Clinical pharmacokinetics of quinupristin/dalfopristin

Quinupristin/dalfopristin is a streptogramin antibacterial with a wide spectrum of Gram-positive antibacterial activity. The drug has minimal oral absorption and is administered intravenously as a fixed 30 : 70 ratio of quinupristin to dalfopristin. A linear relationship has been observed between the dose administered and maximum plasma concentrations. Single-dose administration of 7.5 mg/kg produced a maximal plasma concentration of 2.3-2.7 mg/L for quinupristin and 6.1-8.2 mg/L for dalfopristin. The area under the concentration-time curve (AUC) obtained with the same dose was 2.7-3.3 and 6.5-7.7 mg. h/L for quinupristin and dalfopristin, respectively. Repeated administration results in 13-21% increases in maximum plasma concentrations and 21-26% increases in AUC for both quinupristin and dalfopristin. Quinupristin and dalfopristin exhibit steady-state volumes of distribution of 0.46-0.54 and 0.24-0.30 L/kg, respectively. Quinupristin exhibits higher protein binding (55-78%) than dalfopristin (11-26%), though both entities distribute well into tissues. Concentrations exceeding those in blood have been reported for the kidney, liver, spleen, salivary glands and white blood cells of primates. Extravascular penetration, as measured in blister fluid, is 40-80%. Both quinupristin and dalfopristin are extensively metabolised via nonenzymatic reactions. Quinupristin is conjugated to form two active compounds, a cysteine moiety and a glutathione moiety. Dalfopristin is hydrolysed to the active metabolite pristinamycin IIA. The metabolites exert antibacterial activity similar to that of the parent compounds. Quinupristin/dalfopristin is excreted primarily in the faeces (75-77%), with lesser renal excretion (15-19%). The elimination half-lives of quinupristin and dalfopristin are similar, and are 0.7-1.3 hours after single doses. The metabolites have slightly longer half-lives, ranging from 1.2 to 1.8 hours. With repeated doses, plasma clearance of quinupristin and dalfopristin is reduced by approximately 20% compared with single doses, resulting in clearances of 0.7-0.8 L/h/kg. Saturable protein binding has been hypothesised as a causative mechanism. Quinupristin/dalfopristin is an inhibitor of cytochrome P450 3A4, resulting in multiple drug interactions. Ciclosporin AUC increased by 5-222% when coadministered with quinupristin/dalfopristin. Careful monitoring of patients receiving drugs that are substrates of cytochrome P450 3A4 is suggested.Quinupristin/dalfopristin is administered at 7.5 mg/kg every 8-12 hours, depending upon the severity of infection. The pharmacodynamic parameter linked with antibacterial activity for quinupristin/dalfopristin appears to be the ratio of AUC to the minimal inhibitory concentration. The additional activity of a prolonged post-antibiotic effect may also be important for efficacy.

Linezolid and quinupristin/dalfopristin: novel antibiotics for gram-positive infections of the skin

With the continuing development of clinical drug resistance among bacteria, the need for new, effective agents to treat multi-drug-resistant Gram-positive infections remains important. With treatment options limited, it has become critical to identify antibiotics with novel mechanisms of activity. Several new drugs have emerged as possible therapeutic alternatives. This review focuses on agents newly introduced and FDA-approved for the treatment of skin and skin structure infections: linezolid and quinupristin/dalfopristin.

Treatment of life-threatening multiresistant staphylococcal and enterococcal infections in patients with end-stage renal failure with quinupristin/dalfopristin: preliminary report

BACKGROUND

Life-threatening infections with multiresistant gram-positive bacteria are increasing. Treatment with quinupristin/dalfopristin (Q-D) has turned out to be effective against such resistant pathogens.

PATIENTS AND METHODS

We report on treatment of six patients on dialysis (four with additional liver injury) and of one renal graft recipient with normal renal function who had severe infections caused by multiresistant Staphylococus epidermidis (1/7), methicillin-resistant Staphylococcus aureus (4/7) and vancomycin-resistant Enterococcus faecium (2/7).

RESULTS

Six out of seven patients were cured by therapy with Q-D in adjusted doses lasting for 10 to 34 days. Pharmacokinetics of Q-D and its metabolites were determined and remained within the therapeutic range, despite a modest increase of all compounds at the presumed steady state. The concentrations of the metabolites of Q-D were clearly lower than the parent drugs, including those of quinupristin-conjugated derivatives, which has not been reported previously.

CONCLUSION

These preliminary results suggest that: a) neither quinupristin nor dalfopristin or its metabolites accumulated despite the long duration of treatment; b) no adjustment of the standard dosage regimen (three times 7.5 mg/kg/day) is necessary in end-stage renal disease.

[Novel compounds active on staphylococci]

Research efforts to discover new compounds active against staphylococci are more than ever justified today. The incidence of methicillin-resistant staphylococci remains very high in hospitals, and the solution provided by glycopeptides is far from being satisfactory. These compounds exhibit mediocre pharmacokinetic and pharmacodynamic properties. Their ease and safety of use are poor. Finally, strains with diminished sensitivity to these antibiotics are beginning to appear. This article examines the opportunities offered by two new anti-staphylococcal agents: quinupristine-dalfopristine (Synercid) and linezolide (not marketed in France).

Clearance of quinupristin-dalfopristin (Synercid) and their main metabolites during continuous veno-venous hemofiltration (CVVH) with or without dialysis

BACKGROUND

Quinupristin-dalfopristin (Q/D) is often utilized in critically ill patients, some of whom require CVVH. This study was undertaken to determine the clearance of O/D and their main active metabolites (RPR 100391, RP 69012, RP 12536) via CVVH in the swine model.

METHODS

Q/D 7.5 mg/kg was intravenously administered over 0.5 h to 12 swine after induction of acute renal failure by ligation of the renal arteries. At 0.5 h post injection, the CVVH procedure was initiated and continued for 8 hours at the following pump rates: (1)100 mL/min, (2)180 rnL/min, and (3)100 mL/min with dialysis (flow rate: 1 L/h). Blood and ultrafiltrate samples were collected at 1 h intervals and assessed by a validated HPLC method.

RESULTS

Plasma analysis suggests rapid metabolism to the main active metabolites which are appreciably cleared as demonstrated by high clearance and sieving coefficient estimates. Mean clearance estimates for RP 69012, RP 100391, and RP 12536 are 729, 777, and 578 mL/h in the 100 mL/min CVVH group, 772, 785, 685 mL/min in the 180 mL/min CVVH group, and 753, 791, 616 mL/min in the 100 mL/min CVVH group with 1 L/h dialysis, respectively.

