Drug interactions

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DRUG INTERACTIONS Prof. R.K.DIXIT Dept Of Pharmacology & Therapeutics KGMU, Lucknow dixitkumarrakesh@gmail.com

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“ Drug interaction refers to modification of response to one drug by another when they are administered simultaneously or in quick succession” Once should be vigilant about Drug-Drug Interactions Drug – Herbs Interactions (Indigenous Medicines) Drug-Food Interactions (Grape Fruit Juice etc.) Drug-Environment Interactions Drug-Pollutant Interactions Drug-and..............................Interactions

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The modification in response may be Quantitative (In Intensity), i.e. Increased or Decreased It may be qualitative Abnormal or a different (New) type of response is produced. Possibility arises whenever a patient receives more than one drug , and Chances increase with More number of drugs. Patient with multiple diseases Patient treated by multiple doctors Patients with compromised physiology Patients with extreme of age ( Elderly and Children)

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Every Drug Interaction is Harmful ???? NO Several drug interactions are deliberately employed in therapeutics, e.g. ACE inhibitors + diuretics to treat hypertension or Sulfamethoxazole + Trimethoprim to treat bacterial infection or Furosemide + amiloride to prevent hypokalaemia.

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Doctor should elicit a detailed drug history of the patient and record all the medication that he/ she is currently on.

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Drugs more likely to be involved in drug interactions With Narrow therapeutic index (Low Safety Margin) Aminoglycosides Digitalis Lithium Affecting vital physiology of the body Antihypertensive drugs Anti-diabetic drugs Anticoagulants With high plasma protein binding capacity NSAIDs Warfarin Sulfonylureas Metabolized by Zero Order Kinetics or Saturation Kinetics Phenytoin Theophyllin

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MECHANISM OF DRUG INTERACTIONS Drug interactions can be broadly divided into Pharmaceutical Interaction During dosage form preparation or at time of administrations. Dissolving the drug in solvent, Mixing drugs in powder, solution or injection forms. Pharmacokinetic (ADME) Absorption (Complex or Chelate formation, Altered stomach pH, Ionization, GIT motility, First Pass Metabolism) Distribution (Protein binding) Metabolism ( Enzyme induction/inhibition) Excretion (Altered pH, Ionization, Entero-hepatic recirculation) Pharmacodynamic ( At receptor or tissue level)

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ABSORPTION Insoluble and poorly absorbed complexes in the gut Example:- Tetracyclines and calcium/iron salts, antacids or sucralfate Phenytoin absorption is decreased by sucralfate Minimized by administering the two drugs with a gap of 2-3 hours. Alteration in Entero-hepatic recirculation Antibiotics like Tetracyclines (Broad Spectrum) markedly reduce gut flora that normally deconjugates oral contraceptive steroids secreted in the bile as glucuronides and permits their Entero-hepatic recirculation . C ontraceptive failure when concurrent use of antibiotics due to lowering of the contraceptive blood levels.

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DISTRIBUTION Primarily due to displacement of one drug from its binding sites on plasma proteins by another drug. Drugs highly bound to plasma proteins that have a relatively small volume of distribution like oral anticoagulants, sulfonylureas, certain NSAIDs and anti-epileptics are particularly liable to displacement interactions The drug which is in unbound form is active while portion which is in bound form woks as temporary storage. When the drug is displaced by the other drug or chemical the unbound form of the active drug becomes more leading to toxic level in the blood and presenting as toxicity.

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METABOLISM Certain drugs reduce or enhance the rate of metabolism of other drugs and affect the bioavailability . Inhibition of drug metabolism may be due to competition for the same CYP450 iso-enzyme or cofactor, and attains clinical significance mostly for drugs that are metabolized by saturation kinetics.

