Enhancing Functional Platelet Release In Vivo from in Vitro-Grown Megakaryocytes Using the Protein Kinase Inhibitor SU6656

The clinical demand for platelet transfusions is increasing, threatening the ability to obtain sufficient healthy donors to provide these platelets. Advances in regenerative medicine research have opened the possibility of generating sufficient in vitro-grown megakaryocytes and consequent platelets to supply a portion of the clinical platelet transfusion demand. We have shown that infusing megakaryocytes for obtaining released, functional platelets is a viable alternative strategy than trying to release platelets in vitro. However, for both approaches, in vitro-cultured megakaryocytes have lower ploidy and release fewer platelets than likely occurs in vivo by primary cells. SU6656 inhibitor, a Src kinase inhibitor, has been shown to influence ploidization in several megakaryocyte-like line with purported increase in proplatelets release. However, in our hands, other agents - such as the ROCK inhibitor Y27632 - while increasing polyploidization markedly, inhibited platelet release per infused megakaryocyte in vivo. We grew megakaryocytes from CD34⁺ cells for 12 days with or without SU6656 (2.5 µM) supplementation during the last 4 days. We found that the SU6656 inhibitor only increased the number of CD34⁺-derived megakaryocytes by ~15% at the end of the 12 day growth, but more markedly increase the percent of large megakaryocytes measured by FSC parameter in flow cytometry evaluation from 28 up to 41% and percent of high granular megakaryocytes from 27 to 45%. These changes were accompanied with a shift in average ploidy from 4.9 to 6.9 (p<0.0003, N=6). Notably, SU6656-treated megakaryocytes released ~4-fold more platelets per infused megakaryocytes in immunocompromized NSG mice than untreated similarly in vitro-grown megakaryocytes. By 24 hrs, there were 6.5-fold platelets from the infused SU6656-treated megakaryocytes than control untreated (p<0.037, N=6). Released platelets from the drug-treated and untreated megakaryocytes had similar levels of percent thiazole orange positivity as an indication that they were young platelets. Importantly, baseline annexin V, CD62p and PAC1 binding prior to agonist exposure were also similarly and increased to the same extent after thrombin (1U/ml) stimulation. Additionally, incorporation into a growing cremaster laser injury-induced thrombus in vivo was similar further indicating retained function by the platelets released from the drug-treated megakaryocytes. A number of strategies such as modifying the level of transcription factors have been proposed to increase the size, ploidy or proplatelets release from in vitro-grown megakaryocytes. In none of these cases have these released platelets in vivo biology been examined and demonstrated to replicate high release number per megakaryocyte and retained functionality. We show that terminal exposure of in vitro-grown megakaryocytes to the non-specific inhibitor SU6656 significantly increases in vivo yield while leaving in vivo half-life and functionality intact. The exact pathway affected by SU6656 that leads to these results is now being pursued.