The results for 4 and the other examples in Table 3 reveal a high antiviral EC50/enzyme IC50 ratio, which may arise from poor cell permeability. Indeed, these HMK inhibitors exhibit very low permeability and high levels of efflux beyond the sensitivity of the Caco-2 in vitro assay. However, the impact of high efflux on antiviral potencies from Vero 76 cells (derived from monkey kidney) versus disease-relevant human lung epithelial cells is unknown. To better understand the observed high antiviral EC50/enzyme IC50 ratio for 4, we evaluated the role of efflux in the Vero 76 cell line by the in vitro experimental design discussed further below. Concurrently, a strategy to design molecules reducing efflux by active transporters, such as P-glycoprotein, was pursued to decrease high antiviral EC50/enzyme IC50 ratios. An analysis of the physicochemical properties of 4 suggested that increasing logP, reducing polar surface area (PSA), and reducing the number of hydrogen bond donors/acceptors were design strategies with the potential to improve cellular permeability and reduce efflux.