Reducing the risk of antimicrobial resistance: Building the right dosing regimen (Proceedings)

Dosing regimens for antimicrobials should be related to MIC. However, simply achieving the MIC in the patient is not likely to be sufficient for a variety of reasons.

Bactericidal versus bacteriostatic drugs:. The term "bactericidal" is so often abused that the distinction from bacteriostatic should be underemphasized. Although it is appropriate for clinicians to reach for a drug that is "cidal" rather than "static", it is not appropriate to assume that the ability of that drug to kill rather than simply inhibit an organisms will enhance therapeutic efficacy. This may be true, but only if concentrations of the drug achieved at the site of infection are sufficient to kill the microbe. The term "bactericidal" is an in vitro definition and is based on killing rates (eg, 99.9% reduction in bacterial inoculum within a 24 hr period of exposure) as well as the proximity of the minimum bactericidal concentration (MBC) of a drug to the MIC. The MBC is determined based on kill curves, or following tube dilution procedures: tubes with no observable growth are inoculated on agar gel. If no organism grows on the agar, the organisms were killed in the test tube. The tube with the lowest concentration of drug that yields no growth on the agar gel contains the MBC of drug. For drugs considered "bactericidal", the MBC is within one tube dilution of the MIC, meaning, the organisms were not simply inhibited, but rather, were killed. For "bacteriostatic" drugs, growth on the agar plate will occur for several tube dilutions above the MIC, indicating that organisms were not killed. However, bacteriostatic drugs are capable of killing (eg, some organisms are exquisitely sensitive to the effects of selected drugs; some "static" drugs are accumulated to concentrations that are likely to be cidal [eg, macrolides and lincosamides in phagocytes; urine concentration). However, killing concentrations are generally more likely to be achieved for a cidal drugs compared to a static concentration. On the other hand, a "cidal" drug may not kill if concentrations are not sufficient, or if conditions preclude its actions (eg, slow growth in an anerobic environment; combination with growth inhibitors). Thus, cidal effects will occur only if adequate concentrations (ie, MIC/MBC) are achieved at the site. The bactericidal nature of a drug often reflects its mechanism of action. Drugs which target ribosomes (eg, tetracyclines, macrolides, lincosamides, chloramphenical) often simply inhibit the growth of the organism, and, because a much higher drug concentration is necessary to kill the organism, in vitro, the MIC is distant from the MBC. Clinically, host defenses must eradicate the infection following treatment with these drugs unless exceptionally high concentrations (ie, the MBC) of these drugs are achieved in tissues. An exception is made for aminoglycosides, whose ribosomal inhibition is so effective that the organism dies. Drugs which target cell walls (beta lactams including penicillins and cephalosporins; vancomycin), cell membranes (bacitracin, polymixin and colistin), and DNA (enrofloxacin, metronidazole), RNA (rifampin) are defined in vitro as bactericidal. Combinations of static drugs can often result in cidal actions. For example, sulfonamides (which target folic acid synthesis) are static, but when used in combination with diaminopyramidines (eg, trimethoprim), the combination is defined in vitro as cidal. Attaining bactericidal concentrations of an antimicrobial is critical for those infections for which host killing is likely to be impaired. These include but are not limited to infections in immune compromised animals (eg, viral infections [parvovirus, panleukopenia, FIV, FeLV), patients receiving glucocorticoids), or in systems characterized by derangements in local immunity (ie, CNS infection for which an marked inflammatory response can be life threatening; osteomyelitis; peritonitis, bacteremia/sepsis, many chronic infections).

