A Phase I, Multicenter Dose Escalation Study in Patients With Hairy Cell Leukemia

Inclusion Criteria: Confirmed diagnosis of HCL, Measurable disease, Participant's must have had at least 2 prior systemic therapies. There must have been at least 2 prior courses of purine analog, or 1 if the response to this course lasted less than (\<) 2 years, or if the participant had unacceptable toxicity to purine analog, Eastern Cooperative Oncology Group (ECOG) performance status of 0-2, Participant's with other cancers who meet eligibility criteria and have less than 5 years of disease free survival will be considered on a case-by-case basis, Life expectancy of greater than 6 months, as assessed by principal investigator, Must be able to understand and sign informed consent, Must be at least 18 years old, Female and male participants must agree to use an approved method of contraception during the study.

Exclusion Criteria: - History of allogeneic bone marrow transplant, Documented and ongoing central nervous system involvement with their malignant disease (history of central nervous system (CNS) involvement is not an exclusion criteria), Pregnant or breast-feeding females, HIV positive serology (due to increased risk of severe infection and unknown interaction of CAT-8015 with antiretroviral drugs, Hepatitis B surface antigen positive. - Hepatic function: Serum transaminases (either alanine transaminase (ALT) or aspartate transaminase (AST)) or bilirubin: greater than or equal to (\>=) Grade 2, unless bilirubin is due to Gilbert's disease. -Renal function: Serum creatinine clearance less than or equal to (\<=) 60 millilitre per minute (mL/min) as estimated by Cockroft-Gault formula. -Hematologic function: The absolute neutrophil count (ANC) \< 1000/ cubic millimetres (cmm), or platelet count \<50,000/cmm, if these cytopenias are not judged by the investigator to be due to underlying disease (i.e. potentially reversible with anti-neoplastic therapy). -Pulmonary function: Participant's with \< 50% of predicted forced expiratory volume (FEV1) or \<50% of predicted diffusing capacity for carbon monoxide (DLCO), corrected for hemoglobin concentration and alveolar volume. Uncontrolled pulmonary infection, presence of pulmonary edema, Oxygen saturation at rest \< 88% measured by pulse oximetry or partial pressure of oxygen (PaO2) \< 55 millimetre(s) of mercury (mm Hg), Serum albumin \< 2 g/dL, Radioimmunotherapy within 2 years prior to enrollment in study.