Toxicology – Selected Antiviral Drugs

Core Concept

This older chapter combines several pharmacologically unrelated antiviral drugs:

• Acyclovir / valacyclovir
• Famciclovir
• Amantadine / rimantadine
• Cidofovir
• Foscarnet
• Ganciclovir / valganciclovir
• Ribavirin

They do not produce a common antiviral toxidrome. Toxicity is best remembered by the characteristic organ injury of each drug:

• Acyclovir/valacyclovir → crystal nephropathy + neurotoxicity
• Amantadine → CNS + cardiac toxicity
• Cidofovir → proximal tubular nephrotoxicity/Fanconi syndrome
• Foscarnet → nephrotoxicity + major electrolyte disturbances
• Ganciclovir/valganciclovir → bone-marrow suppression + renal accumulation
• Ribavirin → hemolytic anemia

There is no universal antidote for this group.

1. Acyclovir and Valacyclovir

Acyclovir inhibits viral DNA polymerase after activation within infected cells.

Valacyclovir is the oral prodrug of acyclovir and has substantially greater oral bioavailability.

Famciclovir is similarly converted to the active antiviral penciclovir.

Acute uncomplicated oral overdose is often mild, but excessive systemic exposure—especially with renal dysfunction—can cause important renal and neurologic toxicity.

Acyclovir Crystal Nephropathy

Acyclovir has limited urinary solubility.

High tubular concentrations can result in:

Acyclovir precipitation → intratubular obstruction → crystal nephropathy → AKI

Risk is greatest with:

• High systemic exposure
• Rapid IV administration
• Dehydration
• Preexisting renal impairment
• Inadequate dose adjustment

Clinical Features of Acyclovir Nephrotoxicity

Possible findings include:

• Rising creatinine
• Reduced urine output
• Hematuria
• Crystalluria
• Flank discomfort
• AKI

Renal dysfunction then reduces acyclovir elimination, potentially increasing systemic exposure further.

Acyclovir/Valacyclovir Neurotoxicity

Accumulation can cause:

• Confusion
• Agitation
• Hallucinations
• Tremor
• Myoclonus
• Dysarthria
• Ataxia
• Somnolence
• Seizures
• Coma

The risk is particularly important in patients with renal impairment.

Acyclovir Neurotoxicity vs Viral Encephalitis

This distinction can be difficult because acyclovir is often being administered specifically for suspected CNS herpes infection.

Features favoring drug accumulation include:

• Development after acyclovir/valacyclovir exposure
• Renal dysfunction
• Myoclonus or prominent neuropsychiatric abnormalities
• Improvement after withdrawal and clearance of the drug

However, these findings are not sufficiently specific to exclude active viral encephalitis.

Clinical context, CSF studies, imaging, renal function, and treatment history must be integrated.

Acyclovir Management

Important measures include:

• Stop or appropriately reduce further exposure
• Correct dehydration when present
• Monitor renal function and urine output
• Avoid additional nephrotoxins
• Adjust future dosing for kidney function
• Treat neurologic complications supportively

Routine forced diuresis is inappropriate.

Hemodialysis and Acyclovir

Acyclovir is substantially dialyzable.

Hemodialysis can be useful in selected patients with:

• Severe renal failure
• Marked drug accumulation
• Severe or persistent neurotoxicity

It is not routinely required after a minor ingestion with normal renal function.

2. Famciclovir

Famciclovir is converted to penciclovir.

Acute overdose experience is limited, but uncomplicated exposures generally produce relatively mild effects such as:

• Nausea
• Headache
• GI discomfort
• Dizziness

Because active drug is eliminated renally, renal dysfunction can increase exposure.

Severe toxicity is considerably less characteristic than with drugs such as amantadine or foscarnet.

3. Amantadine

Amantadine deserves separate attention because its toxicity is substantially different from that of nucleoside antiviral agents.

Although historically used against influenza A, amantadine is no longer recommended for routine influenza treatment because of widespread viral resistance.

It remains clinically relevant primarily for neurologic indications such as Parkinsonian syndromes and drug-induced movement disorders.

