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Vitamin D5 is a form of vitamin D.

Calcitriol analogues have been proposed for use as antitumor agents. Studies on vitamin D3 have shown inhibition of cell proliferation in prostate cancer, but vitamin D3 results in hypercalcemia. The effects of vitamin D5 on prostate cancer have also been studied, and unlike vitamin D3, vitamin D5 does not cause hypercalcemia while inhibiting tumor cell proliferation.

Background
Perhaps the most notable analogue of vitamin D5 is 1α-Hydroxyvitamin D5, synthesized first in 1997 by researchers in the Department of Chemistry at the University of Chicago, under Robert M. Moriarty and Dragos Albinescu. By 2005, the group had revised its synthesis method for a more streamlined, higher yield-producing route. It involved the photochemical conversion of precursor 7-dehydrositosteryl acetate to contain a conjugated triene system, a hallmark of this analogue, followed by hydroxylation, photoisomerization and deprotection. Their overall yield was 48%. The motive to develop 1α-Hydroxyvitamin D5 stemmed from the tendency of 1,25 dihydroxy vitamin D3 (a natural metabolite produced in the kidney), or calcitriol, to cause toxic hypercalcemia in patients when dosed at concentrations needed to interrupt prostate cancer cells' cycle and stimulate apoptosis. And while supplementation with dexamethasone decreases hypercalcemia, bypassing it with an equally effective tumor suppressant would reduce patient cost and stress. Thus, the therapeutic effects of 1α-Hydroxyvitamin D5 as a potential antitumor agent without the side effects of calcitriol became a topic of study.