Cell Wall Inhibitors -29

1. Cell Wall Inhibitors-29

3. Introduction: Some antimicrobial drugs selectively interfere with synthesis of the bacterial cell wall—a structure that mammalian cells do not possess. The cell wall is composed of a polymer called peptidoglycan that consists of glycan units joined to each other by peptide cross-links. To be maximally effective, inhibitors of cell wall synthesis require actively proliferating microorganisms. II. Penicillins The basic structure of penicillins consists of a core four-membered β-lactam ring, which is attached to a thiazolidinering and an R side chain. Members of this family differ from one another in the R substituent attached to the 6-aminopenicillanic acid residue (Figure 29.2). The nature of this side chain affects the: Antimicrobial spectrum. Stability to stomach acid. Cross-hypersensitivity. Susceptibility to bacterial degradative enzymes (β-lactamases).

5. A. Mechanism of action Penicillins interfere with the last step of bacterial cell wall synthesis, which is the cross-linking of adjacent peptidoglycan strands by a process known as transpeptidation. Since penicillins structurally resemble the terminal portion of the peptidoglycan strand, they compete for and bind to enzymes called penicillin-binding proteins (PBPs), which catalyze transpeptidase and facilitate cross-linking of the cell wall (Figure 29.3). The result is the formation of a weakened cell wall and ultimately cell death. For this reason, penicillins are regarded as bactericidal and work in a time-dependent fashion. B. Antibacterial spectrum The antibacterial spectrum of the various penicillins is determined, in part, by their ability to cross the bacterial peptidoglycan cell wall to reach the PBPs in the periplasmic space. Factors determining PBP susceptibility to these antibiotics include size, charge, and hydrophobicity of the particular β-lactam antibiotic. In general: Gram-positive microorganisms have cell walls that are easily traversed by penicillins, and, therefore, in the absence of resistance, they are susceptible to these drugs. Gram-negative microorganisms have an outer lipopolysaccharide membrane surrounding the cell wall that presents a barrier to the water-soluble penicillins. However, gram-negative bacteria have proteins inserted in the lipopolysaccharide layer that act as water-filled channels (called porins) to permit transmembrane entry.

7. Natural penicillin: Penicillin G and penicillin V are obtained from fermentations of the fungus Penicilliumchrysogenum. 1. PenicillinG(benzylpenicillin) • Has activity against a variety of gram-positive organisms, gram-negative organisms, and spirochetes (Figure 29.4). • The potency of penicillin G is 5-10 X greater than that of penicillin V against both Neisseria spp. and certain anaerobes. • Most streptococci are very sensitive to penicillin G, but penicillin-resistant are: viridansstreptococci and Streptococcus pneumonia. • The vast majority of Staphylococcus aureus (greater than 90%) are now penicillinase producing and therefore resistant to penicillin G. • The drug of choice for the treatment of gas gangrene (Clostridium perfringens) and syphilis (Treponemapallidum). 2. Penicillin V • Only available in oral formulation, more acid stable • Has a spectrum similar to that of penicillin G. • Not used for treatment of severe infections because of its limited oral absorption. • Employed in the treatment of less severe infections.

9. 2. Semisynthetic penicillins • Ampicillin and amoxicillin (also known as aminopenicillins or extended spectrum penicillins). • Created by chemically attaching different R groups to the 6-aminopenicillanic acid nucleus. This will extends the gram-negative antimicrobial activity of aminopenicillins to include Haemophilusinfluenzae, Escherichia coli, and Proteus mirabilis (Figure 29.5A). • Ampicillin (with or without the addition of gentamicin) is the drug of choice for the gram-positive bacillus Listeria monocytogenes and susceptible enterococcalspecies. • Widely used in the treatment of respiratory infections. • Amoxicillin is employed prophylactically by dentists in high-risk patients for the prevention of bacterial endocarditis. • Both are coformulated with β-lactamase inhibitors, such as clavulanic acid or sulbactam, to combat infections caused by β-lactamase–producing organisms. • Methicillin sensitive Staphylococcus aureus (MSSA) is resistant to ampicillin and amoxicillin. • Resistance in the form of plasmid-mediated penicillinases is a major clinical problem, which limits use of aminopenicillins with some gram negative organisms.

