Efficacy and Safety Study of REM-001 Photodynamic Therapy for Treatment of Cutaneous Metastatic Breast Cancer (CMBC)

Inclusion Criteria: * Adult participants 18 years of age or greater. * Participants able and willing to sign informed consent. * Histopathologically confirmed breast cancer metastasis to the skin. * Cutaneous metastasis not suitable for surgical resection. * Eastern Cooperative Oncology Group (ECOG) performance status 0 to 2. * Symptomatic lesions (including discomfort, pain, discharge, ulceration). * Cutaneous, subcutaneous soft tissue, or superficial lymphatic metastasis that is amenable to PDT: * Lesion(s) \> 10 mm and \< 60 mm in longest dimension. * Lesion(s) exhibit at least one of the following symptoms: ulcerated, bleeding, discharging, itchy, painful. * Judged by investigator as eligible for PDT. * Participants are radiotherapy refractory (have received a radiation dose of 60 gray (Gy) or greater to the ipsilateral thorax) or are not otherwise amenable to radiotherapy. * Disease progression on at least 2 courses of systemic therapy: * HR positive/HER2 negative participants: should be refractory to endocrine therapy (at least 2 different regimens including at least one CDK4/6 inhibitor). Maintenance endocrine therapy at the clinician's discretion is allowed. * HER2 positive participants should have had disease progression on at least 2 different regimens of HER2 targeted therapies. Maintenance therapy on trastuzumab (HERCEPTIN®) is allowed. * If participants are on systemic therapy at enrollment, they must meet the following: * Participants must have undergone a minimum of two cycles of systemic therapy prior to enrollment. * The systemic therapy must be one from the Treatment of Physician's Choice (TPC) list, as follows: : eribulin mesylate (Halaven®); capecitabine (Xeloda®); Gemcitabine (Gemzar®); a taxane \[either docetaxel (Taxotere®), nab-paclitaxel (Abraxane®), or paclitaxel (Taxol®)\]; vinorelbine (Navelbine®); an antibody-drug conjugate \[either sacituzumab-govitecan (Trodelvy®), trastuzumab-deruxtecan (Enhertu®), or ado-trastuzumab emtansine (KADCYLA®)\]; pembrolizumab (Keytruda®); or carboplatin (Paraplatin®). * Patients who are not on any chemotherapy will also be eligible. * At the time of enrolment, participants should have no known plans to change or modify their TPC regimen while receiving study treatment. Note: TPC regimen may be changed or modified after treatment with REM-001 therapy, but this should be done in consultation with the Sponsor Medical Monitor. * Adequate renal function, as evidenced by estimated glomerular filtration rate (eGFR) \> 45 mL/min/1.73 m2 using the CKD-EPI Creatinine Equation without race. * Adequate bone marrow function as evidenced by absolute neutrophil count (ANC) ≥ 1.5 × 10\^9/L, hemoglobin ≥ 8.5 g/dL and platelet count ≥ 100 × 10\^9/L; INR \< 1.5. * Adequate liver function as evidenced by bilirubin ≤ 2.0 times the upper limits of normal (ULN) and alkaline phosphatase, alanine aminotransferase (ALT), and aspartate aminotransferase (AST) ≤ 3 × ULN (in the case of liver metastases ≤ 5 × ULN) \[Participants with known Gilbert's Syndrome who have serum bilirubin \< 1.5 x ULN (NCI CTCAE v5.0 Gr 2) may be enrolled\]. * QTCF \< 470msec on baseline ECG * Woman of childbearing potential (WOCBP) must have a negative serum pregnancy test documented within 7 days prior to registration and must agree to practice adequate contraception * Male patients must be sterile or willing to use an approved method of contraception from the time of treatment with REM-001 until 90 days after study drug treatment. Exclusion Criteria: * Participants who have received local cryotherapy, radiotherapy, intra-lesional chemotherapy, systemic or topical PDT, or surgery to study lesion fields within the past 12 weeks. * Participants with progressive brain or subdural metastases, or leptomeningeal disease. * Participants with previously treated brain or subdural metastases may participate provided: * Previously treated brain metastases are stable and without evidence of progression, as determined by contrast-enhanced CT or MRI brain scan, for at least 4 weeks prior to the first dose of study treatment. * There is no evidence of new brain metastases * They have completed local therapy and discontinued the use of corticosteroids for this indication for at least 4 weeks prior to first dose of study treatment. * Any neurologic symptoms attributed to brain metastases must have been stable for at least 4 weeks prior to study enrollment * History of allergic or hypersensitivity reactions to light, egg proteins or egg yolk; history of porphyria, systemic lupus erythematosus, or xeroderma pigmentosum. * Known disorder of lipoprotein metabolism or clearance (cholesterol\> 400 mg/dl, and/or triglycerides \> 500 mg/dl). * Participants who have received investigational agents within the past 4 weeks or within 4 half-lives of the investigational agent (whichever is shorter) before the first study drug dose. * Participants with inflammatory breast cancer. * Known human immunodeficiency virus (HIV) infection with detectable virus titer. * Active or chronic hepatitis B or C infection. * Active or ongoing infection requiring systemic treatment. * Participants who have undergone major surgery within 4 weeks of study treatment, or have planned surgery within 4 weeks of anticipated initiation of treatment with REM-001 therapy. * Participants with otherwise unexplained weight loss (\> 10% body weight) in the last 30 days prior to Screening. * History of other malignancy treated with curative intent within the last 3-5 years. Exceptions are: Curatively treated basal cell/squamous cell skin cancer; carcinoma in situ of the cervix; superficial transitional cell bladder carcinoma * Patients with other major or uncontrolled medical conditions, e.g., myocardial infarction or New York Heart Association (NYHA) Class III/IV heart failure within the last 6 months, stroke, uncontrolled diabetes, uncontrolled autoimmune disease. * WOCBP that is pregnant or breast-feeding, or planning to become pregnant or breast-feed during protocol treatment and for 3 months after last dose of REM-001 therapy. * WOCBP unwilling to use effective contraception during protocol treatment and for 3 months after last dose of REM-001 therapy.