Helicase-primase as a target of new therapies for herpes simplex virus infections

The seminal discovery of acyclovir 40 years ago heralded the modern era of truly selective antiviral therapies and this drug remains the therapy of choice for herpes simplex virus infections. Yet by modern standards, its antiviral activity is modest and new drugs against novel molecular targets such as the helicase-primase have the potential to improve clinical outcome, particularly in high-risk patients. A brief synopsis of current therapies for these infections and clinical need is provided to help provide an initial perspective. The function of the helicase-primase complex is then summarized and the development of new inhibitors of the helicase-primase complex, such as pritelivir and amenamevir, is discussed. We review their mechanism of action, propensity for drug resistance, and pharmacokinetic characteristics and discuss their potential to advance current therapeutic options. Strategies that include combinations of these inhibitors with acyclovir are also considered, as they will likely maximize clinical efficacy.

Determination of appropriate dosing of influenza drugs in pediatric patients

Dose-finding studies of influenza antiviral drugs are challenging because it is difficult to enroll subjects in pediatric interventional studies and also because of the lack of concentration (or toxicity)–response relationships, the short duration of antiviral therapy, and the continually developing metabolic profiles of infants and young children. The evaluation of influenza antiviral agents in premature infants adds even more complexity. Recent advances in exposure-targeted study designs and modeling and simulations have aided in addressing some of these challenges.

Cloning and characterization of recombinant rhesus macaque IL-10/Fc(ala-ala) fusion protein: a potential adjunct for tolerance induction strategies

The powerful anti-inflammatory and immunosuppressive activities of IL-10 make it attractive for supplemental therapy in translational tolerance induction protocols. This is bolstered by reports of IL-10-mediated inhibition of innate immunity, association of human stem cell and nonhuman primate (NHP) islet allograft tolerance with elevated serum IL-10, and evidence that systemic IL-10 therapy enhanced pig islets survival in mice. IL-10 has not been examined as adjunctive immunosuppression in NHP. To enable such studies, we cloned and expressed rhesus macaque (RM) IL-10 fused to a mutated hinge region of human IgG1 Fc to generate IL-10/Fc(ala-ala). RM IL-10/Fc(ala-ala) was purified to approximately 98% homogeneity by affinity chromatography and shown to be endotoxin-free (<0.008 EU/microg protein). The biological activity of IL-10/Fc(ala-ala) was demonstrated by (1) costimulation of the mouse mast cell line, MC/9 proliferation in a dose-dependent fashion, (2) suppression of LPS-induced septic shock in mice and (3) abrogation of LPS-induced secretion of proinflammatory cytokines/chemokines in vitro and in vivo in NHP. Notably, RM IL-10/Fc(ala-ala) had significantly greater potency than human IL-10/Fc(ala-ala) and exhibited a circulating half-life of approximately 14 days. The availability of this reagent will facilitate definitive studies to determine whether supplemental therapy with RM IL-10/Fc(ala-ala) can influence tolerance outcomes in NHP.

Ganciclovir Population Pharmacokinetics in Neonates Following Intravenous Administration of Ganciclovir and Oral Administration of a Liquid Valganciclovir Formulation

Cytomegalovirus (CMV) is the most common viral congenital infection, producing both sensorineural hearing loss and mental retardation. Our objective was to assess the population pharmacokinetics of a research-grade oral valganciclovir solution in neonates with symptomatic congenital CMV disease. Twenty-four neonates received 6 weeks of antiviral therapy. Ganciclovir and valganciclovir were measured by liquid chromatography/tandem mass spectroscopy. NONMEM version VI beta was used for population analyses. All profiles were consistent with a one-compartment model. Postnatal age, body surface area, and gender did not improve the model fit after body weight was taken into account. The typical value of clearance (l/h), distribution volume (l), and bioavailability of ganciclovir were 0.146 x body weight (WT)(1.68), 1.15 x WT, and 53.6%, respectively. Although these results cannot be extrapolated to extemporaneously compounded valganciclovir preparations, they provide the foundation on which a commercial-grade valganciclovir oral solution may be a viable option for administration to neonates.

Pharmacokinetic enhancement of protease inhibitors

Protease inhibitor (PI)-enhanced regimens are becoming a standard of care in therapy for HIV infection. Many important questions remain regarding the optimal use of this treatment strategy. Multiple physiologic and pathologic factors influence the pharmacokinetic and pharmacodynamic profiles of PIs. Specifically, alterations in drug metabolism associated with inhibition or stimulation of the 3A4 isozyme of the cytochrome P-450 enzyme system, activity of the P-glycoprotein intracellular transport system, and degree of plasma protein binding are all recognized as having important roles in influencing overall plasma PI concentrations and, ultimately, efficacy. Available pharmacokinetic data should include interpatient variation in plasma PI concentrations in addition to mean or median results. Viral inhibitory concentrations that have been corrected for the effect of protein binding should also be standardized. Studies to establish the concentration-response relationship for PIs may also prove beneficial in determining optimal plasma PI concentrations. Currently, therapeutic drug monitoring is not routinely recommended because of a lack of convincing clinical data as well as of a standard approach to collection and interpretation of drug concentrations. Large prospective studies are needed to further advance the usefulness of therapeutic drug monitoring.

