Microdialysis as a tool to determine the local tissue concentration of dicloxacillin in man.

AIMS: The most common pathogen to cause postoperative infections in Denmark is Staphylococcus aureus. Despite using prophylactic antibiotics, infections are still seen. Whether the tissue concentration is above the minimal inhibitory concentration (MIC) for the pathogen is unknown. Thus, the concentration of dicloxacillin in muscle and adipose tissue was measured after intravenous administration, in healthy men. METHODS: MIC for dicloxacillin against S. aureus was determined using the broth macrodilution method. A microdialysis (MD) catheter was placed in the subcutaneous tissue of the abdomen and in the lateral vastus muscle of the thigh of six healthy male volunteers. They were given 2 g dicloxacillin intravenously. Samples from blood and MD fluid were collected. The unbound dicloxacillin was isolated from plasma. Samples were analysed with high performance liquid chromatography (HPLC). RESULTS: The maximum concentration was reached in muscle tissue after 0.5 h and in adipose tissue after 0.8 h. AUC0-6h for the dicloxacillin concentration in adipose tissue was significantly lower when compared to the unbound dicloxacillin concentration in plasma. The dicloxacillin concentration was above the MIC for sensitive S. aureus for a minimum of 2.3 h and a median of 4.1 h in muscle tissue and a minimum of 1.8 h and a median of 3.2 h in adipose tissue. CONCLUSIONS: The unbound dicloxacillin concentration in adipose and muscle tissue remained above the MIC for sensitive S. aureus, for a period sufficient for many orthopaedic procedures. Whether this is true in patients with compromised circulation remains to be investigated.

Determining optimal dosing regimen of oral administration of dicloxacillin using Monte Carlo simulation.

BACKGROUND: Dicloxacillin, a semisynthetic isoxazolyl penicillin, exhibits antimicrobial activity against a wide variety of Gram-positive bacteria, as well as stability against penicillinases and low level of toxicity. The objective of this study was to obtain optimal dosing regimen of oral administration of dicloxacillin by analyzing the pharmacokinetic (PK) index in healthy volunteers and in vitro antibacterial activity by using Monte Carlo simulation. MATERIALS AND METHODS: A total of 867 clinical isolates from community-onset infections were collected from 31 secondary hospitals in People's Republic of China. The minimum inhibitory concentration (MIC) values of dicloxacillin were determined by the agar dilution method. Based on the MICs and the PK parameters of different dosage regimens, Monte Carlo simulation was performed to simulate the PK/pharmacodynamic indices of 250 mg once-daily (qd), 500 mg qd, 1,000 mg qd, 2,000 mg qd, 250 mg every 6 hours (q6h), and 500 mg q6h, respectively. The probability of target attainment was estimated at each MIC value, and the cumulative fraction of response (CFR) was calculated to evaluate the efficacy of these regimens. RESULTS: Dicloxacillin showed poor antibacterial activity against Haemophilus influenzae, Moraxella catarrhalis, and Streptococcus pneumoniae. Resistance to dicloxacillin was observed in 7.5% of coagulase-negative Staphylococcus (CNS) isolates and 9.2% of other Streptococcus isolates, whereas 1.5% of methicillin-sensitive Staphylococcus aureus (MSSA) was resistant to dicloxacillin. Multiple-dose regimens could obtain higher CFR than single-dose regimens against H. influenza and S. pneumoniae. However, all dosing regimens against MSSA achieved CFR ≥$90%. Meanwhile, dosing regimen of 2,000 mg qd, 250 mg q6h, and 500 mg q6h could achieve >90% of CFR for CNS. For other Streptococcus isolates, multiple-dose regimens achieved CFR ≥90%. CONCLUSION: Dicloxacillin has a significant antibacterial activity against MSSA, CNS, and other Streptococcus isolates. The simulation results suggest that dicloxacillin 250 mg q6h and 500 mg q6h dosing regimens may be recommended for clinical applications, especially for community-onset infections.

