Toxicology – HIV Reverse Transcriptase Inhibitors

Core Concept

Reverse transcriptase inhibitors are major components of combination antiretroviral therapy (ART) for HIV.

The older source focuses on:

Nucleoside/nucleotide reverse transcriptase inhibitors – NRTIs

• Zidovudine
• Lamivudine
• Didanosine
• Stavudine
• Zalcitabine

Modern practice also commonly includes:

• Tenofovir
• Emtricitabine
• Abacavir

Non-nucleoside reverse transcriptase inhibitors – NNRTIs

• Nevirapine

Other NNRTIs include:

• Efavirenz
• Etravirine
• Rilpivirine
• Doravirine

Several drugs emphasized in the older chapter—particularly zalcitabine, stavudine, and didanosine—are now obsolete or rarely used because safer antiretroviral regimens are available.

Acute isolated overdose is usually mild, but clinically important toxicity can result from mitochondrial injury, hepatic injury, metabolic acidosis, marrow suppression, pancreatitis, renal injury, hypersensitivity, or agent-specific CNS/cardiac effects.

Mechanism of Antiviral Action

Reverse transcriptase converts viral RNA into DNA.

NRTIs

After intracellular activation, NRTIs mimic normal nucleosides/nucleotides.

Their incorporation into viral DNA interferes with continued DNA synthesis and inhibits reverse transcription.

NNRTIs

NNRTIs bind directly to HIV-1 reverse transcriptase at an allosteric site and alter enzyme function.

They do not require incorporation into viral DNA.

Why Older NRTIs Cause Mitochondrial Toxicity

Several older NRTIs inhibit human mitochondrial DNA polymerase-γ to varying degrees.

This can impair mitochondrial DNA replication and oxidative phosphorylation.

The resulting toxicity may include:

• Lactic acidosis
• Hepatic steatosis
• Peripheral neuropathy
• Myopathy
• Pancreatitis

The older “d-drugs”—particularly didanosine and stavudine—have substantially greater mitochondrial toxicity than most modern NRTIs.

Acute Overdose

A single acute ingestion of many NRTIs produces limited immediate toxicity.

Possible symptoms include:

• Nausea
• Vomiting
• Diarrhea
• Headache
• Fatigue
• Dizziness
• Somnolence

Serious toxicity is more often associated with:

• Chronic treatment
• Repeated dosing errors
• Drug interactions
• Renal/hepatic impairment
• Older, more toxic NRTIs

1. Zidovudine – AZT

Zidovudine was one of the earliest effective antiretroviral drugs.

Its most important toxicities include:

• Bone-marrow suppression
• Anemia
• Neutropenia
• Myopathy
• Mitochondrial toxicity
• Lactic acidosis in severe chronic toxicity

Zidovudine Bone-Marrow Toxicity

Clinically important findings may include:

• Macrocytic anemia
• Neutropenia
• Leukopenia

Macrocytosis is common during zidovudine treatment and does not by itself establish dangerous toxicity.

Severe anemia or neutropenia requires assessment of both the medication and other potential causes.

Zidovudine Myopathy

Long-term exposure can produce mitochondrial skeletal-muscle injury.

Possible manifestations include:

• Proximal weakness
• Myalgia
• Exercise intolerance
• Elevated CK in some patients

This is primarily a chronic toxicity rather than the expected consequence of an isolated overdose.

2. Lamivudine – 3TC

Lamivudine generally has a relatively favorable toxicity profile.

Acute overdose is usually mild.

Possible effects include:

• Nausea
• Vomiting
• Headache
• Fatigue
• GI discomfort

Serious mitochondrial toxicity is much less characteristic than with didanosine or stavudine.

Lamivudine and Renal Function

Lamivudine is substantially renally eliminated.

Reduced kidney function can increase exposure, so dosing may require adjustment depending on the formulation and treatment regimen.

Lamivudine and Hepatitis B

Lamivudine also has activity against hepatitis B virus (HBV).

An important clinical issue is that stopping HBV-active therapy in a person with HBV infection can lead to hepatitis exacerbation.