Pharmacokinetics of saquinavir-SGC in HIV-infected pregnant women

PURPOSE

To evaluate saquinavir (SQV) pharmacokinetics, tolerance, and safety in 10 HIV-infected pregnant women between 14-32 weeks gestation.

METHOD

This was a phase I, prospective, area-under-the-curve (AUC) targeted study. Antepartum treatment consisted of SQV 1200 mg tid, lamivudine 150 mg bid, and zidovudine 200 mg tid. The SQV targeted exposure was an 8-hour AUC (AUC(8)) of 3000 ng. h/mL; the study was to be halted if the first 4 participants did not achieve this AUC(8). Cord blood and plasma samples were collected in neonates at birth.

RESULTS

Four women completed the SQV pharmacokinetic assessments. Exposure in all 4 patients was below the target AUC(8). Median (range) AUC(8) and trough (C8H) were 1672 (738-2614) ng. h/mL and 60 (<15-332) ng/mL, respectively. Oral clearance (CL/F) was 9.3 (5.1-16.6) L/h/kg and C(max) was 599 (177-953) ng/mL. Cord and neonate plasma concentrations were mostly undetectable; 1 of 5 infants was HIV-infected at 24 weeks.

CONCLUSION

These data suggest highly variable SQV pharmacokinetics in pregnant women, and exposure at 1200 mg tid may not be adequate for longer term therapy; both the AUC(8) and C8H were considerably below average. Because ritonavir has been shown to significantly increase SQV concentrations, this combination should be further explored in this population.

Acyclovir for treatment of postherpetic neuralgia: efficacy and pharmacokinetics

Postherpetic neuralgia is the most common complication of herpes zoster (shingles) in the immunocompetent host. Its mechanism is incompletely understood, but one postulate is that continuous replication of varicella-zoster virus (VZV) in nerve tissues may be responsible for the pain. If this is so, antiviral treatment could be advantageous. To test this hypothesis, we performed a randomized, double-blind, placebo-controlled trial of intravenous acyclovir (10 mg/kg every 8 h [q8h]) for 14 days, followed by oral acyclovir (800 mg q6h) for 42 days in 10 subjects (median age, 71 years) who had experienced at least 6 months of severe pain (median duration of postherpetic neuralgia before enrollment, 3.2 years). Intensive and sparse pharmacokinetic sampling occurred during both dosing phases of the study. One- and two-compartment models were fitted to the oral and intravenous concentration-time data, respectively. The four men and four women assigned to acyclovir during either or both dosing phases tolerated it well. Pharmacokinetic results were similar to those previously reported in younger individuals. The mean oral clearance and elimination half-life following oral dosing were 1.47 liters/h/kg and 2.78 h, respectively. Total clearance and terminal half-life following intravenous administration were 0.16 liters/h/kg and 3.67 h, respectively. Only 1 of 10 participants reported definite improvement in the severity of postherpetic pain, and treatment had no effect on titers of humoral antibody to VZV. We concluded that 56 days of intravenous and oral acyclovir therapy were well tolerated but had little or no effect on the clinical course of postherpetic neuralgia.

Combination therapy with indinavir, ritonavir, and delavirdine and nucleoside reverse transcriptase inhibitors in patients with HIV/AIDS who have failed multiple antiretroviral combinations

PURPOSE

Ritonavir (RTV) and delavirdine (DLV) are inhibitors of cytochrome P450 (CYP) 3A4, the specific CYP that metabolizes indinavir (IDV). We hypothesized that patients who have failed multiple therapies containing protease inhibitors would still respond to IDV if high plasma concentrations were achieved. We retrospectively examined the antiviral efficacy of the combination of RTV, DLV, and IDV in heavily antiretroviral-experienced patients.

METHOD

A chart review of patients treated with IDV/RTV/DLV and two nucleoside reverse transcriptase inhibitor (NRTI) drugs was performed. Only patients who failed at least three highly active antiretroviral therapy (HAART) regimens and remained on IDV/RTV/DLV therapy for at least 2 months were included. Plasma concentrations for IDV and RTV were obtained if patients were still on therapy.

