Ch 56 - General Principles of Antimicrobial Therapy PDF

VII Section Chemotherapy of Infectious Diseases Section Editor: Conan MacDougall Chapter 56. General Principles of Antimicrobial Therapy / 1127 Chapter 57. DNA Disruptors: Sulfonamides, Quinolones, and Nitroimidazoles / 1137 Chapter 58. Cell Envelope Disr...

VII Section Chemotherapy of Infectious Diseases Section Editor: Conan MacDougall Chapter 56. General Principles of Antimicrobial Therapy / 1127 Chapter 57. DNA Disruptors: Sulfonamides, Quinolones, and Nitroimidazoles / 1137 Chapter 58. Cell Envelope Disruptors: b-Lactam, Glycopeptide, and Lipopeptide Antibacterials / 1147 Chapter 59. Miscellaneous Antibacterials: Aminoglycosides, Polymyxins, Urinary Antiseptics, Bacteriophages / 1167 Chapter 60. Protein Synthesis Inhibitors / 1179 Chapter 61. Antifungal Agents / 1193 Chapter 62. Antiviral Agents (Nonretroviral) / 1211 Chapter 63. Treatment of Viral Hepatitis (HBV/HCV) / 1227 Chapter 64. Antiretroviral Agents and Treatment of HIV Infection / 1245 Chapter 65. Chemotherapy of Tuberculosis and Nontuberculous Mycobacteria, Including Leprosy / 1267 Chapter 66. Chemotherapy of Malaria / 1289 Chapter 67. Chemotherapy of Protozoal Infections: Amebiasis, Giardiasis, Trichomoniasis, Trypanosomiasis, Leishmaniasis, and Other Protozoal Infections / 1309 Chapter 68. Chemotherapy of Helminth Infections / 1325 https://ebooksmedicine.net/ This page intentionally left blank  56 Chapter General Principles of Antimicrobial Therapy Conan MacDougall ANTIMICROBIAL CHEMOTHERAPY: CLASSES AND ACTIONS TYPES AND GOALS OF ANTIMICROBIAL THERAPY Viruses Parasites BASIS FOR SELECTION OF DOSE AND DOSING SCHEDULE Primary Prophylaxis Preemptive Therapy MECHANISMS OF RESISTANCE TO ANTIMICROBIAL Empiric Therapy Definitive Therapy AGENTS Posttreatment Suppressive Therapy and Secondary Prophylaxis Resistance Due to Reduced Concentration of Drug at Its Target Site Resistance Due to Alteration or Destruction of Antibiotic THE PHARMACOKINETIC BASIS OF ANTIMICROBIAL Resistance Due to Altered Target Structure THERAPY Heteroresistance and Viral Quasi-Species IMPACT OF SUSCEPTIBILITY TESTING ON SUCCESS EVOLUTIONARY BASIS OF RESISTANCE EMERGENCE OF ANTIMICROBIAL AGENTS Development of Resistance via Mutation Selection Resistance by External Acquisition of Genetic Elements Bacteria Fungi Antimicrobial Chemotherapy: Classes Types and Goals of Antimicrobial Therapy and Actions A useful way to organize the types and goals of antimicrobial therapy This chapter reviews the general classes of antimicrobial drugs, their is to consider where antibiotics are initiated with respect to the dis- mechanisms of action and mechanisms of resistance, and principles of ease progression timeline (Figure 56–1); therapy can be classified as drug selection. Chapters 57 through 68 present the pharmacological primary prophylaxis, preemptive, empirical, definitive, or suppressive/ properties and uses of individual classes of antimicrobials. secondary prophylaxis. Microorganisms of medical importance fall into four categories: bacteria, viruses, fungi, and parasites. The broad classification of Primary Prophylaxis antibiotics—a term we will use colloquially to encompass all manner of Prophylaxis involves administering antibiotics to patients who are not yet antimicrobial agents—follows this classification closely, so that we have infected or have not yet developed disease. The goal of primary prophy- antibacterial, antiviral, antifungal, and antiparasitic agents. However, laxis is to prevent a first episode of infection in patients without evidence there are many antibiotics that work against more than one category of infection. Primary prophylaxis can significantly reduce the likelihood of of microbes, especially those that target evolutionarily conserved clinically significant infection but must be balanced against the risks of dis- pathways. Classification of an antibiotic can be performed along sev- ruption of the microbiome, selection for antibiotic-resistant variants, tox- eral dimensions, including the class and spectrum of microorganisms icity, and cost. Thus, primary prophylaxis should be reserved for patients it kills, the biochemical pathway it interferes with, and the chemical at significant risk, using antibiotics of the narrowest appropriate spectrum, structure of its pharmacophore. for the shortest duration appropriate to provide adequate protection. Antimicrobial molecules should be viewed as ligands whose receptors The most common use of antibiotics for primary prophylaxis is the are typically microbial proteins. The term pharmacophore, introduced administration of antibiotics in the perioperative period to prevent sur- by Ehrlich, defines that active chemical moiety of the drug that binds gical site infections. Wound infection results when a critical number to the microbial receptor. The microbial proteins targeted by the antibi- of bacteria are present in the wound at the time of closure, and che- otic are essential components of biochemical reactions in the microbes, moprophylaxis can be used to prevent wound infections after surgical and interference with these physiological pathways inhibits the replica- procedures. Antibiotics directed against the invading microorganisms tion of or directly kills the microorganisms. The biochemical processes may reduce the number of viable bacteria below the critical level and commonly inhibited include cell wall synthesis, cell membrane syn- thus prevent infection. thesis and function, ribosomal translation, nucleic acid metabolism, In some cases, primary prophylaxis may be initiated several days in topoisomerase-mediated chromosomal conformational changes, viral advance of the surgical procedure, as with the use of intranasal mupirocin proteases, viral integrases, viral envelope entry/fusion proteins, folate and topical chlorhexidine gluconate baths to reduce the burden of Staphy- synthesis, and parasitic chemical detoxification processes. Recently, lococcus aureus ahead of cardiac and orthopedic surgeries, among those antisense antibiotics have been developed; these work by inhibiting gene found to be colonized with this organism on preprocedure screening expression in bacteria in a sequence-specific manner. Furthermore, (Schweizer et al., 2015). More commonly, for patients and procedures interferon-based products work by inducing specific antiviral activities that carry significant risk of surgical site infection, antibiotics are of the infected human cells. administered in the perioperative period (Berrios-Torres et al., 2017). https://ebooksmedicine.net/  1128 Abbreviations Primary prophylaxis may also be used in immunosuppressed patients such as those with HIV-AIDS or those status post solid organ transplan- tation on antirejection immunosuppressants. In these groups of patients, ABC: ATP binding cassette specific antiparasitic, antibacterial, antiviral, and antifungal therapy is AUC: area under the Cp-time curve administered based on the typical patterns of pathogens that are major causes of morbidity during immunosuppression. A risk-benefit