The study will not be submitted to authority for IND application.

This study assess the safety, tolerability, and efficacy in reducing viral load and proviral DNA of UB-421 administered as an add-on to the ART in ART-experienced viremic HIV-1 subjects.

Inclusion Criteria:

1. HIV-1 seropositive
2. Male with body weight ≥ 50 kg or female with body weight ≥ 45 kg.
3. have been receiving antiretroviral therapy (ART) for more than 2 years

Exclusion Criteria:

1. Any previous exposure to a mAb within 12 weeks prior to the first dose of UB-421 treatment.
2. Any significant diseases (other than HIV-1 infection) or clinically significant findings, including psychiatric and behavioral problems, determined during the screening period, medical history, and/or physical examination that, in Investigator's opinion, would preclude the subject from participating in this study.
3. History of anaphylaxis to monoclonal antibodies.
4. Any vaccination within 8 weeks prior to the first dose of UB-421.