Pharmacokinetics

Quinupristin and dalfopristin are the main active components circulating in plasma in human subjects. Quinupristin and dalfopristin are converted to several active major metabolites: two conjugated metabolites for quinupristin (one with glutathione and one with cysteine) and one non-conjugated metabolite for dalfopristin (formed by drug hydrolysis).

Pharmacokinetic profiles of quinupristin and dalfopristin in combination with their metabolites were determined using a bioassay following multiple 60-minute infusions of Synercid in two groups of healthy young adult male volunteers. Each group received 7.5 mg/kg of Synercid intravenously q12h or q8h for a total of 9 or 10 doses, respectively. The pharmacokinetic parameters were proportional with q12h and q8h dosing; those of the q8h regimen are shown in the following table:

The clearances of unchanged quinupristin and dalfopristin are similar (0.72 L/h/kg), and the steady-state volume of distribution for quinupristin is 0.45 L/kg and for dalfopristin is 0.24 L/kg. The elimination half-life of quinupristin and dalfopristin is approximately 0.85 and 0.70 hours, respectively.

The total protein binding of quinupristin is higher than that of dalfopristin. Synercid does not alter the in vitro binding of warfarin to proteins in human serum.

Penetration of unchanged quinupristin and dalfopristin in noninflammatory blister fluid corresponds to about 19% and 11% of that estimated in plasma, respectively. The penetration into blister fluid of quinupristin and dalfopristin in combination with their major metabolites was in total approximately 40% compared to that in plasma.

In vitro, the transformation of the parent drugs into their major active metabolites occurs by non-enzymatic reactions and is not dependent on cytochrome-P450 or glutathione-transferase enzyme activities.

Synercid has been shown to be a major inhibitor (in vitro inhibits 70% cyclosporin A biotransformation at 10 μg/mL of Synercid) of the activity of cytochrome P450 3A4 isoenzyme. (See WARNINGS.)

Synercid can interfere with the metabolism of other drug products that are associated with QTc prolongation. However, electrophysiologic studies confirm that Synercid does not itself induce QTc prolongation. (See WARNINGS.)

Fecal excretion constitutes the main elimination route for both parent drugs and their metabolites (75 to 77% of dose). Urinary excretion accounts for approximately 15% of the quinupristin and 19% of the dalfopristin dose. Preclinical data in rats have demonstrated that approximately 80% of the dose is excreted in the bile and suggest that in man, biliary excretion is probably the principal route for fecal elimination.

Special Populations

Elderly: The pharmacokinetics of quinupristin and dalfopristin were studied in a population of elderly individuals (range 69 to 74 years). The pharmacokinetics of the drug products were not modified in these subjects.

Gender: The pharmacokinetics of quinupristin and dalfopristin are not modified by gender.

Renal Insufficiency: In patients with creatinine clearance 6 to 28 mL/min, the AUC of quinupristin and dalfopristin in combination with their major metabolites increased about 40% and 30%, respectively.

In patients undergoing Continuous Ambulatory Peritoneal Dialysis, dialysis clearance for quinupristin, dalfopristin and their metabolites is negligible. The plasma AUC of unchanged quinupristin and dalfopristin increased about 20% and 30%, respectively. The high molecular weight of both components of Synercid suggests that it is unlikely to be removed by hemodialysis.

Hepatic Insufficiency: In patients with hepatic dysfunction (Child-Pugh scores A and B), the terminal half-life of quinupristin and dalfopristin was not modified. However, the AUC of quinupristin and dalfopristin in combination with their major metabolites increased about 180% and 50%, respectively. (See DOSAGE AND ADMINISTRATION and PRECAUTIONS.)

Obesity (body mass index ≥30): In obese patients the Cmax and AUC of quinupristin increased about 30% and those of dalfopristin about 40%.

Pediatric Patients: The pharmacokinetics of Synercid in patients less than 16 years of age have not been studied.

