Bortezomib induces thrombocytopenia by the inhibition of proplatelet formation of megakaryocytes

Abstract

Bortezomib is a potent proteasome inhibitor that has been extensively used to treat multiple myeloma. One of the most common grade 3 adverse events is cyclic thrombocytopenia. In this study, we studied the mechanism by which bortezomib induces thrombocytopenia in a mouse model. After the intravenous administration of bortezomib (2.5 mg/kg) via tail vein, platelet counts significantly decreased on days 2-4 and recovered to the normal range on day 6. Bortezomib (2.5 mg/kg) injected into mice in vivo did not affect colony-forming unit-megakaryocytes (CFU-Mk) or megakaryocytes in the bone marrow. However, proplatelet formation (PPF) significantly decreased on days 2 and 4, after bortezomib administration to mice. Meanwhile, CFU-Mk formation and the ploidy distribution of cultured megakaryocytes in vitro were not affected by bortezomib used at concentrations of ≤ 1 ng/mL. The PPF of megakaryocytes in vitro significantly decreased with 0.1, 1, 10, and 100 ng/mL bortezomib. Considering the bortezomib concentration in clinical studies, these data strongly suggest that decreased PPF activity induces thrombocytopenia. To elucidate the mechanism behind decreased PPF, Western blot was performed. Activated Rho expression increased after the incubation of murine platelets with bortezomib. Decreased PPF activity was eliminated by the addition of Y27632, a Rho kinase inhibitor, in vitro. Given that the Rho/Rho kinase pathway is a negative regulator of PPF, bortezomib increases activated Rho, inducing decreased PPF, which results in decreased platelet count.

Proteasome function is required for platelet production

“…In agreement with the rescue of proplatelet formation observed in bortezomib-treated human megakaryocytes, mouse megakaryocytes treated with bortezomib plus Y27632 or with bortezomib plus fasudil, a more clinically relevant p160ROCK inhibitor, formed proplatelets ( Figure 4D). These results in mouse megakaryocytes were similar to a recent report by Murai et al (19).…”

Bortezomib-Based Therapy for Newly Diagnosed Mantle-Cell Lymphoma

“…32 Rates of thrombocytopenia were increased in the VR-CAP group, but there was no betweengroup difference in rates of clinically significant bleeding events, similarly low rates of cycle delays due to thrombocytopenia, and no trend toward cumulative toxicity on thrombopoiesis, reflecting the previously reported transient, cyclical nature of platelet-count reduction with bortezomib. [33][34][35][36] Notably, supportive therapies, which were allowed according to the protocol, were a key aspect of patient treatment in this study. The higher rate of platelet transfusion in the VR-CAP group may have been associated with the aim of maximizing bortezomib dose intensity; data indicate that the primary use of platelet transfusion was prophylactic (as allowed according to the protocol at the investigators' discretion) to avoid withholding the bortezomib dose on day 11 because of a platelet count of less than 25,000 per cubic millimeter, rather than therapeutic, because of a platelet count of less than 10,000 per cubic millimeter.…”

Targeting Bortezomib to Bone Increases Its Bone Anabolic Activity and Reduces Systemic Adverse Effects in Mice

“…(10,11) However, Btz has significant side effects, including peripheral neuropathy and thrombocytopenia, (12) which restrict its use as a bone anabolic drug. In mice, Btz causes death of thymic cells, (11) thrombocytopenia, (13,14) and damage to dorsal root ganglia (DRG). (15,16) These adverse effects result from the systemic distribution of the drug.…”