Antiviral Drugs — Herpes, HIV, Influenza

Herpesvirus Replication and Drug Mechanisms

• Viral DNA synthesis and thymidine kinase dependency: HSV (types 1 and 2) and varicella-zoster virus (VZV) replicate through a well-characterized molecular pathway exploitable by nucleoside analogs. The virus encodes its own thymidine kinase (TK) enzyme, which phosphorylates acyclovir and related nucleosides to their monophosphate form. Cellular kinases then catalyze further phosphorylation to the active triphosphate. This acyclovir triphosphate acts as an obligate chain terminator because it lacks the 3'-OH group necessary for DNA chain elongation. Critically, the viral DNA polymerase has higher affinity for acyclovir triphosphate than for deoxythymidine triphosphate (dTTP), ensuring selectivity. TK-deficient viral mutants escape this mechanism and drive resistance; this is clinically important in immunocompromised patients receiving prolonged therapy.

• Valacyclovir and famciclovir as prodrugs: Both agents are ester prodrugs that undergo first-pass hepatic metabolism (valacyclovir by esterases and aldehyde dehydrogenase; famciclovir by deamination and oxidation) to yield acyclovir and penciclovir, respectively. Valacyclovir achieves 3-5 fold higher serum acyclovir levels than oral acyclovir due to improved bioavailability (~54% vs 15-20%), allowing less frequent dosing and better CNS penetration. Famciclovir similarly improves penciclovir bioavailability. These pharmacokinetic advantages make them preferred for most indications. The viral thymidine kinase requirement remains essential for all three agents' antiviral activity.

• Mechanism of viral DNA polymerase inhibition: Once phosphorylated to the triphosphate form, acyclovir and penciclovir inhibit viral DNA polymerase through two mechanisms: (1) competitive inhibition of substrate (dTTP) binding and (2) irreversible chain termination following incorporation into nascent viral DNA. The viral polymerase is 10-30 fold more sensitive to acyclovir triphosphate inhibition compared to host DNA polymerases (α, δ, and ε), accounting for the selective toxicity. Importantly, this selectivity is relative, not absolute—high doses can inhibit host cell mitochondrial DNA synthesis and cause toxicity.

HIV Replication Cycle and Antiretroviral Targets

• Reverse transcription as critical vulnerability: HIV, an enveloped retrovirus, enters CD4+ T cells via gp120/gp41-mediated fusion. The viral reverse transcriptase (RT) then synthesizes a DNA copy from the viral RNA genome—a step with no cellular counterpart and thus ideal for selective inhibition. Nucleoside/nucleotide reverse transcriptase inhibitors (NRTIs/NtRTIs) (e.g., tenofovir, emtricitabine) function identically to herpesvirus nucleoside analogs: phosphorylation to triphosphates, competitive inhibition of dTTP binding, and chain termination. Non-nucleoside reverse transcriptase inhibitors (NNRTIs) (e.g., efavirenz, rilpivirine) bind allosterically to a hydrophobic pocket near the active site of RT, causing conformational distortion and direct inhibition without requiring phosphorylation. This allosteric mechanism explains why resistance emerges rapidly (single mutations can confer 100-1000 fold resistance) and why NNRTIs cannot be used as monotherapy.

• Protease inhibition and virion maturation: Following reverse transcription, the viral RNA integrates into the host genome via integrase. Translation of integrated proviral DNA yields a polyprotein precursor that must be cleaved by HIV protease into functional subunits (gag, pol, env proteins) during virion budding. Protease inhibitors (PIs) (e.g., ritonavir, darunavir, atazanavir) competitively inhibit this aspartic protease, producing immature, non-infectious virions. The high genetic barrier to resistance (multiple mutations required) makes PIs powerful agents, though they carry significant drug interactions due to cytochrome P450 (CYP3A4) metabolism.

• Integrase strand transfer inhibition: Integrase inhibitors (INSTIs) (e.g., dolutegravir, bictegravir) block the catalytic activity of integrase during the strand transfer step, preventing proviral DNA from integrating into the host chromosome. This blocks the establishment of persistent infection. INSTIs have a higher genetic barrier to resistance than NNRTIs and excellent CNS penetration; they are increasingly used as first-line agents.

• Entry and fusion inhibition: Fusion inhibitors (e.g., enfuvirtide) bind to gp41 and prevent conformational changes necessary for virion-host cell fusion, blocking early infection. These are reserved for treatment-experienced patients with multidrug-resistant virus due to high cost and injection route (except ibalizumab, an intravenous monoclonal antibody to CD4).