10. 3. Antistaphylococcalpenicillins(Methicillin, nafcillin, oxacillin, dicloxacillin): • β-lactamase penicillinase-resistant penicillins. • The penicillinase-resistant penicillins have minimal to no activity against gram-negative infections. • Restricted to the treatment of infections caused by penicillinase-producing staphylococci, including MSSA. • Methicillin-resistant Staphylococcus aureus (MRSA) is currently a source of serious community and nosocomial (hospital-acquired) infections and is resistant to most commercially available β-lactam antibiotics. • Methicillin is not used clinically in the United States (interstitial nephritis). • 4. Antipseudomonal penicillin (Piperacillin): • Active against Pseudomonas aeruginosa (Figure 29.5B). • Piperacillin+ tazobactam extends the antimicrobial spectrum to include penicillinase-producing organisms (Enterobacteriaceae and Bacteroidesspecies). • Figure 29.6 summarizes the stability of the penicillins to acid or the action of penicillinase.

11. C. Resistance • Survival of bacteria in the presence of β-lactam antibiotics occurs due to the following: • 1. β-Lactamase production • β-Lactamases either are constitutive, mostly produced by the bacterial chromosome or, • More commonly, are acquired by the transfer of plasmids. • Some of the β-lactam antibiotics are poor substrates for β-lactamases and resist hydrolysis, thus retaining their activity against β-lactamase–producing organisms. • Certain organisms may have chromosome-associated β-lactamases that are inducible by β-lactam antibiotics (for example, second- and third-generation cephalosporins). • Gram-positive organisms secrete β-lactamases extracellularly, whereas gram-negative bacteria inactivate β-lactam drugs in the periplasmic space. • 2. Decreased permeability to the drug • Decreased penetration of the antibiotic through the outer cell membrane of the bacteria prevents the drug from reaching the target PBPs. • In gram-positive bacteria, the peptidoglycan layer is near the surface of the bacteria and there are few barriers for the drug to reach its target. • Reduced penetration in gram-negative organisms, which have a complex cell wall that includes aqueous channels called porins. An excellent example of a pathogen lacking high permeability porins is Pseudomonas aeruginosa. • The presence of an efflux pump, (for example, Klebsiellapneumoniae). • 3. Altered PBPs • These are bacterial enzymes involved in the synthesis of the cell wall and in the maintenance of morphologic features of the bacterium. • Antibiotic exposure can prevent cell wall synthesis and can lead to morphologic changes or lysis of susceptible bacteria. • The number of PBPs varies with the type of organism. • Modified PBPs have a lower affinity for β-lactam antibiotics, ex. MRSA resistance to most commercially available β-lactams. Figure 29.6 Stability of the penicillins to acid or the action of penicillinase. *Available only as parenteral preparation.

12. D. Pharmacokinetics • 1. Administration • The route of administration of a β-lactam antibiotic is determined by the stability of the drug to gastric acid and by the severity of the infection. • 2. Routes of administration • The combination of ampicillin + sulbactam, piperacillin + tazobactam, and the antistaphylococcalpenicillinsnafcillin, oxacillin must be administered intravenously (IV) or intramuscularly (IM). Penicillin V, amoxicillin, and dicloxacillin are available only as oral preparations. Others are effective by the oral, IV, or IM routes (Figure 29.6). [Note: The combination of amoxicillin with clavulanic acid is only available in an oral formulation in the United States.] • 3. Depot forms • Procaine penicillin G and benzathine penicillin G are administered IM and serve as depot forms. They are slowly absorbed into the circulation and persist at low levels over a long time period. • 4. Absorption • The acidic environment within the intestinal tract is unfavorable for the absorption of penicillins. In the case of penicillin V, only one-third of an oral dose is absorbed under the best of conditions. Food decreases the absorption of the penicillinase-resistant penicillin dicloxacillin because as gastric emptying time increases, the drug is destroyed by stomach acid. Therefore, it should be taken on an empty stomach. Conversely, amoxicillin is stable in acid and is readily absorbed from the gastrointestinal (GI) tract. • 5. Distribution • The β-lactam antibiotics distribute well throughout the body. All the penicillins cross the placental barrier, but none have been shown to have teratogenic effects. However, penetration into bone or cerebrospinal fluid (CSF) is insufficient for therapy unless these sites are inflamed (Figures 29.7 and 29.8). • Inflamed meninges are more permeable to the penicillins, resulting in an increased ratio of the drug in the CSF compared to the serum. • Penicillin levels in the prostate are insufficient to be effective against infections. • 6. Metabolism • Host metabolism of the β-lactam antibiotics is usually insignificant, but some metabolism of penicillin G may occur in patients with impaired renal function. Nafcillin and oxacillin are exceptions to the rule and are primarily metabolized in the liver. • 7. Excretion • The primary route of excretion is through the organic acid (tubular) secretory system of the kidney as well as by glomerular filtration. Patients with impaired renal function must have dosage regimens adjusted. • Because nafcillinand oxacillin are primarily metabolized in the liver, they do not require dose adjustment for renal insufficiency. • Probenecid inhibits the secretion of penicillins by competing for active tubular secretion via the organic acid transporter and, thus, can increase blood levels. • The penicillins are also excreted in breast milk.

