CD44 and cancer: A gatekeeper was designed on the fact that hyaluronic acid was degraded by the intracellular hyaluronidase (HAase) enzyme, selectively internalized in the CD44-overexpressing melanoma cells, and the guest–host interactions between ferrocene (Fc) and β-cyclodextrin (β-CD), were significantly affected by the hydrogen peroxide (H2O2) environment in cancer cells [20,32,33].