Influenza Virus Replication and Antiviral Targets

• Neuraminidase function and sialidase inhibition: Influenza viruses (types A, B, and C) possess eight gene segments encoding structural and non-structural proteins. The surface protein neuraminidase (NA) functions as a viral sialidase, cleaving sialic acid receptors on the host cell surface to release newly assembled virions and prevent virion aggregation. Neuraminidase inhibitors (oseltamivir, zanamivir, peramivir, baloxavir) mimic the sialic acid structure and competitively bind to the active site of NA with remarkable selectivity, blocking virion release. The high structural conservation of the NA active site across influenza strains results in broad activity and a moderate genetic barrier to resistance (typically multiple mutations needed). Oseltamivir and zanamivir are FDA-approved; baloxavir marboxil (a prodrug activated by serum esterase) represents a newer single-dose option that inhibits cap-dependent endonuclease within the viral nucleocomplex, a novel mechanism with potential for rapid resistance emergence.

• M2 channel inhibitors (adamantanes): The viral M2 protein functions as an ion channel essential for viral uncoating in the endosome after entry. Adamantanes (amantadine and rimantadine) block this channel, preventing acidification-mediated disruption of the viral ribonucleoprotein complex. However, widespread resistance (>95% of circulating influenza A H3N2 strains) limits their current utility; they are rarely used clinically. The genetic barrier to resistance is low (single L26F mutation confers resistance), explaining the rapid emergence of resistant strains in the 1990s-2000s.

• Polymerase complex dysfunction from cap-snatching inhibition: Baloxavir's mechanism—inhibition of viral endonuclease and blocking cap-snatching (a process where the virus steals 5' caps from host pre-mRNAs to prime viral mRNA synthesis)—represents a fundamentally different approach from neuraminidase inhibition, potentially useful in neuraminidase-inhibitor resistant strains.

Herpes Simplex Virus (HSV) Infection Requiring Antivirals

• Primary and recurrent HSV-1 and HSV-2 infection: Herpes simplex virus establishes latency in sensory nerve ganglia after primary mucosal infection. Approximately 67% of the global population is seropositive for HSV-1 (typically orolabial); HSV-2 prevalence varies geographically (13-27% in developed nations, higher in resource-limited settings). Recurrences are triggered by stress, immunosuppression, fever, or UV exposure. Antiviral treatment is indicated for primary genital herpes, severe recurrent HSV (>6 episodes per year), immunocompromised patients (transplant recipients, advanced HIV), HSV encephalitis, neonatal herpes, and HSV-associated severe mucocutaneous disease.

• Varicella-zoster virus (chickenpox and herpes zoster): VZV causes varicella in primary infection and establishes dorsal root ganglion latency. Reactivation in older adults (particularly age >50) or immunocompromised hosts produces herpes zoster (shingles). Risk factors for severe zoster include age >60, immunosuppression, malignancy, and HIV with CD4 <50 cells/μL. Antiviral therapy reduces duration of acute pain, post-herpetic neuralgia (PHN), and visceral complications.

• Cytomegalovirus (CMV) disease in immunocompromised hosts: While not a primary focus, ganciclovir and valganciclovir are crucial for CMV retinitis and other manifestations in advanced HIV (CD4 <50 cells/μL) or organ transplant recipients. These agents require viral protein kinase UL97 (not cellular thymidine kinase) for activation, permitting activity against TK-deficient HSV mutants.

HIV Infection Epidemiology and Transmission Risk

• High-risk behaviors and populations: Sexual transmission (heterosexual, homosexual) accounts for >80% of new HIV infections globally; needle-sharing among people who inject drugs (PWID) and mother-to-child transmission (MTCT) are additional routes. Men who have sex with men (MSM), female sex workers, transgender persons, and incarcerated individuals face disproportionate risk. Plasma viral load is the strongest predictor of transmission risk; undetectable viral load (goal <50 copies/mL) achieved with ART is equivalent to non-transmissible (U=U: undetectable = untransmittable), justifying early initiation of therapy.

• Advanced immunosuppression without ART: CD4+ T cell depletion (reflecting viral-mediated destruction and impaired thymic output) increases risk for opportunistic infections (OIs). The threshold CD4 count defining advanced HIV disease is <50 cells/μL (risk for CMV disease, disseminated MAC) and <200 cells/μL (risk for Pneumocystis pneumonia, toxoplasma encephalitis). Immediate ART initiation is warranted regardless of CD4 count.