Therefore, abnormal liver tests after discontinuation should not automatically be interpreted as direct lamivudine hepatotoxicity.

3. Didanosine – ddI

Didanosine is now rarely used because of significant toxicity.

Important adverse effects include:

• Pancreatitis
• Peripheral neuropathy
• Lactic acidosis
• Hepatic injury
• Noncirrhotic portal hypertension

Didanosine Pancreatitis

Pancreatitis was one of the most important dose-limiting complications.

Possible findings include:

• Severe epigastric pain
• Nausea/vomiting
• Elevated lipase
• Systemic inflammatory complications in severe disease

Lipase is generally more useful than amylase alone when pancreatitis is suspected.

Didanosine Peripheral Neuropathy

Mitochondrial toxicity can produce a painful, predominantly distal sensory neuropathy.

Symptoms include:

• Burning
• Tingling
• Numbness
• Distal pain

Toxicity may be increased when combined with other neurotoxic medications.

Didanosine Hepatic/Vascular Toxicity

Chronic didanosine exposure has been associated with noncirrhotic portal hypertension.

This may lead to:

• Splenomegaly
• Thrombocytopenia
• Esophageal varices
• Portal hypertensive bleeding

This is an important later-recognized toxicity absent from many older references.

4. Stavudine – d4T

Stavudine is another older NRTI now rarely used because of substantial mitochondrial toxicity.

Major complications include:

• Peripheral neuropathy
• Lactic acidosis
• Hepatic steatosis
• Pancreatitis
• Lipodystrophy

Stavudine and Lactic Acidosis

Stavudine is among the NRTIs most strongly associated with mitochondrial dysfunction and severe hyperlactatemia.

Possible manifestations include:

• Progressive weakness
• Nausea/vomiting
• Abdominal discomfort
• Dyspnea
• Tachypnea
• Hepatic dysfunction
• High anion-gap metabolic acidosis
• Elevated lactate

Severe cases may progress to multiorgan dysfunction.

5. Zalcitabine – ddC

Zalcitabine is an obsolete antiretroviral agent.

Its historical toxicities included:

• Painful peripheral neuropathy
• Pancreatitis
• Oral/esophageal ulceration
• Hepatic injury
• Cytopenias

It has essentially no role in contemporary HIV therapy.

6. Tenofovir

Tenofovir is highly relevant to modern ART.

Two major prodrug formulations are:

• Tenofovir disoproxil fumarate – TDF
• Tenofovir alafenamide – TAF

TDF produces greater systemic tenofovir exposure and has greater renal and bone toxicity than TAF.

Tenofovir Nephrotoxicity

Tenofovir can injure proximal renal tubular cells.

Possible manifestations include:

• Rising creatinine
• Proteinuria
• Normoglycemic glycosuria
• Phosphate wasting
• Hypophosphatemia
• Bicarbonate wasting
• Fanconi syndrome
• AKI

Long-term tubular phosphate wasting may contribute to bone disease.

TDF vs TAF

TAF generally has less renal and bone toxicity than TDF because it achieves lower circulating tenofovir concentrations while delivering active drug intracellularly.

This distinction is important when evaluating modern antiretroviral toxicity.

7. Emtricitabine – FTC

Emtricitabine is structurally related to lamivudine and is generally well tolerated.

Acute toxicity is usually limited.

A characteristic benign adverse effect during therapy can be:

• Skin hyperpigmentation, particularly involving palms or soles

Like lamivudine and tenofovir, it also has activity against HBV, making treatment interruption relevant in patients with hepatitis B.

8. Abacavir

Abacavir has an important distinctive toxicity:

Potentially severe hypersensitivity reaction

Risk is strongly associated with HLA-B*57:01.

Abacavir Hypersensitivity

Possible manifestations include combinations of:

• Fever
• Rash
• Malaise
• GI symptoms
• Respiratory symptoms

The reaction can worsen rapidly with continued administration.

Patients are therefore screened for HLA-B*57:01 before starting abacavir.

Critical Abacavir Principle

If true abacavir hypersensitivity is suspected:

Abacavir should be stopped and must not be restarted.