analysis CHAPTER 56 GENERAL PRINCIPLES OF ANTIMICROBIAL THERAPY CCR5: chemokine receptor type 5 CD4: T-helper cells determines choice and duration of prophylaxis. Prophylaxis of oppor- CMV: cytomegalovirus tunistic infections in patients with HIV-related immunosuppression Cp: plasma concentration is typically started when the CD4 count falls below 200 cells/mm3 and CPmax: peak plasma concentration may be discontinued with sustained increases in the CD4 count above CYP: cytochrome P450 this threshold in response to antiretroviral therapy. In posttransplant DHFR: dihydrofolate reductase patients, use of prophylaxis depends on the type of transplantation, time since the transplant procedure, and type and dose intensity of immu- DHPS: dihydropteroate synthase nosuppressive therapy. Prophylaxis may be discontinued in patients E: effect based on benchmarks either of time since transplantation or reduction EC: effective concentration of immunosuppression. Examples of pathogens against which primary ELF: epithelial lining fluid prophylaxis may be used include Pneumocystis jirovecii, Toxoplasma Emax: maximal effect gondii, Candida species, Aspergillus species, cytomegalovirus (CMV), HIV: human immunodeficiency virus and other Herpesviridae. Doses used for primary prophylaxis are often IC: inhibitory concentration lower than when the same drug is used for acute treatment. MALDI-TOF MS: matrix-assisted laser desorption/ionization An emerging area of primary prophylaxis is termed preexposure pro- time-of-flight mass spectrometry phylaxis (PrEP) and is employed in patients at increased risk of contract- MEC: minimum effective concentration ing HIV infection (Mayer and Allan-Blitz, 2019). PrEP involves taking MIC: minimum inhibitory concentration oral antiretroviral drugs on a regular basis (a once-monthly injectable PAE: postantibiotic effect regimen is also under study) to prevent establishment of HIV infection PCR: polymerase chain reaction upon exposure, typically through sex or injection drug use. PrEP may be PK/PD: pharmacokinetics-pharmacodynamics used among patients who are part of a known HIV-serodiscordant couple PrEP: preexposure prophylaxis or to reduce risk of transmission when the status of sexual partners is rpoB: RNA polymerase not known. PrEP has been shown to significantly reduce the risk of new diagnoses of HIV, although regular monitoring for drug-related adverse effects is necessary. The perioperative antimicrobial dose should be administered intrave- Other examples of primary prophylaxis include antiretroviral postex- nously within 60 min prior to the surgical incision, so that local drug con- posure prophylaxis following needlestick exposures, administration of centrations are above the minimum inhibitory concentration (MIC) of rifampin to contacts of patients with meningococcal meningitis, use of likely pathogens at the time of incision. The frequency of redosing during anti-influenza antivirals in household contacts of influenza cases, and the procedure is based on the half-life of the drug to ensure adequate anti- administration of macrolides to close contacts of cases of pertussis. biotic concentrations above the MIC until closure of the surgical incision. This is especially important for those β-lactam antibiotics that have short Preemptive Therapy half-lives; these should be redosed at intervals of two times the half-life. Preemptive therapy is used as a substitute for primary prophylaxis and as For the majority of procedures, a single perioperative dose suffices to early targeted therapy in high-risk patients in whom a laboratory or other prevent infection, and administration of postoperative doses is associated test indicates infection despite a lack of symptoms. The principle is that with no significant increased benefit and increased risks of adverse effects delivery of therapy prior to development of symptoms aborts impending and Clostridioides difficile superinfection (Branch-Elliman et al., 2019). disease, and such therapy is used for a short and well-defined duration. The systemic antibiotic used is chosen based on the pathogen most likely Preemptive therapy may be particularly useful when there are concerns to contaminate the incision, which in turn depends on the site where for drug toxicity with long-term use as prophylaxis. This strategy’s most surgery is being performed. The most common pathogens infecting prominent application is in prevention of CMV disease after hemato- incision sites after clean surgery are staphylococci, specifically S. aureus poietic stem cell transplants and solid-organ transplantation, where and coagulase-negative staphylococci. In clean-contaminated surgery detection of low-level viremia via PCR is possible and current antiviral over the abdomen and pelvis, the same organisms remain important, but therapies (e.g., valganciclovir) carry significant risks of cumulative toxic- Enterococcus species and gram-negative rods are also common. ity (Razonable et al., 2019). Categories of antimicrobial therapy Primary Suppression/Secondary Prophylaxis Preemptive Empiric Definitive Prophylaxis No infection Infection Symptoms Pathogen Resolution isolation Stages of disease progression Figure 56–1 Categories of antimicrobial therapy in relation to disease progression. Empiric Therapy risks of untoward effects (e.g., toxicities or drug interactions), costs, prac- 1129 ticality (e.g., number of doses administered per day), and the desire to Empiric therapy is administered when an infection is suspected but the spe- minimize the contribution to patient- and population-level antimicro- cific causative organism and its susceptibility in that patient are not known, bial resistance. The latter consideration suggests that narrower-spectrum with the antibiotics used based on the typical pathogens associated with antibiotics are preferred over broader-spectrum agents when the other SECTION VII CHEMOTHERAPY OF INFECTIOUS DISEASES the infectious syndrome(s). Most use of antibiotics in clinical medicine is factors are more or less equivalent. When the initial empiric regimen is empiric use. The rationale for this is 2-fold: broader in spectrum than the definitive regimen, this consideration is Definitive identification and susceptibility of the causative micro- often known as streamlining or de-escalation. organism(s) is typically delayed by at least 24 to 48 h from patient For definitive therapy, combination antibiotic therapy is an exception, presentation (when microbiological results are able to be obtained), and rather than a rule. Once a pathogen has been isolated, monotherapy is For many infections, a delay in treatment until definitive identification preferred unless compelling data exist in favor of combination therapy. of the infecting pathogen would be considered harmful to the patient. Using multiple antibiotics where a single agent should suffice can lead to increased toxicity and unnecessary damage to the patient’s protective For many subacute or chronic infections or acute infections of low fungal and bacterial flora. There are, however, special circumstances