Hemendra Ghimire ¹ Srideshikan Sargur Madabushi ¹ Justin Vercellino ² Jamison Brooks ³ Darren Zuro ⁴ Ji Eun Lim ¹ Paresh Vishwasrao ¹ Amr M. Abdelhamid ⁵ Guy Storme ⁶ Gary Eichenbaum ⁷ Monzr M. Al Malki ⁸ Chandan Guha ² Susanta K. Hui ^1*

• ¹ Department of Radiation Oncology, City of Hope National Medical Center, California, United States
• ² Department of Radiation Oncology, Montefiore Medical Center, Albert Einstein College of Medicine, Bronx, United States
• ³ Department of Radiation Oncology, Mayo Clinic, Jacksonville, United States
• ⁴ Department of Radiation Oncology, University of Oklahoma Health Sciences Center, Oklahoma City, United States
• ⁵ Department of Oncology and Nuclear Medicine, Faculty of Medicine, Ain Shams University, Cairo, Beni Suef, Egypt
• ⁶ Department of Radiotherapy Oncology, Ghent University Hospital, Ghent, Belgium
• ⁷ Johnson & Johnson Pharmaceutical Research and Development, New Brunswick, United States
• ⁸ Department of Hematology and Hematopoietic Cell Transplantation, City of Hope National Medical Center, Duarte, California, United States

Background: There is a need for therapies that can mitigate bone marrow dysfunction and organ toxicity that occur following myeloablative injury and reduced intensity conditioning regimens used in patients undergoing bone marrow transplantation (BMT). The pathogenesis of adverse effects from BMT conditioning has been linked to injury to the vascular endothelium, bone marrow (BM), and other organs.Objective: To evaluate the impact of the thrombopoietin mimetic drug JNJ-26366821 (TPOm) on BM vascular recovery in mice undergoing myeloablative radiation conditioning followed by BMT.Study Design: TPOm (doses: 0 µg, 300 µg, 1000 µg per Kg body weight) was administered on Days 0 and 7 after BMT, in mice receiving a total body irradiation (TBI) conditioning regimen (5.5 Gy x 2) before congenic BMT. BM donner cell engraftment was analyzed using flow cytometry on Days 7, 14, and 30 post-BMT. The morphological and biophysical properties of the BM vasculature were evaluated by intravital multiphoton microscopy (MPM) and immunofluorescence confocal imaging. Herein, morphological properties involve microvascular density (MVD), vessel diameter, and vascular area, while biophysical properties include transfer rate (Ktrans) of contrast within the BM vascular niche, as well as the fractional volume (vec) of extracellular extravascular tissue (EES).Results: No significant difference in donor chimerism was observed at days 7, 14, and 30 post-BMT, between TPOm and PBS-treated mice. TPOm intervention improved BM vasculature regeneration in transplanted mice. The MVD, Ktrans, and BM vasculature as well as vascular endothelial growth factor receptor-2 (VEGFR2) in the BM, showed a dose dependent improvement in mice treated with TPOm. On day 14 post-BMT, the group receiving 1000 µg/Kg TPOm showed significant shifts (p-value < 0.05) in MVD, Ktrans, and VEGFR2 expression from their corresponding control types (TPOm dose 0 µg) towards levels comparable to healthy controls.TPOm intervention augments BM vascular structure and function, which may be important for hematopoietic recovery and bone marrow function in radiation conditioned hematopoietic stem cell transplant patients, in addition to enhancing platelet recovery.