The goal of this clinical trial is to learn whether subcutaneous piperacillin/tazobactam can provide drug exposure similar to standard intravenous piperacillin/tazobactam in adults who need treatment for an infection. It will also assess safety and clinical outcomes. The main questions it aims to answer are:

• Does subcutaneous piperacillin/tazobactam achieve pharmacokinetic exposure comparable to intravenous treatment?
• Is subcutaneous piperacillin/tazobactam safe and feasible in this population? Researchers will compare subcutaneous continuous infusion with intravenous continuous infusion to see whether the subcutaneous route is not clinically worse than the standard intravenous route.

Participants will:

• Receive piperacillin/tazobactam by subcutaneous or intravenous continuous infusion after randomization
• Have blood samples collected for pharmacokinetic assessments
• Undergo safety, clinical, and end-of-treatment assessments during the study period

Inclusion Criteria:

• Free, written and informed consent signed by the participant;
• At least 18 years old on the day of inclusion;
• Male and female participants;
• Clinical indication for treatment with piperacillin/tazobactam 18g/die in continuous infusion, including severe pneumonia, neutropenic fever suspected to be caused by bacterial infection or other severe bacterial infections for which, based on investigator's judgment, this dosage regimen is appropriate;
• Evidence of postmenopausal status, defined as no menses for at least 12 consecutive months without an alternative medical cause and, if clinically indicated, confirmed by serum FSH levels in the postmenopausal range; or Negative serum pregnancy test at the screening visit (sensitivity ≥25 mIU/mL) and negative urine β-HCG test at the End-of-Treatment (D8) for females of childbearing potential who are sexually active with a non-sterilized male partner.

Women of childbearing potential (WOCBP) are defined as all women who are not surgically sterile (bilateral salpingectomy, bilateral oophorectomy, or complete hysterectomy) and who are not postmenopausal;

- Admitted to one of the wards authorized for the enrollment.

Exclusion Criteria:

• Known hypersensitivity to penicillins, cephalosporins, other β-lactamase inhibitors, or any component of the formulation;
• Pregnant or breastfeeding women, and women intending to become pregnant during the study or within the End-of-Treatment visit;
• Women of childbearing potential (WOCBP) unwilling or unable to use at least one highly effective method of contraception, as defined in Section 7.4, throughout study participation and for at least at the End-of-Treatment visit;
• No indication for treatment with TZP;
• Known resistance to TZP;
• Any other disease, metabolic dysfunction, physical examination finding, or clinical laboratory finding that contraindicates the use of an investigational drug, may affect the interpretation of the results, or may render the patient at high risk from treatment complications;
• Participation in another clinical trial with administration of an investigational medicinal product (IMP) within 30 days prior to screening or within 5 half-lives of that IMP, whichever is longer.