Immunotherapy in High-risk Ductal Carcinoma in Situ (DCIS)

PRIMARY OBJECTIVES: DOSE ESCALATION (Monotherapy Messenger RNA-2725 (mRNA-2752): I. To determine the efficacy of intralesional mRNA-2752 monotherapy in participants with ductal carcinoma in situ (DCIS) of the breast as measured by the change in the MRI tumor size/volume/enhancement. Absence of tumor on biopsy and increase in immune infiltrates as measured by immune multiplex assays. II. To characterize the safety of mRNA-2752 and feasibility of intralesional administration of mRNA-2752 in patients with high-risk DCIS. DOSE EXPANSION (mRNA-2752 with or without an immune checkpoint inhibitor): I. To determine the Magnetic resonance imaging (MRI) response rate and complete pathologic response rate with either mRNA-2752 monotherapy or combined therapy in high risk DCIS. ENROLLMENT IN THE PREVIOUS COHORTS HAS BEEN COMPLETED.