Polymyxin E2 Therapy for Carbapenem-Resistant Hospital- or Ventilator-Associated Pneumonia

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;
* Individuals with allergic reactions to polymyxins or carbapenems;
* 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.