RESULTS

Ten participants were identified who qualified for this study. The median plasma HIV RNA prior to initiating IDV/RTV/DLV was 359,300 copies/mL. Nine of the 10 patients had failed nonnucleoside reverse transcriptase inhibitor (NNRTI)-containing regimens in the past. Eight out of 10 patients had at reduction in HIV RNA. Four of eight patients maintained the 1 log(10) reduction in HIV RNA past 6 months. Mean CD4 cell count increased from 142+/-99 to 273+/-126 cells/mm(3). Genotypic data available on six patients showed multiple protease gene mutations. Plasma concentration of IDV in three patients (two troughs and one 7 hours postdose) were >1,000 ng/mL.

CONCLUSION

Our data suggests that in heavily antiretroviral drug-treated patients, partial antiretroviral response to RTV/IDV/DLV can still be achieved. The use of IDV/RTV/DLV and two NRTIs as salvage therapy has merit in patients who have no viable treatment options. A prospective trial utilizing this drug combination is warranted.

Pharmacological basis for concentration-controlled therapy with zidovudine, lamivudine, and indinavir

Conventional antiretroviral therapy involves administration of standard fixed doses to adults and adolescents. This approach ignores interindividual variability in pharmacokinetics and results in substantial differences in systemic concentrations among patients. Thus, variability in systemic concentrations contributes to variability in response to therapy. This study was designed to evaluate the feasibility and safety of a regimen of zidovudine, lamivudine, and indinavir designed to achieve select target concentrations versus standard dose therapy. Twenty-four antiretroviral-naïve subjects completed the 24-week study; 13 received standard therapy, and 11 received concentration-controlled therapy. There were no differences in baseline characteristics. Oral clearance for all three drugs was not different between weeks 2 and 28; average ratios of week 2 oral clearance to week 28 oral clearance were 0.95, 1.09, and 1.06 for zidovudine, lamivudine, and indinavir, respectively, with 95% confidence intervals including 1. The selected target concentrations were average steady-state concentrations of 0.19 mg/liter for zidovudine and 0.44 mg/liter for lamivudine and a trough concentration of 0.15 mg/liter for indinavir; mean concentrations achieved at week 28 in the concentration-controlled arm were 0.20, 0.54, and 0.19 mg/liter, respectively. Concentration-controlled therapy significantly reduced interpatient variability in zidovudine concentrations and significantly increased indinavir concentrations. There was no difference in adverse drug effects or adherence. This investigation has provided a pharmacologic basis for concentration-controlled therapy by demonstrating that it is feasible and has a safety profile no different from that of standard therapy. Additional studies to evaluate the virologic effect of the concentration-controlled approach to antiretroviral therapy are warranted.

Plasma population pharmacokinetics and penetration into cerebrospinal fluid of indinavir in combination with zidovudine and lamivudine in HIV-1-infected patients

OBJECTIVES

To evaluate plasma population pharmacokinetics and penetration into cerebrospinal fluid (CSF) by indinavir (IDV) in HIV-infected individuals receiving IDV, zidovudine and lamivudine.

METHODS

Plasma population pharmacokinetic analysis was performed on 805 IDV plasma values from 171 patients, using a non-linear mixed-effects modeling approach. CSF data from 19 patients were analyzed using an individual approach.

RESULTS

Mean individual Bayesian estimates for oral clearance (CL) and volume of distribution (V) by the final model that incorporated interoccasion variability were 0.75 l/h per kg [coefficient of variation (CV) 54.8%] and 1.74 l/kg (CV 82.7%), respectively. Mean model-predicted plasma IDV level at 8 h, maximal level, area under the plasma level-time curve up to 8 h and plasma half-life were 0.42 micromol/l (CV 57.5%), 9.51 micromol/l (CV 47.3%), 29.56 micromol/l x h (CV 46.9%) and 1.50 h (CV 20.9%), respectively. The mean IDV CSF level was 0.11 micromol/l (CV 49.7%) and the mean CSF:plasma concentration ratio was 0.017.

CONCLUSIONS

Population estimates of pharmacokinetic parameters of IDV and its CSF penetration were in excellent agreement with previously reported data from individual analyses. Intraindividual interoccasion variability of IDV pharmacokinetics was estimated to be of similar order of magnitude to its interindividual variability, which may affect response to long-term antiretroviral therapy involving IDV. CSF levels of IDV exceeded its in vitro 95% inhibitory concentration of HIV replication. Given that CSF is virtually free of protein, viral suppression in the central nervous system should be achievable with an IDV-containing regimen.

Competing drug-drug interactions among multidrug antiretroviral regimens used in the treatment of HIV- infected subjects: ACTG 884

OBJECTIVE

To evaluate the steady state concentrations of saquinavir, ritonavir, nelfinavir, delavirdine, and adefovir in six different three- and four-drug combination regimens.

