What is the site of action of amphotericin B in fungi?

Correct Answer: D. Cell membrane

Amphotericin B is a polyene macrolide antibiotic whose primary mechanism of action is binding to ergosterol, the principal sterol component of the fungal cell membrane. Unlike bacterial cell walls (which contain peptidoglycan) or mammalian cell membranes (which contain cholesterol), fungi uniquely depend on ergosterol for membrane integrity and function. Amphotericin B inserts itself into the lipid bilayer and forms pores or channels, disrupting membrane permeability and causing leakage of intracellular contents (K+, Na+, amino acids, nucleotides). This leads to cell death through osmotic imbalance and loss of essential cellular components. The selective toxicity arises because fungal ergosterol differs structurally from mammalian cholesterol, though some mammalian toxicity (nephrotoxicity, hypokalemia) still occurs due to partial binding to cholesterol in human cell membranes. This is why amphotericin B remains the gold standard for serious systemic mycoses in India (cryptococcal meningitis, aspergillosis, candidiasis) despite its toxicity profile, and why lipid formulations (liposomal amphotericin B) are preferred in resource-rich settings to reduce nephrotoxicity while maintaining fungicidal activity.

Why the other options are wrong

A. Cytoplasm — This is wrong because amphotericin B does not directly target cytoplasmic enzymes or metabolic pathways. While the drug's action ultimately affects cytoplasmic osmolarity and causes cytoplasmic leakage, the primary site of binding and initial action is the cell membrane, not the cytoplasm itself. This is a distractor that confuses the consequence (cytoplasmic damage) with the mechanism (membrane disruption). B. Ribosome — This is wrong because amphotericin B is not a protein synthesis inhibitor. Ribosomal inhibitors in fungi include echinocandins (which inhibit β-glucan synthesis, not ribosomes) and some other agents, but amphotericin B does not bind to fungal ribosomes. This option may trap students who confuse amphotericin B with other antifungals or who recall that some antibiotics target ribosomes. C. Nucleic acid — This is wrong because amphotericin B does not intercalate into DNA or RNA or inhibit nucleic acid synthesis. While flucytosine (5-FC) is converted to 5-fluorouracil and disrupts nucleic acid synthesis, amphotericin B has no direct interaction with fungal nucleic acids. This is a classic NBE trap pairing antifungals with nucleic acid targets when the actual mechanism is membrane-based.

High-Yield Facts

Mnemonics

AmB = Membrane Breaker Amphotericin B → Membrane (ergosterol binding) → Breaks cell integrity. Think: 'Amphotericin B breaks the fungal membrane barrier.' Ergosterol ≠ Cholesterol Fungi have Ergosterol, Humans have Cholesterol → AmB targets Ergosterol selectively, but some cross-reactivity with Cholesterol causes side effects. Use when comparing selectivity of antifungals.

NBE Trap

NBE pairs amphotericin B with nucleic acid or ribosomal targets (options C and B) to exploit students' confusion between different antifungal classes—flucytosine targets nucleic acids, echinocandins target cell wall synthesis, but amphotericin B is uniquely a membrane disruptor. The trap is reinforced by the fact that many antibiotics DO target ribosomes or nucleic acids, making those options superficially plausible.

Clinical Pearl

In Indian clinical practice, amphotericin B remains the first-line agent for cryptococcal meningitis (common in HIV+ patients) and invasive aspergillosis in immunocompromised hosts. The classic triad of amphotericin B toxicity—fever/chills during infusion, nephrotoxicity (rising creatinine), and hypokalemia—is directly linked to its non-selective binding to mammalian cholesterol in renal tubules and cell membranes. Premedication with paracetamol and NSAIDs, slow infusion, and electrolyte monitoring are standard practice in Indian hospitals to mitigate these membrane-related side effects.

_Reference: KD Tripathi Pharmacology Ch. 47 (Antifungal Agents); Harrison's Principles of Internal Medicine Ch. 212 (Antifungal Therapy)_