The prospective inhibitor [9], designed by simultaneous modifications of both the 4-amino side chain and the C6 bulky hydrophobic side chain of Tamiflu, was shown to concomitantly exploit the experimentally identified location of the 150-cavity in H5N1 NA, as proposed by Russell et al. [5] (Figure 2(b)), and maintain a structural resemblance to sialic acid. Note that the side chain at the C4 position was a slight modification of the C6 side chain of Relenza (Figure 3) and as such was involved in three electrostatic interactions with the guanidinium side chain of Arg156 upon forming the complex with H5N1 NA [9].