DESIGN

Randomized, partially double-blinded, multicenter study in a population of indinavir-experienced subjects with virologic failure. The first seven subjects enrolled in each of the six treatment arms from 10 participating sites were entered into this pharmacokinetic evaluation.

SETTING

Multicenter study of the AIDS Clinical Trials Group (ACTG).

PATIENTS

HIV-infected subjects.

INTERVENTIONS

A 12-hour pharmacokinetic study was conducted after 2 weeks of drug administration.

MAIN OUTCOME MEASURES

Area under the concentration-time curve with statistical comparisons to evaluate the effect of the second protease inhibitor and the effect of the non-protease inhibitors.

RESULTS

There was no difference in saquinavir concentrations according to whether the second protease inhibitor was ritonavir or nelfinavir. Saquinavir concentrations in the groups receiving the combination of delavirdine plus adefovir dipivoxil were reduced by approximately 50% compared with those receiving delavirdine. Delavirdine concentrations were reduced by approximately 50%, in the delavirdine plus adefovir dipivoxil arms compared with the delavirdine arms.

CONCLUSIONS

Saquinavir concentrations were significantly lower in the arms containing the combination of delavirdine and adefovir dipivoxil compared with the arms containing delavirdine. Delavirdine concentrations were significantly lower when coadministered with adefovir dipivoxil. These drug-drug interactions were not expected, the mechanism(s) is (are) not clear, and additional studies are warranted. This study illustrates the need to understand more fully the pharmacokinetic characteristics of complex combination antiretroviral regimens prior to use in patient management.

Phase I dose escalation pharmacokinetics of O-(chloroacetylcarbamoyl) fumagillol (TNP-470) and its metabolites in AIDS patients with Kaposi's sarcoma

The pharmacokinetics of TNP-470 and its major metabolites were investigated in AIDS patients enrolled in a phase I dose escalation trial for the treatment of Kaposi's sarcoma. The patients received TNP-470 by 1-h intravenous infusion in dose cohorts of 10, 20, 30, 40, 50 and 70 mg/m2. The parent drug and metabolites, MII and MIV, were measured by high-performance liquid chromatography/mass spectrometry (HPLC/MS) in plasma samples collected during and out to 168 h after the beginning of the infusion. Both metabolites were detected in all patients' plasma, while the parent drug was undetectable at time-points as early as 5 min after the end of infusion for some patients. A large interpatient variability of pharmacokinetic parameters among the dosing cohorts was observed for TNP-470, with a mean (+/- SD) plasma elimination half-life (t1/2) of 0.06 +/- 0.04 h, plasma clearance (CL) of 1487 +/- 1216 l/h and an area under the concentration versus time curve (AUC) of 49.9 +/- 35.8 ng/ml x h. Time to maximum plasma concentration (Tmax) typically occurred before the end of the infusion. The predominant plasma metabolite was MII with a t1/2 of 1.21 +/- 0.43 h, AUC of 1226 +/- 2303 l/h and a Tmax occurring between 5 and 15 min after infusion. The reported active metabolite MIV had a t1/2 of 0.24 +/- 0.13 h, AUC of 24.9 +/- 32.6 ng/ml x h and a Tmax occurring between the midpoint of the infusion and 15 min after infusion. The parent drug was undetectable by HPLC/MS/MS in urine samples collected and pooled between 0-6 and 6-24 h from the beginning of drug administration. Metabolite MIV was present in the 0-6-h urine pool of two patients enrolled in the highest dosing cohorts, equivalent to 0.4% of the administered dose. Metabolite MII was present in all 0-6-h samples analyzed and represented 1.12 +/- 0.9% of the administered dose. Renal clearance (CLR) for MII was 140 +/- 70 ml/h.

Continued lamivudine versus delavirdine in combination with indinavir and zidovudine or stavudine in lamivudine-experienced patients: results of Adult AIDS Clinical Trials Group protocol 370

OBJECTIVE

To compare the virologic activity of continued lamivudine (3TC) versus a switch to delavirdine (DLV) when initiating protease inhibitor therapy in nucleoside-experienced patients.

DESIGN

Randomized, open-label, multi-center study.

SETTING

Adult AIDS clinical trials units.

PATIENTS

Protease and non-nucleoside reverse transcriptase inhibitor-naive patients who had received 3TC plus zidovudine (ZDV), stavudine (d4T), or didanosine (ddl) for at least 24 weeks.

INTERVENTIONS

Patients with plasma HIV-1 RNA levels > 500 copies/ml who previously received d4T + 3TC or ddI + 3TC were randomized to ZDV + 3TC + indinavir (IDV) or ZDV + DLV + IDV.

