Drug – enzymes interactions Prof. M. Kršiak Department of Pharmacology, Third Faculty of Medicine Ruská 87, Prague 10, Subject: General Pharmacology Charles University in Prague, Third Faculty of Medicine Academic year GENERAL MEDICINE 6-YEAR MASTER‘S STUDY PROGRAMME CVSE3P0012 ID15026

Figure 3.1 Types of target for drug action. Downloaded from: StudentConsult (on 5 November :28 PM) © 2005 Elsevier Four major targets for drug action: ENZYMES

2. Other drug-enzymes interactions 1.Enzyme inhibition by drugs Outline:

1.Enzyme inhibition by drugs

EnzymesInhibitors Therapeutic groups, indications Angiotensin-converting enzyme enalapril, ramipril Antihypertensives Cyclo-oxygenaseaspirin, ibuprofen, diclofenac Antiinflammatory and antirheumatic agents, analgesics Acetylcholinesteraseneostigmine, rivastigmin Parasympathomimetics, Anti-dementia- drugs Monoamine oxidasemoclobemide Antidepressants HMG-CoA reductasesimvastatin, atorvastatin Lipid modifying agents; (hypercholesterolaemia) XanthinoxidaseallopurinolDrugs inhibiting uric acid production Phosphodiesterase type VsildenafilDrugs used in erectile dysfunction Dihydrofolate reductase trimethoprim Antimicrobial agents methotrexateAntimetabolites, folic acid analogues NeuroamidaseoseltamivirAntivirals ( influenza virus) Thymidine kinaseaciclovirAntivirals (Herpes virus) HIV proteasesaquinavirAntivirals (HIV), protease inhibitors Many clinically important drugs act by inhibiting enzymes: More details follows

Drugs can inhibit enzymes reversibly (usually a competitive inhibition by non-covalent binding) or irreversibly (enzyme is usually changed chemically by covalent binding) An enzyme inhibitor is a molecule which binds to enzymes and decreases their activity Irreversible inhibitors usually react with the enzyme and change it chemically (e.g. via covalent bond formation). These inhibitors modify key amino acid residues needed for enzymatic activity (e.g. aspirin, acting on cyclo-oxygenase) Competitive inhibition is a form of enzyme inhibition where binding of the inhibitor to the active site on the enzyme prevents binding of the substrate and vice versa. Often, the drug molecule is a substrate analogue (e.g. captopril, acting on angiotensin-converting enzyme)

The active site of angiotensin-converting enzyme. [A] Binding of angiotensin I. [B] Binding of the inhibitor captopril, which is an analogue of the terminal dipeptide of angiotensin I. Downloaded from: StudentConsult (on 6 November :30 PM) © 2005 Elsevier Reversible competitive inhibition of enzyme (inhibition of ACE by captopril)::

Irreversible non-competitive inhibition of enzyme (inhibition of COX-1 or COX-2 by aspirin): This makes aspirin different from other NSAIDs (such as diclofenac and ibuprofen, which are reversible inhibitors). Aspirin acetylates serine residue in the active site of the COX enzyme

Irreversible inhibition of enzyme: Recovery is possible only by synthesis of a new enzyme

As thrombocytes (platelets) do not have nucleus (adequate DNA), they are unable to synthesize new COX once aspirin has irreversibly inhibited the enzyme Endothelial cells have nucleus and are able to recover synthesis of COX Irreversible inhibition of COX in thrombocytes and in endothelium

CONSTITUTIVE ISOENZYME INDUCIBLE ISOENZYME COX-1 COX-2 PHYSIOLOGICAL FUNCTIONS INFLAMMATORY RESPONSE PROTECTION OF GASTRIC MUCOUS MEMBR. INFLAMMATION INCREASE OF BLOOD FLOW AND SODIUM FEVER EXCRETION IN THE KIDNEY PAIN CYKLO-OXYGENASE COX MECHANISM OF ACTION OF NON-OPIOID ANALGESICS Selective COX-2 inhibitors: COXIBS lower risk of gastropathy COX-1 inhibitors: ibuprofen, diclofenac and other risk of gastropathy

Selective COX-2 inhibitors (Coxibs) have lower gastropathy but a higher risk for heart attack and stroke

COX-1 tromboxan A 2 increases platelet aggregation + vasoconstriction Arachidonic acid COX-2 prostacyclin PGI 2 inhibits platelet aggregation + vasodilatation aspirin aspirin prevents platelet aggregation coxibs = selective COX-2 inhibitors : higher trombotic risk coxibs has protective anti-coagulative effect promotes clotting

Selective COX-2 inhibitors (coxibs) increase in the risk for heart attack and stroke through an increase of thromboxane unbalanced by prostacyclin (which is reduced by COX-2 inhibition)

Non-steroidal Anti-inflammatory Drugs (NSAIDs) Major required effects: Analgesic + Antipyretic +Anti-inflammatory Nonselective (COX-1 and COX-2) ibuprofen, diklofenac … Preferential (COX-2 > COX-1) nimesulide, meloxicam Selective (coxibs) (COX-2 only) celecoxib … Classification of NSAIDs (by selectivity of inhibition of COX-1 and COX -2) :

Acetylcholinesterase inhibitors inhibit the acetylcholinesterase from breaking down acetylcholine, thereby increasing both the level and duration of action of the neurotransmitter acetylcholine. REVERSIBLE physostigmine, neostigmine, rivastigmine Are used medicinally: antidote to anticholinergic poisoning to treat glaucoma to treat myasthenia gravis to treat Alzheimer disease to reverse the effect of non-depolarising muscle relaxants IRREVERSIBLE Are used as weapons in the form of nerve agents Are used as insecticides

Monoamine oxidase inhibitors (MAOIs) Originally irreversible MAOIs were used, now withdrawn Because of potentially lethal dietary („cheese effect“ and drug interactions, hypertensive crisis

serotonin syndrome MAO –A serotonin, noradrenalin (norepinephrine), tyramine moclobemide treatment of depression treatment of anxiety disorders (OCD, panic disorders, phobia) MAO –B dopamine selegiline no dietary restrictions dietary restrictions Monoamine oxidase inhibitors (MAOIs) at present: reversible RIMA treatment of Parkinson‘s disease MAOIs are usually used as a last line of treatment only when other classes of antidepressant drugs have failed.

