Why adding valganciclovir to zidovudine can flatline a bone marrow
BOARD CHALLENGE
A 43-year-old man with HIV (on zidovudine/lamivudine/dolutegravir) presents with bloody diarrhea and abdominal pain. Colonoscopy shows deep ulcers with intranuclear "owl-eye" inclusions on biopsy. He is started on IV ganciclovir, then transitioned to oral valganciclovir.
What is the most likely cause of his lab findings?
A. Disseminated CMV infection
B. Zidovudine-induced pure red cell aplasia
C. HIV-associated immune thrombocytopenia
D. Additive myelosuppression from valganciclovir and zidovudine
E. Dolutegravir-induced pancytopenia
Good thinking: CMV can absolutely mess with the marrow. But here is the thing: his CMV was being treated. He is on valganciclovir. The timeline matters: he was fine before the new drug, then tanked 2 weeks after starting it. Counts that fall after treatment starts point first to the treatment, and valganciclovir is a marrow suppressant even while it treats CMV.
Sharp instinct: zidovudine really does suppress red cell production. But look at his labs: WBCs are down, hemoglobin is down, and platelets are down. That is all three lines: pancytopenia. Isolated anemia can be zidovudine alone, but pancytopenia after valganciclovir means overlapping marrow suppression.
Reasonable thought: HIV itself can cause low platelets through immune destruction. But his WBCs and hemoglobin are also down. Immune thrombocytopenia only takes out platelets. HIV-associated immune thrombocytopenia is isolated thrombocytopenia, not all-three-line pancytopenia.
Correct. Both valganciclovir and zidovudine independently suppress bone marrow. One marrow suppressant can be tolerated; two at once can collapse production. Valganciclovir is a ganciclovir prodrug and guanosine analog. Zidovudine is a thymidine analog. Both interfere with rapidly dividing cells, and bone marrow progenitors divide fast. Valganciclovir plus zidovudine causes additive myelosuppression, so CBC monitoring is mandatory.
Dolutegravir does not suppress bone marrow. It can raise creatinine by blocking tubular secretion, which looks like kidney trouble without true injury, but the marrow stays out of it. INSTIs are marrow-friendly; myelosuppression points to zidovudine, ganciclovir, or valganciclovir.
Why does this happen? Both drugs are nucleoside analogsFake building blocks that look like real DNA bases. Cells grab them during replication, but they jam the machinery because they are not the real thing. → they pretend to be normal DNA building blocks. When rapidly dividing cells (like bone marrow stem cells) grab these fakes and try to use them, DNA replication stalls. The cell cannot divide. It dies.
One drug doing this? Your marrow can usually compensate. Two drugs doing this at the same time? The marrow cannot keep up. Production of ALL cell lines drops:
Cell Line
Lab Finding
Clinical Consequence
White blood cells
Leukopenia (WBC < 4,000)
Infections, sepsis risk
Red blood cells
Anemia (low Hgb)
Fatigue, dyspnea
Platelets
Thrombocytopenia (< 150K)
Bleeding risk
All three at once
Pancytopenia
This is the board answer
From the Attending
Two nucleoside analogs hitting the same marrow at the same time. One is a thymidine analog. One is a guanosine analog. Different fake bases, same rapidly dividing target. The marrow does not care which fake base killed it. If you see pancytopenia after valganciclovir was added to a zidovudine regimen, the interaction is the answer. Every time.
HOW THEY WORK
Watch It Get Activated
Pick a drug, then press Activate (or Step) to add the phosphates one at a time. Flip the resistance switch to watch the gate fail.
Virus has lost its kinase (resistant)
MIX AND MATCH
Drug Interaction Mixer
Tap two drugs to see what happens when you combine them
These are the "fake bricks." HIV uses reverse transcriptaseHIV's personal copying machine. It takes viral RNA and makes DNA from it → the reverse of normal. NRTIs jam this machine by providing defective building blocks. to copy its RNA into DNA. NRTIs look like real nucleosides but lack a 3' hydroxyl group → they get incorporated and stop the chain cold.
The catch: human cells also use nucleosides for their own DNA. So NRTIs can accidentally interfere with our cells too → especially fast-dividing ones like bone marrow.
💡Zidovudine (AZT) = thymidine analog. Board toxicity: myelosuppression (macrocytic anemia, neutropenia). The NRTI most likely to trash your marrow.
💡Lamivudine (3TC) = cytosine analog. Generally well-tolerated. Also used for Hep B. The chill NRTI.
NNRTIs (Non-Nucleoside RTIs)
These bind directly to reverse transcriptase and change its shape so it cannot work. They do not pretend to be nucleosides → they just physically block the machine allosterically.
💡Efavirenz = Board toxicity: vivid dreams, nightmares, CNS effects. If a board vignette mentions a patient with HIV having wild nightmares → it is efavirenz, every time.
