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 oxygenation, or a change in the level of positive end-expiratory pressure support; ④ new respiratory secretions requiring suction; * A specific carbapenem-resistant Gram-negative bacterium was cultured from qualified lower respiratory tract specimens within the first five days/screening period, with in vitro susceptibility testing confirming resistance to carbapenems； * Female subjects without reproductive potential must meet at least one of the following criteria: a) cessation of regular menstruation for at least 12 consecutive months; b) having undergone hysterectomy and/or bilateral oophorectomy. Female subjects with reproductive potential must have a negative serum pregnancy test result at the screening visit and agree to use reliable contraception throughout the study period; * Male subjects must agree to adopt reliable contraceptive measures throughout the entire study period. Exclusion Criteria: * Those who currently suffer from epilepsy/myasthenia gravis or have a history of seizures (excluding febrile seizures in childhood)/myasthenia gravis; * Those who are undergoing hemodialysis or peritoneal dialysis; * Combined infections with other lung microbiota: viral pneumonia, fungal pneumonia, pulmonary tuberculosis, atypical pathogen infections, etc； * Current concurrent infection of other parts/organs; * Patients with concurrent refractory septic shock, who still exhibit persistent hypotension despite adequate fluid resuscitation or vasopressor therapy prior to randomization; * Individuals with immune deficiency or compromised immune function, including but not limited to: human immunodeficiency virus infection, hematological malignancies, bone marrow transplantation, immunosuppressive therapy, and systemic corticosteroid treatment (defined as a daily dose equivalent to prednisone ≥20mg and a treatment duration \>14 days); * During the screening period, any of the following laboratory abnormalities is present: aspartate aminotransferase (AST) and/or alanine aminotransferase (ALT) levels are more than 5 times the upper limit of normal, or AST and/or ALT levels are more than 3 times the upper limit of normal and total bilirubin levels are more than 1.5 times the upper limit of normal, or neutrophil count \< 1.0×10\^9/L, or platelet count \< 60×10\^9/L; creatinine clearance rate (cLcr) ≤ 50 mL/min; * Suffering from lung diseases that can interfere with treatment response assessment; * Patients with lung abscess, empyema, and mechanical obstructive pneumonia; * New York Heart Association (NYHA) class III-IV heart failure; * Transplant patients； * Patients with an estimated survival time of less than 1 month according to the clinical judgment of the researchers; * Those who have allergic reactions to polymyxins or any β-lactams (such as carbapenems, penicillins, monocyclic lactams, cephalosporins); * Sbjects requiring \>2 systemic antimicrobial drugs for the treatment of Gram-negative bacterial infections; * Patients with an Acute Physiology and Chronic Health Evaluation II (APACHE II) score greater than 30; * Women who are pregnant or breastfeeding; * Use potentially effective antibiotics to treat carbapenem resistant gram-negative bacterial infections within 72 hours prior to randomization, and the treatment duration exceeds 24 hours； * When the culture results of samples from the first 5 days/screening period are available, it is found that the subject has Hospital-Acquired Pneumonia/Ventilator-Associated Pneumonia (HAP/VAP) caused by gram-negative bacteria that are expected to have no response to polymyxin drugs; * Subjects who have participated in other clinical trials within 30 days prior to the first dose of medication; * Other factors determined by the researcher that make the subject unsuitable for participating in this study.