CONCLUSION

These data reveal that Q/D is rapidly metabolized and the metabolites are cleared to a large extent via CVVH. Due to the considerable contribution of the metabolites to overall in vivo activities, additional studies are required to fully quantify their removal before final dosage modifications for patients undergoing CVVH can be recommended.

In vitro activity and post-antibiotic effect of quinupristin/dalfopristin (Synercid)

The in vitro activities of quinupristin/dalfopristin (Synercid), ampicillin, erythromycin, clarithromycin, vancomycin, teicoplanin, ciprofloxacin and tetracycline were examined and compared against 526 gram-positive bacteria. The minimal inhibitory concentrations (MICs) for quinupristin/dalfopristin against Staphylococcus aureus, including methicillin-resistant strains, were low (MIC(90) = 0.5 mg/l), and were comparable with those of vancomycin and teicoplanin. This compound was superior to the macrolides and highly active against Streptococcus pneumoniae (both penicillin-sensitive and penicillin-resistant strains), with MIC(90) = 2 mg/l. It was also active against other streptococci, with MIC(90) = 4 mg/l. However, this agent is less active against enterococci (MIC(90) = 32 mg/l). Quinupristin/dalfopristin showed high activity against gram-positive anaerobes, including Clostridium spp., Peptococcus spp. and Peptostreptococcus spp., with MIC(90) < or = 2 mg/l. Quinupristin/dalfopristin was also investigated for its post-antibiotic effect (PAE) and bactericidal kinetics against nine strains of gram-positive organisms, including staphylococci, enterococci and pneumococci. Exponentially growing (log phase) cultures were exposed to quinupristin/dalfopristin at 2 x MIC. Growth kinetics was evaluated using viable counting. The drug was uniformly bactericidal against pneumococci and staphylococci within 2 and 8 h of exposure, respectively. The killing activity against enterococci was weak; there was little or no reduction in bacterial count over 24 h of incubation. PAEs ranging from 2.13 to 3.28 h, 0.92 to 3.02 h and 1.89 to 7.07 h were produced on the tested pneumococci, staphylococci and enterococci, respectively. This study showed that quinupristin/dalfopristin is a promising agent active against gram-positive bacteria. The prolonged PAEs also suggest that the drug could be used intermittently at more widely spaced dosing intervals against gram-positive organisms.

Multiple-dose pharmacokinetics and safety of two regimens of quinupristin/dalfopristin (Synercid) in healthy volunteers

Quinupristin/dalfopristin (Q/D) is a novel streptogramin antibiotic for the treatment of severe gram-positive infections. The purpose of this open, nonrandomized, parallel-group, phase I trial was to evaluate Q/D pharmacokinetics after single and repeated doses under the two different dosing regimens corresponding to the effective doses and to evaluate tolerability. Two groups of 10 healthy volunteers received multiple 1-hour intravenous infusions of 7.5 mg/kg Q/D either every 8 or 12 hours for 4 or 5 days, respectively. Plasma concentrations of Q, D, and metabolites were determined using high-performance liquid chromatography and selective microbiological assays. The two regimens q8h and q12h lead to the same disposition profile after single and repeated administration. Single-dose data confirmed the high plasma clearances of Q and D (about 0.90 l/h/kg) obtained previously. Unchanged drugs were the main components in plasma, with each of the three metabolites representing about 20% (in terms of the AUC ratio) of the parent drugs. Comparable steady-state concentrations were reached from day 2 of both regimens. A similar moderate increase in Cmax and AUC (about 20%) of parent drugs was observed between the first and last day of treatment. This phenomenon, which was also observed for the metabolites, was not expected considering the short terminal disposition half-lives of the parent drugs and trough plasma concentrations of all components mostly below the limits of quantitation at steady state, whatever the dosing regimen. The clearances of parent drugs at steady state were about 20% lower as compared with that observed following the first drug administration (statistically significant difference). No trend suggesting a treatment effect on any laboratory parameter, vital signs, or electrocardiographic parameters was identified. However, 80% of subjects reported venous adverse events probably related to treatment.

The new antibiotics

It is easy to become overwhelmed by the amount of information available on the new antibiotics and difficult to keep abreast of the appropriate indications for each of them. For most patients with community-acquired infections, the first-line agent is usually not one of the newer agents, but a standard regimen, or at times, no antibiotic at all. The development of resistance is likely to parallel the extent to which these agents are prescribed. They should be used only when standard treatment fails, when compliance with treatment is a real and serious issue, or when the patient has a real allergic reaction to the standard regimen.

Use of anti-infective agents during lactation: Part 1--Beta-lactam antibiotics, vancomycin, quinupristin-dalfopristin, and linezolid

Because many antibiotics are excreted into the breast milk, it can be difficult for a practitioner to choose an antibiotic for a lactating patient that will have minimal risks to her nursing infant. This article is the first of a three-part series discussing the use of anti-infective agents during lactation. The authors review general information regarding use and common side effects of several classes of antibiotics. They also summarize information, including documented milk concentrations, milk-to-plasma ratios, and other pharmacokinetic properties, in a table that can help practitioners choose antibiotics that may be considered safe for the lactating mother.

Pharmacokinetics of quinupristin/ dalfopristin in patients with severe chronic renal insufficiency

OBJECTIVE

To compare the pharmacokinetic profile of a single intravenous injection of quinupristin/dalfopristin, a new injectable streptogramin, in healthy young individuals and patients with severe chronic renal insufficiency. A secondary objective was to assess the relative tolerability of this dose in these patients compared with healthy individuals.

PATIENTS AND PARTICIPANTS

13 patients with severe chronic renal insufficiency (creatinine clearance 6 to 28 ml/min/1.73m2) were individually matched for gender, bodyweight and age to a healthy volunteer.

METHODS

Participants received a single dose of quinupristin/dalfopristin 7.5 mg/kg bodyweight as a continuous 1-hour intravenous infusion, followed by serial blood sampling.

