• Histologically or cytologically confirmed mycosis fungoides or Sézary syndrome, stages IB to IVB with measurable disease and/or detectable blood involvement based on the Global Response Criteria for CTCL (Olsen et al., 2022).

• Received at least one prior line of systemic therapy.

• At least 18 years of age.

• ECOG performance status ≤ 2

• Adequate counts and organ function as defined below:

‣ ANC ≥ 0.7 x 109/L, without growth factor support (filgrastim or pegfilgrastim) for at least 14 days

⁃ Platelets ≥ 75 x 109/L, without platelet transfusion for at least 14 days

⁃ Hemoglobin ≥ 8.0 g/dL, with or without transfusion

⁃ Serum total bilirubin ≤ 1.5 x IULN

⁃ AST(SGOT)/ALT(SGPT) ≤ 2.5 x IULN

⁃ Creatinine clearance \> 30 mL/min by Cockcroft-Gault (using actual body weight) for patients with creatinine levels above institutional normal OR serum creatinine ≤ 1.5 x ULN

• Patients with treated brain metastases are eligible if follow-up brain imaging after CNS-directed therapy shows no evidence of progression.

• The effects of DZR123 on the developing human fetus are unknown. For this reason, women of childbearing potential and men must agree to use highly effective methods of contraception for the duration of study participation and for 183 days after the last dose of DZR123 for female patients and female partners of male patients, or for 93 days after the last dose of DZR123 for male patients and male partners of female patients. Should a woman become pregnant or suspect she is pregnant while participating in this study, she must inform her treating physician immediately.

• Ability to understand and willingness to sign an IRB approved written informed consent document.