the bacterial cell wall and beta lactam antibiotics Flashcards
differences in gram - and gram + bacteria
1) in gram + bacteria, drugs can penetrate the outer layers of the cell wall effectively and the pacterial membrane is the main barrier in keeping drugs out. In gram - bacteria, the outer membrane excludes drugs, but some drugs can get through the porins
2) in gram +, B-lactamases are excreted through the cell wall to the external environment and therefore have to be produced in larger quantities. In gram -, the B-lactamases are confined to the periplasmic space
3) in gram +, the peptidoglycan layer is much thicker than in gram -
4) gram + have one membrane while gram - have 2 membranes (inner and outer) and therefore have a more complex cell wall that is more lipoidal
5) in gram -, the peptidoglycan contains a meso-diaminopimelic acid residue (DAP), but in gram +, that residue is replaced by an L-lyside residue (the COOH of DAP is replaced by an H) - (COOH = neg; H = pos)
6) in gram -, the PG is cross-linked by a bridge between the DAP residue of one strand and the terminal D-Ala of another. In gram +, the bridge exists only between the L-Lys strand and the terminal D-Ala of the 2nd molecule. Transpeptidase is the molecule responsible for the linking
transpeptidase mechanism
the transpeptidase that cross-links the PG strands catalyzes a reaction and transpeptidase is regenerated
B-lactam AB MOA
inhibition of transpeptidases that “glue” the PG strands together by cross-linking - trick transpeptidase into thinking it is a D-ala-D-ala
1) the B-lactam antibiotics acylate the transpeptidase Ser reside in the enzyme active sire to form a stable product. This inactivated the enzyme, inhibiting PG cross-linking which results in a defective bacterial cell wall. The resulting cell wall is subject to osmotic stress resulting in lysis and death
2) the reactivity of the B-lactam system is d/t a highly strained 4-membered ring - the N-C-C bond is compressed to 90 degrees, this stress is relieved by conversion of the carbonyl to the tetrahedral intermediate since the bond angle becomes 109 degrees
3) there are many PBPs (i.e. transpeptidase) which differ from species to species, this explains the heterogenicity of response of different bacteria to different penicillins
4) the bacterial transpeptidases do not catalyze reactions with host cell proteins because the bacterial substrate contains unnatural D-ala amino acid residues that are not found in the host cell proteins
resistance to b-lactam ABs
1) resistance may result from decreased cellular uptake of the drug, mutation of the PBPs to decrease their affinity for the penicillins, or the presence of an efflux pump that pumps the AB out of the cell. A more common mechanism is induction or elaboration of bacterial B-lactamases, the B-lactamases catalyze the hydrolysis of the B-lactam moiety - B-lactamase acts as transpeptidase and attacks the AB, the lactamase can the regenerate but leave the AB hydrolyzed
2) resistance to B-lactams is increasingly common
3) hydrolysis of the acylated B-lactamase intermediate is fast, so the enzyme can hydrolyze many drug molecules rapidly
4) many B-lactamases are known with different specificites - more than 300 have been identified to date
Allerginicity
1) 6-8% of the US pop is allergic to B-lactam ABs, reactions can range from rash to anaphylactic shock - this limits the clinical use of them
2) the allerginicity results from the drug acting as a hapten - it acetylates host cell proteins, which then raise antibodies and result in an allergic reaction
3) Since the allergic reaction originates from the pharmacophore of the drug, it cannot be overcome through structural manipulation of the β-lactam.
4) Because cross-sensitivity is common, a person who has demonstrated an allergy to one β- lactam is likely to be allergic to another β-lactam antibiotic. If the symptoms are mild
(itching or rash), a cephalosporin or carbapenem can be tried since cross-reactivity is 5-15%. For more serious, immediate hypersensitivity reactions, cephalosporins and carbapenems are avoided, but aztreonam** can be used.
5) Topical flare and wheal tests for β-lactam allergy are recommended if there is doubt about whether or not a patient is allergic to penicillins.
penicillin degradation
1) Penicillin hydrolysis products have no antibiotic activity.
2) Hydrolysis of the β-lactam is irreversible. Once the ring is opened, it will not close.
3) Electronegative substituents on the side chain carbonyl reduce the nucleophilicity of the side chain amide carbonyl oxygen atom. This stabilizes the penicillin against hydrolysis under acidic conditions, since the first step in the hydrolysis reaction is decelerated.
Example: Penicillin V is more stable to hydrolysis in the stomach than Penicillin G because the electronegativity of the ether oxygen decreases the nucleophilicity of the amide carbonyl.
4) Solutions of penicillins for parenteral use should be used promptly. They are best stored in the refrigerator between pH 6.0 and 6.8.
5) Heavy metal ions catalyze penicillin degradation reactions and should therefore be kept away from penicillin solutions.
serum protein binding
1) Penicillins with more lipophilic side chains are more highly protein bound.
2) Protein binding reduces bioavailability by reducing the effective concentration of the free drug.
