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HY points about each drug followed by a quiz at the end
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Oseltamivir, Peramivir, Zanamivir
MOA of oseltamivir, peramivir, zanamivir?
- Sialic acid analogues that competitively inhibit influenza A+B neuraminidase.
- Prevent viral release from the cell.
When are oseltamivir, peramivir, zanamivir the answer?
- USMLE Q will ask you which agent prevents viral release; answer = oseltamivir.
- USMLE Q will ask which agent best decreases viral spread within a community; answer = oseltamivir.
- 65F + influenza infection + given a drug for Tx + electron microscopy of upper respiratory tract epithelium shows cells “packed with virions”; which drug was she given? –> answer = oseltamivir –> decreased viral release = cells “packed with virions.”
Ribavirin
MOA of ribavirin?
- Many. But USMLE wants two:
- 1) RNA polymerase inhibitor.
- 2) IMP dehydrogenase inhibitor (purine synthesis).
- Basically say to yourself, “Ribavirin is a weird drug with many MOAs in the literature, but RNA polymerase inhibition is most important. I should also be peripherally aware that it can inhibit IMP dehydrogenase (purine synthesis).”
When is ribavirin the answer?
- It’s not. It’s almost always the wrong answer on USMLE for treatment.
- “Wait. Then why am I learning it?” –> USMLE wants you to know the MOA.
- Can be used in select circumstances for RSV bronchiolitis and HepC treatment, but once again, it’s almost always the wrong answer.
- Answer for RSV bronchiolitis on USMLE is simply “observation” or “supportive care” almost always.
- If mentioned as part of a hepatitis C, be aware it can cause hemolytic anemia.
Adverse effects of ribavirin?
- USMLE wants you to know it can cause hemolytic anemia.
- 32M + IV drug user + being managed with multiple medications including ribavirin + low Hb + high indirect bilirubin; why? –> answer = drug-induced hemolytic anemia” (ribavirin).
Acyclovir, valacyclovir
MOA of acyclovir?
- Competitively inhibits HSV1/2 and VZV DNA polymerase.
- Causes “chain termination.”
- Requires phosphorylation with viral thymidine kinase to a monophosphorylated form.
- The monophosphorylated form is then phosphorylated up to the triphosphate form by host kinases.
- Same MOA as ganciclovir for CMV.
What does “val” mean?
- “Val” means increased oral bioavailability. You’ll see this prefix attached to drugs sometimes.
When is acyclovir the answer?
- For any herpes infection (i.e., labialis, genitalis, esophagitis, keratitis, eczema herpeticum etc.). It decreases both duration and recurrence of symptoms.
- 22F + painful vesicular lesions on labia majora; Tx? –> answer = oral acyclovir (herpes genitalis).
- 24M + painful vesicular eruption on the lips; Tx? –> answer = oral acyclovir (herpes labialis).
- 34M + HIV + painful swallowing (odynophagia) + endoscopy shows well-demarcated, punched-out ulcers; Tx? –> answer = oral acyclovir (herpes esophagitis). Presents as “punched-out” ulcers; in contrast, CMV = “linear” or “confluent” ulcers.
- 12F + blurry vision + Hx of cold sore + fluorescein instillation of the eye shows a dendritic pattern; Tx? –> answer = oral acylovir (herpes keratitis).
- 12F + history of eczema + antecubital fossa on the left has a painful, vesicular eruption superimposed on the eczema; Tx? –> answer = oral acyclovir (eczema herpeticum); actual HSV1/2 infection; caused by HSV inoculation into cracked skin of eczema, often from autoinfection from a cold sore. Do not confuse this with dermatitis herptiformis (not HSV infection; seen in Celiac disease).
- 18M + school wrestling team + painful vesicular eruption down the leg; Tx? –> oral acyclovir (also dermatitis herpetiformis).
- 36F + works as a dental hygienist + vesicular eruption on finger; Tx? –> answer = oral acyclovir (herpetic whitlow).
- Shingles (herpez zoster). Should be noted that VZV, not HSV, causes shingles. Herpes zoster is just another name for shingles.
- 48F + painful eruption along a flank, or on the neck; Tx? –> oral acyclovir (shingles; herpes zoster; VZV).
- 50F + diabetic + painful eruption along a flank; Tx? (answers are oral or IV); answer = oral acyclovir.
- 55M + painful vesicles around the ear + Bells palsy; Tx? –> answer = oral acylcovir (herpes zoster oticus; Ramsay-Hunt syndrome type II).
