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Dose limiting haematotoxicity has been associated with a range of therapeutic agents used for the treatment of a number of different conditions. Haematotoxicity is usually assessed as part of the preclinical safety studies in experimental animals, where changes in peripheral blood cell numbers and bone marrow cellularity are determined at the end of the study. Often no information on the mechanism of the haematotoxicity is revealed. This thesis demonstrates how in vitro bone marrow cultures can be utilized to assist in the assessment of haematotoxicity by two different approaches; firstly, in vitro bone marrow cultures can be used to assess the haematopoietic lineage specificity of vincristine sulphate, vinblastine sulphate, hydroxyurea and anagrelide hydrochloride using clonogenic cultures, enabling ranking of these compounds according to their haematotoxicity. Secondly, using in vitro assays only, elucidate the mechanism(s) of the megakaryocytic lineage specific inhibition of anagrelide hydrochloride. To this end both clonogenic cultures and LTBMC offer the ability to elucidate mechanisms of action on multipotent stem cells, lineage specific cells and effects on the bone marrow microenvironment following single and repeated administration. In addition, the combination of cell identification techniques flow cytometry and light microscopy was shown to provide a more detailed understanding of the different cell populations within the non-adherent cell layer. In vivo AN reduces platelet counts only, however, the mechanism of the megakaryocyte specific toxicity by AN is not understood. In these studies, the mechanism (s) of the megakaryocytic lineage haematotoxicity of AN was examined using the established human clonogenic and LTBMC. The action of AN was shown to be focused at a late stage in megakaryocyte (Mk) colony development. Ranking the potential mechanisms of action of AN by concentration at which they were noted, the inability to organize the microtubules appears to be secondary to 1) alteration in cell cycling, 2) surface receptor expression and 3) inhibition in achieving high (greater than 8N) ploidy number. However, identification of the primary mechanism based solely on concentration seems to be very crude and most probably reflects a limitation of in vitro systems. The inhibition of platelet production by AN is most likely a result from a combination of mechanisms; inhibition of cell cycling, disruption in the expression of cell surface receptors, inhibition of the ability of the cells to increase ploidy number and an associated inability to organize microtubules leading to a reduction in platelet release. This work also demonstrated the importance of the selection of the source of bone marrow used in the cultures. The concentration at which 50 percent of Mk colony growth was inhibited (IC50) by AN for murine cells was markedly (46 fold) different (88.6μM) compared to the IC50 with human cord blood (hCB) (1.92μM). This disparity is indicative of differences in species sensitivity possibly related to AN having a greater affinity towards the human c-mpl chrombopoietin (TPO) receptor than the equivalent murine receptor as suggested by McCarty et al (2006). This work highlights the utility of in vitro bone marrow cultures as a tool for investigating the lineage specific haematotoxicity by evaluating compounds used in the treatment of ET. In addition in vitro haematopoietic cultures can successfully be used as a tool to investigate potential mechanism(s) of haematotoxicity as demonstrated herein by providing an insight to mechanism of platelet count reduction by AN.

Essential thrombocythemia (ET) is a rare myeloproliferative neoplasm affecting 43.7 out of every 100,000 people in the United States. The disease is characterized by abnormally high platelet counts, mutational abnormalities in Janus Kinase 2 (JAK2)/Calreticulin (CALR)/myeloproliferative leukemia virus oncogene (MPL), and increased megakaryocyte production and differentiation. The average age of onset for patients with ET is between 65-70 years, but recent studies have demonstrated a downward trend in the age of diagnosis. Mechanistically, ET mutations cause the dimerization of JAK and upregulation of the JAK-STAT pathways. Common treatment approaches seek to use cytoreduction and platelet inhibition to lower the risk of a thrombotic event. Hydroxyurea and low-dose aspirin have been the gold-standard of treatment for ET patients. This thesis sought to compare the current available therapy with second-line treatments and investigational treatments. Anagrelide is a key second-line treatment for ET that is used in the event of intolerance to hydroxyurea. It acts through cytoreductive mechanisms which result in a decreased platelet count. Major bleeding is a severe adverse event associated with anagrelide. Interferons are another second-line defense in the treatment of ET despite a lack of FDA approval for this indication. Interferons act directly to reduce platelet counts and, unlike other drug classes, mount an immunological response against the JAK2 stem cells to reduce the allelic burden. An immunological approach to ET may be key to the sustained treatment of the disorder without a daily dosing regimen. Despite the promise of interferons, severe adverse effects limit the adherence of many patients to this class of drugs. JAK inhibitors are an investigational drug class that acts directly through the JAK-STAT pathway. JAK inhibitors have shown little efficacy in the treatment of ET and may be better suited for treatment in combination therapies. Telomerase inhibitors are one such investigational drug class that may pair well with JAK inhibitors for the treatment of ET. All of these drug classes were compared to hydroxyurea with respect to their pharmacokinetics, pharmacodynamics, and patient evaluation. Hydroxyurea and low-dose aspirin showed superiority in comparison to other drug classes due to their low toxicity profile and minimum adverse side-effects, high oral bioavailability and wide distribution, high adherence, and production of the most uniform response to reducing thrombotic events and platelet counts. The interferon drug class shows unique potential for the treatment of ET and should be placed above the second-line treatment standard of anagrelide due to its benefits in treatment of younger and pregnant patients. Interferons are the only class of drug for the treatment of ET that did not increase the risk of drug-related leukemogenic transformations. Despite non-adherence due to side-effects and lack of an oral administration, interferons are superior to anagrelide due to their longer dosing interval and immunological attack on JAK2 stem cells. Treatment of ET with anagrelide has shown similar efficacy to hydroxyurea and interferons in platelet reduction and rivals hydroxyurea in the prevention of thrombotic risk. Despite this benefit, the risk of bleeding associated with anagrelide is a significant disadvantage. Hydroxyurea and low-dose aspirin remain the current standard of treatment for patients with ET, although new approaches may soon be available.