Principles of Antiinfective Therapy

SECTION 2

Antiinfective Therapy Jane E. Sykes and Mark G. Papich

CHAPTER 6

Principles of Antiinfective Therapy Jane E. Sykes and Mark G. Papich

w KEY POINTS • T he use of antimicrobial drugs (AMDs) can lead to selection of resistant bacterial populations, within the targeted bacterial population and also other commensal bacteria. • Bacterial mechanisms of drug resistance include production of enzymes that inactivate AMDs, drug exclusion through membrane alterations and efflux pumps, and modification of the site of action of the AMD. • Strategies that minimize selection for drug resistance include (1) confirmation of the presence of infection before AMD treatment is commenced, (2) proper identification of the infecting

organism, (3) selection of agents with activity specific to the pathogen present, (4) administration of an adequate dose of AMD, and (5) administration of AMD for the proper duration of time. • Selection of an appropriate AMD should be based on the activity of the agent against the suspected pathogen; knowledge of drug pharmacokinetics, which includes bioavailability and tissue penetration; consideration of host factors such as concurrent illness, medications, or immunosuppression; and knowledge of drug pharmacodynamics, that is, whether the drug exhibits concentration- or time-dependent antibacterial effects.

INTRODUCTION

encouraged to develop antimicrobial guidelines based on evaluation of their local prevalence of AMD resistance. Strategies that minimize selection for antimicrobial resistance include documentation of the presence of infection; proper identification of the infecting organism; and the use of agents that are as specific for the pathogen as possible, at the proper dose and for the proper duration of time. These strategies are described in more detail in this chapter.

Antibiotics have been considered one of the greatest inventions of the 20th century. The modern era of antimicrobial drug (AMD) treatment began with the discovery of sulfonamides in 1935.1 In the 1940s, the therapeutic value of penicillin and streptomycin was discovered, and by 1950, the “golden age” of AMD therapy was underway. With the increasing use of AMDs, there has been a transition to an era of widespread antimicrobial resistance among important veterinary and human pathogens. This has been compounded by a declining rate of development of new classes of AMDs. Antimicrobial resistance genes have existed in microbes long before antimicrobial agents were used in therapy. Microbe populations generally consist of a mixture of genetically susceptible and resistant organisms. The use of AMDs exerts pressure that favors selection of the resistant microbes (“survival of the fittest”), within both the targeted bacterial population and other commensal bacteria (which have the potential to become pathogenic when host defenses are impaired). The chance that resistant bacterial populations will emerge depends on the ability of the population to rapidly acquire mutations, the ability of host defenses to eliminate resistant bacteria, and the AMD concentrations at the site of infection. Guidelines for prudent antimicrobial use have been published by several professional veterinary organizations in order to reduce selection for resistant bacterial pathogens.2-6 Individual hospitals are

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Identification of the Infecting Organism The detection of fever or leukocytosis does not imply bacterial infection and the need for antibiotic treatment, especially given the relatively high prevalence of sterile inflammatory and immune-mediated diseases in dogs and viral infections in cats. Selection of the most appropriate AMD relies on knowledge of (1) the presence of an infectious agent, (2) the type of infectious agent present, and (3) the susceptibility of the agent to different AMDs. In some cases, treatment with antibiotics may not be necessary even though the cause of disease is bacterial. Some infections undergo spontaneous cures without the aid of an antibiotic. For example, a cat bite abscess can often be drained and resolved without the need to administer an antibiotic (see Chapter 57). In some situations, collection of a specimen for cytology or histopathology permits a diagnosis of infection to be established.

