ANTIMICROBIAL VIVAS
Beta-lactams and cell wall inhibitors
2011-2, 2008-1, 2006-2, 2003-2
What is the mechanism of action of penicillins?
- B-lactam antibiotic
- Bacteriocidal
- Covalently binds to the active site of Penicillin-binding protein (PBP)
- Inhibits bacterial cell wall synthesis by interfering with transpeptidation reaction of bacterial cell
wall synthesis (the final step in cell wall synthesis)
- Structural analogue of D-Ala-D-Ala substrate present in cell wall
- Also activates autolytic enzymes that cause lesions in the cell wall
- High intracellular osmotic pressure bursts weakened cell wall
What are the important mechanisms of resistance to pencillins?
1. Inactivation by B-lactamase (e.g. Staph) – overcome w/ b-lactamase inhibitor e.g. clavulanic
acid, sulbactam or tazobactam (the plasmin-encoded, not the inducible chromosomal one formed
by Enterobacter and Pseudomonas)
2. Modification of target PBPs (e.g. MRSA, Pneumococci, Enterococci)
3. Reduced penetration (Gram-ve organisms)
4. Efflux pump (Gram-ve organisms)
In general, what is the anti-microbial spectrum of penicillin G?
Streptococci, meningococci, enterococci, some pneumococci, treponema pallidum, clostridia, nonbetalactamase producing staphylococci
Describe the pharmacokinetics 2006-2
- Variable resistance to gastric acid
- Poorly absorbed with food
- Widely distributed but only enter meninges when inflammed
- Polar so not much metabolism -> excreted unchanged by filtration 10% and secretion 90%
- Tubular secretion can be blocked (probenecid)
- Some excreted in bile (naficillin and ampicillin undergoes enterohepatic circulation)
- t½ 30mins – 1 hour usually
- Procaine and benzathine forms given IM have long t½ b/c slow release to blood stream
2005-1
What are the clinical manifestations of penicillin allergy?
- Hypersensitivity reactions (~5-6% and <1% of people who have been ok w/ previous)
- Anaphylaxis (rare, 0.05% of people)
- Fever
- Skin (Maculopapular rash, Urticarial skin rash , Exfoliative dermatitis)
- Serum sickness
- Stevens Johnson Syndrome (Erythema multiforme major)
What other side effects of penicillin treatment do you know of?
- Renal failure
- Seizures at high doses
- GI disturbance
- Candidal infections
- Hepatitis (fluclox/oxacillin)
- Pseudomembranous colitis
2010-2
What is the mechanism of action of cephalosporins
- Inhibit bacterial cell wall synthesis, cell division and growth
- Similar to penicillins, contain b-lactam ring
- Bactericidal Most effective in rapidly dividing cells.
How does the spectrum of microbiological activity for the 4th generation cephalosporins
compare to that of earlier generations?
- Gram-ve as for 3rd generation e.g. E. coli, H. influenza, Klebsiella
- Some gram+ve (S. pneumonia) but less than 1st generation
- More resistant to B-lactamases than earlier generations
What is the relationship between penicillin allergy and cephalosporin allergy.
- 5-15% possibility of cross-reaction with penicillin allergy.
2010-1, 2008-2, 2004-2
How are cephalosporin’s classified – what are the differences
Based on their order of introduction into clinical practice, but vary in their antibacterial activity:
- 1st gen: GPs (Strep and Staph) – e.g. cefazolin and cephalexin, cephalothin
- 2nd gen: Slightly less GP, but >GN (Haemophilus & Kleb) – e.g. ceflaclor, cefuroxime,
cefamandole, cefotetan, cefoxitin
- 3rd gen: Even less GP but even more GN and can penetrate BBB – e.g. ceftriaxone, ceftazidime,
cefoperazone, ceftizoxime, cefixime (All end in –one or –ime, exceptions: cefuroxime and
cefepime)
- None of the above active against MRSA, Pseudomonas or Enterobacter
- 4th gen: Good GP and GN (Pseudomonas), only one is cefepime
Why are 3rd generation cephalosporin’s used in CNS infection
- Expanded GN activity
- Cross the BBB
- Penetrate body fluids well
- Good toxicity profile
Are there any bacteria responsible or CNS infection that cephalosporins don’t cover
- Listeria
- Resistant pneumococci may need vancomycin
- Resistant E Coli
- Use with aminoglycoside to cover Pseudomonas
What are the adverse effects of the Cephalosporins? 2008-2
- Hypersensitivity reactions (~2%) identical to penicillins: anaphylaxis, fever, skin rashes,
nephritis, granulocytopenia, & hemolytic anemia
- Some individuals with a history of penicillin allergy may tolerate cephalosporin’s
- Frequency of cross-allergenicity uncertain, probably around 5 - 10%
- Severe pain IMI
- Thrombophlebitis IVI
- Renal toxicity: interstitial nephritis & ATN
- Cephalosporins with a methylthiotetrazole group (eg, cefamandole, cefotetan) may cause:
hypoprothrombinemia, bleeding (preventable with Vit K1 10 mg twice weekly) and severe
disulfiram-like reactions with alcohol.
