• Histologically documented diagnosis of MCL as defined in the 2017 edition of the World Health Organization (WHO) classification

• Age ≥ 18 and \< 85 years

• Relapsed/Refractory disease after one, two, three or four lines of treatment

• Bendamustine-naive or relapsed after at least one year after the last cycle of a bendamustine-containing regimen

• Previous treatment with BTKi (Bruton Tyrosine Kinase inhibitors) monotherapy or BTKi containing regimens with R/R disease; and/or patients who discontinued BTKi monotherapy or BTKi containing regimens for adverse events and have active disease necessitating treatment.

• Previous treatment with any anti-CD19 agents is allowed (included CAR-T treatment) If previous anti-CD19 treatment has occurred, tissue CD19 expression must be assessed by histology or flow cytometry

• Venetoclax treated patients are allowed.

• Stem cell transplant eligible patients are allowed.

• Measurable nodal or extranodal disease ≥ 1.5 cm in longest diameter, and measurable in 2 perpendicular dimensions. Note: Patients with bone marrow involvement only are eligible. In case of bone marrow infiltration only, bone marrow aspiration and biopsy are mandatory for all staging evaluations

• ECOG (Eastern Cooperative Oncology Group)/WHO (World Health Organization) performance status ≤ 2 (unless MCL-related)

• The following laboratory values at screening (unless due to bone marrow involvement by lymphoma):

‣ Absolute Neutrophil count (ANC) \> 1.0×109/L

⁃ Platelet count ≥ 75.000/mm3

⁃ Creatinine clearance ≥ 40 mL/min (Cockcroft-Gault formula)

⁃ Aspartate transaminase (AST) and alanine transaminase (ALT) ≤ 3.0 x ULN (upper limit of normal)

⁃ Bilirubin ≤ 1.5 x ULN (unless bilirubin rise is due to Gilbert's syndrome or of non- hepatic origin)

• Subject understands and voluntarily signs an informed consent form approved by an Independent Ethics Committee (IEC), prior to the initiation of any screening or study-specific procedures.

• Subject must be able to adhere to the study visit schedule and other protocol requirements.

• Life expectancy ≥ 3 months.

• Women of childbearing potential (WOCBP) and men must agree to use effective contraception if sexually active.This applies for the time period between signing of the informed consent form and at least 10 months after last loncastuximab tesirine (ADCT-402) dose. Men with female partners who are of childbearing potential must agree to use effective contraception if sexually active. This applies for the time period between signing of the informed consent form and at least 7 months after last loncastuximab tesirine (ADCT-402) dose.