Assessment of Minimum Inhibitory Concentrations of Telavancin by Revised Broth Microdilution Method in Phase 3 Hospital-Acquired Pneumonia/Ventilator-Associated Pneumonia Clinical Isolates

Staphylococcus aureus (S. aureus) infections are a major cause of pneumonia and are especially implicated in hospital-acquired pneumonia (HAP), ventilator-associated pneumonia (VAP), and healthcare-associated pneumonia [1]. The recent Infectious Diseases Society of America guidelines recommend treating HAP/VAP patients with either vancomycin or linezolid [2]. Higher vancomycin minimum inhibitory concentrations (MICs) have been associated with more frequent treatment failure and higher mortality rates [3‐6]. Telavancin (Theravance Biopharma Antibiotics, Inc., George Town, Grand Cayman, Cayman Islands) is a parenteral bactericidal lipoglycopeptide antibiotic that has a dual mechanism of action [7‐10]. Telavancin is approved in the US, Canada, Russia, and Europe for treatment of HAP, including VAP that is caused by susceptible isolates of S. aureus (methicillin-resistant S. aureus [MRSA] only in Europe). In the US, Russia, and Europe, the indication is limited to infections in which alternative medicines are unsuitable [8, 11, 12]. In the Assessment of Telavancin for Treatment of Hospital-Acquired Pneumonia [ATTAIN (ClinicalTrials.gov identifiers NCT00107952 and NCT00124020)] studies, a total of 1503 patients with HAP were treated with telavancin or vancomycin for up to 21 days. Telavancin demonstrated noninferiority to vancomycin, achieving similar cure rates [13].

Drug loss due to binding to plastics in the MIC assay has been found with other lipoglycopeptides and is resolved with the addition of polysorbate 80 (P-80) [14, 15]. To improve the accuracy and precision of telavancin susceptibility testing, a revised broth microdilution method (rBMD) MIC testing method for telavancin was approved by the US Food and Drug Administration (FDA) and published by Clinical and Laboratory Standards Institute (CLSI) in 2014 to include dimethyl sulfoxide (DMSO) in the diluent to improve solubility and P-80 in the broth microdilution assay cation-adjusted Mueller–Hinton broth (CAMHB) to reduce drug loss due to binding to plastic [8, 16, 17]. This revised method has produced up to 8-fold lower telavancin MIC results compared with the previous method [17] and has been used to establish new values for telavancin activity against a variety of clinical isolates [18, 19]. The FDA and CLSI previously have approved ≤0.12 µg/mL as the telavancin MIC susceptibility breakpoint for S. aureus [both methicillin-susceptible S. aureus (MSSA) and MRSA] isolates; intermediate or resistant breakpoints for telavancin have not been established as of 2016, due to the rarity of telavancin-resistant S. aureus isolates [8, 20]. The objective of this study was to determine the effect of the revised method on telavancin MIC values for S. aureus clinical isolates obtained from the ATTAIN HAP/VAP patients and evaluate the clinical outcome by the revised telavancin MICs. These results were part of the data package presented to breakpoint setting committees, including FDA, CLSI, and the European Committee on Antimicrobial Susceptibility Testing for the evaluation of the revised telavancin breakpoints.

The MIC values obtained using the rBMD method for telavancin and control agents tested against ATTAIN S. aureus isolates were within the CLSI-approved and accepted QC ranges determined using S. aureus ATCC 29213 (telavancin 0.03–0.12 µg/mL, ampicillin 0.5–2.0 µg/mL, linezolid 1.0–4.0 µg/mL, vancomycin 0.5–2.0 µg/mL) [16].

Telavancin MICs for all S. aureus isolates ranged from 0.015 to 0.12 μg/mL, with a range of 0.015 to 0.12 μg/mL for MSSA isolates and 0.015 to 0.12 μg/mL for MRSA isolates (Table 1). For the overall set of S. aureus isolates and for MSSA isolates, telavancin MIC_50/90 values were 0.06/0.06 μg/mL; for MRSA, the telavancin MIC_50/90 values were 0.06/0.12 μg/mL. These results represent a substantial downward shift in telavancin MIC values for S. aureus compared with the MICs obtained using the original method (Fig. 1), which produced higher values of MICs for telavancin (MIC_50/90 of 0.25/0.50 µg/mL for the total pool of S. aureus isolates).

