PharmacologyTier 1Medical Sciences concept

Antiviral resistance patterns in chronic hepatitis B treatment

Core concept

HBV cannot be eradicated - cccDNA persists as a stable episome in the hepatocyte nucleus, and integrated HBV DNA persists in the genome
- -> nucleos(t)ide analogues suppress, they do not cure; relapse on cessation is the rule
Resistance is a function of two things: the genetic barrier of the drug, and the completeness of suppression
- HBV reverse transcriptase has no proofreading -> ~10^11 virions/day -> every single-point mutant pre-exists
- Incomplete suppression selects them. Full suppression does not.
*The resistance problem is therefore largely historical* - it belongs to lamivudine, adefovir and telbivudine, not to the current agents

Agent	5-year resistance	Signature mutation
Lamivudine	~70%	rtM204V/I (YMDD) +/- rtL180M
Telbivudine	~25%	rtM204I
Adefovir	~29%	rtN236T, rtA181V
Entecavir (NA-naive)	~1.2%	Requires M204 PLUS >=1 of rtT184/rtS202/rtM250
Entecavir (lamivudine-experienced)	~50%	M204 is a stepping stone - never use entecavir after lamivudine failure**
Tenofovir (TDF or TAF)	~0% - no confirmed clinical resistance	-

*Cross-resistance rule: L-nucleosides (lamivudine, telbivudine, entecavir) share a resistance pathway; tenofovir does not*
- Lamivudine or entecavir failure -> switch to TENOFOVIR
- Adefovir failure (rtA181/rtN236) -> entecavir
Primary non-response = <1 log10 drop at 12 weeks; partial response = detectable DNA at 48 weeks; virological breakthrough = >1 log10 rise from nadir on treatment
- *Check adherence FIRST* - non-adherence causes far more breakthrough than genotypic resistance

First-line: entecavir, tenofovir disoproxil (TDF) or tenofovir alafenamide (TAF) - all high-barrier
- TAF preferred if renal impairment, osteoporosis or age >60; TDF preferred in pregnancy (most safety data) and cheaper
- Entecavir dose-adjust in renal impairment; avoid after lamivudine exposure
- Pegylated interferon - finite 48-week course, no resistance, highest rate of HBsAg loss, but poorly tolerated; contraindicated in decompensated cirrhosis
2025 AASLD/IDSA broadened eligibility - treatment now offered in the indeterminate ("grey zone") phase by shared decision-making, favouring treatment if age >40, male, platelets <180, or significant fibrosis
*Immunosuppression and HBV reactivation*
- Screen HBsAg + anti-HBc before rituximab, anthracyclines, high-dose steroids, TNF inhibitors, transplant
- Anti-HBc positive + HBsAg negative can still reactivate - prophylactic entecavir/tenofovir with anti-CD20
HCC surveillance 6-monthly continues even with full viral suppression in cirrhosis, and in non-cirrhotic Asian men >40, Asian women >50, African >20, or family history

HIV coinfection - lamivudine, emtricitabine and tenofovir are active against both; never treat HBV with a single agent that is also an HIV drug in an untested patient - selects HIV resistance
HDV coinfection - requires HBsAg; bulevirtide (entry inhibitor) is the first specific agent
Hepatitis B in pregnancy - tenofovir from ~28 weeks if HBV DNA >200,000 IU/mL, plus infant HBIG and vaccine at birth
HCV - by contrast is curable with DAAs (no nuclear reservoir); HBV reactivation can occur during HCV DAA therapy
Resistance analogy: HIV (high genetic barrier of integrase inhibitors dolutegravir/bictegravir), influenza (oseltamivir H275Y), CMV (UL97 -> ganciclovir; UL56 -> letermovir)
Vaccine-escape mutants (sHBsAg G145R) - surface gene overlaps the polymerase gene, so polymerase mutations can alter surface antigen detection

3 more sections, plus exam facts

Premium unlocks every note across every specialty, and the full exam fact library behind it.