basiliximab

**Mechanism:** Basiliximab binds the alpha subunit (CD25, Tac antigen) of the high-affinity IL-2 receptor. CD25 is expressed mainly on activated T cells. Blocking it prevents IL-2 from driving clonal expansion of T cells, which is a key step in acute cellular rejection. It is a competitive antagonist and does not deplete lymphocytes. This differs from antithymocyte globulin (ATG) and alemtuzumab, which deplete cells.

**Structure/naming:** The name ends in "-ximab," meaning chimeric (about 25% murine, 75% human IgG1κ). Naming pattern: -omab = murine, -ximab = chimeric, -zumab = humanized, -umab = fully human. Daclizumab was a humanized anti-CD25 antibody. It was withdrawn from the market, and its later MS formulation was also withdrawn.

**Clinical use:** - Induction therapy in renal transplantation, given with cyclosporine (or tacrolimus), corticosteroids, and often mycophenolate. - Typical dosing is 20 mg IV on day 0 (before or at transplant) and day 4. - It is used off-label in other solid organ transplants. - It is generally chosen for lower-immunologic-risk recipients. Higher-risk patients usually get ATG instead.

**Adverse effects:** - It is generally well tolerated and has fewer infusion reactions than ATG or muromonab-CD3 (OKT3). It does not cause cytokine release syndrome. - Hypersensitivity reactions are possible, including anaphylaxis, especially on re-exposure. Human anti-chimeric antibodies can develop. - Infection risk is increased, as with any immunosuppression (CMV, BK virus). Long-term malignancy risk (such as PTLD) is a general concern with immunosuppression. - GI symptoms and electrolyte disturbances are reported. - Only physicians experienced in immunosuppression and transplant management should prescribe it.

**High-yield comparisons:** - Muromonab-CD3 (OKT3) targets CD3 and causes a cytokine release syndrome. It has been discontinued. - Alemtuzumab targets CD52 and depletes lymphocytes. - Rituximab targets CD20 on B cells. - Belatacept is a CTLA4-Ig fusion protein that blocks CD28 costimulation. It is used in kidney transplant maintenance, and its risk of PTLD is higher in EBV-seronegative patients. - Calcineurin inhibitors (cyclosporine, tacrolimus) block IL-2 transcription. Basiliximab blocks the IL-2 receptor, so the two act on the same pathway at different points. - Sirolimus (mTOR inhibitor) blocks the response to IL-2 signaling.

**Board pearl:** A kidney transplant question that mentions "anti-CD25," "IL-2 receptor antagonist," or "induction therapy with minimal side effects" points to basiliximab. Its main role is to prevent acute rejection, which is T-cell mediated, in the first weeks after transplant.