Pharmacokinetics of intermittently administered intravenous foscarnet in the treatment of acquired immunodeficiency syndrome patients with serious cytomegalovirus retinitis

Use of liquid chromatography-tandem mass spectrometry for foscarnet quantification in human serum and cerebrospinal fluid

RATIONALE

Foscarnet (FCV) is used to treat cytomegalovirus, human herpesvirus, and human immunodeficiency virus infections. It is a low-molecular-weight compound containing carboxylate and phosphate groups. There are no reports on the use of liquid chromatography-tandem mass spectrometry (LC/MS/MS) to analyze FCV via bioanalysis. In the present study, we developed the ion-pair LC/MS/MS method to analyze FCV in human serum and cerebrospinal fluid (CSF).

METHODS

FCV was extracted from human serum and CSF by weak anion exchange (WAX) solid-phase extraction. The LC/MS/MS systems were coated with 0.1% phosphoric acid in methanol to avoid nonspecific absorption. FCV was detected using ion-pair LC/MS/MS with dibutylammonium acetate. Selected reaction monitoring transition of FCV in the negative ion mode was selected at m/z 125.1 → 62.9.

RESULTS

FCV was selectively detected in human serum and CSF, and the liner range was 5-1000 μM (R²  = 0.99). The intraday and interday accuracy and precision were within ±15%. FCV was constantly extracted from human serum and CSF by WAX solid-phase extraction (recovery ratio = 76.0-77.9%). No matrix effect was observed.

CONCLUSIONS

A robust LC/MS/MS method to analyze FCV was successfully developed. FCV was selectively measured using LC/MS/MS in very low extract volumes (20 μL). The method will be useful in evaluating the FCV level in human serum and CSF.

Intracellular Metabolism of 3′-Azido-3′-Deoxythymidine in the Presence of Ganciclovir or Foscarnet

Comparisons were made between the intracellular phosphorylation of 3′-Azido-3′-deoxythymidine (AZT) alone and in combination with ganciclovir (GCV) or foscarnet (PFA) in lymphocytes, uninfected fibroblasts and CMV-infected fibroblasts. The effects of AZT and the combinations of AZT with GCV or PFA on cellular dNTP pools were also examined. The phosphorylation of AZT was not altered by the presence of GCV or PFA in lymphocytes, and neither AZT nor the combinations of AZT with GCV or PFA changed the levels of cellular dNTP pools in these cells. AZT was phosphorylated to a greater extent in lymphocytes when compared to fibroblasts, but the proportion of AZT di- and triphosphates was greater in fibroblasts. The infection of fibroblasts with CMV enhanced AZT phosphorylation and increased the levels of cellular dNTP pools. GCV caused a specific reduction in AZT phosphorylation in CMV-infected fibroblasts, with a seven-fold drop in AZT triphosphate compared to AZT alone. GCV did not affect AZT phosphorylation in uninfected fibroblasts, nor did GCV reduce the dTTP pool compared to AZT alone. The effects of GCV upon AZT phosphorylation in CMV-infected cells may shed light on the antagonistic effects of GCV upon the anti-HIV activity of AZT, and are of importance for the development of effective combination therapies for the treatment of AIDS patients infected with CMV.

Prophylactic Efficacy and Bone Toxicity Associated with Phosphonoformate Therapy against Retrovirus Infection

Phosphonoformate (PFA) is a simple pyrophosphate analogue which is a topical and parenteral treatment for human herpes virus infections and is currently undergoing evaluation for treatment of human immunodeficiency virus (HIV) and cytomegalovirus infections associated with (AIDS). In this study, antiretroviral activity of PFA was demonstrated by two separate treatment regimens. In the first, an inoculum of feline leukaemia virus (FeLV) in plasma from viraemic cats was treated with 1024 μM PFA prior to intravenous inoculation into susceptible animals. Three of four cats given the PFA treated inoculum were protected from viraemia by the PFA treatment, while 2 of 2 challenge controls receiving sham treated inoculum and 6 of 6 untreated challenge controls became viraemic. In the second regimen, a long-term continuous intravenous infusion of PFA (1000 mg kg−1 day−1) was administered to 6 young cats beginning 1–2 days prior to and extending 4 weeks following intravenous inoculation with FeLV. Five of the six PFA-treated cats also received heparin intravenously and acetyl salicylic acid (aspirin) orally to reduce risk of thrombosis. Six cats (heparin controls) received only heparin and aspirin and were inoculated with FeLV in an identical manner. Six cats served as untreated challenge controls. Four of 6 PFA-treated cats were protected from FeLV antigenaemia. In contrast, all 6 heparin-control animals and all 6 challenge-control animals became persistently viraemic as evidenced by continuous expression of FeLV p27 antigen. All challenged cats including the 4 protected by PFA treatment developed antibody to FeLV, indicating that PFA did not prevent primary virus infection. Significant toxic effects of PFA treatment were reduced weight-gain and rickets-like bone lesions in the cats receiving the 4 week treatment. Additionally, decreased serum alkaline phosphatase, phosphorus, and calcitriol concentrations, presumably related to the bone lesions, were observed. Results of this study suggest that the antiviral effect of PFA involves an immediate and direct mechanism targeted at cell-free virus and that long-term continuous intravenous infusion of PFA has significant anti-retroviral activity in vivo.

Posterior segment manifestations of HIV/AIDS

The ocular posterior segment manifestations of AIDS may be divided into four categories: retinal vasculopathy, unusual malignancies, neuro-ophthalmologic abnormalities, and opportunistic infections. Microvasculopathy is the most common manifestation. Opportunistic infections, particularly cytomegalovirus retinitis and progressive outer retinal necrosis, are the most likely to result in visual loss due to infection or subsequent retinal detachment. Diagnosis and treatment are guided by the particular conditions and immune status of the patient.

Evaluation of susceptibility of human herpesvirus 8 to antiviral drugs by quantitative real-time PCR

A new in vitro system based on real-time PCR was developed for evaluation of human herpesvirus 8 susceptibility to antiviral agents. Cidofovir had the greatest inhibitory activity against HHV-8 (50% inhibitory concentration [IC(50)], 0.43 microM) followed by ganciclovir (2.61 microM), adefovir (18.00 microM), acyclovir (31.00 microM), and foscarnet (34.15 microM). The potential therapeutic efficacy for HHV-8 (i.e., peak serum drug level/IC(50)) is highest for cidofovir (167) and foscarnet (22).

