Human pharmacokinetics and tolerance of oral ganciclovir

Metabolic and pharmacokinetic studies following oral administration of famciclovir to the rat and dog

1. Drug-related material was well absorbed following oral administration of 14C-famciclovir to the male rat at doses up to 4000 mg/kg and to the male dog at doses up to 250 mg/kg, as judged by the early onset of the peak blood or plasma concentrations of radioactivity (usually < or = 1.5h) and the rapid extensive excretion of radioactivity in the urine (57-76 and 86-89% of dose in rat and dog respectively). 2. Famciclovir underwent extensive first-pass metabolism in both species. In rat, following dosing at 40 mg/kg, famciclovir was rapidly and extensively metabolized to the active antiviral compound penciclovir, which reached peak concentrations in the plasma (mean 3.5 micrograms/ml) at 0.5 h. The 6-deoxy precursor of penciclovir, BRL 42359, was the only other major metabolite detected in rat plasma. Cmax values for BRL 42359 (mean 2.2 micrograms/ml) were also achieved at 0.5 h. In dog, extensive conversion of famciclovir to penciclovir, via BRL 42359, also occurred, but its rate of formation from BRL 42359 was somewhat slower than in rat. In dog, following dosing at 25 mg/kg, Cmax values for penciclovir (mean 4.4 micrograms/ml) occurred at 3 h and were lower than the Cmax values for BRL 42359 (mean 10.0 micrograms/ml) which were achieved at 1h. 3. A dose-dependent decrease in the conversion of BRL 42359 to penciclovir occurred in both species, resulting a changes in the ratios of the plasma concentrations of the two metabolites with increasing dose. In rat, the urinary excretion of penciclovir decreased from 36% of dose at 40 mg/kg to 21% at 4000 mg/kg, and was accompanied by a corresponding increase in the urinary excretion of BRL 42359. In dog, a similar decrease in the urinary excretion of penciclovir occurred on increasing the dose of famciclovir from 25 to 250 mg/kg. 4. Penciclovir and BRL 42359 were the major metabolites detected in urine and faeces. In rat, following dosing at 40 mg/kg, 54 and 22% of dose were recovered in the excreta as penciclovir and BRL 42359 respectively. Corresponding recoveries of the two metabolites in the dog were 34 and 50% of dose. The metabolic fate of famciclovir in these animal species is, therefore, similar to that reported previously in man.

Ganciclovir. An update of its therapeutic use in cytomegalovirus infection

The antiviral nucleoside analogue ganciclovir has demonstrated in vitro activity against human cytomegalovirus and effectively treats infection caused by this organism in various immunocompromised patient groups. The drug prolongs time to progression in patients with acquired immune deficiency syndrome (AIDS)-related cytomegalovirus retinitis although life-long maintenance therapy is required. Direct comparisons between ganciclovir and foscarnet in this indication are few; nevertheless, the 2 drugs appear to have equal therapeutic efficacy in treating cytomegalovirus retinitis although results from 1 study in this indication suggest that foscarnet has an advantage in terms of patient survival. AIDS-related gastrointestinal and, to a lesser extent, pulmonary cytomegalovirus infection also respond to treatment with ganciclovir; maintenance therapy does not appear to be required in these latter 2 indications. Ganciclovir is also useful against cytomegalovirus infection in organ transplant recipients. The drug is most effective when given prophylactically or as early treatment for asymptomatic infection in bone marrow transplant recipients; treatment of established infection is less effective in this patient group. However, established infection in solid organ transplant recipients appears to respond to treatment with ganciclovir. The most common adverse event during ganciclovir therapy is haematological toxicity but this appears to be readily reversible on discontinuation of the drug. In addition, coadministration of granulocyte colony-stimulating factor (G-CSF) or granulocyte-macrophage CSF (GM-CSF) has been shown to prevent ganciclovir-associated neutropenia. Thus, ganciclovir is a valuable treatment for cytomegalovirus infection in patients with AIDS and in organ transplant recipients. Further studies comparing ganciclovir and foscarnet-ideally incorporating the use of G-CSF or GM-CSF to prevent ganciclovir-associated neutropenia and assessing survival as 1 endpoint--should further clarify the relative role of ganciclovir as treatment or prophylaxis for cytomegalovirus infection.

