Diversity within the TME influences therapeutic efficacies and the delivery of systemic chemotherapeutics and immunotherapies. For example, drug delivery-based therapeutic resistance comes into play when delivery of chemotherapeutics into cancer cells is below the threshold levels required for biological effect. Another example is the metastasized tumor’s poor response to immune checkpoint inhibitors (ICIs), which are anti-cancer drugs that bind to programmed death-ligand 1 (PD-L1) and are meant to kill the malignant cells in various cancers, including breast cancer. This poor tumor response leads to limitations in chemotherapeutics, particularly breast cancer therapeutics. Although researchers haven’t yet determined precisely why this is the case, it’s been suggested that heterogeneous tumor and microenvironment types play a central role. The correlation between ICI delivery and therapeutic efficacy will be further investigated in future studies.