This is an open-label, multi-dose, single-arm, Phase 1, dose-escalation study of XmAb5574. The study was conducted to identify the maximum tolerated dose (MTD) and/or recommended dose(s) (RD) for further study, to characterize safety and tolerability, to characterize PK, PD and immunogenicity, and to evaluate preliminary antitumor activity of XmAb5574 in patients with relapsed or refractory CLL/SLL.

Inclusion Criteria:

• relapsed or refractory CLL/SLL
• at least 18 years of age
• able to receive outpatient treatment and follow-up at the treating institution
• completed all CLL therapies > 4 weeks prior to first study dose

Exclusion Criteria:

• previously treated with an anti-CD19 antibody therapy
• undergone prior allogeneic stem cell transplantation within 6 months or having active graft versus host disease
• active Richter's syndrome
• designated Class III or IV by the New York Heart Association (NYHA) criteria
• history of myocardial infarction or stroke within the last 6 months
• active viral, bacterial, or systemic fungal infection requiring treatment
• HIV or Hepatitis C positive
• Hepatitis B infection