Resurgence of colistin: a review of resistance, toxicity, pharmacodynamics, and dosing

Colistin is a polymyxin antibiotic that was discovered in the late 1940s for the treatment of gram-negative infections. After several years of clinical use, its popularity diminished because of reports of significant nephrotoxicity and neurotoxicity. Recently, the antibiotic has resurfaced as a last-line treatment option for multidrug-resistant organisms such as Pseudomonas aeruginosa, Acinetobacter baumannii, and Klebsiella pneumoniae. The need for antibiotics with coverage of these gram-negative pathogens is critical because of their high morbidity and mortality, making colistin a very important treatment option. Unfortunately, however, resistance to colistin has been documented among all three of these organisms in case reports. Although the exact mechanism causing colistin resistance has not been defined, it is hypothesized that the PmrA-PmrB and PhoP-PhoQ genetic regulatory systems may play a role. Colistin dosages must be optimized, as colistin is a last-line treatment option; in addition, suboptimal doses have been linked to the development of resistance. The lack of pharmacokinetic and pharmacodynamic studies and no universal harmonization of dose units, however, have made it difficult to derive optimal dosing regimens and specific dosing guidelines for colistin. In critically ill patients who may have multiorgan failure, renal insufficiency may alter colistin pharmacokinetics. Therefore, dosage alterations in this patient population are imperative to achieve maximal efficacy and minimal toxicity. With regard to colistin toxicity, most studies show that nephrotoxicity is reversible and less frequent than once thought, and neurotoxicity is rare. Further research is needed to fully understand the impact that the two regulatory systems have on resistance, as well as the dosages of colistin needed to inhibit and overcome these developing patterns.

Optimizing use of quinolones in the critically ill

Fluoroquinolones (FQs) are broad spectrum, concentration dependent, bactericidal antimicrobials that have been commonly utilized to treat severe nosocomial infections. FQ activity is derived from their ability to inhibit DNA gyrase and topoisomerase IV; resistance has been shown to develop in target site mutations, alterations in efflux pump systems, and incorporation of plasmids. The probability of preventing emergence of resistance and achieving maximal rates of kill are best related to the ratio of free-drug in the area under the concentration-time curve (AUC) to minimum inhibitory concentration (AUC:MIC). Major dosage adjustments for FQs are not necessary in hepatic insufficiency, accumulation of extracellular fluids, and burn patients. Appropriate dosage adjustments in renal function should be taken into consideration. FQ optimization in the critically ill is a multifactorial process that should be individualized to each patient and should take into account the MIC of the pathogen, pharmacokinetic/pharmacodynamic profile of the FQ, and the patient's pathophysiological state.

Evaluation of daptomycin, telavancin, teicoplanin, and vancomycin activity in the presence of albumin or serum

We evaluated the calculated protein binding based on arithmetic means of MIC values in the presence and absence of protein for daptomycin, telavancin, vancomycin, and teicoplanin against 5 Staphylococcus aureus isolates. The extent of protein binding was lower than expected, particularly for daptomycin and telavancin. These drugs may be more active than predicted based on unbound drug concentrations alone.

Linezolid-associated peripheral and optic neuropathy, lactic acidosis, and serotonin syndrome

Linezolid is an oxazolidinone antibacterial agent indicated for serious gram-positive infections. Only minor adverse effects were seen in phase III trials. However, more serious adverse effects were reported after commercial release, including cases of lactic acidosis, peripheral and optic neuropathy, and serotonin syndrome. Peripheral and optic neuropathy was usually seen after several months of linezolid therapy (median 5 mo), lactic acidosis after several weeks (median 6 wks), and serotonin syndrome after several days (median 4 days). Death occurred in two of seven reported cases of lactic acidosis, and three of 15 reported cases of serotonin syndrome. Improvement or complete recovery occurred in all cases of optic neuropathy, whereas complete recovery failed to occur in any patient with peripheral neuropathy. Linezolid should be discontinued immediately in patients experiencing these adverse effects. Patients receiving linezolid for more than 28 days should be monitored for signs of peripheral and optic neuropathy.

Community- and health care-associated methicillin-resistant Staphylococcus aureus: a comparison of molecular epidemiology and antimicrobial activities of various agents

The epidemiology of methicillin-resistant Staphylococcus aureus (MRSA) is changing. We determined the inhibitory and bactericidal activity of select antimicrobial agents utilizing a well-characterized group of 200 staphylococcal cassette chromosome mec (SCCmec) type IV community-associated MRSA (CAMRSA) and 50 SCCmec type II health care-associated MRSA (HAMRSA). Differences in carriage of the Panton-Valentine leukocidin (PVL) genes, agr group, and agr function in CAMRSA and HAMRSA were also examined. Pulsed-field gel electrophoresis (PFGE) patterns were determined for a subset of study strains. CAMRSA typically belonged to the USA 300 PFGE profile, were associated with high rates of PVL carriage (78%), and primarily were agr group I. Susceptibility to daptomycin, linezolid, teicoplanin, and vancomycin was 100%. In contrast, HAMRSA isolates typically belonged to the USA 100 PFGE profile, were associated with low rates of PVL carriage (8%), and primarily were agr group II. Comparing susceptibilities between the 2 types of MRSA strains, there was a 2-fold increase in MIC for daptomycin, doxycycline, teicoplanin, trimethoprim-sulfamethoxazole (TMP/SMX), and vancomycin in HAMRSA versus CAMRSA. Levofloxacin and clindamycin susceptibly decreased dramatically by 66% and 54%, respectively, against HAMRSA versus CAMRSA. With respect to agr function, 3.5% of CAMRSA and 48% of HAMRSA displayed a down-regulated agr gene cluster. The comparative bactericidal activities of daptomycin were similar to those of vancomycin and clindamycin, but were significantly greater than those of linezolid, teicoplanin, and TMP/SMX against CAMRSA at 24-h terminal end points. Further studies are warranted against a larger number of molecularly defined, geographically diverse CAMRSA to confirm these findings.