Relationship between MIC, Plasma and tissue Drug Concentrations. The bridging of pharmacodynamic (susceptibility) data with pharmacokinetic data can begin by comparing the MIC established by in vitro testing and the concentrations of drug achieved in the patient at the site of infection when the drug is administered at the labeled dose. A number of investigators have examined several parameters that combine pharmacokinetic (PK) and pharmacodynamic (PD) indices as predictors of clinical antimicrobial efficacy. Among those most commonly sited as potentially useful are the ratio of Cmax (peak PDC) to MIC, defined here as IQ; the area under the inhibitory curve (AUIC; the ratio of area under the PDC versus time curve to the MIC of the infecting organism); and the percent time that PDC are above the MIC [T> MIC]. Based on these relationships, two generally categories of drugs have been described.

Time versus Concentration Dependent Drugs. The relationship between MIC and the magnitude and time course of PDC allows drugs to be categorized as to either concentration-dependent (sometimes referred to as dose dependent) or time-dependent; these definitions are supported by primarily by in vitro but also in vivo studies. Concentration dependent drugs, best represented by the fluoroquinolones and aminoglycosides, are characterized by efficacy best predicted by the magnitude of plasma drug concentration (Cmax ) compared to the MIC of the infecting organism (ie, the inhibitory quotient or IQ). For such drugs, the magnitude of the IQ generally should be 8-10 but ideally is higher for more difficult infections (eg, Pseudomonas aeruginosa, or infections caused by multiple organisms. The duration that PDC is above the MIC is not as important as is the IQ; in fact, efficacy may be enhanced by a drug-free period (i.e., a long interval between doses). For concentration dependent drugs, a dose that is too low is particularly detrimental. In a mouse model of E. coli peritonitis, the antibacterial efficacy of ciprofloxacin, but not imipenem, was improved by doubling the dose. As such, concentration-dependent drugs generally can be administered at longer intervals, ie, once a day. However, for the fluorinated quinolones (FQ), efficacy may also be time related. Efficacy of FQ also is predicted in vitro by AUIC: a ratio of < 60 renders the drugs bacteriostatic, whereas ≥ 125 results in (slow) killing but also decreases the risk of resistance and ≥ 250 causing more rapid bacterial killing. Thus, resistance might be less likely to develop for FQ characterized by longer half-lives (or for ENR, by the production of an active metabolite). Twice daily administration of an FQ might be indicated for organisms already characterized by low level resistance (see MPC below); however, the once daily dose should be given twice daily in such situations. In contrast to concentration dependent drugs, efficacy of time-dependent drugs (eg, β-lactams) is enhanced if PDC remain above the MIC for the majority (60 to 70%) of the dosing interval; efficacy is best predicted by percent time that PDC are above the MIC [T> MIC]. For such drugs, simply achieving the MIC (IQ = 1) is insufficient because PDC (and certainly tissue concentrations) fall below the MIC immediately. With time-dependent drugs, increasing the IQ also may be beneficial, even though although efficacy clearly is related to T>MIC, simply because increasing the dose will increase time above the MIC. Since drug concentrations decrease by 50% every drug half-life, an IQ of two will result in PDC below the MIC in one half-life. However, the relationship between dose increase and time above the MIC (in terms of half-life) is logarithmic, not linear. To increase the dosing interval by two half-lives, the same dose (designed to simply achieve the MIC) would need to be quadrupled; to add three half-lives onto the dosing interval, the dose would have to be increased 8 fold. For example, for a Staphylococcus intermedius with an MIC for axoxcillin-clavulanic acid of 4 mcg/ml, assume PDC achieve the breakpoint MIC (32 mcg/ml) when a recommended dose is administered (13 mg/kg). In one half-life (1.5 hr), PDC have dropped to 16; in 3 hr, to 8 and in 4.5 hr, to 4 mcg/ml. Another 25% of the dosing interval (1.5 hr) can elapse before the animal should be redosed. If the dose is doubled, this can be extended to 8 hours; the dose would have to be quadrupled to extend to 9 hrs, and increased 8 fold to reach the standard 12 hrs. This is for an organism with a relatively low MIC; the problem is markedly compounded for organisms still considered "S" but with MIC that approach the breakpoint MIC. Adding a 3rd or 4th dose is a much more cost effective method of increasing efficacy (compared to increasing the dose) of time dependent drugs; as such, efficacy of time dependent drugs may be best facilitated by both a dose increase (to assure PDC are sufficiently higher than MIC) and an interval decrease increase. Constant rate infusion or slow release products might be ideal for time dependent drugs, assuming drug release (from slow release products) is sufficiently fast to allow Cmax to surpass the MIC. Efficacy also should be enhanced for time dependent drugs which persist in selected tissues (again, assuming the MIC is surpassed). An example would be drugs which accumulate in active (unbound) form in tissues (ie macrolides) or drugs that accumulate in phagocytes).

