What are the indications and recommended dosing regimen for leucovorin calcium 15 mg tablets?

Leucovorin Calcium 15 mg Tablet: Indications and Dosing

Leucovorin calcium 15 mg tablets are primarily indicated as rescue therapy following high-dose methotrexate administration, as adjuvant therapy with 5-fluorouracil in colorectal cancer, and as folinic acid supplementation during pyrimethamine-based treatment for toxoplasmosis.

Primary Indications

1. Methotrexate Rescue Therapy

For impaired methotrexate elimination or inadvertent overdosage, leucovorin 15 mg (10 mg/m²) should be administered every 6 hours until serum methotrexate levels fall below 10⁻⁸ M 1. Rescue must begin within 24 hours of methotrexate administration when delayed excretion occurs.

Key dosing algorithm:

• Start leucovorin 15 mg every 6 hours (oral, IM, or IV)
• Monitor serum creatinine and methotrexate at 24-hour intervals
• If 24-hour creatinine increases ≥50% from baseline, OR
• If 24-hour methotrexate level >5 × 10⁻⁶ M, OR
• If 48-hour methotrexate level >9 × 10⁻⁷ M
• Then escalate to 150 mg (100 mg/m²) IV every 3 hours (doses >25 mg require parenteral administration due to saturable oral absorption) 1

Critical caveat: In the presence of gastrointestinal toxicity, nausea, or vomiting, leucovorin must be given parenterally, not orally 1. Concurrent hydration (3 L/day) and urinary alkalinization with sodium bicarbonate (maintain urine pH ≥7.0) are mandatory 1.

2. Colorectal Cancer Chemotherapy

Leucovorin serves as a biochemical modulator of 5-fluorouracil (5-FU) in multiple colorectal cancer regimens. The standard leucovorin dose is 400 mg/m² IV, which is equivalent to 200 mg/m² of levoleucovorin 2.

Common regimens using leucovorin:

• FOLFOX: Leucovorin 400 mg/m² IV over 2 hours on day 1, followed by 5-FU bolus 400 mg/m² then 1200 mg/m²/day × 2 days continuous infusion; repeat every 2 weeks 2

• FOLFIRI: Leucovorin 400 mg/m² IV infusion on day 1 with irinotecan 180 mg/m² and 5-FU (same dosing as FOLFOX); repeat every 2 weeks 2

• Roswell-Park regimen: Leucovorin 500 mg/m² IV over 2 hours on days 1,8,15,22,29, and 36, with 5-FU 500 mg/m² IV bolus 1 hour after leucovorin start; repeat every 8 weeks 2

• Weekly regimen: Leucovorin 20 mg/m² as 2-hour infusion with 5-FU 500 mg/m² bolus 1 hour after leucovorin; repeat weekly 2

Important note on leucovorin shortage: During shortages, lower leucovorin doses (20-25 mg/m²) are equally efficacious as higher doses (400-500 mg/m²) based on multiple studies, or treatment without leucovorin with a modest 10% increase in 5-FU dose is reasonable 3.

3. Toxoplasmosis Treatment

For congenital toxoplasmosis in pregnant women ≥18 weeks gestation and infants, folinic acid (leucovorin) 10-20 mg/day orally once daily is given during and for 1 week after pyrimethamine therapy 4. This prevents hematologic toxicity from pyrimethamine.

Critical distinction: Folic acid must NOT be substituted for folinic acid (leucovorin), as folic acid does not provide the same protective effect 4.

Pediatric dosing for infants with congenital toxoplasmosis:

• Leucovorin 5 mg orally every 3 days during the 1-year treatment course with pyrimethamine and sulfadiazine 4

For HIV-infected children with toxoplasmic encephalitis secondary prophylaxis:

• Leucovorin 5 mg orally every 3 days when given with pyrimethamine 1 mg/kg/day and sulfadiazine 85-120 mg/kg/day 5

4. Other Folic Acid Antagonist Toxicity

For counteracting hematologic toxicity from folic acid antagonists with less affinity for mammalian dihydrofolate reductase than methotrexate (trimethoprim, pyrimethamine), 5-15 mg leucovorin per day is substantially less than methotrexate rescue doses 1.

Oral Absorption Limitations

A critical pharmacokinetic consideration: oral absorption of leucovorin is saturable, and doses greater than 25 mg should NOT be given orally 1. This is because bioavailability decreases significantly above this threshold. For doses exceeding 25 mg, parenteral administration (IV or IM) is required.

Formulation Details

Leucovorin calcium 15 mg tablets contain leucovorin as the calcium salt, equivalent to 16.21 mg of anhydrous leucovorin calcium 1. Tablets are intended for oral administration only 1.

Common Pitfalls to Avoid

1. Do not use oral leucovorin for doses >25 mg - absorption is saturable and inadequate rescue will result 1
2. Do not substitute folic acid for folinic acid (leucovorin) in toxoplasmosis treatment - they are not interchangeable 4
3. Do not delay methotrexate rescue beyond 24 hours - efficacy decreases significantly 1
4. Do not use oral leucovorin if patient has GI toxicity, nausea, or vomiting - switch to parenteral route 1
5. Do not forget concurrent hydration and alkalinization during methotrexate rescue 1