Foscarnet is administered intravenously (into a vein) and requires careful medical supervision, often in a hospital setting. It is not available in oral forms.

• Intravenous Infusion: Foscarnet is given as a slow intravenous infusion. The duration of infusion depends on the dose, typically ranging from 1 to 2 hours.
• Dilution: The 24 mg/mL solution may need to be diluted to 12 mg/mL with 5% dextrose solution or normal saline before peripheral vein administration. It can be given undiluted via a central venous line.
• Hydration: Adequate hydration is crucial to minimize the risk of kidney toxicity. Patients are typically given intravenous fluids (e.g., normal saline) before and during each Foscarnet infusion to promote urine output.
• Dosing Schedule: The dosing regimen (dose and frequency) varies significantly depending on the type and severity of the viral infection, and especially on the patient’s kidney function. It usually involves an initial “induction” phase with higher, more frequent doses (e.g., every 8 or 12 hours) followed by a “maintenance” phase with lower, less frequent doses (e.g., once daily).
• Complete Course: It is vital to complete the full prescribed course of treatment, even if symptoms improve, to prevent the infection from returning and developing drug resistance.

Foscarnet works by directly interfering with the replication of viruses, particularly herpesviruses like CMV and HSV. Its mechanism of action is unique compared to some other antiviral drugs:

• Mimics Pyrophosphate: Foscarnet is a structural mimic of pyrophosphate, a molecule that is essential for DNA synthesis.
• Inhibits Viral DNA Polymerase: It selectively binds to and inhibits the pyrophosphate binding site on viral DNA polymerase, an enzyme that viruses need to copy their genetic material (DNA) and multiply. It also inhibits reverse transcriptase in HIV.
• No Viral Activation Needed: Importantly, Foscarnet does not require activation (phosphorylation) by viral enzymes (like thymidine kinase). This characteristic is crucial because it means Foscarnet remains effective against viral strains that have developed resistance to other antivirals (e.g., acyclovir-resistant HSV or ganciclovir-resistant CMV) due to mutations in these activating enzymes.
• Slows Viral Growth: By blocking viral DNA synthesis, Foscarnet slows down the growth and spread of the virus within the body.

This unique mode of action makes Foscarnet a vital option for treating drug-resistant viral infections in immunocompromised patients.

Foscarnet can cause a range of side effects, some of which can be serious. Careful monitoring is essential during treatment.

• Kidney Problems (Nephrotoxicity): This is the major toxicity of Foscarnet. Symptoms include changes in urine output, swelling, or signs of kidney failure. This risk is reduced with adequate hydration and dose adjustments based on kidney function.
• Electrolyte Imbalances: Foscarnet commonly causes changes in serum electrolytes, including:
  • Low calcium (hypocalcemia) – can lead to tingling around the mouth, muscle cramps, or seizures.
  • Low magnesium (hypomagnesemia)
  • Low potassium (hypokalemia)
  • Low phosphate (hypophosphatemia) These imbalances can contribute to seizures and heart rhythm abnormalities.
• Seizures: Associated with Foscarnet treatment, often linked to electrolyte imbalances, underlying central nervous system (CNS) conditions, or impaired kidney function.
• Gastrointestinal Issues: Nausea, vomiting, diarrhea, abdominal pain, loss of appetite.
• Central and Peripheral Nervous System Effects: Headache, dizziness, paresthesia (tingling/numbness), confusion, anxiety, tremors, and less commonly, seizures, or other neurological disturbances.
• Genital Ulcerations: Local irritation and ulcerations of the penile epithelium in males or vulvovaginal irritation in females, possibly due to drug presence in urine.
• Anemia: Can occur, particularly with prolonged use or in combination with other myelosuppressive drugs.

Report any new or worsening symptoms to your doctor immediately.

Before and during treatment with Foscarnet, several critical warnings and precautions must be understood due to its potential for serious adverse effects.

• Renal Impairment: Foscarnet’s major toxicity is kidney damage. Frequent monitoring of serum creatinine (a measure of kidney function) and adequate hydration are imperative before and during treatment. Dosing must be carefully adjusted based on kidney function.
• Mineral and Electrolyte Abnormalities: Foscarnet can cause severe imbalances in calcium, magnesium, potassium, and phosphate, which can lead to seizures. Patients with pre-existing electrolyte disturbances should be corrected before starting therapy. Frequent monitoring of serum electrolytes is crucial.
• Seizures: Patients are at risk of seizures, especially those with pre-existing CNS conditions, impaired renal function, or severe electrolyte imbalances.
• Dehydration: Dehydration increases the risk of kidney toxicity and electrolyte imbalances. Adequate hydration before and during each infusion is essential.
• Anemia and Other Hematologic Abnormalities: Monitor blood counts, particularly red blood cell and white blood cell counts, especially in combination with other drugs that cause myelosuppression.
• Genital Irritation/Ulceration: Advise patients about potential irritation or ulceration of the genital area, and instruct on proper hygiene after urination.
• Pregnancy and Breastfeeding: Foscarnet is not recommended during pregnancy, and effective contraception should be used during and for up to 6 months after therapy. Breastfeeding is not recommended due to the potential risk to the infant and the risk of HIV transmission if the mother is HIV-positive.
• Pediatric and Geriatric Use: Use with caution in pediatric patients due to potential effects on bone development and in elderly patients due to increased likelihood of age-related kidney problems.

Foscarnet has a significant number of drug interactions, some of which can be serious or life-threatening. It is critical to inform your doctor about all medications, supplements, and herbal products you are taking.