13. E. Adverse reactions: • 1. Hypersensitivity • Approximately 10% of patients self-report allergy to penicillin. Reactions range from rashes to angioedema (marked swelling of the lips, tongue, and periorbital area) and anaphylaxis. • Cross-allergic reactions occur among the β-lactam antibiotics. • 2. Diarrhea • Diarrhea is a common problem that is caused by a disruption of the normal balance of intestinal microorganisms. It occurs to a greater extent with those agents that are incompletely absorbed and have an extended antibacterial spectrum. Pseudomembranous colitis from Clostridium difficile and other organisms may occur with penicillin use. • 3. Nephritis • Penicillins, particularly methicillin, have the potential to cause acute interstitial nephritis. (Methicillin is no longer used clinically). • 4. Neurotoxicity • The penicillins are irritating to neuronal tissue, and they can provoke seizures if injected intrathecally or if very high blood levels are reached. Epileptic patients are particularly at risk due to the ability of penicillins to cause GABAergic inhibition. • 5. Hematologic toxicities • Decreased coagulation may be observed with high doses of piperacillin and nafcillin (and, to some extent, with penicillin G). • Cytopenias have been associated with therapy of greater than 2 weeks, and therefore, blood counts should be monitored weekly for such patients.

15. III. Cephalosporins The cephalosporins are β-lactam antibiotics closely related both structurally and functionally to penicillins. Most cephalosporinsare produced semisynthetically by the chemical attachment of side chains to 7-aminocephalosporanic acid. Structural changes on the acyl side chain at the 7-position alter antibacterial activity and variations at the 3-position modify the pharmacokinetic profile. Cephalosporinshave the same mode of action as penicillins, and they are affected by the same resistance mechanisms. However, they tend to be more resistant than the penicillins to certain β-lactamases. A. Antibacterial spectrum Cephalosporinshave been classified as first, second, third, fourth, and advanced generation, based largely on their bacterial susceptibility patterns and resistance to β-lactamases. [cephalosporinsare ineffective against L. monocytogenes, C. difficile, and the enterococci.]

16. 1. First generation • act as penicillin G substitutes. • They are resistant to the staphylococcal penicillinase(that is, they cover MSSA). • Isolates of S. pneumoniae resistant to penicillin are also resistant to first generation cephalosporins. • modest activity against Proteus mirabilis, E. coli, and K. pneumoniae. • Most oral cavity anaerobes like Peptostreptococcus are sensitive, but the Bacteroidesfragilisgroup is resistant. • 2. Second generation • display greater activity against gram-negative organisms, such as H. influenzae, Klebsiella species, Proteus species, Escherichia coli, and Moraxella catarrhalis. • activity against gram-positive organisms is weaker. • Antimicrobial coverage of the cephamycins (cefotetanand cefoxitin) also includes anaerobes (for example, Bacteroidesfragilis). • They are the only cephalosporinscommercially available with appreciable activity against gram-negative anaerobic bacteria. • neither drug is first line because of the increasing prevalence of resistance among B. fragilis.

