The in vitro evaluation of 25-hydroxyvitamin D-3 and 19-nor-1 alpha,25-dihydroxyvitamin D-2 as therapeutic agents for prostate cancer

Prostate cancer cells contain specific receptors [vitamin D receptors (VDRs )] for 1 alpha,25-dihydroxyvitamin D, (1 alpha,25(OH)(2)D-3), which is know n to inhibit the proliferation and invasiveness of these cells, These findi ngs support the use of 1 alpha,25(OH)(2)D-3 for prostate cancer therapy, Ho wever, because 1 alpha,25(OH)(2)D-3 can cause hypercalcemia, analogues of 1 alpha,25(OH)(2)D-3 that are less calcemic but that exhibit potent antiprol iferative activity would be attractive as therapeutic agents, We investigat ed the effects of two different types of less calcemic vitamin D compounds, 25-hydroxyvitamin D-2 [25(OH)D-3] and 19-nor-1 alpha,25-dihydroxyvitamin D -2 [19-nor-1 alpha,25(OH)(2)D-2], and compared their activity to 1 alpha,25 (OH)(2)D-3 on (a) the proliferation of primary cultures and cell lines of h uman prostate cancer cells; and (b) the transactivation of the VDRs in the androgen-insensitive PC-3 cancer cell line stably transfected with VDR (PC- 3/VDR), 19-nor-1 alpha,25(OH)(2)D-2, an analogue of 1 alpha,25(OH)(2)D-3 th at was originally developed for the treatment of parathyroid disease, has b een shown to be less calcemic than 1 alpha,25(OH)(2)D-3 in clinical trials. Additionally, we recently showed that human prostate cells in primary cult ure possess 25(OH)D-3-1 alpha-hydroxylase, an enzyme that hydroxylates the inactive prohormone, 25(OH)D-3, to the active hormone, 1 alpha,25(OH)(2)D-3 , intracellularly. We reasoned that the hormone that is formed intracellula rly would inhibit prostate cell proliferation in an autocrine fashion, We f ound that 1 alpha,25(OH)(2)D-3 and 19-nor-1 alpha,25(OH)(2)D-2 caused simil ar dose-dependent inhibition in the cell lines and primary cultures in the [H-3]thymidine incorporation assay and that both compounds were significant ly more active in the primary cultures than in LNCaP cells, Likewise, 25(OH )D-3 had inhibitory effects comparable to those of 1 alpha,25(OH)(2)D-3 in the primary cultures. In the chloramphenicol acetyltransferase (CAT) report er gene transactivation assay in PC-3/VDR cells, 1 alpha,25(OH)(2)D-3 and 1 9-nor-1 alpha,25(OH)(2)D-2 caused similar increases in CAT activity between 10(-11) and 10(-9) M. Incubation of PC-3/VDR cells with 5 x 10(-8) M 25(OH )D-3 induced a 29-fold increase in CAT activity, similar to that induced by 10(-8) M 1 alpha,25(OH)(2)D-3. In conclusion, our data indicate that 25(OH )D-3 and 19-nor-1 alpha,25(OH)(2)D-2 represent two different solutions to t he problem of hypercalcemia associated with vitamin D-based therapies: 25(O H)D-3 requires the presence of 1 alpha-hydroxylase, whereas 19-nor-1 alpha, 25(OH)(2)D-2 does not. Both drugs are approved for human use and may be goo d candidates for human clinical trials in prostate cancer.