Antiviral Pharmacology

Correct Answer

B — Tenofovir disoproxil fumarate and tenofovir alafenamide

Rationale

Tenofovir disoproxil fumarate and tenofovir alafenamide are classified as the preferred first-line nucleos(t)ide analogues for chronic hepatitis B virus infection, based on their high resistance barriers and potent suppression of hepatitis B virus replication. Entecavir is an acceptable alternative with an equivalent resistance barrier. Lamivudine and adefovir select resistance mutations in a substantial proportion of patients after several years of monotherapy and have been superseded. Peginterferon alfa-2a is an immune-based therapy, not a nucleos(t)ide analogue, and ribavirin has no activity against hepatitis B virus.

Correct Answer

D — Entecavir

Rationale

Entecavir is classified as a guanosine nucleoside analogue. Tenofovir disoproxil fumarate and tenofovir alafenamide are acyclic phosphonate nucleotide analogues. Adefovir dipivoxil is also an acyclic phosphonate nucleotide analogue. The nucleoside versus nucleotide analogue distinction and the specific nucleoside base are classification vocabulary for these hepatitis B virus agents.

Correct Answer

A — Sofosbuvir

Rationale

Sofosbuvir is classified as an NS5B nucleotide analogue chain terminator. It targets the NS5B ribonucleic acid-dependent ribonucleic acid polymerase, the enzyme responsible for copying the hepatitis C virus genome. Velpatasvir is an NS5A inhibitor. Glecaprevir and grazoprevir are NS3/4A serine protease inhibitors. These target classifications — NS5B, NS5A, and NS3/4A — are the three drug class labels organizing all approved hepatitis C virus direct-acting antiviral agents.

Correct Answer

C — Glecaprevir/pibrentasvir

Rationale

Glecaprevir/pibrentasvir is classified as the preferred hepatitis C virus direct-acting antiviral regimen in severe renal impairment, including in patients on hemodialysis, because neither component is renally eliminated — both are primarily eliminated via biliary excretion. This biliary elimination classification distinguishes glecaprevir/pibrentasvir from sofosbuvir-containing regimens, which are not recommended in patients with estimated glomerular filtration rate below 30 mL/min due to sofosbuvir metabolite accumulation.

Correct Answer

A — Peginterferon alfa-2a

Rationale

Peginterferon alfa-2a is classified as the only immune-based therapy approved for chronic hepatitis B virus infection. Unlike nucleos(t)ide analogues, which suppress viral replication only during treatment, peginterferon can induce durable off-treatment responses including hepatitis B e antigen seroconversion and, rarely, hepatitis B surface antigen loss. Entecavir, tenofovir alafenamide, and lamivudine are all nucleos(t)ide analogues that act by inhibiting hepatitis B virus reverse transcriptase; they are antiviral rather than immune-based therapies.

Correct Answer

D — NS5A inhibitors

Rationale

Velpatasvir and pibrentasvir are classified as NS5A inhibitors. NS5A is a multifunctional phosphoprotein essential for hepatitis C virus ribonucleic acid replication and virion assembly. NS5A inhibitors are the most potent target class in the hepatitis C virus drug armamentarium. NS3/4A serine protease inhibitors include glecaprevir, voxilaprevir, and grazoprevir. Sofosbuvir is the NS5B nucleotide analogue chain terminator. Non-nucleoside NS5B allosteric inhibitors are an older class not in current frontline regimens.

Correct Answer

C — Hepatitis B virus maintains a stable intranuclear reservoir of covalently closed circular deoxyribonucleic acid that persists despite suppression of viral replication and is not targeted by any currently approved agent

Rationale

The fundamental barrier to hepatitis B virus cure is covalently closed circular deoxyribonucleic acid (cccDNA), a stable episomal form of the viral genome that persists in hepatocyte nuclei independently of cytoplasmic viral replication. Nucleos(t)ide analogues act on hepatitis B virus reverse transcriptase in the cytoplasm and do not degrade or eliminate cccDNA. When therapy is stopped, cccDNA serves as a template for viral rebound. No approved agent directly targets cccDNA. Hepatitis B virus does integrate into the host genome, but this integrated form is incapable of producing infectious virus; the cccDNA reservoir — not chromosomal integration — is the primary barrier to cure.

Correct Answer

B — Lamivudine resistance mutations reduce the number of additional mutations needed for high-level entecavir resistance from several to only one or two, substantially lowering the genetic barrier

Rationale

Entecavir has a high resistance barrier in treatment-naive patients, with fewer than 1% developing resistance through five years. However, in patients with pre-existing lamivudine resistance mutations, the genetic barrier to entecavir resistance is dramatically reduced: the lamivudine resistance mutations serve as a partial scaffold, and only one or two additional mutations are needed to produce high-level entecavir resistance. This is why entecavir should not be used in patients with known lamivudine or telbivudine resistance; tenofovir disoproxil fumarate or tenofovir alafenamide, which retain full activity regardless of lamivudine resistance, are the appropriate choices in this setting.

