The choice of the conventional immunotherapy regimen which will be administered to each patient, R-DHAOx (Group A) or R-GDP (Group B), is left at the investigator's decision before patient's inclusion. Different dose levels for selinexor administration will be examined sequentially in each group by the Dose Escalation Committee (DEC): 4 doses of selinexor per 3-week cycle at 20 mg flat (Dose Level -1, DL-1), 40 mg flat (DL1), 60 mg flat (DL2) or 80 mg flat (DL3) will be taken orally by the patient on D1, D3, D8 and D10 of each cycle (dosing weeks = week 1 and week 2 of each 3-week cycle). Dose escalation will begin at DL1 and will continue until the MTD is exceeded or until the highest dose level defined in the study (DL3) is reached.