A double-blind placebo-controlled crossover trial of intravenous magnesium sulfate for foscarnet-induced ionized hypocalcemia and hypomagnesemia in patients with AIDS and cytomegalovirus infection

Foscarnet (trisodium phosphonoformate hexahydrate) is an antiviral agent used to treat cytomegalovirus disease in immunocompromised patients. One common side effect is acute ionized hypocalcemia and hypomagnesemia following intravenous administration. Foscarnet-induced ionized hypomagnesemia might contribute to ionized hypocalcemia by impairing excretion of preformed parathyroid hormone (PTH) or by producing target organ resistance. Prevention of ionized hypomagnesemia following foscarnet administration could blunt the development of ionized hypocalcemia. To determine whether intravenous magnesium ameliorates the decline in ionized calcium and/or magnesium following foscarnet infusions, MgSO(4) at doses of 1, 2, and 3 g was administered in a double-blind, placebo-controlled, randomized, crossover trial to 12 patients with AIDS and cytomegalovirus disease. Overall, increasing doses of MgSO(4) reduced or eliminated foscarnet-induced acute ionized hypomagnesemia. Supplementation, however, had no discernible effect on foscarnet-induced ionized hypocalcemia despite significant increases in serum PTH levels. No dose-related, clinically significant adverse events were found, suggesting that intravenous supplementation with up to 3 g of MgSO(4) was safe in this chronically ill population. Since parenteral MgSO(4) did not alter foscarnet-induced ionized hypocalcemia or symptoms associated with foscarnet, routine intravenous supplementation for patients with normal serum magnesium levels is not recommended during treatment with foscarnet.

Intravitreal, retinal, and central nervous system foscarnet concentrations after rapid intravenous administration to rabbits

Retinal, vitreous humor, brain, and cerebrospinal fluid (CSF) foscarnet levels were measured by high-performance liquid chromatography after administration of an intravenous dose of 120 mg/kg of body weight to 32 pigmented rabbits. A pharmacokinetic analysis was done using a two-compartment model. The penetration ratios, defined as ratios of retinal, vitreous humor, brain, and CSF areas under the concentration-time curve from 0 to 2 h were 110% +/- 1%, 12.3% +/- 0.7%, 118% +/- 1%, and 20.2% +/- 2.2%, respectively. These results suggest a good penetration of foscarnet into the retinal and brain tissues, reaching higher concentrations than those estimated from vitreous humor and CSF levels.

Cytomegalovirus viraemia has poor predictive value for the development of cytomegalovirus disease in patients with advanced HIV-infection

OBJECTIVE

Cytomegalovirus (CMV) continues to be one of the most important opportunistic infections associated with human immunodeficiency virus (HIV) infection. This study investigated the value of CMV-viraemia in predicting the development of clinical CMV disease in patients with advanced HIV infection.

METHODS

This was a prospective observational study performed over a 2-year period between 1994-96 in the Department of Infection and Tropical Medicine at Leicester Royal Infirmary. Adult HIV-positive patients attending a hospital clinic were included if they were CMV-seropositive with CD4 counts < or =50 cells/mm3. Subjects were seen at approximately 6-weekly intervals in the clinic and were reviewed by an experienced ophthalmologist. Serum for CMV PCR was taken and stored at regular intervals and qualitative and quantitative PCR was performed at the end of the study period. The value of PCR in predicting the development of CMV disease was then assessed.

RESULTS

Twenty-six patients were followed up during the study period and 77 evaluable specimens were analysed for CMV PCR. Twenty-three (30%) samples were positive and 54 negative. Seven (27%) patients developed CMV disease (five retinitis alone, and two with retinitis and oesophagitis) during the study period. Viraemia was often intermittent and there was no significant difference in the proportions of patients with positive or negative tests who subsequently developed CMV disease. The sensitivity, specificity, positive and negative predictive values of the qualitative PCR were 71%, 47%, 33% and 82% respectively and 57%, 74%, 44% and 82% respectively for the quantitative PCR (>10(3) copies/ml).

CONCLUSIONS

The results from this study, which was performed before the introduction of protease inhibitors, found that cytomegalovirus PCR was of limited clinical value in predicting the patients at greatest risk of developing CMV-disease and provided little useful prognostic information.

Neoglycolipid conjugates of foscarnet with enhanced antiviral activity in cells infected with human cytomegalovirus and herpes simplex virus type 1

The synthesis of a series of neoglycolipid conjugates of foscarnet as potential drug targeting forms or lipophilic prodrugs of foscarnet is described. The compounds were obtained from suitably protected neoglycolipids, in which the lipid chain consisted of 12 to 20 carbon atoms, by ethoxycarbonylphosphonylation at the 6-hydroxyl or 4-hydroxyl group followed by deprotection. The in vitro antiviral activity of the compounds was determined in human foetal lung cells infected with human cytomegalovirus (HCMV) or herpes simplex virus type 1 (HSV-1). Compounds in which the lipid chain consisted of 14 to 20 carbon atoms showed pronounced antiviral activity against HCMV and HSV-1, the highest activity being shown by trans-9-octadecen-1-yl 6-O-carboxyphosphonyl-alpha-D-glucopyranoside against HCMV (approximately 50 times that of foscarnet) and by eicosyl 6-O-carboxyphosphonyl-beta-D-galactopyranoside against HSV-1 (approximately 15 times that of foscarnet). Cytotoxicity was determined by assessing the capability of mitochondrial enzymes to metabolise MTT and gave TC50 values for the compounds that were 30 to 350 times higher than their IC50 values against HCMV and 5 to 15 times higher than their IC50 values against HSV-1. Foscarnet was not liberated on incubation of n-tetradecyl 6-O-carboxyphosphonyl-alpha-D-glucopyranoside with rat liver or intestine homogenate, neither could the neoglycolipid conjugate nor foscarnet be detected in rat plasma following oral administration. Further metabolic and pharmacokinetic studies are required in order to determine whether neoglycolipid conjugates of foscarnet can find a use as drug targeting forms of foscarnet.

