# JAK Inhibitor Selectivity in Detail > JAK inhibitor selectivity in MBBS Pharmacology: JAK1-3 and TYK2 map, tofacitinib, baricitinib, deucravacitinib and class boxed warnings. - Canonical URL: https://prepelephant.com/topics/mbbs/pharmacology/jak-selectivity-detail - Exam / course: MBBS · Subject: Pharmacology - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "JAK Inhibitor Selectivity in Detail", PrepElephant, https://prepelephant.com/topics/mbbs/pharmacology/jak-selectivity-detail ## Direct answer Four Janus kinases transcribe more than fifty cytokine signals, and every JAK inhibitor is a bet on which one to silence. JAK2 carries erythropoietin and thrombopoietin, so JAK2 blockade costs anaemia and cytopenias; JAK3 sits on the common gamma chain of lymphocyte cytokines (IL-2, 4, 7, 9, 15, 21); JAK1 channels most inflammatory signals; TYK2 handles IL-12, IL-23 and type I interferons. Tofacitinib (JAK1/3) treats rheumatoid arthritis, psoriasis and ulcerative colitis; baricitinib (JAK1/2) and upadacitinib (JAK1) treat RA and atopic dermatitis; ruxolitinib (JAK1/2) treats myelofibrosis — where blocking JAK2 is the point, not the price. The 2021 ORAL Surveillance trial drove class-wide boxed warnings: serious infection, malignancy, venous thromboembolism and major cardiac events versus TNF inhibitors in vulnerable patients. ## What you must remember - **Selectivity map:** tofacitinib JAK1/3, 5 mg twice daily; baricitinib JAK1/2, 4 mg daily; upadacitinib JAK1-selective, 15 mg; abrocitinib JAK1, for atopic dermatitis; ruxolitinib JAK1/2 (myelofibrosis, polycythaemia vera, topical for vitiligo); fedratinib JAK2 (myelofibrosis). - **Toxicity logic:** JAK2 inhibition → anaemia, neutropenia, thrombocytopenia; JAK1/JAK3 broadly → infection and malignancy surveillance; the profile follows the cytokines, not the brand. - **Class boxed warnings (post-ORAL Surveillance):** serious infections, malignancy, venous thromboembolism, major adverse cardiac events — tofacitinib versus TNF inhibitors in patients aged 50 and above with cardiovascular risk rewrote the class label. - **Deucravacitinib:** allosteric TYK2 inhibitor binding the pseudokinase (JH2) domain rather than the active site — cleaner selectivity, approved for psoriasis; the exam's favourite recent addition. - **Topical ruxolitinib:** the first FDA-approved therapy to repigment vitiligo (2022) — a dermatology crossover worth quoting. - **Screening before starting:** latent TB, hepatitis B and C, zoster history (shingles risk is high); full blood count and lipids at baseline and follow-up; avoid live vaccines. - **Dose adjustments:** reductions in moderate renal impairment and hepatic impairment; cytopenia thresholds gate initiation. - **Repurposing footnote:** baricitinib won COVID-19 approval (ACTT-2) — JAK inhibition as anticytokine therapy, with AAK1 effects speculated. ## Reading the selectivity like a haematologist The map becomes intuitive when each JAK is attached to its owners. Myelofibrosis patients suffer from constitutively active JAK2 signalling — spleen enlargement, night sweats, cachexia — so ruxolitinib, a JAK1/2 inhibitor, treats the disease's engine even though it drops haemoglobin and platelets; the monitoring is blood counts, because the target is haematopoietic. Rheumatoid arthritis wants JAK1's inflammatory cytokines and JAK3's lymphocyte signalling, but wants EPO and TPO left alone — hence upadacitinib's JAK1 selectivity buys anaemia-avoidance, and why haemoglobin checks matter more on baricitinib than on tofacitinib. Deucravacitinib pushes the logic to its end: by binding TYK2's pseudokinase domain allosterically, it spares the catalytic site shared across the family, cutting cross-reactive toxicity — the pharmacological argument for why "selectivity" now means binding-site choice as much as target choice. Add ORAL Surveillance and the caution: selectivity mitigates haematologic cost, not the class-wide infection, VTE and malignancy signal in older, risk-laden patients. The student who walks this chain — cytokine, JAK, drug, monitorable toxicity — answers any JAK question thrown at them. ## Where students slip The first slip is equating selectivity with safety wholesale — JAK1-selective drugs avoid anaemia but inherit the boxed warnings; ORAL Surveillance used tofacitinib, and the class label followed. The second is misassigning targets: baricitinib is JAK1/2 (not JAK3), tofacitinib is JAK1/3 — the pairs matter because JAK2 ownership of haematopoiesis is the toxicity axis. Third, students forget ruxolitinib's dermatology crossover — topical vitiligo repigmentation — which is now a favourite one-liner. Fourth, the screening set is treated as oncology-flavoured ritual: TB, hepatitis, zoster and baseline counts are as mandatory here as for any biologic, with shingles the signature risk. Fifth, deucravacitinib gets filed as "another JAK inhibitor" without its allosteric distinction — the one structural innovation in the class that changed the toxicity conversation. Finally, thrombosis: the VTE signal concentrates in older patients with risk factors, so the practical rule is age-plus-comorbidity gating, not blanket fear — and saying exactly that is the mark. ## Frequently asked questions ### Which JAK does tofacitinib inhibit? JAK1 and JAK3 — matching its inflammatory and lymphocyte-cytokine indications while sparing JAK2-driven haematopoiesis partially. ### Why does JAK2 inhibition cause anaemia? Erythropoietin and thrombopoietin signal through JAK2, so blockade suppresses red cell and platelet production. ### What did the ORAL Surveillance trial change? Tofacitinib showed higher rates of serious infection, malignancy, VTE and major cardiac events than TNF inhibitors in older at-risk patients, driving class-wide boxed warnings. ### How is deucravacitinib different from other JAK inhibitors? It binds TYK2's pseudokinase domain allosterically rather than the conserved catalytic site, achieving cleaner selectivity. ### Which JAK inhibitor repigments vitiligo? Topical ruxolitinib — the first FDA-approved treatment to restore pigment in nonsegmental vitiligo.