Amantadine Toxicity

Amantadine overdose can produce severe:

• CNS excitation or depression
• Delirium
• Hallucinations
• Seizures
• Cardiac conduction abnormalities
• Ventricular dysrhythmias

It also has antimuscarinic-like properties.

Antimuscarinic Features

Possible findings include:

• Mydriasis
• Dry mouth
• Tachycardia
• Reduced bowel motility
• Urinary retention
• Agitation
• Hallucinations
• Hyperthermia

However, amantadine poisoning should not be reduced to a pure anticholinergic syndrome because cardiac and neurologic toxicity can dominate.

Cardiotoxicity

Severe amantadine poisoning can produce:

• Tachycardia
• QRS abnormalities
• QT abnormalities
• Ventricular ectopy
• Ventricular tachyarrhythmias
• Hypotension
• Cardiac arrest

Electrolyte disturbances can worsen electrical instability.

Hypokalemia

Hypokalemia has been described in severe amantadine poisoning and can contribute to ventricular dysrhythmias.

Therefore significant poisoning warrants careful monitoring of:

• Potassium
• Magnesium
• ECG

Amantadine and Renal Function

Amantadine is predominantly eliminated through the kidneys.

Renal impairment can therefore produce substantial accumulation even during apparently conventional treatment.

Older adults with declining kidney function are particularly vulnerable to:

• Hallucinations
• Confusion
• Myoclonus
• Delirium
• Other CNS toxicity

Amantadine Withdrawal

Abrupt discontinuation after chronic use can occasionally produce a withdrawal syndrome with:

• Delirium
• Agitation
• Worsening Parkinsonism

Rare severe syndromes resembling neuroleptic malignant syndrome have also been reported.

This is distinct from acute amantadine overdose.

Rimantadine

Rimantadine is another adamantane antiviral historically used for influenza A.

Because of widespread resistance, it is also no longer recommended for routine influenza therapy.

Its toxicity can include:

• CNS effects
• Antimuscarinic-like findings
• GI symptoms

Severe cardiac toxicity is generally more strongly associated with amantadine.

Physostigmine – Important Update

The older recommendation to consider physostigmine broadly for severe amantadine toxicity requires substantial caution.

Amantadine overdose can involve:

• QRS/conduction abnormalities
• Ventricular dysrhythmias
• Seizures

Therefore physostigmine is not a routine antidote.

It should only be considered in highly selected predominantly antimuscarinic delirium after ECG and seizure-risk assessment and with expert toxicology guidance.

Dialysis and Amantadine

Despite renal elimination, conventional hemodialysis removes relatively little amantadine because of its pharmacokinetic characteristics.

Thus the older claim that dialysis meaningfully lowers amantadine concentrations is misleading.

Management is primarily supportive.

4. Cidofovir

Cidofovir is a nucleotide analog historically important in treatment of selected severe viral infections, particularly CMV in immunocompromised patients.

Its defining toxicity is:

Dose-dependent proximal tubular injury

Cidofovir Nephrotoxicity

Proximal tubular dysfunction can produce:

• Rising creatinine
• Proteinuria
• Glycosuria despite normal blood glucose
• Phosphate wasting
• Bicarbonate wasting
• Electrolyte abnormalities
• Fanconi syndrome
• AKI

Cidofovir Risk Factors

Risk increases with:

• Preexisting renal dysfunction
• High exposure
• Other nephrotoxic drugs
• Inadequate renal-protective measures

Renal function and urinalysis are central to monitoring.

Probenecid and Cidofovir

Probenecid reduces active tubular uptake of cidofovir and is used with IV cidofovir to reduce nephrotoxicity.

Hydration is also part of standard nephroprotection.

However, probenecid should be understood primarily as a preventive component of prescribed cidofovir therapy, not as a universally established antidote after an overdose has already occurred.

5. Foscarnet

Foscarnet directly inhibits viral DNA polymerase without requiring intracellular phosphorylation.

Its major toxicities are:

• Nephrotoxicity
• Electrolyte disturbances

These can interact to produce severe neurologic and cardiac complications.

