Efficacy and Safety of Apixaban in the Treatment of Heparin Induced Thrombocytopenia (HIT)

Inclusion Criteria: * Hospitalized patients at MGH and participating Dana Farber/Harvard Cancer Center (DF/HCC) sites. * Patient must have a diagnosis of HIT or high clinical suspicion of HIT, with a 4 T score of ≥ 5 as calculated by the following criteria: * Thrombocytopenia * Platelet count fall \>50 percent and nadir ≥20,000/microL - 2 points * Platelet count fall 30 to 50 percent or nadir 10 to 19,000/microL - 1 points * Platelet count fall \<30 percent or nadir \<10,000/microL - 0 points * Timing of platelet count fall * Clear onset between days 5 and 10 or platelet count fall at ≤1 day if prior heparin exposure within the last 30 days - 2 points * Consistent with fall at 5 to 10 days but unclear (eg, missing platelet counts), onset after day 10, or fall ≤1 day with prior heparin exposure within 30 to 100 days - 1 point * Platelet count fall at \<4 days without recent exposure - 0 points * Thrombosis or other sequelae * Confirmed new thrombosis, skin necrosis, or acute systemic reaction after intravenous unfractionated heparin bolus - 2 points * Progressive or recurrent thrombosis, non-necrotizing (erythematous) skin lesions, or suspected thrombosis that has not been proven - 1 point * None - 0 points * Other causes for thrombocytopenia * None apparent - 2 points * Possible - 1 point * Definite - 0 points * Prior to or immediately subsequent to enrollment, the patient must have the diagnosis confirmed by Heparin-PF4 EIA or other accepted confirmatory test to remain on study. * Patients can be treated with argatroban, bivalirudin or fondaparinux for up to 72 hours prior to enrollment. * Age 18 years or older. * ECOG performance status ≤2 (Karnofsky ≥60%) * Participants must have organ and marrow function as defined below: * absolute neutrophil count ≥1,500/mcL * AST(SGOT) and ALT(SGPT) ≤2.5 × institutional upper limit of normal * creatinine clearance ≥25 mL/min as was used in the AMPLIFY trial16 * The effects of apixaban on the developing human fetus are unknown. For this reason, women of child-bearing potential and men must agree to use adequate contraception (hormonal or barrier method of birth control; abstinence) prior to study entry and for the duration of study participation. Should a woman become pregnant or suspect she is pregnant while she or her partner is participating in this study, she should inform her treating physician immediately. Men treated or enrolled on this protocol must also agree to use adequate contraception prior to the study entry, for the duration of the study participation, and 4 months after completion of apixaban administration. * Ability to understand and the willingness to sign a written informed consent document. Exclusion Criteria: * Patient requires anticoagulation for a mechanical heart valve. * Patient requires: * anticoagulation for another indication for long-term anticoagulation therapy, * dual antiplatelet therapy, * treatment with aspirin at a dose of more than 162 mg daily * Patient has signs of active or ongoing clinically significant hemorrhage. * Patient has hereditary or acquired coagulopathy or bleeding disorder. * Patient has a contraindication to apixaban. * Participants receiving any medications or substances that are inhibitors or inducers of cytochrome P-450 3A4 or p-glycoprotein are ineligible. Because this list of these agents are constantly changing, it is important to regularly consult a frequently-updated list. As part of the enrollment/informed consent procedures, the patient will be counseled on the risk of interactions with other agents and what to do if new medications need to be prescribed or if the patient is considering a new over-the-counter medicine or herbal product. * Patient has severe renal insufficiency (CrCl \<25 ml/min-as used in the AMPLIFY trial) * Patient has hepatic disease (including Child-Pugh B and C) associated with coagulopathy or clinically relevant bleeding risk. * Recent (previous seven days), or complicated lumbar puncture or epidural catheter placement or removal. * Patient has high potential need to undergo a surgical or major invasive procedure in the near future. * Patient has a history of uncorrected cerebral aneurysm, intracranial tumor or hemorrhagic cerebrovascular accident. * Patient refuses to receive transfused blood products should this intervention become clinically indicated. * Patient is taking or has been taking an investigational drug within the previous 30 days prior to enrollment. * In the judgment of the investigator, any disease or circumstance that would interfere with the objectives of the study. * Participants with known brain metastases. * History of allergic reactions attributed to compounds of similar chemical or biologic composition to apixaban. * Uncontrolled intercurrent illness. * Pregnant women are excluded; breastfeeding should be discontinued if the mother is treated with apixaban. * Prior treatment with a non-heparin anticoagulant while awaiting study enrollment is not an exclusion.