Medical uses

Ibrutinib is used to treat chronic lymphocytic leukemia (CLL), Waldenström’s macroglobulinemia, and as a second-line treatment for mantle cell lymphoma, marginal zone lymphoma, and chronic graft vs host disease.^[3][4] BTK inhibition by ibrutinib impairs the BCR signaling pathway, which is a key dependency of malignant B cells, but it has been shown that BTK inhibition also interferes with CD40 signaling, B cell adhesion and migration.

In the United States ibrutinib is indicated for the treatment of adults with mantle cell lymphoma (MCL) who have received at least one prior therapy, adults with chronic lymphocytic leukemia (CLL)/small lymphocytic lymphoma (SLL) with or without 17p deletion, adults with Waldenström’s macroglobulinemia (WM), adults with marginal zone lymphoma (MZL) who require systemic therapy and have received at least one prior anti-CD20-based therapy, adults with chronic graft versus host disease (cGVHD) after failure of one or more lines of systemic therapy.^{[4][7][8][9][10]}

It is a first line treatment in those with CLL who require treatment and are newly diagnosed.^[11] It may also be used in CLL that relapses.^[11]

Adverse effects

Very common (>10% frequency) adverse effects include pneumonia, upper respiratory tract infection, sinusitis, skin infection, low neutrophil count, low platelet counts, headache, bleeding, bruising, diarrhea, vomiting, inflammation of mouth and lips, nausea, constipation, rash, joint pain, muscle spasms, musculoskeletal pain, fever, and edema.^[3]

Common (1–10% frequency) adverse effects include sepsis, urinary tract infection, non-melanoma skin cancer (basal-cell carcinoma, squamous cell carcinoma), low leukocyte count, low lymphocyte count, interstitial lung disease, tumor lysis syndrome,^[12] high uric acid levels, dizziness, blurred vision, atrial fibrillation, subdural hematoma, nosebleeds, small bruises from broken blood vessels, high blood pressure, hives, and skin redness or blushing.^[3]

Pharmacology

Ibrutinib oral bioavailability is 3.9% in a fasting state, 8.4% in a fed state, and 15.9% after consumption of grapefruit juice.^[13]

Mechanism

Ibrutinib is a potent, irreversible inhibitor of Bruton’s tyrosine kinase (BTK). It forms a covalent bond with the cysteine residue C481 in the BTK active site, leading to sustained inhibition of BTK enzymatic activity. BTK is an important signaling molecule of the B-cell antigen receptor (BCR) pathway, which is plays a role in the pathogenesis of several B-cell malignancies including mantle cell lymphoma (MCL), diffuse large B-cell lymphoma (DLBCL), follicular lymphoma, and chronic lymphocytic leukemia (CLL). Preclinical studies have shown that ibrutinib effectively inhibits malignant B-cell proliferation and survival in vivo as well as cell migration and substrate adhesion in vitro.^[5]

In early clinical studies, the activity of ibrutinib has been described to include a rapid reduction in lymphadenopathy accompanied by a transient lymphocytosis, suggesting that the drug might have direct effects on cell homing or migration to factors in tissue microenvironments.^[14]

In preclinical studies on chronic lymphocytic leukemia (CLL) cells, ibrutinib has been reported to promote apoptosis, inhibit proliferation, and also prevent CLL cells from responding to survival stimuli provided by the microenvironment.^[15] This also leads to a reduction of MCL1 levels (anti-apoptotic protein) in malignant B cells.^[15] Treatment of activated CLL cells with ibrutinib resulted in inhibition of BTK tyrosine phosphorylation and also effectively abrogated downstream survival pathways activated by this kinase including ERK1/2, PI3K, and NF-κB. Additionally, ibrutinib inhibited proliferation of CLL cells in vitro, effectively blocking survival signals provided externally to CLL cells from the microenvironment including soluble factors (BAFF, IL-6, IL-4, and TNF-α), fibronectin engagement and stromal cell contact.

Ibrutinib has also been reported to reduce chronic lymphocytic leukemia cell chemotaxis towards the chemokines CXCL12 and CXCL13, and inhibit cellular adhesion following stimulation at the B-cell receptor (BCR).^[16][17] Additionally, ibrutinib down-modulates the expression of CD20 (target of rituximab/ofatumumab) by targeting the CXCR4/SDF1 axis.^[15] Together, these data are consistent with a mechanistic model whereby ibrutinib blocks BCR signaling, which drives cells into apoptosis and/or disrupts cell migration and adherence to protective tumour microenvironments.