severity, the risk in waiting a few days for definitive pathogen identifica- where evidence favors combination therapy: tion is low, and these patients can wait for more definitive microbiological evidence of infection without empirical treatment. If the risks of waiting Preventing emergence of resistance to monotherapy (e.g., combination are high, based on the nature of the infection, the patient’s severity of ill- antiretroviral therapy for HIV, multidrug regimens for treatment of ness, or the patient’s immune status, then initiation of empiric antibiotic active Mycobacterium tuberculosis infection) therapy should rely on the likely infectious syndrome, patient-specific risk Accelerating the rapidity or extent of microbial kill (e.g., combining factors (e.g., prior antibiotic use), and local epidemiology (e.g., prevalence penicillins and aminoglycosides for treatment of severe enterococcal of drug-resistant organisms). In some cases, it may be necessary to use a infections, combining amphotericin B and flucytosine in patients with combination of antibiotics in order to achieve an adequate spectrum of cryptococcal meningitis) activity for empiric coverage of the likely pathogens. Reducing toxicity—when sufficient efficacy of a single antibacterial If the treating clinician wants to obtain samples for microbiological agent can be achieved only at doses that are toxic to the patient and a analysis to guide therapy, these samples are typically obtained during second drug is coadministered to permit lowering the dose of the first this period. It is optimal to obtain these samples before antimicrobials drug (e.g., the use of reduced-dose combinations of ganciclovir and are administered, to improve the diagnostic yield; however, in some cir- foscarnet in treatment of some resistant CMV infections) (Mylonakis cumstances, it is not feasible to delay antimicrobial administration until et al., 2002) diagnostic samples can be obtained. In some cases, the antibiotic combination is already incorporated into Preliminary microbiology results may be available to allow tailor- standard pharmaceutical preparations. For example, the combination of ing of therapy before final microbiological data are available. The most a sulfonamide and an inhibitor of DHFR, such as trimethoprim, is syn- valuable and time-tested method for early identification of bacteria is ergistic owing to the inhibition of sequential steps in microbial folate examination of the infected secretion or body fluid with Gram stain synthesis; the combination formulation of sulfamethoxazole and trime- to identify the presence of gram-positive or gram-negative organisms. thoprim is more commonly used than either agent separately. Similarly, The predictive value of Gram staining varies by infection and specimen many combination anti-HIV regimens are now completely coformulated, type but may be useful in reevaluating empirically selected regimens often in a single daily pill. (e.g., if an empiric regimen has poor gram-positive coverage, the find- ing of gram-positive organisms in a sample may warrant expansion of the spectrum of activity). In malaria-endemic areas or in travelers Posttreatment Suppressive Therapy and returning from such an area, a simple thick-and-thin blood smear Secondary Prophylaxis may mean the difference between a patient’s survival on appropriate In some patients, the infection is controlled but not completely eradicated therapy or death while on the wrong therapy for a presumed bacterial by the initial round of antimicrobial treatment and/or the immunological infection. Rapid point-of-care diagnostic testing is increasingly avail- or anatomical defect that led to the original infection is still present. In able for a number of viral and bacterial infections. New technologies such patients, antibiotics may be continued as suppressive therapy, differ- such as matrix-assisted laser desorption/ionization time-of-flight mass entiated from definitive therapy by use of a lower dose, different route spectrometry (MALDI-TOF MS), nucleic amplification techniques, of administration, or different antibiotic. Examples include treatment of microarray detection, and morphokinetic cellular analysis may not cryptococcal meningitis or treatment of infections of implanted prosthetic be rapid enough to allow deferral of empiric therapy but can shorten materials (e.g., a prosthetic hip) that cannot be removed and against which the empiric therapy phase by providing microbiological information definitive therapy is unlikely to eradicate the infection. Among immuno- more rapidly than traditional approaches (Bauer et al., 2014). compromised hosts, suppressive therapy may eventually be discontinued Notably, in many situations, patients will receive empiric therapy for if the patient’s immune system reconstitutes (e.g., with a sustained ele- the entire duration of their treatment because the actual organism caus- vation in an HIV-infected patient’s CD4 count). Some patients in whom ing the patient’s infection is never determined. This may be due to the pathogen eradication may be achieved may still be candidates for ongoing high cost or invasiveness of microbiological sampling, short duration of antibiotic use in the form of secondary prophylaxis if they are at high risk antibiotic therapy, high predictability of the causative pathogen based on for a new infection. Risks of toxicity from prolonged use of suppressive symptom presentation, or failure of microbiological samples to detect the therapy and secondary prophylaxis can be significant, and assessment for pathogen. In these situations, monitoring for symptomatic response will potential discontinuation should be performed regularly. determine whether more aggressive approaches to determine the micro- biological etiology are required. Definitive Therapy The Pharmacokinetic Basis of Antimicrobial If a pathogen has been identified and susceptibility results are available, Therapy the optimal antibiotic regimen for that patient should be selected—the Typically, a pathogen causes disease not in the whole body but in specific definitive therapy. This may or may not require adjustment of the empiric organs. Within an infected organ, only specific pathological compart- regimen if one was initiated. Selecting an optimal definitive regimen ments may be infected. Antibiotics are often administered orally or requires balancing the need for potent activity against the pathogen, parenterally, far away from these sites of infection. Therefore, in choosing https://ebooksmedicine.net/  1130 an antimicrobial agent for therapy, a crucial consideration is whether the A 100 drug can penetrate to the site of infection. For example, the antibiotic levofloxacin achieves a ratio of peak concentrations in the skin tissue 80 to plasma (CPmax ratio) of 1.4, a ratio of epithelial lining fluid (ELF) to plasma of 2.8, and a urine to plasma ratio of 67 (Chow et al., 2002; Conte et al., 2006; Wagenlehner et al., 2006). The two most important factors Response 60 CHAPTER 56 GENERAL PRINCIPLES OF ANTIMICROBIAL THERAPY in predicting successful clinical and microbiological outcomes using levofloxacin are the site of infection and achieving a CPmax level of 12 times 40 the MIC (CPmax/MIC ≥12). The failure rate of therapy is 0% in patients with urinary tract infections, 3% in patients with pulmonary infections, and 16% in patients with skin and soft-tissue infections (Preston et al., 20 1998). Clearly, the poorer the penetration into the anatomical compart- ment, the higher is the likelihood of failure. 