• Patients with diffuse large B-cell lymphoma (de novo or DLBCL transformed from an indolent lymphoma (follicular lymphoma, chronic lymphocytic leukemia \[Richter syndrome\]) or high-grade B-cell lymphoma (HGBL): (DLBCL patients)

‣ Defined as relapsed or refractory DLBCL or high-grade B-cell lymphoma (HGBL) following at least one or more prior chemoimmunotherapy regimen (with at least one course including an anthracycline and CD20-directed therapy) following diagnosis of de novo DLBCL/HGBL or DLBCL arising from indolent lymphoma and requiring further treatment and deemed to be candidates for standard of care CAR-T therapy. This includes patients with primary refractory disease (failure to achieve complete response (CR) to first-line therapy), relapsed disease within 12 months of first line chemoimmunotherapy or relapsed/refractory disease after 2 or more prior lines of systemic therapy.

⁃ Patients must have at least one FDG-avid (PET-avid) measurable lesion.

⁃ Relapsed or refractory disease must be confirmed with a repeat biopsy within the last 12 months.

⁃ For patients who have received treatment for confirmed relapsed or refractory disease otherwise meeting criteria 1.i.-1.iii. as above within 6 weeks of study enrollment, active disease does not need to be re-confirmed or present immediately prior to Screening 1 (Section 5.2) for the patient to be eligible for leukapheresis. However, detectable evidence of residual malignancy must be present at Screening 2 (Section 5.3) for the patient to be eligible for 131I-Apamistamab and CAR T-cell therapy.

• Age ≥ 18 years of age

• Creatinine clearance ≥50 mL/min as calculated by the Cockroft-Gault formula.

• Total bilirubin ≤1.5x upper limit of normal , AST and ALT ≤3x upper limit of normal (ULN), unless liver dysfunction is thought to be related to underlying malignancy or secondary to Gilbert's disease in which case the direct bilirubin should be ≤3.0 mg/dL, and AST and ALT ≤5x ULN.

• Adequate pulmonary function as assessed by ≥92% oxygen saturation on room air or per institutional guidelines.

• Thyroid function tests (TSH, FT4) ≤2x upper limit of normal (ULN)

• Adequate bone marrow function meeting the following criteria as defined below, without requiring blood product or granulocyte-colony stimulating factor support in the 7 days prior to screening and start of 131I-Apamistamab treatment.

∙ Absolute neutrophil count ≥1.0k/µL,

‣ Platelets ≥50k/µL,

‣ Hemoglobin ≥8g/dL.

• Performance status: ECOG performance status 0-2.

• All men, as well as women of child-bearing potential must agree to use adequate contraception (hormonal or barrier method of birth control, and/or abstinence) prior to study entry, and for the duration of study treatment, and for 30 days following completion of therapy. Should a woman become pregnant or suspect she is pregnant while participating in this study, she should inform her treating physician immediately.

• A female of child-bearing potential is any woman (regardless of sexual orientation, marital status, having undergone a tubal ligation, or remaining celibate by choice) who meets the following criteria:

⁃ Has not undergone a hysterectomy or bilateral oophorectomy; or

⁃ Has not been naturally postmenopausal for at least 12 consecutive months (i.e., has had menses at any time in the preceding 12 consecutive months).

⁃ Ability to understand and the willingness to sign a written informed consent.

⁃ For patients undergoing bridging therapy after leukapheresis and prior to 131I-Apamistamab infusion a repeat PET/CT scan will be performed 10-14 days prior to the 131I-Apamistamab infusion. They will also be required to meet additional inclusion criteria as written within specific sections of the protocol within 10-14 days prior to the planned infusion of 131I-Apamistamab. This will be considered eligibility Screening 2 and will be approved by the Sponsor-Investigator.