Correct Answer

A — Rifampin induces P-glycoprotein, increasing efflux of sofosbuvir from intestinal epithelial cells and substantially reducing sofosbuvir plasma concentrations

Rationale

Sofosbuvir is a substrate of P-glycoprotein, an efflux transporter expressed in intestinal epithelial cells that pumps substrates back into the gut lumen. Rifampin is a potent inducer of P-glycoprotein; co-administration increases P-glycoprotein-mediated efflux, substantially reducing sofosbuvir absorption and plasma concentrations to levels that may be subtherapeutic. Other P-glycoprotein inducers — carbamazepine and St. John's Wort — carry the same contraindication with sofosbuvir-containing regimens. Rifampin is a CYP3A4 inducer, not an inhibitor; option C incorrectly describes it as an inhibitor.

Correct Answer

D — The covalently closed circular deoxyribonucleic acid reservoir persists in hepatocyte nuclei and continues to serve as a template for hepatitis B surface antigen production even during viral suppression

Rationale

Covalently closed circular deoxyribonucleic acid (cccDNA) is the stable intranuclear template that drives hepatitis B surface antigen transcription. Even when nucleos(t)ide analogues effectively suppress hepatitis B virus replication in the cytoplasm, cccDNA in the nucleus continues to be transcribed, producing hepatitis B surface antigen and sub-viral particles. Eliminating cccDNA — or silencing it through immune-mediated epigenetic mechanisms — is required for hepatitis B surface antigen loss. While chromosomal integration also contributes to hepatitis B surface antigen production, this plays a secondary role; cccDNA clearance is the primary limiting step. No approved agent directly targets or degrades cccDNA.

Correct Answer

B — Glecaprevir inhibits hepatic uptake transporters, reducing hepatic rosuvastatin clearance and raising systemic rosuvastatin concentrations to levels associated with myopathy risk

Rationale

Glecaprevir inhibits hepatic uptake transporters — including organic anion transporting polypeptides — that are responsible for delivering rosuvastatin into hepatocytes for metabolism and biliary clearance. Inhibition of these transporters reduces rosuvastatin elimination, dramatically raising systemic rosuvastatin plasma concentrations and increasing the risk of myopathy and rhabdomyolysis. Rosuvastatin is contraindicated with glecaprevir/pibrentasvir. Other statins such as pravastatin and fluvastatin may be used at reduced doses; atorvastatin should not exceed 20 mg daily with glecaprevir/pibrentasvir. This interaction is transporter-mediated, not cytochrome P450-mediated, distinguishing it from boosted protease inhibitor statin interactions.

Correct Answer

C — Hepatitis C virus suppresses hepatitis B virus replication through interferon-stimulated gene signaling; elimination of hepatitis C virus by direct-acting antiviral therapy removes this suppression, allowing hepatitis B virus rebound

Rationale

In patients co-infected with hepatitis B virus and hepatitis C virus, hepatitis C virus typically dominates virologically and suppresses hepatitis B virus replication through interferon-stimulated gene signaling. When direct-acting antiviral therapy eliminates hepatitis C virus, this interferon-mediated suppression of hepatitis B virus dissipates, allowing hepatitis B virus replication to rebound from the cccDNA reservoir. This paradoxical reactivation occurs in approximately 1 to 2 percent of HBsAg-positive patients starting direct-acting antiviral therapy without concurrent hepatitis B virus-active treatment, with rare cases of acute liver failure documented. All patients initiating hepatitis C virus direct-acting antiviral therapy must be screened for HBsAg and anti-HBc; HBsAg-positive patients require concomitant hepatitis B virus-active therapy throughout treatment and for a defined period afterward.

Correct Answer

A — Entecavir has sufficient anti-HIV activity to exert selective pressure on HIV reverse transcriptase, selecting resistance mutations that compromise future lamivudine and emtricitabine efficacy

Rationale

Entecavir has intrinsic anti-HIV activity — sufficient to exert selective pressure on HIV reverse transcriptase but insufficient to suppress HIV replication. When used as monotherapy in an HIV-positive patient without full antiretroviral suppression, this sub-suppressive anti-HIV activity selects for the M184V resistance mutation in HIV reverse transcriptase. M184V confers high-level resistance to lamivudine and emtricitabine, two of the most important backbone agents in antiretroviral therapy. This makes entecavir monotherapy in HIV/hepatitis B virus co-infected patients a pharmacological hazard that can permanently compromise antiretroviral options. All HIV-positive patients requiring hepatitis B virus treatment must receive a complete suppressive antiretroviral regimen that includes a hepatitis B virus-active agent such as tenofovir plus emtricitabine or lamivudine.

