Methotrexate (MTX)

Chemical Identity and Classification

• An antimetabolite (folate antagonist)
• A conventional synthetic DMARD (csDMARD)
• A cytotoxic/immunosuppressive agent

Mechanism of Action

• Prevents conversion of FH2 (dihydrofolate) to FH4 (tetrahydrofolate)
• Blocks transmethylation reactions essential for cellular function
• Results in NF-kB downregulation and increased sensitivity to apoptosis
• Dominant mechanism at HIGH (oncologic) doses

• AICAR accumulates, which inhibits AMP deaminase (AMPDA) and adenosine deaminase (ADA)
• Leads to extracellular adenosine accumulation
• Adenosine (via A1a, A2a, A2b, A3 receptors) is a potent inhibitor of inflammation, suppressing neutrophils, macrophages, dendritic cells, and lymphocytes
• This adenosine-mediated anti-inflammatory pathway is the key mechanism at LOW (rheumatic) doses

• Decreases pyrimidine synthesis
• Contributes to antiproliferative effect

At rheumatic disease doses, DHFR inhibition is relatively minor; the AICAR/adenosine pathway dominates. - Katzung's Basic and Clinical Pharmacology, 16th Ed.

Pharmacokinetics

To prevent renal toxicity from crystalluria, maintain alkaline urine and adequate hydration, especially with high-dose regimens. - Lippincott Illustrated Reviews: Pharmacology

Therapeutic Uses

Oncology

• Acute lymphocytic leukemia (ALL) - cornerstone therapy
• Burkitt lymphoma (children)
• Breast cancer (combination)
• Head and neck carcinomas
• Osteosarcoma (high-dose with leucovorin rescue)
• Mycosis fungoides
• CNS lymphoma / meningeal leukemia (intrathecal)
• Bladder, choriocarcinoma, gestational trophoblastic disease

Rheumatology / Immunology

• Rheumatoid arthritis - first-line csDMARD; reduces rate of new erosions
• Psoriatic arthritis (peripheral joint disease)
• Juvenile idiopathic arthritis (JIA)
• Ankylosing spondylitis (peripheral joint involvement)
• Systemic sclerosis (SSc) - decreases skin involvement
• Vasculitis, Takayasu arteritis, Polymyositis/Dermatomyositis
• Combined with rituximab for RA

Dermatology

• Severe psoriasis (plaque, pustular, erythrodermic)
• Crohn disease (low dose)

Dosing

• Splitting the dose within a 12-hour window can improve tolerability at doses >15 mg/week
• SC or IM routes have better bioavailability than oral, especially at higher doses

Adverse Effects

Major Toxicities

• Leukopenia, thrombocytopenia, anemia
• Most dangerous with high doses or in folate-deficient states
• Leucovorin rescue required with high-dose regimens

• Elevated transaminases (common, usually reversible)
• Hepatic fibrosis/cirrhosis with chronic low-dose use (cumulative dose-related)
• Fibrosis risk increased by alcohol use, obesity, pre-existing liver disease

• Oral ulcers/stomatitis, nausea, vomiting, diarrhea
• Reduced by leucovorin or folic acid supplementation (without significantly decreasing efficacy)

• MTX pneumonitis - hypersensitivity-like syndrome; can be severe and potentially fatal
• Symptoms: dry cough, dyspnea, fever
• Requires immediate drug discontinuation
• Incidence is rare but serious

• 7-OH metabolite crystallizes in renal tubules at acidic pH
• Managed with alkalinization + hydration (especially high-dose)
• MTX excreted renally - dose must be reduced in renal impairment

• Pregnancy Category X - absolutely contraindicated in pregnancy
• Causes neural tube defects, fetal death
• Contraception required during treatment and for 3-6 months after stopping

• Leukoencephalopathy, arachnoiditis with intrathecal administration

Summary Table of Adverse Effects by System

Leucovorin (Folinic Acid) Rescue

• High-dose MTX: Leucovorin rescue is mandatory; administered ~24-36h after MTX infusion
• Leucovorin (or folic acid) also reduces stomatitis and GI toxicity with low-dose MTX
• Note: leucovorin supplementation may decrease MTX efficacy by 10-18% in rheumatic disease
• Folic acid 1 mg/day (rather than leucovorin) is given routinely with low-dose MTX in RA to reduce side effects while preserving efficacy

