Cell wall inhibitors week 4

Question 1

What is the typical structure of penicillins?

What do penicillinases/β-lactamases do to penicillins?

Answer 1

Question 2

How were the various derivatives of penicillin initially characterized?

What is the prototype/standard penicillin? What are the ways in which it is administered? How do the various ways of adminstration differ?

Which form of penicillin is typically administered orally?

Answer 2

1. The various derivative of penicillin was initially characterized on the basis of their activity and were termed penicillin A, B, C, etc. Benzylpenicillin (G) has become the standard. Because initially the doses were defined in terms of units of biologic activity, penicillin G is prescribed in terms of units. Other penicillins are prescribed in terms of mg. One unit of penicillin G is equivalent to 0.6 mcg.
2. Penicillin G has been formulated into various preparations. The intravenous formulation is potassium penicillin G, which contains 1.7 mEq of potassium per 1 million units. Repository formulations of penicillin for intramuscular use are designed to be slowly absorbed over a prolonged time period. Procaine penicillin G allows the slow release of penicillin G over 12 hours. Benzathine penicillin G allows the slow release of penicillin G over days.
3. First Aid ‘15: Penicillin V is orally administered.

Question 3

What is the mechanism of action of B-lactam agents (penicillin)?

Where is the target of B-lactam agents located? What are the various types (of B-lactam targets)?

How do the outcomes differ when penicillin acts on one type of target vs another?

What are autolysins?

Answer 3

Mechanism of Action of Beta-Lactam Agents:

Bacterial cell walls are required for growth and stability. Stability is provided by the rigid peptidoglycan component of the cell wall. Transpeptidase is the enzyme responsible for the terminal cross-linking of the glycopeptide polymer. The terminal glycine residue of the pentaglycine is linked to the fourth residue of the pentapeptide (D-alanine). Penicillin acylates transpeptidase with cleavage of the beta-lactam ring. Penicillin-Binding Proteins (PBPs) are found in the plasma membrane and are targets for beta-lactams. Transpeptidase is one of the PBPs, but others are involved in structure integrity of the cell wall at cell division. Inhibition of PBP1 causes lysis; inhibition of PBP3 causes production of long filamentous forms. Beta-lactam antibiotics vary in their affinity for PBP. Autolysins are cell wall enzymes responsible for the normal breakdown of the cell wall in processes such as cell division. Interference with peptidoglycan assembly in the presence of autolysis can lead to cell lysis.

Question 4

List 3 mechanisms by which bacteria can gain resistance to penicillin?

Answer 4

1. Altered target site
2. Decreased penetration (permeability of penicillin)
3. Enzymatic inactivation

Question 5

Explain how bacteria gain resistance via altering the target of penicillin.

Give examples of bacteria that have this mechanism of resistance to penicillin.

Answer 5

Altered target site: Certain bacteria may produce PBP which have decreased affinity for beta-lactams. They may be reflected as intrinsic resistance or developed resistance. The production of altered PBP is chromosomally mediated. Examples:

a. Methicillin-resistant staphylococci.
b. Penicillin-resistant pneumococci.
c. Relative resistance of Enterococcus to penicillin.

Question 6

Explain how bacteria gain resistance via decreasing permeability to penicillin.

Give examples of bacteria that have this mechanism of resistance to penicillin.

Answer 6

Decreased penetration: Gram-positive bacteria have their peptidoglycan located near the surface and beta-lactams have easy access to target sites. In gram-negative bacteria, the outer membrane is more complex. The outer membrane is relatively impenetrable. Beta-lactams diffuse through aqueous channels in the outer membrane called porins. The size and number of the porins differ among various bacteria. Porins are formed by proteins in the outer membrane. Beta-lactams differ in the speed with which they can cross the porins. Absence of porins for a particular betalactam may cause intrinsic resistance. Alteration in porin proteins may also cause induced resistance.

Question 7

Explain how bacteria gain resistance via enzymatic inactivation.

Give examples of bacteria that have this mechanism of resistance to penicillin.

Answer 7

Enzymatic inactivation. Bacteria may produce enzymes, beta-lactamases, which hydrolyze the beta-lactam bond and render the antibiotic inactive. Different organisms produce different types and amounts of betalactamases. Beta-lactamases may be substrate specific, such as a penicillinase or cephalosporinase, or they may be broader spectrum.

a. Staphylococcus aureus produces large amounts of an extracellular penicillinase encoded by a plasmid.
b. In gram-negative bacteria, beta-lactamases are found in the periplasmic space and are relatively broad spectrum. Their synthesis can be encoded by plasmids or chromosomes and they may cause intrinsic or acquired resistance.

