Inclusion Criteria:
* Adult participants \> 18 years of age.
* Histologically confirmed HR-/HER2- metastatic TNBC using ASCO/CAP criteria, Stage IIIB-IV.
* Must have experienced disease progression on prior standard of care (SoC) therapy, be ineligible for SoC due to intolerance or contraindications, or have no remaining SoC treatment options available.
* Be willing to provide tissue from a newly obtained core or excisional biopsy of an accessible tumor lesion if archival tissue is not available.
* Must not be eligible for or able to enroll in any trials investigating leronlimab that are currently recruiting.
* Participant must have adequate organ and bone marrow function within 14 days prior to enrollment as determined by the investigator.
* ECOG performance status 0-2.
* Participants of childbearing potential (POCBP) and males must agree to use two medically accepted methods of contraception with hormonal or barrier method of birth control, or abstinence, prior to study entry, for the duration of study participation and 4 months after the last dose of study drug.
* Willingness and ability to comply with protocol requirements and attend follow-up visits.
* Participants must have the ability to understand and the willingness to sign a written informed consent prior to registration on study.
Exclusion Criteria:
* Participants who have a history of allergic reactions attributed to compounds of similar chemical or biologic composition to leronlimab (PRO 140) are not eligible.
* Have known active central nervous system (CNS) metastases and/or carcinomatous meningitis. Note: Subjects with previously treated/stable brain metastases may participate provided they are stable (without evidence of progression by imaging for at least four weeks prior to the first dose of trial treatment and any neurologic symptoms have returned to baseline) and have no evidence of new or enlarging brain metastases. This exception does not include carcinomatous meningitis which is excluded regardless of clinical stability.
* Any uncontrolled concurrent illness (e.g., infection, heart failure) that, in the investigator's judgment, would interfere with the patient's ability to safely receive treatment. Note: Patients must complete any treatment with antibiotics prior to registration.
* Concurrent participation in another interventional clinical trial.
* Pregnant or breastfeeding women.
* Any condition that, in the investigator's judgment, would interfere with the patient's ability to safely receive treatment.
* Has known psychiatric or substance abuse disorders that would interfere with cooperation with the requirements of the trial.
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