CHAPTER 6  Principles of Antiinfective Therapy The identity of some infectious agents, such as rickettsial and fungal pathogens, may be presumptively identified to the genus or species level based on their morphology. If skin cytology from a dog reveals cocci (a Gram stain is not necessary to confirm the presence of gram-positive cocci), then the presence of Staphylococcus pseudintermedius is highly probable. If, on the other hand, cytology from an ear swab reveals rods, the possibility of Pseudomonas aeruginosa must be considered. In either case, this knowledge is valuable for selecting initial antibacterial therapy. The use of molecular diagnostic techniques, such as PCR assays, can also allow rapid identification of an infecting organism, which can help to guide antimicrobial drug selection. For example, identification of a vector-borne pathogen (e.g., Rickettsia spp., Ehrlichia canis) is helpful because doxycycline is usually the first drug of choice for the majority of these pathogens. Unfortunately, unless susceptibility to a particular AMD is predictable, these techniques do not typically provide information about AMD susceptibility. Given the emergence of widespread AMD resistance, especially among bacteria and some fungi, collection of specimens for culture and susceptibility before treatment with AMDs is commenced has become more and more important. Although there is an initial cost to the client for culture and susceptibility testing, long-term costs to the client may be reduced considerably because of decreased prescription of unnecessary or inappropriate AMDs, improved likelihood of rapid resolution of the underlying disorder with early diagnosis and appropriate treatment, and reduced need for more expensive, second- or third-line AMDs. Administration of AMDs without identification of the infectious agent can also decrease subsequent chances of successful culture and proper diagnosis if the disorder does not resolve. In animals with life-threatening infections, institution of AMD treatment is necessary before the results of culture and susceptibility become available. The choice of treatment at this stage can be based on cytology and knowledge of the type of infections that most commonly occur at the anatomic site involved (also known as bacteriologic statistics). Because of the life-threatening nature of these infections, the practice of “de-escalation” (as opposed to “escalation”) is employed. This is the practice of initially selecting a highly active agent that has a high probability of successful elimination of the pathogen; then once the infectious agent has been identified, the drug spectrum should be changed or narrowed according to culture and susceptibility results. If the infection is life threatening, highly active agents such as an aminoglycoside or fluoroquinolone with or without a β-lactam can be selected initially. Then, if culture and susceptibility show susceptibility to β-lactams, treatment should be continued with these agents alone.

Classification of Antimicrobial Drugs Antimicrobial drugs may be classified based on their structure (e.g., β-lactams, fluoroquinolones), spectrum of activity (gram-positive versus gram-negative bacteria), mechanism of action (Table 6-1, Figure 6-1), whether they are bacteriostatic (inhibit or slow growth) or bactericidal drugs, and their pharmacodynamic properties (see Pharmacodynamics of Antimicrobial Drugs, later). Bactericidal drugs interfere with cell wall or nucleic acid synthesis, whereas bacteriostatic drugs inhibit protein synthesis or cause changes in bacterial physiology. When a bacteriostatic drug is removed, in the absence of host defense mechanisms, organism growth resumes, and any

