Antimicrobials in practice: treating resistant infections (Proceedings)

Antimicrobial drugs are the most frequently prescribed drugs in veterinary medicine. They are also frequently used incorrectly, which can lead to treatment failure and the development of resistant bacteria. Appropriate use of antimicrobials requires accurate diagnoses and a basic knowledge of the physical characteristics of the drug that relates to its effectiveness against pathogenic organisms. Furthermore, by incorporating pharmacodynamic data (ie MICs), we can develop more accurate dosing regimens for specific diseases.

Treatment failure versus resistant infection

Many veterinarians (and human doctors) use antimicrobials incorrectly including use of these drugs for the treatment of fever or untreatable disease (ie viral), treatment without appropriate surgical intervention, and use of irrational antibiotic combinations. Other reasons for antimicrobial treatment failure can include an incorrect diagnosis, incorrect dose, route or frequency of administration, immunocompromised patients, poor owner compliance, development of drug resistance, presence of a foreign body, and pharmacokinetic (PK) or pharmacologic antagonism. Additional PK problems can be associated with therapeutic failure, such as poor oral absorption and intestinal metabolism of the drug. Therapeutic success relies on a complex interaction of factors attributed to the host, the drug and the bacteria. As practitioners, we must administer the appropriate drug in sufficient doses, with sufficient frequency, to maintain concentrations that will eliminate the bacteria at the site of the infection.

Pharmacokinetic-pharmacodynamic interactions

Pharmacokinetic-pharmacodynamic (PK-PD) interactions have been developed in order to predict the success of antimicrobial therapies. These interactions vary with each class of drug and must be related back to characteristics of the bacteria, namely the concentration of the antibiotic that is necessary for therapeutic success.

To achieve a cure, the drug concentration at the site of the infection should be maintained above the MIC, or some multiple of the MIC, for at least a portion of the dose interval. In general, guidelines for antibiotic therapy in are as follows:

• Aminoglycosides: CMAX: MIC ratio of 8-10

• Fluoroquinolones: AUC24:MIC ratio of 100-125

• Beta-lactams: T > MIC for 50% of dosing interval

• Bacteriostatic antibiotics: T > MIC for the entire dosing interval

More recent work has emphasized the importance of drug concentrations at the site of the infection, rather than in the plasma, however. For most tissues, antibiotic drug concentrations in the serum or plasma can predict the drug concentration in the extracellular space (interstitial fluid). This is because there is no physical barrier that impedes drug diffusion from the vascular compartment to extracellular tissue fluid. Pores (fenestrations) or microchannels in the endothelium of capillaries are large enough to allow drug molecules to penetrate. One important limitation is for drugs that are highly protein bound in the blood. Some examples of drugs for which this may be important in veterinary medicine include doxycycline and itraconazole. In some tissues a lipid membrane (such as tight junctions in capillaries) presents a barrier to drug diffusion. In these instances, a drug must be sufficiently lipid-soluble, or be actively carried across the membrane in order to reach effective concentrations in tissues.

Intracellular infections also present a problem. Examples of drugs that accumulate in leukocytes, fibroblasts, macrophages, and other cells are fluoroquinolones, tetracyclines, macrolides (erythromycin, clarithromycin), and the azalides (azithromycin). β-lactam antibiotics and aminoglycosides do not reach effective concentrations within cells.

Drug diffusion into an abscess or cavitated lesion may be delayed because the volume into which the drug must diffuse is higher resulting in a lower surface area - volume (SA:V) ratio, lower drug concentrations, and slower equilibrium between plasma and tissue. Therefore, observed slow equilibrium or a low peak drug concentration in this case is more a factor of the geometry of the tissue (low SA:V ratio), rather than a physical barrier to diffusion. For an abscess or granuloma, penetration by antibiotics also is impaired because drug penetration relies on simple diffusion and the site of infection may lack an adequate blood supply.