Effect of renal disease and hemodialysis on foscarnet pharmacokinetics and dosing recommendations

BACKGROUND

Foscarnet is an antiviral agent commonly used for managing patients with cytomegalovirus infection. Despite its clinical usefulness, foscarnet is associated with substantial adverse effects including nephrotoxicity. Moreover, foscarnet is primarily eliminated unchanged through the kidneys, thus requiring aggressive dose adjustment during kidney failure. To develop specific dosage guidelines, information on the disposition of this compound in patients with varying degrees of renal function and those requiring dialysis is essential.

DESIGN

Twenty-six subjects were enrolled in this study and divided into five groups depending on their degree of renal dysfunction. Group 1 included subjects with normal renal function; group 5 included subjects requiring maintenance hemodialysis. Nondialysis study subjects received a single 60-mg/kg intravenous dose of foscarnet whereas hemodialysis subjects received two intravenous doses, separated by 1 week, to compare the effects of conventional and high-flux dialysis methods.

RESULTS

Mean plasma clearance in control subjects averaged 2.1+/-0.7 ml/minute/kg and declined proportionally with changing renal function as indicated by the regression equation: Clp (ml/minute/kg) = 1.48 [CrCl (ml/minute/kg)]-0.08 (r2 = 0.82). Mean half-life averaged 1.9+/-0.1 hours in normal subjects and increased to a mean of 25+/-19 hours in study subjects with severe impairment not on dialysis. Foscarnet dialysis clearance (based on dialysate recovery) averaged 183 ml/minute with conventional dialysis methods and 253 ml/minute during high-flux procedures, which resulted in removal of 37% and 38% of a dose for the two methods, respectively.

CONCLUSIONS

These data indicate that substantial dosage adjustments must be made in renal failure patients. Therefore, a patient-specific dosage nomogram has been developed.

The effect of increasing gastric pH upon the bioavailability of orally-administered foscarnet

For systemic use, the anti-cytomegalovirus (CMV) agent foscarnet must be given intravenously because oral administration results in unmeasurable or barely measurable plasma levels. At low pH, foscarnet decomposes via an acid-catalyzed decarboxylation; therefore, poor oral bioavailability might be due to decomposition of foscarnet in gastric acid. We evaluated whether increasing gastric pH with ranitidine would enhance the absorption of oral foscarnet in six asymptomatic HIV-infected individuals. Each volunteer received two oral 4000-mg (60 mg/kg) doses of foscarnet, preceded intravenously by a 20-min infusion of either ranitidine 50 mg in D5W or D5W alone in a randomized, double-blind, cross-over study. Intragastric pH monitoring revealed that subjects had evidence of gastric acid production (pH < 2.0) prior to administration of ranitidine and increased gastric pH (pH > 6.0) following ranitidine administration. Most foscarnet plasma levels were below the assay limit of detection (33 microM) with only 4/30 levels detectable after D5W and 8/30 after ranitidine. Urinary recovery of foscarnet increased after ranitidine pretreatment. A mean recovery of 9.9% of the drug was realized in the urine in 24 h following ranitidine pretreatment compared to 6.2% of the dose after D5W pretreatment (P < 0.03). We estimate that 9.9% recovery in the urine in 24 h is equivalent to absorption of 17.1% of the oral dose. In spite of the enhanced bioavailability associated with ranitidine pretreatment, the degree of absorption is still insufficient to achieve effective plasma concentrations for the treatment of CMV or acyclovir-resistant herpes viruses. We conclude that gastric acidity is a determinant of foscarnet absorption, albeit not a major one. Oral foscarnet is unlikely to be clinically useful even if administered in the setting of increased gastric pH.

Is there a pharmacokinetic interaction between foscarnet and zalcitabine during concomitant administration?

Foscarnet, an antiviral agent used in the treatment of cytomegalovirus infection, and zalcitabine, an antiretroviral nucleoside analogue used in the treatment of human immunodeficiency virus infection, are commonly used concomitantly. Foscarnet and zalcitabine may interact pharmacokinetically, as both compounds are partially eliminated by renal tubular secretion. Owing to dose-related toxicities associated with these two drugs, it is essential that we have data regarding their pharmacokinetic disposition during concomitant therapy. Twelve patients randomly received either foscarnet (four doses) or zalcitabine (five doses) (Phase 1), followed by concomitant foscarnet (four doses) and zalcitabine (six doses) (Phase 2), followed by dosing with the drug not received in Phase 1 (Phase 3). Following the last dose in each phase of the study, serial plasma samples were collected over 8 hours for zalcitabine and over 12 hours for foscarnet to determine the pharmacokinetics of each drug using noncompartmental analysis. Foscarnet plasma and urine levels were determined using high-performance liquid chromatography, and zalcitabine levels were determined using radioimmunoassay. No clinically significant alterations in the pharmacokinetics of foscarnet or zalcitabine occurred in this study. Thus despite the potential for foscarnet and zalcitabine to compete for renal tubular secretion, no apparent pharmacokinetic interaction exists between these two drugs at the clinically relevant doses studied.