Rechallenge can provoke a rapid, potentially life-threatening reaction.

This is not an overdose phenomenon but is one of the most important safety issues involving an NRTI.

9. Nevirapine

Nevirapine is an NNRTI.

Acute overdose experience is limited, and isolated exposure often produces relatively mild effects.

More important therapeutic toxicities are:

• Hepatotoxicity
• Severe cutaneous hypersensitivity

Nevirapine Hepatotoxicity

Nevirapine can cause clinically significant hepatitis, sometimes associated with systemic hypersensitivity.

Possible findings include:

• Fatigue
• Nausea
• Abdominal discomfort
• Elevated transaminases
• Jaundice
• Severe hepatic injury

Risk is particularly important during the early phase of treatment.

Nevirapine Severe Cutaneous Reactions

Possible reactions include:

• Maculopapular rash
• SJS
• TEN
• DRESS-like systemic hypersensitivity

Warning findings include:

• Painful skin
• Blistering
• Mucosal involvement
• Facial edema
• Fever
• Eosinophilia
• Internal-organ involvement

These require immediate drug cessation and urgent evaluation.

10. Efavirenz

Efavirenz is an NNRTI with characteristic CNS and psychiatric adverse effects.

Possible manifestations include:

• Dizziness
• Abnormal dreams
• Insomnia
• Impaired concentration
• Ataxia
• Confusion
• Hallucinations

These effects are most prominent early during treatment.

11. Rilpivirine

Rilpivirine generally has a favorable toxicity profile but can affect cardiac repolarization at excessive exposure.

Potential concern:

• QT prolongation

Risk increases with interacting medications or other QT-prolonging drugs.

Mitochondrial Toxicity Syndrome

The classic severe NRTI toxicity syndrome consists of:

Mitochondrial dysfunction → impaired oxidative phosphorylation → lactate accumulation ± hepatic steatosis

Possible findings include:

• Malaise
• Weakness
• Nausea
• Vomiting
• Abdominal pain
• Weight loss
• Dyspnea/tachypnea
• Hepatomegaly
• Metabolic acidosis

This is much more strongly associated with older NRTIs than with most current regimens.

Lactic Acidosis

Suspected severe mitochondrial toxicity warrants assessment of:

• Lactate
• Electrolytes
• Bicarbonate
• Anion gap
• Blood gas when clinically indicated
• Glucose
• Renal function
• Liver function

Marked lactate elevation should not automatically be attributed to an NRTI.

Also consider:

• Sepsis
• Shock
• Seizures
• Hypoxia
• Metformin
• Cyanide
• Other mitochondrial toxins

Peripheral Neuropathy

The older NRTIs most strongly associated with toxic neuropathy include:

• Didanosine
• Stavudine
• Zalcitabine

Symptoms are usually:

• Distal
• Symmetric
• Sensory
• Painful

Modern NRTIs have a substantially lower neuropathy burden.

Pancreatitis

Historically important agents include:

• Didanosine
• Stavudine

If pancreatitis is suspected, evaluate:

• Symptoms
• Lipase
• Hydration
• Electrolytes
• Organ dysfunction

Other causes of pancreatitis should also be considered.

Hepatotoxicity

Liver injury can occur through several different mechanisms:

• Direct drug toxicity
• Mitochondrial toxicity
• Hypersensitivity
• HBV flare after withdrawal of HBV-active therapy
• Drug interactions
• Underlying viral hepatitis

The mechanism therefore matters when interpreting elevated liver enzymes.

Bone-Marrow Toxicity

Most strongly associated historically with zidovudine.

Monitor for:

• Anemia
• Neutropenia
• Other cytopenias

CBC abnormalities in a person with HIV have a broad differential and should not automatically be attributed to ART.

Drug Interactions

Reverse transcriptase inhibitors vary considerably in interaction potential.

NNRTIs may induce or inhibit CYP enzymes, while many NRTIs have relatively fewer CYP-mediated interactions.

Always review:

• Complete ART regimen
• Antimicrobials
• Antiseizure medications
• Psychiatric medications
• Cardiovascular drugs
• Supplements
• Recreational substances

Modern fixed-dose combination tablets make identification of every active ingredient particularly important.

