A 45-year-old HIV-positive male presents to his primary care physician complaining of decreased libido. He reports that he has been unable to maintain an erection for the past two weeks. He has never encountered this problem before. He was hospitalized four weeks ago for cryptococcal meningitis and has been on long-term antifungal therapy since then. His CD4 count is 400 cells/mm^3 and viral load is 5,000 copies/ml. He was previously non-compliant with HAART but since his recent infection, he has been more consistent with its use. His past medical history is also notable for hypertension, major depressive disorder, and alcohol abuse. He takes lisinopril and sertraline. His temperature is 98.6°F (37°C), blood pressure is 120/85 mmHg, pulse is 80/min, and respirations are 18/min. The physician advises the patient that side effects like decreased libido may manifest due to a drug with which of the following mechanisms of action?

Inhibition of ergosterol synthesis

Inhibition of beta-glucan synthesis

Formation of pores in cell membrane

Disruption of microtubule formation

Inhibition of pyrimidine synthesis

A 76-year-old man comes to the physician for a follow-up examination. One week ago, he was prescribed azithromycin for acute bacterial sinusitis. He has a history of atrial fibrillation treated with warfarin and metoprolol. Physical examination shows no abnormalities. Compared to one month ago, laboratory studies show a mild increase in INR. Which of the following best explains this patient's laboratory finding?

Increased gastrointestinal absorption of warfarin

Increased non-protein bound warfarin fraction

A 72-year-old woman with type 2 diabetes mellitus comes to the physician because she is concerned about the appearance of her toenails. Examination shows yellowish discoloration of all toenails on both feet. The edges of the toenails are lifted, and there is subungual debris. Potassium hydroxide preparation of scrapings from the nails shows multiple branching septate hyphae. Treatment with oral terbinafine is begun. Which of the following is the primary mechanism of action of this drug?

Inhibition of squalene epoxidase

Formation of pores in cell membrane

Inhibition of β-glucan synthesis

Interference with mitosis during metaphase

Prevention of lanosterol to ergosterol conversion

A 59-year-old man comes to the physician because of a painful, burning red rash on his face and hands, which developed 30 minutes after going outside to do garden work. He wore a long-sleeved shirt and was exposed to direct sunlight for about 10 minutes. The patient is light-skinned and has a history of occasional sunburns when he does not apply sunscreen. The patient was diagnosed with small cell lung carcinoma 2 months ago and is currently undergoing chemotherapy. He is currently taking demeclocycline for malignancy-associated hyponatremia and amoxicillin for sinusitis. He has also had occasional back pain. He takes zolpidem and drinks 1–2 glasses of brandy before going to sleep every night. He has smoked a pack of cigarettes daily for 20 years. His pulse is 72/min and his blood pressure is 120/75 mm Hg. Physical examination shows prominent erythema on his forehead, cheeks, and neck. Erythema and papular eruptions are seen on the dorsum of both hands. Which of the following is the most likely cause of this patient's symptoms?

Adverse reaction to amoxicillin

Azoles (imidazoles and triazoles) for USMLE Step 2 CK: High-Yield Pharmacology Lessons

Mechanism & Classes - Ergosterol Blockers

Inhibit fungal cytochrome P450 enzyme, lanosterol 14-α-demethylase.
Blocks conversion: Lanosterol $\rightarrow$ Ergosterol, a vital component of the fungal cell membrane.
Leads to a leaky and dysfunctional membrane.

Antifungal drug mechanisms on fungal cell

Classes
- Imidazoles (generally topical): Clotrimazole, Miconazole, Ketoconazole.
- Triazoles (generally systemic): Fluconazole, Itraconazole, Voriconazole.

⭐ All azoles inhibit human cytochrome P450 (CYP450) to some extent, creating a high risk for drug-drug interactions (e.g., increasing levels of warfarin, statins).

The Systemic Triazoles - The 'Conazole' Crew

Mechanism: Inhibit fungal cytochrome P450 (lanosterol 14-α-demethylase) → ↓ ergosterol synthesis → disrupts fungal cell membrane.
Class Side Effects: Hepatotoxicity, GI distress, rash. Potent inhibitors of human CYP450 enzymes → numerous drug-drug interactions (DDIs).

Drug	Primary Uses	Key Features / Side Effects
Fluconazole	Candida (not krusei), Cryptococcus neoformans.	Excellent CNS penetration. Fewest DDIs in class.
Itraconazole	Dimorphic fungi (Histoplasma, Blastomyces, Coccidioides), Onychomycosis.	Requires acidic environment for absorption. ⚠️ Black Box Warning: Can worsen heart failure.
Voriconazole	Drug of Choice: Invasive Aspergillosis.	Good CNS penetration. ⭐ Side effects: transient visual disturbances (photopsia), photosensitivity rash.
Posaconazole & Isavuconazole	Broadest spectrum: Aspergillus, Candida, and Mucorales. Prophylaxis in neutropenic patients.	Posaconazole: Take with a high-fat meal. Isavuconazole: Fewer DDIs & side effects vs. Voriconazole.

Antifungal Drug Classification by Target and Mechanism

Adverse Effects & D-D-Is - The P450 Problem

Primary Mechanism: All azoles inhibit cytochrome P450 (CYP) enzymes, particularly CYP3A4, reducing the metabolism of many drugs.
General Adverse Effects:
- GI upset (nausea, vomiting).
- Hepatotoxicity: ↑ LFTs. Monitor liver function. Fluconazole has the lowest risk.
Drug-Specific Side Effects:
- Ketoconazole: Potent P450 inhibitor. Causes gynecomastia, ↓ libido, and menstrual irregularities by inhibiting steroid synthesis.
- Voriconazole: Visual disturbances (blurriness, color changes), photosensitivity.
- Itraconazole: Can exacerbate heart failure (negative inotrope).
Critical Drug-Drug Interactions (DDIs):
- ↑ levels of: Warfarin, Statins (except pravastatin), Cyclosporine, Phenytoin.

⭐ Exam Favorite: Azole-induced inhibition of warfarin metabolism can lead to a supratherapeutic INR and significantly ↑ bleeding risk. Always check for this interaction.

High‑Yield Points - ⚡ Biggest Takeaways

Azoles inhibit fungal P450 (14-α-demethylase), blocking the conversion of lanosterol to ergosterol and disrupting the fungal cell membrane.

As potent inhibitors of human CYP450 enzymes, they cause numerous significant drug-drug interactions.

Hepatotoxicity is a major class-wide adverse effect requiring LFT monitoring.

Voriconazole is the drug of choice for invasive Aspergillosis but can cause visual disturbances.

Fluconazole has excellent CNS penetration, making it effective for fungal meningitis.

Itraconazole carries a black box warning for worsening heart failure.

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