Pharmacokinetics and absolute bioavailability of oral foscarnet in human immunodeficiency virus-seropositive patients

The pharmacokinetics, absolute bioavailability, accumulation, and tolerability over 8 days of an oral formulation of foscarnet (90 mg/kg of body weight once daily [QD] [n = 6], 90 mg/kg twice daily [BID] [n = 6], and 180 mg/kg QD [n = 31) were investigated in 15 asymptomatic, human immunodeficiency virus-seropositive male patients free of active cytomegalovirus infection and with normal upper gastrointestinal function. Peak plasma drug concentrations were (mean +/- standard deviation) 46.4 +/- 10.8 microM (90 mg/kg QD), 45.7 +/- 6.9 microM (90 mg/ kg BID), and 64.9 +/- 31.7 microM (180 mg/kg QD) on day 1 and rose to 86.2 +/- 35.8, 78.7 +/- 35.2, and 86.4 +/- 25.0 microM, respectively, on day 8. The mean peak concentration in plasma following the intravenous administration of foscarnet (90 mg/kg) was 887.3 +/- 102.7 microM (n = 13). The terminal half-life in plasma remained unchanged, averaging 5.5 +/- 2.2 h on day 1 (n = 15) and 6.6 +/- 1.9 h on day 8 (n = 13), whereas it was 5.7 +/- 0.7 h following intravenous dosing. Oral bioavailabilities were 9.1% +/- 2.2% (90 mg/kg QD), 9.5% +/- 1.7% (90 mg/kg BID), and 7.6% +/- 3.7% (180 mg/kg QD); the accumulation ratios on the 8th day of dosing were 2.1 +/- 1.1, 1.8 +/- 0.4, and 1.7 +/- 0.7, respectively. The overall 24-h urinary excretion of oral foscarnet averaged 7.8% +/- 2.6% (day 1) and 13.4% +/- 6.0% (day 8), whereas it was 95.0% +/- 4.9% after intravenous dosing. The glomerular filtration rate and creatinine clearance remained constant, and the mean 24-h renal clearances of foscarnet for the entire study group were 96 +/- 18 ml/min (day 1), 88 +/- 13 ml/min (day 8), and 103 +/- 16 ml/min after intravenous dosing. Adverse effects were largely confined to gastrointestinal disturbances, with all subjects experiencing diarrhea that was dose dependent in its severity. The results suggest that the formulation studied would require significant improvement with respect to tolerability and bioavailability to gain clinical acceptance.

Treatment of gastrointestinal cytomegalovirus infection with twice-daily foscarnet: a pilot study of safety, efficacy, and pharmacokinetics in patients with AIDS

Ten patients with AIDS and cytomegalovirus (CMV) gastrointestinal infection were included in an open-label study to evaluate the safety, efficacy, and pharmacokinetics of 90 mg of intravenous foscarnet/kg of body weight twice daily accompanied by (pre)hydration of 500 to 750 ml. Efficacy was documented endoscopically, while safety was evaluated clinically by patient reports and physical and laboratory observation. The pharmacokinetics of foscarnet was evaluated after the first dose and following approximately 20 days of therapy. Nine patients (90%) responded histopathologically, nine (90%) responded endoscopically, and nine (90%) responded symptomatically to foscarnet therapy. Adverse events resulted in discontinuance of medication in the case of one patient. The mean maximal concentration was 621 microM following the first dose and 687 microM at steady state (P = 0.11). The apparent elimination rate constant and elimination half-life were not different between dose 1 and steady state. There were no significant changes in foscarnet excretion or renal clearance between dose 1 and steady state. The steady-state volume of distribution was 23.4 liters following the first dose and 19.0 liters at steady state (P < 0.002). Twice-daily foscarnet appeared to be safe and efficacious in the treatment of CMV gastrointestinal disease in this study, resulting in endoscopic or histologic improvement in 9 of the 10 (90%) patients. Minor changes in clearance and volume of distribution noted at steady state compared to single-dose administration are readily explained by study design, known information about foscarnet pharmacokinetics, and changes in body weight and creatinine clearance in the patients.

Renal excretion and pharmacokinetics of foscarnet in HIV sero-positive patients: effect of probenecid pretreatment

AIMS

The present study was undertaken to test whether the anti-viral agent foscarnet undergoes significant tubular secretion, by using probenecid, an inhibitor of the organic acid secretory pathway in the proximal segment of the nephron.

METHODS

The pharmacokinetics and renal excretion of foscarnet (90 mg kg-1 infused over 2 h) have been investigated, in the absence and presence of probenecid pretreatment (1 g twice daily for 3 days) in a group of 10 HIV seropositive patients.

RESULTS

Mean (+/-s.d.) peak plasma concentrations were 904 +/- 65 microM (foscarnet) and 862 +/- 97 microM (foscarnet+probenecid) whilst the plasma AUC values were 3326 +/- 451 microM h and 3133 +/- 476 microM h respectively. Terminal elimination half-life remained unchanged at 5.6 +/- 0.7 h and the respective volumes of distribution at steady state were 23 +/- 31 (foscarnet) and 25 +/- 31 (foscarnet+probenecid). Mean total body clearance was 110 +/- 17 ml min-1 (foscarnet) and 113 +/- 13 ml min-1 (foscarnet+probenecid) and the corresponding renal clearances of foscarnet were 102 +/- 5 ml min-1 and 105 +/- 5 ml min-1 respectively. There were no significant differences in the total amount of foscarnet excreted by the kidney with 95 +/- 5% (foscarnet) and 91 +/- 6% (foscarnet+probenecid) of the intravenous dose excreted within 24 h. Glomerular filtration rates at 109 +/- 12 ml min-1 (foscarnet) and 100 +/- 13 ml min-1 (foscarnet+probenecid) and respective creatinine clearances at 120 +/- 15 and 119 +/- 10 ml min-1 remained unchanged throughout the study.

CONCLUSIONS

The study shows that foscarnet is not transported via the probenecid-sensitive organic acid secretory pathway in the proximal tubule; the renal elimination of foscarnet is via glomerular filtration.

Dose-related difference in progression rates of cytomegalovirus retinopathy during foscarnet maintenance therapy. AIDS Clinical Trials Group Protocol 915 Team

PURPOSE

A previous dose-ranging study of foscarnet maintenance therapy for cytomegalovirus retinopathy showed a positive relationship between dose and survival but could not confirm a relationship between dose and time to first progression. This retrospective analysis of data from that study was undertaken to determine whether there was a relationship between dose and progression rates, which reflects the amount of retina destroyed when progression occurs.

METHODS

Patients were randomly given one of two foscarnet maintenance therapy doses (90 mg/kg of body weight/day [FOS-90 group] or 120 mg/kg of body weight/day [FOS-120 group] after induction therapy. Using baseline and follow-up photographs and pre-established definitions and methodology in a masked analysis, posterior progression rates and foveal proximity rates for individual lesions, selected by prospectively defined criteria, were calculated in each patient. Rates were compared between groups.

