Mechanism of Action

FOSCAVIR is an organic analogue of inorganic pyrophosphate that inhibits replication of herpes viruses in vitro including cytomegalovirus (CMV) and herpes simplex virus types 1 and 2 (HSV-1 and HSV-2).

FOSCAVIR exerts its antiviral activity by a selective inhibition at the pyrophosphate binding site on virus-specific DNA polymerases at concentrations that do not affect cellular DNA polymerases. FOSCAVIR does not require activation (phosphorylation) by thymidine kinase or other kinases and therefore is active in vitro against HSV TK deficient mutants and CMV UL97 mutants. Thus, HSV strains resistant to acyclovir or CMV strains resistant to ganciclovir may be sensitive to FOSCAVIR. However, acyclovir or ganciclovir resistant mutants with alterations in the viral DNA polymerase may be resistant to FOSCAVIR and may not respond to therapy with FOSCAVIR. The combination of FOSCAVIR and ganciclovir has been shown to have enhanced activity in vitro.

Antiviral Activity in vitro and in vivo

The quantitative relationship between the in vitro susceptibility of human cytomegalovirus (CMV) or herpes simplex virus 1 and 2 (HSV-1 and HSV-2) to FOSCAVIR and clinical response to therapy has not been established and virus sensitivity testing has not been standardized. Sensitivity test results, expressed as the concentration of drug required to inhibit by 50% the growth of virus in cell culture (IC50), vary greatly depending on the assay method used, cell type employed and the laboratory performing the test. A number of sensitive viruses and their IC50 values are listed below (Table 1).

Statistically significant decreases in positive CMV cultures from blood and urine have been demonstrated in two studies (FOS-03 and ACTG-015/915) of patients treated with FOSCAVIR. Although median time to progression of CMV retinitis was increased in patients treated with FOSCAVIR, reductions in positive blood or urine cultures have not been shown to correlate with clinical efficacy in individual patients.

Resistance

Strains of both HSV and CMV that are resistant to FOSCAVIR can be readily selected in vitro by passage of wild type virus in the presence of increasing concentrations of the drug. All FOSCAVIR resistant mutants are known to be generated through mutation in the viral DNA polymerase gene. CMV strains with double mutations conferring resistance to both FOSCAVIR and ganciclovir have been isolated from patients with AIDS. The possibility of viral resistance should be considered in patients who show poor clinical response or experience persistent viral excretion during therapy.

Pharmacokinetics

The pharmacokinetics of foscarnet has been determined after administration as an intermittent intravenous infusion during induction therapy in AIDS patients with CMV retinitis. Observed plasma foscarnet concentrations in four studies (FOS-01, ACTG-015, FP48PK, FP49PK) are summarized in Table 3:

Distribution

In vitro studies have shown that 14 – 17% of foscarnet is protein bound at plasma drug concentrations of 1 – 1000 μM.

The foscarnet terminal half-life determined by urinary excretion was 87.5 ± 41.8 hours, possibly due to release of foscarnet from bone. Postmortem data on several patients in European clinical trials provide evidence that foscarnet does accumulate in bone in humans; however, the extent to which this occurs has not been determined. In animal studies (mice), 40% of an intravenous dose of FOSCAVIR was deposited in bone in young animals and 7% was deposited in adult animals.

Special Populations

CMV Retinitis

A prospective, randomized, controlled clinical trial (FOS-03) was conducted in 24 patients with AIDS and CMV retinitis comparing treatment with FOSCAVIR to no treatment. Patients received induction treatment of FOSCAVIR, 60 mg/kg every 8 hours for 3 weeks, followed by maintenance treatment with 90 mg/kg/day until retinitis progression (appearance of a new lesion or advancement of the border of a posterior lesion greater than 750 microns in diameter). All diagnoses and determinations of retinitis progression were made from masked reading of retinal photographs. The 13 patients randomized to treatment with FOSCAVIR had a significant delay in progression of CMV retinitis compared to untreated controls. Median times to retinitis progression from study entry were 93 days (range 21 – >364) and 22 days (range 7 – 42), respectively.

