4. Antibacterials Flashcards
1
Q
Antimicrobials
A
Substances that kill or inhibit bacteria
2
Q
Antibiotic
A
Substances produced by micro-organisms that kills other micro-organisms
3
Q
History of antibacterials/infection
A
- Pre antibiotics:
+ Immune response
+ Civilisations using mouldy bread or soil compressed on wounds - Germ theory in 19th century - identifies microorganisms ad the cause of infection
- Ehrlich uses dyes to cure infections including syphillis - development of Salvarsan in 1910
- 1928: Fleming discovers penicillin from Penicillium mould on staphylococci
4
Q
Antibacterial development
A
- 1932: Domagk discovers the active component in a dye - Protonsil (Bayer) the 1st sulphonamide
- Florey & Chain purify penicillin from mould broth
- Further compounds from soil bacteria are isolated & repurposed for human use e.g. streptomycin, tetracycline, & erythromycin
5
Q
Drug targets
A
Ideally unique bacterial targets not present in mammalian cells & usefully therapeutic with minimal mammalian damage
Unique bacterial targets:
- Cell wall
- Cell membrane
- Protein synthesis apparatus
- DNA/RNA transition apparatus
Semi-unique targets:
- Metabolic folate pathway
6
Q
Development to resistance
A
- Identification of resistance
- Fleming predicted inappropriate use of penicillin would lead to antibiotic resistance in 1945
- Resistance is identified in nature before ‘development’ as many drugs are repurposed natural defence mechanisms, but resistance then proliferates in clinical isolates
7
Q
Slowing innovation
A
- Development slowed/stopped while resistance is increasing
- Larger economic returns on chronic diseases & societal willingness to pay for cancer treatments
- University lead research > pharma
- Antibiotics not drug like:
+ Large molecules, reactive moieties, solubility = hard to formulate
+ E.g. Opposite properties for Gram negatives
8
Q
Antimicrobial PK/PD
A
MIC: Min inhibitory concnetration
MBC: Min concentration to kill organism
- PKPD targets are specific for the bacteria + antibiotic + clinical condition
- Streptococcal cellulitis - Penicillin 30-40 % T > MIC
- Streptococcal endocarditis - Penicillin 90-100% T > MIC
- Different strategies can achieve equivalence depending on target
9
Q
Bioavailability (BA/F)
A
- Proportion of given dose in blood/serum
+ IV = 100% or 1F
+ PO ≤ 100% (usually less) limited by tolerability - Benefits of IV:
+ Ensures predictable high concentrations
+ Important in sepsis with reduced flow to site of absorption
- Benefits of PO: \+ Practical for administration \+ Reduced costs \+ Reduced length of stay \+ Reduced length of infection from cannula
10
Q
Distribution
A
- Critical for access to infection sites
- Tissue penetration higher with lipophilic drugs
- High Vd suggests penetration > extracellular fluid
- Eyes, prostate & brain have pores
+ Difficult access so lipophilic better
+ Inflammation changes barrier permeability - Further issues may arise at site of infection e.g. increased pH, biofilms, prosthesis, intracellular pathogens
11
Q
Metabolism
A
- Least critical component for infection PK
- Some antibiotics are formulated as pro-drugs for increased absorption or half life
- Will the drug be activated if used in a non-standard administration route e.g. direct injection in CNS or inhaled?
12
Q
Elimination
A
- Excretion is important in determining length of treatment effect for T > MIC or AUC:MIC
- Influences dosing regimens & TDM monitoring
- Elimination also forms part of the distribution pathway for urinary tract or gut lumen
13
Q
Post-antibiotic effect (PAE)
A
- Phenomena of PAE when persistent suppression of bacterial growth after exposure of antibiotic (concentration below MIC) in absence of host defences
- Postulated that:
+ Persistence of antibiotic at bacterial binding sites (relative to systemic concentration)
+ Non-lethal damage by antibiotics
14
Q
Killing effect - Bactericidal or bacteriostatic?