RESULTS

The disposition profile of unchanged quinupristin was similar in the 2 groups. However, the elimination of quinupristin derivatives in patients with renal impairment tended to be decreased: mean peak plasma drug concentration (Cmax) and area under the concentration-time curve from zero to infinity (AUCinfinity) of quinupristin plus its active derivatives were about 1.4 times higher in the patients with renal impairment compared with healthy volunteers. The mean Cmax and AUCinfinity of both unchanged dalfopristin and dalfopristin plus its active derivatives were about 1.3 times higher in renally impaired patients than in healthy volunteers. Adverse events were generally mild and transient. No severe or serious adverse events were reported and no participants prematurely discontinued the study. Venous tolerability tended to be better in healthy volunteers than in the patients with renal impairment.

CONCLUSION

These results suggest that no formal reduction in the dosage of quinupristin/dalfopristin is necessary in patients with severe chronic renal impairment.

Antibiotics for gram-positive bacterial infections. Vancomycin, teicoplanin, quinupristin/dalfopristin, and linezolid

Vancomycin is a safe, effective antibiotic for a variety of serious gram-positive infections. Because of emerging resistance in enterococci and staphylococci and the emerging threat of spread of vancomycin-resistant genes to other gram-positive organisms, judicious use of vancomycin should be promoted. Quinupristin/dalfopristin, a streptogramin antibiotic, and linezolid, an oxazolidinone, show promise against some strains of gram-positive bacteria that are resistant to vancomycin.

Quinupristin/dalfopristin: spectrum of activity, pharmacokinetics, and initial clinical experience

In recent years, the prevalence of multiple drug-resistant strains of common Gram-positive pathogens has grown in many regions of the world. Increasingly, methicillin-resistant Staphylococcus aureus, coagulase-negative staphylococci, vancomycin-resistant enterococci, and penicillin-resistant pneumococci have been identified as causative organisms in serious and life-threatening infections. This increase in resistance highlights the need for new antimicrobial agents to expand the therapeutic armamentarium. Quinupristin/dalfopristin is the first of a unique class of antibiotics called streptogramins. It is characterized by a unique mechanism of action, intracellular activity, synergistic activity of its components, broad spectrum of activity against most Gram-positive cocci, common respiratory pathogens, and anaerobes, and demonstrated postantibiotic effect. Clinical evidence to date indicates that quinupristin/dalfopristin may be effective for the treatment of multidrug-resistant infections, especially those due to vancomycin-resistant enterococci and methicillin-resistant staphylococci. This article reviews the pharmacology, microbiology, and clinical experience with quinupristin/dalfopristin to date.

[Comparative diffusion of fusidic acid, oxacillin, and pristinamycin in dermal interstitial fluid after repeated oral administration]

OBJECTIVE

The aim of this study was to use the suction bullae technique to compare skin diffusion of 3 antibiotics commonly used for skin infections (fusidic acid, oxacillin, pristinamycin) and to estimate their potential activity at the site of skin infections.

SUBJECTS AND METHODS

This comparative open study was conducted in 12 healthy volunteers using a repeated latin square experimental scheme. Antibiotic concentrations in serum and suction bullae fluid were measured by high performance liquid chromatography after 5.5 days of repeated oral administration of fusidic acid (1 g/d), oxacillin (2 g/d), and pristinamycin (2 g/d).

RESULTS

Mean antibiotic concentrations in serum and interstitial fluid (suction bullae fluid) were highest for fusidic acid with a Cmax at 91.3 +/- 23.0 mg/l and 45.5 +/- 18.0 mg/l respectively (interstitial fluid/serum ratio=49 +/- 10 p. 100). For oxacillin, Cmax was 8.3 +/- 3.6 mg/l and 0.98 +/- 0.49 mg/l (ratio 13 +/- 5 p. 100). Pristinamycin concentrations were low with a Cmax at 0.51 +/- 0.40 and 0.26 +/- 0.15 mg/l (ratio 73 +/- 57 p. 100). Comparing the area under the interstitial fluid and the serum concentration-time curves showed that the best diffusion was obtained with pristinamycin (114 +/- 61 p. 100), followed by fusidic acid (57 +/- 13 p. 100) and oxacillin (48 +/- 25 p. 100).

DISCUSSION

These data were used to calculate indicators of potential efficacy in the interstitial dermal fluid: inhibitor quotient (Cmax/MIC) and AUIC (ASC/MIC), indicator of the time antibiotic concentrations are maintained above the minimal inhibitor concentration (MIC). This showed that fusidic acid was potentially more active against all staphylococci. For streptococci, the observed interstitial concentrations of pristinamycin and of fusidic acid should theoretically inhibit streptococci A growth, but oxacillin was the most adapted antibiotic.

Quinupristin/dalfopristin: a review of its use in the management of serious gram-positive infections

Quinupristin/dalfopristin is the first parenteral streptogramin antibacterial agent, and is a 30:70 (w/w) ratio of 2 semisynthetic pristinamycin derivatives. The combination has inhibitory activity against a broad range of gram-positive bacteria including methicillin-resistant staphylococci, vancomycin-resistant Enterococcus faecium (VREF), drug-resistant Streptococcus pneumoniae, other streptococci, Clostridium perfringens and Peptostreptococcus spp. The combination also has good activity against selected gram-negative respiratory tract pathogens including Moraxella catarrhalis, Legioniella pneumophila and Mycoplasma pneumoniae. Quinupristin/dalfopristin has poor activity against E. faecalis. The combination is bactericidal against staphylococci and streptococci, although constitutive erythromycin resistance can affect its activity. As for many other agents, quinupristin/dalfopristin is generally bacteriostatic against E. faecium. In patients with methicillin-resistant S. aureus (MRSA) or VREF infections participating in prospective emergency-use trials, quinupristin/dalfopristin 7.5 mg/kg every 8 or 12 hours achieved clinical or bacteriological success in > or =64% of patients. Emergence of resistance to quinupristin/dalfopristin was uncommon (4% of patients) in those with VREF infections. Quinupristin/dalfopristin 7.5 mg/kg 8- or 12-hourly also achieved similar clinical success rates to comparator agents in patients with presumed gram-positive complicated skin and skin structure infections or nosocomial pneumonia (administered in combination with aztreoman) in 3 large multicentre randomised trials. Systemic adverse events associated with quinupristin/dalfopristin include gastrointestinal events (nausea, vomiting and diarrhoea), rash and pruritus. Myalgias and arthralgias also occur at an overall incidence of 1.3%, although higher rates (2.5 to 31%) have been reported in patients with multiple comorbidities. Venous events are common if the drug is administered via a peripheral line; however, several management options (e.g. use of central venous access, increased infusion volume) may help to minimise their occurrence. Hyperbilirubinaemia has been documented in 3.1% of quinupristin/dalfopristin recipients versus 1.3% of recipients of comparator agents. Quinupristin/dalfopristin inhibits cytochrome P450 3A4 and therefore has the potential to increase the plasma concentrations of substrates of this enzyme.