In another prospective clinical trial of CMV retinitis in patients with AIDS (ACTG-915), 33 patients were treated with two to three weeks of FOSCAVIR induction (60 mg/kg TID) and then randomized to either 90 mg/kg/day or 120 mg/kg/day maintenance therapy. The median times from study entry to retinitis progression were not significantly different between the treatment groups, 96 (range 14 – >176) days and 140 (range 16 – >233) days, respectively.

In study ACTG 129/FGCRT SOCA study 107 patients with newly diagnosed CMV retinitis were randomized to treatment with FOSCAVIR (induction: 60 mg/kg TID for 2 weeks; maintenance: 90 mg/kg QD) and 127 were randomized to treatment with ganciclovir (induction: 5 mg/kg BID; maintenance: 5 mg/kg QD). The median time to progression on the two drugs was similar (Fos=59 and Gcv=56 days).

Relapsed CMV Retinitis

The CMV Retinitis Retreatment Trial (ACTG 228/SOCA CRRT) was a randomized, open-label comparison of FOSCAVIR or ganciclovir monotherapy to the combination of both drugs for the treatment of persistently active or relapsed CMV retinitis in patients with AIDS. Subjects were randomized to one of the three treatments: FOSCAVIR 90 mg/kg BID induction followed by 120 mg/kg QD maintenance (Fos); ganciclovir 5 mg/kg BID induction followed by 10 mg/kg QD maintenance (Gcv); or the combination of the two drugs, consisting of continuation of the subject's current therapy and induction dosing of the other drug (as above), followed by maintenance with FOSCAVIR 90 mg/kg QD plus ganciclovir 5 mg/kg QD (Cmb). Assessment of retinitis progression was performed by masked evaluation of retinal photographs. The median times to retinitis progression or death were 39 days for the FOSCAVIR group, 61 days for the ganciclovir group and 105 days for the combination group. For the alternative endpoint of retinitis progression (censoring on death), the median times were 39 days for the FOSCAVIR group, 61 days for the ganciclovir group and 132 days for the combination group. Due to censoring on death, the latter analysis may overestimate the treatment effect. Treatment modifications due to toxicity were more common in the combination group than in the FOSCAVIR or ganciclovir monotherapy groups (see ADVERSE REACTIONS section).

Mucocutaneous Acyclovir Resistant HSV Infections

In a controlled trial, patients with AIDS and mucocutaneous, acyclovir-resistant HSV infection were randomized to either FOSCAVIR (N=8) at a dose of 40 mg/kg TID or vidarabine (N=6) at a dose of 15 mg/kg per day. Eleven patients were nonrandomly assigned to receive treatment with FOSCAVIR because of prior intolerance to vidarabine. Lesions in the eight patients randomized to FOSCAVIR healed after 11 to 25 days; seven of the 11 patients nonrandomly treated with FOSCAVIR healed their lesions in 10 to 30 days. Vidarabine was discontinued because of intolerance (N=4) or poor therapeutic response (N=2). In a second trial, forty AIDS patients and three bone marrow transplant recipients with mucocutaneous, acyclovir-resistant HSV infections were randomized to receive FOSCAVIR at a dose of either 40 mg/kg BID or 40 mg/kg TID. Fifteen of the 43 patients had healing of their lesions in 11 to 72 days with no difference in response between the two treatment groups.

CMV Retinitis

FOSCAVIR is indicated for the treatment of CMV retinitis in patients with acquired immunodeficiency syndrome (AIDS). Combination therapy with FOSCAVIR and ganciclovir is indicated for patients who have relapsed after monotherapy with either drug. SAFETY AND EFFICACY OF FOSCAVIR HAVE NOT BEEN ESTABLISHED FOR TREATMENT OF OTHER CMV INFECTIONS (e.g., PNEUMONITIS, GASTROENTERITIS); CONGENITAL OR NEONATAL CMV DISEASE; OR NONIMMUNOCOMPROMISED INDIVIDUALS.