A
- Theoretical concern which has no influence on practice
+ ‘cidal’ - MBC:MIC within 4 dilutions
+ ‘static’ MBC: MIC > 4 dilutions difference (usually») - In practice innate immune system has a role - concern in immunosuppression
- Systematic review shows no difference in outcomes
15
Q
Synergy
A
- When 2 antibiotics achieve better killing together (> 100% inclusive) than either agent alone
- Beta lactam & aminoglycoside
16
Q
Antagonism
A
- Controversial & limited evidence to support
- Competitive similarly located binding sites e.g. chloramphenicol & macrocodes
17
Q
Antibiotic spectra
A
Broad/narrow terms used to help choose the best antibiotics to reduce effect on commensal flora
ALL RELATIVE:
- Penicillin narrow vs co-amoxiclav broad
- Co-amoxiclav narrow vs meropenem broad
18
Q
Mechanisms of resistance
A
- Alter antibiotic targets
- Antibiotic-degrading enzyme
- Enzyme binding to antibiotic
- Efflux pumps
- Alter permeability
- Increased targets
- Remove the necessity for target enzyme
19
Q
Cell wall - Key differences
A
- Outer membrane layer
- Thickness of cell wall
- Porins
20
Q
Beta-lactams (Penicillins, cephalosporins, carbapenems & monobactams)
A
- Peptidoglycan cross links to strengthen cell wall
- Transpeptidase (PBP):
+ Mediated by PBP
+ Recognises side chain (D-ala-D-ala)
+ Cross links to second peptide chain or forms a glycine bridge
+ Different PBPs by bacterial species - Core structure is beta-lactam ring
- Interactions with transpeptidase as a mimic of the D-ala-D-ala residue
- Beta-lactam sterically strained & rapid catalytic reaction
- Enzyme is reversibly inactivated
- Inhibition of cell wall replication or remodelling:
+ Autolysins released (help break down cell wall)
+ Osmotic instability
+ Cell death - Side chains (R1 R2):
+ Provide steric protection from beta-lactamases
+ Allow penetration through porins
+ Confer pharmacological stability
+ Reduce acid lability (improve bioavailability)
21
Q
Main types of resistance to Beta-lactams (Beta-lactamases)
A
- Penicillinase (S. aureus) - flucloxacillin developed using R1 to prevent binding
- Use of beta-lactamase inhibitors e.g. clavulanic acid (weak inhibition & slowly hydrolysed)
- Extended spectrum beta-lactamase (ESBL)
- Metallo-beta-lactamase
PKPD target beta-lactams is fT > MIC
22
Q
Penicillins
A
- Naturally purified from mould (naturally…) – activity against many organisms but many now produce beta-lactamases
- Anti-staph – R1 modified to treat staphylococci (penicillinase)
- Amino – added amino group increases hydrophilicity allowing increased porin transfer (more Gram-negative activity)
- Extended spectrum penicillins – bigger side chains prevent more beta-lactamases’ activity and allows greater porin transfer
- Beta-lactamase inhibitors are generally added to Amino/ES penicillins to confer greater coverage
23
Q
Cephalosporins
A
- The added ring makes the compound more resistant to degradation by enzymes
- “Generations” relate to staged development of cephalosporins with different activity & chemistry
- 1st & 2nd generation relatively ‘narrow’ spectrum
- 3rd R1 side chain allows increased penetration for Gram-negatives, had increased affinity for PBPs & increased resistance to beta-lactamases
- 4th R1 & R2 side chains increase Gram-negative spectrum greatly
- 5th MRSA activity due to R2 side chain binding not beta-lactam ring
24
Q
Carbapenems
A
- Most broad-spectrum antibiotic class
- Modified beta-lactam ring & significant side chain changes
- Relatively small molecules with good porin access
- Resistant to most beta-lactamases
- Affinity for many if not all PBPs
- Imipenem co-formulated with cilastatin (not an antibiotic) as it inhibits the destruction by dehydropeptidase I in the renal brush border
- Meropenem chemically modified R1 so intrinsically resistant to DHP I – pharmacokinetics similar to imipenem
- Ertapenem modified R2 group prolongs the half-life but confers weak activity against Pseudomonas
25
Q
Monobactams
A
- Aztreonam – totally synthetic – drug shortages
- Chemically designed to bind well to (has high affinity for) Gram-negative PBP3
- Novel structure with dissimilar side chains shows no allergic cross-reactivity with other beta-lactams
+ Side chains hypothesis formed after this
26
Q
Manipulating beta-lactam PK/PD
A
Probenecid inhibits tubular secretion of weak organic acids including penicillins & some cephalosporins:
- Prolongs serum half-life, increasing serum concentration
