Effects of Seocalcitol (EB1089) on Nitrosomethyl Urea-Induced Rat Mammary Tumors

Although 1,25-dihydroxyvitamin D3 is a potent cell-differentiating agent, its use in cancer prevention or therapy is precluded because it induces hypercalcemia. Synthetic analogs have been developed which inhibit tumor progression in animal models of breast cancer. One analog, Seocalcitol (EB1089) has been shown to be effective in causing regression of N-methyl-nitrosourea-induced rat mammary tumors. However, at the most effective oral dose, a significant increase in serum and urinary calcium levels were observed. In order to compare the efficacy of different dosing schedules of Seocalcitol, rats were treated either 6 times weekly (1 µg/kg) or by intermittent dosing to achieve the same total weekly dose. All dosing schedules of Seocalcitol were effective in inhibiting tumor progression. Once daily dosing was significantly more effective than intermittent dosing but was associated with a greater rise in serum calcium concentration. In order to evaluate alternative treatment strategies to limit calcemic effects, we assessed the efficacy of limiting vitamin D-induced hypercalcemia using bisphosphonates. Seocalcitol (2.5 µg/kg daily p.o. for 4 weeks) alone and in combination with pamidronate (APD 0.4 mg/kg per day s.c.) or the same dose of the bisphosphonate EB1053 caused substantial tumor regression. No statistically significant difference was seen between combination treatment and Seocalcitol treatment alone. Co-treatment with APD or EB1053 did not limit the rise in serum calcium induced by Seocalcitol alone. Cessation of treatment or administration of a lower dose (1 µg/kg twice weekly) reversed hypercalcemia, hypercalciuria and weight loss induced by high dose Seocalcitol. However, reduction in tumor volume was maintained in the majority of animals.