Pharmacokinetics and intrapulmonary concentrations of high-dose tigecycline in critically ill patients with severe infections

Background In critically ill patients, the use of high tigecycline dosages (HD TGC) (200 mg/day) has been recently increasing but few pharmacokinetic/pharmacodynamic (PK/PD) data are available. We investigated plasmatic and pulmonary concentrations of HD TGC in the treatment of severe infections. This was a single centre, prospective, observational study that was conducted in the twenty-bed mixed ICU of a 1,500- bed teaching hospital in Rome, Italy. In all patients admitted to the ICU between 2015 and 2018, who received TGC (200 mg loading dose, then 100 mg q12) for the treatment of documented infections, serial blood samples were collected to measure TGC concentrations. Moreover, epithelial lining fluid (ELF) concentrations were determined in patients with nosocomial pneumonia. distribution volume; CL, drug clearance; t1/2, elimination half-life; IAI, intra-abdominal infection; SSTI, skin-soft tissue infection; MALDI-TOF, matrix-assisted laser desorption ionization-time-of-flight; EUCAST, European Committee on Antimicrobial Susceptibility Testing; SOFA, Sequential Organ Failure Assessment; ARF, acute respiratory failure; AKI, acute kidney injury; MV,

TGC is currently approved by the U.S. food and Drug Administration (FDA) for complicated skin and skin-structure infections, complicated intra-abdominal infections, community-acquired pneumonia with an initial dose of 100 mg, followed by 50 mg every 12 hours. Nevertheless, due to an increased risk of death compared to other antimicrobials, its use has recently been restricted in situations when alternative treatments are not suitable [5].
However, the alarming increase in antimicrobial resistance among the nosocomial pathogens is leading the clinicians to consider the use of TGC as an important therapy in the management of difficult to treat infection, particularly in critically ill patients. This is also supported by recent studies suggesting that previous failures of TGC therapy in critically ill patients were likely due to a drug underdosage [6,7] and that standard doses provide serum concentrations that are below the minimum-inhibitory concentrations (MICs) of most MDR pathogens. Moreover, it has been reported an increased effectiveness of high-dose TGC (HD TGC) regimen to improve the clinical outcome, without safety issues [8][9][10][11].
Therefore, we designed this prospective observational study to describe the pharmacokinetic/pharmacodynamic (PK/PD) profile of HD TGC in a cohort of critically ill patients with severe infections.

Patients and study design
This was a prospective, observational study that was performed between 2015 and 2018 in the 20bed ICU of a 1,500-bed teaching hospital in Rome, Italy. The protocol was approved by the Catholic University's Ethical Committee (approval number Prot.sf 8431/13). Written informed consent was obtained from the patients' legally authorized representative. Critically ill adult patients were considered eligible for the study when the attending physician prescribed TGC as empirical treatment (within 12 h from microbiological sampling) of a possible MDR infection, or as targeted therapy based on definitive results, in the absence of any exclusion criteria: known TGC allergy, creatinine clearance less than 40 mL/min (calculated according to the Cocrockft-Gault formula) apart from those ones who were anuric and on continuous renal replacement therapy (CRRT), hyperbilirubinemia (bilirubin level higher than 3 mg/dL), severe hepatic failure (Child-Pugh C), little chance of survival as defined by the Simplified Acute Physiology 2 (SAPS 2) score > 80, concomitant treatment with other drugs that can potentially interfere with TGC. TGC was administered intravenously at loading dose (LD) of 200 mg over 30-min, followed by 100 mg over 30-min bid. On day four after the commencement of the HD TGC, at steady state, pharmacokinetic analyses of the study group were performed. Clinical and demographic data were recorded upon enrolment. Safety and adverse events were determined through the observed biochemical abnormalities, documented according to the Department of Health and Human Services-Common Terminology Criteria for Adverse Events (DHHS-CTCAE v.3.0) classification [12].
Clinical cure was defined as the complete resolution of all signs and symptoms of the infection by the end of TGC therapy. In case of ventilator associated pneumonia (VAP), improvement or lack of progression of all abnormalities on chest radiographs was also required [13]. Clinical outcomes were independently evaluated by two physicians (GDP, MSV) who were blinded to the treatment. When judgments were discordant (about 5% of patients), the reviewers reassessed the data and reached a consensus decision. Adequate source control included drainage of infected fluid collections, debridement of infected solid tissue, removal of devices/foreign bodies, and definitive measures to correct anatomic derangements resulting in on-going microbial contamination and to restore optimal function within 48 h after diagnosis [14].

Sample collection
Blood samples were collected after the seventh dose (on day 4 of treatment) at T0 (immediately before the initiation of the infusion) and 1, 1.5, 2, 4, 6, 8, 10, 12 after the start of infusion. According to patients' respiratory status, one mini-bronchoalveolar lavage (BAL) (40 mL sterile 0.9% saline solution was blindly instilled through a telescopic catheter and immediately aspirated in a trap) was performed at steady state.