RESULTS

The following median rates were greater for the FOS-90 group (N = 8) than for the FOS-120 group (N = 10): greatest maximum rate at which lesions enlarged in a posterior direction (43.5 vs 12.5 microns/day; P = .002); posterior progression rate for lesions closest to the fovea (42.8 vs 5.5 microns/day; P = .010); and maximum foveal proximity rate for either eye (32.3 vs 3.4 microns/day; P = .031).

CONCLUSION

Patients receiving higher doses of foscarnet have slower rates of progression and therefore less retinal tissue damage during maintenance therapy. A foscarnet maintenance therapy dose of 120 mg/kg of body weight/day instead of 90 mg/kg of body weight/day may help to preserve vision in patients with cytomegalovirus retinopathy.

Foscarnet and ganciclovir pharmacokinetics during concomitant or alternating maintenance therapy for AIDS-related cytomegalovirus retinitis

INTRODUCTION

The use of foscarnet and ganciclovir as a combination treatment for cytomegalovirus retinitis is increasing because of limitations associated with single agent therapy.

METHODS

The pharmacokinetics of foscarnet and ganciclovir were determined in 13 patients receiving either concomitant therapy (regimen A) or daily alternating therapy (regimen B) for maintenance of cytomegalovirus disease. For regimen A, 60 mg/kg intravenous foscarnet and 3.75 mg/kg ganciclovir were sequentially administered daily; for regimen B, 120 mg/kg foscarnet and 6 mg/kg ganciclovir were administered on alternating days. For both regimens, serial blood sampling for pharmacokinetic analysis was performed for each drug alone (day 1 or 2) and after 2 weeks of combination therapy. Plasma samples for foscarnet and ganciclovir analysis were performed by means of high-performance liquid chromatography. Pharmacokinetic analysis was performed with noncompartmental methods.

RESULTS

For regimen A, the plasma clearance (CL) of foscarnet did not change in the presence of ganciclovir, averaging 0.12 +/- 0.08 and 0.11 +/- 0.02 L/hr/kg on study days 2 and 14, respectively (p = 0.34). The volume of distribution (VSS) and mean residence time (MRT) also did not change significantly. CL and MRT of foscarnet did not change for regimen B, although a slight increase in VSS was observed before (0.38 +/- 0.05 L/kg) and after (0.46 +/- 0.07 L/kg) alternating therapy (p = 0.03). Ganciclovir CL did not change for either regimen, with mean values of 0.21 +/- 0.10 and 0.25 +/- 0.10 L/hr/kg (regimen A, p = 0.17) and 0.32 +/- 0.10 and 0.34 +/- 0.11 L/hr/kg (regimen B, p = 0.24). MRT and VSS were also not significantly different.

CONCLUSION

These plasma data suggest that further dosage adjustments are unnecessary for or alternating maintenance therapy.

Foscarnet. A reappraisal of its antiviral activity, pharmacokinetic properties and therapeutic use in immunocompromised patients with viral infections

The DNA polymerase of human herpes viruses, including cytomegalovirus (CMV), and the reverse transcriptase of human immunodeficiency virus (HIV) are selectively inhibited in vitro by the pyrophosphate analogue foscarnet. Inhibition is reversible on withdrawal of foscarnet and additive or synergistic effects have been demonstrated in vitro with other antiviral drugs, including ganciclovir and zidovudine. Foscarnet appears to have negligible effects on host enzymes and cells. Complete or partial clinical resolution of ocular symptoms is obtained in more than 89% of patients with acquired immunodeficiency syndrome (AIDS) and CMV retinitis during foscarnet induction therapy, but relapse occurs soon after ceasing treatment. Maintenance treatment given daily can extend the period of remission considerably. Foscarnet and ganciclovir monotherapy had similar efficacy in the treatment of CMV retinitis in patients with AIDS in several studies, and have been used concomitantly in immunocompromised patients with recalcitrant CMV infections. In 1 trial, patients receiving foscarnet survived for significantly longer than those receiving ganciclovir. Foscarnet has been used successfully in the treatment of limited numbers of immunocompromised patients with CMV-associated gastrointestinal (improvement in over 67% of patients) and other infections. Aciclovir-resistant herpes simplex infections in immunocompromised patients have also been treated successfully with foscarnet. Almost 90% of a foscarnet dose is excreted in the urine. Reversible nephrotoxicity is common during foscarnet therapy, but may be reduced by dosage adjustment and adequate hydration. Anaemia, nausea and vomiting, disturbances in electrolyte levels and genital ulceration have also been associated with administration of the drug. The different tolerability profiles of foscarnet and zidovudine facilitate the use of these agents in combination in patients with AIDS and CMV infection; whereas ganciclovir, like zidovudine, is associated with dose-limiting haematological toxicity. The apparent survival benefits seen in these patients when receiving foscarnet and zidovudine (possibly linked to synergy between zidovudine and foscarnet and/or the inherent anti-HIV activity of foscarnet), appear to offer potentially important advantages for foscarnet over ganciclovir in the treatment of selected patients with AIDS and CMV infections.

Antineurofilament and antiretinal antibodies in AIDS patients with cytomegalovirus retinitis

Sera obtained from AIDS patients with cytomegalovirus (CMV) retinitis before and after treatment with foscarnet, AIDS patients with human immunodeficiency virus (HIV) retinopathy, AIDS patients without retinal disease, and normal healthy controls with and without positive CMV serologies were assayed for the presence of antibodies against the 200-kDa outer, 160-kDa middle, and 68-kDa core subunits of the neurofilament triplet. Additional studies were performed to determine the presence of antibodies reactive with proteins extracted from crude human retinal antigen preparations. Antibodies against the 200-, 260-, and 68-kDa proteins of the neurofilament triplet were detected in 15 of 15 AIDS patients with CMV retinitis. The expression of these antibodies was unaffected, qualitatively, by successful treatment with foscarnet. In contrast, only 30% of patients with HIV retinopathy unrelated to CMV, fewer than 35% of AIDS patients with positive CMV titers but without evident retinitis, and fewer than 25% of healthy controls with positive or negative CMV titers possessed antibodies against any of the triplet proteins (P < 0.001). Antibodies against several clusters of retinal antigens were also identified in the sera of patients with CMV retinitis. In summary, the data indicate that retinal elements damaged by CMV infection induce an antibody response against the 200-, 160-, and 68kDa components of the neurofilament triplet as well as other, as yet undefined retinal antigens.

