A 48-year-old woman admitted to the ICU with ventilator-associated pneumonia (VAP) has a tracheal aspirate culture positive for Klebsiella pneumoniae. Antibiotic susceptibility testing shows resistance to imipenem and meropenem, but susceptibility to colistin and tigecycline. The organism produces a metallo-β-lactamase (MBL) on molecular testing. She is currently on day 2 of empiric meropenem monotherapy. What is the most appropriate immediate next step in management?