47

decline in bacterial numbers that occurs when the drug concentration drops below the minimum inhibitory concentration (MIC) reflects clearance by the host immune response. However, the line that separates bactericidal and bacteriostatic is not as precise as previously thought. Some bacteriostatic drugs become bactericidal when present in high concentrations at the site of infection, or if concentrations are maintained above the MIC for the entire dosing interval. Even macrolides and chloramphenicol, which are traditionally considered as bacteriostatic drugs, can be bactericidal against some organisms. A drug may be bactericidal against gram-positive cocci, but bacteriostatic against gram-negative bacilli. Therefore, from a clinical perspective, it is more useful to describe drugs as time versus concentration-dependent, rather than as bactericidal or bacteriostatic. This allows the clinician to consider the dosage regimen and dosing interval to optimize therapy. There are rare instances where a bactericidal agent is preferred over a bacteriostatic one. It may be preferable to prescribe a bactericidal drug to treat life-threatening infections (such as vegetations in endocarditis or in a systemically immunocompromised host).7 Antibacterial drugs have also been classified for veterinary use as first-line, second-line, or third-line drugs.8 First-line drugs are those that could be used for empirical selection in the absence of or pending the results of culture and susceptibility testing, and include amoxicillin, cephalexin, doxycycline, and trimethoprim-sulfonamides. Second-line drugs are those to be used on the basis of culture and susceptibility testing and because of the lack of any appropriate first-line options. These include ticarcillin, piperacillin, amikacin, and third-generation cephalosporins. Fluoroquinolones were also included in this group because in human medicine, excessive fluoroquinolone use has been associated with emergence of antimicrobial resistance and treatment failures.9 Fluoroquinolones could be considered as first-line drugs for dogs and cats suspected to have serious gram-negative bacterial infections that require treatment pending the results of culture and susceptibility testing. The use of third-line drugs, including vancomycin, linezolid, and carbapenems such as imipenem and meropenem, is usually reserved for situations when certain criteria are met8: 1. Infection must be documented based on clinical abnormalities and culture. 2. The infection is serious and has the potential to be lifethreatening if left untreated. 3. Resistance is documented to all other reasonable first- and second-line options. 4. The infection is potentially treatable. 5. The clinician may seek advice from an infectious disease clinician or a clinical microbiologist to discuss antimicrobial susceptibility test results, and to discuss the use of these agents if there is unfamiliarity with their use. In some instances, there may be other viable options (e.g., topical therapy).

Pharmacodynamics of Antimicrobial Drugs Pharmacokinetic and pharmacodynamic parameters (PK-PD exposure relationships) are important determinants of the efficacy of AMDs and serve as the basis for determination of clinically effective dosage regimens and susceptibility breakpoints and for the development of guidelines for AMD use for specific types of infections.10 The killing of microbes by AMDs may be classified as concentration dependent or time dependent. AMDs that exhibit

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SECTION 2  Antiinfective Therapy

TABLE 6-1 Classification of Antimicrobial Drugs Based on Their Mechanism of Action Mechanism of Action

Antimicrobial Drug

Examples

Inhibition of cell wall synthesis

β-Lactams, glycopeptides

Penicillin, cephalexin, meropenem, vancomycin

Protein synthesis inhibition

Tetracyclines, aminoglycosides, chloramphenicol, lincosamides, macrolides

Doxycycline, gentamicin, erythromycin, ­clindamycin, azithromycin

Inhibition of DNA replication

Fluoroquinolones

Enrofloxacin, marbofloxacin

Inhibition of folic acid metabolism

Trimethoprim-sulfonamides

Trimethoprim-sulfamethoxazole

FIGURE 6-1  Sites of action of antimicrobial drugs within the bacterial cell. concentration-dependent killing are fluoroquinolones and aminoglycosides. These drugs work optimally when the peak concentration in the plasma (Cmax), or the area under the curve (AUC) exceeds the MIC by a defined factor (Figure 6-2). These are typically bactericidal agents and the ability to kill bacteria increases as the AMD concentrations increase. Target concentrations have been defined for laboratory animals and humans and have been extrapolated to veterinary patients. For example, the peak concentration to MIC ratio (Cmax/MIC) should exceed 8 to 10 for optimum dosing of aminoglycosides. The AUC for a 24-hour interval to MIC ratio (AUC24/MIC) should exceed 100 for fluoroquinolones. For some infections, a ratio of 125 is better, and in some patients (e.g., immunocompromised patients with life-threatening infections), a ratio of 250 may be desirable. Drugs that exhibit concentrationdependent killing also have postantibiotic effect (PAE). The PAE is the persistence of antimicrobial effects after drug concentrations at the site of infection fall below the MIC. The exact cause of the PAE varies with the drug, but it may result

from irreversible binding of a drug to the target site. The duration of the PAE is influenced by the duration of antibiotic exposure, the drug concentration, the bacterial species present, and the class of antibiotic. For concentration-dependent antibiotics, administration of the total daily dose as a single dose every 24 hours is preferred to a smaller divided dose, in order to maximize Cmax or AUC. For aminoglycosides, this also reduces drug toxicity.7 β-lactams (penicillins and cephalosporins), macrolides, and lincosamides (clindamycin) exhibit time-dependent killing. As concentrations of these drugs increase, bacterial killing plateaus, and outcome is correlated with the time that the AMD concentration spends above the MIC at the site of infection. This is usually expressed as the percent of time above the MIC during a 24-hour interval (T > MIC). For β-lactam drugs apart from carbapenems, PAEs are minimal. As a result, these drugs work best when administered multiple times a day or as continuous-rate infusions, in order to maintain drug concentrations at the site of infection. For some drugs (e.g., some

