This study is sponsored by Sierra Oncology, Inc. formerly ProNAi Therapeutics, Inc. It is a multi-center, nonrandomized, open label, phase II investigation of PNT2258 to characterize anti-tumor activity and collect safety data on patients with relapsed or refractory (r/r) diffuse large B-cell lymphoma.

Inclusion Criteria:

Histologically confirmed diffuse large B-cell lymphoma that is refractory to prior therapy or relapsed after prior therapy.

FDG PET-CT (disease) positive baseline scan with measurable disease.

The patient must have received prior therapy that included:

• CD20-targeted therapy (for example, rituximab),
• Alkylating agent (for example, cyclophosphomide), and
• Steroid, unless the patient is steroid intolerant

Exposure to at least 1 or 2 (but no more than 3) prior systemic cytotoxic chemotherapeutic regimens.

Note: Only those subjects who are not eligible for high-dose chemotherapy and autologous stem cell transplant (HD-ASCT), or who refuse HD-ASCT, are eligible with exposure to only 1 prior cytotoxic chemotherapeutic regimen.

ECOG performance status of 0-1.

The patient must be a stable baseline with CTCAE grade ≤ 2 regarding any acute or chronic toxicity associated with prior therapy, and have discontinued prior anti-cancer therapy for ≥ 14 days prior to C1D1; mitomycin-C for at least 6 weeks prior to C1D1; SCT ≥ 2 months prior to C1D1.

Note: Palliative steroids for control of disease-related symptoms are allowed and maintenance hormone therapy is allowed.

Adequate organ function including:

• Hematologic: ANC ≥ 0.5 x 10^9/L. and platelets ≥ 50 x 10^9/L.
• Hepatic: Total Bilirubin ≤ 2 x ULN (patients with Gilbert's syndrome must have total bilirubin ≤ 3 x ULN) and serum transaminase levels ≤ 2.5 x ULN. In the case of known liver metastasis (i.e., radiological or biopsy documented), serum transaminase levels must be ≤ 5 x ULN.
• Renal: Serum creatinine ≤ 2 x ULN, or creatinine clearance ≥ 60 mL/min/1.73 m2 for subjects with serum creatinine levels above 2 x ULN.

Willingness to: 1.) undergo pre-treatment biopsy to obtain adequate tissue for analysis (e.g., core needle, excisional or incisional tumor biopsy) or 2.) provide archived tumor (e.g., FFPE block) for analysis.

Exclusion Criteria:

Eligibility for high-dose chemotherapy (HDT) and stem cell transplant (SCT). Note: Subjects who progressed ≥ 2 months after HDT/SCT are eligible

Concurrent malignancies requiring treatment.

Primary mediastinal (thymic) large B-cell lymphoma

Symptomatic CNS or leptomeningeal involvement of lymphoma.

Concurrent clinically significant illness, medical condition, surgical history, physical finding, electrocardiogram or laboratory finding that, in the opinion of the investigator, could adversely affect the safety of the patient or impair the assessment of the study results.

Signs or symptoms of heart failure characterized as greater than NYHA Class II or other significant cardiac abnormalities.

Pregnant or breast-feeding.

Prior exposure to PNT2258.

Life expectancy less than 3 months.