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SOME IMPORTANT INHIBITORS OF METABOLISM OF MULTIPLE DRUGS (MAO-QC) Macrolide antibiotics, Azole antifungals, Chloramphenicol, Omeprazole, SSRIs, HIV -protease inhibitors, Cimetidine, Quinolones (Ciprofloxacin) Metronidazole . Risk of statin induced myopathy is increased by fibrates, niacin, erythromycin, azole anti-fungals and HIV -protease inhibitors, due to inhibition of statin metabolism.

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Induction involves gene mediated increased synthesis of certain CYP450 isoenzymes. It takes 1-2 weeks of medication with the inducer to produce maximal effect. Effects regresses gradually over 1-3 weeks after discontinuation of the inducer

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IMPORTANT MICROSOMAL ENZYME INDUCERS (RBC) Barbiturates, Phenytoin Carbamazepine Rifampin Cigarette smoking Chronic alcoholism P ollutants

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Instances of failure of antimicrobial therapy with metronidazole, doxycycline or chloramphenicol have occurred in patients who are on long-term medication with an inducing drug. Contraceptive failure and loss of therapeutic effect of many other drugs have occurred due to enzyme induction (Patient taking Rifampicin) Toxic dose of paracetamol is lower in chronic alcoholics and in those on enzyme inducing medication, because one of the metabolites of paracetamol is responsible for its overdose hepatotoxicity

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EXCRETION Interaction involving excretion are important mostly in case of drugs actively secreted by tubular transport mechanisms. The alteration of urinary pH alters the process of reabsorption of the drug leading to increase or decrease excretion. Probenecid inhibits tubular secretion of penicillins and cephalosporins . Aspirin blocks the uricosuric action of probenecid and decreases tubular secretion of methotrexate . Alkalization of urine increases the excretion of barbiturates

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PHARMACODYNAMIC INTERACTIONS These interactions derive from modification of the action of one drug at the target site by another drug, independent of a change in its concentration . This may result in an enhanced response (synergism ), an attenuated response (antagonism) or an abnormal response .

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Examples:- Excessive sedation, respiratory depression, motor incoordination due to concurrent administration of a benzodiazepine (diazepam), a sedating antihistaminic (promethazine), a neuroleptic (chlorpromazine), an opioid (morphine).

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Excessive fall in BP and fainting due to concurrent administration of α 1 adrenergic blockers, vasodilators, ACE inhibitors, high ceiling diuretics and cardiac depressants. Excessive platelet inhibition resulting in bleeding due to simultaneous use of aspirin/ ticlopidine / clopidogrel and carbenicillin. Increased risk of bleeding due to concurrent use of antiplatelet drugs (aspirin, clopidogrel) with anticoagulants (warfarin).

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Abnormal responses sometimes result from pharmacodynamic interaction between certain drugs. (Mechanism may be explainable or unexplainable) e.g. Metronidazole and Cefoperazone inhibit the enzyme aldehyde dehydrogenase resulting in bizarre distressing symptoms if the patient drinks alcohol. (Disulfiram Like reaction) The basis of certain interactions is not explained, e.g. ampicillin has produced high incidence of skin rashes in patients treated with allopurinol.

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DRUG INTERACTIONS BEFORE ADMINISTRATION Certain drugs react with each other and get inactivated if their solutions are mixed before administration . In practice situations, these in vitro interactions occur when injectable drugs are mixed in the same syringe or infusion bottle.

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Some examples are: • Penicillin G or ampicillin mixed with gentamicin or another aminoglycoside antibiotic. • Thiopentone sodium when mixed with succinylcholine or morphine. • Heparin when mixed with penicillin gentamicin/hydrocortisone.

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Not all patients taking interacting drugs experience adverse consequences, but it is advisable to take due precautions to avoid mishaps in all cases where interactions are possible. Two drugs have the potential to interact does not necessarily contraindicate their concurrent use. In many cases, knowledge of the nature and mechanism of the possible interaction may permit their concurrent use provided appropriate dose adjustments are made or other corrective measures are taken

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“ It is prudent to consider the possibility of drug interaction whenever two or more drugs are prescribed to a patient, or any drug is added to what the patient is already taking”

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