MAIN OUTCOME MEASURES

Primary endpoints were the proportion of patients with plasma HIV-1 RNA levels < or = 200 copies/ml at 24 weeks, and occurrence of serious adverse events. The proportion of patients with plasma HIV-1 RNA levels < or = 200 copies/ml at week 48 was a secondary endpoint.

RESULTS

At week 24, 58% of subjects in the ZDV + 3TC + IDV arm and 73% in the ZDV + DLV + IDV arm had plasma HIV-1 RNA levels < or = 200 copies/ml (P = 0.29). At week 48, plasma HIV-1 RNA levels were < or = 200 copies/ml in 48% and 83%, respectively (P = 0.007). Rash and hyperbilirubinemia occurred more frequently in the DLV arm than in the 3TC arm. Steady-state plasma IDV levels were higher among patients in the DLV arm as compared with the 3TC arm.

CONCLUSIONS

Substituting DLV for 3TC when adding IDV improved virologic outcome in nucleoside-experienced patients. This result might be explained, in part, by the positive effect of DLV on IDV pharmacokinetics.

Pharmacodynamics of human immunodeficiency virus type 1 protease inhibitors

Many factors are involved in the success or failure of antiretroviral therapy. Recent data suggest that there are significant differences in drug absorption and disposition for the protease inhibitor class of antiretroviral drugs, and relationships between plasma concentrations and their antiviral effect have been described. Consequently, the issue of whether therapeutic drug monitoring should be employed for patients receiving treatment with these drugs has arisen. Several criteria must be met before a drug is considered a candidate for therapeutic drug monitoring. These criteria include pharmacological, clinical, and analytic components. Although not all the necessary criteria have yet been met, some of these components have been defined, and additional data are being generated. However, prospectively designed clinical trials must be completed to determine if monitoring protease inhibitor plasma concentrations provides additional clinical benefit to the patient.

Indinavir concentrations and antiviral effect

STUDY OBJECTIVES

To determine the variability of indinavir pharmacokinetics in patients attending an outpatient clinic, and to explore relationships between indinavir exposure and antiviral effect.

DESIGN

Open, formal pharmacokinetic evaluation.

SETTING

University-affiliated clinical research center.

PATIENTS

Forty-three adults infected with the human immunodeficiency virus (HIV) receiving therapy with indinavir and concomitant nucleoside reverse transcriptase inhibitors.

INTERVENTION

Indinavir concentrations were measured after patients were observed taking an 800-mg oral dose, and pharmacokinetic parameters were determined using a one-compartment oral absorption model. Virologic and pharmacologic characteristics were compared in a subset of 23 patients who were protease inhibitor naive before receiving indinavir.

MEASUREMENTS AND MAIN RESULTS

Mean indinavir pharmacokinetics were similar to those reported previously. Significant intersubject variability in systemic exposure was observed in patients receiving the same dosage; the 8-hour area under the curve (AUC8) ranged from 5.4-68.0 microM x hour. In protease inhibitor-naive subjects, the indinavir AUC8 was statistically higher in those with undetectable plasma HIV RNA (30.7 microM x hr) versus detectable plasma HIV RNA (22.4 microM x hr, p=0.035). Measured concentrations 5 hours after the dose and extrapolated 8-hour concentrations were also significantly higher in patients with undetectable plasma HIV RNA (both p=0.007).

CONCLUSIONS

Indinavir plasma concentrations were highly variable among patients receiving the same dosage. Patients with an undetectable plasma HIV RNA level who were protease inhibitor naive had statistically higher indinavir concentrations and slower oral clearance than the group with detectable HIV RNA. Relationships between indinavir concentrations and anti-HIV effect provide a basis for quantifying the pharmacologic contribution to the heterogeneity in therapeutic response.

Concentration-controlled zidovudine therapy

BACKGROUND

Heterogeneity in the response to antiretroviral therapy has been attributed to pharmacologic, immunologic, and virologic differences between patients. Currently available antiretroviral agents used for the treatment of human immunodeficiency virus (HIV) infection in adults are administered in standard fixed doses. The active moiety of nucleoside anti-HIV drugs is the intracellular anabolite. Therefore the heterogeneity in response to nucleoside agents may arise as a result of pharmacologic variability at both the systemic and cellular level.

OBJECTIVES

To determine whether a novel concentration-controlled zidovudine regimen could improve anti-HIV response compared with the standard fixed-dose approach.

DESIGN

At the Outpatient Clinic of the General Clinical Research Center at the University of Minnesota, 20 persons with HIV infection received an oral regimen of zidovudine designed to achieve a target concentration in plasma of 0.7 mumol/L and the 500 mg/day standard dose in a randomized, crossover 24-week study.