Safety and pharmacokinetics of dicloxacillin in healthy Chinese volunteers following single and multiple oral doses.

BACKGROUND: Dicloxacillin, a semisynthetic isoxazolyl penicillin antibiotic, has antimicrobial activity against a wide variety of gram-positive bacteria including Staphylococcus aureus, Streptococcus pyogenes, Streptococcus pneumonia, Streptococcus epidermidis, Streptococcus viridans, Streptococcus agalactiae, and Neisseria meningitidis. The objective of this study was to evaluate the safety and pharmacokinetic profile of dicloxacillin after single and multiple oral dose in healthy Chinese volunteers. METHODS: A single-center, open-label, randomized, two-phase study was conducted in 16 subjects. In the single-dose phase, subjects were randomly assigned to receive single doses of 0.25, 0.5, 1.0, and 2.0 g of dicloxacillin sodium capsule in a 4-way crossover design with a 5-day washout period between administrations. In the multiple-dose phase, subjects were assigned to receive 0.25 or 0.5 g every 6 hours for 3 days in a 2-way crossover design. Plasma and urine pharmacokinetic samples were assayed by a validated high-performance liquid chromatography-tandem mass spectrometry method. Pharmacokinetic parameters were calculated and analyzed statistically. Safety assessments were conducted throughout the study. RESULTS: Following a single oral dose of 0.25-2.0 g dicloxacillin sodium, the maximum plasma drug concentration (Cmax) and the corresponding values for the area under the concentration- time curve from 0 to 10 hours (AUC0-10 h) increased in a dose-proportional manner. The mean elimination half-life (t1/2) was in the range of 1.38-1.71 hours. Dicloxacillin was excreted in its unchanged form via the kidney, with no tendency of accumulation, and varied from 38.65% to 50.10%. No appreciable accumulation of drug occurred with multiple oral doses of dicloxacillin. No serious adverse events were reported. Adverse events were generally mild. CONCLUSION: Dicloxacillin was safe and well tolerated in the volunteers and displayed linear increases in the Cmax and AUC0-10 h values.

Hollow-fiber pharmacodynamic studies and mathematical modeling to predict the efficacy of amoxicillin for anthrax postexposure prophylaxis in pregnant women and children.

Amoxicillin is considered an option for postexposure prophylaxis of Bacillus anthracis in pregnant and postpartum women who are breastfeeding and in children because of the potential toxicities of ciprofloxacin and doxycycline to the fetus and child. The amoxicillin regimen that effectively kills B. anthracis and prevents resistance is unknown. Fourteen-day dose range and dose fractionation studies were conducted in in vitro pharmacodynamic models to identify the exposure intensity and pharmacodynamic index of amoxicillin that are linked with optimized killing of B. anthracis and resistance prevention. Studies with dicloxacillin, a drug resistant to B. anthracis beta-lactamase, evaluated the role of beta-lactamase production in the pharmacodynamic indices for B. anthracis killing and resistance prevention. Dose fractionation studies showed that trough/MIC and not time above MIC was the index for amoxicillin that was linked to successful outcome through resistance prevention. Failure of amoxicillin regimens was due to inducible or stable high level expression of beta-lactamases. Studies with dicloxacillin demonstrated that a time above MIC of ≥94% was linked with treatment success when B. anthracis beta-lactamase activity was negated. Recursive partitioning analysis showed that amoxicillin regimens that produced peak concentrations of <10.99 µg/ml and troughs of >1.75 µg/ml provided a 100% success rate. Other amoxicillin peak and trough values produced success rates of 28 to 67%. For postpartum and pregnant women and children, Monte Carlo simulations predicted success rates for amoxicillin at 1 g every 8 h (q8h) of 53, 33, and 44% (30 mg/kg q8h), respectively. We conclude that amoxicillin is suboptimal for postexposure prophylaxis of B. anthracis in pregnant and postpartum women and in children.

