved.

Glycine forms conjugates with nicotinic acid and

salicylates.

Glutamine forms conjugates with p-aminosalicylates.

ACETYLATION:

Acetate derived from acetyl coA conjugates with drugs

like isoniazid, sulfonamides.

This activity resides in the cytosol and occurs in the

leucocytes, gastrointestinal epithelium and the liver.

GLUCURONIDATION:

Catalysed by UDP- glucuronyl tranferase enzyme.

Conjugation reactions between glucuronic acid and carboxyl groups are involved in the metabolism of bilirubin, diazepam etc.

Deficiency of glucuronide formation

Excess unconjugated bilirubin

Non hemolytic jaundice

METHYLATION:

Proceeds by a pathway involving S-adenosyl methionine as methyl donor to drugs with free amino, hydroxyl or thiol groups.

Eg: Catechol-O-methyl transferase.

Present in the cytosol.

Methylates the terminal – NH2 residue of noradrenaline to form adrenaline in the adrenal medulla

Catalyses the transfer of a methyl group tocatecholamines, inactivating noradrenaline, dopamine and adrenaline.

ENZYME INDUCTION

Some P450 substrate drugs, on repeated administration induce P450 expression by enhancing the rate of its synthesis.

Leads to accelerated drug metabolism leading to:

Decreased plasma drug concentrations.

Decreased drug activity if metabolite is inactive.

Increased drug activity if metabolite is active.

Decreased therapeutic drug effect.

CLINICAL RELEVANCE

Drug- drug interaction:

Eg: Phenytoin accelerates Vitamin D3 metabolism Osteomalacia.

Failure of OCP if potent inducers like rifampicin or phenytoin are used.

Drug toxicity:

Eg: Risk of hepatotoxicity is more in Ethanol drinkers than in those having Paracetamol overdose.

ENZYME INHIBITION

One drug may inhibit the metabolism of another drug resulting in an increase in the circulating levels of the slowly metabolized drug.

A drug may inhibit one isoenzyme while itself being a substrate of another isoenzyme.

Eg: Quinidine is metabolized mainly by CYP3A4 but it inhibits CYP2D6.

Inhibition of CYP isoenzyme activity is an important source of drug interactions that leads to serious adverse events.

Eg: Omeprazole is a potent inhibitor of 3 CYP isoenzymes responsible for warfarin metabolism.

Inhibition of drug metabolism

Increased plasma levels over time and with long term medications.

Prolonged pharmacological drug effect.

Increased drug induced toxicities.

FIRST PASS METABOLISM

All drugs taken orally pass through GIT and portal system before reaching the systemic circulation.

In first pass metabolism, metabolism of drugs occur before the drug enters systemic circulation.

Net result is decreased bioavailabilty of the drug leading to diminished therapeutic response.

EXCRETION OF DRUGS

Most drugs and drug metabolites are eliminated from the body through renal (most common) and biliary excretion.

Relies on the lipophilic character of the drug or metabolite.

RENAL EXCRETION OF DRUGS:

Renal blood comprises 25% total systemic blood flow.

Rate of drug elimination through kidneys depend on: balance of drug filtration secretion reabsorption rate.

Afferent arteriole Free drug and plasma protein bound drug glomerulus.

However only the free drug is filtered into the renal tubule.

Renal blood flow, GFR and drug binding to plasma protein affect the amount of drug entering the tubule at the glomerulus.

Rapid excretion of the drug is caused by:

Enhancing the blood flow.

Increasing the GFR

Decreasing plasma protein binding.

GLOMERULAR FILTRATION: Drugs enter the kidney through renal arteries which divide to form glomerular capillary plexus.

Free drug flows through the capillary slits into the Bowman’s space as a part of glomerular filtrate.

Glomerular capillaries allow drug molecules of molecular weight below 20,000.

Lipid solubility and pH do not influence passage of drugs into the glomerular filtrate.

TUBULAR SECRETION: Upto 20% of renal plasma flow is filtered through the glomerulus.

80% pass on to the peritubular capillaries of the proximal tubules.

Here, the drug molecules are transferred to the tubular lumen by two independent and relatively non selective carrier systems- OAT and OCT.

OAT transports acidic drugs while OCT handles organic bases.

Unlike glomerular filtration, carrier mediated transport can achieve maximal drug clearance even when most of the drug is bound to plasma protein.

Many drugs compete for the same transport system leading to drug interactions.Eg: Probenecid prolongs the action of penicillin by retarding its tubular secretion.

TUBULAR REABSORPTION:

The concentration of the drug increases as it moves towards the distal convoluted tubule.

If the drug is uncharged, it may diffuse out of the nephric lumen back into the systemic circulation.

For an ionised drug, reabsorption in the tubule can be enhanced or inhibited by chemical adjustment of urinary pH.

Weak acids can be eliminated by alkanisation of urine while weak bases can be eliminated by acidification of urine- ion trapping.

Eg: Phenobarbitol overdose can be treated with sodium bicarbonate. It alkanises the urine, keeps the drug ionised and decreases its reabsorption.

If overdose is with a weak base, such as cocaine, acidification of the urine with NH4Cl leads to protonation of the drug and an increase in its clearance.

BILIARY EXCRETION:

Various hydrophilic drug conjugates particularly glucuronides are concentrated in the bile and delivered to the intestine.

Here the glucuronide is hydrolysed, releasing the active drug once more.

This free drug is reabsorbed and the cycle is repeated- enterohepatic circulation.

CLEARANCE Defined as the rate of elimination of the drug in relation to its concentration.

Clearance = Rate of elimination

Concentration

Elimination of the drug may involve processes occuring in the kidney, liver, lungs etc..

Clearance(total) = Clearance(renal) + Clearance (hepatic) + Clearance(others).

KINETICS OF ELIMINATION Most of the elimination reactions (includes both metabolism and excretion) follow Michaelis- Menten kinetics:

Rate of elimination= E= Vmax [C]

Km+ C

Where, Vmax Maximum rate of drug elimination.

Km drug concentration at which rate of elimination is ½ Vmax (Michaelis constant).

C Concentration of the drug in the plasma.

FIRST ORDER KINETICS: Here the concentration of the drug is much less than the Michaelis constant Km.

Hence the equation reduces to,

E = Vmax [C] Km

That is, rate of drug elimination is directly proportional to the concentration of the free drug.

ZERO ORDER KINETICS:

In a few drugs like aspirin, ethanol and phenytoin, [C] is much greater than Km.

Hence the equation reduces to,E = Vmax [C] = Vmax

[C]

Rate of elimination remains constant over time.

REFERENCES: Rang and Dales pharmacology.

Basic and clinical pharmacology – Katzung.

Lippincott’s illustrated reviews.

David E Golan’s Principles of pharmacology.

Text book of clinical pharmacolgy- James Ritter

HL Sharma

KD Tripathi

THANK YOU…