Although it cannot be inferred that the high planarity and lipophilicity of naphthalene rings contribute to the increase in lipophilicity and can lead to the worsening of its solubility, several sulfone groups, which were not included in the suggested substructure, were considered to contribute to the decrease in lipophilicity and could lead to the worsening of membrane permeability. This was supported by the high solubility of suramin (>50 mg/mL) . Based on these results, the important substructures estimated using the Chemprop framework partially agreed with the results of previous reports. contributes to an increase, decrease, and maintenance of the lipophilicity balance against the entire structure, respectively. In (D), each substructure contributed significantly to the decrease in lipophilicity compared with the whole structure. In (E), the estimated effect of each substructure on the whole structure is not consistent with the knowledge of medicinal chemistry.