Protease Inhibitors (PIs)
After HIV makes its protein chains, proteaseAn enzyme that cuts long protein chains into functional pieces. Without it, HIV produces useless, unassembled viral parts. PIs block this last assembly step. has to cut them into functional pieces. PIs block this final assembly step. All PIs end in -navir.
💡CYP3A4 interactions: PIs are strong CYP3A4 inhibitors. Ritonavir is used as a pharmacokinetic boosterLow-dose ritonavir does not fight HIV itself. It blocks the liver enzyme that would clear other PIs, keeping their blood levels high enough to work. Cobicistat works the same way. → it blocks the enzyme that clears other drugs, boosting their levels. This is why PIs interact with statins (simvastatin, lovastatin), inhaled steroids (fluticasone), and methadone.
INSTIs (Integrase Strand Transfer Inhibitors)
HIV uses integraseThe enzyme that splices HIV's newly made DNA into your chromosomes. Once integrated, the virus can hide there permanently. INSTIs prevent this insertion step. to splice its DNA into your chromosomes. INSTIs block this step. They are the newest class and generally the best tolerated. All end in -tegravir.
💡Dolutegravir, Raltegravir → minimal side effects. Dolutegravir can falsely raise creatinine (blocks tubular secretion) but does NOT actually damage kidneys or marrow. Main concern: chelation with polyvalent cations (calcium, iron, magnesium) → separate dosing by 2 hours.
THE BOARD TRAP
Acyclovir vs Ganciclovir
Same drug family, opposite target organs
CMV (cytomegalovirus)A herpesvirus that is usually harmless in healthy people. In immunocompromised patients (HIV with low CD4, transplant recipients), it reactivates and attacks the colon, retina, or lungs. is an opportunistic infection in HIV patients with CD4 counts below ~50-100. It causes colitis (bloody diarrhea), retinitis (vision loss), and pneumonitis.
Valganciclovir is the oral prodrug of ganciclovir. It is a guanosine analogLooks like the DNA base guanosine. Gets phosphorylated by a viral kinase first (UL97 in CMV), then by host kinases. The triphosphate form jams viral DNA polymerase, but it also gets into host cell DNA, which is why it is toxic to marrow.. CMV has a kinase (UL97) that activates ganciclovir inside infected cells. But once activated, the drug also gets incorporated into host cell DNA during replication. Bone marrow progenitor cells divide rapidly → they grab more of the drug → myelosuppression.
Acyclovir is also a guanosine analog but is activated by HSV/VZV thymidine kinase (not CMV UL97). It has much less affinity for host cell kinases → less incorporation into host DNA → the marrow is spared. Instead, the poorly soluble drug can precipitate as crystals in the renal tubules, especially when the patient is dehydrated.
⚠
BOARD TRAP: The Guanosine Analog Swap
Drug
Treats
Key Toxicity
Why
Acyclovir
HSV, VZV
Crystal nephropathy
Poorly soluble → precipitates in renal tubules as crystals
Ganciclovir / Valganciclovir
CMV
Myelosuppression
Incorporates into host DNA → kills dividing marrow cells
The board loves swapping these. If the vignette says "kidney damage after starting an antiviral" → acyclovir (crystals). If it says "dropping blood counts after starting an antiviral" → ganciclovir/valganciclovir (marrow). Same drug family, opposite targets.
From the Attending
Both are guanosine analogs. Both need a viral kinase to get started. But acyclovir needs the HSV/VZV thymidine kinase → CMV does not have that kinase, so acyclovir cannot touch CMV. Ganciclovir uses the CMV UL97 kinase. No viral kinase, no activation, no antiviral effect. That is why acyclovir does not treat CMV. Know your kinases.
⚡Acyclovir resistance: HSV can mutate its thymidine kinase gene → acyclovir cannot get phosphorylated → no activation → no effect. Salvage therapy: foscarnet (does not need viral kinase activation, but causes electrolyte wasting).
Self-test: cover the cells and recall each one. Tap a cell to check, or reveal the whole row.
Drug
Toxic organ
Mechanism of injury
Classic clue
Management
Acyclovir / valacyclovir
Kidney
Poorly soluble drug precipitates as crystals in the renal tubules
Rising creatinine with needle-shaped birefringent crystals after IV dosing in a dehydrated patient
IV hydration before and during therapy; adjust dose for renal function
Ganciclovir / valganciclovir
Bone marrow
Triphosphate is incorporated into host marrow DNA and kills dividing progenitors
Neutropenia first, then thrombocytopenia and anemia (a falling ANC)
Serial CBC monitoring; reduce dose or add G-CSF for severe neutropenia
A attacks the kidney, G guts the marrow.
MORE TOXICITIES
Other High-Yield Antiviral Toxicities
The rest of the board bait
⚡NRTIs + Mitochondrial toxicity: NRTIs can inhibit mitochondrial DNA polymerase gammaMitochondria have their own DNA and their own copying enzyme (pol gamma). NRTIs, especially older ones like didanosine and stavudine, jam this enzyme, starving cells of energy production. → hepatic steatosis (fatty liver), lactic acidosis, peripheral neuropathy, pancreatitis. Older NRTIs (didanosine, stavudine) were the worst offenders.