Question 8

State the following facts about Penicillin G and V:

Mechanism of action

Clinical use

Toxicities

Mechanism of resistance

Answer 8

Penicillin G, V Penicillin G (IV and IM form), penicillin V (oral). Prototype β-lactam antibiotics.

Mechanism: Bind penicillin-binding proteins (transpeptidases). Block transpeptidase cross-linking of peptidoglycan in cell wall. Activate autolytic enzymes.

Clinical Use: Mostly used for gram-positive organisms (S. pneumoniae, S. pyogenes, Actinomyces). Also used for gram-negative cocci (mainly N. meningitidis) and spirochetes (namely T. pallidum). Bactericidal for gram-positive cocci, gram-positive rods, gram-negative cocci, and spirochetes. Penicillinase sensitive.

Toxicity: Hypersensitivity reactions, hemolytic anemia.

Mechanis of resistance: Penicillinase in bacteria (a type of β-lactamase) cleaves β-lactam ring.

Question 9

Name the penicillinase resistant penicillins.

Answer 9

Oxacillin, methicillin, naficillin, dicloxacillin

Question 10

State the following facts about the penicillinase resistant penicillins:

Mechanism of action

Clinical use

Toxicity

Answer 10

Dicloxacillin, nafcillin, oxacillin (penicillinase-resistant penicillins)

Mechanism: Same as penicillin. Narrow spectrum; penicillinase resistant because bulky R group blocks access of β-lactamase to β-lactam ring.

Clinical Use: S. aureus (except MRSA; resistant because of altered penicillin-binding protein target site).
“Use naf (nafcillin) for staph.”

Toxicity: Hypersensitivity reactions, interstitial nephritis

Question 11

Name the penicillinase sensitive penicillins.

State the following facts about the penicillinase sensitive penicillins:

Mechanism of action

Clinical use

Toxicity

Mechanism of resistance

What is given with these penicillins to increase efficacy?

Answer 11

Amoxicillin, ampicillin (aminopenicillins, penicillinase-sensitive penicillins)

Mechanism: Same as penicillin. Wider spectrum; penicillinase sensitive. Also combine with clavulanic acid to protect against destruction by β-lactamase.

AMinoPenicillins are AMPed-up penicillin. AmOxicillin has greater Oral bioavailability than ampicillin.

Clinical Use: Extended-spectrum penicillin—H. influenzae, H. pylori, E. coli, Listeria monocytogenes, Proteus mirabilis, Salmonella, Shigella, enterococci. Coverage: ampicillin/amoxicillin HHELPSS kill enterococci

Toxicity: Hypersensitivity reactions, rash, pseudomembranous colitis

Mechanism of resistance: Penicillinase in bacteria (a type of β-lactamase) cleaves β-lactam ring.

Question 12

State the following facts about Piperacillin, ticarcillin, carbenicillin:

Mechanism of action

Clinical use

Toxicity

What is given with these penicillins to increase efficacy?

Answer 12

Piperacillin, ticarcillin (antipseudomonals)

Mechanism: Same as penicillin. Extended spectrum.

Clinical Use: Pseudomonas spp. (such as P. aeruginosa) and gram-negative rods; susceptible to penicillinase; use with β-lactamase inhibitors

Toxicity: hypersensitivity reactions

Question 13

Following oral administration of penicillins, where do they go? What are areas of poor penetration?

Most penicillins are eliminated via what route? Which penicillin is an exception to his?

What is the relative half life? What does this mean for frequency of dosing?

Answer 13

1. Following oral administration, all penicillins are protein bound. The free drug may readily diffuse into tissues and certain body fluids. Areas of poor penetration include:
   a. CSF
   b. Brain
   c. Ocular fluid
   d. Prostate
   e. Phagocytic cells
2. Most penicillins are not metabolized significantly by the liver. An exception is oxacillin.
3. Penicillins are excreted in the urine. The process involves glomerular filtration and tubular secretion. This results in urine levels, which are markedly higher than serum levels.
4. Serum half-life is relatively short (30-60 minutes) and thus penicillins must be administered frequently (i.e., every 2-4 hours).

Question 14

The major side effects of penicillins are hypersensitivity reactions. What are the types of hypersensitivity reactions that can occur? Which is the most frequent?

What is the mechanism by which penicillins cause hypersensitivity reactsions?

What are other adverse reactions that may occur with penicillins?

Answer 14

Generally, penicillins are considered among the safest antimicrobial agents. The major sideeffects are hypersensitivity reactions. When a specific penicillin is administered to a large group of patients, about 5% will develop some type of hypersensitivity reaction.