- t1/2 from 1.6 hours to 2.7 hours, increases T > MIC
27
Q
Glycopeptides (Vancomycin & telicoplanin)
A
- Same peptidoglycan pathway as beta-lactams but very different mechanisms – overcomes resistance in PBP
- Binds to D-ala-D-ala dipeptide of the peptidoglycan side chain which prevents transpeptidase accessing due to the bulk of the drug
- Resistance can easily occur (e.g. in staphylococci & enterococci)
+ D-ala-D-ala dipeptide enzymatically changed to D-ala-D-lac to reduced binding affinity - Large molecules with poor penetration – no Gram-negative activity
- Very broad Gram-positive cover as no PBP involved
- Vancomycin originally very toxic due to poor purification process now increased technique & reduced ADR profile
28
Q
Glycopeptide - Key ADRs
A
- Renal toxicity
- Infusion related “Red Man Syndrome”
+ Histamine release from mast cells
+ Speed of infusion related, reduced rates 10 mg/min to reduce likelihood
29
Q
Vancomycin PK/PD
A
PK/PD target AUC:MIC 400:
- Validated in MRSA pneumonia cohort (n = 103) with improved outcomes & bacterial resistance
- Trough concentration correlated to AUC:
+ 15 mg/L = ~400
- Loading doses used in practice to achieve therapeutic levels earlier (important in sepsis)
30
Q
Vancomycin - Therapeutic monitoring
A
- Outcomes are related to concentrations we need to ensure adequate exposure
- Serum levels (HPLC) measured at Css
+ t1/2 6 hours = 30 hours - Vancomycin clearance linearly related to CrCl – dose adjustment simple if clearance stable
- Monitoring for efficacy so administration should continue whilst awaiting results
31
Q
Daptomycin
A
- Novel cyclic polypeptide
- Inserts lipid ‘tail’ into cytoplasmic membranes forming an ion conducting channel
+ 4 molecules aggregate to form
+ Potassium leaks out, depolarising membrane potential, leading to cell death - Gram-positive activity only as poor penetration
- Concentration dependent - AUC:MIC 400 for stasis & 1000 for killing
Key ADRs:
- Myositis so monitor Creatinine Kinase
- Deactivated by pulmonary surfactant, so no use in pneumonia
32
Q
Polymyxins
A
- Positively charged colistin binds to negatively charged lipopolysaccharides (LPS) in outer membranes (Gram-negative) displacing Ca2+ & Mg2+ stabilising ions
- Fatty acid side chains inserts into outer membrane disrupting the LPS molecules increasing permeability
- PK/PD fAUC:MIC 20-49
- Initial drug dosing licensing found to less effective
+ Ongoing work to identify best approach using loading doses, maintenance dosing based on CrCl - Nephrotoxicity & neurotoxicity required close monitoring of patients
33
Q
Fosfomycin
A
- Binds & covalently modifies cysteine residues inactivating bacterial cell wall enzyme MurA (MurA catalyses 1st step of peptidoglycan synthesis)
- Efficacious for Gram positive & negative bacteria
- Requires cAMP for active transport internally so no anaerobic activity
- Useful for ESBL organisms as cell wall active but not beta-lactam related so unaffected by enzymes
- Different AUC:MIC required but still concentration dependent - intracellular enzyme not in cell wall budding
2 different salts/formulations:
- Trometal has good absorption & predominately urinary excretion (60%) so used as an oral mega-dose for UTI
- Calcium/sodium used with IV dosing, lower urinary excretion (10%)
34
Q
Metabolic pathway inhibition [Sulpha drugs (sulfamethoxazole & dapsone)]
A
- Microorganisms (except enterococci) do not absorb folate like humans & are reliant on synthesis from para-aminobenzoate (PABA)
- Sulphanomides structurally similar to PABA & inhibit DHFR stopping
- Trimethoprim inhibits DHFR stoppung conversion to active form required for DNA synthesis
- No good PK/PD data on effect (1950s/1960s)
- Has effect on creatinine (renal function marker)
+ Sulfa direct renal toxicity
+ Trimethoprim inhibits tubular secretion of Cr without an effect on GFR - expect to see an increase of 20% in Cr - Commonly causes dermatological toxicity from rash to SJS
- Less commonly causes haematological toxicity & bone marrow suppression
35
Q
Protein production - ribosomal inhibition
A
- Protein is inhibited by various antibiotic classes (mRNA to protein)
Key antibiotic targets on ribosomal subunits (30S + 50S = 70S)
- Prevent assembly of the 70S ribosome
- Block tRNA binding
- Mismatch reading of mRNA & tRNA
- Block exit of produced peptide
36
Q
Tetracyclines (Tetracycline, doxycycline, minocycline & tigecycline)
A
