A Study Assessing Adverse Event and How Oral ABBV-453 Moves Through the Body in Adult Participants With Relapsed or Refractory (R/​R) Chronic Lymphocytic Leukemia (CLL)/​Small Lymphocytic Lymphoma (SLL) (453 Ph1 CLL)

Chronica long-lasting disease that changes slowly over time lymphocytic leukemia (CLL) is the most common leukemia in Western countries. The purpose of this study is to assess how well ABBV-453 works adult participants with relapsed/refractory (R/R) untreated CLL/small lymphocytic lymphomacancers of the lymphatic system (SLL). Adverse events, pharmacokinetics, and change in disease activity will be assessed.

ABBV-453 is an investigational drug for the treatment of CLL and SLL. There are 2 parts to this study. In part A participants will be placed 1 of 5 cohorts with a specific target dosethe amount of medication taken for each cohort and receive obinutuzumab during the debulkingremoval of as much of the tumour as possible when complete tumour removal is not possible, also known as cytoreduction period followed escalating doses of ABBV-453, until the appropriate target dose is achieved. In part B participants will be placed in 2 cohorts and receive up to the maximum dose in part A, with cohort 2.1 including a debulking period (obinutuzumab) as in part A. Approximately 80 adult participants with previously R/R CLL/SLL will be enrolled in the study in approximately 40 sites across the world.

Participants in part A will placed into 1 of 5 cohorts with a specific target dose for each cohort and will receive intravenousinto or within a vein (IV) obinutuzumab as part of the debulking period, followed by escalating doses of oral ABBV-453 until the appropriate target dose is achieved. Participants in part B will be place in one of 2 cohorts. Participants in cohort 2.1 will receive IV obinutuzumab as part of the debulking period, followed by escalating doses of oral ABBV-453 until the maximum target dose from part A is achieved. Participants in cohort 2.2 will receive no treatment during the the debulking period, followed by escalating doses of oral ABBV-453 until the maximum target dose from part A is achieved. The estimated study duration is 5 years.

There may be higher treatment burden for participants in this trial compared to their standard of care. Participants will attend regular visits during the study at a hospital or clinic. The effect of the treatment will be checked by medical assessments, blood teststesting done to measure the levels of certain substances in the blood, and checking for side effects.