1. Antimicrobial Pharmacotherapy in Children
Paul C. Walker, Pharm.D.
Manager, Clinical Pharmacy Services
Detroit Medical Center
and
Clinical Assistant Professor
College of Pharmacy and School of Nursing
University of Michigan

2. Classifying Antimicrobial Agents
Inhibition of cell wall synthesis
Altering cell membrane permeability
Reversibly inhibiting protein synthesis
Irreversibly disrupting protein synthesis
Disruption of nucleic acid metabolism
Blocking essential metabolic events

3. Peptidoglycan Synthesis
Peptidoglycan is composed of chains of peptidoglycan monomers (NAG-NAM-tetrapeptide). These monomers join together to form chains and the chains are then joined by cross-links between the tetrapeptides to provide strength.

4. Peptidoglycan Synthesis
New peptidoglycan synthesis occurs at the cell division plane by way of a collection of cell division machinery known as the divisome.
Bacterial enzymes called autolysins, located in the divisome, break both the glycosidic bonds at the point of growth along the existing peptidoglycan, as well as the peptide cross-bridges that link the rows of sugars together.
Transglycosidase enzymes then insert and link new peptidoglycan monomers into the breaks in the peptidoglycan.
Finally, transpeptidase enzymes reform the peptide cross-links between the rows and layers of peptidoglycan to make the wall strong

5. Structure of Bacterial Cell Walls
Peptidoglycan Cross-links
Comparison of the structure and composition of gram positive and gram negative bacterial cell walls

6. Inhibitors of Cell Wall Synthesis
Beta Lactam Antibiotics
Penicillins
Cephalosporins
Carbapenems
Monobactams
Vancomycin
The beta lactam ring of penicillin

7. How Penicillins Inhibit Peptidoglycan Synthesis
During normal bacterial growth, bacterial enzymes called autolysins put breaks in the peptidoglycan in order to allow for insertion of peptidoglycan building blocks (monomers of NAG-NAM-peptide). These monomers are then attached to the growing end of the bacterial cell wall with transglycosidase enzymes. Finally, transpeptidase enzymes join the peptide of one monomer with that of another in order to provide strength to the cell wall. Penicillins and other -lactam antibiotics bind to the transpeptidase enzyme and block the formation of the peptide cross-links. This results in a weak cell wall and osmotic lysis of the bacterium.

8. Beta Lactam Antibiotics: The Penicillins
Natural Penicillins
Penicillin G
Penicillin V
Aminopenicillins
Ampicillin
Amoxicillin
Carboxypenicillins
Ticarcillin
Carbenicillin
Penicillinase-Resistant Penicillins
Cloxacillin
Dicloxacillin
Methicillin
Nafcillin
Oxacillin
Ureidopenicillins
Mezlocillin
Piperacillin

9. Beta Lactam Antibiotics: The Cephalosporins
First Generation
Cephalothin
Cefazolin
Cephalexin
Cephapirin
Cefadroxil
Cephradine
Second Generation
Cefaclor
Cefoxitin
Cefuroxime
Cefotetan
Cefpoxodime
Cefprozil
Cefonicid
Cefmetazole
Third Generation
Cefotaxime
Ceftriaxone
Cefoperazone
Cefipime*
Cefmenoxime
Ceftizoxime
Ceftazidime
Cefdinir
Cefixime
Ceftibutin
This is classified as a “fourth” generation agent; it has gram negative activity similar to other third generation agents, but better gram positive coverage.

10. Beta Lactam Antibiotics: The Carbapenems and Monobactams
Imipenem/Cilastatin
Meropenem
Ertapenem
Monobactams
Aztreonam

11. Side Effects and Adverse Reactions
Beta lactam Antibiotics
Hepatic dysfunction
Acute interstitial nephritis
azotemia, hematuria, proteinuria, fever, rash, eosinophilia
Neurotoxicity
Transient blood dyscrasias
Allergic or hypersensitivity reactions
Coagulopathy

12. Vancomycin
Indications: serious gram positive infections where -lactams are inappropriate (MRSA, MRSE, allergy, etc.)
Toxicities and Side Effects
Nephrotoxicity
Ototoxicity
Red Man Syndrome