Patients characteristics
The clinical details of the 32 non-obese patients included in the study are listed in Table 1. Albumin levels were quite low with an overall positive fluid balance at enrolment. Median SAPS II score was 53.5 and the most relevant comorbidities were cardiovascular diseases, chronic obstructive pulmonary disease, chronic renal failure and neoplasm (Table 1). Median SOFA score was 7 and many patients were in septic shock or presented with acute respiratory failure (ARF) and acute kidney injury

Pharmacokinetic results
A one-compartment model with first-order disposition processes adequately described the  (Table 1). Figure 1 (Fig. 3). Conversely, no significant differences were found comparing mean±SE ELF/plasma ratio at 1 h and 12 h (281±107.6 vs. 298.3±60.7; p = 0.9) (Fig. 4).   Table 2). Similar to plasma 1 h and 12 h, pulmonary concentrations (0.78 mg/L and 0.36 mg/L, respectively) were observed with a good median ELF/plasma ratio of 152.9% (Table 2, Fig. 3). This high-dose regimen was associated with a 65.6% of treatment success rate in a normal weight population including 60% of VAP, 31% of cIAI and 9% of SSTI. TGC was used in half of the cases as targeted regimen for a median duration of 12 days. The rate of septic shock, acute respiratory failure requiring MV and acute kidney injury requiring CRRT was also high, with a mortality rate of 28.1% (Table 1).
The pharmacokinetics/pharmacodynamics and tissue penetration of tigecycline have been extensively studied in various in vitro and human models [17]. However, these studies were generally carried out in healthy volunteers, and few pharmacokinetic data concerning infected patients are available, which may present pathophysiologic conditions influencing the pharmacokinetic profile of this molecule.
Additionally, the majority of available data in infected patients derive from studies where normal doses are used, although for severe nosocomial infections a double-dose regimen is warranted [18,19] Recently, standard dose TGC pharmacokinetics in ten critically ill patients have been studied [6]. The authors observed that a larger body mass index was associated with increased TGC Cl, but standard doses produced satisfactory plasmatic levels for VAP and cIAI treatment due to Enterobacter cloacae, Esherichia coli, Klebsiella pneumoniae and methicillin-resistant Staphylococcus aureus. However higher dosages were required for the treatment of SSTI, especially in obese patients.
Eleven out of 32 patients in our cohort were receiving CRRT while being treated with high-dose TGC.
Interestingly, in a recent paper, Broeker and cow. [20] described the PK/PD of standard dose TGC in eleven patients on continuous veno-venous hemodialysis (CVVHD) or hemodiafiltration (CVVHDF).
TGC dialysability, as expressed by saturation coefficients (0.79 and 0.9 for CVVHD and CVVHDF, respectively) was very high, but the contribution of CRRT TGC clearance was minimal (about 2 L/h), compared with the total body clearance (18.3L/h). Peak drug concentrations were below 1 mg/L and trough levels about 0.2 mg/L. The authors, considering the AUC0-24/MIC referral value for cIAI (6.96), observed that such target was accomplished in 88% of the case if MIC was ≤ 0.5.
Indeed, our results are in line with current available data, underlying the plus-value of increased dosages while treating critically ill patients especially with severe cIAI and SSTI. In addition, there is a high need of PK/PD data on TGC administered at higher than approved dosages, in light of the wide spread of increased resistance to TGC among Gram-negative rods and Acinetobacter spp. The first investigation on PK/PD of HD TGC derives from Ramirez et al who conducted a randomized phase 2 trial to evaluate the clinical efficacy of two high-dosage regimen of TGC (75 mg bid and 100 mg bid) versus imipenem-cilastatin for the treatment of nosocomial pneumonia [8]. In the clinically evaluable population, clinical cure with TGC 100 mg bid was higher than with 75 bid and imipenem-cilastatin (85% vs. 69.6% vs. 75%). Mean peak TGC concentration was about 1 mg/mL, declining to less than 0.5 mg/ml after 8 hours, observing a safety profile comparable to that one known for the approved those. The only other study investigating the PK/PD of HD TGC profile was conducted by Borsuk-De Moor et al in 37 ICU patients with severe infections [21]. Acinetobacter pneumonia [23]. Conversely, the majority of lung penetration occurs in alveolar cells, than in ELF, as suggested by Welte et al. in three cases of MDR lung infections [24]. Finally in a recent study on 58 healthy subjects treated with standard TGC dose, the ratio of ELF and AUC to total plasma concentration of tigecycline was 1.71 and 20.8, respectively [25].
Our study has several limitations. First we adopted a single high-dose of tigecycline and we do not

Availability of data and materials:
The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.

Competing interests:
The authors declare that they have no competing interests