Among the 528 isolates with reduced susceptibility to vancomycin (MIC ≥1 µg/mL), telavancin MIC_50/90 values obtained using the rBMD method were 0.06/0.12 µg/mL. Among these samples, 166 were MSSA and 362 were MRSA, with MIC_50/90 values of 0.06/0.06 µg/mL for MSSA and 0.06/0.12 µg/mL for MRSA, respectively (Table 1). These values were lower than the telavancin MIC_50/90 values for isolates with reduced vancomycin susceptibility generated by the original method, which were 0.25/0.5 µg/mL and 0.5/0.5 µg/mL for MSSA and MRSA, respectively. The isolates with reduced susceptibility to vancomycin remained susceptible to telavancin (rBMD MIC_50/90) when using the revised FDA-approved breakpoint of <0.12 µg/mL [8].

A total of 183 isolates exhibited a ±1 dilution change in the vancomycin MIC ≥1 µg/mL upon retesting. Of those, 1 increased from 0.25 to 0.5 µg/mL, 110 increased from 0.5 to ≥1 µg/mL, 11 increased from 1 to 2 µg/mL, 50 decreased from 1 to 0.5 µg/mL, and 11 decreased from 2 to 1 µg/mL. A total of 2 isolates exhibited a 2-dilution change in the vancomycin MIC upon retesting with 1 increasing from 0.5 to 2.0 µg/mL and 1 decreasing from 2.0 to 0.5 µg/mL. These changes can be attributed to random testing differences between two laboratories (Table 1).

The clinical cure rate by revised MIC was assessed in the microbiologically evaluable population of telavancin-treated patients (195 patients). The overall telavancin clinical cure rate for S. aureus was 78% (153/195 patients), with clinical cure rates of 83% (62/75 patients) and 76% (91/120 patients) for MSSA- and MRSA-infected patients, respectively (Table 2). The clinical cure rate did not decrease with increasing telavancin MIC values for S. aureus obtained using the rBMD method, which ranged from 0.03 to 0.12 µg/mL, as clinical cure rates remained ≥81% and ≥70% for MSSA- and MRSA-infected patients, respectively, for all MIC values (Table 2). Among the 166 microbiologically evaluable telavancin-treated patients with S. aureus isolates that demonstrated reduced susceptibility to vancomycin (MIC ≥1 µg/mL), the clinical cure rate was 79% (131/166 patients) for telavancin. The microbiological eradication rates for the overall S. aureus, individual MSSA and MRSA isolates, and isolates with reduced vancomycin susceptibility (MICs ≥1 µg/mL) were comparable (Table 2) [13].

Lipoglycopeptide drug loss due to its binding to plastics is known to affect the MIC determination. The addition of P-80 to the MIC assay has resolved this issue and the FDA-approved rBMD method includes DMSO and P-80 to improve drug solubility and reduce drug loss, respectively [8, 14‐17]. Telavancin is a lipoglycopeptide active against a wide range of susceptible Gram-positive pathogens, including S. aureus [13]. The objective of this study was to re-evaluate the ATTAIN clinical isolates with the rBMD method that has previously demonstrated lower telavancin MIC values [8, 16, 17].

Telavancin MIC values for MSSA and MRSA obtained using the rBMD method were lower than those that have been published elsewhere using the original method [21, 22], indicating that telavancin is more active in vitro against S. aureus isolates than previously considered. In this study, the rBMD method produced an overall 4-fold decrease in the telavancin MIC_50/90 for S. aureus compared with the original method. All isolates were susceptible to telavancin, consistent with other studies reporting telavancin susceptibility of S. aureus using the rBMD method [18, 19]. Telavancin also retained its in vitro potency against isolates with reduced susceptibility to vancomycin (MIC ≥1 µg/mL). Furthermore, the reassessment of ATTAIN study clinical isolates using the rBMD method demonstrated robust clinical cure and microbiological eradication even at the highest telavancin MIC (0.12 µg/mL) observed in the study.

This study is limited by its retrospective nature. The patients were not prospectively stratified by MIC and the reduced sample size prevented extensive statistical analyses of the clinical cure rates. However, the Clopper–Pearson (Exact) confidence interval method [22] was applied to demonstrate that the clinical cure rates were comparable across all S. aureus isolates and telavancin MICs.

Reassessment of the ATTAIN isolates using the rBMD method demonstrated increased in vitro potency of telavancin against S. aureus. These data support lowering the telavancin susceptibility breakpoint to 0.12 µg/mL for S. aureus. Moreover, these results suggest that published studies using the previous BMD underestimated the in vitro potency of telavancin [23, 24].