Foscarnet Nephrotoxicity

Foscarnet can cause:

• Rising creatinine
• Reduced GFR
• AKI
• Renal tubular dysfunction

Risk increases with:

• Dehydration
• High exposure
• Preexisting renal dysfunction
• Other nephrotoxins

Adequate hydration and renal dose adjustment are important during therapy.

Foscarnet Electrolyte Toxicity

Foscarnet can chelate divalent cations and disturb mineral metabolism.

Possible abnormalities include:

• Hypocalcemia
• Hypomagnesemia
• Hypokalemia
• Hypophosphatemia
• Sometimes hyperphosphatemia

Clinical Effects of Hypocalcemia

Patients may develop:

• Perioral tingling
• Paresthesias
• Muscle cramps
• Tetany
• Seizures
• QT prolongation
• Dysrhythmias

Ionized calcium can be particularly useful because total serum calcium may not accurately reflect the physiologically active fraction.

Foscarnet Monitoring

Significant toxicity warrants:

• Creatinine
• Potassium
• Magnesium
• Calcium, preferably ionized when clinically relevant
• Phosphate
• ECG

Correct clinically important electrolyte abnormalities carefully.

Dialysis and Foscarnet

Foscarnet can be removed to some extent by dialysis, but evidence supporting extracorporeal treatment specifically for overdose remains limited.

Renal replacement therapy should be considered according to:

• Renal failure
• Severe electrolyte abnormalities
• Conventional dialysis indications
• Overall clinical toxicity

6. Ganciclovir and Valganciclovir

Ganciclovir inhibits viral DNA synthesis and is particularly important against CMV.

Valganciclovir is its orally bioavailable prodrug and should be included in a modern discussion.

The defining toxicity is:

Bone-marrow suppression

Ganciclovir Hematologic Toxicity

Possible abnormalities include:

• Neutropenia
• Thrombocytopenia
• Anemia
• Pancytopenia

Severe neutropenia increases the risk of serious infection.

This toxicity is particularly important during prolonged therapeutic exposure.

Ganciclovir and Renal Function

Ganciclovir is substantially renally eliminated.

Renal impairment increases drug exposure and therefore increases the risk of:

• Marrow suppression
• Neurologic toxicity

Dose adjustment according to renal function is essential.

Ganciclovir Neurotoxicity

Less commonly, excessive exposure can cause:

• Confusion
• Tremor
• Seizures

A severe presentation warrants evaluation for renal accumulation and other neurologic causes.

Hemodialysis and Ganciclovir

Ganciclovir is dialyzable.

Dialysis may be useful in selected severe accumulation, particularly when renal failure is present.

Routine dialysis is not indicated after minor exposure.

7. Ribavirin

Ribavirin is a nucleoside analog with uses that have changed considerably since the older source.

Its most characteristic systemic toxicity is:

Hemolytic anemia

Ribavirin Hemolysis

Systemic ribavirin accumulates within erythrocytes and can produce oxidative/metabolic injury.

Possible findings include:

• Falling hemoglobin
• Fatigue
• Pallor
• Dyspnea
• Indirect hyperbilirubinemia
• Compensatory reticulocytosis

Patients with significant underlying cardiac disease may tolerate anemia poorly.

Aerosolized Ribavirin

The severe respiratory events described historically largely relate to specific clinical circumstances involving aerosolized therapy and severely ill infants.

They should not be generalized as the expected syndrome after an ordinary oral exposure.

Ribavirin and Pregnancy

Ribavirin has important embryotoxic and teratogenic potential.

Pregnancy-related precautions are particularly important because systemic ribavirin and its metabolites may persist for a prolonged period.

The old FDA pregnancy letter system is obsolete, but the reproductive risk remains clinically important.

Diagnosis

Because these drugs have very different toxicities, first identify the exact antiviral.

