Clinical Trial of Paricalcitol in Myelodysplastic Syndrome.

Myelodysplastic syndrome is often a pernicious disorder associated with pancytopenia in the elderly. Therapeutic approaches need to balance their toxicities versus the side-effects of the disease. 1,25(OH)₂-vitamin-D₃ inhibits proliferation and induces differentiation of leukemic cells in vitro. Small clinical trials have shown slight efficacy in MDS. Hypercalcemia prevents the administration of doses of this seco-steroid, which have been shown to be effective in vitro. This has provided a stimulus to identify vitamin D analogs that have anti-leukemic activity with minimal hypercalcemic effects. Paricalcitol (19-nor-1,25(OH)₂D₂, Zemplar) has been approved by the FDA for the treatment of secondary hyperparathyroidism; the drug is unique because it has little hypercalcemic potential; but in vitro, it has strong antileukemic effects. We conducted a clinical trial of oral paricalcitol to twelve MDS patients whose disease varied between an IPSS of low to high. Therapy began at 8 μg per day and increased at two week intervals until serum calcium was slightly above normal level; at which point, dose was decreased by 4-8 μg qd. The amount of paricalcitol taken varied between 8 μg qod to 54 μg qd (average 16 μg qd). We confirmed that the drug was having biologic activity in vivo by examining a target of the activated vitamin D₃ receptor, 1,25-(OH)₂-vitamin-D₃-24(OH)ase mRNA. Each patient had prominent induction of this transcript in his or her white blood cells. Furthermore, in selected patients serum paricalcitol was measured and confirmed to be prominently present. The drug was well tolerated in all patients. Two of the 12 patients showed a clinical response. One patient’s platelet counts rose from 50,000 to 120,000/ul blood over 5 weeks; however, the patient succumbed to a fatal fungal infection. The second patient responded by a decrease in RBC transfusions associated with a rise in his hemoglobin, which lasted for about 5 months. Eventually, his hemoglobin began to fall, and erythropoietin therapy was substituted for paricalcitol. In summary, high doses of paricalcitol were well tolerated in all patients. Two patients had a partial clinical response. In general, paricalcitol given as a single agent to individuals with MDS is not therapeutically very efficacious; further trials should examine it in combination with other approaches.