13. Prokaryotes vs. Eukaryotes: Ribosomes

14. Disrupters of Protein Synthesis
Bind to the ribosomal subunits to impair protein synthesis
Aminoglycosides
Chloramphenicol
Macrolides
Erythromycin
Clarithromycin
Azithromycin
Clindamycin

15. Aminoglycosides bind to the 30s subunit to impair protein synthesis.
The Aminoglycosides
Structure of the antibiotic gentamicin C1a bound to its RNA target. Aminoglycoside antibiotics cause misreading of the genetic code.
Kanamycin
Gentamicin
Tobramycin
Amikacin
Netilmicin
Sisomycin
Blocks initiation of protein synthesis
Blocks translation to cause premature termination
Causes incorporation of incorrect amino acid
Aminoglycosides bind to the 30s subunit to impair protein synthesis.

16. Agents that Bind to the 50S Ribosome
Chloramphenicol
spectrum of activity
S. pneumonia
H. influenza
Neisseria spp.
Salmonella
Bordetella
Enterobacteriaceae
some anaerobes

17. Agents that Bind to the 50S Ribosome
Macrolides
Erythromycin
S. pneumonia, S. pyogenes, Legionella, Chlamydia trachomatis, M. catarrhalis, H. influenza, Mycoplasma pneumonia
Clarithromycin
MAC
Azithromycin
Clindamycin
aerobic gram-positive bacteria
anaerobes, especially B. fragilis
used in combination with aminoglycosides to treat intra-abdominal and gynecologic infections

18. Side Effects and Adverse Reactions
Chloramphenicol
Gray syndrome
Dose-dependent bone marrow suppression
Aplastic anemia, pancytopenia
Macrolides
GI complaints
Rash
Clindamycin
Diarrhea
Pseudomembranous colitis
Rash, urticaria
Hypotension

19. Disrupters of Nucleic Acid Metabolism
Metronidazole
Quinolones:
Ciprofloxacin
Levfloxacin
Moxifloxacin
Norfloxacin
Ofloxacin
Trovafloxacin
Gatifloxacin
Grepafloxacin

20. Disrupters of Nucleic Acid Metabolism
Metronidazole
Participates in redox reactions; it is activated by a reduction of the nitro group to an anion radical. In the case of metronidazole, reduced ferredoxin appears to be the primary electron donor responsible for its reduction The anion radical is highly reactive and will form adjuncts with proteins and DNA leading to a loss of function. In particular, the reactions with DNA result in strand breakage and inhibition of replication and will lead to cell death.

21. Disrupters of Nucleic Acid Metabolism
Quinolones: inhibit DNA-gyrase and topoisomerase II
Ciprofloxacin
Levfloxacin
Moxifloxacin
Norfloxacin
Ofloxacin
Trovafloxacin
Gatifloxacin
Grepafloxacin

22. Side Effects and Adverse Reactions
Metronidazole
dizziness
paresthesias
peripheral neuropathy
disulfiram-like reaction
blood dyscrasias
Quinolones
headache
rash, photosensitivity
GI complaints
arthralgias
confusion
liver dysfunction

23. Inhibition of folate metabolism by sulfonamides and trimethoprim
Antimetabolites
Trimethoprim
Sulfonamides
Sulfamethoxazole
Sulfisoxazole
Inhibition of folate metabolism by sulfonamides and trimethoprim

24. Side Effects and Adverse Reactions
Sulfonamides
Dizziness, headache
Rash
Blood dyscrasias
Crystalluria
Acute nephropathy
Bilirubin displacement

25. Proper Antimicrobial Selection: Factors to Consider
Identity of infecting organism
Susceptibility of infecting organism
Host Factors

26. Major Mechanisms of Antimicrobial Resistance
Bypass (TMP/SMX)
Efflux (macrolides, quinolones)
Decreased permeability (-lactams)
Target site modification (intracellular or extracellular; -lactams, macrolides, quinolones, glycopeptides)
Enzymatic degradation (intracellular or extracellular; -lactams, aminoglycosides) X
Bacteria have developed numerous methods by which they can circumvent the deleterious effects of antimicrobials. The most common of these resistance mechanisms are:
reduced permeability of the bacterial outer membrane / wall to the antimicrobial (eg trimethoprim–sulfamethoxazole)
pumping the antimicrobial out of the cell by an efflux mechanism (eg macrolides and quinolones)
enzymatic degradation of the antimicrobial either inside or outside the organism (eg by -lactamase)
modification of the drug’s target site, so as to prevent binding (eg penicillin, macrolides and quinolones)
metabolic bypass of the cellular step affected by the antibiotic (eg bacteria can produce a new dihydrofolate reductase that is not inhibited by trimethoprim–sulfamethoxazole).
The majority of antibiotics used to treat community-acquired respiratory tract pathogens are affected by multiple mechanisms of resistance.