Microbiology

The streptogramin components of Synercid, quinupristin and dalfopristin, are present in a ratio of 30 parts quinupristin to 70 parts dalfopristin. These two components act synergistically so that Synercid's microbiologic in vitro activity is greater than that of the components individually. Quinupristin's and dalfopristin's metabolites also contribute to the antimicrobial activity of Synercid. In vitro synergism of the major metabolites with the complementary parent compound has been demonstrated.

Mechanism of Action

The site of action of quinupristin and dalfopristin is the bacterial ribosome. Dalfopristin has been shown to inhibit the early phase of protein synthesis while quinupristin inhibits the late phase of protein synthesis. Synercid is bactericidal against isolates of methicillin-susceptible and methicillin-resistant staphylococci. The mode of action of Synercid differs from that of other classes of antibacterial agents such as ß-lactams, aminoglycosides, glycopeptides, quinolones, macrolides, lincosamides and tetracyclines. Therefore, there is no cross resistance between Synercid and these agents when tested by the minimum inhibitory concentration (MIC) method.

Mechanism of Resistance

Resistance to Synercid is associated with resistance to both components (i.e., quinupristin and dalfopristin). In non-comparative studies, emerging resistance to Synercid has occurred.

Interaction with other Antibacterials

In vitro combination testing of Synercid with aztreonam, cefotaxime, ciprofloxacin, and gentamicin against Enterobacteriaceae and Pseudomonas aeruginosa did not show antagonism.

In vitro combination testing of Synercid with prototype drugs of the following classes: aminoglycosides (gentamicin), β-lactams (cefepime, ampicillin, and amoxicillin), glycopeptides (vancomycin), quinolones (ciprofloxacin), tetracyclines (doxycycline) and also chloramphenicol against enterococci and staphylococci did not show antagonism.

Synercid has been shown to be active against most isolates of the following bacteria, both in vitro and in clinical infections, as described in the INDICATIONS AND USAGE section.

Gram-positive bacteria
Staphylococcus aureus (methicillin-susceptible isolates only)
Streptococcus pyogenes

The following in vitro data are available, but their clinical significance is unknown.

The combination of quinupristin and dalfopristin (Synercid) exhibits in vitro minimum inhibitory concentrations (MIC's) of ≤1.0 μg/mL against most (≥90%) isolates of the following microorganisms; however, the safety and effectiveness of Synercid in treating clinical infections due to these bacteria have not been established in adequate and well-controlled clinical trials.

Gram-positive bacteria
Corynebacterium jeikeium
Staphylococcus aureus (methicillin-resistant isolates)
Staphylococcus epidermidis (including methicillin-resistant isolates)
Streptococcus agalactiae

Dilution Techniques

Quantitative methods are used to determine antimicrobial minimum inhibitory concentrations (MICs). These MICs provide estimates of the susceptibility of bacteria to antimicrobial compounds. The MICs should be determined using a standardized procedure. Standardized procedures are based on a dilution1 method (broth or agar) or equivalent using standardized inoculum concentrations, and standardized concentrations of quinupristin/dalfopristin (Synercid) in a 30:70 ratio made from powder of known potency. The MIC values are interpreted according to the following criteria:

A report of "Susceptible" indicates that the pathogen is likely to be inhibited if the concentration of the antimicrobial compound in the blood reaches usually achievable levels. A report of "Intermediate" indicates that the result should be considered equivocal, and if the bacterium is not fully susceptible to alternative, clinically feasible drugs, the test should be repeated. This category implies possible clinical applicability in body sites where the drug is physiologically concentrated or in situations where high dosage of drug can be used. This category provides a buffer zone which prevents small uncontrolled technical factors from causing major discrepancies in interpretation. A report of "Resistant" indicates that the pathogen is not likely to be inhibited if the antimicrobial compound in the blood reaches the concentrations usually achievable; other therapy should be selected.

