Question

A $68$-year-old man is evaluated in the intensive care unit for a new-onset fever and increased respiratory secretions. He has been receiving mechanical ventilation for $7$ days following a severe cerebrovascular accident. His temperature is $38.8^\circ\text{C}$ ($101.8^\circ\text{F}$), blood pressure is $110/70\text{ mmHg}$, and pulse is $105/\text{min}$. Physical examination reveals coarse crackles in the right lower lung field. A chest radiograph demonstrates a new right lower lobe infiltrate. Endotracheal aspiration is performed, and empiric intravenous therapy with cefepime, tobramycin, and vancomycin is initiated.

Forty-eight hours later, the patient's temperature is $37.2^\circ\text{C}$ ($99.0^\circ\text{F}$) and his oxygenation has steadily improved. The endotracheal aspirate culture grows $10^6\text{ CFU/mL}$ of *Pseudomonas aeruginosa*. Antimicrobial susceptibility testing indicates the organism is susceptible to cefepime, meropenem, and tobramycin, but resistant to levofloxacin. No other organisms are isolated. Which of the following is the most appropriate next step in the management of this patient's pharmacotherapy?