Metabolic and pharmacokinetic studies following oral administration of 14C-famciclovir to healthy subjects

1. Following oral administration of 14C-famciclovir (500 mg) to three healthy male subjects, drug-related material was rapidly absorbed as judged by peak plasma concentrations of radioactive material being achieved by 0.75 h (6.7 +/- 0.9 microgram equiv./ml (mean +/- SD). 2. Famciclovir underwent extensive first-pass metabolism and was only detected in the plasma of one subject at low concentrations (0.5 microgram/ml). Famciclovir was rapidly and extensively metabolized to the active antiviral compound penciclovir, which reached peak concentrations in the plasma of 3.6 +/- 0.7 microgram/ml (0.75 h). The plasma elimination half-life value for penciclovir was 2.1 +/- 0.1 h. The 6-deoxy precursor of penciclovir, BRL 42359, was the only other relatively major metabolite detected in plasma. Peak plasma concentrations of BRL 42359 (1.0 +/- 0.1 microgram/ml) were achieved at 0.5 h. 3. After 3 days, 73.0 +/- 6.1% of the radioactive dose was excreted in urine, showing that good absorption of drug-related material occurred. Renal excretion was rapid since 60.2 +/- 4.2 and 72.3 +/- 5.7% of the dose was recovered in the urine samples collected up to 6 and 24 h, respectively. A good recovery of the administered radioactive dose was obtained since a further 26.6 +/- 5.1% of the dose was excreted in the faeces over a 72-h period. 4. Penciclovir and BRL 42359 were the major metabolites detected in urine and faeces. Penciclovir accounted for 59.2 +/- 4.9 and 4.2 +/- 1.4% of the dose in 0-24 h urine and 0-48 h faeces, respectively. Corresponding values for BRL 42359 were 5.0 +/- 0.5 and 17.0 +/- 6.2%, respectively. These metabolites were identified in the biological samples using hplc-ms and ms-ms with thermospray ionization.

Ocular complications of the acquired immunodeficiency syndrome. Focus on the treatment of cytomegalovirus retinitis with ganciclovir and foscarnet

The most common ocular complication in patients with the acquired immunodeficiency syndrome (AIDS) is cytomegalovirus retinitis. Incidence figures vary from 20 to 76%. Patients with cytomegalovirus may suffer from mild visual impairment of one or both eyes, but as the disease progresses the retinitis will almost certainly lead to blindness. Although cytomegalovirus retinitis is not a life-threatening infection, it can largely diminish the patient's quality of life. Clinical trials for the treatment of cytomegalovirus retinitis with a number of antiviral drugs have resulted in two drugs of choice, ganciclovir and foscarnet. Both drugs have an initial efficacy with induction therapy of 80-90%, but maintenance therapy is always needed to prevent a relapse. To exclude systemic side-effects of ganciclovir, intravitreal administration has been investigated with good results. Combination therapy of foscarnet and ganciclovir may be worthwhile in resistant cytomegalovirus retinitis.