17. 3. Third generation • These cephalosporins have assumed an important role in the treatment of infectious diseases. • have enhanced activity against gram-negative bacilli, including β-lactamase producing strains of H. influenzae and Neisseria gonorrhoeae. • they are less potent than first-generation cephalosporins against MSSA. • The spectrum of activity of this class includes enteric organisms, such as Serratiamarcescensand Providencia species. • Ceftriaxone and cefotaximehave become agents of choice in the treatment of meningitis. • Ceftazidime has activity against P. aeruginosa (resistance is increasing). • Must be used with caution, as they are associated with significant “collateral damage,” including the induction of antimicrobial resistance and development of Clostridium difficileinfection. [Fluoroquinoloneuse is also associated with collateral damage.]. • 4. Fourth generation • Cefepime: administered parenterally. • has a wide antibacterial spectrum, with activity against streptococci and staphylococci (but only those that are methicillin susceptible). • effective against aerobic gram-negative organisms, such as Enterobacterspecies, E. coli, K. pneumoniae, P. mirabilis, and P. aeruginosa. • When selecting an antibiotic that is active against P. aeruginosa, clinicians should refer to their local antibiograms (laboratory testing for the sensitivity of an isolated bacterial strain to different antibiotics) for direction.

18. 5. Advanced generation • Ceftarolineis a broad-spectrum, advanced-generation cephalosporin. • It is the only β-lactam in the United States with activity against MRSA. The unique structure allows ceftaroline to bind to PBPs found in MRSA and penicillin-resistant Streptococcus pneumoniae. • indicated for the treatment of complicated skin and skin structure infections and community-acquired pneumonia. • In addition to its broad gram-positive activity, it also has similar gram-negative activity to the third-generation cephalosporin ceftriaxone. • Important gaps in coverage (not effective): include P. aeruginosa, extended-spectrum β-lactamase (ESBL)-producing Enterobacteriaceae, and Acinetobacterbaumannii. • The twice-daily dosing regimen also limits use outside of an institutional setting. • B. Resistance • Resistance to the cephalosporins is either due to the hydrolysis of the beta-lactam ring by β-lactamases or reduced affinity for PBPs.

19. C. Pharmacokinetics 1. Administration Many of the cephalosporins must be administered IV or IM because of their poor oral absorption. 2. Distribution • All cephalosporins distribute very well into body fluids. • adequate therapeutic levels in the CSF, regardless of inflammation, are achieved with only a few cephalosporins. For example, ceftriaxone and cefotaxime are effective in the treatment of neonatal and childhood meningitis caused by H. influenzae. • Cefazolin used for surgical prophylaxis due to its activity against penicillinase-producing S. aureus, along with its good tissue and fluid penetration. 3. Elimination • eliminated through tubular secretion and/or glomerular filtration. Therefore, doses must be adjusted in renal dysfunction. • ceftriaxone, which is excreted through the bile into the feces and, therefore, is frequently employed in patients with renal insufficiency.

21. D. Adverse effectsallergic reactions are a concern. Patients who have had an anaphylactic response, Stevens-Johnson syndrome, or toxic epidermal necrolysis to penicillins should not receive cephalosporins. Cephalosporins should be avoided or used with caution in individuals with penicillin allergy. Current data suggest that the cross-reactivity between penicillin and cephalosporins is around 3% to 5% and is determined by the similarity in the side chain, not the β-lactam structure. The highest rate of allergic cross-sensitivity is between penicillin and first-generation cephalosporins.IV. Other β-Lactam AntibioticsA. CarbapenemsCarbapenems are synthetic β-lactam antibiotics that differ in structure from the penicillins in that the sulfur atom of the thiazolidine ring has been externalized and replaced by a carbon atom. Imipenem, meropenem, doripenem and ertapenem are drugs in this group.

22. 1. Antibacterial spectrum • Imipenem resists hydrolysis by most β-lactamases, but not the metallo-βlactamases. • This drug plays a role in empiric therapy because it is active against β-lactamase–producing gram-positive and gram-negative organisms, anaerobes, and P. aeruginosa. • Meropenemand doripenem have antibacterial activity similar to that of imipenem. • Doripenemmay retain activity against resistant isolates of Pseudomonas. • Ertapenemlacks coverage against P. aeruginosa, Enterococcus species, and Acinetobacter species. 2. Pharmacokinetics • Imipenem, meropenem, and doripenem are administered IV. Ertapenem is administered IV once daily. [Doses adjusted with renal insufficiency.] • penetrate well into body tissues and fluids, including the CSF when the meninges are inflamed. Meropenem is known to reach therapeutic levels in bacterial meningitis even without inflammation. • These agents are excreted by glomerular filtration. Imipenem undergoes cleavage by a dehydropeptidase found in the brush border of the proximal renal tubule. Compounding imipenemwith cilastatin protects the parent drug from renal dehydropeptidase and, thus, prolongs its activity in the body. The other carbapenems do not require coadministration of cilastatin.

