Aztreonam-Avibactam in Immunocompromised Hosts

The goal of this observational study is to learn about the effectiveness and safety of aztreonam-avibactam, an antibiotic used to treat infections caused by highly resistant bacteria, in adults with weakened immune systems. The main questions this study aims to answer are: How well does aztreonam-avibactam work in treating these infections? How many participants recover, and how many die or develop another infection caused by the same bacteria? What side effects or medical problems occur during or after treatment? Researchers will also compare participants treated with aztreonam-avibactam with a group of participants previously treated with ceftazidime-avibactam plus aztreonam. This comparison will help researchers understand differences in treatment outcomes. The study will include adults with weakened immune systems, such as people with cancer, organ transplants, or those receiving medicines that suppress the immune system. Participants will: Receive antibiotic treatment chosen by their treating physicians as part of routine medical care. Have their medical records reviewed to collect information about their infections, treatments, and outcomes. Be followed for up to 60 days after starting treatment to assess recovery, survival, side effects, and recurrence of infection. Researchers will not assign treatments or change participants' medical care.

Inclusion Criteria: 1. Adult participants aged 18 years or older with microbiologically confirmed infections caused by metallo-β-lactamase-producing Enterobacterales (MBL-E). 2. Participants must meet at least one of the following immunocompromising conditions: * Solid organ transplant recipients receiving immunosuppressive treatment. * Participants with hematological malignancies or solid tumors who received chemotherapy within the previous 30 days. * Treatment with antilymphocyte or B-cell-depleting agents (e.g., rituximab or alemtuzumab) within the previous 6 months. * Use of biological or targeted therapies associated with immunosuppression (e.g., BTK inhibitors, CAR-T cells, immune checkpoint inhibitors with immunosuppressive management, or bispecific antibodies) within the previous 3 months. * Exposure to high-dose corticosteroids, defined as at least 20 mg/day of prednisone or equivalent for at least 14 days within the previous 30 days. * Allogeneic hematopoietic stem cell transplantation (HSCT), with graft-versus-host disease at any time or without graft-versus-host disease during the first year after transplantation. * Autologous HSCT within 6 months after transplantation. * Autoimmune diseases receiving immunosuppressive treatment. 3. Monomicrobial infection caused by MBL-E. 4. Definitive antimicrobial treatment with aztreonam-avibactam (prospective cohort) or ceftazidime-avibactam plus aztreonam (retrospective cohort) for at least 72 hours. Exclusion Criteria: 1. Recurrent infections. 2. Infections requiring prolonged antibiotic treatment exceeding 28 days (e.g., osteoarticular infections). 3. Positive microbiological specimens that do not represent a true infection, including: * Asymptomatic bacteriuria. * Respiratory specimens without clinical criteria for pneumonia. * Superficial skin and soft tissue cultures. * Blood cultures obtained through a catheter without confirmation in peripheral blood cultures. 4. Participants receiving palliative care.