Local tissue factors may decrease antimicrobial effectiveness. For example, pus and necrotic debris may bind and inactivate vancomycin or aminoglycoside antibiotics, causing them to be ineffective. The acidic environment of infected tissue may decrease the effectiveness of erythromycin, fluoroquinolones, and aminoglycosides. Penicillin and tetracycline activity is not affected as much by tissue pH, but hemoglobin at the site of infection will decrease the activity of these drugs. An anaerobic environment decreases the effectiveness of aminoglycosides because oxygen is necessary for drug penetration into bacteria. Trimethoprim-sulfonamide combinations are sometimes not effective in vivo despite in vitro results that suggest susceptibility. This may be due to thymidine and para-aminobenzoic acid (PABA) in the tissues, which are inhibitors of the action of trimethoprim and sulfonamides, respectively. Effective antibacterial drug concentrations may not be attained in tissues that are poorly vascularized (eg, necrotic lung tissue).

Bacterial susceptibility testing

As previously stated, most of our dosing regimens are based on the MIC of the bacteria. Therefore, it is important to know how the MIC was determined, and how to interpret that data. It is common for laboratories to directly measure the MIC of an organism with an antimicrobial dilution test. The test is most often performed by inoculating the wells of a plate with the bacterial culture and adding multiple dilutions of antibiotics across the rows of the plate. The MIC is determined by observing the exact concentration required to inhibit bacterial growth. In some laboratories other methods to measure the MIC are being used, such as the E-test (epsilometer test) by AB Biodisk. The E-test is a quantitative technique that identifies the MIC via direct measurement of bacterial growth along a concentration gradient of the antibiotic contained in a test strip. The Kirby-Bauer disk diffusion method is frequently used for sensitivity testing, however it does not report an MIC and the results are not as accurate as dilution testing.

When the MIC is measured, resistance and susceptibility are determined by comparing the organism's MIC to the drug's breakpoint that has been standardized by the CLSI. If bacteria have a MIC equal to or below the "susceptible" breakpoint, treatment with this drug should produce a cure unless there are other factors independent of the drug's activity. A MIC equal to or above the "resistant" breakpoint indicates that the organism is resistant regardless of the dose administered or location of the infection. An MIC in the "intermediate" range means that the organism is resistant to the drug unless dosing modifications are used, or unless the drug concentrates at the site of infection as with topical treatment or in the lower urinary tract for drugs excreted via the kidney. Even though we believe that an MIC determination is valuable to guide therapy, there are some limitations. One of the most important limitations for interpreting susceptibility information for pathogens infecting veterinary species is that interpretive criteria to establish susceptibility breakpoints are only available for drugs approved for use in a given species. For other drugs, human interpretive criteria are used and the breakpoints may not necessarily correspond to plasma concentrations in animals. Other pitfalls to be aware of when interpreting MIC results include the following:

• Assumes equal plasma and tissue concentrations

• Overestimates activity in CSF, eye, prostatic fluid and mammary tissue

• Underestimates activity following topical therapy, regional perfusion, or in the urinary tract

• Does not take into account subtherapeutic effects, PAE, or synergisitic drug combinations

• Does not consider local tissue factors

Types of resistant bacteria

Methicillin-resistant staphylococcus aureus

Some strains of S. aureus have developed resistance to the anti-staphylococcal penicillins, such as methicillin. Because of this, they are termed methicillin-resistant S. aureus, or MRSA. MRSA has become a serious cause of nosocomial infections in human and veterinary medicine worldwide. The resistance in MRSA is mediated via the mecA gene which encodes for penicillin binding protein 2a (PBP2a) that has a very low affinity for beta-lactam antibiotics. The incidence of MRSA infections in dogs and cats has increased in recent years, and the majority are associated with post-operative infections, open wounds, and surgical implants. In horses, isolation rates have been reported to be as high as 5.3% in tertiary care facilities. Resistance patterns and genetic screening of MRSA from dogs and humans are virtually identical, therefore cross-contamination from human to animal and animal to human is likely to occur.

Extended spectrum beta-lactamase bacteria

Gram-negative bacteria commonly produce beta-lactamases that inactivate penicillins, but not the extended spectrum cephalosporins. In the last few decades, bacteria (mainly E. coli) have been discovered that produce what are known as extended-spectrum beta-lactamases (ESBL). These enzymes hydrolyze the extended-spectrum cephalosporins that have an oxyimino side chain. These cephalosporins include cefotaxime, ceftriaxone and ceftazidime. The expression of ESBLs is plasmid mediated, and these plasmids often encode for other genes that infer resistance to antimicrobials of other classes (ie aminoglycosides).