Influenza Susceptibility and Seasonal Transmission

• Seasonal epidemiology and high-risk groups: Influenza typically peaks in temperate climates during winter months (December-February in Northern Hemisphere), though it circulates year-round in tropics. Antiviral therapy is most beneficial within 48 hours of symptom onset (before viral shedding declines) but can be considered up to 5 days in hospitalized or severely ill patients. High-risk groups for severe influenza include age >65, chronic medical conditions (cardiovascular, diabetes, lung disease), pregnancy, immunosuppression, and residents of long-term care facilities. Neuraminidase inhibitors reduce disease duration by 1-2 days if initiated early and may prevent complications in hospitalized patients.

• Pandemic and zoonotic strains: Novel influenza strains (e.g., H5N1 avian influenza, H1N1 pandemic strain) pose pandemic risk. Antiviral resistance is a major concern; 2007-2008 saw worldwide resistance to adamantanes, and oseltamivir resistance has emerged in scattered cases (H1N1 and H3N2), requiring surveillance and consideration of neuraminidase inhibitor resistance mechanisms when choosing therapy.

Herpes Simplex Virus Disease

• Primary genital herpes: Presents 2-10 days after exposure with dysuria, painful vesicular eruption, inguinal lymphadenopathy, and systemic symptoms (fever, myalgia). Lesions progress from macules to papules to fluid-filled vesicles (highly infectious, >100 virions per vesicle) over 7-10 days, then crust and heal over 2-4 weeks. Meningitis or myelitis can occur. Cervicitis and prostatitis may develop. Antiviral therapy reduces symptom duration by 2-3 days and viral shedding by 5-7 days.

• Recurrent HSV: Preceded by prodrome (tingling, burning) at the affected site, recurrent episodes are typically milder and shorter (7-10 days) than primary infection. Recurrent HSV shedding is intermittent and asymptomatic in 50% of seropositive individuals. Suppressive antiviral therapy (continuous acyclovir, valacyclovir, or famciclovir) reduces recurrence frequency by 70-80% and transmission risk by >50% in HSV-discordant couples.

• Herpes zoster: Unilateral dermatomal rash with burning pain (dermatomal distribution following dermatome of latently infected ganglion), preceded by 1-3 days of hyperesthesia. Lesions evolve similarly to primary HSV but typically resolve in 2-4 weeks. Post-herpetic neuralgia (PHN)—persistent pain in the affected dermatome—affects 10-50% of patients age >50 and can persist for months to years. Antiviral therapy reduces PHN incidence and duration, particularly when initiated within 72 hours.

• HSV encephalitis: Presents with fever, altered mental status, seizures, and focal neurologic deficits (often temporal lobe involvement, explaining anomia, behavioral changes). CSF shows lymphocytic pleocytosis with elevated protein and normal glucose. PCR detection of HSV DNA in CSF is diagnostic (sensitivity 95%, specificity 100%). This is a medical emergency; antiviral therapy must be initiated on clinical suspicion before confirmed diagnosis, as delayed treatment increases mortality and morbidity.

HIV Infection Presentations

• Acute retroviral syndrome (ARS): Occurs 2-4 weeks post-infection in 40-90% of new infections; resembles infectious mononucleosis (fever, pharyngitis, rash, lymphadenopathy, malaise). Viral load is extremely high (often >100,000 copies/mL), and CD4 count may transiently decline. Mucocutaneous ulcers, meningitis, and hepatosplenomegaly can occur. Recognition is critical as early ART initiation during ARS limits viral reservoir seeding, reduces long-term CD4 depletion, and may preserve immune function.

• Asymptomatic chronic HIV: Without treatment, most patients remain asymptomatic for 5-10 years (though viral replication continues). CD4 count declines ~50 cells/μL per year; viral load slowly increases. Symptoms emerge when CD4 <200 cells/μL.

• AIDS-defining illnesses:
• Opportunistic infections (CD4 <200): Pneumocystis jirovecii pneumonia (PCP) with subacute dyspnea and elevated LDH; toxoplasma encephalitis (CD4 <100) with focal CNS lesions; disseminated MAC (CD4 <50) with fever, diarrhea, and hepatosplenomegaly
• AIDS-defining malignancies: Kaposi sarcoma (HHV-8 coinfection), diffuse large B-cell lymphoma, invasive cervical cancer
• Severe immunosuppression manifestations: Cryptococcal meningitis, CMV retinitis, esophageal candidiasis, progressive multifocal leukoencephalopathy (PML) from JC virus

• Immune reconstitution inflammatory syndrome (IRIS): Occurs 2-12 weeks after ART initiation in patients with very low CD4 counts. Paradoxical worsening of OI symptoms or unmasking of previously subclinical infections (e.g., MAC, CMV, tuberculosis

