A screening period of 3 to 6 weeks will precede the treatment period. At the baseline visit, patients whose eligibility is confirmed will be treated with ZA and followed for 12 months. Safety and efficacy will be assessed at regular intervals (day one, day 10, month 2, month 6, month 9 and month 12). Renal safety will be assessed prior to the i.v. dose of study medication, day 10 after the i.v. dose of study medication and at 12 months. Bone density at the lumbar spine (L1-4) and total hip will be performed at 6 months and at the end of the 12 month treatment period. Biomarker analyses for secondary endpoint will be performed for at day 10, month 2, month 6, month 9 and month 12.