Acyclovir/valacyclovir

Focus on:

• Renal function
• Hydration
• Mental status

Amantadine

Focus on:

• Mental status
• ECG
• Potassium/magnesium
• Renal function

Cidofovir

Focus on:

• Creatinine
• Urinalysis
• Proximal tubular function

Foscarnet

Focus on:

• Renal function
• Calcium
• Magnesium
• Potassium
• Phosphate
• ECG

Ganciclovir/valganciclovir

Focus on:

• CBC
• Renal function

Ribavirin

Focus on:

• Hemoglobin
• Evidence of hemolysis

Laboratory Evaluation

Small uncomplicated exposures may require little or no testing.

For clinically important exposure, testing is agent specific rather than using one universal antiviral panel.

Possible studies include:

• CBC
• Glucose
• Electrolytes
• Creatinine/BUN
• Magnesium
• Calcium
• Phosphate
• Liver tests
• Urinalysis

ECG is particularly important for amantadine toxicity and significant foscarnet-associated electrolyte abnormalities.

Serum Antiviral Concentrations

Routine serum concentrations of these agents are generally not useful for emergency overdose management.

Clinical status, renal function, CBC, ECG, and electrolytes usually provide more actionable information.

Initial Management

General approach:

Airway/breathing → circulation → identify exact antiviral → determine renal function → ECG/electrolytes when indicated → evaluate CNS and hematologic toxicity → supportive care

Because several of these drugs are renally eliminated, renal function is particularly important.

GI Decontamination

Do not induce vomiting.

Ipecac has no modern role.

Routine gastric lavage is obsolete.

Activated charcoal may occasionally be considered after a substantial recent oral exposure when:

• The drug is adsorbable
• Airway protection is adequate
• Aspiration risk is acceptable
• Expected benefit justifies treatment

Many minor exposures need no GI decontamination.

Seizures

For toxicologic seizures:

Benzodiazepines are first-line.

Persistent seizures may require additional benzodiazepines, phenobarbital, or appropriate anesthetic therapy.

Always correct contributing abnormalities such as:

• Hypoglycemia
• Hypocalcemia
• Hypomagnesemia
• Hypokalemia
• Hypoxia
• Acid–base disturbance

This is particularly important with foscarnet.

Hyperthermia

If severe hyperthermia occurs:

• Control agitation/seizures
• Use active external cooling
• Correct dehydration and metabolic complications
• Monitor for rhabdomyolysis and organ injury

Antipyretics do not treat toxicologic hyperthermia unless fever from a hypothalamic set-point change is actually present.

The older term “malignant hyperthermia” should not be casually applied to amantadine toxicity.

Amantadine Dysrhythmias

Management should be based on:

• Rhythm
• QRS
• QT
• Potassium/magnesium
• Blood pressure
• Acid–base status

Avoid medications likely to worsen the existing conduction or repolarization abnormality.

The older blanket statement to avoid class IA drugs remains directionally reasonable, but modern management should be physiology- and ECG-directed rather than based on one antiarrhythmic class alone.

Enhanced Elimination – Summary

Acyclovir

Hemodialysis can meaningfully enhance elimination in selected severe accumulation.

Amantadine

Poorly removed by conventional hemodialysis; supportive care predominates.

Cidofovir

No routine extracorporeal antidotal strategy.

Foscarnet

Some dialysis removal is possible, but clinical overdose evidence is limited.

Ganciclovir

Dialyzable; may be useful in selected severe accumulation with renal dysfunction.

Ribavirin

Dialysis is not an effective routine antidotal strategy.

Monitoring and Observation

A universal 4–6 hour observation period is inappropriate.

Monitoring depends on:

• Exact antiviral
• Dose
• Route
• Symptoms
• Renal function
• ECG
• Electrolytes
• CBC
• Clinical trajectory

Some toxicities are delayed.

For example:

• Ganciclovir marrow suppression may evolve later.
• Ribavirin anemia is not excluded by a normal early hemoglobin.
• Acyclovir accumulation can persist when renal function is impaired.