Intra- and extracellular activities of dicloxacillin against Staphylococcus aureus in vivo and in vitro.

Antibiotic treatment of Staphylococcus aureus infections is often problematic due to the slow response and recurrences. The intracellular persistence of the staphylococci offers a plausible explanation for the treatment difficulties because of the impaired intracellular efficacies of the antibiotics. The intra- and extracellular time- and concentration-kill relationships were examined in vitro with THP-1 cells and in vivo by use of a mouse peritonitis model. The in vivo model was further used to estimate the most predictive pharmacokinetic/pharmacodynamic (PK/PD) indices (the ratio of the maximum concentration of drug in plasma/MIC, the ratio of the area under the concentration-time curve/MIC, or the cumulative percentage of a 24-h period that the free [f] drug concentration exceeded the MIC under steady-state pharmacokinetic conditions [fT(MIC)]) for dicloxacillin (DCX) intra- and extracellularly. In general, DCX was found to have similar intracellular activities, regardless of the model used. Both models showed (i) the relative maximal efficacy (1-log-unit reduction in the numbers of CFU) of DCX intracellularly and (ii) the equal relative potency of DCX intra- and extracellularly, with the MIC being a good indicator of the overall response in both situations. Discordant results, based on data obtained different times after dosing, were obtained from the two models when the extracellular activity of DCX was measured, in which the in vitro model showed a considerable reduction in the number of CFU from that in the original inoculum (3-log-unit decrease in the number of CFU after 24 h), whereas the extracellular CFU reduction achieved in vivo after 4 h did not exceed 1 log unit. Multiple dosing of DCX in vivo revealed increased extra- and intracellular efficacies (2.5 log and 2 log units of reduction in the numbers of CFU after 24 h, respectively), confirming that DCX is a highly active antistaphylococcal antibiotic. PK/PD analysis revealed that fT(MIC) is the index that is the most predictive of the outcome of infection both intra- and extracellularly.

Intracellular activity of antibiotics against Staphylococcus aureus in a mouse peritonitis model.

Antibiotic treatment of Staphylococcus aureus infections is often problematic due to the slow response to therapy and the high frequency of infection recurrence. The intracellular persistence of staphylococci has been recognized and could offer a good explanation for these treatment difficulties. Knowledge of the interplay between intracellular antibiotic activity and the overall outcome of infection is therefore important. Several intracellular in vitro models have been developed, but few experimental animal models have been published. The mouse peritonitis/sepsis model was used as the basic in vivo model exploring a quantitative ex vivo extra- and intracellular differentiation assay. The intracellular presence of S. aureus was documented by electron microscopy. Five antibiotics, dicloxacillin, cefuroxime, gentamicin, azithromycin, and rifampin (rifampicin), were tested in the new in vivo model; and the model was able to distinguish between their extra- and intracellular effects. The intracellular effects of the five antibiotics could be ranked as follows as the mean change in the log(10) number of CFU/ml (Delta log(10) CFU/ml) between treated and untreated mice after 4 h of treatment: dicloxacillin (3.70 Delta log(10) CFU/ml) > cefuroxime (3.56 Delta log(10) CFU/ml) > rifampin (1.86 Delta log(10) CFU/ml) > gentamicin (0.61 Delta log(10) CFU/ml) > azithromycin (0.21 Delta log(10) CFU/ml). We could also show that the important factors during testing of intracellular activity in vivo are the size, number, and frequency of doses; the time of exposure; and the timing between the start of infection and treatment. A poor correlation between the intracellular accumulation of the antibiotics and the actual intracellular effect was found. This stresses the importance of performing experimental studies, like those with the new in vivo model described here, to measure actual intracellular activity instead of making predictions based on cellular pharmacokinetic and MICs.

Lack of effect of P-glycoprotein inhibition on renal clearance of dicloxacillin in patients with cystic fibrosis.