Diagnosis

Determine:

• Exact drug or combination product
• Amount
• Timing
• Acute vs chronic/repeated exposure
• Renal function
• Hepatic function
• Other ART
• Coingestants
• Drug interactions
• HBV coinfection when relevant

Symptoms may overlap with HIV itself, opportunistic infections, and adverse effects from other medications.

Laboratory Evaluation

Minor asymptomatic acute exposures may need little investigation.

For significant or symptomatic exposure, testing can include:

• CBC
• Glucose
• Electrolytes
• Bicarbonate
• BUN/creatinine
• Liver tests

Depending on the suspected agent or syndrome:

• Lactate
• Blood gas
• Lipase
• CK
• Phosphate
• Urinalysis

ECG

Routine cardiac monitoring is not required for every NRTI exposure.

Obtain an ECG when there is:

• Syncope
• Palpitations
• Significant overdose
• Electrolyte disturbance
• Suspected QT-active NNRTI
• Relevant coingestant

The older recommendation for universal cardiac monitoring of nucleoside analog toxicity is unnecessarily broad.

Serum Drug Concentrations

Routine serum NRTI/NNRTI concentrations generally do not guide acute overdose treatment.

Management is based primarily on:

• Clinical status
• Acid–base findings
• Renal/hepatic function
• CBC
• Agent-specific complications

Initial Management

General approach:

Airway/breathing → circulation → identify every ART component → evaluate renal/hepatic function → assess metabolic toxicity → identify coingestants/interactions → supportive care

Most isolated acute ingestions require conservative treatment.

GI Decontamination

Do not induce vomiting.

Ipecac is obsolete.

Routine gastric lavage is obsolete.

Activated charcoal may occasionally be considered after a clinically important recent ingestion when the expected benefit exceeds aspiration risk and the airway is safe.

Routine decontamination is unnecessary for most minor exposures.

Seizures

Seizures are uncommon with uncomplicated NRTI overdose.

If they occur:

Benzodiazepines are first-line.

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

Correct:

• Hypoglycemia
• Hypoxia
• Electrolyte disturbances
• Severe acid–base abnormalities

Also investigate coingestants and CNS disease.

Hypotension

Significant hypotension is not a characteristic uncomplicated reverse transcriptase inhibitor toxidrome.

If present, consider:

• Dehydration
• Sepsis
• Lactic acidosis
• Anaphylaxis/hypersensitivity
• Coingestants
• Other medication toxicity

Use isotonic fluid when appropriate.

Persistent vasodilatory shock generally favors norepinephrine rather than routine dopamine-first treatment.

Trendelenburg positioning is outdated.

Enhanced Elimination

There is no universal role for extracorporeal removal.

Dialyzability varies substantially between drugs.

The older suggestion that didanosine dialysis should routinely be used after overdose is not supported as a general management strategy.

Renal replacement therapy remains appropriate for conventional indications such as:

• Severe renal failure
• Refractory electrolyte disturbance
• Severe acid–base disturbance

Agent-specific toxicology guidance should determine whether extracorporeal drug removal adds meaningful benefit.

No Universal Antidote

There is no general antidote for NRTI or NNRTI poisoning.

Management consists of:

Stop further exposure + supportive care + correct metabolic abnormalities + manage agent-specific organ toxicity

Observation and Follow-Up

A universal observation period is inappropriate.

Acute symptoms may resolve quickly, while other toxicities can be delayed or result from cumulative treatment.

Follow-up may require:

• CBC
• Liver tests
• Renal function
• Lactate
• Lipase
• Neurologic examination

depending on the implicated drug.

Admission

Hospitalization may be appropriate for:

• Significant lactic acidosis
• Hepatic failure
• Pancreatitis
• Severe cytopenias
• AKI/Fanconi syndrome
• Severe peripheral neurologic toxicity
• Seizures
• Persistent altered mental status
• SJS/TEN or systemic hypersensitivity
• Hemodynamic instability

ICU care is appropriate for severe metabolic acidosis, shock, respiratory failure, status epilepticus, fulminant hepatic failure, or multiorgan dysfunction.