Admission

Hospitalization may be appropriate for:

• Significant AKI
• Severe acyclovir neurotoxicity
• Amantadine delirium or seizures
• QRS/QT abnormalities
• Ventricular dysrhythmia
• Significant foscarnet electrolyte disturbances
• Symptomatic hypocalcemia
• Severe cytopenias
• Serious hemolytic anemia
• Persistent altered mental status
• Respiratory compromise
• Hemodynamic instability

ICU care is appropriate for malignant dysrhythmia, shock, status epilepticus, respiratory failure, or severe electrolyte-driven cardiac instability.

Pregnancy and Breastfeeding

The historical FDA pregnancy letter categories are obsolete.

Risk assessment should be drug specific.

Important considerations include:

• Maternal infection
• Gestational age
• Expected therapeutic benefit
• Alternative therapies
• Drug-specific fetal effects

Ribavirin remains particularly important because of its reproductive toxicity.

A blanket category-based approach should not replace contemporary drug-specific guidance.

Safeguarding

Rigid age cutoffs for assuming neglect, abuse, or intentional poisoning are outdated.

Assess pediatric exposures using:

• Developmental capability
• Access to medication
• Exposure circumstances
• Consistency of history
• Recurrent events
• Broader safeguarding concerns

Prognosis

Most minor antiviral exposures have favorable outcomes.

Important exceptions include:

• Severe amantadine cardiotoxicity
• Acyclovir/valacyclovir neurotoxicity with renal failure
• Persistent cidofovir-associated proximal tubular injury
• Severe foscarnet renal/electrolyte toxicity
• Profound ganciclovir-associated marrow suppression
• Clinically important ribavirin-associated hemolytic anemia

Outcome depends more on the specific antiviral and affected organ system than on the broad label “antiviral poisoning.”

Important Modernization of the Older Source

• These antivirals should not be treated as one toxicologic class.
• Valganciclovir belongs with ganciclovir in a modern discussion.
• Acyclovir/valacyclovir can cause crystal nephropathy plus neurotoxicity, especially with renal impairment.
• Hemodialysis can enhance acyclovir elimination in selected severe accumulation.
• Amantadine is no longer routinely used for influenza because of widespread resistance but remains relevant in neurologic practice.
• Amantadine toxicity includes CNS, antimuscarinic-like, and potentially severe cardiac effects.
• Renal impairment markedly increases amantadine toxicity.
• Physostigmine is not a routine amantadine antidote, particularly when conduction abnormalities or seizure risk exist.
• Conventional hemodialysis removes relatively little amantadine.
• Cidofovir → proximal tubular injury/Fanconi syndrome.
• Probenecid is principally preventive nephroprotection during cidofovir therapy, not a proven universal rescue antidote after overdose.
• Foscarnet → nephrotoxicity plus calcium, magnesium, potassium, and phosphate disturbances.
• Ionized calcium can be particularly informative during symptomatic foscarnet toxicity.
• Ganciclovir/valganciclovir → neutropenia, thrombocytopenia, anemia, with increased toxicity during renal impairment.
• Ribavirin → hemolytic anemia and important reproductive toxicity.
• The historical FDA pregnancy categories are obsolete.
• Ipecac and routine gastric lavage are obsolete.
• A fixed 4–6 hour observation period is inappropriate because several toxicities may be delayed.
• There is no universal antiviral antidote; management is agent-specific and complication-directed.

Key Points

• Acyclovir/valacyclovir → AKI/crystal nephropathy + neurotoxicity.
• Amantadine → delirium/seizures + potentially dangerous ventricular dysrhythmias.
• Cidofovir → proximal tubular injury/Fanconi syndrome.
• Foscarnet → AKI + major electrolyte disturbances, especially hypocalcemia and hypomagnesemia.
• Ganciclovir/valganciclovir → bone-marrow suppression.
• Ribavirin → hemolytic anemia.
• Renal dysfunction is a major amplifier of toxicity for several antiviral agents.
• Hemodialysis is useful for selected severe acyclovir or ganciclovir accumulation, but not as a universal antiviral treatment.
• There is no single specific antidote for this group.
• Treatment is primarily withdrawal of the offending drug, supportive care, correction of organ-specific complications, and appropriate renal dose adjustment.