2009-1
What classes of antibiotic are used in staphylococcal infection
Beta-lactamase negative staph: Penicillin, 1st Generation Cephalosporins
Beta-lactamase positive staph:
- Beta-lactamase resistant penicillins (Methicillin, Naficillin) or Isoxazolyl Penicillins (dicloxacillin,
flucloxacillin)
- 1st Generation Cephalosporins
- Beta-lactamase inhibitor with penicillin combination – amoxicillin with clavulinic acid, sulbactam,
tazobactam
- Vancomycin (MRSA), Aminogycosides, Macrolides
What are the mechanisms of resistance in MRSA
- Beta-lactam antibiotics normally bind to PBP's (Penicillin Binding Proteins) causing inhibition of
transpeptidation, thus blocking cell wall synthesis and lead to cell wall death
- MRSA produce PBP's that have a low affinity for binding beta-lactam antibiotics and hence render
them ineffective
- May be overcome if used in high enough concentrations, but not clinically achievable
What are the adverse effects of vancomycin
- Local phlebitis
- Chills & fever
- Flushing due to histamine release (Red Man Syndrome)
- Ototoxicity / nephrotoxicity (esp if administered with aminoglycoside)
Additional: Vancomycin
- PD: Bactericidal glycoprotein, binds D-Ala-D-Ala, inhibits cross linking
- Resistance by replacing terminal D-ala with D-lactate
- Narrow spectrum, used for MRSA, or w/ 3rd gen for penicillin resistant pneumococci, or as backup
agent for C. dif
- PK: No oral absorbtion (IV only), widely distributed, excreted unchanged, needs dose monitoring
What circumstances encourage the development of bacterial resistance to antimicrobial
agents? 2005-2
Resistance is an example of natural selection, and arises through spontaneous mutations or DNA
exchange between different species of bacteria (either directly by piasmids or via bacteriophages)
Therefore resistance is promoted by:
1. Dirty hospital environments with multiple species of bacteria co-existing and "exchanging" between
environment, patients and staff
2. A course of antibiotics that only partially treats a target population (inadequate potency, dose or
duration)
(Thus, paradoxically, both under-use and overuse of antibiotics plays a role in the development of
resistance!)
Overall, however, total consumption of antibiotics within a human population is the critical factor in
development of resistant strains
Inhibitors of microbial protein synthesis
2007-1
What are the adverse effects of chloramphenicol
- GIT: nausea, vomiting and diarrhoea
- Bone marrow suppression: reversible RBC suppression, idiosyncratic aplastic anaemia:
1/24000 – 1/40000
- Newborn: gray baby syndrome
- Drug interaction: Phenytoin, chlorpropamide ,warfarin prolongs half life and raises concentration
Which bacteria does it affect: What is mechanism of action?
- Aerobic and anaerobic Gram+ve and Gram-ve, also rickettsia but not Chlamydia
- Potent inhibitor of microbial protein synthesis-binds to 50S subunit of bacterial ribosome by
inhibiting peptidyl transferase
- Bacteriostatic
2003-2, 2003-1(doxycycline)
How do tetracyclines exert their antimicrobial activity?
