Control of a sulfadoxine/trimethoprim combination in the competition horse: Elimination, metabolism and detection following an intravenous administration.

The combination of sulfadoxine (SDO) with trimethoprim (TMP) is widely used in veterinarian medicine. The aim of the present study was to compare excretion profiles and detection time windows of SDO and TMP in plasma and urine by means of a validated quantitative method. Eight horses received a single intravenous (i.v.) dose of 2.7 mg TMP and 13.4 mg SDO per kg bodyweight. Plasma and urine samples were collected up to 15 and 70 days post-administration, respectively. While urine samples underwent an enzymatic hydrolysis, plasma samples were proteolysed before further analysis. After solid-phase extraction, samples were analysed by liquid chromatography/electrospray ionisation tandem mass spectrometry in positive ionisation mode. The applied multiple reaction monitoring (MRM) method allowed the detection of SDO and TMP with a lower limit of detection of 0.03 ng/mL in plasma and 0.2 (SDO) and 0.4 ng/mL (TMP) in urine, respectively. In the present study, detection times for SDO were 15 days in plasma and 49 days in urine, respectively. TMP was detected for up to 7 days in plasma and up to 50 days in urine, respectively. The detection via the TMP metabolite 3-desmethyl-trimethoprim was possible for 70 days in urine. Detection times of the other confirmed metabolites N4 -acetylated sulfadoxine, hydroxytrimethoprim, trimethoprim-1-oxide and trimethoprim-3-oxide were significantly lower. In order to postulate reasonable screening limits (SLs) to control specific withdrawal times, a Monte Carlo simulation was performed for SDO. The proposed SL of 10 ng/mL SDO in blood and 300 ng/mL urine corresponds to a detection time of 4 days.

Kinetic disposition of diazepam and its metabolites after intravenous administration of diazepam in the horse: Relevance for doping control.

In horses, the benzodiazepine diazepam (DIA) is used as sedative for pre-medication or as an anxiolytic to facilitate horse examinations. As the sedative effects can also be abused for doping purposes, DIA is prohibited in equine sports. DIA is extensively metabolized to several active metabolites such as nordazepam, temazepam and oxazepam (OXA). For veterinarians, taking into account the detection times of DIA and its active metabolites is needed for minimizing the risk of an anti-doping rule violation. Therefore, a pharmacokinetic study on 6 horses was conducted using a single intravenous (IV) dose of 0.2 mg/kg DIA Plasma and urine samples were collected at specified intervals until 16 and 26 days post-administration, respectively. Samples were analysed by a sensitive liquid chromatography-electrospray ionization/tandem mass spectrometry method. DIA showed a triphasic elimination pattern in the horse. The mean plasma clearance of DIA was 5.9 ml/min/kg, and the plasma elimination half-life in the terminal phase was 19.9 h. Applying the Toutain model approach, an effective plasma concentration of DIA was estimated at 24 ng/ml, and irrelevant plasma concentration (IPC) and irrelevant urine concentration (IUC) were computed to 0.047 and 0.1 ng/ml, respectively. The detection time according to the European Horserace Scientific Liaison Committee (EHSLC), that is the time for which observed DIA plasma concentrations of all investigated horses were below the IPC was 10 days. Using Monte Carlo Simulations, it was estimated that concentrations of DIA in plasma would fall below the IPC 18 days after the DIA administration for 90% of horses. However, in the present study, a single administration of DIA could be detected for 24 days in urine via the presence of OXA, its dominant metabolite.

Determination of the pharmacokinetic-pharmacodynamic cut-off values of marbofloxacin in horses to support the establishment of a clinical breakpoint for antimicrobial susceptibility testing.

BACKGROUND: Marbofloxacin (MBX), a fluoroquinolone (FQ), is considered as a critical antibiotic requiring antimicrobial susceptibility testing (AST) for prudent use. No clinical breakpoint (CBP) currently exists to interpret the results of such tests in horses. OBJECTIVES: To compute PK/PD cut-offs (PK/PDCO ) that is one of the three minimum inhibitory concentrations (MICs) considered establishing a CBP for antimicrobial susceptibility test interpretation. STUDY DESIGN: A meta-analysis conducted by combining five sets of previously published pharmacokinetic data, obtained in clinical and nonclinical settings. METHODS: Horses (n = 131) received MBX intravenously at doses of either 2 or 10 mg/kg BW. They were richly sampled (five or six samples per horse). A population model was built to generate a virtual population of 5000 MBX disposition curves by Monte Carlo simulations (MCS) over 24 hours. The selected PK/PD index was the ratio of Area Under the free plasma concentration-time Curve divided by the MIC (fAUC/MIC). The PK/PDCO , which is the highest MIC for which 90% of horses can achieve an a priori selected critical value for the numerical value of the PK/PD index, was established for Gram-positive and Gram-negative bacteria for a dose of 2 mg/kg. RESULTS: The PK/PDCO of MBX in horses was 0.125 mg/L for Gram-positive pathogens and 0.0625 mg/L for Gram-negative pathogens. MBX MICs determined by broth microdilution for 54 Escherichia coli and 189 Streptococcus equi isolates are reported. MAIN LIMITATION: No clinical data are taken into account in the determination of a PK/PDco . CONCLUSION: The computed PK/PDco predicts that MBX may be efficacious in horses to treat infections associated with Enterobacteriaceae but unlikely to those involving Staphylococcus aureus or Streptococcus equi.

