Antiviral Pharmacology

Correct Answer

D — Rituximab

Rationale

Rituximab is classified as a chimeric anti-CD20 monoclonal antibody. It targets the CD20 antigen expressed on B lymphocytes and depletes CD20-positive cells through antibody-dependent cellular cytotoxicity, complement-dependent cytotoxicity, and direct apoptosis induction. Natalizumab is an anti-alpha-4-integrin monoclonal antibody. Cidofovir is an acyclic nucleoside phosphonate antiviral. Valganciclovir is the oral L-valyl ester prodrug of ganciclovir.

Correct Answer

B — Cidofovir

Rationale

Cidofovir is classified as having antiviral activity against adenovirus, in addition to cytomegalovirus and other DNA viruses. Ganciclovir's spectrum is limited to herpesviruses and does not include adenovirus. Acyclovir is active against herpes simplex virus and varicella-zoster virus but not adenovirus. Oseltamivir is a neuraminidase inhibitor active against influenza and has no activity against DNA viruses such as adenovirus.

Correct Answer

A — Foscarnet

Rationale

Foscarnet is classified as having antiviral activity against human herpesvirus 6 (HHV-6), along with ganciclovir. Both agents are used clinically for HHV-6 disease. Ribavirin has no established antiviral activity against HHV-6. Oseltamivir is a neuraminidase inhibitor active only against influenza viruses. Acyclovir has insufficient activity against HHV-6B to be clinically useful, distinguishing HHV-6 from herpes simplex virus and varicella-zoster virus where acyclovir is effective.

Correct Answer

C — Valganciclovir

Rationale

Valganciclovir is the L-valyl ester oral prodrug of ganciclovir, classified as the standard oral agent for cytomegalovirus prophylaxis and pre-emptive therapy in solid organ and hematopoietic stem cell transplant recipients. After oral absorption it is hydrolyzed to ganciclovir, achieving plasma concentrations comparable to intravenous ganciclovir. Valacyclovir is the prodrug of acyclovir. Brincidofovir is the lipid conjugate prodrug of cidofovir. Famciclovir is the prodrug of penciclovir, used for herpes simplex virus and varicella-zoster virus infections.

Correct Answer

D — Natalizumab

Rationale

Natalizumab is classified as an anti-alpha-4-integrin monoclonal antibody. Alpha-4-integrin is an adhesion molecule required for lymphocyte trafficking across the blood-brain barrier and gut endothelium; natalizumab blocks this migration and is used for relapsing multiple sclerosis and Crohn's disease. Rituximab is a chimeric anti-CD20 monoclonal antibody. Palivizumab is a humanized monoclonal antibody that targets the respiratory syncytial virus fusion protein. Nirsevimab is a long-acting monoclonal antibody targeting a prefusion-specific epitope on the respiratory syncytial virus fusion protein.

Correct Answer

B — Brincidofovir

Rationale

Brincidofovir is classified as a lipid conjugate prodrug of cidofovir. The lipid ether linkage allows intracellular delivery via lipid transport pathways, substantially reducing proximal tubular cidofovir accumulation and the severe nephrotoxicity that limits cidofovir clinical use. Brincidofovir retains cidofovir's broad deoxyribonucleic acid virus activity including adenovirus, BK polyomavirus, cytomegalovirus, and orthopoxviruses. Valganciclovir is the L-valyl ester prodrug of ganciclovir. Valacyclovir is the L-valyl ester prodrug of acyclovir. Foscarnet is a pyrophosphate analogue, not a prodrug of cidofovir.

