Vancomycin is one of the mainstays of therapy for MRSA infections. In adults, IV vancomycin at a dose of 15-20 mg/kg/dose (max 2 g/dose) every 8-12 h based on renal function is recommended with a loading dose of 25-30 mg/kg in seriously ill patients[7]. Therapeutic drug monitoring (TDM) is recommended to ensure adequacy of dosing, with most infections necessitating trough concentrations of 10-20 μg/mL, with concentrations at the higher end of this range (i.e., 15-20 μg/mL) reserved for difficult to penetrate sites such as pulmonary and central nervous system. However, in skin and skin structure infections (SSTIs), trough monitoring may not be necessary and vancomycin in a dose of 1 mg every 12 h may be adequate[6].

Over the years of vancomycin use, resistance is now beginning to emerge in MRSA isolates[8]. Vancomycin has several limitations. First is the ratio of minimum bactericidal to inhibitory concentration (MBC: MIC ratio). A study from Sader et al[9] demonstrated that 20.1% of tested MRSA strains (n = 900) were vancomycin tolerant defined by MBC: MIC ratio of ≥ 32. This varied from 10.0% to 43.0% among different centres evaluated[9]. Secondly, the accessory gene regulator pathway is associated with regulation of quorum sensing and endotoxin production[10]. Development of polymorphisms or loss of function of accessory gene regulator (agr) pathway is associated with failure of vancomycin therapy[11]. Thirdly, the “MIC creep” phenomenon wherein there is a gradual reduction in susceptibility of S. aureus to vancomycin despite concentrations in the susceptible range (≤ 2 mg/L) can develop with continued use of vancomycin[8]. A study from California by Wang et al[12], demonstrated a gradual shift of MIC from ≤ 0.5 to 1.0 µg/mL over 5 years to vancomycin in MRSA strains (n = 6002). The proportion of isolates with MIC 1 µg/mL increased from 19.9% to 70.4% over study duration (Figure 1). Fourth concern is development of hetero-resistance to vancomycin (hVISA). In this phenomenon, from among the isolated MRSA, a subpopulation demonstrates intermediate level of vancomycin resistance, but the colony as a whole remains susceptible. The mechanisms for this remains unclear but may involve thickening of cell wall avoiding penetration of vancomycin, and alteration in agr pathway[10]. A study from Sader et al[9] involving nine hospitals in the United States showed hVISA prevalence of 13.4%. The development of hVISA was more common (45.6%) in MRSA isolates with MIC ≥ 1 mg/L. Fifth, the extensive protein binding of vancomycin leads to variable tissue penetration which can further be different in comorbidities like diabetes, meningitis, etc[10]. Sixth, the pharmacodynamics of vancomycin has been considered to be an important aspect in determining efficacy. The area under the curve (AUC) and MIC ratio of 400 or more is believed to provide therapeutic effectiveness for which vancomycin trough concentration should reach 15-20 mg/L especially in severe MRSA infections[7]. For achieving AUC: MIC ratio of 400 or more at MIC of 1 mg/L, dose of 3-4 mg/d is necessary. For MIC of 2 mg/L, achieving target AUC: MIC ratio is not possible even when higher doses are used. This can result in poor clinical and microbiological cure.

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Figure 1 Minimal inhibitory concentration creep - Proportion of MRSA isolates with vancomycin minimal inhibitory concentration of 1 µg/mL[12].

Nephrotoxicity is an important adverse effect associated with vancomycin. The reported incidence varies from nearly 14% in children to 35% in adults. In adults, trough concentration beyond 15 µg/mL is associated with increased risk of renal injury. Attaining AUC: MIC ratio of ≥ 400 is therefore harmful especially wherein the isolate MIC is > 2 mg/L. In such cases, use of alternative agents is advised[13].

