antibacterials 2 a Flashcards
how do antibacterials get into bacteria (gram +ve)?
a have active peptide transporters
– dipeptide, tripeptide and OligoPePtide transporters (Opp)
– transport most hydrophilic antibacterials directly into the cytoplasm
– hydrophobic antibacterials can use passive diffusion
how do antibacterials get into gram negative bacteria?
Gram negative bacteria have Outer Membrane Proteins (porins, Omp) that act as
molecular sieves
– e.g. main porins in E. coli: OmpC and OmpF – allow hydrophilic molecules up to about 600 Da (1.1 – 1.2 nm diameter) to pass into periplasmic space
who targets the cell wall?
b-lactams
vancomycin
teicoplanin
isoniazid
what are b-lactam antibiotics?
Broad spectrum bactericidal activity against – both G+ve (except aztreonam) and G-ve bacteria
what are the characteristics of b-lactam antibiotics?
time dependent action
good distribution:
- increased with greater lipophilicity
-remember polarity
must have low PAE
largely renal excretion
why do we usually not use b-lactams for UTIs?
largely renal excreted
– not often used for UTIs to reduce increase in resistance
* treat with other suitable agents, if possible
* can be used for complicated UTIs, if necessary
what is the penicillin SAR?
- Amide and carboxylic acid are involved in binding
- Carboxylic acid binds as the carboxylate ion (pKa ~ 2.5 – 3.0)
- Mechanism of action involves the b-lactam ring
- Activity related to b-lactam ring strain
(subject to stability factors) - Bicyclic system increases b-lactam ring strain
- Not much variation in structure is possible
- Variations are limited to the side chain (R)
what are some areas for developements for b-lactams?
1* to increase chemical stability for oral administration (stomach acid)
* addition of electron withdrawing groups (R)
* prodrugs (-CO2H)
2* to increase resistance to b-lactamases
* addition of bulky groups (R)
3* to increase the range of activity
* addition of ionisable groups (R)
* addition of ureido groups (R)
what happens to b-lactam structure at physiological ph?
All have >99.9% ionised ‘acid’ at physiological pH
what is the MOA of b-lactams?
Act upon synthesis of bacterial cell membrane
b-Lactams interfere with peptidoglycan (components of call wall) formation through their interaction with the transpeptidase enzyme [penicillin binding proteins (PBPs)]
what is the consequence of b-lactams inhibiting the formation of cross-links in cell wall?
– reduces integrity and strength, eventually leads to cell lysis / death
* high osmotic pressure inside bacterial cells
* when cell wall weakened, easily bursts
what is prokaryotic cell wall composed of?
peptidoglycan
– polymer
– sugar units (NAG/NAM)
– cross-linked by peptide uni
what are 3 reasons for potential acid sensitivity?
- (1) Ring strain – acid-catalysed ring opening relieves ring of β-lactam ring
- (2) Highly reactive β-lactam carbonyl group – resonance stabilisation is not possible for β-lactam ring
- (3) Influence of the acyl side chain (neighbouring group participation)
what is a solution to acid sensitivity?
- β-lactam ring is vital for antibacterial activity and only the third factor can be
addressed
– reduce amount of neighbouring group participation
– use electron-withdrawing group in side chain to draw electrons away from the
carbonyl oxygen and reduce its nucleophilicity
– stability improvements can allow oral administration
what are the 4 mechanisms of resistance to B-lactam agents?
- Most resistance to b-lactam antibiotics due to specific enzymes
– b-lactamases
– break open b-lactam ring, which is essential to activity
– b-lactamase resistance to penicillin first seen in 1940, before clinical use started! - Modification of the target protein (PBP)
– most common in G+ve bacteria, e.g. Haemophilus and Neisseria strains
– less common in G-ve bacteria - Limited uptake into the bacterial cell, especially G-ve bacteria
– polar molecules, so poor passive diffusion, use porins
– mutations to porins reduce uptake and confer resistance - Increased efflux pump activity
– up-regulation of efflux pumps causes multi-drug resistance - affects many antibiotics (e.g. fluroquinolones, aminoglycosides)
what are the 4 classes b-lactamases are divided into?
– active-site serine β-lactamases (SBLs; classes A, C and D)
– zinc-dependent or metallo-β-lactamases (MBLs; class B
how do extended spectrum b-lactamases (ESBLs) work?
- hydrolyse even third generation cephalosporins, but not usually carbapenems
how do carbapenemases work?
- confer resistance to carbapenemases, alongside almost all clinical b-lactam
antibiotics
how do metallo b-lactamases work?
- broad range of b-lactam substrates, including carbapenems (but not aztreonam)
what are b-lactamases in gram +ve bacteria?
Beta-lactamases are inducible in G+ve bacteria – produced in response to presence of b-lactam antibiotic
what are b-lactamases in gram -ve bacteria?
constitutive in G-ve bacteria – they produce the enzyme all the time, whether or not beta-lactam antibiotics are present
– but, only if the G-ve bacterium has the b-lactamase gene
– located in periplasmic space
how do b-lactamases work?