Mucocutaneous Acyclovir Resistant HSV Infections

FOSCAVIR is indicated for the treatment of acyclovir-resistant mucocutaneous HSV infections in immunocompromised patients. SAFETY AND EFFICACY OF FOSCAVIR HAVE NOT BEEN ESTABLISHED FOR TREATMENT OF OTHER HSV INFECTIONS (e.g., RETINITIS, ENCEPHALITIS); CONGENITAL OR NEONATAL HSV DISEASE; OR HSV IN NONIMMUNOCOMPROMISED INDIVIDUALS.

Renal Impairment

THE MAJOR TOXICITY OF FOSCAVIR IS RENAL IMPAIRMENT (see ADVERSE REACTIONS section). Renal impairment is most likely to become clinically evident during the second week of induction therapy, but may occur at any time during FOSCAVIR treatment. Renal function should be monitored carefully during both induction and maintenance therapy (see PATIENT MONITORING section). Elevations in serum creatinine are usually, but not always, reversible following discontinuation or dose adjustment of FOSCAVIR. Safety and efficacy data for patients with baseline serum creatinine levels greater than 2.8 mg/dL or measured 24-hour creatinine clearances <50 mL/min are limited.

SINCE FOSCAVIR HAS THE POTENTIAL TO CAUSE RENAL IMPAIRMENT, DOSE ADJUSTMENT BASED ON SERUM CREATININE IS NECESSARY. Hydration may reduce the risk of nephrotoxicity. It is recommended that 750–1000 mL of normal saline or 5% dextrose solution should be given prior to the first infusion of FOSCAVIR to establish diuresis. With subsequent infusions, 750–1000 mL of hydration fluid should be given with 90-120 mg/kg of FOSCAVIR, and 500 mL with 40–60 mg/kg of FOSCAVIR. Hydration fluid may need to be decreased if clinically warranted.

After the first dose, the hydration fluid should be administered concurrently with each infusion of FOSCAVIR.

Mineral and Electrolyte Abnormalities

FOSCAVIR has been associated with changes in serum electrolytes including hypocalcemia, hypophosphatemia, hyperphosphatemia, hypomagnesemia, and hypokalemia (see ADVERSE REACTIONS section). FOSCAVIR may also be associated with a dose-related decrease in ionized serum calcium which may not be reflected in total serum calcium. This effect is likely to be related to chelation of divalent metal ions such as calcium by foscarnet. Patients should be advised to report symptoms of low ionized calcium such as perioral tingling, numbness in the extremities and paresthesias. Particular caution and careful management of serum electrolytes is advised in patients with altered calcium or other electrolyte levels before treatment and especially in those with neurologic or cardiac abnormalities and those receiving other drugs known to influence minerals and electrolytes (see PATIENT MONITORING and Drug Interactions sections). Physicians should be prepared to treat these abnormalities and their sequelae such as tetany, seizures or cardiac disturbances. The rate of FOSCAVIR infusion may also affect the decrease in ionized calcium. Therefore, an infusion pump must be used for administration to prevent rapid intravenous infusion (see DOSAGE AND ADMINISTRATION section). Slowing the infusion rate may decrease or prevent symptoms.

Seizures

Seizures related to mineral and electrolyte abnormalities have been associated with FOSCAVIR treatment (see WARNING section; Mineral and Electrolyte Abnormalities). Several cases of seizures were associated with death. Risk factors associated with seizures included impaired baseline renal function, low total serum calcium, and underlying CNS conditions.

General

Care must be taken to infuse solutions containing FOSCAVIR only into veins with adequate blood flow to permit rapid dilution and distribution to avoid local irritation (see DOSAGE AND ADMINISTRATION). Local irritation and ulcerations of penile epithelium have been reported in male patients receiving FOSCAVIR, possibly related to the presence of drug in the urine. Cases of male and female genital irritation/ulceration have been reported in patients receiving FOSCAVIR. Adequate hydration with close attention to personal hygiene may minimize the occurrence of such events.

