This is a 2-part Phase 2a, non-randomized, multicenter, open-label, single and multi-dose study to assess pharmacokinetics (PK), safety, and tolerability of ATM-AVI in hospitalized neonates and infants aged birth, including preterm birth, to \<9 months. A total of 48 participants will be enrolled in 4 age cohorts of 12 each, 6 Part A and 6 Part B. Part A will begin enrolling before Part B and no infant may participate in both parts. Cohort 1 will include full term infants age ≥13 weeks to \<39 weeks and preterm infants postmenstrual age ≥50 weeks to \<75 weeks. Cohort 2 will include full term infants age ≥28 weeks to \<13 weeks and preterm infants postmenstrual age ≥40 weeks to \<50 weeks and ≥28 days of age. Cohort 3 will include full term infants age birth to \<28 days. Cohort 4 will include preterm infants age birth to \<28 days or postmenstrual age \<40 weeks.

Participants in Part A must be hospitalized and receiving intravenous antibiotic treatment for a suspected or confirmed bacterial infection. Participants will receive a single 3 hour intravenous infusion of ATM-AVI and have 3 ATM-AVI blood level assessments during and up to 5 hour after the infusion. Participants will be observed for 48 hours following the infusion to assess safety and toleration and will have a final follow-up safety assessment which may be conducted by telephone 4-5 weeks following the infusion. The single infusion of ATM-AVI is administered to assess the safety, tolerability, and pharmacokinetics of a single dose of ATM-AVI and is not intended as treatment for the bacterial infection. The total duration of study participation in Part A is expected to be 5 weeks through the end of the final safety follow-up.

Participants in Part B must be hospitalized with suspected or confirmed aerobic gram-negative bacterial infection requiring intravenous antibacterial therapy. Part B participants will receive multiple 3 hour intravenous infusions of ATM-AVI every 6 hour (8 hours for preterm infants) for 3-14 days as treatment for their bacterial infection and to assess ATM-AVI pharmacokinetics, safety, tolerability, and efficacy. Participants with complicated intra-abdominal infection (cIAI) will also receive intravenous metronidazole and all participants will have the option to receive other intravenous antibiotic treatment for gram-positive bacteria, as appropriate. Participants who have a good clinical response after 72 hours of intravenous ATM-AVI treatment may be switched to a different orally administered antibiotic, if clinically appropriate. Part B participants will have a total of 5 ATM-AVI blood level assessments over the first 2 or more days following the start of ATM-AVI infusions and will have their clinical response assessed at the End of Treatment (intravenous and oral, if applicable), and at a Test-of-Cure (TOC) evaluation 7 to 14 days after the last antibiotic treatment (intravenous or oral). A final safety assessment which may be conducted by telephone will occur 4-5 weeks after the last dose of ATM-AVI. The total duration of study participation in Part B is expected to be up to 7 weeks through the end of the final safety follow-up.

Additional safety monitoring will be provided by an independent external Data Monitoring Committee (DMC). Enrollment for the study will begin with Part A, single dose, cohorts 1-3. Part A Cohort 4 (preterm neonates) will commence enrollment after sponsor and DMC review of plasma drug levels and safety for a least 2 participants in Part A Cohort 3 and review of ATM-AVI safety and tolerability for all participants enrolled at that time. Enrollment in the multidose Part B cohorts will be delayed until preliminary information is obtained regarding ATM-AVI multidose safety, tolerability, and drug levels for the participants 9 months to 2 years of age in the separate ongoing ATM-AVI pediatric study C3601008 \[NCT05639647\].