0 The penetration of a drug into an anatomical compartment depends on the physical barriers that the molecule must traverse, the chemical prop- B 100 erties of the drug, and the presence of multidrug transporters. Chapters 2 (pharmacokinetics) and 4 (membrane transporters) provide excellent dis- 80 cussions of these concepts. A unique consideration for drug penetration in treatment of infections is the presence of microorganism-produced Response biofilms. Examples of biofilms include endocardial vegetations on heart 60 valves in endocarditis; biofilms formed by bacteria and fungi on pros- thetic devices such as artificial heart valves, long-dwelling intravascular 40 Emax catheters, and artificial hips; and the biofilms formed within the lungs of patients suffering from cystic fibrosis. Bacterial and fungal biofilms are colonies of slowly growing cells enclosed within an exopolymer matrix. 20 The exopolysaccharide is negatively charged and can bind positively charged antibiotics and restrict their access to the intended target. To 0 be effective against infections in these compartments, antibiotics must 0 100 penetrate the biofilm and endothelial barriers (Sun et al., 2013). [Antimicrobial] Figure 56–2 Changes in sigmoid Emax model with increases in drug resistance. Impact of Susceptibility Testing on Success of An increase in resistance may show changes in IC50: In A, the IC50 increases from 70 (orange line) to 100 (green line) to 140 (blue line). An increase in Antimicrobial Agents resistance may also show a decrease in Emax: In B, efficacy decreases from full The microbiology laboratory plays a central role in the decision to choose response (orange line) to 70% (green line). a particular antibiotic agent over others. First, identification and isolation of the culprit organism take place when patient specimens are sent to the microbiology laboratory. Once the microbial species causing the disease has been identified, a more rational choice of the class of antibiotics likely warrant higher doses of drug to achieve particular effect. The change in IC50 to work in the patient can be made. The microbiology laboratory then may become so large that it is not possible to overcome the concentration plays a second role, which is to identify what antibiotics the organism deficit by increasing the antimicrobial dose without causing toxicity to isolated from that sample is susceptible to, allowing for definitive therapy. the patient. At that stage, the organism is now “resistant” to the particular Millions of individuals across the globe become infected by many dif- antibiotic. ferent isolates of the same species of pathogen. Evolutionary processes A second possible change in the curve is decrease in Emax (Figure 56–2B), cause each isolate to be slightly different from the next, so that each may such that increasing the dose of the antimicrobial agent beyond a certain have a unique susceptibility to antimicrobial agents. As the microor- point will achieve no further effect; that is, changes in the microbe are ganisms divide within the patient, they may undergo further evolution such that eradication of the microbe by the particular drug can never between the time of infection and the time of diagnosis. Therefore, one be achieved. This occurs because the available target proteins have been observes a distribution of concentrations of antimicrobial agents that can reduced or the microbe has developed an alternative pathway to over- kill the pathogens. Often, this distribution is Gaussian, with a skew that come the biochemical inhibition. For example, maraviroc is an allosteric, depends on local susceptibility patterns. noncompetitive antagonist that binds to the CCR5 receptor of a patient’s Because antimicrobial agents are ligands that bind to their targets to CD4 cells to deny HIV entry into the cell. Viral resistance occurs by a produce effects, the relationship between drug concentration and effect mechanism that involves HIV adapting to use of the maraviroc-bound on a population of organisms is modeled using the standard Hill-type CCR5, which results in a decrease of Emax in phenotypic susceptibility curve for receptor and agonist (see Chapters 2 and 3), characterized by assays (Hirsch et al., 2008). three parameters: Bacteria IC50 (also termed EC50), the inhibitory concentration that is 50% effec- For bacteria, diffusion tests use antimicrobial-impregnated disks placed tive, a measure of the antimicrobial agent’s potency on a solid growth medium upon which the bacterium of interest has been Emax, a measure of the maximal effect plated and allowed to incubate for 12 to 24 h (Figure 56–3A). The size H, the slope of the curve, or Hill factor of the zone of inhibition (area without bacterial growth) around each With changes in susceptibility, the sigmoid Emax curve shifts in one disk is given a categorical interpretation of susceptible, intermediate, or of two basic ways. The first is a shift to the right, an increase in IC50 resistant. These interpretations are based on consensus breakpoints that (Figure 56–2A), meaning that much higher concentrations of antimicro- have been established that relate the size of the zone of inhibition to the bials than before are now needed to show specific effect. Susceptibility predicted clinical utility of the drug. In contrast, dilution tests employ tests for bacteria, fungi, parasites, and viruses have been developed to antibiotics in serially diluted concentrations in a liquid broth medium determine whether these shifts have occurred at a sufficient magnitude to that contains a culture of the test microorganism (Figure 56–3B). A Disk diffusion test 1131 Piperacillin + tazobactam (PTZ36) SECTION VII CHEMOTHERAPY OF INFECTIOUS DISEASES Meropenem Gentamicin (MEM10) (GMN10) Intermediate susceptible 32 mm range 11 mm Amikacin (AKN30) 17 mm 34 mm 21 mm Ciprofloxacin 17 mm (CIP5) Clinical 9 mm breakpoints Cefepime (FEP30) Ceftazidime (CZD10) Inhibition zones B Minimal Inhibitory Concentration (MIC) A B C D E F G H Ampicillin/ 1 128 32 Sulbactam Piperacillin 2 128 16 4 Piperacillin/ 3 16 128 4 Tazobactam Ceftazidime 4 16 8 2 5 1 Aztreonam 16 2 Meropenem 6 16 2 0.25 Gentamicin 7 32 4 0.5 Amikacin 8 64 8 1 Colistin 9 16 2 1 Ciprofloxacin 10 8 2 0.5 Tigecycline >8 8 Trimethoprim/ Sulfamethoxazole >8 8 Clinical breakpoints (mg/L) Minimum inhibitory concentrations Figure 56–3 Antibiotic susceptibility testing methods. Once a bacterium is isolated from a clinical sample and grown in culture, it is incubated with antibiotics to determine if those drugs inhibit its growth. In A, disk diffusion testing is used. A solid agar