Role of increased vascular permeability in chemotherapy‐induced alopecia: In vivo imaging of the hair follicular microenvironment in mice

Chemotherapy‐induced alopecia is one of the most difficult adverse events of cancer treatment for patients. However, it is still unknown why anticancer drugs cause hair loss. We aimed to clarify the mechanism of chemotherapy‐induced alopecia in mice using an in vivo imaging technique with a two‐photon microscope, which enables observation of the deep reaction in the living body in real time. In this study, ICR mice were injected intraperitoneally with cyclophosphamide (120 µg/g). Changes in the hair bulb morphology, subcutaneous vessel permeability, and vessel density were evaluated by two‐photon microscopy and conventional methods. In order to determine whether there is a causal relationship between vascular permeability and hair loss, we combined cyclophosphamide (50 µg/g) with subcutaneous histamine. We found that the hair bulbs became smaller, blood vessels around the hair follicle decreased in diameter, and vascular permeability was confirmed by two‐photon microscopy and conventional examination at 24 h after cyclophosphamide injection. Apoptosis occurred in vascular endothelial cells around the hair follicle. Additionally, hair loss was exacerbated by temporarily enhancing vascular permeability with histamine. In conclusion, cyclophosphamide caused a decrease in vascular density and an increase in vascular permeability, therefore increased vascular permeability might be one of the causes of chemotherapy‐induced alopecia.