Evaluation of Pharmacokinetics, Safety, and Tolerability of Ceftazidime-avibactam in Neonates and Infants.

Inclusion Criteria (All Subjects): 1. Evidence of a personally signed and dated informed consent document indicating that the subject's parent(s), legal guardian, or legally acceptable representative has been informed of all pertinent aspects of the study. 2. Willing and able to comply with scheduled visits, treatment plan, laboratory tests, and other study procedures. 3. Male or female neonates and infants with age at Screening: Cohort 1: Full term infants (gestational age ≥ 37 weeks) with chronological age \>28 days to \<3 months (\<89 days) or pre-term infants with corrected age \>28 days to \<3 months (\<89 days). A maximum of 3 pre-term corrected age infants may be enrolled in each part (A and B) of Cohort 1. Sites will be notified in writing if this limit is reached. Cohort 2: Full term neonates (gestational age ≥ 37 weeks) from birth to ≤ 28 days. Cohort 3: Pre-term neonates (gestational age ≥ 26 to \<37 weeks) from birth to ≤ 28 days. Corrected age = Subtract the number of weeks born before 40 weeks of gestation from the chronological age. Inclusion Criteria for Part A Subjects Only: 1\. Hospitalized and receiving intravenous antibacterial therapy for the treatment of a suspected or confirmed bacterial infection. Inclusion Criteria for Part B Subjects Only: 1. Hospitalized with suspected or confirmed aerobic Gram-negative bacterial infection requiring intravenous antibacterial therapy. 2. Subjects must meet at least 1 clinical and 1 laboratory criterion or meet at least 2 of the clinical criteria: Clinical Criteria: 1. Hypothermia (\<36ºC) OR fever (\>38.5ºC); 2. Bradycardia OR tachycardia OR rhythm instability; 3. Urine output 0.5 to 1 mL/kg/h OR hypotension OR mottled skin OR impaired peripheral perfusion; 4. Petechial rash OR sclerema neonatorum; 5. New onset or worsening of apnea episodes OR tachypnea episodes OR increased oxygen requirements OR requirement for ventilation support; 6. Feeding intolerance OR poor suckling OR abdominal distension; 7. Irritability; 8. Lethargy; 9. Hypotonia. Laboratory Criteria: 1. White blood cell count ≤ 4.0 × 10\^9/L OR ≥ 20.0 × 10\^9/L; 2. Immature to total neutrophil ratio \>0.2; 3. Platelet count ≤ 100 × 10\^9/L; 4. C reactive protein (CRP) \>15 mg/L OR procalcitonin ≥ 2 ng/mL; 5. Hyperglycemia OR Hypoglycemia; 6. Metabolic acidosis. Exclusion Criteria (All Subjects): 1. Investigator site staff members directly involved in the conduct of the study and their family members, site staff members otherwise supervised by the Investigator, or subjects who are Pfizer employees, including their family members, directly involved in the conduct of the study. 2. Participation in another clinical study involving investigational drug(s) within 30 days prior to study entry and/or during this study participation or have previously participated in the current study or in another study of CAZ-AVI (in which an active agent was received). 3. Use of potent inhibitors of organic anion transporters OAT1 and/or OAT3 (eg, probenecid, p-aminohippuric acid (PAH), or teriflunomide) are prohibited. This prohibition of OAT1 and/or OAT3 inhibitors also applies to the mothers of any neonates or infants who are breast feeding during the trial. 4. Other acute or chronic medical or laboratory abnormality that may increase the risk associated with study participation or investigational product administration or may interfere with the interpretation of study results and, in the judgment of the Investigator, would make the subject inappropriate for entry into this study. 5. Documented history of any hypersensitivity or allergic reaction to any beta-lactam antibiotic. 6. Refractory septic shock within 24 hours before screening that does not resolve after 60 minutes of vasopressor therapy. 7. Moderate or severe renal impairment defined as serum creatinine ≥ 2 times the upper limit of normal (ULN) for age OR urine output \<0.5 mL/kg/h (measured over at least 8 hours) OR requirement for dialysis. Deterioration of renal function after enrollment during Part B of the study will be handled on a case-by-case basis in discussion with the Medical Monitor. 8. Evidence of progressively fatal underlying disease, or life expectancy of ≤ 60 days. 9. Documented history of seizure. 10. Active acute viral hepatitis or acute hepatic failure. 11. Known Clostridium difficile associated diarrhea. 12. Requiring or currently taking antiretroviral therapy for human immunodeficiency virus (HIV) or known HIV positive mother. 13. Any condition (eg, cystic fibrosis, urea cycle disorders), antepartum/peripartum factors, or procedures that would, in the opinion of the Investigator, make the subject unsuitable for the study, place a subject at risk, or compromise the quality of data. 14. Treatment with ceftazidime within 12 hours of CAZ-AVI administration. Exclusion Criteria for Part A Subjects Only: 1. Subject received a blood or a blood component transfusion within 24 hours of the start of CAZ AVI infusion. 2. Subject is expected to be discharged less than 24 hours after the start of CAZ AVI infusion. Exclusion Criteria for Part B Subjects Only: 1. At study entry, subject has confirmed or strongly suspected infection with a pathogen known to be resistant to CAZ-AVI or only a Gram-positive pathogen or viral, fungal, or parasitic pathogens as the sole cause of infection. 2. Confirmed or suspected central nervous system (CNS) infection (eg, meningitis, brain abscess, subdural abscess). 3. Anticipated need for antibacterial therapy longer than 14 days (eg, osteomyelitis, endocarditis). This applies to both study treatment with CAZ-AVI as well as adjunctive IV antibacterial treatment for suspected co infection with Gram-positive organisms or multi drug resistant Gram-negative organisms. 4. Receipt of more than 24 hours of nonstudy systemic antibacterial treatment for Gram-negative organisms after culture and before administration of study doses of CAZ-AVI. Empiric coverage with an aminoglycoside for suspected multidrug resistant organisms is permitted, provided CAZ-AVI is initiated within 24 hours after culture. 5. Intravenous treatment with chloramphenicol within 24 hours of administration of study doses of CAZ-AVI. 6. Subject is expected to be discharged less than 48 hours after the start of CAZ-AVI infusion.