Phase IIa portion of the study: The Phase IIa study is an open-label, single-dose level study. All participants who are eligible to receive treatment on study will receive the same dose level for both BP1001 (the HTD or 1 level below the HTD) as determined by the Phase Ib study and Das (140 mg). There will be no randomization for this study. The Phase IIa study will compare the efficacy of the BP1001 in combination with Das to historical response rates documented for Das alone, and will evaluate the PK of BP1001, when given alone or in combination with Das.