CHAPTER 6  Principles of Antiinfective Therapy

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In animals with renal failure, AMD clearance may be greatly diminished. In this case, T > MIC and AUC/MIC are achieved with a reduced dose or less frequent dosing. For β-lactam antibiotics, it is logical to increase the dose interval. For fluoroquinolones, it is better to decrease the dose.

Site of Infection

A

B FIGURE 6-2  A, Antimicrobial drug (AMD) pharmacodynamics. Concentration-­ dependent drugs are most active when the peak concentration (Cmax), or the area-underthe-curve (AUC) in the plasma exceeds the MIC by a defined factor. For time-dependent drugs, outcome is correlated with the time that the AMD concentration at the site of infection spends above the MIC. This is usually expressed as the percent of time above the MIC during a 24-hour interval (T > MIC). B, As AMD concentrations increase, the ability of concentration-dependent drugs such as fluoroquinolones (in this case, ciprofloxacin) to kill bacteria increases. As concentrations of time-dependent drugs such as β-lactam drugs (in this case, ticarcillin) increase, bacterial killing plateaus. MIC, minimum inhibitory concentration.

cephalosporins), a long T > MIC is achieved by virtue of a long half-life. The required time above the MIC for an AMD varies with the pathogen present, the drug, and the site of infection, but in general it should be 40% to 50% of the dosing interval.7,10 Drug dosages that achieve concentrations in excess of an MIC for these periods can be calculated based on pharmacokinetic data. For time-dependent AMD with long half-lives (such as macrolides, clindamycin, and tetracyclines), efficacy is determined primarily by the extent to which the area under the curve exceeds the MIC. As the MIC of an organism increases, it becomes more difficult to achieve these PK-PD targets, and organisms are regarded as resistant.

In general, the concentration of an AMD at the site of infection should at least equal the MIC for the infecting organism, but the exact concentration and the duration of the concentration at the site varies with the class of antimicrobial. For some AMDs, the serum or plasma total (bound and unbound) drug concentrations may not reflect the drug concentration in tissues. Examples are the highly lipophilic macrolide antibiotics (e.g., azithromycin) for which the tissue drug concentration greatly overestimates the plasma drug concentration. On the other hand, tissue concentrations greatly underestimate the plasma or serum drug concentration for poorly lipophilic agents such as aminoglycosides or β-lactams. Antimicrobial selection for different body sites is outlined in Box 6-1. The concentration of an AMD at the site of infection is affected by factors such as lipid solubility, protein binding, and other factors reducing permeability, such as the presence of large amounts of pus, scar tissue, foreign material, devitalized tissue, and bone. Lipid-soluble drugs such as chloramphenicol, rifampin, fluoroquinolones, and trimethoprim are most adept at crossing membranes, including the blood-brain, alveolar, and prostatic barriers. In contrast, poorly lipophilic, polar drugs such as penicillins and aminoglycosides do not cross lipid membranes and will not achieve adequate tissue levels in tissues in which there is a barrier to penetration (e.g., prostate, brain, eye, or intracellular sites). In human patients, aminoglycosides and amphotericin B have been administered intrathecally to treat central nervous system infections.11,12 In order to use β-lactam antibiotics to treat these infections, high doses are required because of a poorly penetrated blood-brain barrier. Even in the face of inflammation, the brain/blood concentration ratio of these drugs is low. Water-soluble AMDs that are excreted in active form by the liver and concentrated in bile, such as doxycycline and ampicillin, make excellent choices for susceptible hepatobiliary infections (see Chapter 88). Urinary tract infections are ideally treated with drugs such as amoxicillin and trimethoprim-sulfonamides, which are highly concentrated in the urine (see Chapter 89).