plate is covered with a lawn of bacteria, and antibiotic-impregnated disks are placed on it. The area around the bacteria without visible growth is the zone of inhibition and used to determine susceptibility. In B, dilution testing is used. A sample of the bacterium is added to microtiter plates containing variable concentrations of antibiotics of interest. After incubation, the wells with the lowest concentration of an agent that has prevented visible growth are considered to represent the minimum inhibitory concentration (MIC) for the drug-organism pair. If the MIC is less than the clinical breakpoint, the isolate is considered to be susceptible to that antibiotic. https://ebooksmedicine.net/  1132 The lowest concentration of the agent that prevents visible growth are not standardized. These tests are primarily used in the research setting after 18 to 24 h of incubation is the MIC, which is typically measured and not for individualization of therapy. in doubling concentrations. In clinical laboratories, dilution tests are performed on commercial platforms that automate many of the steps in preparation, incubation, and interpretation. Interpretations are made by Basis for Selection of Dose and Dosing Schedule comparing the MIC obtained for each drug to the consensus breakpoints CHAPTER 56 GENERAL PRINCIPLES OF ANTIMICROBIAL THERAPY Although susceptibility testing in the laboratory is central to decision established for that drug-organism pair. When the MIC measured for making, it does not completely predict patient response. In susceptibility an isolate is at or below the breakpoint for that drug-organism pair, the tests, the drug concentration is constant; by contrast, in patients, the drug isolate is considered to be susceptible to that drug. concentration is dynamic and ever changing. Antibiotics are prescribed Recently, nucleic acid amplification–based reactions of specific bac- at a certain schedule (e.g., three times a day) so that there is a periodi- terial genes have been used in the clinic for rapid diagnosis of drug city in the fluctuations of drug at the site of infection, and the microbe resistance. The genes targeted are those encoding known drug resistance is exposed to a particular shape of the concentration-time curve. Harry proteins or processes. For example, rifampin resistance in M. tuberculo- Eagle performed studies on penicillin and discovered that the shape of sis has been difficult to ascertain in a timely fashion: The bacteria take the concentration-time profile was an important determinant of the effi- 2 to 3 weeks to grow in order to identify them as a cause of disease, and cacy of the antibiotic. This important observation was forgotten until then a similar amount of time is needed to perform some version of the William Craig and colleagues rediscovered it and performed systematic broth dilution tests. Small PCR reactors at points of care can purify and studies on several classes of antibiotics, initiating the era of antimicro- concentrate a patient’s fluid sample, perform nucleic acid amplification bial PK/PD (Ambrose et al., 2007; Craig, 2007). These findings have now of a target gene, identify mutations, and provide a result in less than 2 h. been extended to combination therapy and to microbes that require long Similarly, PCR-based rapid identification of the mecA gene responsible treatment durations, such as M. tuberculosis and HIV. for methicillin resistance in S. aureus in clinical or surveillance samples is As an example, consider an antibiotic with a serum t1/2 of 3 h that is frequently employed for infection control and clinical care in hospitals. being used to treat a bloodstream infection by a pathogen with an MIC of 0.5 mg/L; the antibiotic is administered with a dosing interval of 24 h Fungi (that is, a once-daily schedule). Figure 56–4A depicts the concentration- For fungi that are yeasts (i.e., Candida), susceptibility testing methods time curve of the antibiotic, with definitions of CPmax, AUC, and the are similar to those used for bacteria. However, the definitions of MIC fraction of the dosing interval for which the drug concentration remains differ based on drug and the type of yeast, so there are cutoff points of above the MIC (T > MIC), as shown. The AUC is a measure of the total 50% decrease in turbidity compared to controls at 24 h, 80% at 48 h, or total clearance of the turbidity. Susceptibility tests and MICs for triazoles (e.g., fluconazole) have been extensively shown to correlate with clin- 5 A ical outcomes. Standardized tests for echinocandin antifungals and amphotericin B–based compounds are also available. CPmax = maximum concentration Susceptibility tests for molds have been developed, especially for 4 Aspergillus species. Different terminology is required when evaluating echinocandins against molds because the fungal burden cannot be read- 3 ily measured, given that hyphae will break up into unpredictable num- bers of discrete fungi when under antifungal pressure. Furthermore, 2 AUC0–24h = 24h area under the echinocandins often do not completely inhibit mold growth, but instead cause damage reflected by morphological changes in hyphae. Thus, the concentration-time curve minimum effective concentration (MEC) for echinocandins is the lowest Drug concentration (mg/L) 1 drug concentration at which short, stubby, and highly branched hyphae T > MIC Microbe’s MIC are observed on microscopic examination. 0 Viruses In HIV phenotypic assays, the patient’s HIV-RNA is extracted from 5 B plasma, and genes for targets of antiretroviral drugs such as reverse tran- scriptase and protease are amplified. The genes are then inserted into a 4 standard HIV vector that lacks analogous gene sequences to produce a recombinant virus, which is coincubated with a drug of interest in a mam- 3 malian cell viability assay (Hanna and D’Aquila, 2001; Petropoulos et al., 2000). Growth is compared to a standardized wild-type control virus. Phe- notypic assays are laborious and time-consuming, and genotypic testing 2 CPmax is more commonly employed. These tests aim to detect the presence of AUC8–16h AUC16–24h AUC0–8 mutations that are predicted to result in reduced phenotypic susceptibility. 