Abatacept is a biologic medicine used to treat moderate to severe rheumatoid arthritis, psoriatic arthritis and polyarticular juvenile idiopathic arthritis in people whose disease has not responded well enough to other medicines. It belongs to a group of drugs called selective T-cell costimulation modulators. Abatacept works by calming down overactive T-cells, a type of white blood cell that drives joint inflammation and damage in these autoimmune conditions. It is not a tablet or capsule. It is given as an intravenous (IV) infusion into a vein or as a subcutaneous (under-the-skin) injection, usually alongside other arthritis medicines such as methotrexate [1].

Abatacept is a fusion protein, not a small chemical molecule like most tablets. It combines part of a natural human protein called CTLA-4 with a fragment of human immunoglobulin (antibody) [3]. This design lets it sit on the surface of immune cells and block a step called "costimulation," which T-cells need before they can become fully active and start attacking the body's own joint tissue [2][3].

By interrupting this step early, abatacept reduces the inflammation, pain and joint damage seen in rheumatoid arthritis, psoriatic arthritis and polyarticular juvenile idiopathic arthritis in children aged 2 years and above [1][3]. Because it works on the immune system in this targeted way, it is classed as a disease-modifying antirheumatic drug (DMARD), specifically a biologic DMARD, rather than a painkiller.

Abatacept does not permanently resolve these conditions. It is used long-term to control disease activity, and its full benefit is usually judged after several months of regular treatment rather than after a single dose. It is generally prescribed by a rheumatologist and used under ongoing medical supervision, because it changes how the immune system works and needs monitoring for infection and other effects described later in this article [1].

Abatacept is available as a powder that is reconstituted and given as an IV infusion, and also as a ready-to-use prefilled syringe or autoinjector for subcutaneous injection that some patients learn to give themselves at home [1][4].

The active ingredient is abatacept, a soluble fusion protein made of the extracellular domain of human CTLA-4 joined to a modified Fc portion of human immunoglobulin G1 (IgG1) [1][3]. It is produced using recombinant DNA technology, not extracted from a plant or chemically synthesised like conventional tablets. The IV formulation is supplied as a lyophilised (freeze-dried) powder that is reconstituted with sterile water before infusion, while the subcutaneous formulation is a ready-to-use liquid solution in a prefilled syringe or autoinjector [1]. In terms of marketed strength, the IV formulation comes as a single-use vial containing 250 mg of abatacept powder for reconstitution, and the subcutaneous formulation is supplied as a 125 mg/mL solution in a prefilled syringe or autoinjector [1].

In Adults

• Moderate to severe active rheumatoid arthritis, usually when other DMARDs have not worked well enough [1][3]
• Active psoriatic arthritis [1][3]

In Children (2 Years and Older)

• Active polyarticular juvenile idiopathic arthritis [1][3]

In Transplant Patients

• Prevention (prophylaxis) of acute graft-versus-host disease in certain transplant patients aged 2 years and older, as an add-on to standard treatment [1]

1. Targets T-cells directly. Abatacept blocks the costimulation signal that T-cells need to become active, addressing a root driver of joint inflammation rather than only easing pain [2][3].
2. Two ways to take it. It is available as an IV infusion given at a clinic or as a subcutaneous injection that many patients can learn to give at home, allowing some flexibility in routine [1][4].
3. Option when other DMARDs are not enough. It gives doctors another treatment path for people whose rheumatoid, psoriatic or juvenile idiopathic arthritis has not responded adequately to standard DMARDs [1][3].
4. Established long-term use. Abatacept has been authorised and used in rheumatoid arthritis since 2005, giving a long track record of clinical experience [4][3].