Foscarnet for suppression of human immunodeficiency virus replication

The effect of foscarnet against human immunodeficiency virus (HIV) was evaluated in nine HIV-infected individuals; six completed 28 days of induction therapy. The overall mean increase in CD4+ lymphocytes was 64 cells per mm3. The mean decline in the HIV antigen concentration was 108 pg/ml (P = 0.03), and suppression was related to systemic foscarnet exposure by a maximum-effect pharmacodynamic model.

Esophageal infections: risk factors, presentation, diagnosis, and treatment

Infections of the esophagus are unusual in the general population and strongly imply immunodeficiency, although immunocompetent individuals are not exempt. HIV infection is predominant among risk factors for infectious esophagitis. For all immunocompromised patients, the most frequently identified esophageal pathogens are Candida, CMV, and HSV. Peculiar to HIV-infected patients are idiopathic esophageal ulcers as well as unusual bacteria and parasites. Patterns of presentation differ with each infecting organism, and clinical features should be used as a guide in achieving a correct diagnosis. For example, a patient with AIDS presenting with esophageal symptoms and thrush, along with abdominal pain, nausea, vomiting, and fever, is unlikely to resolve all symptoms with empiric antifungal therapy alone. Parsimony of diagnosis does not hold among immunodeficient patients in whom concurrent infections are common. Accurate and timely diagnoses are essential as effective treatments are available for particular etiologies. Finally, among immunocompromised patients, all esophageal symptoms are not necessarily due to an infection, and possible diagnoses of pill esophagitis, acid-peptic injury, or structural and functional abnormalities should not be overlooked.

Antiviral susceptibility testing of cytomegalovirus from primary culture using shell vial assay to detect the late viral antigen

Susceptibility testing of 68 cytomegalovirus (CMV) peripheral blood isolates to Ganciclovir (DHPG) and 11 blood isolates to Foscarnet (PFA), was performed on primary culture isolates using the shell vial assay methodology (SVA-IFA, that is, quantitation of fluorescent focus units, FFUs), with an anti-CMV monoclonal antibody to the late viral antigen. A positive reaction in monolayer cultures of MRC-5 cells was characterized by cytoplasmic fluorescence with inclusions at both or more commonly off one end of the elongated fibroblast nucleus. Isolates from conventional MRC-5 tube cultures displaying a 1+ (10% cytopathic effect) were inoculated into shell vials containing DHPG concentrations of 0, 1.5, 3, 6, 12, or 24 microliters/ml shell vials containing 400, 500, 800, or 1200 microM PFA. The optimal readability of monolayers (expressed as FFUs per monolayer) occurred at 96 h after treatment with DHPG and at 36-48 h with PFA. Resistance to DHPG was determined at the concentration of antiviral agent necessary to reduce the number of FFUs to 90% or 50% of the control [that is, the 90% minimum inhibitory concentration (MIC90) or MIC50]. Six of 68 isolates showed an MIC90 > 12 or an MIC50 > 1.5 microgram/ml, and were considered DHPG resistant. Three of the six isolates were from AIDS patients with late-stage disease who had never received DHPG therapy. All but one (specimen 2400) DHPG-resistant isolates revealed MIC90 values to a PFA concentration of 500 microM, which is considered an achievable peak plasma level in patients undergoing PFA therapy. The single DHPG- and FPA-resistant isolate was obtained from a patient displaying marked clinical resistance to both drugs.(ABSTRACT TRUNCATED AT 250 WORDS)

Pathophysiology and treatment of clinically resistant cytomegalovirus retinitis

PURPOSE

To determine the incidence, pathophysiology, clinical outcome, and survival in patients with clinically resistant retinitis.

METHODS

Cytomegalovirus (CMV) retinitis was prospectively studied in 100 patients with acquired immune deficiency syndrome (AIDS). In 11 of these patients, clinically resistant retinitis developed, defined as new activity or progression, despite at least 8 consecutive weeks of induction doses of either foscarnet or ganciclovir. Fundus photography, pharmacokinetics, CMV cultures and sensitivities, and survival analyses were studied. The therapeutic interventions attempted after clinically resistant retinitis was identified included continuing a high dose (induction level) of the same antiviral drug, changing the antiviral drug, and combining antiviral therapy with foscarnet and ganciclovir.

RESULTS

Clinically resistant retinitis occurred in 11 (11%) of 100 patients with CMV retinitis and appeared to be a manifestation of acquired CMV antiviral drug resistance. Drug metabolism and pharmacokinetics in these patients were normal. The use of combination therapy with foscarnet and ganciclovir was effective in halting the progression of retinitis in three (75%) of four patients (6 of 7 eyes able to be evaluated) receiving combination therapy.

CONCLUSION

Clinically resistant retinitis is a manifestation of infection by CMV that has acquired drug resistance. In these patients, combination antiviral drug treatment should be considered. It is likely that clinically resistant retinitis will become more frequent as patients with CMV retinitis and AIDS survive longer.

Antiviral therapy in human immunodeficiency virus infections. Current status (Part II)

Part I of this article reviewed the targets against which anti-HIV drugs can be directed, problems in assessing active compounds (e.g. resistance development and use of surrogate end-points). and nucleoside analogues effective against HIV reverse transcriptase. Intensive research is currently being undertaken in laboratories and hospitals to design and evaluate new inhibitors of HIV. In this work, combining different drugs is one important approach, both to decrease toxicity and to offset the rate of resistance development, which seems to be a major problem associated with therapy directed against the ever-changing HIV. Therapeutic vaccines and immunomodulators are other modalities being actively evaluated against HIV and AIDS, although this effort has not yet yielded any licensed treatment. It appears likely that new antiviral drugs and immunotherapies will be forthcoming during the next 5 years, that they will be used in a variety of combinations, and that the treatment options available for opportunistic infections will increase. These developments should improve the survival and the quality of life of patients with HIV infection.