Other Host Factors Other host factors that should be considered in AMD selection and route of administration include the following: 1. History of adverse drug reactions. These are most commonly gastrointestinal in dogs and cats, but may include hypersensitivity reactions. 2. Age. With the exception of doxycycline, the use of systemic tetracyclines is contraindicated in young animals because of the potential for teeth discoloration (see Figure 8-5). Fluoroquinolones have the potential to cause cartilage and joint toxicity in dogs between the ages of 7 and 28 weeks. The clinical significance of this has been questioned.13,14 3. Species. Dogs and cats differ in their susceptibility to adverse drug reactions. For example, dogs can develop severe cutaneous reactions to 5-flucytosine. Cats are susceptible to acute retinal degeneration following treatment with high doses of

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SECTION 2  Antiinfective Therapy

BOX 6-1 Examples of Appropriate Initial Antimicrobial Drug Choices for Treatment of Bacterial Infections in Different Anatomic Sites* Skin

Cephalexin Amoxicillin-clavulanate Clindamycin

Urinary Tract

Amoxicillin Trimethoprim-sulfamethoxazole

Prostate

7. Pregnancy. Penicillins, cephalosporins, aminoglycosides, and macrolides are generally considered safe during pregnancy. The volume of distribution in pregnancy is greater, and so higher doses may be required to achieve equivalent serum concentrations. 8. Renal and hepatic function. The dose of AMDs that are dependent on renal elimination may require reduction in these animals with impaired renal function in order to minimize toxicity. For example, fluoroquinolones may be more likely to cause seizures or retinal toxicity in animals with renal failure. The use of nephrotoxic drugs such as amphotericin B or aminoglycosides may be relatively contraindicated in animals with renal dysfunction. Adverse effects may be more common when drugs that require hepatic metabolism, such as metronidazole or chloramphenicol, are administered to animals with impaired liver function.

Trimethoprim-sulfamethoxazole

Route of Administration

Respiratory tract

The most common routes of administration for AMDs in dogs and cats are topical, intravenous, intramuscular, subcutaneous, and oral. For localized infections of the skin, eye, or ear canal, topical administration has the advantage of delivering a high concentration of the AMD to the site, which may overcome bacterial resistance mechanisms, and avoid any systemic exposure to the drug. Parenteral administration, especially intravenous administration, provides maximum bioavailability and is recommended for (1) treatment of life-threatening infections, (2) systemic administration of antimicrobials with poor oral bioavailability (such as aminoglycosides), and (3) when gastrointestinal signs or malabsorption preclude effective drug administration via the oral route. Because parenteral AMDs are generally administered in hospital, compliance with this mode of administration is also likely to be greater. Absorption may be delayed after subcutaneous or intramuscular administration when perfusion of these tissues is impaired, and so these routes are not recommended for use when intravenous administration is possible. Peak plasma drug concentrations are achieved quickly after intravenous bolus administration. Therefore, bolus administration is an optimum approach for a concentration-dependent antimicrobial provided rapid injection does not produce toxicity (such as may occur with fluoroquinolones). For a time-dependent drug such as a β-lactam, a slow infusion (even a slow constant-rate infusion) optimizes the time-dependent activity. Provided gastrointestinal function is healthy, administration of many oral AMDs results in acceptable bioavailability, even though many popular antimicrobials have oral absorption that is far less than 100%. Drugs with the highest oral bioavailability in dogs and cats are the fluoroquinolones; β-lactams have low and variable bioavailability. Nevertheless, once the animal can tolerate oral medications and its clinical condition becomes stable, a switch can be made from parenteral to oral antimicrobials. Peak plasma concentrations are always lower with oral administration than those achieved with intravenous administration, but for time-dependent drugs or for drugs that achieve efficacy on the basis of an AUC/MIC exposure relationship, this route is acceptable for a cure. Early switch from intravenous to oral therapy in human patients has gained increased favor in recent years because of reduced lengths of hospitalization, lowered costs, fewer intravascular catheter-related infections, and decreased selective pressure on nosocomial bacterial infections.15,16 The oral bioavailability of some drugs can be maximized if they are administered with food (e.g., clavulanic acid-amoxicillin) or

