AbstractBackground and objective
1,25-dihydroxyvitamin D3[1α,25(OH)2D3, calcitriol], and its less calcaemic synthetic analogues have therapeutic potential in several diseases, including hyperparathyroidism (HPT). We have suggested that non-1α-hydroxylated (nonactive) vitamin D analogues may present an alternative in tumour cells expressing 25-hydroxyvitamin D3 1α-hydroxylase (1α-hydroxylase). The aim of this study was to investigate biological effects of a non-1α-hydroxylated vitamin D analogue in normal and tumour parathyroid cells.Patients and methods
Effects of vitamin D analogues and ketoconazole on parathyroid hormone (PTH) secretion (radioimmunoassay) and PTH mRNA expression (reverse transcription-polymerase chain reaction) were studied in primary bovine parathyroid cells. Proliferation of tumour cells isolated from HPT patients was determined by thymidine incorporationResults
EB1285, non-1α-hydroxylated precursor of the vitamin D analogue EB1089, suppressed PTH secretion and PTH mRNA level as well as increased expression of 25-hydroxyvitamin D3-24-hydroxylase (24-hydroxylase) in bovine parathyroid cells. EB1285 also inhibited cell proliferation of parathyroid tumour cells from primary (pHPT) and secondary HPT (sHPT) patients. Combined treatment with the cytochrome P450-dependent enzyme inhibitor ketoconazole and EB1285 or with active vitamin D compounds potentiated the suppressive effect on PTH secretion from bovine parathyroid cells. Ketaconazole alone displayed PTH suppression and increased 24-hydroxylase expression.Conclusion
The results support the idea that a non-1α-hydroxylated vitamin D analogue may elicit vitamin D receptor (VDR) effects in 1α-hydroxylase expressing parathyroid tumour cells. Further studies are warranted to elucidate whether precursor vitamin D analogues as well as inhibitors of 24-hydroxylase present therapeutic alternatives in patients suffering from HPT.