Platelet factor 4 platelet levels are inversely correlated with steady-state platelet counts and with platelet transfusion needs in pediatric leukemia patients.

Biomarkers of steady-state platelet count and risk for thrombocytopenia post-chemotherapy have not been previously identified. Thrombocytopenia can interfere with chemotherapy by limiting doses and delaying therapy[1], potentially resulting in poorer outcomes.[2] Therefore strategies to predict risk and mitigate chemotherapy-induced thrombocytopenia (CIT) are needed. Additionally, the ability to identify patients at high risk of thrombocytopenia and new strategies to influence platelet production may permit more intensive therapy, while avoiding severe thrombocytopenia and its sequelae. To date, no therapies are established for treatment of CIT. Use of thrombopoietin (TPO), the cytokine most related to megakaryocyte and platelet production, is not recommended because of risk of immunogenicity.[3] Other cytokines [e.g., stromal derived factor-1 (SDF-1, CXCL12), interleukin (IL)-3 and IL-6] have limited utility due to toxicity.[4] TPO receptor agonists are in clinical trials to examine their efficacy in this setting; however, experience with recombinant human (h) TPO suggests that in settings where bone marrow suppression is severe, these agents may have limited efficacy.[3] Another candidate to ameliorate thrombocytopenia, especially after marrow injury[5, 6], is platelet factor 4 (PF4, CXCL4), a platelet and megakaryocyte-specific chemokine stored in alpha granules and released from activated platelets. In vitro studies have shown that PF4 is a negative regulator of human and murine megakaryopoiesis via the LDL-receptor related protein-1 (LRP-1).[6–8] In mice, we have shown that steady-state platelet counts are inversely related to platelet (p) PF4 content.[6] In CIT and radiation-induced thrombocytopenia (RIT) models, mice with high levels of pPF4 developed significantly worse thrombocytopenia and delay in platelet count recovery.[5,6] Blocking PF4 using anti-PF4 antibodies was able to improve platelet count recovery in both CIT and RIT; however, PF4's importance in defining platelet counts in humans has not been examined. We present the first evidence that pPF4 levels inversely correlate with steady-state platelet counts in individuals and recovery after human bone marrow injury. By studying the relationship between pPF4 and recovery from CIT in standard risk acute lymphoblastic leukemia (SR-ALL), we found that steady-state and post-CIT nadir platelet counts were inversely related to pPF4. There was a significant difference in pPF4 content for those patients requiring transfusions compared to those that did not. The clinical implications of these findings are discussed. In a single institution, cohort study of patients with National Cancer Institute SR-ALL (age >1 year or <10 years; white blood cell count <50,000/mcL at diagnosis; morphological, flow cytometry and surface marker studies consistent with SR-ALL) who had completed therapy since January 1999, blood samples were collected during a routine follow-up visit 4 months to 10 years after completion of chemotherapy. Eligible patients were off therapy and were treated at our institution so that medical records were complete and available for review. Patients with SR-ALL were selected because of the relative uniformity of therapy. Medical records were examined to determine platelet count nadir and transfusion requirements during delayed intensification (DI) by investigators (AR, AG) blinded to pPF4 levels. Transfusion data were obtained from the hospital blood bank; only transfusions given during DI were analyzed. Control subjects were recruited from the healthy pediatric population referred for routine laboratory studies or minimal risk surgical procedures through the Anesthesia Resource Center. This study was approved by the Institutional Review Board. One hundred and one patients with SR-ALL were enrolled between October 2008 and September 2011. Ninety-seven had evaluable PF4 levels and were used in analysis of PF4 with respect to race, gender and age. Ninety subjects had complete medical records available for review and were used in the transfusion and nadir platelet count analysis. Fifteen subjects who were seen more than once during the study period also had repeated PF4 measurements. The gender/age/racial distribution within this study group reflected the SR-ALL patient population at our institution. Ninety-seven healthy, control subjects were recruited to compare their PF4 levels to those of SR-ALL patients (Supplemental Table 1).