Daptomycin, a branched cyclic anionic lipopeptide exerts bactericidal action via calcium-dependent modification in membrane potential causing leaking of intracellular ions and cell death[14]. It has shown similar efficacy to vancomycin in MRSA bacteraemia, endocarditis, complicated SSTIs, but not in pneumonia due to inactivation by alveolar surfactant[10]. However, point mutation in MprF gene (L431F substitution) identified in clinical isolates was associated with reduced negative cell membrane charge, thicker cell wall, and longer doubling time. This was found to confer increased resistance to daptomycin and vancomycin[15]. Daptomycin-non-susceptible (DAP-NS) phenotype has also been reported in MRSA infections. Among 2.4% DAP-NS strains (n = 208), one was sequence type 72 (ST72) and other four were ST5. Three of these strains were also found to be hVISA. The resistance mechanism in ST72 was charge repulsion, ST5 showed charge independent mechanisms. Changes in cell wall thickness were not found in any of the DAP-NS strains[16]. DAP-NS isolates were not sensitive to high-dose of daptomycin[17]. Increased MIC of daptomycin was found to be associated with increased mortality in patients with MRSA bacteraemia[18]. Finally, daptomycin has been associated with elevated creatine kinase and rhabdomyolysis, which is problematic in critically ill patients already at risk of such increases and sequalae thereof, such as renal injury[19].

Linezolid, a synthetic antibiotic, binds to ribosomal RNA on both 30S and 50S subunits and thereby inhibits protein synthesis. Additionally, it inhibits formation of initiation complex and reduce the rate of translation process[20]. Occurrence of serious adverse drug reactions like thrombocytopenia, optic neuropathy, peripheral neuropathy, lactic acidosis, and potential serotonin syndrome through monoamine oxidase inhibition have important therapeutic limitations resulting in poor adherence to therapy[21]. MIC creep with linezolid similar to that of vancomycin has also been reported[22]. Being a bacteriostatic agent, its first line use in severe invasive infections especially bacteraemia and endocarditis is avoided[10].

In persistent MRSA bacteraemia (> 7 d) despite therapy with glycopeptides like vancomycin or teicoplanin, shifting to linezolid failed to show superiority in microbiologic response, treatment success, and mortality compared to the patients who continued glycopeptides[23].

Teicoplanin: TDM may be necessary in ascertaining the teicoplanin concentrations as daily dosages of 4 mg/kg have been reported to result in treatment failure compared to a 6 mg/kg dose. Also, trough concentrations of > 10, > 20, and > 30 mg/L have been reported to be necessary for successful treatment of S. aureus septicemia, MRSA endocarditis, and MRSA osteomyelitis, respectively[24]. Also, given its important role in MRSA management, there is more need to generate evidence on pharmacokinetics and clinical pharmacodynamics[24].

Trimethoprim-sulfamethoxazole (TMP-SMX): In MRSA infections, its utility is limited by development of resistance and poor efficacy[25,26]. Therefore, TMP-SMX is mainly confined to treatment of uncomplicated skin and skin structure infections from an MRSA standpoint[27].

Clindamycin: Clindamycin has bacteriostatic activity and high rates of inducible and constitutive resistance, limiting its utility for MRSA infections[28,29]. Further, risk of Clostridium difficile infection (CDI) might deter use of clindamycin as sole agent for MRSA as duration of exposure has been identified as an important determinant of CDI[30].

Tetracyclines: Tetracyclines such as doxycycline and minocycline are limited to uncomplicated SSTIs by community-acquired MRSA. Bacteriostatic activity and limited spectrum limits utility in severe invasive MRSA infections[10].

Fucidin (fusidic acid): Fusidic acid inhibits bacterial protein synthesis via action on RNA. As a topical agent, it has been used for treatment of skin infection, though there has been recent interest in rectifying its use in combination with rifampicin for infected joint prostheses. This however has been limited by significant drug-drug interactions resulting in ineffective fusidic acid exposure[31].

Tigecycline: Tigecycline has shown promise in MRSA infections equivalent to vancomycin[32]. It is effective in SSTIs and complicated intraabdominal infections[33]. However, high protein binding can result in low serum levels thereby limiting effectiveness in MRSA bacteraemia. Black box warning issued from the US Food and Drug Administration for all-cause mortality, mortality imbalance and lower cure rates in VAP and pancreatitis is a concern with tigecycline[34].

Quinupristin/Dalfopristin: Quinupristin/Dalfopristin is considered among the effective agents in Staphylococcal infections and may be effective in MRSA bacteremia[35]. However, occurrence of side effects like infusion-site inflammation, pain, and edema, thrombophlebitis, arthralgia, myalgia, nausea, diarrhoea, vomiting, and rash limit its use. Also, inhibition of cytochrome P450 3A4 with qui-nupristin/dalfopristin warrants caution with use of drugs metabolized through this enzymatic pathway[36]. Interference with other drugs metabolism may result in QTc prolongation with use of quinupristin/dalfopristin.

