Flow cytometric analysis of the murine placenta to evaluate nanoparticle platforms during pregnancy

Clinically approved therapeutics for obstetric conditions are extremely limited, with over 80% of drugs lacking appropriate labeling information for pregnant individuals. The pathology for many of these obstetric conditions can be linked to the placenta, necessitating the development of therapeutic platforms for selective drug delivery to the placenta. When evaluating therapeutics for placental delivery, literature has focused on ex vivo delivery to human placental cells and tissue, which can be difficult to source for non-clinical researchers. Evaluating in vivo drug delivery to the placenta using small animal models can be more accessible than using human tissue, but robust, quantitative methods to characterize delivery remain poorly established. Here, we report a flow cytometric method to evaluate in vivo drug delivery to the murine placenta. Specifically, we describe techniques to identify key cell types in the murine placenta - trophoblasts, endothelial cells, and immune cells - via flow cytometric analysis. While we have employed this method to detect lipid nanoparticle-mediated nucleic acid delivery, this approach can extend to a variety of drug carriers (e.g., liposomes, exosomes, polymeric and metallic nanoparticles) and payloads (e.g., small molecules, proteins, other nucleic acids). Similarly, we describe the application of this method toward immunophenotypic analysis to assess changes in the placental immune environment during disease or in response to a therapeutic. Together, the techniques reported herein aim to broaden the accessibility of placental research in an effort to encourage collaboration between physician-scientists, engineers, placental biologists, and clinicians for developing novel therapeutics to treat placental conditions during pregnancy.