Comparative pharmacokinetics of antiviral nucleoside analogues

The recent development of nucleoside analogues with antiviral activity has expanded the small but useful armamentarium for the treatment of certain viral diseases such as the human immunodeficiency virus, cytomegalovirus and others. Their intracellular site of action and need for sequential phosphorylation require that traditional pharmacokinetic parameters be used in conjunction with an understanding of intracellular metabolism when designing dosage regimens. This review summarises the available pharmacokinetic literature for zidovudine, didanosine, zalcitabine, aciclovir, ganciclovir, vidarabine and ribavirin. After oral administration, didanosine, aciclovir and ribavirin are < 50% bioavailable and ganciclovir is < 6% absorbed. In contrast, zidovudine and zalcitabine are > 60% bioavailable, although zidovudine undergoes considerable and variable first-pass hepatic glucuronidation while zalcitabine has no first-pass effect. Zidovudine, zalcitabine and didanosine are absorbed rapidly in the fasted state, with peak plasma concentrations exceeding their respective in vitro antiretroviral inhibitory concentrations. All reviewed agents except ribavirin have a relatively short plasma half-life (approximately 0.5 to 4h), with each agent demonstrating a different intracellular enzymatic activation scheme. For example, the rate-limiting step for formation of zidovudine triphosphate is the conversion of the monophosphate to the diphosphate, while didanosine is ultimately converted to dideoxyadenosine triphosphate which has the longest intracellular half-life (approximately 12 to 24h) among these agents. These drugs are not highly protein bound and they distribute into tissues with an apparent volume of distribution at steady-state ranging from 0.3 to 1.2 L/kg. They vary in the extent to which they enter cerebrospinal fluid, ranging from a low of < 25% for didanosine to a high of > 70% of a concurrent plasma concentration for ribavirin and vidarabine. These agents also vary with regard to degree of renal excretion of the parent drug, with the lowest noted for vidarabine (1 to 3%) and the highest for zalcitabine (approximately 75%) and ganciclovir (> 90%). With the increasing number of clinically useful nucleoside analogues, it is essential for the clinician to appreciate the subtle differences among these agents to ensure that optimal therapeutic outcomes may be attained with minimal toxicity.

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.

Cytomegalovirus infection of the larynx in the acquired immunodeficiency syndrome

CMV is the most important viral opportunist in patients with AIDS. Nearly all patients with AIDS have been exposed to CMV and the virus can be isolated from many. When CMV-related disease occurs in AIDS, it has usually involved the eye or gastrointestinal tract. In lesions in which CMV is the suspected etiology, evidence of direct pathogenicity of the virus should be obtained, usually histologically. Accurate diagnosis is essential because long-term therapy with a relatively toxic drug (ganciclovir) is indicated. We have presented a case of CMV infection of the larynx that responded, albeit slowly, to ganciclovir. Additional cases may arise as the prevalence of AIDS increases. Because of the potential for airway compromise and the tendency toward chronicity, CMV infection of the larynx should be of a significant concern both for the patient and the otolaryngologist.

Cytomegalovirus infection in AIDS. Patterns of disease, response to therapy and trends in survival

Among 347 AIDS patients seen at St Mary's Hospital, London between 1983 and 1989, cytomegalovirus (CMV) disease was observed in 75 (22%). Of these, 58 (77%) had CMV retinitis, 26 (35%) CMV colitis, and 12 (16%) had CMV infection diagnosed at other sites. Relapse occurred in 71%. A favourable response to the use of ganciclovir as induction therapy for CMV retinitis was observed in 92%. Relapse of CMV retinitis occurred in 54% at a median time of 97 days. Neutropenia was the most frequent and serious side-effect of ganciclovir, 76% patients having neutrophil counts less than 1.0 x 10(9)/l and 48% less than 0.5 x 10(9)/l at some stage of therapy. Thrombocytopenia was also common, and platelet counts of less than 50 x 10(9)/l occurred in 43% patients on ganciclovir. The concurrent use of zidovudine made the development of severe neutropenia and thrombocytopenia more likely. Median survival following the diagnosis of CMV disease increased from 5-8 months between 1984 and 1987, to over 12 months in 1988. Patients with CMV colitis had a worse prognosis than patients with CMV retinitis, with median survival of 4.5 and 7 months respectively. In conclusion, CMV is an important opportunist infection in AIDS and both the disease and its treatment cause considerable morbidity. Hence, it is important to develop more effective and less toxic forms of therapy for CMV infection.