Pregnancy and Breastfeeding

The historical FDA pregnancy letter categories are obsolete.

Modern HIV care strongly emphasizes maintaining effective ART during pregnancy because viral suppression protects both:

• Maternal health
• The fetus/newborn from perinatal HIV transmission

Drug selection depends on:

• Current treatment guidelines
• Resistance profile
• Previous ART
• Viral suppression
• Drug interactions
• Pregnancy pharmacokinetics
• Maternal/fetal safety data

Older blanket Category B/C descriptions should not guide modern therapy.

Safeguarding

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

Assess pediatric exposures according to:

• Developmental ability
• Access to medication
• Circumstances
• Consistency of history
• Recurrent unexplained exposures
• Broader safeguarding concerns

Prognosis

Most isolated acute NRTI/NNRTI overdoses have a favorable outcome.

Serious morbidity is more likely from:

• Mitochondrial lactic acidosis
• Severe hepatotoxicity
• Pancreatitis
• Profound marrow suppression
• Severe hypersensitivity
• Renal tubular injury
• Chronic peripheral neuropathy

The older NRTIs account for many of the historically severe mitochondrial complications and are now rarely used.

Important Modernization of the Older Source

• Modern reverse transcriptase inhibitors include many agents absent from the historical chapter, especially tenofovir, emtricitabine, and abacavir.
• Didanosine, stavudine, and zalcitabine are obsolete or rarely used because of toxicity.
• NRTI mitochondrial toxicity results partly from inhibition of mitochondrial DNA polymerase-γ.
• Older NRTIs can cause lactic acidosis, hepatic steatosis, neuropathy, myopathy, and pancreatitis.
• Zidovudine → anemia/neutropenia and chronic myopathy.
• Didanosine → pancreatitis, neuropathy, mitochondrial toxicity, and noncirrhotic portal hypertension.
• Stavudine → neuropathy, lactic acidosis, hepatic steatosis, and lipodystrophy.
• Tenofovir, particularly TDF → proximal tubular injury/Fanconi syndrome and bone toxicity.
• TAF generally has less renal and bone toxicity than TDF.
• Abacavir → HLA-B*57:01-associated hypersensitivity; suspected true hypersensitivity means no rechallenge.
• Nevirapine → severe hepatotoxicity and serious cutaneous hypersensitivity.
• Efavirenz is particularly associated with CNS/neuropsychiatric adverse effects.
• Rilpivirine can contribute to QT prolongation at excessive exposure.
• Stopping lamivudine, emtricitabine, or tenofovir in a patient with HBV can precipitate a hepatitis B flare.
• Universal cardiac monitoring is unnecessary for uncomplicated NRTI exposure.
• Serum antiretroviral concentrations rarely guide acute overdose management.
• Ipecac and routine gastric lavage are obsolete.
• Trendelenburg and routine dopamine-first shock management are outdated.
• Routine dialysis is not a universal treatment for these drugs.
• Historical FDA pregnancy categories are obsolete.
• Modern ART toxicity must be interpreted in the context of fixed-dose combinations, drug interactions, HIV itself, and coinfections.

Key Points

• Reverse transcriptase inhibitors are divided into NRTIs and NNRTIs.
• Acute isolated overdose is usually relatively mild; important toxicity is often agent-specific or cumulative.
• Older NRTIs → mitochondrial toxicity, including lactic acidosis, neuropathy and hepatic injury.
• Zidovudine → marrow suppression.
• Didanosine → pancreatitis + neuropathy.
• Stavudine → neuropathy + lactic acidosis.
• Tenofovir → proximal renal tubular toxicity/Fanconi syndrome, especially TDF.
• Abacavir → potentially life-threatening hypersensitivity; never rechallenge after true hypersensitivity.
• Nevirapine → hepatotoxicity + severe cutaneous reactions.
• There is no universal specific antidote.
• Management is primarily supportive, agent-specific, and guided by metabolic, hepatic, renal, hematologic, and neurologic complications.