- Bacteriostatic
- Enter cells by diffusion and active transport
- Bind irreversibly to 30S sub-unit of the ribosome
- Block binding of tRNA to mRNA – ribosome complex
- Stop addition of amino-acids to peptide (and thus inhibitor of protein synthesis)
- PD: used for Mycoplasma, Chlamydia, Rickettsia and some protoza e.g. malaria
- Used for: Atypical pneumonia, STDs, H. pylori infections, Acne, Malaria prophylaxis
Describe the pharmacokinetics of the tetracyclines.
- Variable oral absorption depending on which drug
- Generally greater than 60% absorbed
- Absorption occurs mainly in upper small intestine.
- Food, calcium, dairy products and alkaline pH impair absorption
- 40-80% protein bound
- Distributed widely to tissues except CSF, and cross placenta
- Chelate to Ca and are bound to growing teeth and bones
- Excreted in bile and in urine
- Concentrated in bile (up to 10x serum conc.)
- Undergo enterohepatic circulation
- Doxycycline excreted mainly in bile/faeces, the others mostly in the urine
Are their any group of patients where tetracyclines are contraindicated and why?
- Pregnancy, children < 8 yrs, breast feeding
- Cause tooth enamel dysplasia and irregularities in bone growth
How does resistance to doxycycline develop? 2003-1
1. Decreased intracellular accumulation: by impaired influx and increased efflux by active
transport protein pump that is encoded on plasmid (commonly encodes resistance genes for
other drugs: aminoglycosides, sulfonamides, CAM etc)
2. Ribosome protection: proteins interfere with tetracycline binding to ribosome
3. Enzymatic inactivation of tetracyclines
2011-2, 2010-2 (erythromycin), 2009-1, 2005-1(azithromycin)
Give some examples of macrolide antibiotics
Erythromycin (prototype drug), roxithromycin, azithromycin, clarithromycin
What is their mechanism of action?
- Inhibit protein synthesis by binding to 50S ribosomal RNA blocking transpeptidation
- Erythromycin may be inhibitory or bacteriocidal at higher concentrations
What organisms are usually sensitive to macrolides 2009-1
- Gram+ve cocci: Pneumocci and Staphyloccci
- Gram-ves: Chlamydia, Legionella, Neisseria, Bordatella pertussis, Campylobacter, Treponema
pallidum
- Mycoplasma
- Clarithromycin in triple therapy for H. pylori
What are the adverse effects of erythromycin?
- Gastrointestinal (anorexia, nausea, vomiting, diarrhoea)
- Liver toxicity (acute cholestatic hepatitis, particularly with estolate)
- Allergic reaction (fever, eosinophilia, rash)
- Drug interactions (inhibits cytochrome P450 – except azithromycin)
- QT prolongation
What is the mechanism for the drug interactions associated with erythromycin & give some
examples? 2010-2
- Inhibits hepatic CYP3A4 => inhibits metabolism of other drugs causing increased activity
- Examples: benzodiazepines, carbamazepine, cisapride (cardiotoxicity), digoxin, warfarin,
theophylline, cyclosporin, tacrolimus
How does azithromycin differ from other antibiotics in its class? 2005-1
- Absorption affected by food
- Much longer t½ - 2-4 days
- Levels considereably higher in tissues and phagocytes c.f. plasma
- Doesn't activate cytochrome P450
- Very active against Chlamydia
- Excreted in urine unchanged
Additional: Pharmacokinetics
- Good oral bioavailability
- Good tissue distribution (esp azithromycin)
Erythromycin: biliary excretion, t½ 2hr
Clarithromycin: hepatic -> urinary t½ 6hr
Additional: Clindamycin
Pharmacodynamics
- A lincosamide w/ similar mechanism to macrolides via binding the 50s subunit
- Structurally dissimilar
- Gram-ve aerobes resistant b/c poor penetration of outer membrane
- Used for anaerobes, MRSA, penicillin allergy, Pneumocystis jiroveci???