1. Types of hypersensitivity reactions:
   a. Skin rashes are the most frequent and can vary from mild to quite severe.
   b. Anaphylaxis is an IgE-mediated reaction, which may be associated with angioedema. This is a serious reaction, which can be fatal. It is estimated that 1 in 100,000 patients who are treated die from anaphylaxis.
   c. Serum sickness is an immune complex-mediated reaction to a penicillin.
   d. Allergic vasculitis.
   e. Fever: may cause one to think there is another source of infection
   f. Eosinophilia.
2. Mechanism: Penicillins and their breakdown products act as haptens.
3. An allergic reaction to penicillin markedly increases the possibility that re-administration of any penicillin derivative will case an untoward reaction. Patients with an IgE-mediated hypersensitivity reaction should not receive any penicillin.
4. Other adverse reactions:
   a. Bone marrow suppression.
   b. Thrombophlebitis.
   c. Seizures -inhibition of GABA receptors.
   d. Hyperkalemia -1.7 mEq of K+ per 1 million units of penicillin G

Question 15

Name the β-lactamase inhibitors and what penicilins they are administered with.

Answer 15

β-lactamase inhibitors are CAST: Cluvalanic Acid, Sulbactam, & Tazobactam

Cluvalanic Acid + Amoxicillin

Sulbactam + Ampicillin

Tazobactam + Piperacillin (ticarcillin, carbenicillin)

Question 16

What was the first source of cephalosporin abx?

Answer 16

Similar to the development of the penicillins, the cephalosporins were discovered fortuitously by Brotzu from the sea near a sewage outlet in Sardinia. The first source was a mold, Cephalosporium acremonium.

Question 17

There are several drugs within the cephalosporin class. What is the basic strucutre and how is it modified?

How are the different types of cephalosporins classified?

Answer 17

Chemistry: The basic structure of 7-aminocephalosporanic acid may be modified at two main positions creating a whole family of cephalosporins. Modifications alter antimicrobial activity and pharmacokinetic properties.

Classification: on basis of spectrum of activity

Question 18

What is the mechanism of cephalosporin antibiotics?

There are 5 generations of cephalosporins. Name abx in each class and the clinical uses of each.

Which is most commonly used at Rush?

Answer 18

1. First generation have good activity against gram-positive organisms including penicillinase-producing staphylococci and gram-negative organisms such as E.coli and K. pneumoniae.
   * First aid: cefazolin, cephalexin. Treat PEcK: Proteus mirabilis, E. coli, Klebsiella pneumoniae. Cefazolin is used prior to surgery to prevent S. aureus wound infections.
2. Second generation have increased activity against gram-negative bacteria, but less than third generation.
   * First Aid: cefoxitin, cefaclor. Treat HEN PEcKS: Haemophilus influenzae, Enterobacter aerogenes, Nesseria spp., Proteus mirabilis, E. coli, Klebsiella pneumoniae, Serratia marcescens
3. Third generation cephalosporins are very resistant to broad spectrum beta lactamases and have activity against most gram-negative bacteria, including P. aeruginosa. One third generation cephalosporin, ceftazidime, is the most frequently used antibiotic at Rush. Third generation cephalosporins have good penetration into the CSF and are drugs of choice for meningitis.
   * First aid: treats serious gram negative infections resistant to other B-lactams. Ceftriaxone: meningitis, gonorrhea. Ceftazidime: Pseudomonas

4th generation: cefepime: gram negative organisms with increased activity agains Pseudomonas and gram positive orangisms

5th generation: ceftaroline: broad gram positive and gram negative organism coverage, including MRSA, does not cover Pseudomonas

Question 19

What is the mechanism of resistance to cephalosporins? What organisms are resistant?

What are the toxicities?

What is the primary site of elmination? What is different about the distribution of 3rd generation cephalosporins?

Answer 19

Mechanism of resistance is a structural change in penicillin binding proteins (transpeptidases). Note that Cephalosporins are less susceptible to penicillinases and are bactericidal. MRSA and enterococci are resistant to cephalosporins.

Toxicities: Hypersensitivity reactions (similar to those discussed for penicillin), bone marrow suppression, autoimmune hemolytic anema, disulfiram-like reaction, vit K deficiency, increase nephrotoxicity of aminoglycosides

Pharmacologic Properties

1. May be metabolized in the liver.
2. All excreted by kidneys.
3. Good CSF penetration by third generation.

Question 20

State the following facts about monobactams:

Drugs in this class

Mechanism of action

Clinical use

Toxicity

Answer 20

Monobactams: Aztreonam

Mechanism: Less susceptible to β-lactamases. Prevents peptidoglycan cross-linking by binding to penicillin binding protein 3 (PBP 3). Synergistic with aminoglycosides. No cross-allergenicity with penicillins. Clinical Use: Gram-negative aerobic rods only—no activity against gram-positives or anaerobes. For penicillin-allergic patients and those with renal insufficiency who cannot tolerate aminoglycosides.

