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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 related to mineral and electrolyte abnormalities have been associated with FOSCAVIR treatment (see WARNINGS 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.
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.
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.
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, Category C: FOSCAVIR did not adversely affect fertility and general reproductive performance in rats. The results of periand 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 postpartum 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.
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.
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.
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).This monograph has been modified to include the generic and brand name in many instances.
Last reviewed on RxList: 7/13/2016
Additional Foscavir Information
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