Correct Answer

A — In cytomegalovirus colitis, the virus replicates locally within the colonic mucosal epithelium without generating sufficient systemic viremia to be detected in peripheral blood; colonoscopy with biopsy is required to establish the diagnosis

Rationale

Cytomegalovirus colitis is a local end-organ manifestation in which the virus replicates within colonic mucosal epithelial cells and lamina propria macrophages. In many patients with isolated gastrointestinal cytomegalovirus disease, this local replication does not produce systemic viremia at levels detectable by peripheral blood polymerase chain reaction. Diagnosis therefore requires colonoscopy with biopsy demonstrating the characteristic cytomegalovirus cytopathic effect — enlarged cells with intranuclear and intracytoplasmic inclusions — confirmed by immunohistochemistry or in situ hybridization. Treatment of confirmed cytomegalovirus colitis is intravenous ganciclovir. Reliance on blood polymerase chain reaction alone would result in missed diagnoses and delayed treatment in this population.

Correct Answer

C — Cytomegalovirus pneumonitis in hematopoietic stem cell transplant recipients involves an immune-mediated inflammatory component; intravenous immunoglobulin provides immunomodulatory benefit beyond direct antiviral activity, which ganciclovir alone cannot address

Rationale

Cytomegalovirus pneumonitis in hematopoietic stem cell transplant recipients is partly a virally driven and partly an immune-mediated inflammatory process. The high mortality (30 to 50 percent despite treatment) reflects not only the viral infection but also the exuberant immune response in the lung during immune reconstitution. Intravenous immunoglobulin provides immunomodulatory effects — including neutralizing inflammatory mediators, modulating complement, and potentially providing passive anti-cytomegalovirus antibody — that address the immune-mediated component of lung injury. This is distinct from the direct antiviral mechanism of ganciclovir, which suppresses viral replication but cannot modulate the inflammatory injury. The survival benefit of adding intravenous immunoglobulin to ganciclovir is based on retrospective cohort data rather than randomized controlled trial evidence.

Correct Answer

D — Post-transplant lymphoproliferative disorder is driven by unconstrained proliferation of Epstein-Barr virus-infected B cells; rituximab depletes CD20-positive B lymphocytes including Epstein-Barr virus-infected B cells, eliminating the proliferating population

Rationale

In immunocompetent individuals, Epstein-Barr virus-specific cytotoxic T lymphocytes maintain tight surveillance over latently infected B cells. Transplant-related T-cell immunosuppression — calcineurin inhibitors, anti-thymocyte globulin, T-cell depletion — removes this surveillance, allowing Epstein-Barr virus-infected B cells to proliferate uncontrolled. Because Epstein-Barr virus-infected B cells express CD20, rituximab's anti-CD20 mechanism directly depletes this proliferating population. This is not an antiviral mechanism — rituximab does not inhibit Epstein-Barr virus replication — but a cellular depletion strategy targeting the cell type that Epstein-Barr virus exploits. Rituximab does not restore T-cell function or inhibit calcineurin.

Correct Answer

B — No antiviral has demonstrated robust efficacy for BK polyomavirus nephropathy; reducing immunosuppression allows partial recovery of antiviral T-cell immunity that can control BK polyomavirus replication more effectively than available drugs

Rationale

BK polyomavirus nephropathy has no antiviral agent with proven efficacy in prospective randomized trials. Cidofovir and leflunomide have been used empirically but without strong evidence of clinical benefit. The primary and most effective intervention is reduction of immunosuppression — typically by lowering calcineurin inhibitor target levels — which allows partial recovery of BK polyomavirus-specific T-cell immunity. This immune recovery can control viral replication more effectively than any currently available drug. The risk of this approach is increased rejection, which must be balanced against the risk of progressive nephropathy. Option D is inaccurate because cidofovir is used for BK polyomavirus in some centers despite nephrotoxicity concerns; the main reason for not using it routinely is lack of proven efficacy, not absolute contraindication.