1 Where they are accessible, genotypic assays are a standard of care for HIV management and are also used to detect resistance-associated mutations MIC T1, MIC T2, MIC T3, MIC in pathogens such as CMV. 0 0 3 6 9 12 15 18 21 24 Parasites Time in hours Susceptibility testing for parasites, especially those that cause malaria, Figure 56–4 Effect of different dose schedules on shape of the concentration- has been performed in the laboratory. Plasmodium species in the patient’s time curve. The same total dose of a drug was administered as a single dose blood are cultured ex vivo in the presence of different dilutions of anti- (panel A) and in three equal portions every 8 h (panel B). The total AUC for malarial drug. A sigmoid Emax curve for effect versus drug concentration the fractionated dose in B is determined by adding AUC0–8h, AUC8–16h, and is used to identify IC50 and Emax. These susceptibility tests are usually field AUC16–24h, which totals to the same AUC0–24h in A. The time that the drug tests at sentinel sites that are used to determine if there is drug resistance concentration exceeds MIC in B is also determined by adding T1 > MIC, in a particular area. In general, susceptibility tests for parasitic infections T2 > MIC, and T3 > MIC, which results in a fraction greater than that for A. concentration of drug and is calculated by taking an integral between two model does not apply to resistance suppression (Gumbo et al., 2007b; 1133 time points, 0 to 24 h (AUC0–24) in this case. Tam et al., 2007). Now, if one were to change the dosing schedule of the same antibiotic amount by splitting it into three equal doses administered at 0, 8, and 16 h, the shape of the concentration-time curve changes to that shown Mechanisms of Resistance to Antimicrobial SECTION VII CHEMOTHERAPY OF INFECTIOUS DISEASES in Figure 56–4B. Because the same cumulative dose has been given for the dosing interval of 24 h, the AUC0–24 will be similar whether it was Agents given once a day or three times a day. For the same pathogen, therefore, Antibiotics were viewed as miracle cures when first introduced into clin- the change in dose schedule does not change the AUC0–24/MIC. However, ical practice. However, as became evident soon after the discovery of the CPmax will decrease by a third when the total dose is split into thirds penicillin, resistance eventually develops and dims the luster of the mira- and administered more frequently (Figure 56–4B). Thus, when a dose cle. Today, every major class of antibiotic is associated with the emergence is fractionated and administered more frequently, the CPmax/MIC ratio of significant resistance. When a microbial species is subjected to an exis- decreases. In contrast, the time that the drug concentration persists above tential threat, chemical or otherwise, that pressure will select for random MIC (T > MIC) will increase with the more frequent dosing schedule, mutations in the species’ genome that permit survival. This evolution is despite the same cumulative dose being administered. greatly assisted by poor therapeutic practices by healthcare workers and Some classes of antimicrobial agents exert greater antimicrobial the indiscriminant use of antibiotics in agriculture and animal husbandry. effects when their concentration persists above the MIC for longer dura- Antimicrobial resistance can develop at any one or more of steps in the tions of the dosing interval. Indeed, increasing the drug concentration processes by which a drug reaches and combines with its target. Major beyond four to six times the MIC does not increase microbial kill for mechanisms of antibiotic resistance include: such antibiotics. Two good examples are β-lactam antibacterials (e.g., penicillin) and the antifungal agent 5-fluorocytosine (Ambrose et al., Reduced concentration of the antibiotic at its target site 2007; Andes and van Ogtrop, 2000). There are usually biochemical Production of microbial enzymes that alter or destroy the antibiotic explanations for this pattern; the clinical implication, however, is that Alteration of antibiotic targets in ways that reduce antibiotic affinity a drug optimized by T > MIC should be dosed more frequently; given A host of less-common mechanisms have been discovered as as a prolonged, instead of rapid, infusion; or have its t1/2 prolonged by well, including bypass of inhibited metabolic pathways, excision of other drugs (as with the coadministration of probenecid with penicil- antibiotic-target complexes, and overproduction of target enzymes. lin), so that drug concentrations persist above MIC (or EC95) as long Organisms may also express resistance elements that interfere with the as possible. Thus, the effectiveness of penicillin is enhanced when it is immune response; this can result in an effect similar to antibiotic resis- given as a continuous infusion. Some antibiotics, such as ceftriaxone tance, as antibiotics typically work in concert with the immune system to (t1/2 = 8 h), have long half-lives, such that infrequent dosing still allows clear infections (Sun et al., 2021). More than one mechanism may work maintenance of an adequate T > MIC. HIV protease inhibitors are often in concert to confer resistance to an individual antibiotic. “boosted” with ritonavir or cobicistat. This “boosting” inhibits the metabolism of the protease inhibitors by CYPs 3A4 and 2D6, thereby Resistance Due to Reduced Concentration prolonging time above EC95. Conversely, the peak concentration is most predictive of efficacy for other of Drug at Its Target Site antimicrobial agents. Persistence of concentration above the MIC has less The outer membrane of gram-negative bacteria is a semipermeable bar- relevance for these drugs—described as having “time-independent killing,” rier that excludes large polar molecules from entering the cell. Small meaning that these drugs can be dosed more intermittently. Aminogly- polar molecules, including many antibiotics, enter the cell through pro- cosides are a prime example of this class; aminoglycosides are highly effec- tein channels called porins. Absence of, mutation in, or loss of a favored tive when given once a day at sufficient dosage, despite their short half-lives. porin channel can slow the rate of drug entry into a cell or prevent entry These CPmax/MIC–linked drugs can often be administered less frequently due altogether, effectively reducing drug concentration at the target site. If the to their long duration of PAE (post antibiotic effect), with effectiveness con- target is intracellular and the drug requires active transport across the cell tinuing long after antibiotic concentrations decline below the MIC. membrane, a mutation or phenotypic change that slows or abolishes this Rifampin is such another such drug (Gumbo et al., 2007a). The entry transport mechanism can confer resistance. of rifampin into M. tuberculosis increases with increased concentration Once an antibiotic crosses the cell membrane, its concentration may in the bacillus microenvironment, likely because of a saturable trans- be reduced below the effective concentration through the action of efflux port process. Once inside the bacteria, the drug’s macrocyclic ring pumps, energy-dependent transporters that expel antibiotics to which the binds the β subunit of DNA-dependent RNA polymerase (rpoB) to form microbes would otherwise be susceptible. There are five major systems of a stable drug-enzyme complex within 10 min, a process not enhanced efflux pumps that are relevant to antimicrobial agents: by longer incubation of drug and enzyme and only slowly reversed. The multidrug and toxin extruder The PAE of the rifampin is long and concentration dependent (Gumbo The major facilitator superfamily transporters et al., 2007a). The small multidrug resistance system There is a third group of drugs for which it is the cumulative dose that The resistance nodulation division exporters matters most and for which the daily dosing schedule has no effect on effi- ABC