Inclusion Criteria:

          -  Patient has advanced (loco-regionally recurrent not amenable to curative therapy or
             metastatic) breast cancer.

          -  Patient must have measurable disease, i.e., at least one measurable lesion as per
             RECIST 1.1 criteria (Tumor lesions previously irradiated or subjected to other
             loco-regional therapy will only be considered measurable if disease progression at the
             treated site after completion of therapy is clearly documented)

          -  Patient progressed after adjuvant or 1 prior systemic treatment in the metastatic
             setting. Patients with de novo metastatic disease are eligible if they received 1
             prior line of therapy

          -  Patient must have received prior systemic treatment that included taxane-based
             chemotherapy for adjuvant or metastatic disease

          -  Patient must have a site of disease amenable to biopsy, and must be willing to undergo
             a new tumor biopsy at screening and during therapy on this study, the latter if
             medically feasible. Patients with an available archival tumor tissue do not need to
             perform a tumor biopsy at screening if patient has not received anti-cancer therapy
             since the biopsy was taken.

          -  Patient has histologically and/or cytologically confirmed diagnosis of advanced TNBC
             (based on most recently analyzed biopsy from locally recurrent or metastatic site,
             local lab) meeting the following criteria: HER2 negative in situ hybridization test or
             an IHC status of 0 or 1+, and ER and PR expression is <1 percent as determined by
             immunohistochemistry (IHC)

        Exclusion Criteria:

          -  Patient has received prior immune checkpoint inhibitors as anticancer treatment such
             as anti-LAG-3, anti-PD-1, anti-PD-L1, or anti-PD-L2 antibody (any line of therapy).

          -  Patient received prior neoadjuvant or adjuvant therapy with a platinum agent or
             mitomycin and experienced recurrence within 12 months after the end of the
             platinum-based or mitomycin containing therapy or received Platinum or mitomycin for
             metastatic disease

          -  Patient has had major surgery within 14 days prior to starting study treatment or has
             not recovered to grade 1 or less from major side effects.

          -  Patient with presence of CTCAE grade 2 toxicity or higher due to prior cancer therapy.
             Exception to this criterion; patients with any grade of alopecia are allowed to enter
             the study..

          -  Patient has received radiotherapy ≤ 4 weeks prior to randomization (≤ 2 weeks for
             limited field radiation for palliation), and has not recovered to grade 1 or better
             from related side effects of such therapy (with the exception of alopecia).

          -  Patient has a known hypersensitivity to other monoclonal antibodies,
             platinum-containing compounds, or to any of the excipients of LAG525, spartalizumab,
             or carboplatin.

          -  Patient has symptomatic central nervous system (CNS) metastases or CNS metastases that
             require local CNS-directed therapy (such as radiotherapy or surgery), or increasing
             doses of corticosteroids within the 2 weeks prior to first dose of study treatment.
             Patients with treated brain metastases should be neurologically stable and witout CNS
             progression for at least 12 weeks prior to randomization and have discontinued
             corticosteroid treatment (with the exception of < 10 mg/day of prednisone or
             equivalent for an indication other than CNS metastases) for at least 4 weeks before
             first dose of any study treatment.

        Other protocol-defined inclusion/exclusion criteria may apply