Mean pharmacokinetic parameters following oral administration of marbofloxacin tablets to aduilt beagle dogs and MIC values of marbofloxacin against bacteria isolated

Postantibiotic Effect :The post antibiotic effect (PAE) describes the continued inhibition of microbial growth after a short exposure of the organisms to the drug. The impact of the PAE on antimicrobial efficacy can be profound, particularly for concentration-dependent drugs. It is the PAE that allows some of these drugs to be administered at long intervals. The PAE may be absent for some organisms or some patients (e.g., some immunocompromised patients). In general, concentration- dependent drugs appear to exhibit longer PAE; further, the duration of the PAE may vary with the magnitude of the peak PDC (ie, longer with higher PDC) and is enhanced by combination antimicrobial therapy. PAEs vary with each drug and each organism.

Penetrating the site of infection: Interpretation of C&S is based on the assumption that the MIC should be achieved in plasma. Basing MIC interpretation on plasma drug concentrations (PDC) might result in over or under estimation of drug efficacy. For tissues which concentrate the drug (or if the drug can be applied topically), and for drugs which can be concentrated by phagocytes and thus transported to the site of infection, concentrations may markedly exceed PDC, resulting in underestimation of efficacy. In such circumstances, a drug noted as "I", or,under the appropriate circumstances (eg, topical therapy) a drug designated as "R" might actually be effective. The more likely scenario is overestimating drug efficacy because of failure to achieve the anticipated PDC at the site of infection. Most infections occur in extracellular tissues; because most tissues are characterized by fenestrated capillaries, antimicrobials can move unimpeded from plasma into ECF . However, whether or not concentrations in ECF are equivalent to that in plasma is not clear. Much of the data for water soluble drugs (volume of distribution [Vd] generally ≤ 0.3 L/kg) suggests antimicrobial concentrations may be 30% or less of PDC in some tissues, although the method of fluid collection used among the studies (eg, tissue homogenate versus tissue cages versus ICF collection) may impact the results. However, studies do suggest for some tissues, doses may need to be increased several fold to target effective concentrations. Certainly tissues characterized by non-fenestrated capillaries (including the prostate, eye, brain, testicles, cartilage) will be difficult to penetrate. In humans, recommended doses of beta-lactams drugs (water soluble) are increased 5 to 10 fold when treating infections of the central nervous system. For difficult to penetrate tissues, intracellular infections, or perhaps infections characterized by marked inflammatory debris, lipid soluble drugs (Vd generally ≥ 0.6 l/kg) may be more likely to distribute to sites of infection at concentrations necessary to kill organisms. However, note that a Vd > 0.6 L/kg does not necessarily indicate better penetration of a drug; Vd is a theoretical compartment that describes how much tissue dilutes a drug, but provides no indication as to where the drug distributions. In the presence of marked inflammation, use of a drug that accumulates in phagocytes (eg FQs, macrolides, lincosamides) is likely to increase distribution of the drug to the site.