27. Enzyme Inactivation of Penicillins
1 = Site of action of penicillinase
2 = Site of action of amidase
A = Thiazolidine ring
B = -lactam ring 2 1
Structure of penicillins and interaction with beta lactamase

28. Resistance to Penicillin in N. gonorrhea
Beta lactamase

29. Bacterial Resistance: What Problems are We Seeing?
Gram Negative Organisms
H. Influenza
M. Catarrhalis
Enterobacter
Klebsiella
Citrobacter
Serratia
Gram Positive
Staphylococcus
S. aureus
S. epidermidis
Streptococcus
S. pneumoniae
Vancomycin
Enterococci
E. faecalis
E. faecium
Over the last 20 years, a heterogeneous group of aerobic gram negative pathogens that are resistant to one or more class of conventional antibiotics has emerged. These organisms primarily affect critically ill hospitalized patients, particularly those in tertiary care hospitals. These patients include critically ill neonates in neonatal ICUs. The species of bacteria most frequently implicated are the gram negative rods: Enterobacter, Klebsiella, Citrobacter, Pseudomonas, Acinetobacter, and recently Stenotrophomonas. Resistant E. coli have also been reported.
Resistant gram (+) organisms are also a concern. All of use are familiar with the emergence of resistant staphylococci, and over the last several years, resistant streptococci have been reported with increasing frequency.
One specific area of concern is the emergence of vancomycin-resistant enterococci. These organisms are intrinsically resistant to a number of antibiotics, including aminoglycosides, penicillins, cephalosporins, azetreonam, and clindamycin. Further, resistance may be acquired to other b-lactams, aminoglycosides, chloramphenicol, and erythromycin. Therapy generally consists of combination penicillin/aminoglycoside therapy. In spite of the emerging resistance to all these antibiotics, clinicians were always comfortable relying on the enterococcus being universally susceptible to vancomycin. So even if combination therapy failed, we always had an ace-in-the-hole.
While the incidence of enterococci as a cause of neonatal sepsis is very low, as many of you are aware, there has been an alarming increase in the number of nosocomial isolates of enterococci that are now resistant to vancomycin. From 1989 to 1993, the CDC reported a significant increase in the rate of resistance, from 0.3% to 7.9%. In ICU’s, the rate increased even more, going from 0.4% to 13.6%. many VRE are also resistant to penicillin/aminoglycoside therapy. This obviously complicates our choices of antibiotic therapy and may result in increasing morbidity and mortality from enterococcal sepsis. There is also concern that the enterococci may transfer resistance to staphylococci.

30. Other Important Factors: MICs and MBCs Fail to Tell the Whole Story
Antimicrobial Pharmacodynamics
attempt to characterize the relationship between ANTIMICROBIAL EXPOSURE (concentration, dose, AUC) and ANTIMICROBIAL EFFECT (eg., rate, extent, and duration of antimicrobial activity)
There are several other pharmacodynamic effects. The first is the post-antibiotic effect. This effect is seen with the concentration dependent antibiotics. This effect is defined as delayed regrowth of surviving bacteria following limited exposure to an antimicrobial agent. Unlike the concentration dependent agents, the time dependent agents do not usually have a clinically significant PAE; while most antibiotics can demonstrate PAE, this effect with the time-dependent agents is very short-lived and clinically insignificant.
The causative mechanism underlying the PAE is caused is not clear. It is known, however, that antimicrobials exert many different effects on surviving bacteria which are detectable after the drug has been removed. These include delayed recovery of enzyme and non-enzyme protein functions within the bacterial cell, prolonged changes in bacterial cell morphology, metabolism, growth and regeneration, changes in cell receptors, and susceptibility to phagocytosis.
PAE may be attributed to
recovery of reversible, non-lethal damage to cell structures
limited persistence of drug at binding sites or within the cell
bacterial need to synthesize new enzyme systems before regrowth.

