Foscarnet Dosing for CMV Infection
For CMV infection in adults and children with normal renal function, foscarnet should be dosed at 60 mg/kg IV every 8 hours (or 90 mg/kg every 12 hours) for 14-21 days induction, followed by 90-120 mg/kg once daily for maintenance therapy, with mandatory dose adjustment based on creatinine clearance. 1
Induction Dosing
Adults (Normal Renal Function)
- CMV retinitis/disease: 60 mg/kg IV every 8 hours OR 90 mg/kg IV every 12 hours for 14-21 days 1
- Each dose must be infused over 1-2 hours (no faster than 1 mg/kg/minute) 2, 1
- An infusion pump is mandatory to control infusion rate 1
Pediatric Patients
- 60 mg/kg IV every 8 hours for 14-21 days 2, 3
- Infuse over 1-2 hours 2
- Same rate restrictions apply (no faster than 1 mg/kg/minute) 2
The pediatric dosing is based on CDC/NIH/IDSA guidelines specifically for HIV-infected children with CMV disease, representing the most authoritative pediatric dosing recommendation available.
Maintenance Therapy
Adults
- Start at 90 mg/kg once daily IV over 2 hours 1
- May escalate to 120 mg/kg once daily if early reinduction needed due to retinitis progression 1
- The 120 mg/kg dose has not demonstrated superiority in controlled trials and carries higher toxicity risk 1
Pediatric Patients
- 90-120 mg/kg once daily for chronic suppression 3
- Lifelong maintenance therapy required after initial induction 2
Renal Dose Adjustment
Critical: All doses MUST be individualized based on creatinine clearance (CrCl) in mL/min/kg 1
Calculating CrCl in mL/min/kg
Use modified Cockcroft-Gault equation:
- Males: [140 - age] / (serum creatinine × 72) = mL/min/kg
- Females: Multiply result by 0.85 1
Dose Adjustment Table for Induction (CMV)
| CrCl (mL/min/kg) | 60 mg/kg Q8h equivalent | 90 mg/kg Q12h equivalent |
|---|---|---|
| >1.4 | 60 Q8h | 90 Q12h |
| >1.0-1.4 | 45 Q8h | 70 Q12h |
| >0.8-1.0 | 50 Q12h | 50 Q12h |
| >0.6-0.8 | 40 Q12h | 80 Q24h |
| >0.5-0.6 | 60 Q24h | 60 Q24h |
| >0.4-0.5 | 50 Q24h | 50 Q24h |
| <0.4 | Not recommended | Not recommended |
Dose Adjustment Table for Maintenance (CMV)
| CrCl (mL/min/kg) | 90 mg/kg/day | 120 mg/kg/day |
|---|---|---|
| >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 |
Important Renal Considerations
- Discontinue foscarnet if CrCl falls below 0.4 mL/min/kg during therapy 1
- Hydrate patient and monitor daily until renal function recovers 1
- Foscarnet is not recommended in hemodialysis patients (no established dosing guidelines) 1
- Limited safety/efficacy data for baseline serum creatinine >2.8 mg/dL or CrCl <50 mL/min 1
- Even patients with normal serum creatinine may require dose adjustment—calculate CrCl at baseline and frequently thereafter 1
Critical Safety Measures
Mandatory Hydration Protocol
- Establish diuresis with saline fluid loading both before and during each infusion 2, 1
- This is essential to minimize nephrotoxicity 2
- Do not hydrate if clinical contraindications exist 1
Monitoring Requirements
- Renal function: Up to 30% of patients develop increased serum creatinine 2
- Electrolytes (calcium, phosphorus, magnesium, potassium): Approximately one-third of patients develop metabolic abnormalities 2
- Monitor for hypocalcemia-related seizures and cardiac dysrhythmias 2
- Check liver transaminases 2
Common Toxicities
The main toxicity is decreased renal function, occurring in up to 30% of patients 2. Foscarnet binds divalent metal ions (calcium), leading to:
Special Clinical Scenarios
Ganciclovir-Resistant CMV
Foscarnet is the alternative drug of choice for ganciclovir-resistant CMV infections 2. Use standard induction and maintenance dosing as above.
Combination Therapy
For sight-threatening disease or retinitis failing monotherapy, combine ganciclovir (5 mg/kg Q12h) with foscarnet (60 mg/kg Q8h) 2, 3. However, these drugs are physically incompatible and must NOT be mixed 2.
CNS Disease
For CMV CNS disease, use ganciclovir 5 mg/kg Q12h PLUS foscarnet 60 mg/kg Q8h until symptoms improve, followed by chronic suppressive therapy 3.
Acyclovir-Resistant HSV
For acyclovir-resistant HSV (not CMV), dose is 40 mg/kg Q8h or Q12h for 2-3 weeks 1.
Key Clinical Pearls
Foscarnet has survival advantages: When used for suppression, foscarnet has been associated with increased length of survival relative to ganciclovir in HIV-infected adult patients 2. This is a critical consideration when choosing between agents.
Infusion rate is non-negotiable: Never exceed 1 mg/kg/minute infusion rate 2, 1. Use an infusion pump for all doses 1.
Renal dosing is mandatory: Even with "normal" creatinine, calculate weight-based CrCl and adjust accordingly 1. This is not optional.
Hydration prevents nephrotoxicity: Aggressive saline hydration before and during infusions significantly reduces the 30% baseline risk of renal dysfunction 2.