Host Factors: The impact of host response to infection can be profound. Problems contributing to therapeutic failure include immunocompromise (design a dosing regimen that will assure bactericidal concentrations of the chosen drug reach the site of infection), inflammatory response (debride or otherwise appropriate clean/drain accessible infections, select a drug that distributes into tissues well and ideally accumulates in phagocytes and increase the dose appropriately). Even tissues traditionally considered "well perfused" might be of concern. For example, drugs do not penetrate bronchial secretions well, despite the fact that the lungs are well perfused. Amoxicillin is often used to treat respiratory tract infections. Yet, only 3% of the amoxicillin that is in plasma is distributed to bronchial secretions. Theoretically, one must dose amoxicillin 30X the recommended dose to achieve targeted PDC in bronchial secretions. Most water soluble drugs (beta-lactams and aminoglycosides) reach only 20 to 25% of PDC in bronchial secretions whereas over 50% of lipid soluble drugs reach bronchial secretions. Dosing adjustments also are necessary for those infections that are intracellular or complicated by host response to infection.

Microbial Factors: Microbial resistance is addressed in another manuscript in this same proceedings. Materials released from microbes facilitate invasion, impair cellular phagocytosis, and damage host tissues. Most staphylococci associated with canine pyoderma produce "slime," a material that facilitates bacterial adhesion to cells. Soluble mediators released by organisms (hemolysin, epidermolytic toxin, leukocidin) may damage host tissues or alter host response. Staphylococcal organisms contain protein A, which impairs antibody response, activates complement, and causes chemotaxis. Nocardia stimulates the formation of calcium-containing "sulfur granules" that impair drug penetration to the organisms Pseudomonas and other gram-negative organisms produce a glycocalix, or biofilm, that protects the organism.. Biofilms are microcolonies of pathogenic and host microbes embedded in a polysaccharide matrix ("slime" or "glycocalyx") produced by the bacteria; dental plaque is the prototypic example. Normal microflora of the skin or mucous membranes in the biofilm are lost with shedding of the skin surface or by the excretion of mucus; new cells and mucus are rapidly colonized by biofilm forming bacteria. Translocation of the normal microflora to otherwise sterile tissues (which can be facilitated by the presence of foreign bodies) may lead to acute infections (again, associated with biofilm) and accompanying inflammatory response. Persistent, chronic bacterial infections may reflect biofilm producing bacteria; persistent inflammation associated with immune complexes contributes to clinical signs. Unfortunately, bacteria growing in biofilms more easily resist antimicrobial killing and immune defenses of the host. In addition to debridement or other methods of cleansing should facilitate antimicrobial penetration; dose modification (increase) may be indicated to compensate for debris.

Drug Factors: In addition to drug characteristics previously addressed (eg, concentration versus time dependent, static versus cidal, drug distribution), pharmaceutical manufacturers have been able to manipulate antimicrobial drugs in a variety of ways such that efficacy and thus bacterial killing is enhanced such that resistance might be reduced. For example, efficacy has been decreased by synthesizing smaller molecules that can penetrate smaller porins (e.g., the extended spectrum penicillins ticarcillin and piperacillin); "protecting" the antibiotic (e.g., with clavulanic acid, which "draws" the attention of the β-lactamase away from the penicillin); modifying the compound so that it is more difficult to destroy (e.g., amikacin, which is a larger and more difficult to reach molecule than gentamicin); and developing lipid-soluble compounds that are more able to achieve effective concentrations at the site of infection (e.g., doxycycline compared with other tetracyclines). However, with each innovative approach to reducing resistance, microbes are able to circumvent the drug in a disconcertingly short time. The use of pro-biotics or pre-biotics to minimize emergence of resistance in the gastrointestinal tract is controversial and requires additional scientific evidence.

