Recently the active form of vitamin D, 1a,25-dihydroxyvitamin D 3 , [1,25(OH) 2 D 3 ] and its synthetic analogues have been shown to exert anti-tumor effects both in vitro and in vivo. [1][2][3][4][5] As summarized by Johannes et al.,6) many investigations have reported that 1,25(OH) 2 D 3 and its analogues affect tumor cells due to: a) blockade of the cells in the G 1 /G 0 phase and reduction of the number of cells in S phase of cell cycle progression, 7-9) b) induction of apoptosis, 10,11) c) expression and regulation of oncogenes and tumor suppressor genes, [12][13][14] d) interaction with tumor-or stroma-derived growth factors for growth inhibition [e.g. transforming growth factor (TGFb), insulin-like growth factor (IGF)], [15][16][17] and e) induction of differentiation. 18,19) However, most studies have been completed in human tumors with little information available in other species on the effects of 1,25(OH) 2 D 3 and its analogues.This study examined the effects of 1,25(OH) 2 D 3 and its analogues [22,24-diene-24a,26a,27a-trihomo-1a ,25-dihydroxyvitamin D 3 (EB1089) and 1,25-dihydroxy-16-ene-23-yne-vitamin D 3 (analog V)] on the canine adenocarcinoma (CAC-8) model of humoral hypercalcemia of malignancy (HHM) in nude mice. Canine adenocarcinoma derived from apocrine glands of the anal sac is commonly associated with cancer-associated hypercalcemia. [20][21][22] HHM is a paraneoplastic syndrome in dogs that significantly contributes to the morbidity and mortality of cancer patients. It is well documented that an autonomous overproduction of parathyroid hormone-related protein (PTHrP) is the principal humoral factor that causes hypercalcemia in dogs with this carcinoma due to increased release of calcium from bone and enhanced reabsorption of calcium in the kidney. 22,23) The inhibitory effect of 1,25(OH) 2 D 3 and its analogues on PTHrP production thus far has been reported on both normal human cells 24,25) and human tumor cells. [26][27][28][29][30] In order to achieve the inhibitory effect of 1,25(OH) 2 D 3 on PTHrP production, high doses and/or prolonged treatment with this potent active vitamin D metabolite most likely will be needed, which has the potential to enhance the hypercalcemia. 1,25(OH) 2 D 3 itself is also an important regulator of bone development, bone metabolism, calcium homeostasis and increases blood calcium. 31) In order to minimize this potential toxic effect, vitamin D analogues have been synthesized with noncalcemic effect. Among these noncalcemic analogues, EB1089 and analog V, have been studied on tumor growth.1,2,32) The effects of these two analogues (EB1089 and analog V) on canine adenocarcinoma model of HHM in nude mice were evaluated in this study. The specific objectives were to determine the effects of 1,25(OH) 2 D 3 and its analogues on tumor growth and body weight, changes in plasma ionized calcium, PTHrP production, bone resorption, and the distribution of the 1,25(OH) 2 D 3 receptor (VDR) on tumors in CAC-8-bearing mice. * To whom correspondence should be addressed...