Generally, have a similar mechanism of action, cleaving the b-lactam ring leading to inactivation of b-lactam antibiotics
how are SBLS related to PBPs?
– share a similar SER-based amino acid sequence
– Use serine as the nucleophile to hydrolyse β-lactams
how do MBLs work?
MBLs (class B) have a different hydrolytic mechanism
– use a metal-activated water nucleophile to to hydrolyse β-lactams
what are the catalytic important residues of serine b-lactamases in serine?
SER64/70
what additional mechanism do b-lactamases have?
- Additionally, are hydrolysed to recycle the b-lactamase enzyme
– not irreversibly bound to b-lactam
– this continues activity
what are carbapenemases?
- Carbapenems: important class of antibiotics
– stable to most b-lactamases due to the hydroxyl-containing group
– active against most clinically relevant bacteria (especially Gram-ve bacteria)
what is the real threat to carbapenemases?
Real threat from pan-drug resistant G-ve bacteria
what does NDM-1 do?
NDM-1 hydrolyses and inactivates all b-lactam antibiotics, except aztreonam
what should be done for an NDM expressing bacterial infection?
– no b-lactam antibiotic can treat
– few, if any, other agents that can treat
– resistant to (almost) all clinical antibacterials
– currently, colistin active against most NDM
-expressing G-ve bacteria
– colistin resistance is increasing
– examples of otherwise healthy (and young) patients dying from a pan
-drug resistant bacterial infection are increasing (very few in UK so far)
how do b-lactamase resistant penicillins occur?
Steric shields used to block the penicillin derivative from accessing the penicillinase
or β-lactamase active site
* Introduce a bulky group on the side chain
how do steric shields work?
- β-lactam ring interacts with both β-lactamase and transpeptidase target enzyme in the
same way – difficult problem
– If steric shield is too bulky, then also prevents the penicillin derivative from
interacting with transpeptidase target enzyme
– large enough to not react with b-lactamase enzyme
– small enough to allow reaction with target enzyme
what factors affect whether a particular strain is susceptible to a penicillin?
– structure
– ability to cross the cell membrane of Gram-negative bacteria
– susceptibility to β-lactamases
– affinity for the transpeptidase target enzyme – rate at which it is pumped back out of cells b
how do the hydrophobic groups affect spectrum of activity?
– favour activity against G+ve bacteria
– poor activity against G-ve bacteria
– as hydrophobic character increases, little effect on G+ve activity, but drops
against G-ve bacteria
how do hydrophilic groups affect spectrum of activity?
– little (or reduce) effect on G+ve activity
– increase activity against G-ve bacteria
– enhancement of G-ve activity greatest if hydrophilic group (e.g. NH2, OH,
CO2H) is attached to carbon α to the carbonyl group on side chain
give examples of carbapenems/ penems
- Biapenem
- Doripenem
- Ertapenem
- Imipenem
- Meropenem
Penems - Faropenem
- Ritipenem
what is currently the only monobactam in clinical use?
Aztreonam IV use only
strong against gram -ve bacteria
no activity against gram +ve
resistant to many b-lactamase enzymes including NDM
inactivated by extended spectrum b-lactamases and carbapenemases
what are b-lactamase inhibitors traditionally based on?
b-lactam core
what are the properties of sulbactam?
Sulbactam – a penam
– not active vs P. aeruginosa or AmpC cephalosporinases – ampicillin and sulbactam (US)
what are the properties of tazobactam?
- Tazobactam – a penam
– used to inhibit ESBL-expressing bacteria in combination with Piperacillin – piperacillin and tazobactam
– ceftolozane and tazobactam
how do b-lactamase inhibitors also attack?
- Are also attacked by b-lactamase enzyme….
– BUT, form stable covalently bonded complex – enzyme not released to inactivate other b
-lactams
– suicide inhibitors of b-lactamase enz
what is diazabicycloocyanones?
– bicyclic core structure that still reacts at serine of β-lactamases – ceftazidime and avibactam
– imipenem, cilastatin and relebactam
what are boronate-based compounds?
– boron can adopt a tetrahedral geometry
– interacts at serine of β-lactamases as transition state inhibitor mimicking the
transient tetrahedral species formed during hydrolytic reaction
– meropenem and vaborbacta
what is imipenem?
– largely excreted unchanged into kidneys
– metabolised by dehydropeptidase-I in kidneys – metabolite is toxic to kidneys
what is cilastatin?
- Cilastatin is a dehydropeptidase-I inhibitor – protective to kidneys and works as a booster of imipenem (also boosted with
relebactam)
why do we use prodrugs?
Prodrugs used to improve oral stability and uptake
– penicillins particularly acid sensitive, decreased bioavailability
– many zwitterionic / very polar and poor uptake
– ester groups help to increase logP and uptake
