2. is unfit for standard chemoimmunotherapy meeting at least 1 of the following co-morbidity criteria: a. CIRS ≥ 6; b. Creatinine Clearance < 70 mL/min; c. Subject is not a candidate for a chemoimmunotherapy in the opinion of the investigator. The reason for not being a candidate must be documented in CRF. ii. Dose Expansion Phase: Subjects must not have received prior treatment with ibrutinib (or any other approved BTK inhibitors) and must have high risk CLL. High risk is defined as: 1) 17p- and/or TP53 mutation positive in treatment naïve CLL; or 2) 17p- and/or TP53 mutation positive, and/or complex karyotype, and/or progression < 24 months after completion of 1st line chemoimmunotherapy in R/R CLL c. Subjects with R/R SLL or CLL with bulky disease (at least one lymph node measuring > 5.0 cm in diameter) are considered at higher risk for developing a TFR and may only be enrolled upon discussion with the sponsor's medical monitor and agreement to close medical management.