Doxycycline Fluoroquinolones

Intestinal Tract (e.g., for animals with hepatic encephalopathy) Ampicillin Neomycin

Biliary Tree

Amoxicillin-clavulanate Doxycycline Fluoroquinolones

Brain

Metronidazole Fluoroquinolones Trimethoprim-sulfonamides

Bone

Clindamycin Cephalosporins Amoxicillin-clavulanate *The identity of the pathogen present must also be used to guide antimicrobial drug selection.

enrofloxacin. Cats also are prone to esophageal ulceration from oral doxycycline hyclate or clindamycin unless the medication is administered with a bolus of water or food. 4. Breed. Doberman pinschers may be more susceptible to hypersensitivity reactions following trimethoprim-­sulfonamide administration. 5. Gastric acidity. The absorption of some AMDs, such as ketoconazole and itraconazole, is impaired by medications that suppress gastric acid production. 6. Concurrent medications. Drug interactions, such as concurrent use of drugs that require metabolism using cytochrome P450 enzymes, can affect the choice or dose of AMD used. For example, chloramphenicol and ketoconazole are wellknown P450 enzyme inhibitors, whereas rifampin is an enzyme inducer.

CHAPTER 6  Principles of Antiinfective Therapy

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FIGURE 6-3  Genetic mechanisms of resistance in bacteria. without food (e.g., azithromycin). Some drugs are simply not absorbed orally and must be administered parenterally or by the topical route. These drugs include penicillin G, piperacillin, ticarcillin, aminoglycosides, many cephalosporins (cefazolin, cefotaxime, cefoxitin), meropenem, imipenem, and vancomycin. Some drugs are not absorbed orally unless administered as a prodrug. An example is cefpodoxime proxetil. The proxetil portion of the molecule is cleaved off at the time of intestinal absorption to allow for adequate systemic concentrations.17

Antimicrobial Resistance Infectious agents may be intrinsically resistant to an AMD, or be resistant as a result of genetic variations. Intrinsic or innate resistance refers to the innate ability of an organism to resist the effects of an AMD owing to structural or functional characteristics of that organism. For example, enterococci are intrinsically resistant to cephalosporins, because they lack the penicillinbinding proteins that bind these antibiotics.18 Anaerobic bacteria are intrinsically resistant to aminoglycosides because oxygen is required for entry of the drug to the bacteria. The microbiology laboratory may “suppress” the results of susceptibility tests for antimicrobials to which organisms are intrinsically resistant (i.e., they are not listed in the susceptibility report to the clinician). Resistance can also be defined via pharmacokineticpharmacodynamic targets. For example, an organism may not express complete resistance to an antimicrobial, but the organism is effectively resistant when standard dosage regimens cannot reach a particular peak concentration or AUC. Genetic resistance to an AMD may occur as a result of point mutations, DNA rearrangements, or acquisition of foreign DNA. Point mutations can result in alteration of the target site of an AMD, such as the susceptibility of bacterial DNA gyrase to fluoroquinolones.19 DNA rearrangements may include inversions, deletions, duplications, insertions, or transpositions (including chromosome to plasmid) of large segments of DNA.

