Last reviewed 2026-04-20 · How we verify

Litfulo (pf-06651600)

Litfulo works by blocking the ITK/TSK enzyme, which helps to stop the immune system from attacking hair follicles.

Litfulo (PF-06651600) is a small molecule developed by Pfizer Inc, targeting the tyrosine-protein kinase ITK/TSK. It was approved by the FDA in 2023 for the treatment of alopecia areata. The commercial status of Litfulo is patented, as it is owned by Pfizer Inc. Key safety considerations are not publicly available. Litfulo works by inhibiting the ITK/TSK enzyme, which plays a role in the immune system's response to hair loss.

At a glance

Mechanism of action

LITFULO is kinase inhibitor. Ritlecitinib irreversibly inhibits Janus kinase (JAK3) and the tyrosine kinase expressed in hepatocellular carcinoma (TEC) kinase family by blocking the adenosine triphosphate (ATP) binding site. In cellular settings, ritlecitinib inhibits cytokine induced STAT phosphorylation mediated by JAK3-dependent receptors. Additionally, ritlecitinib inhibits signaling of immune receptors dependent on TEC kinase family members. The relevance of inhibition of specific JAK or TEC family enzymes to therapeutic effectiveness is not currently known.

Approved indications

• Severe alopecia areata

Boxed warnings

• WARNING: SERIOUS INFECTIONS, MORTALITY, MALIGNANCY, MAJOR ADVERSE CARDIOVASCULAR EVENTS (MACE), and THROMBOSIS WARNING: SERIOUS INFECTIONS, MORTALITY, MALIGNANCY, MAJOR ADVERSE CARDIOVASCULAR EVENTS (MACE), and THROMBOSIS See full prescribing information for complete boxed warning. • Increased risk of serious bacterial, fungal, viral, and opportunistic infections that may lead to hospitalization or death, including tuberculosis (TB). Interrupt treatment if serious infection occurs until the infection is controlled. LITFULO should not be given to patients with active tuberculosis. Test for latent TB before and during therapy; start treating latent TB prior to use. Monitor all patients for active TB during treatment, even patients with initial negative, latent TB test. ( 5.1 ). Monitor all patients for signs and symptoms of infection during and after treatment with LITFULO. ( 5.1 ) • Higher rate of all-cause mortality, including sudden cardiovascular death with another Janus kinase inhibitor (JAK) vs. TNF blockers in rheumatoid arthritis (RA) patients. LITFULO is not approved for use in RA patients. ( 5.2 ) • Malignancies were reported in patients treated with LITFULO ( 5.3 ). Higher rate of lymphomas and lung cancers with another JAK inhibitor vs. TNF blockers in RA patients. • Higher rate of MACE (defined as cardiovascular death, myocardial infarction, and stroke) with another JAK inhibitor vs. TNF blockers in RA patients. ( 5.4 ). • Thrombosis has occurred in patients treated with LITFULO. Increased incidence of pulmonary embolism, venous and arterial thrombosis with another JAK inhibitor vs. TNF blockers. ( 5.5 ) • Increased risk of serious bacterial, fungal, viral and opportunistic infections leading to hospitalization or death, including tuberculosis (TB). Interrupt treatment if serious infection occurs until the infection is controlled. LITFULO should not be given to patients with active tuberculosis. Test for latent TB before and during therapy; treat latent TB prior to use. Monitor all patients for active TB during treatment, even patients with initial negative, latent TB test. ( 5.1 ) • Higher rate of all-cause mortality, including sudden cardiovascular death with another Janus kinase inhibitor (JAK) vs. TNF blockers in rheumatoid arthritis (RA) patients. LITFULO is not approved for use in RA patients. ( 5.2 ) • Malignancies have occurred in patients treated with LITFULO [see Warnings and Precautions (5.3) ] . Higher rate of lymphomas and lung cancers with another JAK inhibitor vs. TNF blockers in RA patients. ( 5.3 ) • Higher rate of MACE (defined as cardiovascular death, myocardial infarction, and stroke) with another JAK inhibitor vs. TNF blockers in RA patients. ( 5.4 ) • Thrombosis has occurred in patients treated with LITFULO. Increased incidence of pulmonary embolism, venous and arterial thrombosis with another JAK inhibitor vs. TNF blockers. ( 5.5 )

Common side effects

• Headache
• Diarrhea
• Acne
• Rash
• Urticaria
• Folliculitis
• Pyrexia
• Dermatitis atopic
• Dizziness
• Blood creatine phosphokinase increased
• Herpes zoster
• Red blood cell count decreased

Drug interactions

• CYP3A substrates
• CYP1A2 substrates
• Strong CYP3A inducers (e.g., rifampin)

Key clinical trials

• Long-Term PF-06651600 for the Treatment of Alopecia Areata (PHASE3)
• Ritlecitinib (PF-06651600) in Participants With Chronic Spontaneous Urticaria (PHASE2)
• A 16-Week Study to Learn About the Study Medicine Called Ritlecitinib in Adults With Long Lasting Painful Red Skin Lumps, Known by the Medical Term, Hidradenitis Suppurativa, or HS. (PHASE2)
• Pre-approval Single-patient Expanded Access for Ritlecitinib (PF-06651600)
• A Study to Learn About Ritlecitinib for the Potential Treatment of Chronic Spontaneous Urticaria in Adults. (PHASE2)
• A Study of 2 Doses of Ritlecitinib in People 12 Years of Age and Older With Alopecia Areata (PHASE3)
• Ritlecitinib in CTCL (PHASE2)
• PLACEBO-CONTROLLED SAFETY STUDY OF RITLECITINIB (PF-06651600) IN ADULTS WITH ALOPECIA AREATA (PHASE2)

Patents

Primary sources

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Competitive intelligence

For the full competitive landscape — auto-detected comparators, recent regulatory actions across the set, upcoming PDUFA, patent timeline, sponsor landscape:

• Litfulo CI brief — competitive landscape report
• Litfulo updates RSS · CI watch RSS
• Pfizer portfolio CI