Antimicrobials in practice part 1: pharmacokinetics and pharmacodynamic principles (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 Failures

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. 

Definitions

• Minimum Inhibitory Concentration (MIC):  the lowest concentration of drug that inhibits visible bacterial growth.  Typically, the MIC90 should be used, which indicates the concentration for inhibiting 90% of the bacteria.

• Minimum Bactericidal Concentration (MBC):  The lowest concentration of a drug that kills 99.9% of bacteria.

• Cmax:MIC:  ratio of the maximum plasma concentration (Cmax) to the MIC.

• T>MIC:  Duration (hrs) that the plasma concentration is above the MIC during a 24 hour period.

• AUC:MIC:  ratio of the area under the plasma concentration versus time curve (AUC) to the MIC.  The AUC is limited to a 24 hour period.

• Bactericidal:  the antibiotic can be safely given to achieve concentrations at or above the MBC.  The MBC:MIC ratio for bactericidal drugs is small.

• Bacteriostatic:  the antibiotic only reaches concentrations at or above the MIC. The MBC:MIC ratio for bacteriostatic drugs is large.  Bacteriostatic drugs should be avoided in patients that are immunocompromised or receiving immunosuppressive drugs, or in cases of bacterial endocarditis and surgical prophylaxis.

• Post-antibiotic effect (PAE):  persistent drug effect on bacteria after the drug concentrations have declined below the MIC.  Mechanisms of PAE may include decreased virulence of the bacteria, development of abnormal cell wall or septum needed for bacteria to replicate, decreased adherence of bacteria to mucosal surfaces, increased susceptibility to host defenses (post-antibiotic leukocyte effect).

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 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

Strategies for Therapy Based on Infection Site

Rational first-line antibiotic choices can be made based on the site of the infection and the likelihood of a bacterium causing that infection. This information should not be used as an alternative to culture and sensitivity, but rather as a guide to choosing an appropriate initial antibiotic to begin treatment while waiting on the culture results.

Soft Tissue and Skin Infections

Most of these infections will be extracellular, therefore the most important characteristic of a drug that may limit penetration to the site of the infection would be plasma protein binding.  The most likely bacterium to be cultured from superficial infections is a Staph. intermedius.  Initial drug choices for superficial infections include amoxicillin-clavulanate combinations or 1st generation cephalosporins. Deep infections or otitis often culture Pseudomonas or other Enterobacteriaceae. These can be more difficult to treat, but should be susceptible to aminoglycosides.  Of the beta-lactam antibiotics, ticarcillin (+/- clavulanic acid) and ceftazidime are the best choices.  Of the fluoroquinolones, ciprofloxacin has the highest activity against Pseudomonas sp. however high doses are recommended.  Bite wound infections usually culture organisms from the oral cavity, including Pasteurella sp. and streptococci. Pasteurella organisms are susceptible to most antibiotics.

Urinary Tract Infections

Since many drugs are excreted via the kidney and concentrate in the urine, even though a drug is listed as intermediate on culture and sensitivity, it may still be effective for treating UTI.  The most common bacteria causing UTI include E. coli, and staphylococci. Most beta-lactam antibiotics will reach effective urine concentrations.  Cephalosporins and amoxicillin (+/- calvulanate) are good first choices.  If you are unsuccessful in treating the infection the first time, some reasons for failure may include inadequate duration of treatment (10-14 days for uncomplicated, >4 weeks for infections involving the kidney or the prostate is recommended), the presence of concurrent urinary or endocrine disease (particularly in cats), or prolonged corticosteroid therapy.  Local factors in the urine that may affect drug efficacy include pH, which decreases the activity of aminoglycosides and fluoroquinolones, sequestration of bacteria in calculi or epithelium, and the presence of cations which may chelate tetracyclines and fluoroquinolones. It is not recommended that drugs listed as resistant on culture and sensitivity be used for urinary tract infections.

Bone and Joint Infections

There are typically no barriers to antibiotic penetration into bone and joint fluid, as this is an extension of the extracellular fluid. Exceptions to this include joint infections with excessive purulent debris, and devitalized bone fragments.  Surgical implants can also present a problem as they may develop a biofilm that is non-vascularized and shields the bacteria from the effects of the antibiotics.  Staphylococci, E. coli, Pseudomonas and anaerobic bacteria are likely.  Good first choice antibiotics include cephalosporins, amoxicillin-calvulanate, trimethoprim-sulfa combinations, and fluoroquinolones.  Although tetracyclines reach adequate concentrations in the bone, they become chelated by the calcium in the bone matrix and become inactivated. Local or regional therapies, as well as surgical intervention, should also be used when possible.

Respiratory Tract Infections

There are relatively few barriers to drug penetration into the lungs.  Exceptions include the non-fenestrated capillaries in the alveoli, and the presence of consolidation in the lung parenchyma. Most respiratory infections are mixed, and there is a high likelihood of aerobic, as well as anaerobic bacteria, therefore the initial antibiotic choice should be broad spectrum. Combination therapy with a beta-lactam and an aminoglycoside or fluoroquinolone is reasonable in mixed infections. Special considerations include Mycoplasma, and Bordetella bronchiseptica. Mycoplasma species are often present in mixed infections, and their role in respiratory tract disease is debated.  Tetracyclines and macrolides are good choices for therapy.  If infections with B. bronchiseptica are severe enough to require treatment, a drug with good penetration into the pulmonary epithelial lining fluid should be used.  Good choices are macrolides, tetracyclines and chloramphenicol.

CNS, Ocular, Prostatic Infections

These infections can be difficult to treat and drugs must be sufficiently lipid-soluble in order to cross the protective barriers.  Drugs that penetrate into these protected sites would include tetracyclines, metronidazole, chloramphenicol, fluoroquinolones, and macrolides.  Drugs that do not penetrate well into these sites include beta-lactams (except some 3rd generation cephalosporins) and aminoglycosides.  High doses should be administered frequently to increase the chance of successful therapy.