- 55M + painful vesicles around eye + no prior Hx of herpes infections + fluorescein instillation of the eye shows a dendritic pattern; Tx? –> oral acyclovir (herpes zoster ophthalmicus).
- 45F + Hodgkin lymphoma currently on chemo + painful eruption on the neck; Tx? (answers are oral or IV); answer = IV acyclovir.
- 2M + painful vesicular eruption on the leg; Tx? –> answer = oral acyclovir (pediatric shingles; yes, it’s a “thing,” and it’s tested; can occur in immunocompromised peds cases if prior Hx of either VZV infection or vaccine).
- Always choose oral (not topical) acyclovir if posed with both as answer choices.
- If you’re confronted with both oral and IV, choose oral almost always. IV will be if the patient is immunocompromised (i.e., HIV, lymphoma, on chemotherapy).
- Choose oral, not IV, if the patient merely has diabetes.
- If the patient is septic, give IV acyclovir.
- Sepsis = SIRS + infection.
- SIRS = systemic inflammatory response syndrome = two or more of the following: temp <36 or >38; HR >90; RR >20; WBCs <4,000 or >12,000).
- Acyclovir safe to give during pregnancy. Give intrapartum if expectant mother has Sx (even just tingling without visible lesions), then deliver via C-section. If no Sx peripartum, choose vaginal delivery.
- 38 weeks gestation + typically gets herpes outbreak every 3-4 months + last outbreak was 4 months ago + currently aysmptomatic; what should we do? –> answer = prophylactic acyclovir.
Adverse-effects of acyclovir?
- Crystal-nephropathy (kidney stones).
- Acyclovir deposits as crystals in the urine.
Mechanism of resistance to acyclovir?
- Altered or absent viral thymidine kinase.
- Same as resistance mechanism against ganciclovir by CMV.
Why can’t acyclovir be used for CMV; likewise, why can’t ganciclovir be used for HSV1/2 and VZV?
- The viral thymidine kinases have different affinities for the respective drugs.
- Btw, note the difference in spellings of the drugs.
If patient has acyclovir-resistant HSV, which drug can be given?
- In theory, cidofovir. But never an answer on USMLE. I’ve never once seen this drug come up.
- “Then why the fuck are you mentioning it to me. I don’t need info overload.” –> Because resources sometimes mention it, so rather than wondering if the drugs I’ve mentioned to you are an incomplete list, I’d rather tell you, “Yes, this drug is mentioned sometimes, but you don’t need to know jack shit about it.”
Ganciclovir, valganciclovir
MOA of ganciclovir?
- Competitively inhibits CMV DNA polymerase.
- Causes “chain termination.”
- Requires phosphorylation with viral thymidine kinase to a monophosphorylated form.
- The monophosphorylated form is then phosphorylated up to the triphosphate form by host kinases.
- Same MOA as acyclovir for HSV1/2 + VZV.
When is ganciclovir the answer?
- 32F + HIV with CD4 count of 80/uL + blurry vision; Tx? –> answer = ganciclovir (CMV keratitis).
- 45M + IV drug user + CD4 count of 37/uL + blood in stool + colonoscopy shows linear ulcerations; Tx? –> answer = ganciclovir (CMV colitis).
- USMLE wants you to know CMV colitis comes in with a CD4 count <50/uL.
- 40F + HIV + painful swallowing + endoscopy shows confluent ulcerations; Tx? –> answer = ganciclovir (CMV esophagitis).
- HSV = “punched out” ulcers; CMV = “linear” or “confluent” ulcers.
Adverse-effects of ganciclovir?
- Neutropenia (agranulocytosis).
- Neutropenia presents as mouth ulcers, fever, sore throat.
- Other HY drugs causing neutropenia: methotrexate, clozapine, methimazole, propylthiouracil, ticlopidine.
Mechanism of resistance to ganciclovir?
- Altered or absent viral thymidine kinase.
- Same as resistance mechanism against acyclovir by HSV1/2 or VZV.
Why can’t acyclovir be used for CMV; likewise, why can’t ganciclovir be used for HSV1/2 and VZV?
- Once again: viral thymidine kinases have different affinities for the respective drugs.
- Reiterating this point because of yieldness on the Step 1.
If patient has ganciclovir-resistant CMV, which drug can be given?
- Foscarnet.
- Probably one of the lowest yield drugs that resources propagate as though it’s HY.
- Mentioning it here for the sake of completion, but never actually shows up on NBME material anywhere.
- Pyrophosphate (PPi) analogue; therefore bypasses the need for viral thymidine kinase to perform initial monophosphorylation.
- Can cause hypocalcemia due to chelation.
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