Cytomegalovirus infection in acquired immune-deficiency syndrome

Cytomegalovirus (CMV) infection is very common in acquired immune deficiency syndrome (AIDS) with approximately 40% of patients presenting CMV visceral involvement at advanced stage disease. The most common localizations are retinitis and gastrointestinal involvement; central nervous system disorders are likely to be underestimated. Diagnosis of CMV disease should be assessed, for all localizations except retinitis, on the association of clinical symptoms with presence of CMV inclusions in biopsy specimens. Two drugs, ganciclovir and foscarnet, are currently licensed for treatment. Both are administered intravenously. Since progression of CMV disease is very common particularly in retinitis, maintenance therapy should be given throughout remaining life. Unfortunately, maintenance therapy does not entirely prevent the risk of relapses. Drug toxicity is mainly bone marrow suppression with ganciclovir and renal insufficiency with foscarnet. Since quality of life is severely impaired by long-term daily intravenous treatment, orally active drugs are urgently needed both for treatment and primary prophylaxis of CMV infection.

Pharmacokinetics of foscarnet after twice-daily administrations for treatment of cytomegalovirus disease in AIDS patients

The pharmacokinetics of foscarnet were evaluated in 11 AIDS patients with cytomegalovirus disease after twice-daily infusion of 90 mg/kg of body weight for 2 weeks. All patients were hydrated during foscarnet infusion. Blood and urine samples were collected on days 1, 7, and 14 of therapy. Foscarnet concentrations were measured by high-pressure liquid chromatography. Despite large interindividual variations, no significant differences were seen between day 1, day 7, and day 14 concentrations in plasma. Mean peak and trough concentrations on day 14 of therapy were 605 +/- 118 and 52 +/- 59 microM, respectively. In all patients, peak concentrations were well above those necessary to inhibit cytomegalovirus. Pharmacokinetic parameters remained stable throughout the study. On day 14, the mean half-life was 3.4 h, total and renal clearances were 118 and 92 ml/min, respectively, and the volume of distribution was 0.6 liter/kg. These data and previous clinical trials demonstrate that this more convenient dosage regimen can be safely used for patients with cytomegalovirus disease. The side effects were comparable to those reported with other dosage regimens, although no renal impairment was seen in this study, probably because of the hydration.

Pharmacokinetics of concomitantly administered foscarnet and zidovudine for treatment of human immunodeficiency virus infection (AIDS Clinical Trials Group protocol 053)

Foscarnet and zidovudine (ZDV) pharmacokinetic parameters were not altered in five patients receiving 14 days of concomitant therapy. Foscarnet clearance in plasma averaged (+/- standard deviation) 0.16 +/- 0.03 and 0.13 +/- 0.05 liter/h/kg of body weight in the absence and presence of ZDV. ZDV parameters were also not significantly altered, with a mean (+/- standard deviation) oral clearance of 2.7 +/- 1.0 and 2.6 +/- 0.8 liter/h/kg for the 2 study days, respectively.

Systemic antiviral drugs used in ophthalmology

Over the past two decades, the recognition of viral enzymes and proteins that can serve as molecular targets of drugs has revolutionized the treatment of viral infections. Beginning with acyclovir, a number of systemically administered agents which are both relatively safe and effective for the treatment of herpetic infections and human immunodeficiency virus (HIV) infections have become widely available. Because of increased numbers of herpes virus infections, as well as the rising epidemic of HIV infections, the ophthalmologist is, more likely than ever before to be involved in the treatment of severe and frequent ocular infections caused by herpes viruses. In addition, the acute retinal necrosis (ARN) syndrome has been demonstrated to be caused by herpes viruses and a once rare retinal infection caused by cytomegalovirus is common in patients with the acquired immunodeficiency syndrome (AIDS). In this article, four systemic antiviral drugs (Vidarabine, Acyclovir, Ganciclovir, and Foscarnet) that have demonstrated usefulness in the treatment of ophthalmic disease are reviewed in detail with regard to their mechanisms, applications, effectiveness, and side effects.

Antiviral therapy: current concepts and practices

Drugs capable of inhibiting viruses in vitro were described in the 1950s, but real progress was not made until the 1970s, when agents capable of inhibiting virus-specific enzymes were first identified. The last decade has seen rapid progress in both our understanding of antiviral therapy and the number of antiviral agents on the market. Amantadine and ribavirin are available for treatment of viral respiratory infections. Vidarabine, acyclovir, ganciclovir, and foscarnet are used for systemic treatment of herpesvirus infections, while ophthalmic preparations of idoxuridine, trifluorothymidine, and vidarabine are available for herpes keratitis. For treatment of human immunodeficiency virus infections, zidovudine and didanosine are used. Immunomodulators, such as interferons and colony-stimulating factors, and immunoglobulins are being used increasingly for viral illnesses. While resistance to antiviral drugs has been seen, especially among AIDS patients, it has not become widespread and is being intensely studied. Increasingly, combinations of agents are being used: to achieve synergistic inhibition of viruses, to delay or prevent resistance, and to decrease dosages of toxic drugs. New approaches, such as liposomes carrying antiviral drugs and computer-aided drug design, are exciting and promising prospects for the future.

Clinical pharmacology: foscarnet

Foscarnet exerts its antiviral effects via reversible inhibition of viral polymerases. Pharmacodynamic data indicate that herpesvirus and human immunodeficiency virus replication is inhibited by therapeutically achievable concentrations of foscarnet; however, the concentrations of foscarnet required for such inhibition have been found to vary widely. Pharmacokinetic data indicate that foscarnet is eliminated via the renal route, undergoes negligible metabolism, and appears to be distributed widely from the circulation. However, the available data indicate that the pharmacokinetics of the drug varies among patients and within the individual patient, particularly with regard to plasma drug levels; furthermore, such factors as the intracellular kinetics of the drug have yet to be well characterized. It is thus difficult to formulate optimal dosing regimens on the basis of what is known of foscarnet pharmacodynamics and pharmacokinetics. Nevertheless, dosages that produce clear-cut therapeutic benefits without unacceptable toxicity have been identified in clinical trials of foscarnet in acquired immunodeficiency syndrome (AIDS) patients with cytomegalovirus (CMV) retinitis.