Hemopoietic System

Anemia has been reported in 33% of patients receiving FOSCAVIR in controlled studies. Granulocytopenia has been reported in 17% of patients receiving FOSCAVIR in controlled studies; however, only 1% (2/189) were terminated from these studies because of neutropenia.

Information for Patients

Drug Interactions

A possible drug interaction of FOSCAVIR and intravenous pentamidine has been described. Concomitant treatment of four patients in the United Kingdom with FOSCAVIR and intravenous pentamidine may have caused hypocalcemia; one patient died with severe hypocalcemia. Toxicity associated with concomitant use of aerosolized pentamidine has not been reported.

Since FOSCAVIR decreases serum concentrations of ionized calcium, concurrent treatment with other drugs known to influence serum calcium concentrations should be used with particular caution. Fatalities have been reported in post-marketing surveillance during concomitant therapy with FOSCAVIR and Pentamidine.

Because of foscarnet's tendency to cause renal impairment, the use of FOSCAVIR should be avoided in combination with potentially nephrotoxic drugs such as aminoglycosides, amphotericin B and intravenous pentamidine (see above) unless the potential benefits outweigh the risks to the patient.

Abnormal renal function has been observed in clinical practice during the use of FOSCAVIR and ritonavir, or FOSCAVIR, ritonavir, and saquinavir. (See DOSAGE and ADMINISTRATION.)

Carcinogenesis, Mutagenesis, Impairment of Fertility

Carcinogenicity studies were conducted in rats and mice at oral doses of 500 mg/kg/day and 250 mg/kg/day. Oral bioavailability in unfasted rodents is < 20%. No evidence of oncogenicity was reported at plasma drug levels equal to 1/3 and 1/5, respectively, of those in humans (at the maximum recommended human daily dose) as measured by the area-under-the-time/concentration curve (AUC).

FOSCAVIR showed genotoxic effects in the BALB/3T3 in vitro transformation assay at concentrations greater than 0.5 mcg/mL and an increased frequency of chromosome aberrations in the sister chromatid exchange assay at 1000 mcg/mL. A high dose of foscarnet (350 mg/kg) caused an increase in micronucleated polychromatic erythrocytes in vivo in mice at doses that produced exposures (area under curve) comparable to that anticipated clinically.

Pregnancy: Teratogenic Effect

Pregnancy, Category C: FOSCAVIR did not adversely affect fertility and general reproductive performance in rats. The results of peri- and post-natal studies in rats were also negative. However, these studies used exposures that are inadequate to define the potential for impairment of fertility at human drug exposure levels.

Daily subcutaneous doses up to 75 mg/kg administered to female rats prior to and during mating, during gestation, and 21 days post-partum caused a slight increase (< 5%) in the number of skeletal anomalies compared with the control group. Daily subcutaneous doses up to 75 mg/kg administered to rabbits and 150 mg/kg administered to rats during gestation caused an increase in the frequency of skeletal anomalies/variations. On the basis of estimated drug exposure (as measured by AUC), the 150 mg/kg dose in rats and 75 mg/kg dose in rabbits were approximately one-eighth (rat) and one-third (rabbit) the estimated maximal daily human exposure. These studies are inadequate to define the potential teratogenicity at levels to which women will be exposed.

There are no adequate and well controlled studies in pregnant women. Because animal reproductive studies are not always predictive of human response, FOSCAVIR should be used during pregnancy only if clearly needed.

Nursing Mothers

It is not known whether FOSCAVIR is excreted in human milk; however, in lactating rats administered 75 mg/kg, FOSCAVIR was excreted in maternal milk at concentrations three times higher than peak maternal blood concentrations.

