Immunosuppressive Therapy — Antimetabolites

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Antimetabolite Drug Classification
Methotrexate
Azathioprine
Mycophenolate Mofetil
High-Yield Pearls
Related Topics

Antimetabolite Drug Classification

Antimetabolites are a class of steroid-sparing immunosuppressants that interfere with nucleotide synthesis pathways, preferentially affecting rapidly dividing cells — including the activated lymphocytes that drive chronic ocular inflammation. Methotrexate and azathioprine are the two classic antimetabolites in this sense, distinguishing them from calcineurin inhibitors (cyclosporine, tacrolimus), biologics (adalimumab, rituximab, tocilizumab), and alkylating agents (cyclophosphamide). Mycophenolate mofetil shares the same underlying strategy — starving lymphocytes of the nucleotide precursors they need to proliferate — even though it targets a different point in that pathway, as detailed below.

Methotrexate

Methotrexate is a folate antagonist. It competitively inhibits dihydrofolate reductase (DHFR), preventing conversion of dihydrofolate to tetrahydrofolate — a cofactor required for purine and pyrimidine synthesis. By blocking this conversion, methotrexate impairs DNA and RNA synthesis specifically in rapidly dividing immune cells, reducing the proliferative capacity driving ocular inflammation.

Because methotrexate is hepatotoxic, baseline and periodic liver function tests are essential before and throughout treatment, to catch transaminase elevation or fibrosis risk before it becomes clinically significant. This monitoring requirement is one of the most practical, board-relevant facts about methotrexate use in uveitis — it's not a "start and forget" drug.

Azathioprine

Azathioprine is a prodrug metabolized to 6-mercaptopurine, which incorporates into purine synthesis pathways as a false substrate, inhibiting purine nucleotide synthesis and thereby suppressing proliferating lymphocytes.

Common Pitfall — Skipping TPMT Testing

Before initiating azathioprine for refractory uveitis, checking thiopurine methyltransferase (TPMT) activity is essential. Patients with low or absent TPMT activity cannot adequately metabolize azathioprine, leading to accumulation of toxic thiopurine metabolites and severe, potentially life-threatening bone marrow suppression. Starting azathioprine without checking TPMT status first is a real and dangerous omission, not just a theoretical board-exam trap.

Mycophenolate Mofetil

Mycophenolate mofetil is converted in the body to mycophenolic acid, its active form, which inhibits inosine monophosphate dehydrogenase (IMPDH). This blocks the de novo purine synthesis pathway — the pathway that activated lymphocytes rely on more heavily than most other cell types, which is what gives mycophenolate its relative selectivity for suppressing immune activation over other rapidly dividing tissue.

The most common practical limitation of mycophenolate mofetil is gastrointestinal side effects — diarrhea and cramping — caused by direct mucosal effects of the drug. Switching to an enteric-coated formulation (mycophenolate sodium) can reduce these symptoms while maintaining immunosuppressive efficacy, which is a useful next step before abandoning the drug entirely for GI intolerance.

High-Yield

Methotrexate inhibits DHFR; mycophenolate mofetil inhibits IMPDH — different enzymes, same downstream goal of starving nucleotide synthesis.
Methotrexate needs baseline and periodic liver function tests for hepatotoxicity monitoring.
Azathioprine requires TPMT testing before starting — low/absent activity risks severe, potentially fatal myelosuppression.
Enteric-coated mycophenolate sodium is the practical fix for dose-limiting GI side effects on mycophenolate mofetil.

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