A Phase 1, Open-Label, Sequential Cross-over, Bioavailability/Bioequivalence Study to Compare the Pharmacokinetics of Oral Cladribine With the Reference Listed Drug, Intravenous Cladribine

To compare an investigational oral form of the drug cladribine to the FDA approved form of the drug when it is given by vein (IV).

Inclusion Criteria: Study participants must meet the following inclusion criteria 1. Patients, both men and women of all races and ethnic groups, aged ≥ 18 years are eligible for enrollment. 2. Provision of written informed consent prior to any study related procedures. 3. Patients with newly diagnosed or previously treated HCL who have indication for therapy and are candidates for cladribine therapy (all 3 cohorts). 4. Patients with previously treated T-cell prolymphocytic leukemia who have received at least one course of alemtuzumab-based treatment, who have documented indication for therapy and who are candidates for cladribine therapy (Cohort 1 and 2) only. 5. Adequate renal and hepatic organ function as indicated by the following laboratory values: 1. Creatinine clearance ≥ 60 mL/min 2. Serum total bilirubin ≤1.5×ULN (with the exception of patients with known Gilbert's syndrome: serum total bilirubin must be \