Pediatric Use

The safety and effectiveness of FOSCAVIR in pediatric patients have not been established. FOSCAVIR is deposited in teeth and bone and deposition is greater in young and growing animals. FOSCAVIR has been demonstrated to adversely affect development of tooth enamel in mice and rats. The effects of this deposition on skeletal development have not been studied. Since deposition in human bone has also been shown to occur, it is likely that it does so to a greater degree in developing bone in pediatric patients. Administration to pediatric patients should be undertaken only after careful evaluation and only if the potential benefits for treatment outweigh the risks.

Geriatric Use

No studies of the efficacy or safety of FOSCAVIR in persons 65 years of age or older have been conducted. However, FOSCAVIR has been used in patients age 65 years of age and older. The pattern of adverse events seen in these patients is consistent across all age groups.This drug is known to be substantially excreted by the kidney, and the risk of toxic reactions to this drug may be greater in patients with impaired renal function. Because elderly patients are more likely to have decreased renal function, care should be taken in dose selection, and renal function should be monitored. (See DOSAGE AND ADMINISTRATION).

Incidence of 5% or Greater

Body as a Whole: fever, fatigue, rigors, asthenia, malaise, pain, infection, sepsis, death

Central and Peripheral Nervous System: headache, paresthesia, dizziness, involuntary muscle contractions, hypoesthesia, neuropathy, seizures including grand mal seizures (see WARNINGS)

Gastrointestinal System: anorexia, nausea, diarrhea, vomiting, abdominal pain

Hematologic: anemia, granulocytopenia, leukopenia (see PRECAUTIONS)

Metabolic and Nutritional: mineral and electrolyte imbalances (see WARNINGS) including hypokalemia, hypocalcemia, hypomagnesemia, hypophosphatemia, hyperphosphatemia

Psychiatric: depression, confusion, anxiety

Respiratory System: coughing, dyspnea

Skin and Appendages: rash, increased sweating

Urinary: alterations in renal function including increased serum creatinine, decreased creatinine clearance, and abnormal renal function (see WARNINGS)

Special Senses: vision abnormalities

Incidence between 1% and 5%

Application Site: injection site pain, injection site inflammation

Body as a Whole: back pain, chest pain, edema, influenza-like symptoms, bacterial infections, moniliasis, fungal infections, abscess

Cardiovascular: hypertension, palpitations, ECG abnormalities including sinus tachycardia, first degree AV block and non-specific ST-T segment changes, hypotension, flushing, cerebrovascular disorder (see WARNINGS)

Central and Peripheral Nervous System: tremor, ataxia, dementia, stupor, generalized spasms, sensory disturbances, meningitis, aphasia, abnormal coordination, leg cramps, EEG abnormalities (see WARNINGS)

Gastrointestinal: constipation, dysphagia, dyspepsia, rectal hemorrhage, dry mouth, melena, flatulence, ulcerative stomatitis, pancreatitis

Hematologic: thrombocytopenia, platelet abnormalities, thrombosis, white blood cell abnormalities, lymphadenopathy

Liver and Biliary: abnormal A-G ratio, abnormal hepatic function, increased SGPT, increased SGOT

Metabolic and Nutritional: hyponatremia, decreased weight, increased alkaline phosphatase, increased LDH, increased BUN, acidosis, cachexia, thirst, hypocalcemia (see WARNINGS)

Musculo-Skeletal: arthralgia, myalgia

Neoplasms: lymphoma-like disorder, sarcoma

Psychiatric: insomnia, somnolence, nervousness, amnesia, agitation, aggressive reaction, hallucination

Respiratory System: pneumonia, sinusitis, pharyngitis, rhinitis, respiratory disorders, respiratory insufficiency, pulmonary infiltration, stridor, pneumothorax, hemoptysis, bronchospasm

Skin and Appendages: pruritus, skin ulceration, seborrhea, erythematous rash, maculo-papular rash, skin discoloration

Special Senses: taste perversions, eye abnormalities, eye pain, conjunctivitis

Urinary System: albuminuria, dysuria, polyuria, urethral disorder, urinary retention, urinary tract infections, acute renal failure, nocturia, facial edema

Selected adverse events occurring at a rate of less than 1% in the five initial U.S. controlled clinical trials of FOSCAVIR include: syndrome of inappropriate antidiuretic hormone secretion, pancytopenia, hematuria, dehydration, hypoproteinemia, increases in amylase and creatinine phosphokinase, cardiac arrest, coma, and other cardiovascular and neurologic complications.