Enterococci

Enterococcal organisms, particularly E. faecalis and E. faecium, are opportunistic pathogens that are considered commensal organisms in the GI tract. They a not typically virulent, but are often multi-drug resistant. They most commonly infect the urinary tract in small animals and are frequently cultured in combination with E.coli. If they are cultured in combination, we recommend treating the other bacterium, and then reculturing the urine. In many instances, clearing the primary organism will allow the immune system to clear the enterococcal infection. If this is the only bacterium cultured and the animal is showing clinical signs, treatment options are often limited. They are typically sensitive to vancomycin, however vancomycin-resistant enterococci (VRE) are increasing.

Antimicrobials for treatment of resistant bacterial infections

Due to the emergence of many resistant bacteria in human medicine, newer antibiotics are being developed to combat these infections. Currently, their use is not recommended in veterinary medicine without documented sensitivity testing. Under no circumstances should these antibiotics be used as a first line treatment for disease.

Carbapenems

The carbapenems are beta-lactam antibiotics and include the drugs imipenem (in combination with cilastatin), meropenem and ertapenem. These drugs have a very broad spectrum of activity that includes bacteria that are resistant to most other drugs. The only notable exceptions include MRSA and resistant Enterococcus faecium. The high activity of these drugs is due to the ability to withstand the beta-lactamases (including ESBL) and the ability to penetrate porin channels that usually exclude other drugs. They are more rapidly bactericidal and less likely to cause endotoxin release following administration to septic animals. Of the drugs in this class, meropenem may be the best choice for use in animals because it is easily administered SC and is less likely than imipenem to be nephrotoxic. The recommended dose for Enterobactericeae and other sensitive organisms is 8.5 mg/kg SC every 12 hr, or 24 mg/kg IV every 12 hr. For infections caused by Pseudomonas aeruginosa, or other similar organisms that may have MIC values as high as 1.0 µg/mL, the recommended dose is 12 mg/kg q8h, SC, or 25 mg/kg q8h, IV.

Glycopeptides

Often, the only effective drug against MRSA and Enterococcus species will be a glycopeptide. Of the glycopeptides, vancomycin is the only one used in veterinary medicine. Vancomycin has been given as an IV infusion administered over 30 to 60 minutes. It is not absorbed orally and is too painful when injected IM. The dose to maintain concentrations within the therapeutic range, and avoid toxicity is 15 mg/kg, q6h, IV. For successful therapy of serious infections, an aminoglycoside such as gentamicin or amikacin should be administered with vancomycin.

Oxazolidinones

Linezolid is the first in the class of oxazolidinones to be used in medicine. It works by binding to the bacterial 23S ribosomal RNA of the 50S subunit and prevents the formation of a functional 70S initiation complex. It is currently being used in people to treat vancomycin resistant gram-positive infections caused by enterococci and streptococci, as well as MRSA. Linezolid is absorbed orally and has been used successfully in dogs and cats at NCSU for the treatment of MRSA at a dose of 10 mg/kg PO or IV q8-12h. Linezolid is available in 400 and 600 mg tablets ($53 per tablet!), oral suspension, and injection.

Glyclyclines

Tigecycline is the first in a new class of glycylcyclines with activity against susceptible or multidrug-resistant staphylococci, enterococci or streptococci as well as most Enterobacteriaceae and anaerobic pathogens. It has recently been approved for use in humans for the treatment of skin, soft-tissue and intra-abdominal infections. Its use in veterinary medicine has not been reported.

Streptogramins

Quinupristin and dalfopristin are straptogramin antibiotics that are used in human medicine for cases of MRSA and VRE. They work in concert at the level of the bacterial ribosome to inhibit protein synthesis. Dalfopristin works in the early phases of protein synthesis, while quinupristin works on the later phases. Veterinary use has not been reported.

Daptomycin

Daptomycin represents a class of antibiotics known as the peptolides. It is active against a wide variety of bacteria, including gram-positives, gram-negatives, MRSA and VRE. The mechanism of action involves the disruption of amino acid transport by the cell membrane and alterations of the cytoplasmic membrane potential. The bactericidal activity of daptomycin is concentration dependent and is influenced by pH and ionized calcium concentrations.