CONCLUSIONS

Quinupristin/dalfopristin, the first parenteral streptogramin, offers a unique spectrum of activity against multidrug-resistant gram-positive bacteria. In serious gram-positive infections for which there are other treatment options available, the spectrum of activity and efficacy of quinupristin/ dalfopristin should be weighed against its tolerability and drug interaction profile. However, in VREF or unresponsive MRSA infections, where few proven treatment options exist, quinupristin/dalfopristin should be considered as a treatment of choice for these seriously ill patients.

Pharmacokinetics of quinupristin-dalfopristin in continuous ambulatory peritoneal dialysis patients

Quinupristin-dalfopristin may be useful for treatment of organisms causing peritoneal dialysis-related peritonitis, including methicillin-resistant coagulase-negative staphylococci, methicillin-resistant Staphylococcus aureus, and vancomycin-resistant enterococci. The pharmacokinetic profiles of single intravenous doses of this combination streptogramin antibiotic of 7.5 mg/kg of body weight were characterized for eight noninfected patients receiving continuous ambulatory peritoneal dialysis. Comparison was made to pharmacokinetic profiles determined for eight healthy volunteers matched by age, sex, and race. Drug was measured in dialysate up to 6 h following the dose. Plasma and dialysate were assayed for parent compounds and metabolites. Mean pharmacokinetic parameters were compared between groups. No statistically significant differences were observed between groups for maximal concentrations in plasma, times to maximal concentration, areas under the curve, distribution volumes, rates of total body clearance, or half-lives in plasma for quinupristin and dalfopristin. No statistically significant differences were observed in maximal concentrations in plasma, times to maximal concentration, areas under the curve, or half-lives for cysteine, the glutathione conjugates of quinupristin, or the pristinamycin IIA metabolite of dalfopristin. The measurements in dialysate of the parent and most metabolites were below the expected MICs. Dialysis clearance was insignificant. Quinupristin-dalfopristin was well tolerated in both groups, causing only mild adverse events that resolved prior to discharge from the study. The disposition of quinupristin, dalfopristin, or their primary metabolites following a single dose was unaltered in patients receiving peritoneal dialysis. Intravenous dosing of this antibiotic combination is unlikely to be adequate for the treatment of peritonitis associated with peritoneal dialysis.

Streptogramins and their potential role in geriatric medicine

Despite advances in antimicrobial chemotherapy over recent decades, morbidity and mortality secondary to infection continues to rise. In addition, the incidence of infection caused by resistant organisms has also increased. Concurrently, the elderly are living longer than prior generations, often with disabling chronic diseases. The more debilitated of the geriatric population are at greater risk for infection, and more likely to acquire or develop antimicrobial resistant organisms. Gram-positive organisms are a source of resistance and commonly cause infection in older patients. Whereas resistance is a concern in all patients, in the elderly this is magnified by limitations in treatment options because of differences in pharmacokinetics and tolerance as compared with younger counterparts. Pharmacokinetic differences include changes in drug distribution and may arise as a result of diminished end organ function. Age-related decreases in renal function often impact on commonly prescribed antimicrobials. In addition, the elderly are more susceptible to drug-drug interactions because polypharmacy is common in this patient population. Streptogramins may offer a useful alternative in the treatment of infections in the elderly due to their coverage of organisms commonly causing infections in this population and because of their favourable pharmacokinetic profiles. While published experience is limited, streptogramins are not appreciably eliminated by the kidney and, therefore, they are less subject to age-related changes in renal elimination. What is required is multi-dose pharmacokinetic analysis of streptogramins in geriatric populations and subset analysis of patient use data on file. The following will provide the reader with the most recently presented data on streptogramin use and their potential. While focusing on potential use in the elderly, we have cited data and issues which we believe will be relevant in the geriatric population.

Treatment of vancomycin-resistant Enterococcus faecium with RP 59500 (quinupristin-dalfopristin) administered by intermittent or continuous infusion, alone or in combination with doxycycline, in an in vitro pharmacodynamic infection model with simulated endocardial vegetations

Quinupristin-dalfopristin is a streptogramin antibiotic combination with activity against vancomycin-resistant Enterococcus faecium (VREF), but emergence of resistance has been recently reported. We studied the activity of quinupristin-dalfopristin against two clinical strains of VREF (12311 and 12366) in an in vitro pharmacodynamic model with simulated endocardial vegetations (SEVs) to determine the potential for resistance selection and possible strategies for prevention. Baseline MICs/minimal bactericidal concentrations (microg/ml) for quinupristin-dalfopristin, quinupristin, dalfopristin, and doxycycline were 0.25/2, 64/>512, 4/512, and 0.125/8 for VREF 12311 and 0.25/32, 128/>512, 2/128, and 0.25/16 for VREF 12366, respectively. Quinupristin-dalfopristin regimens had significantly less activity against VREF 12366 than VREF 12311. An 8-microg/ml simulated continuous infusion was the only bactericidal regimen with time to 99.9% killing = 90 hours. The combination of quinupristin-dalfopristin every 8 h with doxycycline resulted in more killing compared to either drug alone. Quinupristin-dalfopristin-resistant mutants (MICs, 4 microg/ml; resistance proportion, approximately 4 x 10(-4)) emerged during the quinupristin-dalfopristin monotherapies for both VREF strains. Resistance was unstable in VREF 12311 and stable in VREF 12366. The 8-microg/ml continuous infusion or addition of doxycycline to quinupristin-dalfopristin prevented the emergence of resistance for both strains over the 96-h test period. These findings replicated the development of resistance reported in humans and emphasized bacterial factors (drug susceptibility, high inoculum, organism growth phase) and infectious conditions (penetration barriers) which could increase chances for clinical resistance. The combination of quinupristin-dalfopristin with doxycycline and the administration of quinupristin-dalfopristin as a high-dose continuous infusion warrant further study to determine their potential clinical utility.