Correct Answer

A — JC polyomavirus lacks the viral enzymes required for activation of currently available antivirals; immune reconstitution through antiretroviral therapy in HIV or immunosuppression reduction in other settings is the only intervention consistently associated with improved outcomes

Rationale

JC polyomavirus does not encode a thymidine kinase or UL97 equivalent, making it intrinsically resistant to acyclovir and ganciclovir. Cidofovir — which does not require viral enzyme activation — has in vitro activity against JC polyomavirus but has not demonstrated clinical benefit in controlled studies. Cytarabine and mirtazapine have also been tested without meaningful benefit. The only intervention consistently associated with improved outcomes in progressive multifocal leukoencephalopathy is immune reconstitution: in HIV-associated PML, optimizing antiretroviral therapy to suppress HIV and raise the CD4 count above 200 cells per cubic millimeter is the cornerstone of management; in non-HIV settings, reduction of immunosuppression when clinically feasible allows recovery of JC polyomavirus-specific T-cell immunity.

Correct Answer

C — Chromosomally integrated HHV-6 represents the full viral genome integrated into the germline of every nucleated cell; the very high stable polymerase chain reaction signal reflects germline deoxyribonucleic acid amplification rather than active viral replication, and antiviral therapy is not indicated

Rationale

In approximately 1 percent of the general population, the complete HHV-6 genome integrates into human germline chromosomes — chromosomally integrated HHV-6. Because every nucleated cell in the body contains a copy of this integrated genome, quantitative polymerase chain reaction testing of whole blood detects approximately 1 viral copy per host cell, producing very high and stable HHV-6 levels that can be mistaken for active replication. Chromosomally integrated HHV-6 is not associated with active viral disease and does not require antiviral therapy. Distinguishing chromosomally integrated HHV-6 from true active HHV-6 reactivation — which produces rising rather than stable viral loads — is essential to avoid inappropriate foscarnet therapy with its serious nephrotoxicity.

Correct Answer

D — HHV-6B encephalitis; cerebrospinal fluid HHV-6 polymerase chain reaction; intravenous foscarnet

Rationale

HHV-6B encephalitis is the classic opportunistic viral encephalitis in allogeneic hematopoietic stem cell transplant recipients during the first month post-transplant. Its distinctive clinical syndrome — seizures, anterograde amnesia, confusion, and bilateral hippocampal and limbic MRI T2 signal abnormalities — closely resembles autoimmune limbic encephalitis and should prompt immediate cerebrospinal fluid HHV-6 polymerase chain reaction in any HSCT recipient presenting with this pattern. Treatment is intravenous foscarnet for 3 to 6 weeks. Cytomegalovirus encephalitis typically produces periventricular rather than hippocampal changes. Herpes simplex virus encephalitis can affect the temporal lobes but is less common in this specific HSCT context and timing. Progressive multifocal leukoencephalopathy produces subcortical white matter lesions without the bilateral hippocampal pattern described.

Correct Answer

B — Natalizumab blocks alpha-4-integrin, preventing lymphocyte trafficking into the central nervous system and thereby removing the immune surveillance that normally contains JC polyomavirus in the brain

Rationale

Natalizumab is a monoclonal antibody that blocks alpha-4-integrin, an adhesion molecule required for lymphocyte migration across the blood-brain barrier. By preventing lymphocyte entry into the central nervous system, natalizumab depletes the JC polyomavirus-specific T-cell surveillance that normally constrains JC polyomavirus replication in oligodendrocytes. With immune surveillance removed, JC polyomavirus — which is present latently in the kidneys, bone marrow, and lymphoid tissue of most adults — can reach and productively infect oligodendrocytes. Management of natalizumab-associated progressive multifocal leukoencephalopathy centers on drug cessation to allow lymphocyte re-entry into the central nervous system and immune reconstitution, while monitoring for progressive multifocal leukoencephalopathy-immune reconstitution inflammatory syndrome, which can itself cause neurological deterioration.