Toxicity: Usually none, occasional GI upset

Question 21

State the following facts about Carbapenems:

Drugs in this class

Mechanism of action

Clinical use

Toxicity

What must be coadministeredwith one of the drugs in this class? Why?

Answer 21

Carbapenems: Imipenem, meropenem, ertapenem, doripenem

Mechanism: Similar to other B-lactams (binds transpeptidases) but has a specific porin in gram negative bacilli. Imipenem is a broad-spectrum, β-lactamase– resistant carbapenem. Always administered with cilastatin (inhibitor of renal dehydropeptidase I) to  inactivation of drug in renal tubules.
With imipenem, “the kill is lastin’ with cilastatin.” Newer carbapenems include ertapenem (limited Pseudomonas coverage) and doripenem.

Clinical Use: Gram-positive cocci, gram-negative rods, and anaerobes. Includes killing of mixed infections caused by nosocomial infections. Wide spectrum, but significant side effects limit use to life-threatening infections or after other drugs have failed. Meropenem has a decreased risk of seizures and is stable to dehydropeptidase I.

Toxicity: GI distress, skin rash, and CNS toxicity (seizures) at high plasma levels. Imipemem more eliptogenic than other B-lactams.

Question 22

State the following facts about Vancomycin:

Type of molecule

Mechanism of action

Clinical use

Toxicities

Mechanism of resistance

Forms of administration, How eliminated

Answer 22

Vancomycin

Chemistry – Glycopeptide (not a beta-lactam)

Mechanism: Inhibits cell wall peptidoglycan formation by binding D-ala D-ala portion of cell wall precursors. Bactericidal. Not susceptible to β-lactamases.

Clinical Use: Gram-positive bugs only—serious, multidrug-resistant organisms, including MRSA, S. epidermidis, sensitive Enteroccocus species, and Clostridium difficile (oral dose for pseudomembranous colitis).

Toxicity: Well tolerated in general—but NOT trouble free. Nephrotoxicity, Ototoxicity, Thrombophlebitis, diffuse flushing—red man syndrome (can largely prevent by pretreatment with antihistamines and slow infusion rate).

Mechanism of resistance: Occurs in bacteria via amino acid modification of D-ala D-ala to D-ala D-lac. “Pay back 2 D-alas (dollars) for vandalizing ( Inhibits cell wall peptidoglycan formation by binding D-ala D-ala portion of cell wall precursors. Bactericidal. Not susceptible to β-lactamases. cliNical Use Gram-positive bugs only—serious, multidrug-resistant organisms, including MRSA, S. epidermidis, sensitive Enteroccocus species, and Clostridium difficile (oral dose for pseudomembranous colitis).

Toxicity: Well tolerated in general—but NOT trouble free. Nephrotoxicity, Ototoxicity, Thrombophlebitis, diffuse flushing—red man syndrome (can largely prevent by pretreatment with antihistamines and slow infusion rate).

Mecnanism of resistance: Occurs in bacteria via amino acid modification of D-ala D-ala to D-ala D-lac. “Pay back 2 D-alas (dollars) for vandalizing (vancomycin).”

Not aborbed orally. Must be adminstered intravenously (except in cases of pseudomembranous colitis to tx C. difficile). Excretion is entirely renal.

Question 23

State the following facts about Daptomycin:

Type of molecule

Mechanism of action

Clinical use

Toxicities

Mechanism of resistance

Frequency of dosing, How eliminated

Answer 23

Daptomycin

Chemical structure: lipopeptide

Mechanism: Lipopeptide that disrupts cell membrane of gram-positive cocci.

Clinical Use: S. aureus skin infections (especially MRSA), bacteremia, endocarditis, VRE.

Not used for pneumonia (avidly binds to and is inactivated by surfactant).

Toxicity: Myopathy, rhabdomyolysis. Dapto causes rhabdo

Mech of resistance: change in cytoplasmic membrane voltage

Disposition

1. Post-antibiotic effect and serum T 1⁄2 of 8 hours allow once daily dosing.
2. Elimiated primarily through renal excretion.

Question 24

How is bacitracin administered? What are the toxicities?

Answer 24

Bacitracin: restricted to topical usage-rarely causes hypersensitivity