Study on the Safety and Efficacy of Polymyxin E2 Methanesulfonate for Injection in the Treatment of Hospital-Acquired Bacterial Pneumonia/Ventilator-Associated Bacterial Pneumonia Caused by Carbapenem-Resistant Gram-Negative Bacteria

Inclusion Criteria: * Age ≥ 18 years old (based on the date of signing the informed consent form); * The subject (or their guardian) voluntarily signed the informed consent form； * Acute pulmonary infection with hospitalization duration exceeding 48 hours or within 7 days after discharge; or acute pulmonary infection patients who have undergone mechanical ventilation via oral or nasal tracheal intubation for at least 48 hours; * Chest imaging examination (X-ray or CT) within 72 hours prior to randomization reveals characteristics of new or worsening pulmonary infiltration; * At least one of the following physical signs or laboratory abnormalities: ① fever (temperature ≥38℃); ② hypothermia (temperature ≤35℃); ③ elevated peripheral white blood cell count (WBC ≥10×10\^9/L); ④ decreased white blood cell count (WBC ≤4.5×10\^9/L); ⑤ more than 15% of immature neutrophils such as band forms in peripheral blood; * At least one of the following clinical symptoms is present: ① new or acute worsening of pulmonary symptoms or signs, such as cough, dyspnea, increased respiratory rate (respiratory rate \> 25 breaths per minute), expectoration, or the need for mechanical ventilation; ② hypoxemia (arterial blood gas oxygen partial pressure below 60 mmHg at standard atmospheric pressure, or a progressive decrease in the ratio of oxygen partial pressure to inspired oxygen concentration (PaO2/FiO2)); ③ deteriorating oxygenation requiring replacement of ventilation support system to improve oxyg…