- Binds to the 30S subunit & prevents tRNA binding to the A site
- Tigecycline modified to overcome resistance from structural modification of the subunit, decreasing tetracycline binding
+ Also reduces efflux pump excretion
+ Very broad spectrum - PK/PD fAUC:MIC but limited data on targets
- Tetracyclines highly protein bound & well distributed into tissue
+ Tigecycline is bound so well that increased mortality in sepsis due to no circulating antibiotic
Key ADRs:
- Photosensitivity - c.f. malaria
- Oesophageal ulceration & gastritis
- Binding metals e.g. Ca2+ in mineralisation of children’s bone/teeth - lower concern with doxycycline
37
Q
Aminoglycosides (Streptomycin, neomycin, gentamicin, tobramycin, amikacin)
A
- Amino group bound by glycosidic linkage to central 6 membered ring
- Large molecule but smaller than vancomycin so penetrates Gram-negative well
+ Positive charge facilitates transition through the outer membrane by creating transient holes
+ Requires oxygen dependent transport through cytoplasmic membrane therefore poor anaerobic cover
+ Binds to the 30S subunit causing mismatching of mRNA & tRNA leading to protein mistranslation (junk proteins) - Resistance usually occurs from efflux pumps & degradation enzymes rather than ribosomal conformation
38
Q
Aminoglycoside PK/PD
A
- Cmax:MIC target of 8-10
- AUC:MIC pf 70-120 also associated with efficacy
- Vd similar to ECF so changes in PK vastly when affected e..g burns & sepsis
- Once daily dosing is utilised to achieve high Cmax compared to licensed dosing of q12 or q8 - also utilises PAE (increased rates of toxicity)
- Limited resistance but use is restricted by toxicity
Penetrates human cells poorly except proximal renal tubules (&perilymph) where it concentrates
- High rates (5%) of nephrotoxicity/AKL but usually reversible & often significant after prolonged therapy (≥ 5 days) with high Cmax
- Ototoxicity with high frequency hearing loss & vestibular toxicity (mitochondrial weakness)
39
Q
Aminoglycoside TDM
A
- Cmax (peak) - occurs 30 minutes post infusion after distribution to ECF (usually not measured as recommended daily dose are»_space;> MIC)
- 4 to 6 hours post dose - used to calculate an AUC or draw the ADME curve using software
- Cmin (trough/pre-dose)
+ Offers little to no PK benefit for the patient
+ Used to check for clearance & reduces risk of toxicity
40
Q
Chloramphenicol
A
- Broad spectrum but limited by toxicity - limited PK/PD data
- Binds to 50S subunit in the peptide-transferase cavity:
+ Interacts with nucleotides & prevents transpeptidation reaction (peptide chain formation)
+ Resumes on discontinuation - reversible - May also bind to human mitochondrial ribosomes causing toxicity
- Concentration dependent bone marrow suppression
- Idiosyncratic aplastic anaemia (1 in 10-20,000)
- Gray (baby) syndrome:
+ Seen in 1959 with neonatal mortality of 50%
+ Occurs predominately in neonates due to impaired glucuronidation & reduced renal excretion
+ Circulatory collapse from reduced myocardial function with hypotonia, ashen-grey colour, hypotension, hypoperfusion & acidosis
41
Q
Macrolides (erythromycin, clarithromycin, roxithromycin, azithromycin & clindamycin)
A
- Similar overlapping binding sites on 50S ribosome at peptidyl-transferase centre
- Macrolides bind near the exit tunnel for the proteins but also interferes extension of the peptide chain like chloramphenicol
- Clindamycin binds at A & P site preventing the path of the growing peptide chain (physical block)
- Resistance is often enzyme mediated alteration of binding site so when macrolide resistance occurs it often confers clindamycin resistance or can easily develop
- Clindamycin has some limited evidence of benefit in toxic shock syndromes (streptococci or staphylococci) by reducing protein production especially enterotoxins as an adjunct to beta-lactam therapy
- Poor Gram-negative penetration so little activity
+ Slight differences in activity between agents - PK/PD target is fAUC:MIC 25-30 with PAE seen
- Azithromycin is derived from erythromycin for better PK
+ Prolonged serum t1/2 & tissue t1/2 from high host cell tissue concentration - Shorter treatment durations & daily dosing
+ Pertussis 5 days vs 14 days - Prolonger sub-therapeutic concentrations for up to 30 days leading to AMR
- Macrolides, especially azithromycin, being used for immuno-modulatory effects