Selected adverse event data from the Foscarnet vs. Ganciclovir CMV Retinitis Trial (FGCRT), performed by the Studies of the Ocular Complications of AIDS (SOCA) Research Group, are shown in Table 7 (see CLINICAL TRIALS section).

Selected adverse events from ACTG Study 228 (CRRT) comparing combination therapy with FOSCAVIR or ganciclovir monotherapy are shown in Table 8. The most common reason for a treatment change in patients assigned to either FOSCAVIR or ganciclovir was retinitis progression. The most frequent reason for a treatment change in the combination treatment group was toxicity.

Adverse events that have been reported in post-marketing surveillance include: ventricular arrhythmia, prolongation of QT interval, gamma GT increased, diabetes insipidus (usually nephrogenic), renal calculus, and muscle disorders including myopathy, myositis, muscle weakness and rare cases of rhabdomyolysis. Cases of vesiculobullous eruptions including erythema multiforme, toxic epidermal necrolysis, and Stevens-Johnson Syndrome have been reported. In most cases, patients were taking other medications that have been associated with toxic epidermal necrolysis or Stevens-Johnson Syndrome.

Instructions for Administration and Preparation

FOSCAVIR is administered by controlled intravenous infusion, either by using a central venous line or by using a peripheral vein. The rate of infusion must be no more than 1 mg/kg/minute. An individualized dose of FOSCAVIR should be calculated on the basis of body weight (mg/kg), renal function, indication of use and dosing frequency (refer to DOSAGE subsection). To reduce the risk of nephrotoxicity, creatinine clearance (mL/min/kg) should be calculated even if serum creatinine is within the normal range, and doses should be adjusted accordingly.

An individualized dose at the required concentration (24 mg/mL or 12 mg/mL) for the route of administration (central line or peripheral line) needs to be aseptically prepared prior to dispensing. The standard 24 mg/mL solution may be used with or without dilution when using a central venous catheter for infusion. When a peripheral vein catheter is used, the 24 mg/mL injection must be diluted to a 12 mg/mL concentration with 5% dextrose in water or with a normal saline solution prior to administration to avoid local irritation of peripheral veins. Dilutions and/or removals of excess quantities should be accomplished under aseptic conditions. Solutions thus prepared should be used within 24 hours of first entry into a sealed bottle.

Hydration

Hydration may reduce the risk of nephrotoxicity. It is recommended that 750–1000 mL of normal saline or 5% dextrose solution should be given prior to the first infusion of FOSCAVIR to establish diuresis. With subsequent infusions, 750–1000 mL of hydration fluid should be given with 90–120 mg/kg of FOSCAVIR, and 500 mL with 40–60 mg/kg of FOSCAVIR. Hydration fluid may need to be decreased if clinically warranted.

After the first dose, the hydration fluid should be administered concurrently with each infusion of FOSCAVIR.

Compatibility With Other Solutions/Drugs

Other drugs and supplements can be administered to a patient receiving FOSCAVIR. However, care must be taken to ensure that FOSCAVIR is only administered with normal saline or 5% dextrose solution and that no other drug or supplement is administered concurrently via the same catheter. Foscarnet has been reported to be chemically incompatible with 30% dextrose, amphotericin B, and solutions containing calcium such as Ringer's lactate and TPN. Physical incompatibility with other IV drugs has also been reported including acyclovir sodium, ganciclovir, trimetrexate glucuronate, pentamidine isethionate, vancomycin, trimethoprim/sulfamethoxazole, diazepam, midazolam, digoxin, phenytoin, leucovorin, and proclorperazine. Because of foscarnet's chelating properties, a precipitate can potentially occur when divalent cations are administered concurrently in the same catheter.