23. 3. Adverse effects • Imipenem/cilastatin can cause nausea, vomiting, and diarrhea. Eosinophilia and neutropenia are less common than with other β-lactams. • High levels of imipenem may provoke seizures; however, the other carbapenems are less likely to do so. • Carbapenemsand penicillin share a common bicyclic core. Structural similarity may confer cross-reactivity between classes. While those with true penicillin allergy should use carbapenems cautiously, the cross-reactivity rate seen in studies is very low (less than 1%). • B. Monobactams • disrupt bacterial cell wall synthesis, are unique because the β-lactam ring is not fused to another ring. • Aztreonamantimicrobial activity: gram-negative, including the Enterobacteriaceae and P. aeruginosa. • It lacks activity against gram-positive organisms and anaerobes. • Aztreonamis administered either IV or IM and can accumulate in patients with renal failure. • Aztreonamis relatively nontoxic, but it may cause phlebitis, skin rash, and, occasionally, abnormal liver function tests. • little cross-reactivity with antibodies induced by other β-lactams. Thus, this drug may offer a safe alternative for treating patients who are allergic to other penicillins, cephalosporins, or carbapenems.

24. V. β-Lactamase Inhibitors • β-lactam ring: clavulanic acid, sulbactam, and tazobactam, are nearly devoid (lacking) of any antibacterial activity. • hey do not have significant antibacterial activity or cause any significant adverse effects. • Non-β-lactam : Avibactamand vaborbactam. • inactivating β-lactamases, thereby protecting the antibiotics that are normally substrates for these enzymes. • formulated in combination with β-lactamase–sensitive antibiotics, such as amoxicillin, ampicillin, and piperacillin.

25. A. Cephalosporin and β-lactamase inhibitor combinations • Ceftolozane-tazobactam(3rd gen. ceph.) is available only in an IV formulation. • Its niche for use is in the treatment of resistant Enterobacteriaceae and multidrug-resistant Pseudomonas aeruginosa. • β-lactamase–producing bacteria (for example, select strains of ESBLs). • narrow gram-positive and very limited anaerobic activity. • 2. Ceftazidime-avibactam(3rd gen. ceph.)available only in IV formulationis. • broad gram-negative activity including Enterobacteriaceae and P. aeruginosa. • Addition of avibactam allows the drug to resist hydrolysis against broad spectrum β-lactamases (AmpC, ESBL, carbapenemases) with the exception of metallo-β-lactamases. • minimal activity against Acinetobacter as well as anaerobic and gram-positive organisms. • Indications of these combinations : • the treatment of intra-abdominal infections (in combination with metronidazole). • management of complicated urinary tract infections. • reserved for the treatment of infections due to multidrug-resistant pathogens. • B. Carbapenem/β-lactamase inhibitor combination • Meropenem-vaborbactam • It is approved for the treatment of complicated urinary tract infections including pyelonephritis. • Activity against Enterobacteriaceaeproducing a broad spectrum of β-lactamases, except metallo-β-lactamases.

26. VI. Vancomycin • is a tricyclic glycopeptide active against aerobic and anaerobic gram-positive bacteria, including MRSA, methicillin-resistant Staphylococcus epidermidis (MRSE), Enterococcus spp., and Clostridium difficile. • Following cell entry, it binds to peptidoglycan precursors, disrupting polymerization and cross-linking required for maintenance of cell wall integrity. This interaction results in bactericidal activity. • Due to an increase in MRSA, vancomycin is commonly used in patients with skin and soft tissue infections, infective endocarditis, and nosocomial pneumonia. • Frequency of administration is dependent on renal function. Therefore, monitoring of creatinine clearance is required to optimize exposure and minimize toxicity. • Common adverse events include nephrotoxicity, infusion-related reactions (red man syndrome and phlebitis), and ototoxicity. • Emergence of resistance is uncommon within Streptococcus and Staphylococcus spp., but frequently observed in Enterococcus faecium infections. • Resistance is driven by alterations in binding affinity to peptidoglycan precursors. Prudent (cautious) use of vancomycin is warranted. • poor absorption after oral administration, so use of the oral formulation is limited to the management of Clostridium difficile infection in the colon.