Foscarnet. A review of its antiviral activity, pharmacokinetic properties and therapeutic use in immunocompromised patients with cytomegalovirus retinitis

The pyrophosphate analogue, foscarnet, selectively inhibits the DNA polymerase of human herpes viruses, including cytomegalovirus, and the reverse transcriptase of HIV. Viral replication is therefore prevented, but resumes when the drug is cleared from infected cells. In vitro, the combination of foscarnet and zidovudine (azidothymidine) has an additive effect against cytomegalovirus and acts synergistically against HIV. An improvement in cytomegalovirus retinitis is obtained in over 85% of affected AIDS patients during foscarnet induction therapy, but relapse usually occurs within a month of ceasing treatment. There is a similar duration of remission during maintenance therapy given for 5 days each week, but this can be extended 4- to 5-fold with daily administration of higher doses. In allograft recipients, progression of retinitis can be halted by foscarnet until immune function recovers and eradicates the virus. The incidence of acute renal failure, which is common during foscarnet therapy, may be reduced by dosage adjustment and adequate prehydration. Anaemia, phlebitis, nausea and vomiting, and disturbances in serum calcium and phosphate levels, perhaps resulting from uptake of foscarnet into bone or chelation with ionised calcium, have also been associated with administration of the drug. Cytomegalovirus retinitis is difficult to treat, with few therapeutic options available. Although treatment with foscarnet produces some severe adverse effects, with care these can be minimised, and the drug produces clinical improvement in a large proportion of patients; this is a highly encouraging finding at this stage in its development. Preliminary comparative data indicate that foscarnet and ganciclovir are similarly effective, but foscarnet may have some theoretical advantages in AIDS patients since it can be used in combination with zidovudine without potentiating myelosuppression.

Foscarnet sodium

Cytomegalovirus (CMV), a major opportunistic viral pathogen frequently causing disease in immunocompromised patients such as organ transplant recipients and people with AIDS, may present as pneumonitis, gastrointestinal disease, or encephalitis. Its most common manifestation in patients with AIDS is retinitis which, if left untreated, invariably progresses to extensive retinal necrosis and ultimately to blindness. Ganciclovir sodium, currently the only licensed antiviral agent for the treatment of CMV retinitis, effectively controls this infection in a majority of AIDS patients, but significant granulocytopenia or thrombocytopenia related to ganciclovir therapy often limit its clinical application. Myelosuppression may be further exacerbated in AIDS patients by such other agents as zidovudine or trimethoprim/sulfamethoxazole, often necessitating dosage reductions or discontinuation of these agents in patients receiving ganciclovir. Foscarnet sodium, a pyrophosphate analog active against both cytomegalovirus and the human immunodeficiency virus type 1 (HIV), may be an effective alternative to ganciclovir in the management of CMV retinitis. Trials with intravenous foscarnet in CMV retinitis have reported favorable results using initial daily doses of 180-230 mg/kg/d given as intermittent infusions every eight hours, followed by maintenance regimens of 60-90 mg/kg/d given as single daily one- or two-hour infusions. Foscarnet therapy may result in renal impairment, and indefinite intravenous maintenance therapy may be required to prevent recurrence of CMV infection. Despite these drawbacks, foscarnet's lack of major myelosuppressive toxicity, and its activity in suppressing HIV replication, make this a potentially safe and effective alternative agent for the management of CMV infection, especially in AIDS patients.

The prevention of cytomegalovirus disease in renal transplantation

Primary cytomegalovirus (CMV) disease can be prevented in renal transplant recipients with the use of either CMV hyperimmune globulin (CMVIg) or acyclovir. Started within 72 hours of transplantation and continued for 16 weeks posttransplant, CMVIg decreases the incidence of primary CMV disease from 60% to 21%. Acyclovir administered preoperatively and for 3 months thereafter decreases the incidence of CMV disease from 29% to 8% and is the most cost-effective therapy. The effectiveness of these preparations in preventing CMV reinfection or reactivation has not been established. The utility of therapies other than CMVIg or acyclovir for the prevention of CMV disease has not been proven; CMV vaccination is ineffective, polyvalent immunoglobulins require further study, and interferon alpha (IFN-alpha) has been associated with frequent irreversible rejection reactions and therefore should not be used for CMV prophylaxis. Although further investigation is necessary, patients at risk for primary CMV disease should receive prophylactic therapy with either CMVIg or acyclovir.

Age-related differences in pharmacokinetics of phosphonoformate in cats

Phosphonoformate (PFA) is a simple PPi analog which inhibits the activities of a variety of viral DNA polymerase, RNA polymerase, and reverse transcriptase enzymes. PFA is a topical and parenteral treatment for human herpesvirus infections and is currently in phase I trials for treatment of acquired immunodeficiency syndrome. Pharmacokinetic properties of PFA in young (growing) and adult specific-pathogen-free cats were compared. Mean PFA clearance from plasma was twofold higher in young cats (7.52 ml/min per kg of body weight) than in adult cats (3.70 ml/min per kg). Higher PFA clearance from plasma observed in young cats may result from higher renal clearance or enhanced accumulation of PFA in bone tissue of young versus adult cats. No plasma protein binding of PFA was observed. Mean oral bioavailability was 35% in young cats. These data indicate that age-related differences in PFA clearance from plasma occur in cats.

Determination of phosphonoformic acid in human plasma and urine by high-performance liquid chromatography with electrochemical detection

Trisodium phosphonoformate (foscarnet) is used in the treatment of cytomegalovirus infections in immunocompromised patients, such as bone marrow and renal transplant recipients, as well as patients with the acquired immune deficiency syndrome. A simple high-performance liquid chromatographic assay is described using an electrochemical detector. The method is accurate, precise and reproducible. Hydrochlorothiazide is used as the internal standard. This assay allows measurement of foscarnet in biological fluids at concentrations as low as 33 microM. This method is being used for the analysis of samples in clinical trials and is important in the evaluation of the pharmacokinetic disposition of the drug.