Diffusion Techniques

Quantitative methods that require measurement of zone diameters also provide reproducible estimates of the susceptibility of bacteria to antimicrobial compounds. One such standardized procedure1 requires the use of standardized inoculum concentrations. This procedure uses paper disks impregnated with 15 μg quinupristin/dalfopristin in a ratio of 30:70 (Synercid) to test the susceptibility of bacteria to quinupristin/dalfopristin. Reports from the laboratory providing results of the standard single-disk susceptibility test with a 15 μg quinupristin/dalfopristin disk are interpreted according to the following criteria:

Interpretation should be as stated above for results using dilution techniques. Interpretation involves correlation of the diameter obtained in the disk test with the MIC for quinupristin/dalfopristin.

Drug Interactions

In vitro drug interaction studies have demonstrated that Synercid significantly inhibits cytochrome P450 3A4 metabolism of cyclosporin A, midazolam, nifedipine and terfenadine. In addition, 24 subjects given Synercid 7.5 mg/kg q8h for 2 days and 300 mg of cyclosporine on day 3 showed an increase of 63% in the AUC of cyclosporine, an increase of 30% in the Cmax of cyclosporine, a 77% increase in the t1/2 of cyclosporine, and, a decrease of 34% in the clearance of cyclosporine. Therapeutic level monitoring of cyclosporine should be performed when cyclosporine must be used concomitantly with Synercid.

It is reasonable to expect that the concomitant administration of Synercid and other drugs primarily metabolized by the cytochrome P450 3A4 enzyme system may likely result in increased plasma concentrations of these drugs that could increase or prolong their therapeutic effect and/or increase adverse reactions. (See Table below.) Therefore, coadministration of Synercid with drugs which are cytochrome P450 3A4 substrates and possess a narrow therapeutic window requires caution and monitoring of these drugs (e.g., cyclosporine), whenever possible. Concomitant medications metabolized by the cytochrome P450 3A4 enzyme system that may prolong the QTc interval should be avoided.

Concomitant administration of Synercid and nifedipine (repeated oral doses) and midazolam (intravenous bolus dose) in healthy volunteers led to elevated plasma concentrations of these drugs. The Cmax increased by 18% and 14% (median values) and the AUC increased by 44% and 33% for nifedipine and midazolam, respectively.

Table of Selected Drugs That Are Predicted to Have Plasma Concentrations Increased by Synercid +

Antihistamines: astemizole, terfenadine

Anti-HIV (NNRTIs and Protease inhibitors): delavirdine, nevirapine, indinavir, ritonavir

Antineoplastic agents: vinca alkaloids (e.g., vinblastine), docetaxel, paclitaxel

Benzodiazepines: midazolam, diazepam

Calcium channel blockers: dihydropyridines (e.g., nifedipine), verapamil, diltiazem

Cholesterol-lowering agents: HMG-CoA reductase inhibitors (e.g., lovastatin)

GI motility agents: cisapride

Immunosuppressive agents: cyclosporine, tacrolimus

Steroids: methylprednisolone

Other: carbamazepine, quinidine, lidocaine, disopyramide

+ This list of drugs is not all inclusive.

Clostridium difficile associated diarrhea (CDAD) has been reported with use of nearly all antibacterial agents, including Synercid, and may range in severity from mild diarrhea to fatal colitis.  Treatment with antibacterial agents alters the normal flora of the colon leading to overgrowth of C. difficile.

C. difficile produces toxins A and B which contribute to the development of CDAD.  Hypertoxin producing strains of C. difficile cause increased morbidity and mortality, as these infections can be refractory to antimicrobial therapy and may require colectomy.  CDAD must be considered in all patients who present with diarrhea following antibiotic use.  Careful medical history is necessary since CDAD has been reported to occur over two months after the administration of antibacterial agents.

If CDAD is suspected or confirmed, ongoing antibiotic use not directed against C. difficile may need to be discontinued. Appropriate fluid and electrolyte management, protein supplementation, antibiotic treatment of C. difficile, and surgical eva luation should be instituted as clinically indicated.