The treatment of drug-resistant Mycobacterium tuberculosis relies on complex antibiotic therapy. Inadequate antibiotic exposure can lead to treatment failure, acquired drug resistance, and an increased risk of adverse events. Therapeutic drug monitoring (TDM) can be used to optimize the antibiotic exposure. Therefore, we aimed to develop a single-run multiplex assay using high-performance liquid chromatography–mass spectrometry (HPLC–MS) for TDM of patients with multidrug-resistant, pre-extensively drug-resistant and extensively drug-resistant tuberculosis. A target profile for sufficient performance, based on the intended clinical application, was established and the assay was developed accordingly. Antibiotics were analyzed on a zwitterionic hydrophilic interaction liquid chromatography column and a triple quadrupole mass spectrometer using stable isotope-labeled internal standards. The assay was sufficiently sensitive to monitor drug concentrations over five half-lives for rifampicin, rifabutin, levofloxacin, moxifloxacin, bedaquiline, linezolid, clofazimine, terizidone/cycloserine, ethambutol, delamanid, pyrazinamide, meropenem, prothionamide, and para-amino salicylic acid (PAS). Accuracy and precision were sufficient to support clinical decision making (≤±15% in clinical samples and ±20–25% in spiked samples, with 80% of future measured concentrations predicted to fall within ±40% of nominal concentrations). The method was applied in the TDM of two patients with complex drug-resistant tuberculosis. All relevant antibiotics from their regimens could be quantified and high-dose therapy was initiated, followed by microbiological conversion. In conclusion, we developed a multiplex assay that enables TDM of the relevant first- and second-line anti-tuberculosis medicines in a single run and was able to show its applicability in TDM of two drug-resistant tuberculosis patients.

Mycobacterium (M.) tuberculosis mostly spreads from active tuberculosis (TB) patients to human contacts, although human-to-animal and animal-to-human transmission has been described. Here, we present a rare case of rifampicin-resistant tuberculosis (RR-TB) transmission from a companion dog to its owner, highlighting the zoonotic potential of the pathogen. Namely, a 37-year-old Croatian man was diagnosed with RR-TB, with whole-genome sequencing analysis revealing a close genetic link to the strain isolated from his dog, which had died of miliary TB six years earlier. This case emphasizes the complexity of TB transmission dynamics, particularly involving companion animals, and underlines the importance of integrated “One Health” approaches for TB control. Awareness of zoonotic TB risks is essential for the early detection and prevention of cross-species transmission, especially in vulnerable populations and households with close human–animal contact.