Foscarnet Sodium Injection dosing must be individualized according to the patient’s renal function status. Refer to Table 13 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|
|HSV: Equivalent to||CMV: Equivalent to|
|CrCI(mL/min/kg)||80 mg/kg/day total(40 mg/kg Q12h)||120 mg/kg/day total (40 mg/kg Q8h)||180 mg/kg/day total|
|(60 mg/kg Q8h)||(90 mg/kg Q12h)|
|>1.4||40 Q12h||40 Q8h||60 Q8h||90 Q12h|
|>1.0 – 1.4||30 Q12h||30 Q8h||45 Q8h||70 Q12h|
|> 0.8 – 1.0||20 Q12h||35 Q12h||50 Q12h||50 Q12h|
|>0.6 – 0.8||35 Q24h||25 Q12h||40 Q12h||80 Q24h|
|>0.5 – 0.6||25 Q24h||40 Q24h||60 Q24h||60 Q24h|
|> 0.4 – 0.5||20 Q24h||35 Q24h||50 Q24h||50 Q24h|
|<0.4||Not recommended||Not recommended||Not recommended||Not recommended|
*> means “greater than”, †≥ means “greater than or equal to”, ‡< means “less than”
|CMV: Equivalent to|
|CrCI(mL/min/kg)||90 mg/kg/day(once daily)||120 mg/kg/day(once daily)|
|>*1.4||90 Q24h||120 Q24h|
|>*1.0 – 1.4||70 Q24h||90 Q24h|
|>*0.8 – 1.0||50 Q24h||65 Q24h|
|>*0.6 – 0.8||80 Q48h||105 Q48h|
|>*0.5 – 0.6||60 Q48h||80 Q48h|
|≥†0.4 – 0.5||50 Q48h||65 Q48h|
|<‡0.4||Not recommended||Not recommended|
The majority of patients will experience some decrease in renal function due to foscarnet administration. Therefore it is recommended that creatinine clearance, either measured or estimated using the modified Cockcroft and Gault equation based on serum creatinine, be determined at baseline, 2–3 times per week during induction therapy and once weekly during maintenance therapy, with foscarnet dose adjusted accordingly (see Dose Adjustment). More frequent monitoring may be required for some patients. It is also recommended that a 24-hour creatinine clearance be determined at baseline and periodically thereafter to ensure correct dosing (assuming verification of an adequate collection using creatinine index). Foscarnet should be discontinued if creatinine clearance drops below 0.4 mL/min/kg.
Due to foscarnet’s propensity to chelate divalent metal ions and alter levels of serum electrolytes, patients must be monitored closely for such changes. It is recommended that a schedule similar to that recommended for serum creatinine (see above) be used to monitor serum calcium, magnesium, potassium and phosphorus. Particular caution is advised in patients with decreased total serum calcium or other electrolyte levels before treatment, as well as in patients with neurologic or cardiac abnormalities, and in patients receiving other drugs known to influence serum calcium levels. Any clinically significant metabolic changes should be corrected. Also, patients who experience mild (e.g., perioral numbness or paresthesias) or severe (e.g., seizures) symptoms of electrolyte abnormalities should have serum electrolyte and mineral levels assessed as close in time to the event as possible.
Careful monitoring and appropriate management of electrolytes, calcium, magnesium and creatinine are of particular importance in patients with conditions that may predispose them to seizures (see WARNINGS).
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