6.2 Antirheumatic Pharmacotherapy & the Spondyloarthropathies

1. Pharmacotherapy for Rheumatoid Arthritis

The modern management of RA employs an aggressive Treat-to-Target (T2T) strategy aiming for clinical remission (e.g., Clinical Disease Activity Index [CDAI] <=2.8) or low disease activity (CDAI <=10). Disease-Modifying Antirheumatic Drugs (DMARDs) should be initiated immediately upon diagnosis to prevent irreversible joint destruction.

1. Conventional Synthetic DMARDs (csDMARDs)

• Methotrexate (Anchor csDMARD):
  • Dose: 15 mg weekly PO/SC, rapidly titrated to 20–25 mg weekly. Subcutaneous administration improves bioavailability and reduces gastrointestinal adverse effects.
  • Mandatory Co-Prescription: Folic Acid 1 mg daily (or 5 mg weekly) to mitigate mucosal ulcers, nausea, alopecia, and hematologic toxicity without diminishing efficacy.
  • Mechanism: Inhibits dihydrofolate reductase and aminoimidazole carboxamide ribonucleotide (AICAR) transformylase, leading to extracellular adenosine accumulation and anti-inflammatory signaling.
  • Monitoring: Baseline CBC, LFTs, serum creatinine, viral hepatitis serologies, and chest radiograph. Repeat CBC and LFTs every 8 to 12 weeks.
  • Contraindications: Pregnancy (absolute teratogen; must stop >=3 months prior to conception in women and men), severe renal impairment (eGFR <30 mL/min/1.73m2), chronic liver disease / cirrhosis, heavy alcohol consumption, active pulmonary fibrosis / preexisting severe lung disease.
  • Key Toxicities: Hepatotoxicity (transaminitis, hepatic fibrosis), myelosuppression (leukopenia, megaloblastic anemia, thrombocytopenia), oral stomatitis/aphthous ulcers, acute hypersensitivity pneumonitis (dry cough, fever, dyspnea, bilateral infiltrates; requires immediate drug discontinuation and high-dose steroids).
• Hydroxychloroquine (Plaquenil):
  • Dose: 200–400 mg daily (maximum safe dose <= 5.0 mg/kg actual body weight/day).
  • Clinical Role: Used as monotherapy in mild, non-erosive seronegative RA, or as part of "triple therapy" (Methotrexate + Sulfasalazine + Hydroxychloroquine). Safe in pregnancy.
  • Toxicity & Screening: Retinal toxicity (bull's-eye maculopathy) resulting in irreversible central visual field loss. Baseline ophthalmologic evaluation (fundoscopy, visual fields 10-2, spectral-domain OCT) within the first year of therapy, followed by annual screening starting after 5 years of use (or earlier in high-risk patients with renal impairment, tamoxifen use, or concurrent retinal disease).
• Sulfasalazine:
  • Dose: 1,000 mg PO BID (titrated from 500 mg daily).
  • Mechanism: Cleaved by colonic bacteria into 5-aminosalicylic acid (5-ASA) and sulfapyridine.
  • Toxicity: Hemolytic anemia (screen for G6PD deficiency prior to initiation), leukopenia, agranulocytosis, reversible oligospermia in men, transaminitis, hypersensitivity rash.
• Leflunomide (Arava):
  • Dose: 10–20 mg PO daily (loading dose 100 mg daily x 3 days is generally avoided due to GI intolerance).
  • Mechanism: Inhibits dihydroorotate dehydrogenase (DHODH), blocking de novo pyrimidine synthesis in rapidly proliferating T and B lymphocytes.
  • Toxicity & Elimination: Hepatotoxicity, hypertension, peripheral neuropathy, diarrhea, teratogenicity. Enterohepatic recirculation confers an extremely long elimination half-life (~2 years). For planned pregnancy or severe toxicity, execute an accelerated drug elimination procedure with Cholestyramine 8 g PO TID for 11 days, verifying plasma leflunomide metabolite levels <0.02 mg/L on two occasions 14 days apart.

2. Biologic DMARDs (bDMARDs)

Biologic DMARDs are indicated when disease activity remains moderate-to-high despite maximally tolerated methotrexate or in patients with poor prognostic markers (high anti-CCP/RF titers, early marginal erosions).

3. Targeted Synthetic DMARDs (tsDMARDs) — JAK Inhibitors

• Agents: Tofacitinib (Xeljanz, JAK1/JAK3 inhibitor), Baricitinib (Olumiant, JAK1/JAK2 inhibitor), Upadacitinib (Rinvoq, selective JAK1 inhibitor).
• Oral Route: Potent oral small molecules targeting the Janus Kinase - Signal Transducer and Activator of Transcription (JAK-STAT) intracellular pathway.
• ORAL Surveillance Trial & FDA Black Box Warnings:
  • Major Adverse Cardiovascular Events (MACE): Non-fatal MI, stroke, and cardiovascular death.
  • Venous Thromboembolism (VTE): Deep vein thrombosis and pulmonary embolism.
  • Malignancy: Non-melanoma skin cancer and lymphoma.
  • Serious Infections: Opportunistic fungal infections, reactivation of Herpes Zoster (mandates recombinant zoster vaccination [Shingrix] before starting), and reactivation of latent tuberculosis.
  • Guideline Rule: Reserve JAK inhibitors for patients who have had an inadequate response or intolerance to one or more anti-TNF biologics, particularly in patients >=50 years with cardiovascular risk factors or active tobacco use.

2. Seronegative Spondyloarthropathies (SpA)

The seronegative spondyloarthropathies share common genetic, clinical, and radiographic hallmarks:

1. HLA-B27 Genetic Association: Strongest in Ankylosing Spondylitis (90-95%), followed by Reactive Arthritis (70%), Psoriatic Arthritis (50%), and Enteropathic Arthritis (50%).
2. Seronegativity: Absence of Rheumatoid Factor and Anti-CCP antibodies.
3. Axial & Asymmetric Peripheral Joint Disease: Sacroiliitis, spondylitis, and asymmetric oligoarthritis (<5 joints, often large lower extremity joints).
4. Enthesitis: Inflammation at the insertion sites of tendons, ligaments, or joint capsules into bone (most commonly Achilles tendonitis and plantar fasciitis at the calcaneus).
5. Dactylitis ("Sausage Digits"): Uniform diffuse swelling of an entire digit due to simultaneous flexor tenosynovitis and joint synovitis.
6. Extra-articular Inflammation: Acute unilateral anterior uveitis (iritis), aortic root dilation with aortic regurgitation, apical pulmonary fibrosis, and inflammatory bowel disease.

Comparison of Spondyloarthropathy Subtypes