Evaluation of the effect of vincristine on platelet count in dogs with lymphoma

ACVIM consensus statement on the treatment of immune thrombocytopenia in dogs and cats

Management of immune thrombocytopenia (ITP) in dogs and cats is evolving, but there are no evidence-based guidelines to assist clinicians with treatment decisions. Likewise, the overall goals for treatment of ITP have not been established. Immunosuppressive doses of glucocorticoids are the first line treatment, but optimal treatment regimens beyond glucocorticoids remain uncertain. Additional options include secondary immunosuppressive drugs such as azathioprine, modified cyclosporine, and mycophenolate mofetil, usually selected based on clinician preference. Vincristine, human IV immunoglobulin (hIVIg), and transfusion of platelet or red blood cell-containing products are often used in more severe cases. Splenectomy and thrombopoietin receptor agonists are usually reserved for refractory cases, but when and in which patient these modalities should be employed is under debate. To develop evidence-based guidelines for individualized treatment of ITP patients, we asked 20 Population Intervention Comparison Outcome (PICO) format questions. These were addressed by 17 evidence evaluators using a literature pool of 288 articles identified by a structured search strategy. Evidence evaluators, using panel-designed templates and data extraction tools, summarized evidence and created guideline recommendations. These were integrated by treatment domain chairs and then refined by iterative Delphi survey review to reach consensus on the final guidelines. In addition, 19 non-PICO questions covering scenarios in which evidence was lacking or of low quality were answered by expert opinion using iterative Delphi surveys with panelist integration and refinement. Commentary was solicited from multiple relevant professional organizations before finalizing the consensus. The rigorous consensus process identified few comparative treatment studies, highlighting many areas of ITP treatment requiring additional studies. This statement is a companion manuscript to the ACVIM Consensus Statement on the Diagnosis of Immune Thrombocytopenia in Dogs and Cats.

Investigation of platelet measurands in dogs with hematologic neoplasia

INTRODUCTION

Thrombocytopenia has been associated with some neoplastic processes, including hematologic neoplasia. There is no information regarding specific changes in platelet measurands in dogs with hematologic neoplasia compared with healthy dogs. The objectives of our study were to establish RIs, evaluate platelet measurands in dogs with hematologic neoplasia, and compare these measurands in patients with hematologic malignancies with or without thrombocytopenia.

METHODS

This was a retrospective study. Platelet measurands were determined using the ADVIA 120 Hematology analyzer when a CBC was performed and included the platelet count, MPV, platelet distribution width (PDW), plateletcrit (PCT), mean platelet component (MPC), platelet component distribution width (PCDW), mean platelet mass (MPM), platelet mass distribution width (PMDW), and number of large platelets. Reference intervals were determined retrospectively using data from 129 healthy dogs. Patients with hematologic neoplasia (n = 50) were identified through retrospective evaluation of medical records from the Auburn University Veterinary Teaching Hospital and separated into thrombocytopenic (n = 20) and nonthrombocytopenic groups (n = 30).

RESULTS

Platelet count and PCT were significantly higher in older healthy dogs compared with younger dogs. Significant differences were identified when comparing healthy dogs with those with hematologic neoplasia without thrombocytopenia for PDW, PCDW, PMDW, and the number of large platelets, indicating the presence of more heterogeneous platelets. Thrombocytopenic dogs with hematologic neoplasia had significantly decreased MPCs and increased MPVs, MPMs, and PCDWs compared with nonthrombocytopenic dogs with neoplasia.

CONCLUSIONS

Dogs with hematologic neoplasia had more heterogeneous platelets, whereas thrombocytopenic patients with neoplasia had more activated platelets.

Influence of Vincristine, Clinically Used in Cancer Therapy and Immune Thrombocytopenia, on the Function of Human Platelets

Vincristine is a clinically used antimicrotubule drug for treating patients with lymphoma. Due to its property of increasing platelet counts, vincristine is also used to treat patients with immune thrombocytopenia. Moreover, antiplatelet agents were reported to be beneficial in thrombotic thrombocytopenic purpura (TTP). Therefore, we investigated the detailed mechanisms underlying the antiplatelet effect of vincristine. Our results revealed that vincristine inhibited platelet aggregation induced by collagen, but not by thrombin, arachidonic acid, and the thromboxane A₂ analog U46619, suggesting that vincristine exerts higher inhibitory effects on collagen-mediated platelet aggregation. Vincristine also reduced collagen-mediated platelet granule release and calcium mobilization. In addition, vincristine inhibited glycoprotein VI (GPVI) signaling, including Syk, phospholipase Cγ2, protein kinase C, Akt, and mitogen-activated protein kinases. In addition, the in vitro PFA-100 assay revealed that vincristine did not prolong the closure time, and the in vivo study tail bleeding assay showed that vincristine did not prolong the tail bleeding time; both findings suggested that vincristine may not affect normal hemostasis. In conclusion, we demonstrated that vincristine exerts antiplatelet effects at least in part through the suppression of GPVI signaling. Moreover, this property of antiplatelet activity of vincristine may provide additional benefits in the treatment of TTP.