Penciclovir (PCV) and acyclovir are acyclic guanine analogs which inhibit herpes simplex virus (HSV) DNA polymerase. Their 50% infective doses were 0.5 to 0.8 ,ug/ml for clinical isolates of HSV-1 and 1.3 to 2.2 ,ug/ml for HSV-2. Furthermore, HSV-infected cultures receiving 2-h pulses of PCV had 2-to 50-fold less HSV than acyclovir-treated cultures, consistent with the prolonged intracellular half-life of PCV triphosphate.Penciclovir [PCV; 9-(4-hydroxy-3-hydroxymethylbut-1-yl) guanine] is an acyclic guanine derivative with potent activity against herpes simplex viruses type 1 (HSV-1) and 2 (HSV-2), varicella-zoster virus, and Epstein-Barr virus (3). PCV is similar to acyclovir (ACV) in structure, metabolism, and antiviral spectrum. PCV, like ACV, is phosphorylated by a virus-specific thymidine kinase. PCV monophosphate is then phosphorylated by host enzymes to PCV triphosphate, which is a selective inhibitor of viral DNA synthesis. PCV triphosphate has an intracellular half-life of 10 h, which is 14 times longer than that of ACV triphosphate (5), resulting in antiviral activity persisting after the extracellular concentrations drop to low levels. In contrast, intracellular ACV triphosphate is rapidly metabolized to the acyclonucleoside which diffuses out of the cell. PCV triphosphate also achieves higher intracellular concentrations than ACV. The consequences of this difference were demonstrated by experiments showing that PCV reduced HSV yield with shorter periods of in vitro treatment than were required with ACV for the same effect (2). In addition, after removal of PCV from the medium, HSV replication remained inhibited for several days, whereas HSV replication resumed shortly after removing ACV. The persistence of PCV antiviral activity was confirmed by effectively treating HSV-infected mice with single daily doses of PCV (1).We compared PCV with ACV for in vitro activity against additional clinical isolates of HSV-1 and HSV-2 and following intermittent or continuous exposure at concentrations close to their 50% in vitro inhibitory concentrations (ID50s) for HSV.Sensitivity of HSV clinical isolates to ACV and PCV. Twenty clinical isolates each of HSV-1 and HSV-2 from our clinical laboratory were grown and assayed by plaque titration in human embryonic lung fibroblast (HEL) tissue culture. ID50s of the clinical isolates were determined by both plaque reduction assay (PRA) and enzyme-linked immunoassay (EIA) as previously described (4). The mean + standard deviation ID50 values of ACV for HSV-1 isolates * Corresponding author.were 0.5 ± 0.4 ,ug/ml by PRA and 0.7 ± 0.5 ,Lg/ml by EIA (Fig. 1). This was very similar to the sensitivity determined for PCV: 0.6 ± 0.4 ,ug/ml and 0.8 ± 0.3 ,ug/ml, by PRA and EIA, respectively. One HSV-1 isolate had intermediate sensitivity to ACV (ID50 of 2.2 ,ug/ml) and was sensitive to PCV (0.9 ,ug/ml). All others were sensitive to both drugs.

A new enzyme-linked immunosorbent assay (ELISA) respiratory syncytial virus antigen detection kit (Ortho Diagnostics, Inc., Raritan, N.J.) was compared with virus culture and with the indirect fluorescent antibody test (FAT) by using fresh nasal washings from children with suspected respiratory syncytial virus infection. Both uncentrifuged nasal washings and pellets from centrifuged split specimens were examined by ELISA. The ELISA was considered positive when the optical density was greater than the mean background optical density plus 0.15. Specimens positive by ELISA but negative by culture and FAT were reexamined in an ELISA blocking assay. Of 337 uncentrifuged specimens, 124 (37%) were positive by culture, 107 (32%) were positive by FAT, and 166 (49%) were positive by ELISA. Blocking assays showed that 21 of 30 (70%) of the seemingly false-positive ELISA tests were, in fact, true-positives and that the cultures and FATs were falsely negative. A patient specimen was considered positive when it was positive by virus culture, FAT, or blocking assay. The sensitivity, specificity, and positive predictive value of the ELISA test were 88, 94, and 95%, respectively. Centrifugation of nasal washings raised the sensitivity from 88 to 92% but reduced the specificity from 94 to 81%. We conclude that the Ortho ELISA test of uncentrifuged nasal washings is more sensitive than virus culture or indirect FAT and is highly specific.

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We compared the new Abbott TestPack (TP) respiratory syncytial virus (RSV) enzyme immunoassay (EIA) with cell culture and two commercial RSV EIAs (from Abbott Diagnostics and Kallestad Laboratories) by using split samples of fresh nasal washings from children with suspected RSV disease. Two tubes of HEp-2 cells were inoculated and observed for cytopathic effect for 14 days, and isolates were confirmed by immunofluorescence. The TP EIA was performed by following the manufacturer's instructions. Specimens positive by TP EIA but negative by culture were examined in a competitive inhibition (blocking) assay using the TP EIA and rabbit anti-RSV serum. Of 218 specimens, 93 were positive by culture, 105 were positive by TP EIA, 80 were positive by the Abbott Diagnostics EIA, and 87 were positive by the Kallestad Laboratories EIA. The sensitivity, specificity, positive predictive value, and negative predictive value of the TP EIA were 92, 86, 81, and 93%, respectively. Of 20 apparently false-positive TP EIAs, 10 of 14 that were positive when retested were neutralized in the blocking assay, indicating that they were truly positive. The recalculated sensitivity, specificity, positive predictive value, and negative predictive value of the TP EIA were 92, 91, 90, and 93%, respectively. We conclude that the TP EIA is easy to perform, rapid (<0.5 h), and accurate.

We compared a rapid respiratory syncytial virus (RSV) antigen enzyme immunoassay (EIA) (Abbott Diagnostics, North Chicago, Ill.) with virus culture and with the indirect fluorescent-antibody test (FAT) by using nasopharyngeal washings from children with suspected RSV pneumonia or bronchiolitis. Fresh washings were used in ail three tests. Specimens were inoculated into HEp-2 cells and human embryonic lung fibroblasts and observed for cytopathic effect. Cells in the centrifuged sediments of the nasal washes were examined for typical cytoplasmic fluorescence of RSV by FAT. The EIA cutoff was an optical density (OD) at 492 nm that was greater than the mean OD of the negative controls plus 0.1. An OD within +20% of the cutoff was considered borderline, and these specimens were retested. Of 289 specimens, 118 (41%) were positive by culture, 150 (52%) were positive by FAT, and 154 (53%) were positive by EIA. Eight borderline EIAs were all negative when the specimens were retested after storage at-70°C. Of 17 specimens positive by EIA but negative by culture and FAT, 9 were blocked in a competitive EIA, indicating that they were true-positives and that the culture and FAT were falsely negative. The sensitivity, specificity, and predictive value (positive) of the EIA versus culture, FAT, or blocking assay were 90, 94, and 95%, respectively. We conclude that the Abbott RSV antigen EIA is highly sensitive and specific.

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