Correct Answer

D — Sofosbuvir is renally eliminated and its inactive metabolite accumulates at low estimated glomerular filtration rate levels; glecaprevir/pibrentasvir, which is eliminated by biliary excretion, is the preferred alternative in severe renal impairment including dialysis

Rationale

Sofosbuvir undergoes extensive intracellular metabolism to its active triphosphate form in hepatocytes, and the primary circulating metabolite is an inactive dephosphorylated nucleoside that is renally eliminated. In patients with estimated glomerular filtration rate below 30 mL per minute, this metabolite accumulates and its safety at these concentrations is not well characterized in clinical trials. Sofosbuvir-containing regimens are therefore not recommended in this population. Glecaprevir/pibrentasvir, whose components are eliminated by biliary excretion without meaningful renal elimination, is the preferred pangenotypic regimen in severe renal impairment and dialysis, achieving sustained virological response rates above 98% in this population.

Correct Answer

B — Concomitant hepatitis B virus-active therapy should be initiated because eliminating hepatitis C virus removes the interferon-mediated suppression of hepatitis B virus, risking reactivation and potentially fatal hepatic injury

Rationale

In hepatitis B virus and hepatitis C virus co-infected patients, hepatitis C virus suppresses hepatitis B virus replication through interferon-stimulated gene signaling. When direct-acting antiviral therapy eliminates hepatitis C virus, this viral interference dissipates, allowing hepatitis B virus rebound from cccDNA despite an initially undetectable hepatitis B virus deoxyribonucleic acid level. Rare cases of acute liver failure from hepatitis B virus reactivation during hepatitis C virus direct-acting antiviral therapy have been documented. All HBsAg-positive patients must receive concomitant hepatitis B virus-active antiviral therapy — typically tenofovir disoproxil fumarate or tenofovir alafenamide — throughout hepatitis C virus treatment and for at least 12 weeks afterward. Peginterferon alfa-2a is not the appropriate concurrent therapy in this scenario.

Correct Answer

A — Glecaprevir/pibrentasvir is the preferred regimen because both components are eliminated by biliary excretion without renal elimination, allowing standard dosing in dialysis patients

Rationale

Glecaprevir and pibrentasvir are both eliminated via biliary excretion and do not rely on renal elimination. This pharmacokinetic profile makes glecaprevir/pibrentasvir safe at standard doses in patients with severe renal impairment, including those on hemodialysis, achieving sustained virological response rates above 98% in this population. Sofosbuvir's inactive metabolite accumulates at estimated glomerular filtration rate below 30 mL per minute and sofosbuvir-based regimens are not recommended in this setting. Peginterferon and ribavirin carry their own toxicity burden and are not preferred when direct-acting antiviral alternatives are available. Dose reduction is not an established strategy for sofosbuvir in dialysis patients.

Correct Answer

D — The M204I lamivudine resistance mutation serves as a partial scaffold for entecavir resistance, reducing the number of additional mutations needed for high-level entecavir resistance from several to only one or two

Rationale

Entecavir has a high genetic resistance barrier in treatment-naive patients — fewer than 1% develop resistance through five years of therapy. However, the M204I and M204V lamivudine resistance mutations dramatically reduce this barrier by serving as a partial scaffold: in their presence, only one or two additional mutations (typically at residues L180M and I169T or T184 or S202 or M250) are needed to produce high-level entecavir resistance. The virus effectively starts partway up the mutational pathway to entecavir resistance. Tenofovir disoproxil fumarate or tenofovir alafenamide retain full activity regardless of lamivudine resistance mutations and are the appropriate choices after lamivudine failure. Options A, B, and C do not reflect the actual pharmacological relationship between these agents.

Correct Answer

C — Genotype 3 infection with cirrhosis achieves only approximately 88 to 91 percent sustained virological response with sofosbuvir/velpatasvir, below the greater-than-95 percent threshold; adding voxilaprevir improves outcomes in this challenging subgroup

Rationale

Genotype 3 hepatitis C virus with compensated cirrhosis is the most challenging subgroup in contemporary hepatitis C virus therapy. Sofosbuvir/velpatasvir achieves sustained virological response in approximately 88 to 91 percent of these patients — below the greater-than-95 percent threshold achieved in other populations. Adding voxilaprevir, a pangenotypic NS3/4A protease inhibitor, creates a three-class combination targeting NS5B, NS5A, and NS3/4A simultaneously. Sofosbuvir/velpatasvir/voxilaprevir for 12 weeks is the preferred approach in NS5A inhibitor-naive genotype 3 patients with cirrhosis. The rationale for the triplet is suboptimal doublet efficacy in this specific population, not pharmacokinetic rescue or resistance pre-emption. Options A, B, and D do not reflect the actual basis for this treatment decision.