- Toxicity: GI upset, skin rashes, neutropaenia, hepatic dysfunction, C. dif -> pseudomembranous
colitis
Pharmacokinetics
- Good tissue penetration after oral administration
- Hepatic metabolism -> renal excretion of both changed and unchanged drug
Aminoglycosides
2010-1, 2008-2, 2007-1, 2003-2
Describe the mechanism of action of gentamicin
- Bactericidal inhibitor of protein synthesis
- Passive diffusion via porin channels across outer membrane, then active transport into cytoplasm
by an O2 dependant process (anaerobes protected)
- Inside the cell, binds 30S ribosome & inhibits protein synthesis by:
1. Inducing misreading of mRNA thus producing toxic or nonfunctional protein
2. Interfere with initiation complex of peptide formation
3. Cause break up of polysomes into non-functional monosomes
What are the benfits of once daily dosing of gentamicin
1. Concentration-dependant killing - at increased concentration kill increased numbers of bacteria
at a more rapid rate
2. Postantiobiotic effect - activity lasts longer than detectable serum levels
3. Reduced toxicity - toxicity is time and concentration dependant, so time above critical level will
be longer with multi dose than single dose schedule
4. Less nursing time and OPD therapy possible
5. Drug level not required unless >3 day therapy
6. Dosage still needs to be adjusted according to renal function
How do penicillins enhance the efficacy of gentamicin
- Antimicrobial synergism
- Low ECF pH and anaerobic conditions inhibits (O2 dependant) transport
- Transport enhanced by cell wall active drugs e.g. β-lactam, vancomycin
What are the mechanisms of resistance 2007-1
1. Group transferases that inactivates drug, carried by plasmids (esp in Gram-ve)
2. Impaired cell entry (cell wall)
3. Changes in the ribosomal binding site
Regarding gentamicin, outline its pharmacokinetic properties 2003-2
- Polar compounds w/ no oral absorption
- Well absorbed IM and usually given IV
- Highly polar and thus does not enter cells well
- Water soluble
- CSF: 20% plasma levels, Bile: 30% plasma level, Pleural/synovial: 50-90%
- Most tissues low except renal cortex
- Not metabolised
- May be inactivated by bacteria
- Cleared by the kidney
- t½ : 2-3 hours
- 40-60% removed by HD
- Dosage adjustment needed for renal impairment
Additional: Toxicity
- Otoxoticity: risk proportional to plasma levels, e.g. is no dose adjustment for renal impairment
- Nephrotoxicity (ATN), esp if co-aministration of amphotericin B, cephalopsporins, vancomycin
- Neuromuscular blockade (rare w/ high doses => respiratory paralysis, reverse w/ Ca2+ and
neostigmine)
- Skin reactions (contact dermatitis)
Sulphonamides, trimethoprim and fluroquinolones
Sample viva, 2006-2
What is the mechanism of action of trimethoprim?
- Inhibit microbial enzymes involved in folic acid synthesis thus inhibiting growth
- Dihydrofolic acid =(dihydrofolate reductase inhibited by trimethoprim)=> tetrahydrofolic acid =>
purines => DNA
Describe the mechanism of antimicrobial activity of the sulphonamides. 2005-1
- Inhibit microbial enzymes involved in folic acid synthesis thus inhibiting growth
- Structurally similar to PABA and compete with it
- PABA =(dihydropteroate synthase)=> dihydrofolic acid
Why is trimethoprim commonly administered in combination with sulfamethoxazole?
- Antibacterial synergism via sequential blockade of steps in folic acid dependent purine
synthesis
- Usually bacteriostatic but the combination is frequently bacteriocidal
What are mechanisms of bacterial resistance to trimethoprim?
1. Reduced cell permeability
2. Increased production of dihydrofolic reductase
3. Alteration in dihydrofolic acid reductase with reducing binding
2008-1, 2004-2, 2003-1
What is the mechanism of action of fluoroquinolones?
- DNA gyrase inhibitor/blocks DNA synthesis
What are the mechanisms of resistance to fluoroquinolones?
1. Decreased intracellular entry by change in the permeability of the organism or efflux pump
2. One or more point mutations in the quinolone binding region of the target enzyme
What are the clinical uses ciprofloxacin ?