Herpesvirus agents

• Acyclovir/valacyclovir — crystalline nephropathy: Acyclovir is poorly soluble and precipitates in renal tubules when infused rapidly or in volume-depleted patients, producing obstructive crystal nephropathy with birefringent needle-shaped crystals in urine. Prevention (not reversal) is the point: adequate IV hydration, slow infusion, and renal dose adjustment. There is no antidote; acyclovir is dialyzable.
• Acyclovir neurotoxicity: Accumulation in renal impairment causes tremor, myoclonus, confusion, and hallucinations. Distinguish from HSV encephalitis itself — the clue is rising creatinine.
• Ganciclovir/valganciclovir: Dose-limiting myelosuppression (neutropenia, thrombocytopenia) from inhibition of host DNA synthesis; requires serial CBC monitoring. Additive marrow toxicity with zidovudine.
• Foscarnet: Nephrotoxicity plus chelation-mediated electrolyte derangement — hypocalcemia, hypomagnesemia, hypokalemia, hypo- or hyperphosphatemia — with seizures as the feared consequence. Treat symptomatic hypocalcemia with IV calcium; saline-load before dosing.
• Cidofovir: Proximal tubular toxicity; given with probenecid (blocks tubular uptake) and saline prehydration.

Antiretrovirals (per the DHHS/HHS Adult and Adolescent Antiretroviral Guidelines)

• NRTIs: Inhibition of mitochondrial DNA polymerase-γ causes lactic acidosis with hepatic steatosis, myopathy, and (older agents) peripheral neuropathy and pancreatitis. Zidovudine causes macrocytic anemia and neutropenia.
• Abacavir: HLA-B*57:01–restricted hypersensitivity; DHHS mandates genotyping before use, and rechallenge after a reaction can be fatal.
• Tenofovir disoproxil fumarate: Proximal tubulopathy (Fanconi syndrome) and reduced bone mineral density; monitor renal function and phosphate. Tenofovir alafenamide has lower renal/bone toxicity.
• Protease inhibitors: Insulin resistance, dyslipidemia, lipodystrophy; ritonavir/cobicistat are potent CYP3A4 inhibitors. Atazanavir causes benign indirect hyperbilirubinemia (UGT1A1 inhibition).
• Efavirenz: Vivid dreams, dizziness, mood changes; INSTIs: weight gain and insomnia.

Influenza agents (IDSA seasonal influenza guideline)

• Oseltamivir: Nausea/vomiting, taken with food; neuropsychiatric events reported in adolescents.
• Inhaled zanamivir: Bronchospasm — contraindicated in asthma/COPD.
• Amantadine: CNS effects, anticholinergic symptoms, livedo reticularis; renally cleared.

• Acyclovir crystal nephropathy is the classic renal stem: A patient on high-dose IV acyclovir with rising creatinine and needle-shaped urinary crystals. Single best next step: aggressive IV hydration, slow the infusion rate, and dose-adjust for renal function — not switching antivirals.
• Suspected HSV encephalitis = empiric IV acyclovir now: Per IDSA encephalitis guidance, start before CSF PCR returns. Delay increases mortality. The common distractor is "await PCR" or "obtain MRI first" — imaging and LP should not postpone the first dose.
• Acyclovir resistance means thymidine kinase deficiency: In a transplant or advanced-HIV patient with progressive mucocutaneous HSV despite acyclovir, switch to foscarnet, which inhibits viral DNA polymerase directly and requires no viral kinase activation. Watch for hypocalcemia and seizures.
• **HLA-B*57:01 before abacavir, always**: The examiners test this as a pharmacogenomic screening question. Never rechallenge after a hypersensitivity reaction.
• Two benign lab changes you should not act on: Atazanavir causes indirect hyperbilirubinemia (UGT1A1 inhibition) — not hemolysis or hepatotoxicity. Dolutegravir (and cobicistat) raise serum creatinine by blocking tubular creatinine secretion without lowering true GFR — do not stop the drug.
• CMV retinitis toxicity pairing: Ganciclovir → myelosuppression; foscarnet → nephrotoxicity and electrolyte wasting; cidofovir → nephrotoxicity requiring probenecid. Match the toxicity to the agent.
• Influenza timing: Neuraminidase inhibitors work best within 48 hours of symptom onset, but the IDSA seasonal influenza guideline recommends treating hospitalized, severely ill, or high-risk patients even beyond that window — and starting empirically without waiting for test confirmation.
• Two influenza distractors: Inhaled zanamivir is the wrong choice in asthma/COPD (bronchospasm), and adamantanes (amantadine/rimantadine) are not recommended for influenza because of near-universal resistance in circulating strains per CDC surveillance.