Table 3.

Foscarnet Resistance in Serial Clinical Isolates

Virus Isolate	Source/Day of Illness	Phenotypic Resistance to Foscarnet, IC50 (μmol/L)a	Frequency of Resistant Variants (%)b
D-1	Mouth lesion, day 19c	63	9.5 × 10−4 (0.095)
D-2	Eye, day 19	123	3.8 × 10−3 (0.377)
D-3	Oropharyngeal swab, day 19	93	7.4 × 10−4 (0.074)
D-4	Eye, day 53	123	4.3 × 10−4 (0.043)
D-5	Tongue lesion, day 102	153	1.1 × 10−4 (0.011)
D-6	Eye, day 144	83	8.2 × 10−5 (0.008)
KOS	Reference strain	137	8.6 × 10−3 (0.86)
I-309	Reference strain	97	3.1 × 10−5 (0.003)
615.8	Reference strain	>667	0.54 (53.9)

Abbreviation: IC₅₀, median inhibitory concentration.

^aDetermined by plaque reduction using Hill slope method. Breakpoint for resistance is generally IC₅₀ ≥ 333 µmol/L (100 µg/mL) foscarnet [10].

^bDetermined by plating efficiency assay as described in “Methods.”

^cInitial treatment with foscarnet began 19 days into the illness.