Correlation of vancomycin and daptomycin susceptibility in Staphylococcus aureus in reference to accessory gene regulator (agr) polymorphism and function

OBJECTIVES

Recently, an association between the accessory gene regulator (agr) in Staphylococcus aureus and the development of vancomycin resistance secondary to suboptimal exposure has been demonstrated. We investigated the relationship of vancomycin with and without gentamicin or rifampicin, and daptomycin in the development of resistance in agr groups I and II.

METHODS

S. aureus belonging to agr groups I and II was exposed to varying concentrations of vancomycin and daptomycin simulating an fAUC/MIC of 14-460 and 30-239, respectively, in an in vitro pharmacodynamic model.

RESULTS

Vancomycin regimens resulting in fAUC/MIC of 16.1-107.0 resulted in resistance in agr I and agr II knockout strains, whereas regimens resulting in fAUC/MIC of 16.1 resulted in emergence of resistance in agr I- and agr II-positive strains. Overall, agr-null strains demonstrated a higher likelihood of resistance and a greater change in vancomycin susceptibility. The addition of gentamicin and rifampicin to vancomycin at these same exposures prevented the emergence of resistance. At extremely low daptomycin exposures of fAUC/MIC of 22-66, an increase in MIC of 2-3-fold up to a maximum of 0.75 mg/L was observed. However, this was independent of agr group and/or function and still within the susceptible range of daptomycin.

CONCLUSIONS

The combination of vancomycin with either rifampicin or gentamicin prevented the emergence of vancomycin resistance in agr I and II S. aureus isolates. Changes in daptomycin susceptibility were independent of agr group and function. The association between agr and vancomycin resistance in S. aureus requires further investigation.

Evaluation of accessory gene regulator (agr) group and function in the proclivity towards vancomycin intermediate resistance in Staphylococcus aureus

Simulated therapeutic vancomycin exposures were evaluated against agr wild-type and knockout Staphylococcus aureus groups I, II, III, and IV using an in vitro pharmacodynamic model. All agr groups developed intermediate resistance to vancomycin after subtherapeutic exposure. The free unbound fraction of the area under the concentration-time curve (fAUC/MIC) required to suppress resistance was fourfold higher (P < 0.001) in agr dysfunctional strains (112 to 169) than that in parent wild-type strains (28).

Etest synergy testing of clinical isolates of Staphylococcus aureus demonstrating heterogeneous resistance to vancomycin

In search for potential synergistic antimicrobial combinations against Staphylococcus aureus isolates, which display heterogeneous resistance to vancomycin, we evaluated the activities of 21 various combinations involving ampicillin/sulbactam, daptomycin, gentamicin, linezolid, quinupristin/dalfopristin, rifampin, and vancomycin by Etest and time-kill methods. A number of combinations demonstrated either synergistic or additive effects against hGISA SA118 and GISA SA179. Agreement between the Etest and time-kill methods for detecting antimicrobial synergy ranged from 66.6% to 71.4%. The Etest method appears promising, and further investigations are warranted.

Short-course gentamicin in combination with daptomycin or vancomycin against Staphylococcus aureus in an in vitro pharmacodynamic model with simulated endocardial vegetations

The ability to maximize bactericidal activity while minimizing toxicity is a therapeutic goal in the treatment of infective endocarditis. We evaluated the impact of administering short-course regimens of gentamicin in combination with daptomycin or vancomycin against one methicillin-susceptible (MSSA 1199) and one methicillin-resistant (MRSA 494) Staphylococcus aureus isolate using an in vitro pharmacodynamic model with simulated endocardial vegetations over 96 h. Human therapeutic dosing regimens for daptomycin (6 and 8 mg/kg of body weight), vancomycin, and gentamicin were simulated. Short-course combination regimens involving gentamicin were administered either as a single 5-mg/kg dose or three 1-mg/kg doses for only the first 24 h and compared to the regimens administered for the full 96-h duration. For all experiments, physiologic conditions of albumin, calcium, and pH were simulated. Both regimens of daptomycin achieved 99.9% kill by 32 h and maintained bactericidal activity against both isolates, which was significantly different from vancomycin, which displayed bacteriostatic activity (P < 0.05). The effects of all short-course regimens of gentamicin were equal to those of the full-duration regimens in combination with daptomycin. Adding three doses of gentamicin (1 mg/kg) to daptomycin resulted in enhancement and bactericidal activity at 24 h against both MRSA and MSSA. The addition of a single dose of gentamicin (5 mg/kg) enhanced or improved the activity of daptomycin and resulted in early bactericidal activity at 4 h against both isolates. The addition of three doses of gentamicin (1 mg/kg) did not improve the activity of vancomycin. However, the addition of a single 5-mg/kg dose of gentamicin to vancomycin resulted in early enhancement at 4 h and 99.9% kill at 32 h for MRSA. These results suggest that a single high dose of gentamicin in combination with daptomycin or vancomycin may be of utility to maximize synergistic and bactericidal activity and minimize toxicity. Further investigation is warranted.