RESULTS

The concentration-controlled regimen achieved overall higher systemic concentrations with reduced interpatient variability: steady-state average zidovudine plasma concentrations were 0.76 mumol/L (coefficient of variation, 12%) versus 0.62 mumol/L (coefficient of variation, 32%) for the standard regimen. There was no difference in safety and tolerance between regimens. Intracellular zidovudine triphosphate concentrations averaged 160 fmol/10(6) peripheral blood mononuclear cells (PBMCs) with concentration-controlled versus 92 fmol/10(6) PBMCs for standard therapy. The percentage change from baseline in CD4 cells was a 22% increase for the concentration-controlled regimen versus a 7% decrease with standard therapy.

CONCLUSIONS

These data indicate that pharmacologic variability affects antiretroviral response. Furthermore, these findings provide a framework to characterize the pharmacologic determinants of effect and quantitate their contribution to the heterogeneity in clinical response to optimize therapeutic benefit.

Pharmacokinetics and safety of concentration-controlled oral zidovudine therapy

STUDY OBJECTIVE

To evaluate the pharmacokinetics, safety, and feasibility of concentration-controlled oral zidovudine therapy.

DESIGN

Randomized, crossover, open-label study.

SETTING

University-affiliated general clinical research center.

PATIENTS

Eight individuals infected with the human immunodeficiency virus with CD4+ lymphocyte counts of 100 cells/microliter or greater.

INTERVENTION

During the 24-week study, patients received oral zidovudine regimens that consisted of a standard fixed dose of 500 mg/day and a concentration-controlled regimen designed to maintain a steady-state plasma concentration (Css) of 0.187 +/- 0.04 mg/L (0.7 +/- 0.14 microM).

MEASUREMENTS AND MAIN RESULTS

The mean Css during standard therapy was 0.170 +/- 0.024 mg/L versus 0.205 +/- 0.021 mg/L with the concentration-controlled regimen (p = 0.025). Respective mean changes in hemoglobin were -0.02 g/dl (range -0.9-0.9 g/dl) and -0.30 g/dl (range -1.5-0.4 g/dl, p = 0.67). The absolute neutrophil count decreased 0.90 x 10(9)/L during standard therapy and increased 0.40 x 10(9)/L during concentration-controlled therapy (p = 0.07). The regimens did not differ in toxicity.

CONCLUSION

Concentration-controlled oral antiretroviral therapy with zidovudine is feasible and safe, and provides pharmacologic data to determine the regimen's virologic and immunologic benefits.

Valacyclovir

OBJECTIVE

To discuss the clinical pharmacology, antiviral activity, clinical efficacy, and other therapeutic issues associated with valacyclovir use for the treatment of herpesvirus infections.

DATA SOURCE

Literature searches using MEDLINE were prospectively designed to include relevant articles and abstracts between January 1982 and March 1996. The searches focused on valacyclovir pharmacology, clinical efficacy, and issues associated with herpesvirus infections.

STUDY SELECTION

Selection of clinical and basic science studies were limited to those focusing on valacyclovir. All articles with pertinent information relevant to the scope of this article were reviewed.

DATA SYNTHESIS

Valacyclovir is an amino acid ester prodrug of acyclovir. It is currently approved for the treatment of herpes zoster infections in immunocompetent adults (1 g p.o. tid for 7 d) and recurrent episodes of genital herpes in immunocompetent adults (500 mg bid for 5 d). Valacyclovir is rapidly and almost completely hydrolyzed to acyclovir prior to systemic exposure. The bioavailability of valacyclovir is 54% compared to approximately 20% for oral acyclovir. At higher dosages (2 g qid), the plasma AUC of acyclovir following oral valacyclovir administration approximates that seen after intravenous administration of 10 mg/kg every 8 hours. Clinical data indicate that valacyclovir is at least as effective as acyclovir in decreasing the duration of pain associated with postherpetic neuralgia, and in reducing time to genital lesion healing and the length of the episode.

CONCLUSIONS

Valacyclovir has improved bioavailability over acyclovir and is at least as efficacious. The favorable safety profile of acyclovir and increased systemic exposure make it a particularly ideal candidate for further studies of herpes group viral infections in immunocompromised patients.

