Class of 2015 Abstract / Objectives: Argatroban is the current drug of choice for type II heparin induced thrombocytopenia. Primarily metabolized by the liver, this direct thrombin inhibitor has a volume of distribution of approximately 174 mLs per kg. While few studies suggested no differences in coagulation parameters or clinical outcomes between obese and non-obese populations receiving argatroban, a recent case report revealed elevated anticoagulation parameters for 20 days post argatroban discontinuation in a morbidly obese female. The purpose of this study is to assess anticoagulation parameters in obese and non-obese patients in an intensive care unit (ICU) setting who received argatroban treatment during their stay. Methods: This is a retrospective, observational, single-centered study. Participants of the study must be adults, at least 18 years of age. Patient must be an inpatient and have received argatroban for either suspected or confirmed heparin-induced thrombocytopenia (HIT). All patients in the study were screened for the above criteria between November 2008 and September 2013. Patients admitted to the cardiac ICU were excluded from the study. Main anticoagulation parameters post discontinuation evaluated were daily international normalized ratio (INR) and activated partial thromboplastin time (aPTT), while safety outcomes included major, minor and non-bleed events. All data were analyzed with STATA 13 with P less than 0.05 being considered as statistically significant. Results: The study included a total of 51 patients, 37 were non-obese with body mass index (BMI) less than 30 kg per m2 (73 percent), and 14 were obese with BMI greater or equal to 30 kg per m2 (27 percent). Among basic demographic data, no differences were found between age, sex, race, height and SOFA scores at baseline between the two groups, BMI less than 30 kg per m2 and BMI greater or equal to 30 kg per m2. (P equals 0.7, 0.21, 1.0, 0.41, 0.51 respectively). However, as expected, weight was the only characteristic that was different at baseline (P less than 0.01). Primary outcome of time of INR to normalization post argatroban administration (2.73 seconds plus or minus 0.27 seconds) as well as safety outcomes including major, minor, and non-bleed adverse events (P equals 0.61) were statistically non-significant between the two groups. Conclusions: In this retrospective, observational, single centered study, no differences were identified between non-obese and obese groups in terms of argatroban administration, primary anticoagulation parameters, and safety outcomes. The length of time required for coagulation parameters to normalize after discontinuation of argatroban therapy for HIT does not appear to be influenced by BMI. Large, multicenter, and random controlled trials are needed to evaluate obesity on pharmacokinetic parameters and clinical outcomes of argatroban.

Intrinsic structural and conformational mechanisms regulate the functional specificity of the coagulation system. The study of these structure-function relationships is important for understanding the strategies used in the management of clinical thrombosis. Previous studies have shown that the central enzyme in clotting, thrombin, is sequestered inside of a clot, and protected from the natural downregulator antithrombin (AT). This is problematic for anticoagulants like heparin which depend on AT. Subsequently, it was found that the key upstream propagator of thrombin, the prothrombinase enzyme complex, is also resistant to the AT-heparin. Our data show that further upstream of prothrombinase, the intrinsic tenase is only moderately protected, while there is no protection at the level of the initiator complex, extrinsic tenase. This protection phenomenon possibly reflects steric and allosteric mechanisms that ensure maximal activation of the coagulation system once a threshold stimulus is achieved. These mechanisms likely evolved as a result of conformational rearrangement, as evidenced by the proteolytic activation of thrombin activity following proteolysis of prothrombin. Indeed, subtle differences in the structural interaction of ligands with the active site can lead to substantial differences in enzyme activity. The binding of rivaroxaban and apixaban to factor Xa is nearly identical; both interact with the active site with comparable affinity. Despite this, a 3-fold faster rate of the rivaroxaban on-rate yields significantly greater prolongation of the prothrombin time (PT) and activated partial thromboplastin time (aPTT), global tests of coagulation. These small differences in ligand interaction also have allosteric consequences. Structural differences between the direct thrombin inhibitors dabigatran and argatroban yield divergent exosite-mediated thrombin binding to physiologic ligands like yA-fibrin, y'-fibrin, factor Va, and factor VIII, interactions that govern clot-mediated protection from AT inhibition, and the various functions of thrombin. These divergent effects were robust and ligand-dependent, suggesting conserved energetic scaffolds within the thrombin molecule that govern allosteric changes throughout the molecule. Because proteolysis of prothrombin yields significant allosteric and structural rearrangement that capacitates the active site for substrate recognition amd catalytic ability, we investigated the role of Ser195, a key residue in the thrombin catalytic triad in also regulating thrombin allostery. Site directed mutagenesis of Ser195 to Ala yielded a significant increase in the flexibility of the entire thrombin molecule, as evidenced by increased potency of dabigatran and argatroban in terms of their capacity to modulate exosite binding through the active site, and increased interexosite cooperative and competitive allostery. Together, these studies represent an advance in our understanding of the consequences of both small molecule ligation of coagulation proteases, as well as the consequences of subtle structural modification for overall allosteric function. / Thesis / Doctor of Philosophy (PhD)