High-dose methotrexate with calcium leucovorin rescue is part of osteosarcoma chemotherapy. Which statement about the rescue is correct?
- A Leucovorin is started 24 hours after methotrexate administration and continued until serum methotrexate falls below cytotoxic levels ✓
- B Leucovorin is given simultaneously with methotrexate to prevent toxicity
- C Leucovorin rescue is required only if the patient develops mucositis
- D Leucovorin is given as a single bolus 6 hours after methotrexate and then stopped
Correct answer: A. Leucovorin is started 24 hours after methotrexate administration and continued until serum methotrexate falls below cytotoxic levels
Explanation
Folinic acid rescue exploits the gap between tumour and normal cell uptake of methotrexate: it is deliberately withheld for about 24 hours after the methotrexate infusion begins, then administered and titrated against serial serum methotrexate levels until the drug clears below around 0.1 micromol per litre. Simultaneous administration would negate antitumour effect, and rescue guided solely by symptoms is unsafe because delayed clearance predicts severe toxicity.
Reference: Katzung's Basic and Clinical Pharmacology, 15th ed.
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