Inclusion Criteria:

1. Age ≥ 18 years.
2. At least 2 but no more than 4 prior lines of therapy for the treatment of de novo DLBCL and ineligible for (or failed) autologous or allogeneic stem cell transplant (SCT) (salvage therapy, conditioning therapy and maintenance with transplant will be considered one prior treatment). NOTE: For follicular lymphoma transformed to DLBCL (t-FL/DLBCL), single agent non-cytotoxic therapy will not be considered as a line of therapy.
3. Histopathologically confirmed diagnosis of one of the following:
   • RR DLBCL per the 2008 World Health Organization (WHO) classification of hematopoietic and lymphoid tumors (Swerdlow et al, 2008).
   • High grade B-cell lymphoma (HGBL), with MYC and BCL2 and/or BCL6 rearrangements or DLBCL, NOS per the 2016 revision of the WHO classification of lymphoid neoplasms (Swerdlow et al, 2016).
   • Diagnosis of t-FL/DLBCL is allowed. However, other B-cell lymphomas including other transformed indolent lymphomas/DLBCL per the 2008 WHO classification, and Burkitt lymphoma are not eligible.

Exclusion Criteria:

1. Known primary mediastinal, ocular, epidural, testicular or breast DLBCL.
2. Active CNS involvement of their malignancy.
3. Known allergy or hypersensitivity to phosphatidylinositol 3 kinase (PI3K) inhibitors or any component of the formulations used in this study.
4. Cytotoxic anticancer therapy (e.g., alkylating agents, anti-metabolites, purine analogues) or any other systemic anticancer therapy within 2 weeks of study entry.
5. Radiotherapy delivered to non-target lesions within one week prior to starting study treatment or delivered to target lesions that will be followed on the study (note: prior sites of radiation will be recorded).
6. Treatment with experimental therapy within 5 terminal half-lives (t1/2) or 4 weeks prior to enrollment, whichever is longer.
7. Current or planned glucocorticoid therapy, with the following exceptions:
   • Doses ≤ 10 mg/kg/day prednisolone or equivalent is allowed, provided that the steroid dose has been stable or tapering for at least 14 days prior to the first dose of CUDC-907.
   • Inhaled, intranasal, intraarticular, and topical steroids are permitted.