Correct Answer

C — A negative blood cytomegalovirus polymerase chain reaction does not exclude cytomegalovirus colitis because local colonic mucosal replication frequently does not produce detectable systemic viremia; colonoscopy with biopsy is required to diagnose or exclude the condition

Rationale

Cytomegalovirus colitis is a local end-organ manifestation in which viral replication within colonic mucosal epithelial cells and lamina propria may not generate sufficient viremia for detection in peripheral blood. Blood cytomegalovirus polymerase chain reaction is frequently undetectable or low-level in isolated gastrointestinal cytomegalovirus disease, making it an unreliable tool for ruling out this diagnosis. Colonoscopy with mucosal biopsy demonstrating cytomegalovirus cytopathic effect confirmed by immunohistochemistry is the diagnostic standard. Relying on a negative blood polymerase chain reaction to exclude cytomegalovirus colitis in an immunocompromised patient with compatible symptoms risks delayed diagnosis and missed treatment with intravenous ganciclovir.

Correct Answer

A — Reduce immunosuppression, because no antiviral has proven efficacy for BK polyomavirus nephropathy and allowing partial recovery of antiviral T-cell immunity is the most effective approach to controlling viral replication

Rationale

A plasma BK polyomavirus viral load above 10,000 copies per milliliter is the threshold used in most transplant programs to trigger intervention before nephropathy is established by biopsy. The primary intervention is reduction of immunosuppression — typically by lowering calcineurin inhibitor trough targets — which allows partial recovery of BK polyomavirus-specific T-cell immunity. No antiviral agent has demonstrated robust efficacy in prospective randomized trials for this indication. Increasing immunosuppression would worsen viral replication. Valganciclovir has no activity against BK polyomavirus because BK polyomavirus does not encode UL97 or a thymidine kinase equivalent to activate ganciclovir. While cidofovir has in vitro activity against BK polyomavirus, it lacks proven clinical efficacy and carries nephrotoxicity risk that is particularly problematic in this patient population.

Correct Answer

D — Administer pre-emptive rituximab, because Epstein-Barr virus-infected B cells express CD20 and rituximab depletion of this population can prevent post-transplant lymphoproliferative disorder from developing in patients with rising viremia

Rationale

Rising Epstein-Barr virus viral load in a high-risk hematopoietic stem cell transplant recipient — particularly those who received T-cell-depleted grafts or anti-thymocyte globulin conditioning — is an indication for pre-emptive rituximab before frank post-transplant lymphoproliferative disorder develops. Epstein-Barr virus establishes latency in B cells, and the CD20 antigen is expressed on Epstein-Barr virus-infected B cells. Rituximab depletion of this population controls the unconstrained B-cell proliferation that would otherwise progress to lymphoma. Antiviral agents such as ganciclovir, foscarnet, and acyclovir have no meaningful role in Epstein-Barr virus-driven post-transplant lymphoproliferative disorder because Epstein-Barr virus in latently infected B cells does not rely on viral deoxyribonucleic acid polymerase activity that these drugs target. Increasing immunosuppression would worsen Epstein-Barr virus replication.

Correct Answer

B — Initiate antiretroviral therapy promptly to suppress HIV and restore CD4 count, because immune reconstitution is the only intervention consistently associated with improved outcomes in HIV-associated progressive multifocal leukoencephalopathy; monitor for progressive multifocal leukoencephalopathy-immune reconstitution inflammatory syndrome

Rationale

In HIV-associated progressive multifocal leukoencephalopathy, prompt initiation or optimization of antiretroviral therapy to suppress HIV replication and restore CD4 count above 200 cells per cubic millimeter is the only intervention consistently associated with improved neurological outcomes and survival. No antiviral agent — including cidofovir, cytarabine, or mirtazapine — has demonstrated clinical benefit in controlled studies for progressive multifocal leukoencephalopathy. Progressive multifocal leukoencephalopathy-immune reconstitution inflammatory syndrome occurs in 10 to 30 percent of patients initiating antiretroviral therapy with active progressive multifocal leukoencephalopathy, producing transient neurological deterioration as immune function recovers, but this risk does not outweigh the benefit of antiretroviral therapy. Deferring antiretroviral therapy in a patient with CD4 of 30 would leave progressive multifocal leukoencephalopathy without any effective intervention.