Contraindications and Precautions

Monitoring

Drug Interactions

Resistance Mechanisms

• Decreased MTX influx (reduced RFC expression)
• Increased MTX efflux (overexpression of ABC transporters: ABCC1-4, ABCG2)
• Impaired polyglutamation (reduced folyl-polyglutamyl synthetase activity)
• DHFR gene amplification
• Mutations in DHFR reducing MTX binding affinity
• Genetic polymorphisms in transporter proteins can also produce variable MTX response profiles

Key Points Summary

• MTX is a folic acid analogue that enters cells via RFC and is polyglutamated for intracellular retention
• At high (oncologic) doses: DHFR inhibition dominates - blocks folate-dependent DNA/RNA synthesis (S-phase specific)
• At low (rheumatic) doses: AICAR/adenosine pathway dominates - potent anti-inflammatory effect
• Leucovorin rescue bypasses DHFR blockade - given after high-dose MTX to protect normal tissues
• Folic acid supplementation (not leucovorin) used routinely in RA to reduce side effects
• Must be dose-adjusted for renal function; absolutely contraindicated in pregnancy
• Most serious side effects: hepatic fibrosis (chronic), pneumonitis (rare but potentially fatal), myelosuppression

Clinical Interpretation and Treatment Plan

Patient: Mr Akshansh | 28 Years / Male | Sampled: 27 July 2026

Lab Results Summary and Interpretation

Hormonal Pattern: Primary (Hypergonadotropic) Hypogonadism

Step 1: Establish the Cause (Mandatory Before Treatment)

Additional Investigations to Order NOW

• Repeat fasting morning testosterone (8-10 AM) - confirm on a second sample (required by guidelines before starting TRT)
• Free testosterone + SHBG - total T can be borderline; free T gives functional status
• Karyotype (chromosomal analysis) - to rule out Klinefelter syndrome (47,XXY), especially in a young male with this pattern
• Estradiol (E2) - may be elevated in Klinefelter; also guides treatment
• Semen analysis - fertility assessment; important at age 28
• Testicular ultrasound - assess testicular volume, rule out tumor or structural cause
• Inhibin B - marker of Sertoli cell function and spermatogenesis

• PSA (prostate-specific antigen)
• Hematocrit/CBC - TRT can cause polycythemia
• Lipid profile
• Liver function tests
• Sleep apnea screening (TRT can worsen OSA)
• Bone mineral density (DEXA scan) - hypogonadism causes osteoporosis

Step 2: Line of Treatment

First-Line: Testosterone Replacement Therapy (TRT)

Available Formulations

Testosterone undecanoate injections are the preferred replacement therapy in many parts of the world. - Goldman-Cecil Medicine, International Edition

Important: Modified 17α-alkylated androgens (methyl testosterone, most oral anabolic steroids) are NOT recommended - they cause hepatotoxicity, lower HDL, and raise total cholesterol.

Special Consideration: Fertility Planning at Age 28

• Do NOT start standard TRT yet
• Instead, consider:
  • Clomiphene citrate (clomifene) 25-50 mg orally every other day to every day - a selective estrogen receptor modulator (SERM) that blocks estrogen feedback at the pituitary, stimulating endogenous LH/FSH and thus testosterone. However, in primary hypogonadism (testicular failure), this is less likely to work because the defect is in the testes, not the pituitary.
  • Sperm banking/cryopreservation before starting TRT - strongly recommended
  • In the context of primary hypogonadism, the testes have limited reserve, so fertility options are limited; assisted reproductive techniques (ART) such as testicular sperm extraction (TESE) with ICSI (intracytoplasmic sperm injection) may be explored

Note: A 2026 systematic review and meta-analysis comparing clomiphene citrate vs TRT in male hypogonadism found clomiphene more effective at preserving fertility, but TRT superior for symptom control. The choice depends on the patient's priorities.

Step 3: Monitoring During TRT

Step 4: Risks of TRT to Counsel the Patient On

Summary Treatment Algorithm

Low Testosterone (229 ng/dL) + High LH (12.0) → PRIMARY HYPOGONADISM
          ↓
Confirm with repeat morning testosterone + extended workup
(Free T, SHBG, karyotype, semen analysis, E2, testicular USS, PSA, CBC)
          ↓
Counsel on fertility → Sperm banking if desired
          ↓
Start Testosterone Replacement Therapy (TRT)
  → Preferred: Testosterone undecanoate IM (1000 mg at 0 and 6 weeks, then every 12 weeks)
  → Alternative: Testosterone enanthate 150-200 mg IM every 2-3 weeks
  → Alternative: Testosterone gel 50 mg daily
          ↓
Monitor: T levels, hematocrit, PSA, symptoms at 3-6 months then annually