STUDY OBJECTIVE: To determine whether upregulation of P-glycoprotein is responsible for the enhanced renal clearance of dicloxacillin in patients with cystic fibrosis. DESIGN: Single-center, prospective, open-label, randomized, three-part crossover pharmacokinetic study. SETTING: General clinical research center. SUBJECTS: Eleven patients with cystic fibrosis and 11 age-matched healthy volunteers. INTERVENTION: All subjects received a single oral dose of dicloxacillin 500 mg alone, dicloxacillin 500 mg plus probenecid (an organic anion transport inhibitor) 1 g, and dicloxacillin 500 mg plus cyclosporine (a P-glycoprotein inhibitor) 5 mg/kg; each treatment was separated by a washout period of 48 hours. A bolus dose of iothalamate meglumine 456 mg was administered on each study day as a marker of glomerular filtration. MEASUREMENTS AND MAIN RESULTS: Blood and urine samples were taken serially up to 6 hours after each dose. Pharmacokinetics of dicloxacillin and iothalamate were determined by using compartmental and noncompartmental methods. Quantitative polymerase chain reaction was performed on peripheral blood mononuclear cells to measure expression of multidrug resistance 1 (MDR1) messenger RNA (mRNA). Genotyping for ABCB1 was performed to determine the presence of single nucleotide polymorphisms (exons 21 and 26). In both healthy subjects and patients with cystic fibrosis, compared with dicloxacillin alone, coadministration with probenecid produced a significantly lower renal clearance of dicloxacillin, whereas coadministration with cyclosporine resulted in no significant change; renal clearance was not significantly different between the two study groups. No correlation was found between MDR1 mRNA expression and renal clearance of dicloxacillin. The renal excretion of dicloxacillin was significantly greater in subjects with the ABCB1 exon 26 TT polymorphism when compared with subjects with the CT genotype. CONCLUSION: We found no significant difference in the pharmacokinetics of dicloxacillin between patients with cystic fibrosis and healthy volunteers. Renal clearance of dicloxacillin was significantly reduced in the presence of probenecid but not with cyclosporine, suggesting that the rate-limiting step in tubular secretion of dicloxacillin is uptake mediated by the organic anion transporter, and not P-glycoprotein inhibition.

Effect of the MDR1 C3435T variant and P-glycoprotein induction on dicloxacillin pharmacokinetics.

This study investigated 2 hypotheses about genotype-phenotype relationships for the efflux transporter, P-glycoprotein: (1) the presence of a synonymous C3435T variant in exon 26 of the MDR1 gene correlates to higher plasma concentrations of a P-glycoprotein substrate, dicloxacillin, and (2) the effects of genotypic differences decrease under conditions of P-glycoprotein induction by rifampin. Eighteen healthy volunteers received two 1-g doses of dicloxacillin, one on the 1st study day and the other on the 11th day of rifampin dosing (600 mg daily). Dicloxacillin and its 5-hydroxymethyl metabolite were analyzed using liquid chromatography/tandem mass spectrometry. Mean dicloxacillin C(max) measurements were 30.5 +/- 13.5, 33.3 +/- 4.7, and 31.1 +/- 12.8 mug/mL in individuals with the CC, CT, and TT genotype at position 3435 in exon 26 of the MDR1 gene. Following rifampin dosing, the mean dicloxacillin C(max) across genotypes decreased from 31.4 +/- 10.8 to 22.9 +/- 7.0 microg/mL (P < .05), whereas the mean oral clearance increased from 235 +/- 82 to 297 +/- 71 mL/min (P < .001), and the mean absorption time increased from 0.71 +/- 0.55 to 1.34 +/- 0.77 h (P < .05). Rifampin treatment increased the formation clearance, C(max), and AUC of the 5-hydroxymethyl metabolite by 135%, 119%, and 59%, respectively. The C3435T variant had no effect on dicloxacillin pharmacokinetics. The data suggested that rifampin induced intestinal P-glycoprotein and increased dicloxacillin metabolism.