Effect of foscarnet on quantities of cytomegalovirus and human immunodeficiency virus in blood of persons with AIDS

Four intravenous dosages of foscarnet given for 10 days were compared with no therapy in persons with AIDS who had asymptomatic cytomegalovirus (CMV) viremia. CMV viremia was quantitated by endpoint cell dilution microcultures, pp65 antigenemia assay, and measurement of CMV DNA in peripheral blood leukocytes by a quantitative-competitive PCR. Human immunodeficiency virus type 1 (HIV-1) viremia was quantitated by endpoint cell dilution microculture, serum p24 antigen assay, and PCR for HIV-1 RNA in plasma. Twenty-seven subjects who had received a median of 22 months of nucleoside antiretroviral therapy were enrolled. Twenty-two subjects received foscarnet, which was well tolerated and decreased the CMV burden, as reflected by all three indicator assays. During the 10 days of dosing, the level of CMV viremia, as measured by 50 percent tissue culture infective doses, decreased from 117.5 to 12.7 (P = 0.001), the amount of CMV DNA decreased from 20,328 copies to 622 copies per 150,000 leukocytes (P = 0.02), and the level of CMV pp65 antigenemia decreased from 14.9 to 1.6 positive peripheral blood mononuclear cells per 50,000 leukocytes (P = 0.008). A significant pharmacodynamic relationship was found between the peak foscarnet concentration and a decrease in the level of CMV antigenemia (P < 0.05). Foscarnet had no effect on quantitative HIV-1 microcultures during the 10 days of treatment, but the HIV-1 p24 antigen level in serum decreased significantly, from 454 to 305 pg/ml (P = 0.01). Also, a significant pharmacodynamic relationship was seen between plasma HIV-1 RNA concentrations and both peak foscarnet concentration (P < 0.01) and the area under the foscarnet time-concentration curve (P < 0.05). Reductions in the levels of CMV and HIV-1 viremia correlated quantitatively with systemic exposure to foscarnet, whereas control subjects actually experienced an increase in CMV and HIV-1 burdens. The dual antiviral activity of foscarnet shown in this trial encourages investigation of its use in combination with other antiretroviral therapies for persons with AIDS.

Clinical pharmacokinetics of zidovudine. An update

The battle against the acquired immune deficiency syndrome (AIDS) is now into its second decade, and substantial advancements have been made in our understanding of the complex life cycle of, and the immunopathology associated with, human immunodeficiency virus (HIV) infection, as well as of the drugs used to modify the course of disease. Zidovudine was the first agent approved for treatment of HIV disease, and since its widespread availability in 1987 the pharmacokinetic disposition and clinical effects of zidovudine have been extensively evaluated. This article reviews the absorption, distribution, metabolism and elimination characteristics of zidovudine, focusing on more recent information. In addition, factors that may or may not affect zidovudine disposition are discussed. These include selected drug interactions and concomitant disease states such as renal and hepatic insufficiency. Issues such as bodyweight normalisation, maternal-fetal transfer, pregnancy and intracellular phosphorylation are discussed in relation to the pharmacokinetics and clinical efficacy of zidovudine. Finally, information regarding the clinical pharmacodynamics of zidovudine is presented. This includes possible relationships between zidovudine pharmacokinetics and markers of efficacy and toxicity, and the significance of linking pharmacokinetic and pharmacodynamic information.

Gender differences in human pharmacokinetics and pharmacodynamics

Gender differences in pharmacokinetics and pharmacodynamics have long been recognized in animals. In humans, however, little attention has been paid to this field despite at least theoretical reasons to believe that gender may be an important variable in the processes of absorption, distribution, metabolism, and excretion. Gastric acid secretion, gastrointestinal blood flow, proportions of muscular and adipose tissue, amount of drug binding proteins, gender-specific cytochrome P450 isozymes, physiologic and hormonal changes during the menstrual cycle, and renal blood flow are several factors that may contribute to sex-related differences in pharmacokinetics. Clinical investigations have documented greater absorption and subsequent incorporation of iron into erythrocytes, and higher bioavailability of ethanol in females. Women have been shown to have a slower metabolism of mephobarbital and propranolol but an increased biotransformation of methylprednisolone, all three of which are metabolized by enzymes of the cytochrome P450 system. Lastly, the renal excretion of amantadine was inhibited significantly by quinidine and quinine in men but not in women. While gender-specific pharmacodynamic data are meager, evidence also supports the existence of sex-related differences. Women appear to be more prone to develop torsades de points from drugs such as quinidine and procainamide than men. A dimorphism in insulin sensitivity has been demonstrated with males having an enhanced response compared to females. Pharmacokinetic and pharmacodynamic sex-related differences exist and are complex. Future research efforts should be designed to provide more gender-specific information on drug disposition and clinical effect.