3) Protein binding in general protects drugs from degradation.
4) The half-lives of penicillins are generally not affected by protein binding, since their dissociation rates from the protein are fast and the renal excretion rates are rate-limiting.
Excretion
1) Penicillins are rapidly excreted by the renal or biliary routes. For those that are excreted by the kidneys, about 10% of renal excretion is by glomerular filtration and 90% is by tubular secretion.
2) The half-lives of penicillins that are excreted by the kidneys are prolonged in cases of kidney disease or failure.
3) Two mechanisms exist for tubular secretion, one for anions and one for cations. The penicillins are anionic and competition with the anion probenecid for the secretion mechanism causes an increase in half life when probenecid is administered along with the penicillin.
General comments on penicillin PKs
1) The dose range is from 3 g to 12 g per day for an average adult.
2) Dosing is established to create concentrations that are 4-5 times the MIC. In general, the useful penicillin concentrations are greater than 2 µg/mL, and peak serum levels are between 8-25 µg/mL.
3) They are distributed to most tissues except the CSF. However, if the meninges are inflamed, parenteral penicillins can enter the CSF and are therefore useful for treatment of meningitis.***
4) The serum half-lives are generally from 0.5 to 2 hours. Most penicillins are excreted through the kidneys except nafcillin, which is cleared by biliary excretion.
Benzylpenicillin (Penicillin G)
1) Antimicrobial spectrum: Gram-(+) cocci plus Neisseria gonorrhoeae and Haemophilus influenzae [both Gram-(–)]. Since Pen G resistant microorganisms are common, therapy should be guided by susceptibility testing.
2) β-Lactamase sensitivity: yes.
3) Administration: orally in large doses (80% hydrolyzed), although the most effective route is parenteral.
4) Toxicity: acute allergic reactions.
5) Precautions: Pen G should be used with caution in individuals with histories of significant allergies and/or asthma.
6) Notes: Pen G is the drug of choice for treatment of more infections than any other antibiotic.
Benzylpenicillin Benzathine and Benzylpenicllin Procaine (Bicillin C-R)
1) Pharmacokinetics: because of low solubility, the drug is released slowly from the intramuscular* injection site. The duration of action is longer and the blood levels are lower than with other parenteral penicillins. IM injection of 1,200,000 units in adults results in peak plasma levels of 2.1-2.6 units per mL within three hours, which falls off to 0.75 units per mL within 12 hours.
2) Administration: Bicillin® C-R should only be administered by deep IM injection**. Inadvertant IV administration can result in cardiac arrest and death. Injection near a nerve can result in permanent neurological damage.
3) Therapeutic Use: (a) Moderately severe to severe infections of the upper-respiratory tract, scarlet fever, and skin and soft-tissue infections due to susceptible streptococci. (b) Moderately severe pneumonia and otitis media due to susceptible pneumococci.
Phenoxymethyl Penicillin (Penicillin V)
1) The main difference between penicillins G and V is that penicillin V is more stable in acid*. The increase in stability in acid is attributed to the electronegative ether oxygen, which decreases the nucleophilicity of the side chain amide carbonyl and therefore decreases its participation in the β-lactam hydrolysis reaction.
2) Penicillin V is more stable in the stomach and produces higher and more sustained blood levels than penicillin G after oral administration.
3) Notes: the antimicrobial spectrum (gram +, N. gonorrhea, H. influenzae), β-lactamase sensitivity (yes), and toxicity (acute allergic reactions are basically the same as penicillin G.
B-lactamase-resistant parenteral penicillins
methicillin, nafcillin, oxacillin
Methicllin
1) β-Lactamase sensitivity: no. This is due to steric hindrance* of nucleophilic attack by the enzyme on the β-lactam carbonyl. The phenyl ring is directly attached to the amide carbonyl, and the two methoxy substituents are attached ortho to the amide. Penicillins with two similarly placed ortho substituents, which hinder nucleophilic attack on the lactam, are generally stable to hydrolysis by β-lactamases. Removal of one of the methoxyl groups, moving one methoxyl to a para position, or inserting a methylene between the amide carbonyl and the aromatic ring abolishes β-lactamase insensitivity.
2) Methicillin is unstable to acid in the stomach so it must be administered by injection (its half life at pH 2 is 5 min). The instability is due to electron donation toward the amide carbonyl oxygen by the ortho-methoxy groups, making the amide carbonyl oxygen more nucleophilic
3) Antimicrobial spectrum: narrow. It was formerly used primarily for treatment of β- lactamase-producing Staphylococcus aureus. It has been discontinued because many bacteria are resistant to it.
4) Methacillin-resistant Staphylococcus aureus (MRSA) is resistant to methacillin because of a mutation in a PBP (transpeptidase). The gene coding for this protein is called methicillin resistance gene (mecA**), and the penicillin binding protein that it codes for is called PBP2A*.
nafcillin
1) β-Lactamase sensitivity: no.
2) It is slightly more stable than methicillin in acid, but is clinically identical to methicillin