Pharmacodynamics of macrolides, azalides, and streptogramins: effect on extracellular pathogens

The efficacy of macrolides against extracellular pathogens depends on extracellular levels of free drug and the organisms' patterns of susceptibility to the macrolides. The effect of macrolides against most bacteria is considered time-dependent. The size of inoculum affects erythromycin's activity against streptococci and, moreover, against staphylococci. The optimal effect is observed at a pH of 8. A significant postantibiotic effect (PAE), lasting approximately 9 hours, has been shown with erythromycin and roxithromycin against gram-positive cocci. Azalides share the same properties. For the streptogramin synercid, a dose-dependent bactericidal activity within a range of low concentrations has been demonstrated. The serum area under the curve appeared to be the best predictor of in vivo effect on the mouse thigh model. Synercid also exhibited a prolonged PAE (approximately 10 hours) against the main pathogens of its spectrum. A better knowledge of the pharmacodynamic properties of macrolides and streptogramins is essential for definition of proper dosing regimens.

Simultaneous high-performance liquid chromatographic determination of quinupristin, dalfopristin and their main metabolites in human plasma

Quinupristin-dalfopristin (30:70, w/w) is a new streptogramin, which has been developed for intravenous use. A specific and sensitive HPLC method was developed to measure simultaneously quinupristin (RP 57669) and dalfopristin (RP 54476) and their main metabolites in human plasma. The metabolites measured by this method were RP 69012 (glutathione-conjugated) and RPR 100391 (cysteine-conjugated) from quinupristin and RP 12536 (natural pristinamycin IIA), from dalfopristin. Solid-phase extraction with disposable cartridges was combined with reversed-phase HPLC and fluorimetric detection for RP 57669, RP 69012 and RPR 100391 and UV detection for RP 54476 and RP 12536. The method provided good recovery and low limits of quantitation (0.025 mg l(-1) for RP 57669, RP 54476 and RP 12536, and of 0.010 mg l(-1) for RP 69012 and RPR 100391). The validated range of concentrations of the method was: 0.025-5000 mg l(-1) for RP 57669, RP 54476 and RP 12536 and 0.010-0.750 mg l(-1) for RP 69012 and RPR 100391.

Pharmacodynamics of RP 59500 (quinupristin-dalfopristin) administered by intermittent versus continuous infusion against Staphylococcus aureus-infected fibrin-platelet clots in an in vitro infection model

We evaluated the bactericidal activity of RP 59500 (quinupristin-dalfopristin) against fibrin-platelet clots (FPC) infected with two clinical isolates of Staphylococcus aureus, one constitutively erythromycin and methicillin resistant (S. aureus AW7) and one erythromycin and methicillin susceptible (S. aureus 1199), in an in vitro pharmacodynamic infection model. RP 59500 was administered by continuous infusion (peak steady-state concentration of 6 microg/ml) or intermittent infusion (simulated regimens of 7.5 mg/kg of body weight every 6 h (q6h) q8h, and q12h. FPCs were infected with S. aureus to achieve an initial bacterial density of 10(9) CFU/g. Model experiments were run in duplicate over 72 h. Two FPCs were removed from each model at 0, 12, 24, 36, 48, and 72 h, and the bacterial densities (in CFU per gram) were determined and compared to those of growth control experiments. Additional samples were also removed from the model over the 72-h period for pharmacokinetic evaluation. All regimens significantly (P < or = 0.01) decreased bacterial densities in the infected FPCs for both isolates compared to growth controls. This occurred even though MBCs were equal to or greater than the RP 59500 concentrations achieved in the models. There were no significant differences found between the dosing frequencies and levels of killing when examining each isolate separately. However, examination of the residual bacterial densities (CFU per gram at 72 h) and visual inspection of the overall killing effect (killing curve plots over 72 h) clearly demonstrated a more favorable bactericidal activity against 1199 than against the AW7 isolate. This was most apparent when the q8h and the q12h AW7 regimens were compared to all 1199 treatment regimens by measuring the 72-h bacterial densities (P < or = 0.01). Killing (99.9%) was not achieved against the AW7 isolate. However, a 99.9% kill was demonstrated for all dosing regimens against the 1199 isolate. The area under the concentration-time curve from 0 to 24 h was found to be significantly correlated with reduction in bacterial density for the AW7 isolate (r = 0.74, P = 0.04). No resistance was detected during any experiment for either isolate. RP 59500 efficacy against constitutively erythromycin- and methicillin-resistant S. aureus may be improved by increasing organism exposure to RP 59500 as a function of dosing frequency.

The pharmacokinetics of quinupristin/dalfopristin in laboratory animals and in humans

The pharmacokinetics of quinupristin/dalfopristin have been studied in rats, monkeys and humans following intravenous infusion of radiolabelled and unlabelled drug. In rats and monkeys quinupristin and dalfopristin undergo rapid elimination from the blood and wide tissue distribution. Nevertheless, they do not penetrate the central nervous system or cross the placenta to any significant degree and they do not appear to be subject to significant body retention following cessation of administration. The blood elimination half-life of quinupristin was approximately 0.6 h in rats and 0.5 h in monkeys, and that of dalfopristin was approximately 0.6 h and 0.2 h, respectively. Both compounds are primarily eliminated through the bile into the faeces; quinupristin is mainly excreted unchanged whereas dalfopristin is extensively metabolized beforehand. The metabolites include the microbiologically active pristinamycin PIIA for dalfopristin and the microbiologically active glutathione- and cysteine-conjugated derivatives for quinupristin. Quinupristin and dalfopristin appear to be handled in a similar manner by humans. Following intravenous administration both compounds are rapidly cleared from the blood with elimination half-lives of approximately 1 h for quinupristin and 0.4-0.5 h for dalfopristin. The pharmacokinetic profile of quinupristin is dose-independent and so is that of dalfopristin and RP 12536 when considered together. Extravascular diffusion of quinupristin/dalfopristin has been assessed in human non-inflammatory interstitial fluid.

