Acyclovir Research News

Herpes simplex-1 virus thymidine kinase gene is unable to completely eliminate live, nonimmunogenic tumor cell vaccines.

Recent experiments with genetically engineered tumors have generated renewed interest in active cellular immunotherapy as a cancer treatment modality. In order to consider the use of live tumor cells for immunotherapy in human cancer patients, it will be important to ensure that these cells do not themselves produce morbidity in the event the immune system fails to eliminate them. Toward this end, we have examined a strategy for eliminating genetically manipulated nonimmunogenic tumors in vivo. When B16F10 melanoma cells were transfected with the Herpes simplex virus 1 thymidine kinase (HSV-TK) gene, cells were rendered susceptible to killing by the nucleoside analogs acyclovir (ACV) and ganciclovir (GCV). B16-HSV-TK+ tumors established in C57BL6 mice were successfully "suicided" in vivo when GCV was administered by continuous infusion. However, late recurrences were observed even after 1 month of continuous GCV treatment. In vivo growth kinetics suggested that the recurrences resulted from a tiny number (< 20) of cells that had survived the GCV treatment. Interestingly, recurrent tumors were as sensitive to GCV as the parental B16-HSV-TK+ line. While these results demonstrate potential feasibility of the suicide gene strategy for active immunotherapy with live tumor cells, they also illustrate that approaches dependent on the intracellular generation of cell cycle-dependent toxins may fail to eliminate small numbers of cells that temporarily exit cell cycle or that are pharmacologically sequestered.

Journal ISSN: 1053-8550 Issue: 12-4 (1992) Pages: 224-30

2.5 years survival with sequential ganciclovir/foscarnet treatment in a patient with acquired immune deficiency syndrome and cytomegalovirus retinitis.

The case with the longest survival time (30 months) after the diagnosis of cytomegalovirus retinitis in a group of 53 patients with the acquired immune deficiency syndrome and cytomegalovirus retinitis (median survival time 8.4 months) is described. The patient developed cytomegalovirus retinitis in his left eye and received intravenous virustatic treatment for 29.5 months. Treatment was started with ganciclovir. After withdrawal from maintenance treatment, a relapse of cytomegalovirus retinitis occurred, which was again successfully treated with ganciclovir. A secondary cataract developed, and cataract extraction was performed. After 12.5 months on treatment with ganciclovir, a change to foscarnet was necessary because of neutropenia. Maintenance treatment with foscarnet was well tolerated for 17 months. The cytomegalovirus retinitis showed no signs of reactivation during this period. Vision in the right eye was preserved until death, and no sign of cytomegalovirus retinitis developed in the right eye. This case report demonstrates that an unusual long survival time is possible in a patient sequentially treated with ganciclovir and foscarnet.

Journal ISSN: 0941-2921 Issue: 1-2 (1992) Pages: 110-3

Acyclovir-resistant herpes simplex virus keratouveitis after penetrating keratoplasty.

PURPOSE: A case of acyclovir-resistant herpes simplex virus keratouveitis after penetrating keratoplasty is reported. METHODS: Resistance to acyclovir was evident clinically and was confirmed by in vitro susceptibility testing. The susceptibility of the herpes simplex isolates to acyclovir and foscarnet was determined by a dye uptake assay that measured cytopathic effect, and thymidine kinase activity was measured by a plaque autoradiography technique. RESULTS: The viral isolate from postoperative day 22 was susceptible to acyclovir and foscarnet, and showed normal thymidine kinase activity. Isolates from postoperative days 29 and 32 (coinciding with deterioration in clinical appearance) were resistant to acyclovir, susceptible to foscarnet, and deficient in thymidine kinase activity. CONCLUSION: Practitioners should be aware of the potential for the emergence of resistance in this setting; prophylaxis and rational alternate therapies are discussed.

Journal ISSN: 0161-6420 Issue: 99-12 (1992) Pages: 1805-8

Neonatal herpes simplex virus infections: pathogenesis and therapy.

Neonatal herpes simplex virus (HSV) infections are of increasing incidence in North America, now occurring at a rate of approximately one in 3,500 to one in 5,000 deliveries per year. Disease manifests as one of three forms; namely, infection: localized to the skin, eye and mouth (SEM), encephalitis (CNS), or disseminated disease. With the advent of antiviral therapy, it has become possible to decrease mortality and improve morbidity for babies suffering from infection. Advances in antiviral therapy have allowed for prevention of disease progression beyond states of SEM involvement. Furthermore, life threatening infections of the CNS or of multiple organs, have mortality with either acyclovir or vidarabine therapy. Now approximately 15% (CNS) and 50% (disseminated disease) of babies die from neonatal HSV disease. The results of ongoing studies in the United States will summarize the pathogenesis and treatment of neonatal HSV infection.

Journal ISSN: 0369-8114 Issue: 40-7 (1992) Pages: 729-34

Mode of antiviral action of penciclovir in MRC-5 cells infected with herpes simplex virus type 1 (HSV-1), HSV-2, and varicella-zoster virus.

The metabolism and mode of action of penciclovir [9-(4-hydroxy-3-hydroxymethylbut-1-yl)guanine; BRL 39123] were studied and compared with those of acyclovir. In uninfected MRC-5 cells, low concentrations of the triphosphates of penciclovir and acyclovir were occasionally just detectable, the limit of detection being about 1 pmol/10(6) cells. In contrast, in cells infected with either herpes simplex virus type 2 (HSV-2) or varicella-zoster virus (VZV), penciclovir was phosphorylated quickly to give high concentrations of the triphosphate ester. Following the removal of penciclovir from the culture medium, penciclovir-triphosphate remained trapped within the cells for a long time (half-lives, 20 and 7 h in HSV-2- and VZV-infected cells, respectively). In HSV-2-infected cells, acyclovir was phosphorylated to a lesser extent and the half-life of the triphosphate ester was only 1 h. We were unable to detect any phosphates of acyclovir in VZV-infected cells. (S)-Penciclovir-triphosphate inhibited HSV-1 and HSV-2 DNA polymerase competitively with dGTP, the Ki values being 8.5 and 5.8 microM, respectively, whereas for acyclovir-triphosphate, the Ki value was 0.07 microM for the two enzymes. Both compounds had relatively low levels of activity against the cellular DNA polymerase alpha, with Ki values of 175 and 3.8 microM, respectively. (S)-Penciclovir-triphosphate did inhibit DNA synthesis by HSV-2 DNA polymerase with a defined template-primer, although it was not an obligate chain terminator like acyclovir-triphosphate. These results provide a biochemical rationale for the highly selective and effective inhibition of HSV-2 and VZV DNA synthesis by penciclovir and for the greater activity of penciclovir than that of acyclovir when HSV-2-infected cells were treated for a short time.

Journal ISSN: 0066-4804 Issue: 36-12 (1992) Pages: 2747-57