31. Other Important Factors: MICs and MBCs Fail to Tell the Whole Story
Antibiotic Pharmacodynamics
Rate and Extent of Bactericidal Action
Post-antibiotic Effect
Effects of Sub-inhibitory Concentrations
Post-antibiotic Leukocyte Effect
Inoculum Effect
MICs and MBCs fail to tell the whole story. Effectiveness of antibiotic therapy depends on many factors, including delivery of adequate amounts of drug to the site of infection. Generally, the concentration of drug at the site of infection should be equal to or exceed the MIC of the bacteria we are trying to eradicate. However, antibiotic concentrations representing multiples of the MIC are usually believed to be more efficacious. Therefore, ideally, the serum concentration of antibiotic should exceed the MIC 4 to 8 fold. Unfortunately, the concentration of drug at the site of infection may be affected by many factors, and attainment of the ideal concentrations may be very difficult, if not impossible, to achieve.

32. Classification Based on Pharmacodynamic Characteristics
Concentration-Dependent Agents
Bactericidal activity is dependent on concentration above the MIC achieved, increasing with increasing concentration
Time-Dependent Agents
Bactericidal activity is dependent on how long the concentration exceeds the MIC
Bacteriostatic Agents
Abort bacterial growth and allow host defenses to eradicate organisms
The study of pharmacodynamics is concerned with the relationship between concentration of the drug in the body (or tissue), particularly its concentration at the site of action, and the response produced by the drug. With respect to antibiotics, pharmacodynamics related the time course of antibiotic concenrations to the antimicrobial effects at the site of infection and to any toxicologic effects of the drugs. Pharmacodynamics is a tool that helps us understand the effects of antibiotics. A knowledge of these pharmacodynamic effects or characteristics provides a more rational basis for determining how to design a dosing regimen for antibiotics.
Classification of antibiotics can be based on pharmacodynamic characteristics produces 3 groups of drugs
agents that demonstrate concentration dependent bactericidal activity over a wide range of concentrations. The aminoglycosides typify this group
agents that demonstrate time-dependent bactericidal activity that has little relationship to the magnitude of the drug concentration, as long as the concentration exceeds a given level. This group is typified by the b-lactam antibiotics and vancomycin.
agents that exhibit predominantly bacteriostatic effects

33. Concentration-Dependent Killing of
Pseudomonas aeruginosa with Tobramycin
1 / log CFU per mL 1 2 3 4 5 6 7 8
Time (hours)
control
1/4 MIC
1 MIC
4 MIC
16 MIC
64 MIC
Antibiotic conc

34. NON-Concentration-Dependent Killing9
1 / log CFU per mL 1 2 3 4 5 6 7 8
Time (hours)
control
1/4 MIC
1 MIC
4 MIC
16 MIC
64 MIC
Antibiotic conc

35. Pharmacodynamic Properties by Antibiotic Class
CONCENTRATION dependent killing
TIME dependent killing
Aminoglycosides β-lactams
Fluoroquinolones Glycopeptides Azithromycin? Metronidazole
Macrolides (except
Azithromycin)
Chloramphenicol
Rifampin
Tetracyclines
Clindamycin

36. Pharmacodynamic Relationships between Antibiotic Concentration and Antibacterial Effect
Plasma
Conc
MBC
MIC
CIDAL activity
PAE
Bacterial REGROWTH
STATIC activity
Site
Time

37. Pharmacodynamic Relationships between Antibiotic Concentration and Antibacterial Effect
Time
Plasma
Conc
T > MIC
PAE
AUC > MIC
MIC
Cmax
AUC

38.    Eradication / Cure Pharmacokinetics Susceptibility
MIC / MBC
Serum / Tissue Concentrations
 
Pharmacodynamics
Time > MIC
Peak / MIC
AUC > MIC 
Eradication / Cure

39. Antibiotic Pharmacodynamics in Otitis Media: T>MIC
Average percentage of time drug concentration exceeds the minimum inhibitory concentration (%T>MIC) for pediatric dosages of oral ß-lactam agents against penicillin-sensitive (black bars) and penicillin-intermediate (hatched bars) Streptococcus pneumoniae. Rodvold. Pharmacoatherapy. 2001; 21(11s) :319s-330s.