- Broad spectrum
- UTI
- Bacterial diarrhoea caused by Shigella, Salmonella, toxigenic E. coli, Campylobacter
- Soft tissue, bone, joint, intra-abdominal and respiratory tract infections
- Treatment against multidrug-resistant organisms (pseudomonas and enterobacter)
- Prophylaxis and treatment against anthrax
- Gonococcal infection
- Chlamydial urethritis or cervicitis TB and atypical mycobacterial infections Eradication of
meningococcal carrier state Prophylaxis in neutropenic patients
Additional:
- 1st gen = norfloxacin for UTI
- 2nd gen = ciprofloxacin for Gram-ve and gonnococus
- 3rd gen = moxifloxacin – less Gram-ve, but more Gram+ve and even anaerobes
2003-1, 2004-2 (in children)
What are the adverse effects of ciprofloxacin
- Well tolerated
- Nausea, vomiting, diarrhea > headache, dizzy, insomnia, rash, LFT abnormalities
- May damage growing cartilage, cause arthropathy not for use <18 yrs
- Tendinitis in adults – risk of tendon rupture
- Avoid during pregnancy and lactation
Miscellaneous
2011-1
In treatment of a new case of Tuberculosis, what are the important principles of drug use?
1. Multiple drugs used initially (usually 4) ensures efficacy
2. Prolonged course, usually 6 months
3. Close supervision to ensure compliance and detect adverse effects
Describe the pharmacology of Rifampicin
- Bactericidal vs TB – inhibits DNA dependant RNA polymerase
- Resistance emerges rapidly if used alone
- Well absorbed orally
- Highly lipid soluble - widely distributed in tissues
- Metabolism in liver, excreted in faeces
- Induces P450 enzymes – many drug interactions
- Discolouration (orange) of body fluids
- Can be used in prophylaxis
Additional: Isoniazid
PD:
- Structural congener of pyridoxine => inhibition of mycobacteral cell walls
- Rapid resistance if used alone (although is for prophylaxis)
PK:
- Well absorbed orally => penetrates cells to act intracellularly
- Actetylation in the liver (genetic control)
- t½ in fast acetylators = 60-90, in slow = 3-4hr
- Fast (higher proportion in Asians) require higher dosing
Tox:
- Neurotoxic: peripheral neuritis, reslestness, twitching - relieved by pyridoxine
- Hepatotoxic
- Inhibition of metabolism of carbamazepine, phenytoin, warfarin
2011-1, 2009-1
What are the indications for acyclovir in the ED?
1. HSV – encephalitis
2. VZV
3. Patients with HIV
To which class of antiviral drugs does acyclovir belong?
- DNA polymerase inhibitors
- Guanosine analog
- Specificity for virus-infected cell (virus-specific thymidine synthase)
- Inhibition of viral DNA synthesis (irreversible binding to viral DNA polymerase)
Describe the pharmacokinetics of acyclovir?
- Short half life 2.5 hrs (5 times daily dosing oral)
- Low oral bioavailability
- Mostly excreted unchanged in urine
- CSF 50% of plasma
- Wide distribution
2008-2
Describe the pharmacokinetics of metronidazole
- Class: Nitroimidazole antiprotozoal drug
- PD: imidazole derivative -> reductive bioactivation (in anaerobic parasites) -> cytotoxic products
that interfer with nucleic acid synthesis
- Well absorbed orally
- Oral/IV/suppository (99% oral bio-availability)
- Metabolised in liver (can accumulate in hepatic insufficiency) and excreted in kidney
- Low protein binding (10-20%)
- Dosage: 500mg tds or single dose of 2g for vaginitis
- Half life 7.5 hours
What are the adverse effects of metronidazole?
- GI: Nausea, diarrhoea, dry mouth, hairy black tongue
- CNS: Headache, paraesthesia, dizziness, insomnia
- GU: Dysuria, dark urine
- Disulfiram-like effect, hence avoid alcohol
- Interactions: Potentiate the effect of coumarin anticoagulants, Lithium
- Teratogenic effect on mice, but not proven in human
2007-1
What is an antiseptic?
A chemical disinfectant applied to living tissue (skin, mucous membranes and wounds) which
decreases the number of organisms by killing, removing and diluting and has generally low
toxicity to tissues.
Describe the actions and uses of chlorhexidine
- Contains chlorinated phenols
- Slower than alcohols but has residual activity
- Low skin sensitising or irritating capacity
- Low oral toxicity (poorly absorbed from the alimentary tract)
- Active against many bacteria but most effective against Gram+ve cocci) and mycobacteria
- Moderate against fungi & viruses - not inhibited by blood or organic products
- Spore germination is inhibited
When is chlorhexidine contraindicated
- Middle ear surgery (causes sensorineural deafness)
- Neurosurgery as neural toxicity
- Allergy