TABLE 2.

Serial passage of A(H5N1) viruses in the presence of increasing concentrations of zanamivir^a

Virus strain	Passage no.	Zanamivir concn	Zanamivir IC50	Oseltamivir IC50	HA mutationb	NA mutation (%)c
A/Vietnam/1203/2004		0 nM	0.7	0.2
	1	1 nM	1.1	0.3
	2	5 nM	0.6	0.2
	3	25 nM	0.6	0.2
	4	50 nM	0.8	0.2
	5	250 nM	0.6	0.2
	6	1,250 nM	0.6	0.2	K234N	D198G (28)
	7	6,250 nM	1.2	0.4	K234N	D198G (86)
	8	31 μM	7.0	2.1	K234N	D198G (93)
	9	156 μM	7.8	2.3	K234N	D198G (64); E119G (35)
	10	781 μM	40.3	3.0	K234N	E119G (76)
A/Chicken/Laos/26/2006		0 nM	0.2	0.2
	1	1 nM	0.2	0.2
	2	5 nM	0.2	0.2
	3	25 nM	0.2	0.2
	4	125 nM	0.3	0.3
	5	625 nM	0.2	0.2
	6	3,125 nM	0.2	0.3
	7	16 μM	0.2	0.3		E119G (50)
	8	78 μM	4.0	0.9		E119G (82)
	9	390 μM	19.1	6.2	K205E	E119G (87)
	10	1,950 μM	16.7	5.7	K205E	E119G (88)

^aA(H5N1) influenza viruses A/Vietnam/1203/2004 and A/Chicken/Laos/26/2006 were cultured in MDCK cells in the presence of increasing concentrations of oseltamivir. Each passage level was tested for susceptibility to zanamivir and oseltamivir and sequenced to investigate the generation of any HA or NA mutations. IC₅₀ values are based on assays completed in duplicate.

^bBased on H5 residue numbering.

^cBased on N2 residue numbering. The percentages in parentheses indicate the proportion of each individual mutant in the total viral population based on pyrosequencing analysis. Mutations that were present at <20% of the total viral population are not shown.