40. Antibiotic Pharmacodynamics: Ciprofloxacin AUC0-24:MIC and Clinical Outcomes
Percentage of bacteriologic (black bars) and clinical (hatched bars) cures as a function of AUC0-24:MIC in 68 patients with gram-negative infections treated with ciprofloxacin. Note that the bacteriologic and clinical outcomes are better with AUC > 125.

41. Clinical Breakpoints
Clinical breakpoints are supposed to indicate at which MIC the chance of eradication or even clinical success of antimicrobial treatment prevails significantly over failure, given the dosing schedule of the drug. The breakpoint thus is not only dependent on the antimicrobial activity of the drugs itself, but also on its pharmacokinetics and pharmacodynamics.

42. Postantibiotic effect
The period of time where there is persistent suppression of bacterial growth following exposure to an antimicrobial agent, despite removal of the antimicrobial agent.

43. Antibiotic Pharmacodynamics
For drugs with concentration dependent bactericidal activity, the rate and extent of bactericidal action increases with increasing drug concentration above the MBC up to a maximum point, which usually occurs at 5 to 10 times the MBC. for these drugs, the kill rate is greater near the peak concentration than the kill rate engendered by concentrations near the end of the serum concentration curve, which corresponds to the end of the dosing interval. Also, the kill rate changes continuously as drug concentrations change.
This graph demonstrates a time-kill study in broth containing various concentrations of an antimicrobial agent that exhibits concentration-dependent bactericidal activity.
At a concentration equal to the MIC, growth of the bacteria is aborted, but the number of colony forming units remains unchanged. At a concentration equal to the MBC, we get logarithmic kill of bacteria, and the CFU count drops from 108 to 106 organisms. At 2 X the MBC, the rate and extent of logarithmic kill is greater; the CFU count drops from 108 to 104 organisms. And at 3 X MBC, an even greater rate and extent of kill is observed, with the CFU count dropping from 108 to 102 organisms over the same 1 hour period.
One other thing to notice is the sustained reduction in bacterial counts after the the antimicrobial agent is washed from the system. This is an effect known as the post-antibiotic effect, and we will discuss this in just a little while.
Antibiotic 1
Antibiotic 2
MIC = minimum inhibitory concentration
MBC = minimum bactericidal concentration
From: Levinson ME. Infect Dis Clin North Amer. 1995;

44. Antibiotic Combinations: Rationale and Indications
Additive Effects
Synergistic Effects
Antagonistic Effects

45. Antibiotic Synergy and Antagonism

46. Antibiotic Combinations: Rationale and Indications
Prevent emergence of resistance
Polymicrobial infections
Empiric therapy
Reduced drug toxicity
Synergism
Indications
1. Prevention of emergence of resistance - substantial documentation has been developed only for TB; less epidemiological data are available for other infections.
2. Polymicrobial infections - certain infections due to multiple organisms may require more than one antibiotic to adequately eradicate the infection. Newer agents, particularly the carbapenems and the combination products containing a b-lactam plus a b-lactamase inhibitor, may provide effective monotherapy because of their broad spectrum of activity.
3. Empiric therapy - when the nature of the presumed infection is unclear; switch to a single agent as appropriate based on culture and sensitivity results
4. Reduced drug toxicity - combination therapy may allow the amount of each potentially toxic agent to be reduced and thus decrease the potential for dose related toxicity. Unfortunately, supportive data are insufficient to establish beyond a doubt that combination therapy permits a dose reduction sufficient to prevent dose-related toxicity.
5. Synergism - use of synergistic combinations to treat infections due to resistant or relatively resistant organisms may provide a therapeutic advantage. Examples include the treatment of enterococcal or pseudomonal infections.

47. Antibiotic Combinations: Disadvantages of Inappropriate Combination Therapy
Antagonism
Increased drug costs
Adverse drug reactions
Disadvantages of Inappropriate Combination Antimicrobial Therapy
1. Antagonism - numerous reports of antagonistic antibiotic interactions have been published; however, the clinical significance of the interactions is questionable
2. Increased drug costs
3. Adverse Drug Reactions - it is reported that at least 5% of all ADRs occur in patients receiving antibiotics