Liquid chromatographic assay for dicloxacillin in plasma.

A simple high-performance liquid chromatographic method for the determination of dicloxacillin in plasma has been developed. The method only requires 0.5 ml of plasma, phosphate buffer solution (pH = 4.7), acidification with 0.5N hydrochloride acid and liquid extraction with dichloromethane. Posterior evaporation of organic under nitrogen steam and redissolution in mobile phase is carried out. The analysis was performed on a Spherisorb C18 (5 microm) column, using methanol -0.05 M phosphate buffer, pH = 4.7 (75:25; v/v) as mobile phase, with ultraviolet detection at 220 nm. Results showed that the assay is sensitive: 0.5 microg/ml. The response is linear in the range of 0.5 - 10 microg/ml. Maximum inter-day coefficient of variation was 12.4%. Mean extraction recovery obtained was 96.95%. Stability studies showed that the loss was not higher than 10%, samples are stable at room temperature for 6 h, at -20 Celsius for 2 months, processed samples were stable at least for 24 h and also after two freeze-thaw cycles. The method has been used to perform pharmacokinetic and bioequivalence studies in humans.

Antibiotic concentrations in serum and wound fluid after local gentamicin or intravenous dicloxacillin prophylaxis in cardiac surgery.

One important aim of antibiotic prophylaxis in cardiac surgery is preventing mediastinitis and thus it would appear to be relevant to study the antibiotic concentrations in pericardial/mediastinal fluid. Local administration of gentamicin in the wound before sternal closure is a novel way of antibiotic prophylaxis and could be effective against bacteria resistant to intravenous antibiotics. This study measured dicloxacillin concentrations in 101 patients in serum and wound fluid following intravenous administration of dicloxacillin. Similarly, concentrations of gentamicin in serum and wound fluid were determined in 30 patients after administration of 260 mg gentamicin in the wound at sternal closure. Median dicloxacillin concentrations in serum and wound fluid at sternal closure were 59.4 and 55.35 mg/l, respectively. Gentamicin levels in the wound were very high (median 304 mg/l), whereas serum concentrations were low (peak median 2.05 mg/l). Dicloxacillin, 1 g given intravenously, according to the clinical protocol, resulted in levels in serum and wound fluid at sternal closure likely to prevent Staphylococcus aureus infections. Locally administered gentamicin resulted in high local concentrations, potentially effective against agents normally considered resistant.

PKQuest: capillary permeability limitation and plasma protein binding - application to human inulin, dicloxacillin and ceftriaxone pharmacokinetics.

BACKGROUND: It is generally assumed that the tissue exchange of antibiotics is flow limited (complete equilibration between the capillary and the tissue water). This assumption may not be valid if there is a large amount of plasma protein binding because the effective capillary permeability depends on the product of the intrinsic capillary permeability (PS) and the fraction of solute that is free in the blood (fwB). PKQuest, a new generic physiologically based pharmacokinetic software routine (PBPK), provides a novel approach to modeling capillary permeability in which the only adjustable parameter is the PS of muscle. METHODS: All the results were obtained by applying PKQuest to previously published human pharmacokinetic data. RESULTS: The PKQuest analysis suggests that the highly protein bound antibiotics dicloxacillin and ceftriaxone have a significant capillary permeability limitation. The human muscle capillary PS of inulin, dicloxacillin and ceftriaxone was 0.6, 13 and 6 ml/min/100 gm, respectively. The ceftriaxone protein binding is non-linear, saturating at high plasma concentrations. The experimental ceftriaxone data over a wide range of intravenous inputs (0.15 to 3 gms) was well described by PKQuest. PKQuest is the first PBPK that includes both permeability limitation and non-linear binding. CONCLUSIONS: Because of their high degree of plasma protein binding, dicloxacillin and ceftriaxone appear to have a diffusion limited exchange rate between the blood and tissue and are not flow limited as had been previously assumed. PKQuest and all the examples are freely available at http:\\www.pkquest.com.