Common Side Effects (Usually Mild and May Lessen With Continued Use)

• Headache [1][2]
• Upper respiratory tract infection and nasopharyngitis (common cold-type symptoms) [1]
• Nausea [1][2]
• Diarrhoea [2]
• Back pain [2]
• Injection site reactions (redness, swelling or itching) with the subcutaneous form [1]
• Infusion-related reactions such as dizziness, headache or nausea during or shortly after an IV dose [1][2]

Serious Side Effects (Seek Urgent Medical Attention)

• Signs of a serious infection, such as persistent fever, chills, a cough that will not go away, or unusual tiredness; serious infections requiring hospitalisation have occurred in clinical trials [1][2]
• Signs of an allergic reaction, including swelling of the face, lips or throat, severe rash, or difficulty breathing (anaphylaxis has been reported, though rarely) [1]
• Symptoms of tuberculosis reactivation, such as a persistent cough, weight loss, night sweats or low-grade fever [1]
• Signs of liver problems, such as yellowing of the skin or eyes, dark urine, or unusual tiredness [2]
• New or worsening breathing problems, particularly in people with chronic obstructive pulmonary disease (COPD) [1]
• Unusual lumps, swelling, or other possible signs of cancer, which should always be checked by a doctor [1][2]

Abatacept is a biologic medicine and is never swallowed as a tablet. It is given in one of two ways, and the method your doctor chooses depends on your condition and preference [1][4]:

• Intravenous (IV) infusion: A healthcare professional reconstitutes the powder and gives it as a drip into a vein over about 30 minutes. This is usually done in a hospital or clinic. The first three infusions are typically spaced two weeks apart, and further infusions continue every four weeks after that, as decided by the treating doctor [1].
• Subcutaneous (under-the-skin) injection: This comes as a ready-to-use prefilled syringe or autoinjector, usually given once a week. A doctor or nurse will first teach patients or caregivers how to prepare and inject the dose correctly, commonly into the stomach area or thigh, before self-injection at home is attempted [1][4].

There are no special food restrictions with abatacept, and it can generally be used with or without meals unless a doctor advises otherwise [2]. Do not change the injection site pattern, reuse needles or syringes, or alter the schedule without checking with the prescribing doctor. Each dose should be given as close as possible to the day it is due to keep blood levels steady.

Abatacept is a selective T-cell costimulation modulator [1][3]. In rheumatoid, psoriatic and juvenile idiopathic arthritis, T-cells become abnormally activated and drive ongoing joint inflammation. Full activation of a T-cell needs two signals: one from an antigen, and a second "costimulatory" signal delivered when proteins called CD80 and CD86 on antigen-presenting cells bind to a receptor called CD28 on the T-cell [1][3].

Abatacept is built from the extracellular part of human CTLA-4, a natural protein that also binds CD80 and CD86, joined to part of a human antibody [1][3]. By binding CD80 and CD86 first, abatacept blocks their interaction with CD28, so the T-cell does not receive its second activation signal [1][3]. This dampens T-cell activation and reduces the release of inflammatory chemical messengers (cytokines) that would otherwise damage the joints [2][3]. Because this mechanism suppresses part of the immune system rather than acting only on pain pathways, abatacept carries the infection-related precautions described below.

• Abatacept is given by infusion or injection only; it is never a tablet to swallow [1][4].
• Tell your doctor about any current infection, or history of tuberculosis or hepatitis B, before your first dose [1][2].
• Keep every infusion and vaccination appointment as scheduled; do not skip screening blood tests [1][2].
• Learn the correct subcutaneous injection technique from a nurse before self-injecting at home [1][4].
• Report a persistent cough, fever, unusual tiredness, or unusual bruising to your doctor promptly, since these can be early warning signs [1][2].
• Avoid live vaccines while on treatment and discuss any planned vaccination with your doctor first [1].

Store abatacept prefilled syringes or autoinjectors in a refrigerator and do not freeze them [2]. Keep them in their original carton to protect the medicine from light, and keep it out of reach of children [2]. The IV powder is reconstituted just before use by a healthcare professional, and once mixed, the diluted solution should be used within 24 hours [1]. Do not use the medicine past its expiry date, and do not shake the prefilled syringe or autoinjector.

No specific food or drink has been identified that requires abatacept to be avoided, and no special dietary changes are described in the prescribing information [2]. Alcohol is not listed as a direct interaction with abatacept itself, but since abatacept can affect liver enzymes in a small proportion of patients, discuss regular alcohol use with your doctor so liver function can be monitored appropriately [3].