The single most important mechanism of antibacterial resistance is acquisition of foreign DNA by horizontal transfer, especially that carried by plasmids and transposons.20 Plasmids are circular, double-stranded DNA molecules that are capable of independent replication and can be transferred from one bacterial strain or species to another through the process of bacterial conjugation. Transposons are small, mobile segments of DNA that are flanked by inverted repeats and encode one or more resistance genes (Figure 6-3). They depend on the chromosome or plasmids for replication. Plasmid-associated genes that confer AMD resistance are frequently on transposons and can move from the plasmid to the bacterial chromosome. A single plasmid may contain resistance genes for more than five different AMDs.

Mechanisms of Antimicrobial Resistance Antimicrobial Inactivation

Bacterial enzymes capable of inactivation of AMDs include β-lactamase enzymes, enzymes that modify aminoglycoside structure, chloramphenicol acetyltransferase, and erythromycin esterase. β-Lactamase production is the most important mechanism of resistance to β-lactam antibiotics among gram-negative bacteria.20 A vast array of β-lactamase enzymes have been discovered, which have been extensively classified into groups.21,22 Extended-spectrum β-lactamases (ESBLs) can hydrolyze not only penicillins and first-generation cephalosporins, but also extended-spectrum cephalosporins such as cefotaxime and ceftazidime (which contain an oximino group). To date, ESBLs have only been described in gram-negative bacilli and are most commonly produced by Escherichia coli and Klebsiella pneumoniae. ESBLs are often inhibited by β-lactamase inhibitors such as clavulanic acid. The ESBL with the most importance at the present time for small animals are the CTX-M β-lactamases, which hydrolyze cefotaxime and other third-generation cephalosporins. Other types of β-lactamase enzymes include AmpC β-lactamases, which are encoded by chromosomal genes, and metallo-β-lactamases.20,23 These types of β-lactamase enzymes

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SECTION 2  Antiinfective Therapy

FIGURE 6-4  Synergistic, antagonistic, and additive effects of antimicrobial drug combinations. Drugs used are referred to using letters of the alphabet. Control = no drug.

are resistant to clavulanic acid. Metallo-β-lactamases can inactivate carbapenems such as meropenem and imipenem.

Membrane Alteration

Protein channels called porins facilitate the passage of hydrophilic AMDs into gram-negative bacteria. Acquisition of porin mutations can result in conformational changes of or reduced expression of porins. The result is exclusion of AMDs such as β-lactams and fluoroquinolones.24

Efflux Pump Induction

Expression of efflux pumps by bacteria allows them to exclude certain AMDs. This is a major mechanism of resistance to tetracyclines in gram-negative bacteria. A huge variety of efflux pumps have been described.25 Efflux pumps may not be specific for one particular class of antimicrobial. Therefore, these have been termed “multidrug” efflux pumps and mediate multidrug resistance (MDR). The acquisition of MDR mechanisms greatly reduces the effective options for therapy. Multidrug-resistant organisms such as Pseudomonas aeruginosa and Acinetobacter baumannii can utilize multidrug efflux pumps in concert with a variety of other resistance mechanisms, including β-lactamase enzyme production, drug target alterations, and AMD modification enzymes.

Target Modification

Methylation of ribosomal targets by bacterial methyltransferases can lead to resistance to tetracyclines, lincosamides, aminoglycosides, and especially macrolide antibiotics. Expression of altered penicillin binding proteins (PBPs) is an important mechanism of resistance to β-lactam antimicrobials. Expression of PBP2a by staphylococci, which is encoded by the mecA gene, confers resistance to all β-lactam antimicrobials, including β-lactamase resistant penicillins such as methicillin and oxacillin (methicillin-resistant staphylococci). Mutations of DNA gyrase can confer resistance to fluoroquinolones.20 Resistance to trimethoprim sulfonamides can result from altered dihydropteroate synthetase and dihydrofolate reductase enzymes.26