Ganciclovir permeation of the human erythrocyte membrane

The membrane permeation of ganciclovir (DHPG)--a structural analogue of acyclovir (ACV) with activity against cytomegalovirus--was investigated in human erythrocytes at 37 degrees with an "inhibitor-stop" assay. DHPG influx was nonconcentrative, occurred without permeant metabolism, and was rate-saturable. While substantial inhibition of the influx of 13 microM DHPG occurred only in the presence of permeants of the purine nucleobase carrier, nucleosides and inhibitors of nucleoside transport markedly inhibited DHPG influx at higher DHPG concentrations (greater than or equal to 200 microM). Adenine and dilazep (a potent inhibitor of the nucleoside carrier) each inhibited the influx of DHPG only partially; when present together, however, they inhibited DHPG permeation completely. DHPG permeation via the purine nucleobase carrier (Km = 0.89 mM) was characterized by assessing influx in the presence of 1.0 microM dilazep. Adenine and ACV were shown to competitively inhibit this process, while DHPG (Ki = 0.90 mM) was found to competitively inhibit adenine influx. DHPG influx via the nucleoside carrier (Km = 14 mM) was characterized by assessing influx in the presence of 2 mM adenine. DHPG (Ki = 10 mM) also appeared to competitively inhibit the influx of 5-iodo-2'-deoxyuridine. These results indicate that DHPG permeates the human erythrocyte membrane primarily by the purine nucleobase carrier and secondarily by the nucleoside transporter.

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.

Antiviral activity and pharmacokinetics of HOE 602, an acyclic nucleoside, in animal models

The acyclic nucleoside derivative HOE 602 (2-amino-9-[1,3-bis(isopropoxy)-2-propoxymethyl]purine) was evaluated for its antiviral activity in cell culture and for its therapeutic efficacy in mice infected with herpes simplex virus type 1 (HSV-1) and type 2 (HSV-2) or with murine cytomegalovirus (MCMV). HOE 602 was inactive in vitro against a variety of DNA- and RNA-viruses. However it prevented symptoms and mortality in mice systemically infected with HSV-1, HSV-2 or MCMV when administered intraperitoneally or orally at a dosage of 100 mumol/kg twice per day. Pharmacokinetic studies in mice and macaques revealed that HOE 602 was converted via three metabolic steps to ganciclovir, which seemed to be the antivirally active compound. The bioavailability of ganciclovir after oral administration of HOE 602 or ganciclovir was similar in mice, while in rhesus monkeys much higher serum levels of ganciclovir were reached with HOE 602. After intraperitoneal or intravenous administration higher drug levels were obtained with ganciclovir. The excellent therapeutic efficacy in animal models, the high enteral absorption in monkeys, and the favourable physical properties will hopefully lead to an orally active drug against cytomegalovirus and severe herpes infections in man.

Ganciclovir. A review of its antiviral activity, pharmacokinetic properties and therapeutic efficacy in cytomegalovirus infections

Ganciclovir is a nucleoside analogue with antiviral activity in vitro against members of the herpes group and some other DNA viruses. It has demonstrated efficacy against human cytomegalovirus infections and should be considered a first-line therapy in the treatment of life- or sight-threatening cytomegalovirus infection in immunocompromised patients. Clinical efficacy varies with the underlying aetiology of immunocompromise and the site of disease, and prompt diagnosis and early treatment initiation appear to improve the response. In patients with cytomegalovirus pneumonia, particularly bone marrow transplant recipients, concomitant administration of cytomegalovirus immune globulin may significantly improve clinical outcome. Maintenance therapy to prevent recurrence is usually required by bone marrow transplant recipients until the recovery of adequate immune function, whereas AIDS patients may require indefinite ganciclovir maintenance therapy to prevent disease progression, as ganciclovir (like other antivirals) does not eradicate latent viral infection. Haematological effects occur relatively frequently during ganciclovir administration but are usually reversible. Ganciclovir has not been directly compared with other antiviral drugs because of the absence until recently of other effective treatments. However, comparative studies with foscarnet, particularly in cytomegalovirus retinitis, will be of considerable interest. Thus, ganciclovir represents a major advance in the therapy of severe cytomegalovirus infections in immunocompromised patients. Comparative studies, and investigation of ways of reducing toxicity (intravitreal administration; concomitant use of stimulants of haematopoiesis; use in conjunction with other antivirals with differing mechanisms of action), may further expand its eventual role.

