A Study to Investigate the Sequencing Strategy of Pirtobrutinib After Disease Progression on First-line Acalabrutinib Treatment for Adult Participants with Chronic Lymphocytic Leukemia or Small Lymphocytic Lymphoma - BOSS

To assess the efficacy and safety of pirtobrutinib in participants with CLL/SLL who have progressed on first-line treatment with acalabrutinib.

The purpose of this study is to assess the efficacy and safety of pirtobrutinib in participants with CLL/SLL who have progressed on first-line treatment with acalabrutinib. A subset of participants who have disease progression on pirtobrutinib will be retreated with acalabrutinib to assess whether relapsed CLL can be re-sensitized to a covalent irreversible BTK inhibitor such as acalabrutinib, and thereby, remain on treatment within the BTK inhibitor class rather than transition into another CLL/SLL treatment. • The study duration for each participant will be up to 3 years in total. • For participants who receive pirtobrutinib alone, the visit frequency will be approximately every month for the first 6 months. After that, the visit frequency will be reduced to one visit approximately every 3 months for the subsequent 12 months. The final part of the Treatment Phase has 2 visits in the space of 6 months. There is one visit to the site after the Treatment Phase. • Participants who have disease progression on pirtobrutinib and go on to receive acalabrutinib retreatment will visit the site approximately once every month for the first 6 months. After that, the visit frequency will be reduced to 2 visits in the space of 6 months. There is one visit to the site after the Treatment Phase.