Agents for treating human immunodeficiency virus infection

The replicative cycle of the human immunodeficiency virus (HIV) is reviewed, and currently used and investigational agents directed against the virus are discussed. The first step in the replication of HIV is selective binding of the envelope glycoprotein to CD4 receptors located on T lymphocytes. The virion is then uncoated within the cytoplasm, yielding viral genomic RNA. Reverse transcriptase uses the viral RNA as a template to form single-stranded DNA, which is duplicated to form proviral DNA through the activity of ribonuclease H. Host RNA polymerases transcribe the integrated proviral DNA into messenger RNA, and there is subsequent translation to viral proteins. After translation, further modification of precursor polyproteins is necessary to produce functional peptides. The assembled virus then buds from the cell surface and invades other cells. Targets of drug intervention in the replicative cycle include (1) binding and entry, (2) reverse transcriptase, (3) transcription and translation, and (4) viral maturation and budding. Inhibitors of binding and entry include recombinant soluble CD4, immunoadhesins, peptide T, and hypericin. Nucleoside reverse-transcriptase inhibitors include zidovudine, didanosine, zalcitabine, and stavudine. Foscarnet, tetrahydroimidazobenzo-diazepinthione compounds, and nevirapine are some nonnucleoside reverse-transcriptase inhibitors. Inhibitors of transcription and translation include antagonists of the tat gene and GLQ223. Castanospermine, N-butyldeoxynojirimycin, and protease inhibitors interfere with viral maturation and budding. Drug combinations that have been or are being investigated include zidovudine plus interferon alfa, zidovudine plus zalcitabine, and zidovudine plus didanosine. Four agents currently have approved labeling for use against HIV infection: zidovudine, didanosine, zalcitabine, and stavudine. Monotherapy with zidovudine remains the treatment of first choice. Although progress has been made in developing drug therapies for HIV infection, more selective and more potent drugs are urgently needed. The best approach at present is to optimize the use of available agents, continue to investigate new therapies, and educate the public about prevention.

Leukotriene-receptor antagonists and 5-lipoxygenase inhibitors in asthma

OBJECTIVE

To familiarize readers with a potentially new class of compounds for treating asthma. Background information on leukotrienes is provided in addition to an in-depth review of pertinent clinical trials.

DATA SOURCES

Information was obtained from controlled clinical trials, abstracts, and review articles identified through a MEDLINE search of English-language articles.

STUDY SELECTION

Emphasis was placed on early clinical trials that showed some benefit with these compounds as well as more recent studies using newer agents that produced more promising results.

DATA EXTRACTION

Information regarding leukotriene biochemistry was extracted from basic science research and data from human studies were evaluated by the authors according to patient selection, study design, methodology, and therapeutic response.

DATA SYNTHESIS

Leukotrienes have a pathophysiologic role in asthma. Two distinct but pharmacologically similar classes of leukotriene inhibitors are currently being clinically evaluated. These are leukotriene receptor antagonists and 5-lipoxygenase inhibitors. Early clinical trials with these agents yielded unfavorable results primarily because of lack of drug potency and selectivity, poor patient tolerance, and possibly the route of administration. Subsequent studies with more potent and selective agents have further implicated leukotrienes as biochemical mediators in asthma and, consequently, have shown promising clinical outcomes with respect to pulmonary function testing and patient tolerance.

CONCLUSIONS

Advancements in the pathogenesis of asthma are beginning to define a role for the leukotrienes. Although more studies are needed to assess the efficacy of leukotriene inhibitors, recent clinical trials using leukotriene-receptor antagonists and 5-lipoxygenase inhibitors indicate a potential for the expansion of therapeutic regimens currently used in the treatment of asthma.

Thromboxane synthesis and actions in isolated adrenergic nerve (pheochromocytoma-12) cells

The synthesis of cyclooxygenase products by cultured adrenergic neuronal (pheochromocytoma-12) cells was investigated by measuring both the extent of conversion of [3H]arachidonic acid to prostanoids and the immunoreactive prostanoid concentrations in the bathing buffer. Statistically significant amounts of arachidonic acid metabolites migrated with prostaglandins (PGs) E (81 +/- 14 fmol) and F (68 +/- 13 fmol) and thromboxane B (49 +/- 12 fmol) on thin-layer chromatography plates after incubation of differentiated cells with 1 pmol of [3H]arachidonic acid. The conversion of arachidonic acid to these products was lower in undifferentiated cells, although PGE- and PGF-like metabolites were produced in significant amounts. Both immunoreactive PGE and thromboxane B were detected in the media of differentiated cells and their concentrations were elevated when the cells were exposed to arachidonic acid. The potential significance of the thromboxane production by pheochromocytoma-12 cells was investigated by examining the effect of a stable thromboxane mimetic, U46619, on potassium-stimulated norepinephrine release. The U46619 significantly enhanced norepinephrine release from potassium-depolarized cells. These results are indicative of PGE, PGF and thromboxane production by isolated adrenergic neuronal tissue. A thromboxane receptor agonist also was observed to potentiate norepinephrine release.