28. VII. Lipoglycopeptides • Semisynthetic lipoglycopeptideantibiotics: Telavancin, oritavancinand dalbavancinare bactericidal. • with activity against gram-positive bacteria. The spectrum of activity similar to vancomycin, affecting primarily staphylococci, streptococci, and enterococci. • Because of structural differences, they are more potent than vancomycin and may have activity against vancomycin-resistant isolates. • inhibit bacterial cell wall synthesis. The lipid tail is essential in anchoring the drug to the cell walls to improve target site binding. Additionally, telavancinand oritavancin disrupt membrane potential. In combination, these actions improve activity and minimize resistance. • Telavancinis considered an alternative to vancomycin in treating acute bacterial skin and skin structure infections (ABSSSIs) and hospital-acquired pneumonia caused by resistant gram-positive organisms, including MRSA. • The use of telavancin in clinical practice may be limited by its adverse effect profile, which includes: • nephrotoxicity, risk of fetal harm. • interactions with medications known to prolong the QTc interval (fluoroquinolones, macrolides). • infusion-related reactions may occur. • Oritavancinand dalbavancin have prolonged half-lives (245 and 187 hours, respectively), allowing for single-dose administration for the management of ABSSSI. • Oritavancinand telavancin are known to interfere with phospholipid reagents used in assessing coagulation (heparin).

29. VIII. Daptomycin • Daptomycinis a bactericidal concentration-dependent cyclic lipopeptideantibiotic. • An alternative to other agents, such as vancomycin or linezolid, for treating infections caused by resistant gram-positive, including MRSA and vancomycin-resistant enterococci (VRE). • Daptomycinis indicated for the treatment of complicated skin and skin structure infections and bacteremia caused by S. aureus, including those with right-sided infective endocarditis. • Efficacy of treatment with daptomycin in left-sided endocarditis has not been demonstrated. • Daptomycin is inactivated by pulmonary surfactants; thus, it should never be used in the treatment of pneumonia. • Daptomycinis dosed IV once daily.

31. IX. Fosfomycin • Fosfomycinis a bactericidal synthetic derivative of phosphonic acid. • It blocks cell wall synthesis by inhibiting the enzyme enolpyruvyltransferase, a key step in peptidoglycan synthesis. • Indicated for urinary tract infections caused by E. coli or E. faecalis and is considered first-line therapy for acute cystitis. • No cross-resistance with other antimicrobial agents . • Rapidly absorbed after oral administration and distributes well to the kidneys, bladder, and prostate. The drug is • excreted in its active form in the urine and maintains high concentrations over several days, allowing for a one-time • dose. [Note: A parenteral formulation is available in select countries and has been used for the treatment of systemic • infections.] • The most commonly reported adverse effects include diarrhea, vaginitis, nausea, and headache. • X. Polymyxins • The polymyxinspolymyxinB and colistin (polymyxin E): are cation polypeptides that bind to phospholipids on the bacterial cell membrane of gram-negative bacteria. They have a detergent-like effect that disrupts cell membrane integrity, leading to leakage of cellular components and cell death. • Polymyxinsare concentration-dependent bactericidal agents with activity against most clinically important gram-negative bacteria, including P. aeruginosa, E. coli, K. pneumoniae, Acinetobacter spp., and Enterobacter spp. • However, alterations in the cell membrane, lipid polysaccharides allow many species of Proteus and Serratia to be intrinsically resistant. • PolymyxinB is available in parenteral, ophthalmic, otic, and topical preparations. • Colistinis only available as a prodrug, colistimethate sodium, which is administered IV or inhaled via a nebulizer. • The use of these drugs has been limited due to the increased risk of nephrotoxicity and neurotoxicity (for example, • slurred speech, muscle weakness) when used systemically. • However, with increasing gram-negative resistance, they are now commonly used as salvage therapy for patients with multidrug-resistant infections(Careful dosing and monitoring).