NAPLEX Antiviral Practice Questions with Answers

Concept Explanation

NAPLEX antiviral therapy involves the clinical application of pharmacological agents designed to inhibit viral replication, necessitating a deep understanding of mechanisms, dosing, and toxicity profiles for herpesviruses, HIV, hepatitis, and influenza. Mastering these drugs requires clinicians to categorize agents by their specific viral targets and understand the nuances of drug-drug interactions, which are particularly prevalent in antiretroviral therapy. For comprehensive preparation, students should utilize NAPLEX Prep resources to integrate these concepts into their broader clinical knowledge base.

Antiviral agents are broadly categorized by the virus they treat. Key classes include:

• Herpes Simplex/Varicella-Zoster: Acyclovir, valacyclovir, and famciclovir. These require renal dose adjustments.
• Cytomegalovirus (CMV): Ganciclovir and valganciclovir, which carry significant risks of myelosuppression.
• Influenza: Neuraminidase inhibitors like oseltamivir, zanamivir, and baloxavir.
• Hepatitis B/C: Nucleoside/nucleotide reverse transcriptase inhibitors (NRTIs) for HBV; direct-acting antivirals (DAAs) for HCV.
• HIV: Complex regimens involving NRTIs, NNRTIs, PIs, INSTIs, and CCR5 antagonists.

Pharmacists must prioritize safety, such as monitoring for nephrotoxicity with cidofovir or bone marrow suppression with ganciclovir. As noted by the Centers for Disease Control and Prevention, timely administration of antivirals is critical for improving patient outcomes in acute viral infections.

Solved Examples

1. Question: A patient with a creatinine clearance (CrCl) of 45 mL/min is prescribed valacyclovir 1000 mg TID for herpes zoster. What is the appropriate dose adjustment?
   Solution: Valacyclovir dosing for herpes zoster in patients with CrCl 30-49 mL/min is 1000 mg every 12 hours. Therefore, the frequency must be reduced from TID to BID.
2. Question: A patient starting abacavir must undergo HLA-B*5701 testing. Why is this necessary?
   Solution: HLA-B*5701 testing is required to prevent a potentially fatal hypersensitivity reaction. If the patient is positive, abacavir is strictly contraindicated.
3. Question: Calculate the daily dose of oseltamivir for a patient weighing 65 kg for influenza treatment.
   Solution: The standard treatment dose for adults is 75 mg BID for 5 days. Total daily dose = $75 \text{ mg} \times 2 = 150 \text{ mg} \text{ per day}$.

Practice Questions

1. Which antiviral agent is associated with a high risk of nephrotoxicity and requires concurrent administration with probenecid?
2. A patient on a stable HIV regimen of bictegravir/emtricitabine/tenofovir alafenamide (Biktarvy) is started on a polyvalent cation supplement. What is the recommended separation time?
3. Which class of HIV medications is known for causing significant metabolic abnormalities, including dyslipidemia and hyperglycemia?

4. A patient with CMV retinitis is being treated with ganciclovir. Which laboratory parameter must be monitored most closely?
5. Which DAA regimen is considered pangenotypic for the treatment of Hepatitis C?
6. What is the primary mechanism of action for oseltamivir?
7. Which HIV medication requires an "active" screen for the CCR5 co-receptor before initiation?
8. Patients taking tenofovir disoproxil fumarate (TDF) should be monitored for which unique side effects compared to tenofovir alafenamide (TAF)?
9. Which antiviral is the first-line treatment for acyclovir-resistant HSV?
10. Which drug class for HIV is generally associated with the fewest drug-drug interactions?

Answers & Explanations

1. Cidofovir. Probenecid is used to block tubular secretion, reducing the accumulation of cidofovir in renal tubules.
2. 2 hours before or 6 hours after. Bictegravir is an integrase inhibitor that chelates with cations, significantly reducing absorption.
3. Protease Inhibitors (PIs). PIs are famously associated with metabolic syndrome, including insulin resistance and elevated lipids.
4. Complete Blood Count (CBC). Ganciclovir commonly causes neutropenia, anemia, and thrombocytopenia.
5. Sofosbuvir/velpatasvir (Epclusa). This combination covers all six HCV genotypes.
6. Neuraminidase inhibitor. It prevents the release of new viral particles from infected host cells.
7. Maraviroc. It is a CCR5 antagonist, and testing ensures the virus uses the CCR5 receptor rather than CXCR4.
8. Decreased bone mineral density and nephrotoxicity. TDF has a higher systemic concentration than TAF, leading to these specific toxicities.
9. Foscarnet. Foscarnet is the drug of choice for acyclovir-resistant strains, though it carries a risk of electrolyte imbalances.
10. Integrase Strand Transfer Inhibitors (INSTIs). These generally have a more favorable profile compared to PIs and NNRTIs.

1. Which agent is the primary treatment for CMV?

• AAcyclovir
• BGanciclovir
• COseltamivir
• DLedipasvir

Frequently Asked Questions

Why is HLA-B*5701 testing required for abacavir?

Testing is required because patients carrying the HLA-B*5701 allele are at a significantly higher risk of developing a severe, potentially life-threatening hypersensitivity reaction to abacavir.

How do integrase inhibitors interact with divalent cations?

Integrase inhibitors like bictegravir and dolutegravir chelate with cations such as calcium, magnesium, and iron, which prevents the drug from being absorbed in the gastrointestinal tract.

What are the common side effects of protease inhibitors?

Protease inhibitors are frequently associated with metabolic complications, including dyslipidemia, hyperglycemia, and lipodystrophy, as well as gastrointestinal distress.

Why is cidofovir administered with probenecid?

Cidofovir is highly nephrotoxic; probenecid inhibits the active tubular secretion of the drug, thereby reducing its concentration in the renal epithelial cells.

Which antiviral agent requires a tropism assay?

Maraviroc requires a tropism assay to confirm that the patient's HIV strain utilizes the CCR5 co-receptor, as the drug is ineffective against viruses that use the CXCR4 co-receptor.