Ceftaroline: Ceftaroline is an effective agent for severe MRSA infections and provides clinical cure in nearly 74% cases. The major concern with this agent is development of agranulocytosis. Prolonged therapy (≥ 21 d) increases risk of leukopenia and therefore treatment with ceftaroline should be closely monitored in these situations[37].

Telavancin: It is another effective agent in MRSA with resistance to vancomycin, linezolid and daptomycin. However, nephrotoxicity is an important limitation. An increased mortality has been observed in hospital or ventilator associated pneumonia[38].

Oritavancin and Dalbavancin: These lipoglycopeptides have ultra-long half-life upwards of 346 h making them attractive as single-dose antibiotics. This and the inability to remove via dialysis, however also raises a concern as injury resulting from delayed hypersensitivity (if occurs) or other adverse effects may persist for weeks. It’s effectiveness has not been established in bacteraemia, pneumonia, bone and joint infections, or prosthetic infections[39]. While these agents have potential for ambulatory infectious diseases management, particularly in areas of poor clinic access for frequent intravenous infusions, their utility in acute and critical care remains to be proven.

Rifampicin is bactericidal to S. aureus, achieves high intracellular concentration, and penetrates biofilms. A systematic review in 2008 reported that in-vitro findings identified with rifampicin combination did not relate to in-vivo findings[41]. Another review in 2013 reported limited evidence to support adjunctive use of rifampicin in MRSA infections. The increased risk of drug interactions, adverse effects with rifampicin and development of rifampicin resistance are possibilities with use of rifampicin in combination[42]. Latter is especially important in Indian context where the rifampicin is the primary drug against tuberculous infection and burden of tuberculosis is enormous. Currently, IDSA guidelines recommend use of rifampicin in combination only in prosthetic valve endocarditis and in osteoarticular infections associated with prostheses[6]. Rifampicin should not be used as monotherapy for the treatment of MRSA infections.

In vitro studies have demonstrated increased bactericidal activity of vancomycin and animal studies have shown to shorten the duration of bacteraemia. Nephrotoxicity associated with gentamycin can add to the nephrotoxic potential of vancomycin[40].

Laboratory analyses have shown synergism with this combination[10]. However, clinical evidence is restricted to case reports.

Similar to other combination treatments, the evidence from in-vitro studies shows synergistic activity with this combination as well[43,44]. However, clinical evidence is restricted to case reports[45-47]. In time-kill study, addition of gentamicin rather than rifampicin has been shown to provide synergism with daptomycin[48].

With beta-lactams active against MRSA (e.g. ceftaroline), daptomycin has shown synergistic activity[49]. In MRSA strains from endocarditis, ceftaroline in addition to daptomycin also cleared daptomycin non-susceptible strains. Daptomycin at 6 mg/kg every 48 h was and ceftaroline at 200 mg every 12 h enhanced bacterial killing[50]. The finding from this single study demands further careful determination of optimal dosing regimen for effective utilization of active agents like ceftaroline. Another study reported rapid clearance of bacteraemia with addition of high dose nafcillin or oxacillin (2 mg IV every 4 h) to high-dose daptomycin (8-10 mg/d) in 7 cases of vancomycin and daptomycin resistant MRSA[51]. Though this points to enhanced efficacy of beta-lactams, further evaluation in prospective studies is necessary.

An in-vitro study involving pharmacokinetic/pharmacodynamic model of biofilm for 3 d showed greater activity with combination of daptomycin and linezolid than either agent alone suggesting potential for biofilm associated MRSA infections[52]. However, there is lack of clinical studies to substantiate the findings of in-vitro studies.

In combination with rifampicin, time kill studies of linezolid did not show synergism or antagonism but linezolid prevented emergence of mutant resistance in rifampicin[53]. One major issue with this combination is that rifampicin can reduce the linezolid concentration which can be well below the MIC90 for Staphylococci and effect may persist longer than 3 wk even after withdrawal of rifampicin[54,55]. With tedizolid and rifampicin combination, activity is increased but synergy observed was not found to be universal[56].

Poor efficacy, development of resistance and side effects and drug interactions as mentioned above in their individual discussion, render this regimen redundant.

The evidence is very limited for effectiveness and utility of triple drug combinations including beta-lactams, aminoglycosides, and vancomycin, barring isolated case reports[57].