Designing the Dosing Regimen. Examining the relationship between MIC and PDC more closely emphasizes the importance of selecting an adequate dose. If one assumes the recommended dose is designed to achieve the MICBP of the drug ( a reasonable assumption as the MICBP is based, in part, on the Cmax ), clearly this concentration will be insufficient for concentration-dependent drugs (for which PDC should be 8-10 X the MIC) unless the MIC of the infecting organisms is approximately 1/10th of the MICBP . For example, whereas a dose of enrofloxacin at 5 mg/kg (which achieved PDC of 1 μg/ml) might be sufficient to treat an E coli with an MIC of 0.06 μg/ml (0.06 μg/ml * 8 = 0.48 μg/ml which is < 1 μg/ml), a dose of even 20 mg/kg, which achieves PDC of approximately 4 μg/ml may not be sufficient to treat a Pseudomonas aeruginosa organisms with an MIC of 1 (g/ml even though a C&S report might indicate "S" (1 μg/ml * 8 = 8 μg/ml). Organisms whose MIC is approaching the MICBP may reflect organisms that have undergone the first step mutation leading to low level resistance. Achieving an AUIC necessary to kill as well as prevent resistance (ie, > 125) is paramount to therapeutic efficacy, particularly in patients afflicted with infections in immuncompromised states or sites. Administering the same dose twice daily may be important even for concentration-dependent drugs for treatment of first step mutants. Time dependent drugs do not require 8-10 * the MIC for efficacy. However, they do require PDC above the MIC for most of the dosing interval. Yet, if the dose is designed to simply achieve the MICBP of the drug, for organisms whose MIC is close to the MICBP , PDC will rapidly drop below the MIC. For example, if a Staphylococcus aureus has an MIC of 8 μg/ml for a beta-lactam, with an MICBP of 32 μg/ml, only two half-lives of the drug can elapse before the PDC has dipped below the MIC (PDC = 16 μg/ml after the first half-life, and 8 μg/ml after the second). Most beta-lactams have half-lives of 2 hours or less, leaving 4 hours between dosing intervals. Doubling the dose of the drug will add one more half-life to the dosing interval. Thus, for time dependent drugs, both the dose (increased) and the interval (shortened) may need to be altered for organisms whose MIC is close to the MICBP. This approach is based on drug concentrations; modifications become more important and potentially greater for infections further complicated by distribution site, etc. Effective antimicrobial therapy increasingly will require doses that are higher than recommended. Duration of therapy also may need to be be modified: longer therapy is likely to support the advent of resistance; high doses for shorter periods of time (3 to 4 days) may become the norm except for infections caused by slow growing organisms.

Impact of FQ on retinal degeneration in cats

Miscellaneous dosing considerations: Generic Augmentin® (human Clavamox®) is now available. However, the ratio of clavulanic acid to amoxicillin varies among the human tablets and solution, but not the small animal versions. The variability reflects an attempt to minimize vomiting. However, it is not clear if the ratio also impacts efficacy. The 400 mg human capsule has the same ratio as the veterinary ratio. Metronidazole benzoate might be compounded for its palatability in cats. However, the benzoate salt weights more than the hydrochloride salt; accordingly, metronidazole benzoate should be dosed at 1.6 times metronidazole hydrochloride (ie, 16 mg/kg versus 10 mg/kg). Ciprofloxacin oral bioavailability in dogs is 40 to 60% and in cats, 0-20%. Although ciprofloxacin is more potent toward Gram negative organisms, the dose should nonetheless be increased two fold compared to enrofloxacin, and 3 fold for Gram positive. Oral ciprofloxacin should not be used in cats. For other fluoroquinolones in cats, marbofloxacin is among the safest in regards to retinal degeneration.

Drugs, vd, half-life, and doses

In conclusion, increasingly, rationale use of antimicrobial therapy should focus on short term therapy with concentrations sufficiently high to kill the infecting microbes, thus assuring that mutation to resistant organisms does not occur. Design of doses are most appropriately based on culture and susceptibility data that indicates the MIC; the closer the MIC is to the breakpoint MIC, the more important increasing doses (of concentration dependent drugs) and shortening the interval of time dependent drugs. Combination therapy is another method whereby the advent of resistance can be minimized.

Drugs, vd, half-life, and doses (continued)