Quinupristin-dalfopristin (RP 59500): an injectable streptogramin combination

The pharmacology, pharmacokinetics, activity, and potential clinical role of quinupristin-dalfopristin (RP 59500) are described. Quinupristin-dalfopristin is the first injectable formulation of the streptogramin antibiotics. Streptogramin drug products are each composed of two chemically distinct compounds, which when administered together act synergistically by inhibiting bacterial protein synthesis. Quinupristin-dalfopristin has shown activity in vitro against many strains of streptococci and staphylococci, including methicillin- and erythromycin-resistant strains of staphylococci. The combination is more active against Enterococcus faecium than Enterococcus faecalis. It has also shown activity in vitro against certain gram-negative organisms and anaerobes. The mean maximum blood concentration at the end of a one-hour infusion ranged from 0.95 mg/L for a 1.4-mg/kg dose to 24.2 mg/L for a 29.4-mg/kg dose; there was a linear correlation between dose and mean area under the concentration-time curve. Mean half-life ranged from 1.27 to 1.53 hours. The drug is under investigation in the United States in Phase III trials. Of 60 evaluable patients with documented bacteremia involving E. faecium resistant to vancomycin, 40 (67%) had a favorable clinical response. Of 11 patients with bacteremia caused by methicillin-resistant Staphylococcus aureus, 78% had a favorable response. The efficacy of quinupristin-dalfopristin in treating resistant infections has also been suggested by smaller studies and case reports. The drug may be useful in the prophylaxis of endocarditis. Adverse reactions are generally mild and transient. Quinupristin-dalfopristin may be useful in treating serious gram-positive infections, but more clinical study is needed.

Quinupristin-dalfopristin

Quinupristin-dalfopristin (RP 59500) is an injectable streptogramin antibiotic. It possesses a wide spectrum of activity against Gram-positive bacteria including methicillin-resistant staphylococci, glycopeptide-resistant. Enterococcus faecium and penicillin-resistant pneumococci. Quinupristin-dalfopristin has activity against some anaerobes and selected Gram-negative pathogens. Quinupristin-dalfopristin, by way synergism of between its 2 components, is unaffected by most forms of bacterial resistance. Rare forms of macrolide-lincosamide-streptogramin group B resistance may affect its activity; however, at present the incidence of strains with this type of resistance remains low. Quinupristin-dalfopristin is bactericidal against streptococci and staphylococci but has weak or no bactericidal activity against enterococci. In a compassionate use programme, 67% of 95 evaluable patients with vancomycin-resistant Gram-positive infections or intolerant of vancomycin showed improvement with eradication of infection.

Treatment of vancomycin-resistant enterococci, with a focus on quinupristin-dalfopristin

Enterococci are the second most common cause of hospital-acquired infections, and drug resistance among these organisms is a growing problem. Vancomycin-resistant enterococci (VRE) now account for 7.9% of the nosocomial enterococcal infections. There is no standard therapy for VRE. Although some agents have shown in vitro activity alone or in combination, including ciprofloxacin, doxycycline, novobiocin, teicoplanin, chloramphenicol, and rifampin, treatment options are limited to combinations of drugs with marginal efficacy against the pathogens. Quinupristin-dalfopristin is a new investigational agent with activity against gram-positive cocci, including VRE.

Streptogramins. A unique class of antibiotics

Streptogramin antibiotics represent a unique class of antibacterials in that each member of the class consists of at least 2 structurally unrelated molecules: group A streptogramins (macrolactones) and group B streptogramins (cyclic hexadepsipeptides). Both group A and group B streptogramins inhibit protein synthesis at the ribosomal level, and they act synergistically against many isolates, their combination generating bactericidal activities and reducing the possibility of emergence of resistant strains. The mechanisms of acquired resistance to group B streptogramins are similar to those induced by erythromycin, but group A streptogramins remain unaffected by target modifications and active efflux. The pharmacokinetic parameters of group A and group B streptogramins in blood are quite similar. In addition, both the A and B groups penetrate and accumulate in macrophages and in the bacterial vegetations of experimental endocarditis. There are important structural and biological differences between the streptogramins and the macrolides. The main differentiating features are the rapid anti-bacterial killing of streptogramins and the rarity of cross-resistance between the 2 groups of antibiotics.

Quinupristin/dalfopristin (RP 59500): a new streptogramin antibiotic

OBJECTIVE

To review the current knowledge on RP 59500 (quinupristin/dalfopristin, Synercid), a new streptogramin antibiotic, with respect to its pharmacology, pharmacokinetics, pharmacodynamics, mechanism of resistance, and in vitro inhibitory and bactericidal activity.

DATA SOURCES

A MEDLINE search using keywords RP 59500, pristinamycin, virginiamycin, and streptogramin was performed. Relevant abstracts presented at recent scientific conferences also were consulted.

STUDY SELECTION

Because RP 59500 is a relatively new investigational agent, relevant in vitro and animal studies were selected. All available human studies were included as well.

DATA EXTRACTION

Data from in vitro and in vivo studies were included, with particular emphasis on human studies.

DATA SYNTHESIS

RP 59500 is a new injectable streptogramin antibiotic consisting of a mixture o 2 synergistic pristinamycin compounds. RP 59500 possesses in vitro inhibitory and bactericidal activity against most isolates of gram-positive organisms including vancomycin-resistant Enterococcus faecium, selected gram-negative bacteria, and most anaerobic organisms. Based on preliminary data, the drug appears to be metabolized rapidly and extensively while exhibiting a significant postantibiotic effect. Data from ongoing clinical trials suggests that RP 59500 is well-tolerated except for mild injection site irritations. However, before the role of RP 59500 within the vast armamentarium of antimicrobials can be elucidated, additional studies need to be conducted to document its clinical efficacy.

CONCLUSIONS

Based on in vitro susceptibility testing, in vivo studies, and preliminary clinical data, RP 59500 may be an alternative to the glycopeptides, especially for inherently resistant organisms. Further studies are needed to confirm this agent's in vitro activity and to establish its clinical efficacy.

Bactericidal activity and kinetics of RP 59500 in a mouse model of Staphylococcus aureus septicaemia

The bactericidal activity of RP 59500, a semisynthetic streptogramin, was compared with that of vancomycin against Staphylococcus aureus. This activity was evaluated in vitro by the kill curve method and in vivo using a model of mouse septicaemia. In vitro, RP 59500 (MIC = 0.12 mg/L) was more rapidly bactericidal against S. aureus IP 8203 than was vancomycin (MIC = 1 mg/L). In vivo, RP 59500 (120 mg/kg) was bactericidal against staphylococci in the blood of infected mice 1 h after administration, an effect which lasted for up to 7 h, whereas vancomycin at the same dose was bactericidal only 4 h after administration. The serum concentrations of vancomycin were higher than those of RP 59500 for at least 4 h after administration, and the Cmax and AUC of vancomycin were 4.8 and 8.3 times higher, respectively, than those of RP 59500 (Cmax = 13.2 mg/L; AUC0(-1) = 15.2 mg/L/h), although the agents had similar elimination half-lives (about 0.5 h). RP 59500 was also administered to mice as a fractionated dose (30 mg/kg x 4, 40 mg/kg x 3, 60 mg/kg x 2). The onset of its bactericidal effect was delayed by fractionating the dose, but the suppression of bacteria in the blood was prolonged.

