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TABLE 1. Summary of HSV-1 susceptibility to acyclovir and penciclovir by antiviral use and medical history^a

Clinical ACV (Vero cells)
PCV (MRC-5 cells)

No. of subjects Median IC₅₀ (µg/ml)^b P^c No. of subjects Median IC₅₀ (µg/ml)^b P^c

Overall 1,002 0.42 1,004 0.26

Previous use of an antiviral agent (ACV, FCV, PCV, VCV)

    Used an antiviral agent 331 0.41 0.211 333 0.26 0.649

    Never used an antiviral agent 662 0.44 662 0.26

Current antiviral agent usage (oral ACV, FCV, VCV)

    Using an antiviral agent 49 0.31 0.025 49 0.24 0.488

    Not using an antiviral agent 943 0.43 945 0.26

Immune status

    Potential immunocompromise^d 37 0.39 0.274 38 0.25 0.377

    No immunocompromise 959 0.42 960 0.26

No. of RHL episodes in past 12 mo

    <4 443 0.43 0.387 443 0.26 0.284

     $>=$ 4 557 0.42 559 0.26

Duration of history of RHL

    <16 yr 332 0.39 0.115 333 0.26 0.804

     $>=$ 16 yr 651 0.44 652 0.26

Avg duration of an RHL episode

    <6 days 265 0.45 0.223 266 0.27 0.644

     $>=$ 6 days 735 0.41 736 0.26

^a No meaningful comparisons of the potency of acyclovir and penciclovir can be made because the isolates were tested in different cell lines. Abbreviations: ACV, acyclovir; FCV, famciclovir; PCV, penciclovir; VCV, valacyclovir.

^b The mean ± SD IC₅₀ of acyclovir for all isolates was 0.53 ± 0.39 µg/ml; that of penciclovir was 0.28 ± 0.11 µg/ml. For sensitive reference strain SC16, the mean ± SD IC₅₀ of acyclovir in all assays was 0.57 ± 0.34 µg/ml; that of penciclovir was 0.20 ± 0.17 µg/ml. For resistant HSV-1 reference strain DM21, the mean ± SD IC₅₀s of acyclovir ranged from 14.35 to >30 µg/ml; the mean ± SD IC₅₀s of penciclovir for HSV-1 DM21 ranged from 2.26 to >30 µg/ml.

^c Medians within each subgroup were compared by the Wilcoxon rank sum test with the t-test approximation.

^d Based on subjects' reported medication and medical histories.

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