Restoring Control: Real-World Success with Imipenem–Relebactam in Critical MDR Infections—A Multicenter Observational Study

Background: Multidrug-resistant (MDR) Gram-negative infections, particularly those caused by carbapenem-resistant Enterobacterales (CRE) and difficult-to-treat Pseudomonas aeruginosa (DTR-Pa), present a growing global healthcare challenge, especially in critically ill populations. Imipenem–relebactam (I/R), a novel β-lactam/β-lactamase inhibitor combination, has shown efficacy in clinical trials, but real-world data remain limited. Methods: We conducted a multicenter, retrospective–prospective observational study across tertiary-care hospitals in Italy between January 2020 and May 2025. Adult patients (≥18 years) treated with I/R for ≥48 h for suspected or confirmed MDR Gram-negative infections were included. Primary endpoints were clinical success at the end of therapy and 30-day all-cause mortality. Secondary endpoints included microbiological eradication, recurrence, safety, and predictors of treatment failure. Statistical analysis involved descriptive methods and correlation analysis for mortality predictors. Results: Twenty-nine patients were included (median age 66 years; 58.6% ICU admission; 71.4% mechanical ventilation). Clinical success was achieved in 22/29 patients (75.9%), while 30-day mortality was 24.1% (7/29). The most common pathogen was Klebsiella pneumoniae (62.1%), with 41.4% of infections being polymicrobial. Microbiological eradication was confirmed in all the BSIs. Parenteral nutrition (p = 0.016), sepsis at presentation (p = 0.04), candidemia (p = 0.036), and arterial catheter use (p = 0.029) were significantly more frequent in non-survivors. Survivors showed significant reductions in CRP, PCT, and bilirubin at 48 h, while non-survivors did not. Parenteral nutrition (rho = 0.427, p = 0.023), sepsis (rho = 0.378, p = 0.043), and arterial catheter use (rho = 0.384, p = 0.04) were significantly correlated with mortality. Conclusions: In this Italian multicenter cohort of critically ill patients, imipenem–relebactam demonstrated high clinical success and acceptable mortality rates in the treatment of severe MDR Gram-negative infections, particularly those caused by KPC-producing K. pneumoniae. Early biomarker dynamics may aid in monitoring treatment response. Larger prospective studies are needed to confirm these findings and define optimal treatment strategies.