Foscarnet Dose for CMV Retinitis in Renal Dysfunction

 

A. Induction therapy:

 

With any degree of renal dysfunction, the dose must be modified on an individual basis so that serum levels of foscarnet do not rise excessively and thereby potentiate renal toxicity. To individualize doses:

 

a. First, the patient's creatinine clearance must be known or estimated;

b. Then, divide the patient's creatinine clearance by the patient's body weight (kg) to determine the milliliters per minute per kilogram (mL/min/kg) for that patient. Use Table 11.03.06A to determine the actual dose.

For example, if the patient's estimated creatinine clearance is 60 mL/minute, and his weight was 60 kg, then:

 

((estimated creatinine clearance) / (weight in kilograms)) =

= (60 mL per min) / (60 kg) = 1 mL/min/kg

 

Based on Table 1, the proper dose would be 39 mg per dose q8h.

If the patient's creatinine clearance divided by his or her weight is under 0.4 mL/minute/kg, the package insert suggests that foscarnet should be discontinued and the patient monitored (e.g., daily) until resolution of minimal renal function is ensured.

 

Table 1: Induction doses of foscarnet

Cr Cl (mL/min/kg)	Equivalent Dose
>= 1.6	60
1.5	57
1.4	53
1.3	49
1.2	46
1.1	42
1.0	39
0.9	35
0.8	32
0.7	28
0.6	25
0.5	21
0.4	18

 

where:

• equivalent dose is Equivalent to 60 mg/kg dose q8h

 

Source: Physicians' Desk Reference 1993, 47th Edition, published by Medical Economics Data, Montvale, NJ 07645.

 

B. Maintenance therapy.

 

Following induction treatment, the package insert suggests a dose of 90 mg/ kg/day (individualized for renal function) given as an intravenous infusion over 2 hours. Adequate hydration is important. Patients who demonstrate escalation of retinitis on this initial dose or who tolerate the 90 mg/kg/ day dose very well may be candidates for an escalated maintenance dose, if tolerated, to 120 mg/kg/day.

 

See Table 2 for modification of doses depending on the patient's individual renal function as discussed in sec. A above. If the patient's renal function is below 0.4 mL/min/kg, foscarnet should not be used.

 

Table 2:. Maintenance dosing of foscarnet

 

Creat Cl	MD90	MD120
>=1.4	90	120
1.2-1.4	78	104
1.0-1.2	75	100
0.8-1.0	71	94
0.6-0.8	63	84
0.4-0.6	57	76

 

where:

• creatinine clearance is in mL/min/kg

• MD90 = equivalent to 90 mg/kg dose q24h

• MD120 = equivalent to 120 mg/kg dose q24h

 

Source: Physicians' Desk Reference 1993, 47th Edition, published by Medical Economics Data, Montvale, NJ 07645.