- No effect on Pseudomonas but may have indirect effects via immune system e.g. reduced ILs from macrophages & evidence shows reduced CF exacerbations & increased survival in panbronchiolitis
42
Q
Linezolid
A
- Completely synthetic compound
- Binds to 50S subunit preventing assembly of the 70S ribosome
- Unique mechanism so low resistance
- Good activity against Gram-positive organisms only
+ MRSA, VRE, MDRTB - Limited course length due to toxicity after weeks of therapy
+ Neuropathy & bone marrow suppression
+ MAO inhibitor - Could also be used as a toxin suppressor
43
Q
Fluoroquinolones (Ciprofloxacin, norfloxacin, levofloxacin, moxifloxacin)
A
- Chloroquine (antimalarial) modified to target more Gram-negative as quinolones - Fluorine added to enhance therapy
- Inhibits 2 topoisomerases that regulate DNA supercoiling - DNA gyrase + Topoisomerase IV
- Topoisomerase work in uncoiling DNA, replication, transcription, recombination & repair of DNA
- FQs bind near the active enzyme site and create a highly stable complex with DNA at the point of strand breakage but before relegation, leading to double stranded DNA breakage & cell death
Different affinity for each enzyme leads to slight change in activity, but penetration & degradation are also factors:
- Ciprofloxacin – Gram-negative predominance
- Moxifloxacin – Gram-positive predominance
PK/PD fAUC:MIC 30 – 40 (+) or 80 – 100 (-), PAE seen:
- Dosing differences needed to achieve these
- Highly bioavailable, high tissue penetration (inc Vd) including sanctuary sites
Key ADRs (Infrequent due to dissimilar topoisomerases in humans)
- <1% tendinopathies e.g. Achille’s tendon rupture
- Irreversible peripheral neuropathy
- Mental health conditions including delirium, confusion & agitation
44
Q
Rifampicin (also rifambutin, ridfapentine, rifaximin)
A
- Inhibits RNA polymerase (an enzyme that produces mRNA or tRNA from DNA) by binding within the DNA/RNA channel & sterically blocks the RNA transcript at 2 – 3 nucleotides
- Resistance can easily occur with single mutations in binding site - Always used in combination
- Gram-positive & mycobacterial action, poor penetration through outer Gram-negative membrane
- PK/PD AUC:MIC for killing but Cmax:MIC for reducing resistance development
- t1/2 = 3 h but significantly PAE seen especially against mycobacteria
- Zwitterionic at physiological pH (both +ve & -ve charges) gives good distribution & tissue penetration
- Hepatically metabolised via CYP isoenzymes & large number of strong interactions as enzyme inducer - Can also cause hepatotoxicity
ADRs: Secretion staining – benign but scary ADR is unexpected:
- Orange/red staining of all bodily fluids including urine, sweat & tears
- Caution patients on clothing & contact lenses
45
Q
Nitrofurantoin
A
Unknown precise mechanism but postulated several mechanisms (explains low resistance):
- Inhibit bacterial enzymes
- Ribosomal blockage
- Direct DNA damage
- Active against most uropathogens (UTIs)
PKPD – none to speak of!
- Rapid urinary excretion & tissue degradation (t1/2 = 20 minutes) so even with IV only urinary concentrations are measurable
- 2 formulations – standard given QDS & macro-crystals which slow dissolve & are then absorbed slower given BD
- ADRs are rarely seen due to low tissue concentrations but can be seen with chronic use & renal impairment
+ Pulmonary fibrosis & hepatoxicity
46
Q
Metronidazole (also tinidazole & ornidazole)
A
- Unknown precise mechanism but small molecules diffuses into cells & is activated by reduction of the nitro group, leads to anionic free radical production causing DNA damage & cell death
- Only occurs in anaerobic bacteria as oxygen re-oxidises the free radical back to the parent compound producing oxygen radicals instead which are easily scavenged
- PKPD AUC:MIC (70) & Cmax:MIC both show best efficacy parameters
- Well absorbed (>90%) & distributed widely with limited restriction
- Intraluminal metronidazole (& active metabolite) levels of 0 – 15% with reducing levels as diarrhoea resolves
+ Low entero-hepatic recirculation but presumably direct secretion through inflamed mucosa
Key ADRs:
- Neuropathies seem with prolonged (e.g. 4 – 6 weeks) treatment & can be slow to resolve
- Often causes a metallic taste (systemic)
- ‘Disulfuram-like’ reaction with alcohol
+ Inhibition of aldehyde dehydrogenase increases levels of acetyl aldehyde leading to nausea, vomiting, flushing, palpitations
+ So no alcohol while on it – including washout period
