What are the side effects of tenofovir and how should renal and bone toxicity be monitored and managed?

Tenofovir Side Effects: Renal and Bone Toxicity

Tenofovir disoproxil fumarate (TDF) causes renal dysfunction and bone mineral density loss, while tenofovir alafenamide (TAF) has significantly lower rates of these toxicities and should be preferred in patients with risk factors. 1

Primary Side Effects

Renal Toxicity

TDF causes proximal renal tubular dysfunction that manifests as:

• Decreased eGFR (median decline of -5.0 to -7.8 mL/min over 96 weeks) 1
• Acute renal failure and Fanconi syndrome (hypophosphatemia, glycosuria, proteinuria) 2, 3
• Hypophosphatemia (<2.5 mg/dL) 1
• Proteinuria/albuminuria (urine albumin:creatinine ratio >30 mg/g) 1

The mechanism involves mitochondrial toxicity affecting proximal tubular cells 4. This toxicity is dose-dependent and usually reversible if caught early 5.

Bone Toxicity

TDF causes:

• Decreased bone mineral density in spine and hip (reductions of -3.29% and -1.12% respectively over 48-96 weeks) 1
• Osteomalacia with bone pain 4
• Increased fracture risk in long-term users 6

The pathophysiology involves hyperphosphaturia from tubular dysfunction, disrupting bone-kidney-hormone interplay and leading to progressive bone loss 7.

Other Side Effects

• Lactic acidosis and severe hepatomegaly with steatosis (rare but potentially fatal) 3
• Hepatitis B flare upon discontinuation 3
• Gastrointestinal symptoms (nausea, diarrhea) - generally mild 3

Monitoring Protocol

Before Starting TDF 2

• Serum creatinine and calculate eGFR
• Serum phosphorus
• Urine glucose
• Urine protein (spot protein:creatinine or albumin:creatinine ratio preferred; dipstick acceptable in resource-limited settings)
• Bone mineral density (if risk factors present)

During TDF Treatment 2

• First 6 months: Monitor renal function every 3 months
• After 6 months: Monitor at least annually
• High-risk patients: Monitor every 3-6 months

High-risk patients include those with:

• Baseline eGFR <60 mL/min/1.73 m²
• Age >50 years
• Hypertension, diabetes, cardiovascular disease
• Concurrent nephrotoxic medications (NSAIDs, boosted protease inhibitors)
• Proteinuria or hypophosphatemia 1

TAF Monitoring

TAF requires similar baseline assessment but less frequent monitoring due to lower toxicity profile 1, 8.

Management Algorithm

When to Avoid TDF (Use TAF, Entecavir, or Besifovir Instead) 1

Treatment-naïve patients with:

• eGFR <60 mL/min/1.73 m²
• Proteinuria or urine albumin:creatinine >30 mg/g
• Hypophosphatemia (<2.5 mg/dL)
• Osteopenia or osteoporosis
• Chronic steroid use
• Age >50 years with other risk factors

When to Switch from TDF 1

Immediate switch indicated if:

• eGFR decreases by >25% AND drops to <50 mL/min/1.73 m²
• New or worsening proteinuria/albuminuria
• Hypophosphatemia develops
• Bone mineral density significantly decreases
• Fanconi syndrome develops

Switch to:

• TAF (preferred - most data supporting reversibility) 1
• Entecavir (alternative, especially if prior resistance concerns) 1
• Besifovir (alternative in some regions) 1

Recovery After Switching

Switching from TDF to TAF results in:

• Renal function improvement: eGFR increases by +4.2 to +7.3 mL/min within 48-144 weeks 1
• Bone density recovery: BMD returns to baseline by week 240, with improvements visible at 48 weeks 1

Similar improvements occur with switches to entecavir or besifovir 1, 9.

Dose Adjustments for Renal Impairment 1

TDF dosing:

• CrCl ≥50: 300 mg every 24 hours
• CrCl 30-49: 300 mg every 48 hours
• CrCl 10-29: 300 mg every 72-96 hours
• CrCl <10 with dialysis: 300 mg every 7 days after dialysis
• CrCl <10 without dialysis: Not indicated

TAF dosing:

• CrCl ≥15: 25 mg every 24 hours
• CrCl <15 without dialysis: Not indicated

TAF vs TDF: Key Differences

TAF advantages 1, 8, 10:

• 91% lower plasma tenofovir concentrations
• Minimal impact on eGFR (median change -0.3 mL/min vs -5.0 mL/min for TDF)
• Minimal bone density loss (+1.23% vs -3.29% for TDF)
• Equal antiviral efficacy
• 0% resistance rate at 5 years

TAF considerations 10, 11:

• Higher lipid levels than TDF (may require statin therapy)
• Greater weight gain than TDF
• More expensive (though this gap is narrowing)

Critical Pitfalls to Avoid

1. Don't ignore mild eGFR declines: Even a 10% decrease warrants closer monitoring 12
2. Don't use TDF with ritonavir-boosted protease inhibitors in patients with renal risk factors - this combination significantly increases tenofovir levels 1, 13
3. Don't stop monitoring after initial stability: Tubular dysfunction is progressive and cumulative with time on TDF 7
4. Don't delay switching: Early intervention allows for reversibility; advanced dysfunction may be permanent 12, 6
5. Don't forget hepatitis B flare risk: Monitor closely for 3-6 months after any TDF discontinuation 3

The evidence strongly supports preferring TAF over TDF in treatment-naïve patients and switching from TDF to TAF in those who develop toxicity or have risk factors, as this approach optimizes both efficacy and safety. 1, 8