Novel Approaches to Antimicrobial Drug Resistance With rapid spread of multidrug-resistant bacterial pathogens, alternatives to antibiotics have been investigated. One approach is bacteriophage therapy. Bacteriophages are viruses that infect

bacteria, and because of their specificity for certain bacterial species, they can deliver targets to one type of organism without harming others.27,28 Bacteriophage treatment may also be more successful than AMD treatment in the presence of biofilms, which impair antibiotic penetration. Other approaches that are under investigation include use of bacterial cell wall hydrolases, cationic antimicrobial peptides, and antisense antibiotics. Cationic antimicrobial peptides are derived from eukaryotic cells, have immunomodulatory properties, and rapidly kill a variety of bacteria.29 Although it is difficult for bacteria to develop resistance to these peptides, multiple resistance mechanisms have been described. Antisense antibiotics are oligonucleotides that bind to the DNA of pathogenic microorganisms and inhibit gene expression.30

Antimicrobial Drug Combinations When an infectious agent is suspected but has not yet been identified, the use of AMD combinations is tempting because of the added broad spectrum of coverage that they provide. However, use of AMD combinations does not necessarily reduce the chance of selection for a resistant organism, adds expense, and may predispose to drug toxicity. Theoretically, drug combinations may have additive, synergistic, or antagonistic effects ­(Figure 6-4). These properties are not as well established in clinical situations. For example, antagonism between a bacteriostatic antibiotic (such as tetracycline) and a bactericidal antibiotic that requires bacterial growth for efficacy (such as a β-lactam) is also theoretically possible, but there is only one clinical example of this occurrence, published in the 1950s.31 Such antagonism has not been replicated since that time. Several reasons have been suggested for the use of AMD combinations: 1. Some drugs must be administered in combination with other drugs because of rapid development of resistance to them when they are used as a sole agent. Examples are 5-flucytosine and rifampin. 2. If a polymicrobial infection is present, one drug may not have a sufficient spectrum of activity. 3. If the nature of an infection is not known and infection is life-threatening, it is reasonable to commence treatment with a broad-spectrum combination (such as a fluoroquinolone and a β-lactam), based on knowledge of the likely pathogen present and local prevalence of AMD resistance, pending the results of culture and susceptibility testing. Once the results

CHAPTER 6  Principles of Antiinfective Therapy of culture and susceptibility testing are available, deescalation should be performed. 4. Certain combinations of AMD are synergistic for treatment of specific infections. For example, the combination of a β-lactam and an aminoglycoside is synergistic for treatment of enterococcal endocarditis. The relatively slow bactericidal activity of the β-lactam is greatly enhanced by the addition of the aminoglycoside, such that shorter durations of treatment or extension of the dosing interval becomes possible without affecting outcome.7 5. Some drugs are not effective against anaerobic bacteria (e.g., fluoroquinolones with the exception of pradofloxacin). They should be combined with an agent active against anaerobes (e.g., clindamycin or metronidazole) when a mixed infection with anaerobic and aerobic bacteria is suspected. 6. When a vector-borne pathogen is suspected, doxycycline may be administered until the results of further diagnostic tests confirm or eliminate this suspicion. In these cases, patients are frequently administered doxycycline in combination with another active agent to treat alternative bacterial infections that have not yet been ruled out.

Monitoring the Response to Treatment The response to antimicrobial treatment is best monitored based on clinical assessment, which includes follow-up cytologic examination or cultures. Therapeutic drug monitoring can be used to ensure adequate serum drug concentrations when treating with aminoglycosides, vancomycin, and azole antifungal drugs. It should be remembered that in some cases, infections resolve because of the host immune response alone.

SUGGESTED READINGS Hawkey PM, Jones AM. The changing epidemiology of resistance. J Antimicrob Chemother. 2009;64(suppl 1):i3-i10. Levison ME, Levison JH. Pharmacokinetics and pharmacodynamics of antibacterial agents. Infect Dis Clin North Am. 2009;23(4):791-815. Weese JS. Investigation of antimicrobial use and the impact of antimicrobial use guidelines in a small animal veterinary teaching hospital: 1995-2004. J Am Vet Med Assoc. 2006;228(4):553-558.

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