Treatment with Bortezomib or Placebo. Patients allocated to the intervention group will receive two cycles of Bortezomib at an interval of 3 months. Each cycle will consist of Bortezomib 1.3 mg/m2 administered twice weekly on days 1, 4 and days 8, 11. Bortezomib will be given by injection intravenously (within 3-5 seconds). Bortezomib-treated patients will receive oral antiviral prophylaxis to prevent the development of herpes zoster infection: Valacyclovir 500 mg per day (eGFR <30 ml/min/1.73 m2: 250 mg per day) for 3 weeks after initiation of each cycle. Patients allocated to the control group will receive oral Placebo instead of Valacyclovir prophylaxis.