What is the recommended initial dose of methotrexate (MTX) for a 50.10 kg girl with Psoriatic Juvenile Idiopathic Arthritis (JIA) according to the British National Formulary for Children (BNFC)?

Methotrexate Dosing for a 50.10 kg Girl with Psoriatic JIA

For a 50.10 kg girl with Psoriatic Juvenile Idiopathic Arthritis (JIA), the recommended initial dose of methotrexate according to the British National Formulary for Children (BNFC) is 15 mg/m² body surface area once weekly, which would be approximately 10-15 mg weekly.

Dosing Calculation and Administration

The dosing for methotrexate in JIA follows these principles:

• Initial dosing: 10-15 mg/m² body surface area (BSA) per week 1
• Maximum dose: 25 mg weekly 1
• Administration route: Can be given orally or subcutaneously 1

For a 50.10 kg girl:

• Calculate BSA (approximately 1.5 m² for this weight)
• Initial dose would be 15-22.5 mg weekly
• A practical starting dose would be 15 mg once weekly

Evidence-Based Considerations

Efficacy

• Methotrexate is the cornerstone disease-modifying antirheumatic drug (DMARD) for JIA treatment, with proven efficacy in inducing remission in 60-70% of children 2
• Studies show that doses above 15 mg/m² BSA per week do not provide additional therapeutic benefit 1
• A weekly subcutaneous methotrexate dose of 15 mg/m² BSA is associated with high response rates within the first 12 months of treatment 1

Route of Administration

• No clinically significant differences have been observed between oral and parenteral methotrexate administration in terms of efficacy and safety 1
• However, subcutaneous administration may be considered if:
  • Gastrointestinal side effects occur with oral dosing
  • Higher doses are required (>15 mg weekly)
  • Better bioavailability is needed 1

Monitoring Requirements

Baseline assessments before starting methotrexate:

• Complete blood count
• Liver function tests
• Renal function tests
• Serum folate and vitamin B12 levels 3

Ongoing monitoring:

• Blood tests every 1-2 weeks for the first month
• Once stable, monitoring every 4-8 weeks 1, 3
• Clinical assessment for signs of mucositis, myelosuppression at each visit 3

Folic Acid Supplementation

Folic acid supplementation is strongly recommended to reduce the incidence of gastrointestinal and hepatic adverse effects:

• Typically 5 mg weekly, given 24-48 hours after methotrexate dose 1
• Some protocols recommend 1 mg daily except on the day of methotrexate 3

Special Considerations for Psoriatic JIA

Psoriatic JIA requires particular attention as:

• It is associated with less favorable outcomes compared to other JIA subtypes 4
• Patients with psoriatic JIA often require more aggressive treatment 4
• Early identification and treatment is crucial for better outcomes

Common Adverse Effects and Management

Be vigilant for:

• Gastrointestinal symptoms (35.6% of patients): nausea, vomiting, abdominal pain 5
• Behavioral problems (35.6% of patients) 5
• Elevated liver enzymes (common reason for discontinuation) 5

Management of adverse effects:

• Dose modification or route change may resolve adverse effects in approximately 55% of cases 5
• Consider switching to subcutaneous administration if gastrointestinal side effects occur with oral dosing
• Folic acid supplementation helps reduce adverse effects

Treatment Duration

• Continue methotrexate for at least 12 months after achieving remission 1
• Longer duration of inactive disease before discontinuation is associated with lower flare rates 1

Methotrexate remains the first-line treatment for JIA, including psoriatic JIA, with a well-established safety and efficacy profile when properly dosed and monitored.