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Difficulty: Very hardPharmacologic Interventions

An urgent evaluation is requested in the emergency department for a 54-year-old woman presenting with a 3-day history of progressively worsening, painful oral sores, severe fatigue, and fever. She has a history of seropositive rheumatoid arthritis, for which she has taken weekly methotrexate and daily folic acid for the past 4 years with excellent disease control. One week ago, she was evaluated at an urgent care clinic for a purulent spider bite on her right thigh and was prescribed a 10-day course of trimethoprim-sulfamethoxazole.

Vital signs are temperature 39.2°C (102.6°F), blood pressure 105/65 mm Hg, pulse 112/min, and respirations 18/min. Physical examination reveals pale conjunctivae, diffuse petechiae on the lower extremities, and severe, ulcerated stomatitis with hemorrhagic crusting of the lips. The right thigh skin lesion is healing well with no erythema or fluctuance.

Laboratory studies show:
Laboratory TestResult
Leukocyte count900/mm³
Absolute neutrophil count150/mm³
Hemoglobin7.4 g/dL
Platelet count18,000/mm³
Serum creatinine0.9 mg/dL
Alanine aminotransferase (ALT)45 U/L

Blood and urine cultures are drawn. In addition to immediate discontinuation of her current home medications and initiation of empiric broad-spectrum intravenous antibiotics, which of the following is the most appropriate targeted pharmacologic intervention?

  1. Intravenous leucovorinAnswer
  2. B
    Intravenous glucarpidase
  3. C
    High-dose oral folic acid
  4. D
    Subcutaneous pegfilgrastim
  5. E
    Intravenous sodium bicarbonate

Answer

Intravenous leucovorin
The correct answer is intravenous leucovorin. This patient is experiencing severe, acute methotrexate toxicity, precipitated by the recent addition of trimethoprim-sulfamethoxazole (TMP-SMX). Methotrexate is a disease-modifying antirheumatic drug (DMARD) that acts by competitively inhibiting dihydrofolate reductase (DHFR), the enzyme responsible for converting dietary folic acid into its active form, tetrahydrofolate. TMP-SMX is also a folate antagonist (trimethoprim inhibits bacterial DHFR, but at high doses or in combination, affects human DHFR). The co-administration of these two medications causes synergistic DHFR inhibition, leading to a catastrophic halt in purine and pyrimidine synthesis. This manifests rapidly as profound pancytopenia (with severe neutropenia increasing infection risk) and gastrointestinal mucosal damage (stomatitis). The targeted pharmacologic intervention is leucovorin (folinic acid). Leucovorin is a fully reduced folate derivative that does not require DHFR to participate in nucleic acid synthesis. By administering leucovorin, the DHFR blockade is bypassed, rapidly rescuing the actively dividing bone marrow and mucosal cells.

Step-by-Step Solution

1
Identify the primary diagnosis from the clinical presentation and medication history.
The patient has severe pancytopenia and mucositis after adding trimethoprim-sulfamethoxazole (TMP-SMX) to her baseline methotrexate (MTX) regimen.
Both MTX and TMP-SMX inhibit dihydrofolate reductase (DHFR), leading to synergistic, profound bone marrow suppression and mucosal toxicity.
2
Determine the necessary supportive and targeted interventions.
Discontinue offending agents and start empiric antibiotics for neutropenic fever (already done). A targeted rescue agent is needed.
The bone marrow failure is life-threatening and requires immediate reversal of the folate pathway blockade.
3
Select the correct pharmacologic antidote.
Intravenous leucovorin (folinic acid) is chosen.
Leucovorin is a fully reduced folate derivative that does not require DHFR for activation, thereby bypassing the enzymatic block and rescuing normal cells.

Key Concept

Methotrexate toxicity can be catastrophically exacerbated by other folate-antagonizing medications (like TMP-SMX). The specific pharmacologic rescue agent is leucovorin (folinic acid), which bypasses the inhibited dihydrofolate reductase enzyme.
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Pharmacologic Interventions

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