EnzymesInhibitors Therapeutic groups, indications Angiotensin-converting enzymeenalapril, ramipril Antihypertensives Cyclo-oxygenaseaspirin, ibuprofen, diclofenac Antiinflammatory and antirheumatic agents, analgesics Acetylcholinesteraseneostigmine, rivastigmin Parasympathomimetics, Anti-dementia- drugs Monoamine oxidasemoclobemide Antidepressants HMG-CoA reductasesimvastatin, atorvastatin Lipid modifying agents; (hypercholesterolaemia) XanthinoxidaseallopurinolDrugs inhibiting uric acid production Phosphodiesterase type VsildenafilDrugs used in erectile dysfunction Dihydrofolate reductase trimethoprim Antimicrobial agents methotrexateAntimetabolites, folic acid analogues NeuroamidaseoseltamivirAntivirals ( influenza virus) Thymidine kinaseaciclovirAntivirals (Herpes virus) HIV proteasesaquinavirAntivirals (HIV), protease inhibitors Many clinically important drugs act by inhibiting enzymes: Others for example

2. Other drug-enzymes interactions

Inactive (prodrugs)Active drugActive metaboliteToxic metabolite Prednisone →Prednisolone Enalapril →Enalaprilat Diazepam →Nordiazepam → Oxazepam Morphine →Morphine 6-glucuronide Paracetamol → N-Acetyl-p- benzoquinone imine Drugs-enzyme interactions producing active or toxic metabolites:

Those of importance in the metabolism of psychotropic drugs are CYP1A2, CYP2C9, CYP2C19, CYP2D6 and CYP3A4, the last being responsible for the metabolism of more than 90% of psychotropic drugs that undergo hepatic biotransformation. Cytochrome P450 (CYP) enzymes Many psychotropic drugs have a high affinity for one particular CYP enzyme but most are oxidised by more than one Drug - cytochrome P450 interactions The most important enzymes involved in drug interactions are members of the cytochrome P450 (CYP) system that are responsible for many of the phase 1 biotransformations of drugs. These metabolic transformations, such as oxidation, reduction and hydrolysis, produce a molecule that is suitable for conjugation.

Genetic polymorphism The CYP enzymes that demonstrate pharmacogenetic polymorphism include CYP2C9, CYP2C19 and CYP2D6. In clinical practice, the polymorphism produces distinct phenotypes, described as poor metabolisers, extensive metabolisers (the most common type) and ultra-rapid metabolisers. Genetic effects:

CYP enzymes can be induced or inhibited by drugs or other biological substances, with a consequent change in their ability to metabolise drugs that are normally substrates for those enzymes. Drug effects:

Enzymatic induction enzymatic induction can cause a decrease as well as an increase in the drug’s effect The onset and offset of enzyme induction take place gradually, usually over 7– 10 days The most important are inducers of CYP3A4 and include carbamazepine, phenobarbital, phenytoin, rifampicin and St John’s wort (Hypericum perforatum). An example of an interaction in psychiatric practice is the reduced efficacy of haloperidol (or alprazolam) when carbamazepine is started, resulting from induction of CYP3A4.

Enzymatic inhibition enzymatic inhibition can cause an increase as well as a decrease in the drug’s effect Most hazardous drug interactions involve inhibition of enzyme systems, which increases plasma concentrations of the drugs involved, in turn leading to an increased risk of toxic effects. Inhibition of CYP enzymes is the most common mechanism that produces serious and potentially life-threatening drug interactions Inhibition is usually due to a competitive action at the enzyme’s binding site. Therefore, in contrast to enzyme induction, the onset and offset of inhibition are dependent on the plasma level of the inhibiting drug

Amitriptyline + fluoxetine Fluoxetine inhibits 2D6 Amitriptyline is a substrate for 2D6 Amitriptyline + fluoxetine → increased plasma levels of amitriptyline and prolonged t 1/2 → sometimes fatal consequences

OTHER ADVERSE CLINICAL CONSEQUENCES OF DRUG INTERACTIONS* Profound oversedation Severe sedation due to the additive effect (summation) of drugs with sedating properties is a particular problem in elderly and frail people, and it can lead to falls and injuries (especially fractures of the femoral neck). Excessively drowsy patients are also at increased risk of venous thromboembolism and, if confined to bed, of hypostatic pneumonia. In people who drive, increased sedation due to drug interactions carries a correspondingly increased risk of road traffic accidents. Profound and prolonged sedation can be brought about by inhibition of CYP3A4 enzymes that are involved in the metabolism of anxiolytics and sedatives e.g. alprazolam, midazolam + ketoconazole/clarithromycine/grapefruit

Drugs may also act as false substrates, where the drug molecule undergoes chemical transformation to form an abnormal product that subverts the normal metabolic pathway. An example is the anticancer drug fluorouracil, which replaces uracil as an intermediate in purine biosynthesis but cannot be converted into thymidylate, thus blocking DNA synthesis and preventing cell division Drugs as false substrates: