Reduced provirus burden and enhanced humoral immune function in AZT-treated SCID-feline mice inoculated with feline immunodeficiency virus.

The lack of a safe, economical murine lentivirus model for human immunodeficiency virus type 1 (HIV-1) infection of humans has hampered the preclinical evaluation of potential antiviral compounds, vaccines, and biological response modifiers. A small animal model that does not employ HIV-1 is needed to minimize risk of accidental human exposure, enhance efficient use of scarce experimental compounds, and reduce laboratory space necessary to conduct statistically significant in vivo trials. Feline immunodeficiency virus (FIV), an immunosuppressive lentivirus of domestic cats, has been used extensively as an animal model for the pathogenesis and therapy of human HIV-1 infection. Cats, however, are not amenable to large-scale efficacy trials because of their relatively large size, high cost, and limited degree of physiologic characterization, particularly with regard to drug metabolism. To adapt the feline immune system to a small laboratory animal host, severe combined immunodeficient mice (SCID mice) were engrafted with feline lymphoid tissues (forming the SCID-fe mouse) and inoculated with FIV. Two quantitative parameters, the incidence of provirus detection in feline tissue grafts and the level of feline IgG in plasma, were used to demonstrate the antiviral efficacy of 3'-azido-3'-deoxythymidine (AZT, azidothymidine, Retrovir, zidovudine) in the SCID-fe system. Of 17 SCID-fe mice inoculated with 7 x 10(6) peripheral blood mononuclear cells (PBMC) from an FIV-infected cat, eight had detectable FIV provirus in both the feline thymus and feline lymph node implants, as measured by polymerase chain reaction (PCR)/Southern blot analysis. Treatment of these mice with AZT at a dose of 125 mg kg-1 day-1 in drinking water beginning 1 day prior to FIV inoculation and continuing throughout the study interval prevented the dual detection of provirus in feline lymph node and thymus grafts of all mice tested. In a separate experiment, the level of spontaneous feline IgG production was quantified by ELISA 2 weeks after FIV inoculation with and without AZT treatment. Mean plasma feline IgG level of five SCID-fe mice inoculated with 10(3) TCID50 cell-free FIV was 2.23 mg ml-1. Mean feline IgG level of five mice inoculated with the same quantity of FIV and treated with AZT beginning 1 day prior to virus inoculation and continuing for 2 weeks thereafter was 14.98 mg ml-1. AZT significantly (P < 0.05) enhanced feline humoral immune function at a virus inoculum titer of 10(3) TCID50.(ABSTRACT TRUNCATED AT 400 WORDS)

Phosphatidylazidothymidine and phosphatidyl-ddC: assessment of uptake in mouse lymphoid tissues and antiviral activities in human immunodeficiency virus-infected cells and in Rauscher leukemia virus-infected mice.

During the early stages of human immunodeficiency virus (HIV) infection, although symptoms are absent and viral replication in peripheral blood mononuclear cells is low, substantial levels of HIV replication can be documented in lymphoid tissue [G. Pantaleo, C. Graziosi, J.F. Demarest, L. Butini, M. Montroni, C.H. Fox, J.M. Orenstein, D.P. Kotler, and A.S. Fauci, Nature (London) 362:355-358, 1993, and J. Embretsen, M. Zupancic, J.L. Ribas, A. Burke, P. Racz, K. Tenner-Tacz, and A.T. Haase, Nature (London) 362:359-362, 1993]. This observation suggests that earlier treatment of HIV infection may be indicated and that strategies for enhancing drug targeting to the lymphoid tissue reservoris of HIV infection may be beneficial. To address this issue, we synthesized dioleoylphosphatidyl-ddC (DOP-ddC) and dipalmitoylphosphatidyl-3'-azido-3'-deoxythymidine (DPP-AZT), phospholipid prodrugs which form lipid bilayers and which are readily incorporated into liposomes. The anti-HIV activity of DOP-ddC was similar to that of ddC in HIV type 1-infected HT4-6C cells, but DPP-AZT was considerably less active than AZT in HT4-6C cells. Liposomes containing DOP-[3H]ddC or DPP-[3H]AZT administered intraperitoneally to mice produced greater levels of total radioactivity over time in plasma, spleen, and lymphoid tissue relative to the results with [3H]ddC and [3H]AZT, respectively. DPP-AZT administered intraperitoneally in liposomes as a single daily dose to mice infected with Rauscher leukemia virus prevented increased spleen weight and reverse transcriptase levels in serum with a dose-response roughly comparable to that of AZT given continuously in the drinking water. DOP-ddC, DPP-AZT, and lipid conjugates of other antiretroviral nucleosides may provide higher levels of drug over time in plasma and in lymph nodes and spleen, important reservoirs of HIV infection, and may represent an interesting alternative approach to antiviral nucleoside treatment of AIDS.

Feline lymphoid tissues engrafted into scid mice maintain morphologic structure and produce feline immunoglobulin.

To adapt the feline immune system to a small laboratory animal host, severe combined immunodeficient (scid) mice were engrafted with neonatal feline lymphoid tissues, including lymph node, thymus, spleen, and bone marrow. Lymph node and thymus tissue were implanted subcutaneously within the mammary fat pad, and a single-cell suspension of spleen, thymus, and bone marrow was inoculated intraperitoneally (IP). Seven groups of mice (three mice per group) were engrafted on day 0, and members of one group were euthanatized weekly from 2 to 8 weeks after engraftment. For each mouse, graft morphology was evaluated by light microscopy, feline DNA was detected in peripheral blood by polymerase chain reaction (PCR) amplification of feline-specific DNA sequences, and serum IgG concentration was measured by ELISA. Ten of 13 feline grafts evaluated histologically between 3 and 8 weeks after engraftment contained large focal aggregates of lymphocytes bordered by plasma cells. Of 14 thymus grafts evaluated histologically during the same period, 5 were characterized by dense accumulations of small lymphocytes surrounding thymic epithelial cells. Two of these thymus grafts were indistinguishable from age-matched feline thymus. At 2 weeks after engraftment, feline lymph node and thymus contained extensive central necrosis bordered by a narrow zone of lymphocytes and small-caliber blood vessels.(ABSTRACT TRUNCATED AT 250 WORDS)

Evaluation of the potent anti-hepatitis B virus agent (-) cis-5-fluoro-1-[2-(hydroxymethyl)-1,3-oxathiolan-5-yl]cytosine in a novel in vivo model.

A murine model was developed to investigate the in vivo activity of anti-hepatitis B virus (HBV) agents. Mice with subcutaneous tumors of HBV-producing 2.2.15 cells showed reductions in levels of HBV in serum and in intracellular levels of HBV when the mice were orally dosed with (-) cis-5-fluoro-1-[2-(hydroxymethyl)-1,3-oxathiolan-5-yl]cytosine (FTC). No effects on tumor size or alpha-fetoprotein levels were observed. FTC can selectively inhibit HBV replication at nontoxic doses.

Feline immunodeficiency virus infection of cats as a model to test the effect of certain in vitro selection pressures on the infectivity and virulence of resultant lentivirus variants.

Three groups of specific pathogen-free (SPF) domestic cats, each containing 5 animals, were infected with one of three closely related FIV variants and monitored for 36 weeks. A fourth group of 5 cats was sham-infected and served as uninfected controls. FIV variants included: (1) a fully virulent animal passaged FIV-Petaluma; (2) a Crandell feline kidney (CrFK) cell-adapted FIV-Petaluma (FIV-CrFK); and (3) a variant of FIV-CrFK (FIV-CrFKAZT) that had been selected in vitro for resistance to azidothymidine. Cats infected with fully virulent FIV-Petaluma strongly seroconverted, became persistently viremic, and exhibited lymphadenopathy, neutropenia, and inversion of the CD4+:CD8+ T cell ratio. Cats infected with FIV-CrFK seroconverted but the antibody responses were much weaker and more variable; two of the cats became transiently viremic and no hematologic abnormalities or clinical signs of illness other than a very mild lymphadenopathy were observed. None of the five cats inoculated with FIV-CrFKAZT seroconverted, became viremic, or exhibited any gross or hematologic signs of disease, even though proviral DNA was transiently detected in tissue following inoculation. This study demonstrates that the FIV infection model can be used to assess differences in the virulence of FIV variants, including variants selected for antiretroviral drug resistance.

6-Dimethylamino-9-(beta-D-arabinofuranosyl)-9H-purine: pharmacokinetics and antiviral activity in simian varicella virus-infected monkeys.

6-Dimethylamino-9-(beta-D-arabinofuranosyl)-9H-purine (ara-DMAP) effectively prevented the development of rash and appreciably reduced viremia in simian varicella virus-infected monkeys. Doses of 100 and 50 mg/kg/day, administered orally, were highly effective. The lowest dose of 20 mg/kg/day was much less effective in preventing moderate viremia. However, the 20 mg/kg/day did prevent the development of rash in two of three monkeys. All three doses of ara-DMAP reduced liver infection as reflected by lower aspartate aminotransferase values in the sera of the African green monkeys. Orally administered ara-DMAP was rapidly absorbed. However, significant variation among individual monkeys in the AUC values, peak plasma levels, and plasma half-lives were observed.

Sensitivity monitoring of clinical isolates of herpes simplex virus to acyclovir.

Zovirax (acyclovir, ACV) is now widely accepted as a safe and effective treatment for the management of herpes simplex virus (HSV) infections in normal and immunocompromised patients. However, a common concern with regard to the widespread use of any antimicrobial agent is resistance. The virus specific mechanism of action of ACV involves two virus encoded enzymes, thymidine kinase (TK) and DNA polymerase. Any alteration in the genes coding for these two enzymes would therefore be expected to confer resistance. The findings from two extensive resistance monitoring programs have shown that in immunocompetent patients receiving ACV for the management of acute HSV disease, the incidence of resistance is extremely rare. The situation in the immunocompromised is different. In this patient group HSV disease is severe and protracted often requiring prolonged therapy thus increasing the exposure of the virus to drug. As a result HSV isolates resistant to ACV have occasionally been recovered. Biochemical and genetic analysis of the resistant clinical isolates has shown that resistance in the most part is due to an inability of the virus to produce TK which mirrors the findings with cell culture derived resistant virus. Laboratory studies would indicate that TK-deficient virus would have little clinical impact. Significantly, resistance has rarely been attributed to alterations in the substrate specificity of TK or DNA polymerase. The biological significance of these mutants is unclear but to date there has been no evidence of transmission of resistant virus.

Inactivators of herpes simplex virus ribonucleotide reductase: hematological profiles and in vivo potentiation of the antiviral activity of acyclovir.

A1110U (BW 1110U81) is an inactivator of herpesvirus ribonucleotide reductases and a potentiator of the antiviral activity of acyclovir (ACV) (T. Spector, J. A. Harrington, R. W. Morrison, Jr., C. U. Lambe, D. J. Nelson, D. R. Averett, K. Biron, and P. A. Furman, Proc. Natl. Acad. Sci. USA 86:1051-1055, 1989) that was subsequently found to cause hematological toxicity at high oral doses in rats. Eleven structurally related inactivators of herpes simplex virus (HSV) ribonucleotide reductase were therefore tested in vivo for hematological toxicity and for potentiation of ACV. None of the novel ribonucleotide reductase inactivators was hematologically toxic to rats following oral dosing at 60 mg/kg/day for 30 days. Four of these inactivators statistically improved the antiviral topical potency of ACV on HSV type 1-infected nude mice. A promising compound, 2-acetylpyridine 5-[(2-chloroanilino)thiocarbonyl]thiocarbonohydrazone (BW 348U87), was studied more extensively in two in vivo models: dorsum-infected athymic nude mice and snout-infected hairless mice. BW 348U87 significantly potentiated the antiviral activity of ACV against all virus strains tested, i.e., wild-type (ACV-sensitive) HSV type 1 and HSV type 2 strains and three mutant (ACV-resistant) HSV type 1 strains. The latter included a virus expressing a DNA polymerase resistant to inhibition by ACV triphosphate, a virus deficient in thymidine kinase (the enzyme responsible for phosphorylating ACV), and a virus expressing an altered thymidine kinase, which catalyzes the normal phosphorylation of thymidine but not of ACV. BW 348U87 and ACV are currently being developed as a combination topical therapy for cutaneous herpes infections.

In vitro and in vivo characterization of herpes simplex virus clinical isolates recovered from patients infected with human immunodeficiency virus.

A total of 100 herpes simplex viruses isolated from lesions not responding to acyclovir (ACV) therapy were recovered from 51 patients infected with human immunodeficiency virus. In vitro analysis of these isolates included testing their susceptibility to ACV and determining their thymidine kinase (TK) phenotypes. Of the 100 isolates evaluated, 23 were ACV sensitive and 77 were ACV resistant. Seventy-four of these ACV-resistant isolates were of the TK-deficient or low-TK-producer phenotype and three were of the TK-altered phenotype. The TKs isolates that represented each of the different autoradiographic phenotypes were further characterized by enzyme kinetics. The ability of selected isolates to cause disease in vivo was evaluated by using several mouse virulence models. Cutaneous virulence in normal and immunocompromised mice was evaluated, and neurovirulence in normal mice was determined. Latent infections were assayed by the cocultivation of trigeminal ganglia recovered from mice that had survived acute infection. These reactivated viruses were evaluated in vitro and compared with the original infecting isolate. The mechanisms of resistance and pathogenicity of these herpes simplex virus isolates recovered from patients positive for human immunodeficiency virus are similar to those reported for isolates recovered from normal and immunocompromised patients without AIDS.

Synergistic topical therapy by acyclovir and A1110U for herpes simplex virus induced zosteriform rash in mice.

Combination therapy with A1110U, an inactivator of the herpes simplex virus (HSV) and the varicella zoster virus ribonucleotide reductase, and acyclovir (ACV) was evaluated for treatment of cutaneous herpetic disease in athymic mice infected on the dorsum. In this model, infection with HSV produces a 'zosteriform-like' rash that is first visible on day 3 or 4 post-infection (p.i.) and eventually extends from the anterior mid-line to the dorsal mid-line of the affected flank. In untreated mice, the infection is fatal at about day 7 p.i. presumably due to central nervous system involvement. Topical treatment of infections induced by either wild-type (wt) HSV-1 or wt HSV-2 with 3% A1110U in combination with 5% ACV resulted in synergistic (P less than 0.01) reductions in lesion scores. Therapy was also synergistic in mice infected with an ACV-resistant thymidine kinase-deficient mutant and an ACV-resistant TK-altered mutant HSV-1 isolated. Combination therapy was very effective in reducing lesion scores of mice infected with an ACV-resistant HSV-1 DNA polymerase mutant, but did not result in statistically significant synergy (P = 0.07) because of the enhanced efficacy of A1110U alone against this virus. These results provide encouragement that the combination of A1110U and ACV may offer an effective therapy for topical treatment of cutaneous HSV infections in humans.

Acyclovir-resistant herpes simplex virus causing pneumonia after marrow transplantation.

Three marrow transplant patients developed pneumonia due to acyclovir-resistant thymidine-kinase-deficient herpes simplex virus (HSV) type 1. In all three, pneumonia was evident at autopsy by both standard and immunohistology, and virus was recovered from culture of lung tissue. Two patients also had other pulmonary infections at death; one had only HSV pneumonia. All had received prophylaxis and repeated treatment courses with acyclovir, and all initially had acyclovir-sensitive virus. The acyclovir-resistant HSV strains were sensitive to foscarnet, and in at least one case to vidarabine, but as expected were resistant to ganciclovir. These cases represent potentially severe visceral infection in which acyclovir-resistant virus strains were primary or important copathogens. Although acyclovir-resistant HSV is generally considered less virulent, these cases illustrate the potential importance of infection due to acyclovir-resistant HSV in severely immunocompromised patients. They also highlight the need to test HSV strains for antiviral sensitivity and to consider alternative therapies to acyclovir in appropriate clinical situations.

The effect of nicotinamide on microvascular density and thermal injury in rats.

The effects of nicotinamide on the microvasculature and wound healing were examined in rats subjected to thermal injury. Rats (250 g) were treated with 50 mg nicotinamide intraperitoneally twice daily for 21 days and then heart and brain biopsies were taken. Skin biopsies were removed from sites in and adjacent to the injury throughout the course of healing. Tissues were stained for alkaline phosphatase and capillary length density was determined by morphometric analysis. Significant increases were observed in the heart, brain, and dermal tissue of treated animals compared to controls. Capillary density in the injured skin was significantly greater when compared to the injured skin of saline-treated controls. The injuries of the rats that were treated systemically with nicotinamide healed significantly faster than saline-treated as determined by planimetric evaluation of the granulation bed and eschar.

Synergistic therapy by acyclovir and A1110U for mice orofacially infected with herpes simplex viruses.

Clinical effects of the administration of a combination of acyclovir (ACV) and compound A1110U (a 2-acetylpyridine thiocarbonothiohydrazone inactivator of herpes simplex virus [HSV] ribonucleotide reductase) on the development of herpetic skin lesions were studied in athymic and hairless mice infected intracutaneously with different HSV type 1 (HSV-1) strains. ACV was administered topically (5%) or orally (5 mg/ml), while A1110U was applied topically (3%). In all but one experiment, the effect of combination therapy was greater than that calculated for the sum of the individual drug effects in limiting the development of herpetic skin lesions in mice. In several experiments, combination therapy totally eliminated all signs of infection. This synergistic chemotherapeutic efficacy was evident in infections caused by ACV-susceptible as well as several ACV-resistant HSV-1 strains. These results indicate that this combination therapy may provide a significant improvement in clinical responses over single-agent topical therapy.

Mucocutaneous dissemination of acyclovir-resistant herpes simplex virus in a patient with AIDS.

Infections caused by herpes simplex virus (HSV) are a significant source of morbidity in immunocompromised patients. Acyclovir is often used prophylactically and therapeutically in patients with human immunodeficiency virus infection. The emergence of acyclovir-resistant strains of HSV capable of causing disease has been recognized. We report a case in which a thymidine kinase-deficient mutant of HSV caused extensive disease in a patient with AIDS. This case emphasizes that virus recovered from nonhealing lesions should be submitted for further study, which may advance our understanding of the interaction between host defense and drug-resistant strains.

Angiogenic potency of nucleotide metabolites: potential role in ischemia-induced vascular growth.

Hypoxia and ischemia are potent stimuli to vascular growth. The mechanisms by which vascular growth is induced are unknown. During ischemia, such as that which occurs in the heart, purine and pyridine nucleotides are degraded and their metabolites accumulate. At least two of these metabolites, adenosine and nicotinamide, have previously been demonstrated to induce vascular growth. The goal of this study was to determine whether other purine and pyridine metabolites have the potential to stimulate angiogenesis in vivo, to determine the relative angiogenic potency of these metabolites, and to determine if their angiogenic effects is mediated through a direct effect on endothelial cell proliferation. Purine metabolites (adenosine, inosine, hypoxanthine, xanthine, guanosine, uric acid), the pyridine metabolite nicotinamide, and chemical derivatives of nicotinamide, were tested at various concentrations for their ability to stimulate angiogenesis in the chick choriollantoic membrane assay. Although none of the purine metabolites were effective in promoting the angiogenic response, nicotinamide as well as several derivatives of nicotinamide induced an angiogenic response in a dose-dependent manner. Nicotinamide was then evaluated to determine if its angiogenic effect is a result of a direct effect on capillary endothelial cell proliferation. In concentrations of 100 microM to 1 mM nicotinamide was not demonstrated to be mitogenic for bovine capillary endothelial cells. These results demonstrate that pyridine nucleotides are indirect angiogenic agents that do not exert a primary effect on endothelial cell proliferation. The results of this study suggest that increases in vascular growth induced by ischemia and hypoxia might be mediated, at least in part, by pyridine metabolites released from ischemic tissues.

Orofacial infection of athymic mice with defined mixtures of acyclovir-susceptible and acyclovir-resistant herpes simplex virus type 1.

Infection of athymic mice with defined populations of acyclovir-susceptible (thymidine kinase [TK]-positive) and acyclovir-resistant (TK-deficient or TK-altered) herpes simplex virus type 1 strains was used to simulate herpetic skin disease of the immunocompromised host. In vitro characterization of the defined virus mixtures revealed that the dye uptake method was quite sensitive in the detection of small amounts (3 to 9%) of acylovir-resistant virus. Mice infected with homogeneous virus populations exhibited a good correlation between clinical response and the in vitro drug susceptibility of the infecting virus. Animals infected with defined mixtures of viruses exhibited varied patterns of infection and responses to acyclovir treatment. However, disease severity was useful in predicting the TK phenotype of virus recovered from lesions. Pathogenic, TK-altered virus was responsible for progressive disease in animals receiving low-dose (0.25-mg/ml) prophylactic acyclovir or high-dose (1.25-mg/ml) delayed therapy. Although this mutant was recovered infrequently, it was responsible for clinically significant disease in the animals from which it was isolated.

Nucleotide sequence changes in thymidine kinase gene of herpes simplex virus type 2 clones from an isolate of a patient treated with acyclovir.

To identify the nucleotide changes that occur in drug-induced thymidine kinase (TK) mutants of herpes simplex virus type 2 (HSV-2), we compared the nucleotide sequences of the tk genes of two mutant HSV-2 clones isolated from a patient who had been treated with acyclovir [9-(2-hydroxyethoxymethyl)guanine; ACV] with the nucleotide sequence of the parental TK+ HSV-2(8703) strain isolated from the same patient. One of the mutants, TK-altered (TKA) HSV-2(9637), was ACV resistant but induced the incorporation of [14C]thymidine into the DNA of infected rabbit skin cells. The nucleotide sequence of the tk gene of mutant TKA HSV-2(9637) had a single change (G to A) at nucleotide 668, which would cause an arginine-to-histidine substitution at amino acid residue 223 of the TK polypeptide. The second ACV-resistant mutant, TK- HSV-2(8710), did not induce detectable incorporation of [14C]thymidine into the DNA of infected rabbit skin cells. This mutant exhibited a deletion of a single base at nucleotide 217 of its nucleotide sequence. This deletion would cause a frameshift mutation at amino acid residue 73 and chain termination at amino acid residue 86 of the TK polypeptide. The nucleotide sequence of TK+ HSV-2(8703) was the same as that of the laboratory strain, TK+ HSV-2(333). The nucleotide sequence of a bromodeoxyuridine-resistant TK- HSV-2(333) mutant of TK+ HSV-2(333) also exhibited a single-base deletion, but at nucleotide 439. This deletion would cause a frameshift mutation at amino acid residue 147 and chain termination at amino acid residue 182. The frameshift mutations of TK- HSV(8710) and TK- HSV-2(333), respectively, occurred in sequences in which C was repeated three times and G was repeated seven times. The results raise the possibility that TK- frameshift mutations of HSV-2 may be common.

Progressive mucocutaneous herpes simplex infection due to acyclovir-resistant virus in an immunocompromised patient: correlation of viral susceptibilities and plasma levels with response to therapy.

Rapidly progressive disease due to acyclovir-resistant herpes simplex has not been described. We report such a case and detail successful patient management using viral sensitivities and plasma acyclovir levels to guide therapy. Response was correlated with plasma levels above those inhibiting viral growth by 50% (ID50) in vitro.

Clinical isolate of herpes simplex virus type 2 that induces a thymidine kinase with altered substrate specificity.

In vitro and in vivo studies were done on a herpes simplex virus type 2 strain recovered from a patient on acyclovir (ACV) which was ACV resistant but expressed thymidine (dThd) kinase (EC 2.7.1.21) activity. Plaque-purified clones derived from the original clinical sample were heterogeneous with respect to plaque size and drug susceptibility. The heterogeneity of this viral mixture was also evident from varied 125I-labeled 5-iodo-2'-deoxycytidine autoradiographic patterns and from varied expression of dThd kinase-associated phosphorylating activities. Four clones from this mixture were 1-beta-D-arabinofuranosylthymine (ara-T) susceptible and ACV resistant. Extracts of cells infected with these clones catalyzed the phosphorylation of ara-T but little of ACV. The virus-coded dThd kinase was purified from one of these clones to determine whether its substrate specificity was altered. The amount of virus-coded dThd phosphorylating activity with the cell extracts was estimated to be sevenfold lower with the resistant clone than with the MS strain of herpes simplex virus type 2. The dThd kinase eluted from a dThd-agarose affinity column under the same conditions with extracts from both sources and substrate saturations of both enzymes by acyclic nucleoside analog phosphate acceptors were classical hyperbolic functions. However, there were significant differences in the kinetic parameters of substrates between the two enzymes. Apparent Km (Km') values for dThd, deoxycytidine, ara-T, ACV, and the acyclic guanosine analog 9-[[2-hydroxyl-1-(hydroxymethyl)ethoxy]methyl]guaine (BW B759U) were 2- to 60-fold higher with the variant enzyme than with the enzyme from laboratory strain MS. Comparing these two enzymes, relative maximal phosphorylation rates (Vm) were eightfold lower for ACV but unchanged for BW B759U. In contrast, the relative rates for deoxycytidine and ara-T were eight- and twofold higher, respectively. The surprisingly good substrate activity with BW B759U compared with that of ACV (Vm/Km' = 0.39 versus 0.01) coincided with susceptibility of the ACV-resistant virus to BW B759U. This clinical variant retained its pathogenicity for mice and was only moderately less neurovirulent than wild-type virus. Although such mutants have the potential to induce illness less responsive to therapy, the recurrence from which the isolate was obtained was typical for this patient in severity and duration. Since this episode, the patient has been treated successfully with ACV.

Clinical and laboratory experience with acyclovir-resistant herpes viruses.

The emerging field of antiviral sensitivity testing must depend upon some of the lessons learned from prior experience with anti-bacterial sensitivity testing, but also take into account additional factors including intracellular drug distribution and metabolism as well as the unique features of viral replication. There is an urgent need to standardise in-vitro sensitivity testing, and an international collaborative study is essential so that different laboratories may compare and define their sensitivity results in the context of the outcome of therapy in infected patients. It is only by such a study that the influence of patient and therapeutic factors such as immune function, anatomical location of the infection and dosage regimens used can be understood.