transporters cacy. Thus, it is the ratio of the total concentration (AUC) to MIC that is most predictive of effect and not the time that concentration persists Resistance due to reduced concentrations of drug at the site of infec- above a certain threshold. Antibacterial agents such as daptomycin fall tion is a prominent mechanism of resistance for parasites, bacteria, and into this class (Louie et al., 2001). These agents also have a long PAE. fungi, and can function selectively or broadly. For example, in Pseu- The shape of the concentration-time curve that optimizes resistance domonas aeruginosa, resistance to the antipseudomonal carbapenem suppression is often different from that which optimizes microbial kill. In imipenem is significantly mediated through mutational loss of the many instances, the drug exposure required for resistance suppression OprD porin, the primary means by which imipenem crosses the outer is much higher than that for optimal kill. Ideally, this higher exposure membrane (Fernandez and Hancock, 2012). In contrast, meropenem is should be achieved by each dose for optimal effect, rather than the only minimally affected by isolated loss of OprD, but its activity is sig- EC80, as discussed previously. However, this is often precluded by drug nificantly reduced by upregulation of efflux pump activity, such as that toxicity at higher dosages. Second, although the relationship between of the MexA-MexB-OprM system. Upregulation of these efflux systems kill and exposure is based on the inhibitory sigmoid Emax model, has less impact on imipenem but tends to raise MICs for a broader array experimental work with preclinical models demonstrated that this of antibiotics including cephalosporins and aminoglycosides. https://ebooksmedicine.net/  1134 Resistance Due to Alteration or Destruction induction. Mutations are random events that confer a survival advan- tage when drug is present. Mutation and antibiotic selection of resistant of Antibiotic mutants are the molecular basis for resistance for many bacteria, viruses, Drug inactivation is a common mechanism of drug resistance. The most and fungi. Mutations may occur in: prominent example is the enzymatic inactivation of β-lactam antibiot- ics through the function of β-lactamase enzymes (Bush, 2018). Over a A gene encoding the target protein, altering its structure so that it no CHAPTER 56 GENERAL PRINCIPLES OF ANTIMICROBIAL THERAPY thousand distinct β-lactamases have been identified, some of which can longer binds the drug confer resistance to nearly all β-lactams. In some cases, β-lactamase- A gene encoding a protein involved in drug transport mediated resistance can be averted by coadministration of β-lactamase A gene encoding a protein important for drug activation or inactivation inhibitors (e.g., clavulanate, avibactam). Other examples of enzymatic A regulatory gene or promoter affecting expression of the target, inactivation leading to resistance include production of aminoglycoside- a transport protein, or an inactivating enzyme modifying enzymes and esterification of macrolides. In some instances, a single-step mutation results in a high degree of resistance. In M. tuberculosis katG, Ser315 mutations cause resistance to Resistance Due to Altered Target Structure isoniazid; the M814V mutation in the reverse transcriptase gene of HIV-1 A common consequence of either single or multiple point mutations causes resistance to lamivudine; and C. albicans fks1 Ser645 mutations is a change in amino acid composition and conformation of an anti- cause resistance to echinocandins. microbial’s target protein. This change can lead to reduced affinity of In other circumstances, however, it is the sequential acquisition of drug for its target or of a prodrug for the enzyme that activates the multiple mutations that leads to clinically significant resistance. For prodrug. Such alterations may be due to mutation of the natural tar- example, the combination of pyrimethamine (an inhibitor of DHFR) and get (e.g., fluoroquinolone resistance), enzyme-mediated target modifi- sulfadoxine (an inhibitor of DHPS) blocks the folate biosynthetic pathway cation (e.g., ribosomal protection type of resistance to macrolides and in P. falciparum. Clinically meaningful resistance occurs only when there tetracyclines), or acquisition of a resistant form of the native, susceptible is a single-point mutation in the DHPS gene accompanied by at least a target (e.g., staphylococcal methicillin resistance caused by production double mutation in the DHFR gene. of a low-affinity penicillin-binding protein) (Hooper, 2002; Lim and Strynadka, 2002; Nakajima, 1999). In HIV resistance, mutations asso- Resistance by External Acquisition of ciated with reduced affinity are encountered for protease inhibitors, Genetic Elements integrase inhibitors, fusion inhibitors, and nonnucleoside reverse tran- As noted above, drug resistance may be acquired by mutation and selec- scriptase inhibitors (Nijhuis et al., 2009). Similarly, benzimidazoles are tion, with passage of the trait vertically to daughter cells, provided the used against myriad worms and protozoa and work by binding to the mutation is not lethal, does not appreciably alter virulence, and does not parasite’s tubulin; point mutations in the β-tubulin gene lead to modifi- affect replication by the progeny. Drug resistance may also be acquired by cation of the tubulin and drug resistance (Ouellette, 2001). horizontal transfer of resistance determinants from a donor cell, often of another bacterial species, by transduction, transformation, or conjugation. Heteroresistance and Viral Quasi-Species Resistance acquired by horizontal transfer can disseminate rapidly and Heteroresistance occurs when a subset of the total microbial population widely either by clonal spread of the resistant strain or by subsequent is resistant, despite the total population being considered susceptible on transfers to other susceptible recipient strains. Horizontal transfer of testing (Falagas et al., 2008; Rinder, 2001). A subclone that has altera- resistance offers several advantages over mutation selection. Lethal muta- tions in genes associated with drug resistance is expected to reflect the tion of an essential gene is avoided; the level of resistance often is higher normal mutation rates (occurrence in 1 in 106 to 105 colonies). In bac- than that produced by mutation, which tends to yield incremental teria, heteroresistance has been described especially for vancomycin in changes. The gene, which still can be transmitted vertically, can be mobi- S. aureus and Enterococcus faecium; colistin in Acinetobacter baumannii- lized and rapidly amplified within a population by transfer to susceptible calcoaceticus; rifampin, isoniazid, and streptomycin in M. tuberculosis; and cells, and the resistance gene can be eliminated when it no longer offers a penicillin in S. pneumoniae (Falagas et al., 2008; Rinder, 2001). Increased selective advantage. Horizontal transfer of resistance genes is greatly therapeutic failures and mortality have been reported in