Ibrutinib (Imbruvica™) is an irreversible, potent inhibitor of Bruton's tyrosine kinase (BTK). Over the last few years, ibrutinib has developed from a promising drug candidate to being approved by FDA for the treatment of three B cell malignancies, a truly remarkable feat. Few, if any medicines are monospecific and ibrutinib is no exception; already during ibrutinib's initial characterization, it was found that it could bind also to other kinases. In this review, we discuss the implications of such interactions, which go beyond the selective effect on BTK in B cell malignancies. In certain cases, the outcome of ibrutinib treatment likely results from the combined inhibition of BTK and other kinases, causing additive or synergistic, effects. Conversely, there are also examples when the clinical outcome seems unrelated to inhibition of BTK. Thus, more specifically, adverse effects such as enhanced bleeding or arrhythmias could potentially be explained by different interactions. We also predict that during long‐term treatment bone homoeostasis might be affected due to the inhibition of osteoclasts. Moreover, the binding of ibrutinib to molecular targets other than BTK or effects on cells other than B cell‐derived malignancies could be beneficial and result in new indications for clinical applications.

Ca 2ϩ is a universal messenger from bacterial to mammalian cells since its concentration gradients across both organelle and plasma membranes can be efficiently used to communicate biological signals. Therefore, the control of the intracellular free Ca 2ϩ concentration [Ca 2ϩ ] i 3 is of crucial importance for the regulation of many cellular functions, including proliferation, contraction, fertilization, motility, apoptosis, and cell death (1). Receptor-mediated Ca 2ϩ influx from the extracellular space is one important mechanism to control [Ca 2ϩ ] i in non-excitable cells, e.g. leukocytes (2). Although the molecular machinery underlying Ca 2ϩ entry is still poorly defined, cation channels of the transient receptor potential (TRP) family that includes several subfamilies (3-5) are likely candidates for Ca 2ϩ entry pathways under the control of membrane receptors. TRPM2 (formerly LTRPC2 and TRPC7) is a member of the TRPM subfamily. TRPM2 forms non-selective Ca 2ϩ -permeable cation channels and is mainly expressed in brain and in cells of the immune system (6 -8). Opening of the channel is induced by intracellular ADP-ribose (ADPR; Refs. 6 and 7) and enhanced by increased cytosolic Ca 2ϩ (9). Whether NAD also activates TRPM2 currents is still controversial (6, 7, 10, 11). The nudix box in the cytosolic C-terminal region of TRPM2, a conserved motif of enzymes with nucleotide pyrophosphatase activity, seems to be responsible for gating of TRPM2 by ADPR and possibly by NAD (6,7,12). An involvement of TRPM2 in cellular signaling processes has been proposed since the expression of TRPM2 confers susceptibility to oxidant-induced cell death (11).A key question in the field relates to the potential role of the NAD metabolite ADPR as a second messenger. A function of NAD or ADPR as the missing link between specific extracellular signals and Ca 2ϩ influx mediated by TRPM2 has been hypothesized (8,13,14), but experimental proofs are missing so far. To test this hypothesis directly, we developed a method to measure intracellular levels of ADPR in Jurkat T cells (15). We report that cytosolic ADPR concentrations are raised in response to concanavalin A (ConA) and induce Ca 2ϩ entry through TRPM2, thereby significantly increasing [Ca 2ϩ ] i . Inhibition of intracellular ADPR formation or gene silencing of TRPM2 efficiently diminished receptor-mediated Ca 2ϩ influx carried by TRPM2. Moreover, blockade of ADPR formation also efficiently blocked ConA-induced cell death. EXPERIMENTAL PROCEDURESElectrophysiology-Membrane currents were recorded in the wholecell configuration of the patch clamp technique (16) or the perforatedpatch configuration with nystatin (17). An EPC9 patch clamp amplifier was used in conjunction with the PULSE stimulation and data acquisition software (HEKA Elektronik, Lamprecht, Germany). The patch electrodes were made from 1.5-mm diameter borosilicate glass capillaries and filled with intracellular solution. Data were low pass-filtered at 1 kHz and compensated for both fast and slow capacity transients. Se...

Our study fills an important knowledge gap in how acid treatment affects PLT function and interactions with immune cells, paving the way for controlled clinical trials to evaluate acid-treated PLTs as an alternative to HLA-matched donors in PLT refractoriness.