Milk depletion of dicloxacillin residues in cows and sheep following intramammary administration.

The excretion rate of dicloxacillin from milk was studied after intramammary administration of a suspension of the drug active in vegetable oil. Eight cows and eight sheep, four of each group in low and four in high milk production, were dosed with 200 mg dicloxacillin/quarter in cows and 100 mg dicloxacillin/quarter in sheep, three times at 12 h intervals. The dicloxacillin concentrations in milk were quantified by high performance liquid chromatography (HPLC). In cows, time until dicloxacillin was undetectable was 48 h and no difference was observed between the groups. In sheep, dicloxacillin was undetectable 72 h and 84 h after the treatment in low and in high milk production groups, respectively. The implications of several factors affecting the possible milk withdrawal period were studied. The results indicated that the pharmaceutical vehicle and the coefficient of lipid solubility exerted major effects on depletion time.

Dicloxacillin and flucloxacillin: pharmacokinetics, protein binding and serum bactericidal titers in healthy subjects after oral administration.

The pharmacokinetics of dicloxacillin and flucloxacillin were studied in 12 healthy volunteers after oral administration. The participants received a single dose of either dicloxacillin (0.5 g, 0.75 g or 1.0 g) or flucloxacillin (0.75 g) in a cross-over fashion. Antibiotic concentrations were determined in serum and urine by bioassay and followed for 8 and 24 h, respectively. The three dicloxacillin dosages showed no significant differences for the serum elimination half-lives (t1/2 beta, median: 72 min). Comparing 0.75 g flucloxacillin with the same dose of dicloxacillin, no significant differences between the values of Cmax, t1/2 beta and AUC were found. Protein binding as determined by ultrafiltration in pooled serum was 94.7-96.2% for flucloxacillin and 96.4-97.2% for dicloxacillin. The serum bactericidal titers were similar for the two drugs. In conclusion, dicloxacillin and flucloxacillin showed similar pharmacokinetic behavior after 0.75 g doses in human volunteers.

Continuation of warfarin-nafcillin interaction during dicloxacillin therapy.

Interactions between warfarin and penicillins have been infrequently reported. A case report of a single patient who experienced the effects of a warfarin-nafcillin interaction as well as a warfarin-dicloxacillin interaction is presented. Clinical effects of this interaction were documented primarily through changes in prothrombin time (PT) and the need for higher warfarin dosing. While the patient received nafcillin, warfarin doses were increased to as much as 4.5 times the previous amounts needed to provide adequate anticoagulation. During dicloxacillin therapy, warfarin doses were gradually decreased, but stabilized to a maintenance dose higher than the patient's pre-nafcillin dose. The dicloxacillin-warfarin interaction appears similar to that noted during nafcillin-warfarin combination.

Dicloxacillin absorption and elimination in children.

We determined the apparent bioavailability of dicloxacillin in 26 children between the ages of 0.24 and 143 months by comparing the area under the serum concentration versus time curve following intravenous and oral administration of 25 mg/kg. With intravenous infusion the overall mean half-life of elimination was 0.53 +/- 0.20 h, the AUC was 70.15 +/- 32.18 mg.min/l and the apparent volume of distribution was 0.29 +/- 0.09 l/kg. The overall average bioavailability was 59.89%. Children less than 6 months old had a shorter time to peak concentration (1.39 +/- 0.49 h) and the lowest oral bioavailability (64.35 +/- 13.62%) in comparison to children more than 60 months old. In children older than 60 months the time to peak was 1.79 +/- 1.16 h and the average oral bioavailability was 79.38 +/- 32.87%. However, children less than 6 months old had the least variability in absorption, the coefficient of variation (CV) of oral bioavailability was 13.62%, while in children between 6 and 40 months old the CV was 60.4%: children older than 60 months had the most variability in absorption, a CV of 32.87%. This variability was not dependent on the formulation administered. After 5 years of age the bioavailability increased with increasing age.