Treatment of experimental endocarditis due to erythromycin-susceptible or -resistant methicillin-resistant Staphylococcus aureus with RP 59500

RP 59500 is a new injectable streptogramin composed of two synergistic components (quinupristin and dalfopristin) which are active against erythromycin-susceptible and -resistant gram-positive pathogens. The present experiments compared the therapeutic efficacy of RP 59500 with that of vancomycin against experimental endocarditis due to either of two erythromycin-susceptible or two constitutively erythromycin-resistant isolates of methicillin-resistant Staphylococcus aureus. RP 59500 had low MICs for the four test organisms as well as for 24 additional isolates (the MIC at which 90% of the isolates were inhibited was < 1 mg/liter) which were mostly inducibly (47%) or constitutively (39%) erythromycin resistant. Aortic endocarditis in rats was produced with catheter-induced vegetations. Three-day therapy was initiated 12 h after infection, and the drugs were delivered via a computerized pump, which permitted the mimicking of the drug kinetics produced in human serum by twice-daily intravenous injections of 7 mg of RP 59500 per kg of body weight or 1 g of vancomycin. Both antibiotics reduced vegetation bacterial titers to below detection levels in ca. 70% of animals infected with the erythromycin-susceptible isolates (P < 0.05 compared with titers in controls). Vancomycin was also effective against the constitutively resistant strains, but RP 59500 failed against these isolates. Further experiments proved that RP 59500 failures were related to the very short life span of dalfopristin in serum (< or = 2 h, compared with > or = 6 h for quinupristin), since successful treatment was restored by artificially prolonging the dalfopristin levels for 6 h. Thus, RP 59500 is a promising alternative to vancomycin against methicillin-resistant S. aureus infections, provided that pharmacokinetic parameters are adjusted to afford prolonged levels of both of its constituents in serum. This observation is also relevant to humans, in whom the life span of dalfopristin in serum is also shorter than that of quinupristin.

Critical influence of resistance to streptogramin B-type antibiotics on activity of RP 59500 (quinupristin-dalfopristin) in experimental endocarditis due to Staphylococcus aureus

In order to determine the microbiological and pharmacokinetic parameters that best predicted the in vivo antistaphylococcal activity of the streptogramin RP 59500 (quinupristin-dalfopristin), we evaluated the activity in rabbit aortic endocarditis of three regimens of quinupristin-dalfopristin against five strains of Staphylococcus aureus with various streptogramin B-type antibiotic resistance phenotypes and susceptible to streptogramin A-type antibiotics. Quinupristin-dalfopristin was as active as vancomycin against three strains that were susceptible to its streptogramin B component quinupristin, including one strain that was inducibly resistant to erythromycin, but had a significantly decreased activity against two strains that were resistant to quinupristin, for all quinupristin-dalfopristin regimens tested (P < 0.05). The area under the concentration-time curve for quinupristin-dalfopristin in plasma divided by the MIC of quinupristin was the only parameter retained by multilinear regression that predicted the in vivo activity of quinupristin-dalfopristin (P = 0.0001), emphasizing the importance of determining the susceptibility to quinupristin in order to predict the in vivo activity of quinupristin-dalfopristin against S. aureus.

Pharmacokinetics and suction blister fluid penetration of a semisynthetic injectable streptogramin RP 59500 (RP 57669/ RP 54476)

RP 59500 is a new semisynthetic injectable streptogramin with excellent activity against most gram-positive bacteria. In order to assess its potential for the treatment of tissue infections, the pharmacokinetics and penetration into suction blister fluid were studied in a pilot phase I study in six male volunteers following a single infusion of 12 mg/kg over 1 h. Plasma and suction blister fluid concentrations were determined by microbiological assay. The mean peak concentration in plasma was 8.65 mg/l at the end of infusion. The mean plasma elimination half-life was 1.48 h. The mean peak concentration in interstitial fluid was 2.41 mg/l and was reached after 1 h in two volunteers and after 2 h in the other four. The mean percentage penetration for the interval 0-6 h was 82.5%. RP 59500 was still detectable in interstitial fluid at 6 h at a mean concentration of 0.92 +/- 0.25 mg/l. The data of this pilot study demonstrate good penetration of RP 59500 into non-inflammatory interstitial fluid.

In vivo activities and penetration of the two components of the streptogramin RP 59500 in cardiac vegetations of experimental endocarditis

We evaluated the in vivo activity and the diffusion of radiolabelled RP 57669 (RPI) and RP 54476 (RPII), the two components of the injectable streptogramin RP 59500, alone or in combination, in aortic vegetations from experimental endocarditis in rabbits. RPI and RPII demonstrated in vitro bacteriostatic and bactericidal synergy against a clinical strain of Staphylococcus aureus resistant to methicillin and susceptible to erythromycin. In experimental staphylococcal endocarditis, RP 59500 was as effective as vancomycin and significantly more effective than RPI (P < 0.01) and RPII (P < 0.05). Autoradiography studies showed different patterns of distribution into cardiac vegetations infected with Streptococcus sanguis for [14C]RPI and [14C]RPII. [14C]RPI was homogeneously distributed throughout the vegetations whereas [14C]RPII showed a decreasing gradient of concentration between the periphery and the core of the vegetation, with an approximately 2:1 ratio. [14C]RPI diffused approximately 2 to 4 times more than [14C]RPII into the core of the vegetations. Since the injected ratio of RPI and RPII is 30:70 in RP 59500, the actual RPI:RPII ratio in the core of the vegetation may range from 0.8 to 1.7, a ratio which remains compatible with the in vivo synergism demonstrated between the two components.