⚡Protease Inhibitors + Metabolic syndrome: PIs interfere with lipid and glucose metabolism → hyperglycemia, hyperlipidemia, lipodystrophy (fat redistribution: buffalo hump, central obesity, facial wasting).
⚡Acyclovir + Dehydration: Low solubility + poor hydration = crystal precipitation in tubules → obstructive nephropathy. Prevention: IV hydration before and during treatment.
⚡Neuraminidase Inhibitors (oseltamivir, zanamivir): Block neuraminidaseThe influenza surface enzyme that cuts newly formed virus particles free from the host cell. Without it, budding virions stay stuck to the cell and cannot spread. That is why oseltamivir must be started within 48 hours: the virus needs to still be actively budding. on influenza A and B → must start within 48 hours of symptoms. Side effects: nausea, vomiting. Amantadine/rimantadine (M2 blockers) are now obsolete due to near-universal resistance.
INTERACTION CHECK
Starting ART: Check for Interactions
Commit to each call before the rule shows. One co-medication at a time.
Co-medication: a statin
Your patient is starting ritonavir-boosted darunavir and needs a statin for hyperlipidemia. Which statin can you start safely?
Protease inhibitors and boosters (ritonavir, cobicistat) strongly inhibit CYP3A4. Simvastatin and lovastatin are cleared by CYP3A4, so their levels can climb 10 to 50 times and trigger rhabdomyolysis. Rosuvastatin and pravastatin are not meaningfully CYP3A4-metabolized, so they pair safely with a boosted PI. A boosted PI plus a CYP3A4 statin equals rhabdomyolysis risk; reach for rosuvastatin or pravastatin.
Co-medication: rifampin for TB
A patient on a ritonavir-boosted protease-inhibitor regimen is diagnosed with active tuberculosis and must start rifampin. What is the safest move?
Rifampin is a potent CYP3A4 and UGT inducer. It accelerates metabolism so hard that protease-inhibitor and NNRTI levels collapse, risking viral breakthrough and resistance. Stopping ART lets HIV rebound. The fix is to move to an INSTI: dolutegravir can be given 50 mg twice daily to outrun the induction. Rifampin plus a boosted PI fails; switch to twice-daily dolutegravir and never stop ART.
Co-medication: a proton pump inhibitor
A patient on antiretroviral therapy needs a PPI for reflux. Which agent is an absolute contraindication with a PPI?
Rilpivirine needs an acidic stomach to dissolve and absorb. A PPI raises gastric pH enough to drop rilpivirine into the failure range, so the pairing is contraindicated. Dolutegravir tolerates standard PPIs (its real trap is cation chelation, solved by separating doses), and efavirenz absorption does not depend on acid. PPI plus rilpivirine equals absorption failure; switch to efavirenz or an INSTI.
Co-medication: methadone maintenance
A patient on methadone maintenance is starting antiretroviral therapy. Which choice avoids precipitating opioid withdrawal?
Efavirenz and nevirapine induce CYP3A4, the enzyme that clears methadone. Methadone levels can fall 50 to 60 percent and the patient withdraws within days. INSTIs do not induce CYP enzymes, so methadone stays steady, which is why they are preferred in patients on methadone. NNRTI inducers drop methadone into withdrawal; an INSTI keeps it stable.
THE LINEUP
The Antiviral Rogues Gallery
Tap any card to flip and see the full toxicity profile
Teratogenicity: Category X, avoid 6 months after stopping
Modern HCV: largely replaced by direct-acting antivirals
FROM THE LAB
Viruses Under the Microscope
Real electron micrographs and clinical images via Wikimedia Commons
HIV-1 particles on transmission electron microscopyHerpes simplex virions on transmission electron microscopyInfluenza A particles: neuraminidase target lives on the viral surfaceAntiretroviral tablets: interactions start when regimens stackTDF/FTC tablets: kidney monitoring matters with nephrotoxins
🔑 Memory Hooks (tap to reveal)
Acyclovir toxicityA = Attacks kidneys (crystals)
Ganciclovir toxicityG = Guts your marrow (myelosuppression)
AZT + ValganciclovirVZ Day: Victory over your marrow (WWII on the bone marrow)
Efavirenz side effectE-fairy-enz: the fairy that gives you vivid dreams
PI metabolic effectsProtease Inhibitors = PI = Paunchy and Insulin-resistant
INSTI chelationMetals grab -tegravirs: separate by 2 hours
PROVE IT
Clinical Vignettes
One patient at a time. Use the front-side tools, commit, then reveal the chain.
Medically reviewed by Fatima Ali, DO and Kaitlyn Cocuzzo, MD · Last reviewed June 2026
Bone Wizardry is an independent educational resource for visual learning in the medical sciences. It is not affiliated with, endorsed by, or sponsored by any licensing or examination board, contains no real or recalled examination questions, and does not guarantee any educational or examination outcome.
