In vitro antimicrobial activity and MIC quality control guidelines of RPR 106972 (RPR 112808/RPR106950): a novel orally administered streptogramin combination. The Quality Control Study Group

Quinupristin/Dalfopristin

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Quinupristin/dalfopristin

Gram-positive pathogens are associated with both community- and hospital-acquired infections. These infections may be life-threatening in hospitalised patients, especially in those with significant underlying acute or chronic diseases. Prompt and appropriate antimicrobial therapy is essential for avoiding morbidity and mortality. The concept of appropriate therapy is being redefined by increasing antimicrobial resistance, especially amongst Gram-positive pathogens. This has been most dramatic with penicillin-resistant Streptococcus pneumoniae in the community, including cross-resistance to other classes of antimicrobial agents. In the US, the incidence of methicillin-resistant Staphylococcus aureus (MRSA) with community isolates is significant. For hospital-acquired Gram-positive pathogens, MRSA, vancomycin-resistant Enterococcus species and vancomycin-intermediate resistant and -resistant S. aureus are a great concern, particularly as the frequency of recovery of these pathogens from infected patients increases. The net result of these various resistance issues is a reduction in the number of appropriate antimicrobial agents for treating infected patients. Quinupristin/dalfopristin is a parental streptogramin with a spectrum of activity that includes Gram-positive pathogens, including those resistant to other classes of antimicrobial compounds. In this review, data summarising the frequency of recovered Gram-positive pathogens from various infectious diseases, the escalating prevalence of resistance amongst Gram-positive pathogens and the factors making quinupristin/dalfopristin a suitable agent for treating patients infected with Gram-positive organisms will be discussed.

Novel streptogramin antibiotics

Streptogramins represent a unique class of antibiotics remarkable for their antibacterial activity and their unique mechanism of action. These antibiotics are produced naturally as secondary metabolites by a number of Streptomyces species and have been classified into two main groups. They consist of at least two structurally unrelated compounds, group A or M (macrolactones) and group B or S (cyclic hexadepsipeptides). Both groups bind bacterial ribosomes and inhibit protein synthesis at the elongation step and they act synergistically in vitro against many microorganisms. Streptogramins A and B act synergistically in vivo; the mixture of the two compounds is more powerful than the individual components and their combined action is irreversible. The pharmacokinetic parameters of group A and B streptogramins in blood are similar. The major gap, limiting the therapeutic use of the natural compounds, was represented by the lack dissolution in water. The synthesis of water-soluble derivatives of pristinamycin I(A) and II(B) has allowed the development of injectable, first represented by quinupristin/dalfopristin (Synercid) and oral formulations, represented by RPR-106972, streptogramins with fixed compositions. Streptogramins have demonstrated activity against Gram-positive microorganisms in vitro and in vivo, including those with multi-drug resistance. Moreover, the absence of cross-resistance to macrolides in many of these microorganisms and the rarity of cross-resistance between the two groups of antibiotics associated with the rapid bacterial killing are the principal features of the streptogramins, offering the possibility for treating the rising number of infections that are caused by multi-resistant Gram-positive bacteria.

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.

Staphylococcal resistance to streptogramins and related antibiotics

Streptogramin and related antibiotics are mixtures of two compounds, A and B (e.g. Dalfopristin and Quinupristin), particularly against Gram-positive bacteria. Staphylococci resistant to these mixtures are always resistant to the A compounds but are not necessarily resistant to the B compounds. Resistance to A compounds and to the mixtures is conferred by acetyltransferases or ATP-binding proteins via unknown mechanisms. Several genes encoding each of the two categories of protein have been characterized and regularly detected on plasmids. Genes encoding lactonases, which inactivate B compounds, have been occasionally detected on these plasmids. Staphylococci which harbour plasmids conferring resistance to A compounds should not be treated with the mixtures even if they appear susceptible in vitro. Indeed, susceptibility to the mixtures of staphylococci carrying resistance to A compounds has often been attributed to partial loss of the plasmids conferring this resistance. When staphylococci are constitutively resistant to B compounds, the in vitro activities of the mixtures should be evaluated, because they are better correlated than MICs with their efficacy in therapy.