Pharmacodynamic interaction between RP 59500 and gram-positive bacteria infecting fibrin clots

The fibrin clot penetration and in vivo bactericidal activity of RP 59500, a new semisynthetic streptogramin, for two Staphylococcus aureus strains (one methicillin resistant and the other methicillin susceptible), two Staphylococcus epidermidis strains (one methicillin resistant and the other methicillin susceptible), and one Enterococcus faecalis strain were evaluated. The clots, inserted subcutaneously, were infected with a mean of 10(8) CFU of the pathogen per g. For each strain, groups of four rabbits received a single intravenous injection of 50 mg of RP 59500 per kg of body weight over 30 min. The mean peak level of RP 59500 in serum in the infected rabbits was 61.9 +/- 6.3 micrograms/ml. The drug was detectable in serum at a level of 0.8 micrograms/ml up to 4 h after administration. The mean peak fibrin clot drug level at 1 h was 3.3 +/- 0.1 micrograms/g. At 6 h, the level in clots was 1.2 +/- 0.1 micrograms/g. The mean half-life in serum in infected rabbits was 0.34 +/- 0.01 h, while in clots the drug exhibited a longer half-life of 3.8 +/- 0.4 h. In vivo, this new streptogramin sterilized the clots infected with the two S. aureus strains studied in less than 1 h and induced a marked reduction in colony counts of the two S. epidermidis strains studied for up to 24 h. The activity of the streptogramin against E. faecalis was limited. These results suggest that RP 59500 should be further evaluated for the treatment of infection with methicillin-resistant staphylococci.

RP 59500: a proposed mechanism for its bactericidal activity

RP 59500 is a combination of RP 57669 and RP 54476, which are semisynthetic water soluble derivatives of pristinamycin IA (PIA) and pristinamycin IIA (PIIA), respectively. Like their precursors, these molecules are bacteristatic in their own right. In association, they exert bactericidal activity against a variety of Gram-positive bacteria. Experiments involving the binding of these antibiotics to the target bacterial ribosome showed that both the binding sites and the mechanism of action of the components of RP 59500 are identical to those of the parent molecules. By affinity-labelling with a structural analogue of RP 57669, it was demonstrated that L24, a protein of the 70S ribosomal subunit, was specifically labelled. Experiments using radioactive N-ethylmaleimide to label proteins possessing a thiol residue, indicated that proteins L24, L10 and L11 are not only close to each other in the ribosomal structure, but are also adjacent (if not actually part of) the channel through which newly synthesized proteins are extruded. We propose that the mechanism of action of these compounds is to close or narrow the extrusion channel of these proteins, which could lead to their accumulation on the ribosome. We cannot exclude, of course, the possibility that this accumulation disturbs peptidyl-tRNA hydrolase (PHT) activity, thereby depleting free tRNAs within the cell and inhibiting protein synthesis.

Cellular uptake and intracellular bactericidal activity of RP 59500 in murine macrophages

RP 59500, a new antibacterial agent, is a combination of two compounds, RP 54476 and RP 57669. The uptake of radiolabelled RP 59500, i.e. a mixture containing [14C]-RP 54476 plus RP 57669 or [14C]-RP 57669 plus RP 54476, by J 774 murine macrophages was evaluated by a velocity gradient centrifugation technique. After 120 min, the ratios of cellular to extracellular concentration for RP 54476 and RP 57669 were 34 and 50, respectively. The highest intracellular accumulation of RP 59500 was observed at pH 7-7.5. RP 59500 was found to accumulate less at 4 degrees C than at 37 degrees C. The uptake of RP 59500 by dead macrophages was markedly higher than that by live macrophages. As the extracellular concentration of RP 59500 was increased, the intracellular concentration of each component rose, but not proportionally. The metabolic inhibitors sodium cyanide and potassium fluoride both decreased modestly the entry of RP 57669, but not that of RP 54476, into macrophages. After removal of the extracellular antibiotic, RP 54476 and RP 57669 were released rapidly by the cells until equilibrium was established (45% of the original intracellular RP 59500 remained in the cells after 120 min). The intracellular activity of RP 59500 was assessed by incubating macrophages containing ingested Staphylococcus aureus 209P with the drug (10 x MIC: 2.5 mg/L) at 37 degrees C and determining the number of viable cell-associated bacteria. Approximately 70% of the intracellular bacteria were killed within 120 min of incubation. Thus, RP 59500 attains a high intracellular concentration and is active against intracellular S. aureus.

A phase I, double-blind, placebo-controlled study of the tolerance and pharmacokinetic behaviour of RP 59500

The tolerance and pharmacokinetic behaviour of a new injectable streptogramin antibiotic, RP 59500 were evaluated on 26 healthy male volunteers in a double-blind, placebo-controlled, phase I study. The doses used were 1.4, 2.8, 4.6, 7.0, 9.8, 12.6, 16.8, 22.4 and 29.4 mg/kg; each dose was administered as a 1 h infusion to eight subjects, two of them receiving placebo. Blood levels of RP 59500 were measured by both microbiological and HPLC assays. At the end of the infusion, Cmax for RP 59500 ranged from 0.95 +/- 0.22 to 24.20 +/- 8.82 mg/L. The apparent elimination half-life of the compound ranged from 1.27 to 1.53 h. RP 59500 did not significantly affect any of the laboratory or clinical assessments (blood pressure, pulse rate, ECG, peak expiratory flow rate). Reported adverse effects were of mild intensity; the most frequent event was mild pain or burning at the infusion site with doses of 7 mg/kg or higher. These results support further studies of RP 59500 in phase II clinical trials.

Single oral dose pharmacokinetics of the two main components of pristinamycin in humans

A preliminary study of the pharmacokinetics of the two main components of pristinamycin, PIA and PIIA is reported. A single oral dose of 2 g of pristinamycin was administered to six patients with normal renal and hepatic function. The samples were withdrawn during 9.5 h and assayed by HPLC. PIA and PIIA plasma levels evolved in parallel in each subject and their kinetic parameters were comparable: Tmax of 3.25 +/- 1.80 h and 3.08 +/- 1.98 h, Cmax of 0.760 +/- 0.427 mg/l and 0.581 +/- 0.285 mg/l, elimination half-life of 4.03 +/- 2.77 h-1 and 2.83 +/- 0.75 h-1, respectively. The relative proportions of PIA and PIIA, which govern the antibacterial activity of the mixture, were maintained during the study in the range of values leading to 90-100% of optimal activity against Staphylococcus aureus. The sum of the plasma concentrations of PIA and PIIA, which can be considered to be equivalent to the concentration of total pristinamycin, was superior to the MICs for the most susceptible staphylococcal strains during the entire period of study. On the other hand, the sum of PIA and PIIA concentrations exceeded the MICs for the less susceptible strains for only 4 h. These results differ from the scanty pharmacokinetic data presently available, which were obtained by microbiological methods.