patients with facilitated by mobile genetic elements. Mobile genetic elements include heteroresistant staphylococci and M. tuberculosis (Falagas et al., 2008; plasmids and transducing phages. Other mobile elements—transposable Hofmann-Thiel et al., 2009). For fungi, heteroresistance leading to clini- elements, integrons, and gene cassettes—also participate. Transposable ele- cal failure has been described for fluconazole in Cryptococcus neoformans ments are of three general types: insertion sequences, transposons, and and Candida albicans (Marr et al., 2001; Mondon et al., 1999). transposable phages. Viral replication is more error prone than replication in bacteria and fungi. Viral evolution under drug and immune pressure occurs relatively Resistance Transfer in Action easily, commonly resulting in variants or quasi-species that may contain drug-resistant subpopulations. This is often not termed heteroresistance, A startling example of how the transfer mechanisms spread but the principle is the same: A virus may be considered susceptible resistance is the recent description of the plasmid-mediated colistin to a drug because either phenotypic or genotypic tests reveal “lack” of resistance gene (mcr-1), which confers resistance to one of the last- resistance, even though there is a resistant subpopulation just below the resort antibiotics for multidrug-resistant gram-negative bacteria limit of assay detection. These minority quasi-species that are resistant (Liu et al., 2016). Colistin is used in agriculture and animal husbandry. to antiretroviral agents have been associated with failure of antiretroviral Escherichia coli strains carrying the mcr-1 gene were found in pigs, therapy (Metzner et al., 2009). then in pork, and then in patients. The plasmid carrying mcr-1 was mobilized by conjugation to E. coli at a frequency of 10−1 to 10−3 cells per recipient and could be spread and maintained in other gram- EVOLUTIONARY BASIS OF RESISTANCE negative rods of clinical significance. The resistant bacteria were initially identified in China, but within months, isolates were also EMERGENCE identified in North America, South America, Europe, East Asia, and Africa and in other organisms, such as Salmonella typhimurium. Development of Resistance via Mutation Selection The gene has now been demonstrated in gut microbiota of healthy Genetic mechanisms by which antibiotic resistance develops can include individuals, suggesting integration in the human gut and the capacity acquisition of genetic elements that code for the resistant mecha- to spread to organisms in the human microbiome. nism, mutations that develop under antibiotic pressure, or constitutive Acknowledgment: Tawanda Gumbo contributed to this chapter in Lim D, Strynadka NC. Structural basis for the beta lactam resistance of 1135 previous editions of this book. We have retained some of his text in the PBP2a from methicillin-resistant Staphylococcus aureus. Nat Struct current edition. Biol, 2002, 9:870–876. Liu YY, et al. Emergence of plasmid-mediated colistin resistance mechanism MCR-1 in animals and human beings in China: a microbiological and SECTION VII CHEMOTHERAPY OF INFECTIOUS DISEASES molecular biological study. Lancet Infect Dis, 2016, 16:161–168. Bibliography Louie A, et al. Pharmacodynamics of daptomycin in a murine thigh model Ambrose PG, et al. Pharmacokinetics-pharmacodynamics of of Staphylococcus aureus infection. Antimicrob Agents Chemother, antimicrobial therapy: it’s not just for mice anymore. Clin Infect Dis, 2001, 45:845–851. 2007, 44:79–86. Marr KA, et al. Inducible azole resistance associated with a heterogeneous Andes D, van Ogtrop M. In vivo characterization of the pharmacodynamics phenotype in Candida albicans. Antimicrob Agents Chemother, 2001, of flucytosine in a neutropenic murine disseminated candidiasis 45:52–59. model. Antimicrob Agents Chemother, 2000, 44:938–942. Mayer KH, Allan-Blitz LT. PrEP 1.0 and beyond: optimizing a biobehavioral Bauer KA, et al. Review of rapid diagnostic tests used by antimicrobial intervention. J Acquir Immune Defic Syndr, 2019, 82(S2):S113–S117. stewardship programs. Clin Infect Dis, 2014, 59(S3):S134–S135. Metzner KJ, et al. 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Antimicrob Agents Chemother, Informa Healthcare USA, New York, 2007, 1–19. 2000, 44:920–928. Falagas ME, et al. Heteroresistance: a concern of increasing clinical Preston SL, et al. Pharmacodynamics of levofloxacin: a new paradigm for significance? Clin Microbiol Infect, 2008, 14:101–104. early clinical trials. JAMA, 1998, 279:125–129. Fernandez L, Hancock REW. Adaptive and mutational resistance: role of Razonable RR, et al. Cytomegalovirus in solid organ transplant recipients— porins and efflux pumps in drug resistance. Clin Microbiol Rev, 2012, guidelines of the American Society of Transplantation Infectious 25:661–681. Diseases Community of Practice. Clin Transplant, 2019, 33:e13512. Gumbo T, et al. Concentration-dependent Mycobacterium tuberculosis Rinder H. Hetero-resistance: an under-recognised confounder in killing and prevention of resistance by rifampin. Antimicrob Agents diagnosis and therapy? J Med Microbiol, 2001, 50:1018–1020. Chemother, 2007a, 51:3781–3788. Schweizer ML, et al. Association of a bundled intervention with surgical Gumbo T, et al. Isoniazid bactericidal activity and resistance emergence: site infections among patients undergoing cardiac, hip, or knee surgery. integrating pharmacodynamics and pharmacogenomics to predict JAMA, 2015, 313:2162–2171. efficacy in different ethnic populations. Antimicrob Agents Chemother, Sun F, et al. Biofilm-associated infections: antibiotic resistance and novel 2007b, 51:2329–2336. therapeutic strategies. Future Microbiol, 2013, 8:877–886. Hanna GJ, D’Aquila RT. Clinical use of genotypic and phenotypic drug Sun J, et al. Repurposed drugs block toxin-driven platelet clearance by resistance testing to monitor antiretroviral chemotherapy. Clin Infect the hepatic Ashwell-Morell receptor to clear Staphylococcus aureus Dis, 2001, 32:774–782. bacteremia. Sci Trans Med, 2021, 13:eabd6737. Hirsch MS, et al. Antiretroviral drug resistance testing in adult HIV-1 Tam VH, et al. The relationship between quinolone exposures and infection: 2008 recommendations of an International AIDS Society- resistance amplification is characterized by an inverted U: a new USA panel. Clin Infect Dis, 2008, 47:266–285. paradigm for optimizing pharmacodynamics to counterselect Hofmann-Thiel S, et al. Mechanisms of heteroresistance to isoniazid and resistance. Antimicrob Agents Chemother, 2007, 51:744–747. rifampin of Mycobacterium tuberculosis in Tashkent, Uzbekistan. Eur Wagenlehner FM, et al. Concentrations in plasma, urinary excretion Respir J, 2009, 33:368–374. and bactericidal activity of levofloxacin (500 mg) versus ciprofloxacin Hooper DC. Fluoroquinolone resistance among gram-positive cocci. (500 mg) in healthy volunteers receiving a single oral dose. Int Lancet Infect Dis, 2002, 2:530–538. J Antimicrob Agents, 2006, 28:551–519. https://ebooksmedicine.net/ This page intentionally left blank