• TNF inhibitors (etanercept, adalimumab, infliximab, and similar biologics): Combining these with abatacept is not recommended. Clinical trial data showed a higher rate of serious infections with the combination compared with either medicine used alone, without extra benefit for disease control [1][2].
• Other biologic DMARDs (such as anakinra) and JAK inhibitors: Concomitant use with abatacept is not recommended because it further increases the risk of serious infection without established added benefit [1].
• Live vaccines: Live vaccines should not be given during treatment with abatacept, or for a period after stopping it, because the weakened immune response may not handle a live vaccine safely [1].
• Other immunosuppressants: Using abatacept together with other drugs that suppress the immune system may raise infection risk further, so this combination needs close medical supervision [1].

• Active or chronic infections: Abatacept should not be started in a patient who has an active infection, including a localised infection, until it is treated [1].
• Latent tuberculosis: Patients should be screened for latent TB before starting abatacept, because reactivation of latent TB has been reported with this class of medicine [1].
• Hepatitis B: Abatacept can reactivate hepatitis B virus in patients with current or past infection (HBsAg-positive or anti-HBc-positive - blood test results showing current or past hepatitis B infection), sometimes leading to serious liver problems; screening before treatment and monitoring during treatment is recommended, with antiviral prophylaxis considered where appropriate [2][5].
• Chronic obstructive pulmonary disease (COPD): In a clinical trial, patients with COPD treated with abatacept had more frequent respiratory adverse events, including COPD exacerbation, cough, and shortness of breath, than those on placebo, so abatacept should be used with caution and close monitoring in people with COPD [1].
• History of cancer: Because abatacept affects immune surveillance, doctors weigh the risks and benefits carefully in patients with a current or past malignancy; cases of malignancy, including lymphoma, have been observed in patients treated with abatacept and related biologics [1].

Abatacept dosing is decided and adjusted only by the treating doctor, and is not a fixed home-measured daily dose like a tablet. In general terms, based on the prescribing information [1]:

• For IV infusion in adults, the dose is calculated by body weight, with the treating team giving the first three infusions two weeks apart, followed by infusions every four weeks.
• For subcutaneous injection in adults, a fixed-strength prefilled syringe or autoinjector is typically given once a week.
• In children, the subcutaneous dose is adjusted according to body weight bands set by the prescriber.

Always follow the exact dose, strength and schedule your doctor or rheumatology team has prescribed, and do not change the frequency or amount on your own.

In clinical studies, doses higher than the standard recommended amount were given without new or unexpected toxicity being seen, and no specific overdose antidote is described [1]. If more than the prescribed dose is given, or if the injection schedule is not followed correctly, contact a doctor or emergency medical services immediately for advice and monitoring rather than waiting for symptoms to appear [2]. Watch for signs such as unusual dizziness, fainting, a fast heartbeat, or symptoms of an allergic reaction, and seek urgent medical attention if these occur.

If a subcutaneous injection dose is missed, contact your doctor or rheumatology nurse for guidance on when to give the next dose; do not simply double up the next injection to make up for a missed one [2]. If a scheduled IV infusion appointment is missed, contact the clinic or your doctor as soon as possible to arrange a new appointment rather than skipping it altogether [2]. Never adjust the dosing schedule yourself.

Contact your doctor promptly if you notice any of the following while on abatacept:

• Fever, chills, persistent cough, or other signs of infection [1][2]
• Symptoms suggesting tuberculosis, such as a cough lasting several weeks, unexplained weight loss, or night sweats [1]
• Yellowing of the skin or eyes, dark urine, or unusual tiredness that could suggest a liver problem [2][3]
• Swelling of the face, lips, tongue or throat, severe rash, or difficulty breathing during or after a dose [1]
• New or worsening breathlessness or cough, especially if you have COPD [1]
• Any unusual lump, swelling, or other possible sign of cancer [1][2]
• A missed infusion appointment or an injection you are unsure how to manage [2]