Comparison of in vitro static and dynamic assays to evaluate the efficacy of an antimicrobial drug combination against Staphylococcus aureus.

An easily implementable strategy to reduce treatment failures in severe bacterial infections is to combine already available antibiotics. However, most in vitro combination assays are performed by exposing standard bacterial inocula to constant concentrations of antibiotics over less than 24h, which can be poorly representative of clinical situations. The aim of this study was to assess the ability of static and dynamic in vitro Time-Kill Studies (TKS) to identify the potential benefits of an antibiotic combination (here, amikacin and vancomycin) on two different inoculum sizes of two S. aureus strains. In the static TKS (sTKS), performed by exposing both strains over 24h to constant antibiotic concentrations, the activity of the two drugs combined was not significantly different the better drug used alone. However, the dynamic TKS (dTKS) performed over 5 days by exposing one strain to fluctuating concentrations representative of those observed in patients showed that, with the large inoculum, the activities of the drugs, used alone or in combination, significantly differed over time. Vancomycin did not kill bacteria, amikacin led to bacterial regrowth whereas the combination progressively decreased the bacterial load. Thus, dTKS revealed an enhanced effect of the combination on a large inoculum not observed in sTKS. The discrepancy between the sTKS and dTKS results highlights that the assessment of the efficacy of a combination for severe infections associated with a high bacterial load could be demanding. These situations probably require the implementation of dynamic assays over the entire expected treatment duration rather than the sole static assays performed with steady drug concentrations over 24h.

Differential Activity of the Combination of Vancomycin and Amikacin on Planktonic vs. Biofilm-Growing Staphylococcus aureus Bacteria in a Hollow Fiber Infection Model.

Combining currently available antibiotics to optimize their use is a promising strategy to reduce treatment failures against biofilm-associated infections. Nevertheless, most assays of such combinations have been performed in vitro on planktonic bacteria exposed to constant concentrations of antibiotics over only 24 h and the synergistic effects obtained under these conditions do not necessarily predict the behavior of chronic clinical infections associated with biofilms. To improve the predictivity of in vitro combination assays for bacterial biofilms, we first adapted a previously described Hollow-fiber (HF) infection model by allowing a Staphylococcus aureus biofilm to form before drug exposure. We then mimicked different concentration profiles of amikacin and vancomycin, similar to the free plasma concentration profiles that would be observed in patients treated daily over 5 days. We assessed the ability of the two drugs, alone or in combination, to reduce planktonic and biofilm-embedded bacterial populations, and to prevent the selection of resistance within these populations. Although neither amikacin nor vancomycin exhibited any bactericidal activity on S. aureus in monotherapy, the combination had a synergistic effect and significantly reduced the planktonic bacterial population by -3.0 to -6.0 log10 CFU/mL. In parallel, no obvious advantage of the combination, as compared to amikacin alone, was demonstrated on biofilm-embedded bacteria for which the addition of vancomycin to amikacin only conferred a further maximum reduction of 0.3 log10 CFU/mL. No resistance to vancomycin was ever found whereas a few bacteria less-susceptible to amikacin were systematically detected before treatment. These resistant bacteria, which were rapidly amplified by exposure to amikacin alone, could be maintained at a low level in the biofilm population and even suppressed in the planktonic population by adding vancomycin. In conclusion, by adapting the HF model, we were able to demonstrate the different bactericidal activities of the vancomycin and amikacin combination on planktonic and biofilm-embedded bacterial populations, suggesting that, for biofilm-associated infections, the efficacy of this combination would not be much greater than with amikacin monotherapy. However, adding vancomycin could reduce possible resistance to amikacin and provide a relevant strategy to prevent the selection of antibiotic-resistant bacteria during treatments.