Gram-Positive Bacterial Infections

Cardiac infections, including infective endocarditis and cardiovascular implantable electronic device infections, are associated with substantial morbidity and mortality and are commonly caused by gram-positive bacteria. Standard management typically requires prolonged courses of intravenous antibiotics and extended hospitalisation, which are costly, burdensome, and associated with complications related to long-term vascular access. People who inject drugs are disproportionately affected and often experience stigma, barriers to care, and poorer outcomes. Long-acting lipoglycopeptides such as oritavancin maintain therapeutic serum concentrations for prolonged periods and may offer an alternative to conventional intravenous antibiotic regimens. Oritavancin is not TGA-registered in Australia and is accessed as an unregistered medicine (for example, via SAS or clinical trials). It is approved in other jurisdictions, including the United States and European Union, for acute bacterial skin and skin structure infections. Prospective data in cardiac infections remain limited, and optimal dosing strategies, including the role of therapeutic drug monitoring, are uncertain. This multicentre, open-label pilot study will assess the feasibility, pharmacokinetics, safety, acceptability, and preliminary efficacy of oritavancin for gram-positive cardiac infections using both standard fixed dosing and TDM-guided dosing strategies. Findings will inform PK/PD modelling, the potential role of TDM, and the design of future larger-scale trials and models of care, including alternatives to prolonged inpatient intravenous therapy.

Study Rationale: Previous in vitro and retrospective in vivo studies suggest that optimal linezolid concentrations (between 2 and 7 mg/L) achieve clinical efficacy and microbiological eradication while minimizing side effects like thrombocytopenia and the emergence of resistance. No prospective or randomized clinical trial has confirmed these findings, and there is no consensus on how to adjust linezolid dosing to achieve optimal drug concentrations. Objectives: The primary objective is to determine if model-informed precision dosing optimizes linezolid dosing to achieve therapeutic trough concentrations compared to a standard dose. Secondary objectives include assessing the PK/PD profile, investigating the prevalence of linezolid resistance among gram-positive bacteria, assessing microbiological resolution of infection, and evaluating the safety and tolerability of linezolid. Methodology: This study is an open, monocentric pilot randomized controlled trial with two arms: standard dose therapy versus dose adjustment based on model-informed precision dosing using therapeutic drug monitoring and PK/PD targets developed in TMDx software. Sample Size: 28 patients, 14 in each group. Assumptions are based on only 25% of patients in intensive care achieving the optimal therapeutic range with standard dosing, compared to an expected 80% achieving this with model-informed precision dosing. Selection Criteria: Adult patients (18+ years) already starting linezolid treatment for gram-positive infections, expected to require treatment beyond the next calendar day. Exclusions include imminent death, expected or confirmed pregnancy, expected linezolid treatment of less than 4 days or more than 4 weeks. Outcomes: The primary endpoint is defined as the difference in the proportion of patients in the intervention and in the control groups who maintained a trough linezolid concentration of 2 to 7 mg/L on Day 7 and Day 13.

The incidence and prevalence of nontuberculous mycobacteria (NTM) infections have gradually increased over the years worldwide (1-3). In China, Mycobacterium avium complex (MAC) was the most prevalent NTM specie (4), while challenged by long treatment duration, frequent drug-induced adverse events, lack of treatment alternatives, poor treatment outcome and high recurrence rate (5, 6). In order to maximize the efficacy of the few available drugs and prevent the development of drug resistance, ensuring adequate plasma drug concentrations are of importance. Despite the role of pathogen susceptibility, determined by minimum inhibitory concentration (MIC), is non-negligible, the evidences regarding its association with treatment outcome are limited, especially for rifamycin and ethambutol. The difficulties in explaining the clinical values of MIC might partially be attributed to the lack of in vivo drug exposure data, which cannot be accurately predicted by the dose administered because of between-patient pharmacokinetic variability (7). Therapeutic drug monitoring (TDM) is a strategy to guide and personalize treatment by measuring plasma drug concentrations and pathogen susceptibility, which might have the potential to improve treatment response to MAC lung disease. In this observational study, the hypothesis is that the drug exposure and/or MIC of antimycobacterial drugs are correlated to the treatment response of MAC lung disease, which is assessed from the perspective of treatment outcome, mycobacterial culture negative conversion, lung function, radiological presentation and self-reported quality of life. Consenting adult patients with culture-positive MAC lung disease will be recruited in study hospital. Respiratory samples (sputum and/or bronchoalveolar lavage fluid) will be collected regularly for mycobacterial culture on the basis of BACTEC MGIT 960 system and MIC will be determined using a commercial broth microdilution plate. Drug concentrations will be measured at 1 and/or 6 months after treatment initiation using liquid chromatography tandem mass spectrometry (LC-MS/MS). The final treatment outcome is recorded at the end of MAC treatment and defined according to an NTM-NET consensus statement (8).