Selection of an oral prodrug (BRL 42810; famciclovir) for the antiherpesvirus agent BRL 39123 [9-(4-hydroxy-3-hydroxymethylbut-l-yl)guanine; penciclovir]

The limited oral absorption in rodents of the antiherpesvirus agent 9-(4-hydroxy-3-hydroxymethylbut-l-yl)guanine (BRL 39123 [penciclovir; British approved name]) prompted a search for oral prodrugs. The 6-deoxy derivative of penciclovir (BRL 42359) and the corresponding diacetyl and dipropionyl 6-deoxy derivatives (BRL 42810 [famciclovir; British approved name] and BRL 43599) were tested as oral prodrugs. The in vivo absorption (dose, 0.2 mmol/kg) and the conversion to the active compound, penciclovir, were determined in rats. Compared with the sodium salt of penciclovir given intravenously, the bioavailabilities of penciclovir from orally administered penciclovir, BRL 42359, famciclovir, and BRL 43599 were 1.5, 9, 41, and 27%, respectively. These prodrugs and 6-deoxyacyclovir were tested for stability in rat duodenal contents and for metabolism in rat intestinal wall homogenate, liver homogenate, and blood and in the corresponding human fluids and tissues. Famciclovir was much more stable than BRL 43599 in human duodenal contents (half-lives, greater than 2 h and 7 min, respectively) yet was efficiently converted to penciclovir by the tissue homogenates. The major metabolic pathway was by deacetylation followed by oxidation at the 6 position. The rate of oxidation was comparable to that of 6-deoxyacyclovir, which is known to be converted efficiently to acyclovir in humans. Famciclovir was selected for further evaluation and progression to studies in humans. These subsequent studies confirmed that, after oral dosing with famciclovir, more than half the dose was absorbed and rapidly converted to penciclovir.

Evaluation of ganciclovir for cytomegalovirus disease

Cytomegalovirus (CMV) infections are a common cause of morbidity and mortality in immunosuppressed patients. Ganciclovir is an acyclic deoxyguanosine analog structurally similar to acyclovir but with superior activity against CMV. The median ganciclovir concentration required to inhibit viral replication by 50 percent is 2.15 mumol versus 72 mumol for acyclovir. Pharmacokinetic properties of ganciclovir include biexponential decay with a terminal half-life of 2.5 hours, tissue uptake, cerebrospinal fluid penetration, and renal dependence for elimination. CMV treatment approaches have commonly used dosages of 3-15 mg/kg/d. In uncontrolled trials, the response rate of CMV retinitis is approximately 80 percent. The overall response rate for CMV pneumonitis has been approximately 50 percent. However, AIDS (acquired immunodeficiency syndrome) and other immunosuppressed patients appear to respond more favorably (approximately 70 percent) than do marrow transplant recipients. Relapse is common once ganciclovir is stopped and maintenance therapy may be required for sustained benefit. Neutropenia appears to be the drug-limiting adverse reaction. Although the development of ganciclovir-resistant CMV, risk factors for neutropenia, and alternative administration strategies all need further study, ganciclovir appears to have a role in the treatment of cytomegalovirus disease.

Randomized prospective trial of ganciclovir maintenance therapy for cytomegalovirus retinitis

We report the first randomized prospective comparative study of long-term maintenance ganciclovir (9-[2-hydroxy-1-(hydroxymethyl)ethoxymethyl]guanine, BW759U, DHPG) therapy for cytomegalovirus retinitis in patients with the acquired immunodeficiency syndrome (AIDS). Eleven retinitis patients who received a 10-day course of ganciclovir induction therapy and then were randomized to receive either immediate daily ganciclovir maintenance therapy or deferred maintenance (eight deferred maintenance, three immediate maintenance) were evaluated for drug efficacy. Median time to retinitis progression was 42 days for the immediate maintenance group compared with 16 days for the deferred maintenance group, (P = 0.07). After crossing over to maintenance therapy, patients in the deferred group had a median time to retinitis progression of 58 days compared to 16 days while not on maintenance therapy (P = 0.13). Only 9% of cultures obtained while patients received maintenance therapy were positive for cytomegalovirus, vs 40% of those obtained off maintenance (P less than 0.001). We can state then that maintenance therapy with ganciclovir delays, but does not halt, progression of cytomegalovirus retinitis and suppresses, but does not eradicate, cytomegalovirus shedding in patients with AIDS.