• Nephrotoxic Drugs: Co-administration with other medications that can harm the kidneys (nephrotoxic drugs) can significantly increase the risk of kidney damage. Examples include:
  • Aminoglycoside antibiotics (e.g., gentamicin, amikacin)
  • Amphotericin B (an antifungal)
  • Pentamidine (used to treat certain parasitic infections)
  • NSAIDs (nonsteroidal anti-inflammatory drugs)
  • Cidofovir (another antiviral)
• Drugs Affecting Electrolytes: Foscarnet can cause imbalances in serum electrolytes (especially low calcium, magnesium, potassium, and phosphate). Concomitant use with other drugs that affect these electrolytes (e.g., diuretics, laxatives) can worsen these imbalances and increase the risk of seizures.
• QTc Prolonging Drugs: Foscarnet can prolong the QTc interval on an electrocardiogram (ECG), which can lead to serious heart rhythm abnormalities. Co-administration with other drugs that prolong the QTc interval should be avoided or used with extreme caution.
• Zidovudine (AZT): Concurrent use with zidovudine (an antiretroviral) may increase the risk of hematologic toxicity (e.g., anemia).
• Oral Phosphate Binders: May interfere with foscarnet absorption.

Due to the complexity and potential severity of these interactions, careful review of all medications by a healthcare professional is essential.

The dosage of Foscarnet is highly individualized, determined by the treating physician based on the specific viral infection, the patient’s body weight, and crucially, their kidney function. Dosage adjustments for kidney impairment are mandatory.

• Route of Administration: Always administered by slow intravenous (IV) infusion.
• Hydration: Patients must receive adequate hydration (e.g., 0.5 to 1 liter of normal saline or 5% dextrose solution) before the first infusion and with subsequent infusions to prevent kidney toxicity.
• CMV Retinitis:
  • Induction Therapy: Typically 60 mg/kg IV every 8 hours or 90 mg/kg IV every 12 hours for 14 to 21 days. Infusion time should be at least 1 hour for 60 mg/kg doses.
  • Maintenance Therapy: Following induction, 90 to 120 mg/kg IV once daily (infused over 2 hours or more).
• Acyclovir-Resistant HSV:
  • Induction Therapy: Typically 40 mg/kg IV every 8 hours or 60 mg/kg IV twice daily for 14 to 21 days.
• Kidney Impairment: Dosage must be adjusted significantly based on the patient’s creatinine clearance (CrCl). Detailed dosing charts are used to determine the reduced dose for varying degrees of renal function. Foscarnet is generally not recommended for patients with severe kidney impairment (CrCl < 0.4 mL/min/kg).
• Monitoring: Serum creatinine should be monitored frequently (e.g., every second day during induction and weekly during maintenance) to guide dose adjustments.

Foscarnet is a highly specialized, prescription-only medication whose use is restricted to specific severe viral infections, particularly in immunocompromised patients.

• Medical Diagnosis: A confirmed diagnosis of the specific viral infection (e.g., CMV retinitis, acyclovir-resistant HSV) is required. This often involves laboratory tests to identify the virus and its resistance patterns.
• Immunocompromised Status: It is primarily indicated for immunocompromised patients.
• Professional Administration: Due to its intravenous administration, potential for serious side effects, and complex dosing, Foscarnet is typically administered by or under the direct supervision of a healthcare professional in a hospital or specialized clinic setting.
• Extensive Monitoring: Prescribing and administering Foscarnet necessitates extensive monitoring. This includes frequent blood tests (for kidney function, electrolytes, blood counts) and potentially eye exams for CMV retinitis patients.
• Risk-Benefit Assessment: The healthcare provider will carefully weigh the severe nature of the viral infection and the potential benefits of Foscarnet against its significant toxicity profile. Patients and their caregivers will receive detailed counseling on the drug’s risks, proper hydration, and warning signs of side effects.

What is Foscarnet used for? CMV retinitis in AIDS patients; acyclovir-resistant HSV infections

2. What is the active ingredient? Foscarnet sodium

3. What drug class does it belong to? Miscellaneous antivirals

4. Is Foscarnet a controlled substance? No

5. Is Foscarnet available in generic form? Yes

6. How is Foscarnet administered? Intravenous infusion

7. What strengths are available? 24 mg/mL solution (in 250 mL or 500 mL vials)

8. What is the usual dosage for CMV? Induction: 60 mg/kg every 8 hours for 2–3 weeks; maintenance: 90–120 mg/kg once daily

9. Can it be used in children? Yes, with weight-based dosing

10. What are common side effects? Fever, nausea, anemia, electrolyte imbalance, renal dysfunction

11. Can it cause serious reactions? Yes—nephrotoxicity, seizures, electrolyte disturbances (hypocalcemia, hypomagnesemia)

12. Is Foscarnet safe during pregnancy? Use only if clearly needed; consult a healthcare provider

13. Is a prescription required? Yes

14. Is Foscarnet available in Pakistan? Rare; may be available in tertiary care hospitals

15. What precautions should be taken during use? Ensure adequate hydration; monitor renal function and electrolytes

16. What are contraindications? Severe renal impairment; hypersensitivity to foscarnet

17. What monitoring is needed during use? Renal function, serum electrolytes, CBC

18. Can it be used for CMV prophylaxis? Not routinely; reserved for treatment

19. What is the mechanism of action? Direct inhibition of viral DNA polymerase without requiring activation by viral enzymes

20. What are similar antivirals? Ganciclovir, valganciclovir, cidofovir