Background. Maxacalcitol (22-oxacalcitriol
OCT) is a novel vitamin D analogue. In previous clinical studies, OCT was administered three times a week to hemodialysis patients with refractory secondary hyperparathyroidism (2HPT), in whom it acted by inhibiting parathyroid hormone secretion, as well as causing mildly elevated serum calcium. However, intravenous injection of OCT, which requires frequent visits to the outpatient clinic, degrades the quality of life of patients with continuous ambulatory peritoneal dialysis (CAPD) who otherwise visit the clinic only once or twice per month. In the present study, we investigated whether transperitoneal absorption of OCT inhibited intact parathyroid hormone (i-PTH) in CAPD patients when the OCT was added to the peritoneal dialysis fluid. Methods. Peritoneal dialysis fluid containing 20 μg of OCT was injected into the peritoneal cavity of five CAPD patients. The serum and peritoneal fluid levels of OCT, i-PTH, calcium, and phosphate were measured before and after treatment. Results. The mean concentration of OCT in peritoneal dialysis fluid rapidly decreased, from 25268.0 pg/ml at 0 h to 1694.0 pg/ml at 2 h and 44.9 pg/ml at 4 h. In contrast, the mean serum OCT level increased from the pretreatment level, which was below the detection limit of the assay, to 656.0 g/ml at 0.5 h and a peak of 759.0 pg/ml at 1 h, and thereafter gradually decreased, to 713.8 pg/ml at 2 h and 555.8 pg/ml at 4 h. Mean i-PTH significantly decreased, to 83.9% of the baseline level, at 1 h (P &lt
0.05) and thereafter stayed at around 90%. No consistent trends in calcium and phosphate levels were observed in the five patients. Conclusions. By injecting OCT into the peritoneal cavity, i-PTH levels could be significantly decreased. These findings indicate the therapeutic efficacy of intraperitoneal administration of OCT for CAPD patients.