Parenteral drug products must be inspected visually for particulate matter and discoloration prior to administration whenever the solution and container permit. Solutions that are discolored or contain particulate matter should not be used.

Accidental Exposure

Accidental skin and eye contact with foscarnet sodium solution may cause local irritation and burning sensation. If accidental contact occurs, the exposed area should be flushed with water.

Induction Treatment

The recommended initial dose of FOSCAVIR for patients with normal renal function is:

• For CMV retinitis patients, either 90 mg/kg (1-1/2 to 2 hour infusion) every twelve hours or 60 mg/kg (minimum one hour infusion) every eight hours over 2-3 weeks depending on clinical response.
• For acyclovir-resistant HSV patients, 40 mg/kg (minimum one hour infusion) either every 8 or 12 hours for 2-3 weeks or until healed.

An infusion pump must be used to control the rate of infusion. Adequate hydration is recommended to establish a diuresis (see Hydration for recommendation), both prior to and during treatment to minimize renal toxicity (see WARNINGS), provided there are no clinical contraindications.

Maintenance Treatment

Following induction treatment the recommended maintenance dose of FOSCAVIR for CMV retinitis is 90 mg/kg/day to 120 mg/kg/day (individualized for renal function) given as an intravenous infusion over 2 hours. Because the superiority of the 120 mg/kg/day has not been established in controlled trials, and given the likely relationship of higher plasma foscarnet levels to toxicity, it is recommended that most patients be started on maintenance treatment with a dose of 90 mg/kg/day. Escalation to 120 mg/kg/day may be considered should early reinduction be required because of retinitis progression. Some patients who show excellent tolerance to FOSCAVIR may benefit from initiation of maintenance treatment at 120 mg/kg/day earlier in their treatment.

An infusion pump must be used to control the rate of infusion with all doses. Again, hydration to establish diuresis both prior to and during treatment is recommended to minimize renal toxicity, provided there are no clinical contraindications (see WARNINGS).

Patients who experience progression of retinitis while receiving FOSCAVIR maintenance therapy may be retreated with the induction and maintenance regimens given above or with a combination of FOSCAVIR and ganciclovir (see CLINICAL TRIALS section). Because of physical incompatibility, FOSCAVIR and ganciclovir must NOT be mixed.

Use in Patients with Abnormal Renal Function

FOSCAVIR should be used with caution in patients with abnormal renal function because reduced plasma clearance of foscarnet will result in elevated plasma levels (see CLINICAL PHARMACOLOGY). In addition, FOSCAVIR has the potential to further impair renal function (see WARNINGS). Safety and efficacy data for patients with baseline serum creatinine levels greater than 2.8 mg/dL or measured 24-hour creatinine clearances < 50 mL/min are limited.

Renal function must be monitored carefully at baseline and during induction and maintenance therapy with appropriate dose adjustments for FOSCAVIR as outlined below (see Dose Adjustment and PATIENT MONITORING). During FOSCAVIR therapy if creatinine clearance falls below the limits of the dosing nomograms (0.4 mL/min/kg), FOSCAVIR should be discontinued, the patient hydrated, and monitored daily until resolution of renal impairment is ensured.

Dose Adjustment

FOSCAVIR dosing must be individualized according to the patient's renal function status. Refer to Table 9 below for recommended doses and adjust the dose as indicated. Even patients with serum creatinine in the normal range may require dose adjustment; therefore, the dose should be calculated at baseline and frequently thereafter.

To use this dosing guide, actual 24-hour creatinine clearance (mL/min) must be divided by body weight (kg), or the estimated creatinine clearance in mL/min/kg can be calculated from serum creatinine (mg/dL) using the following formula (modified Cockcroft and Gault equation):

For males:             140 – age              (x 0.85 for females) = mL/min/kg
                     Serum creatinine x 72