Effect of Combination Treatment with a Vitamin D Analog (OCT) and a Bisphosphonate (AHPrBP) in a Nude Mouse Model of Cancer-Associated Hypercalcemia

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Abstract
Hypercalcemia represents one of the important paraneoplastic syndromes affecting morbidity and mortality of cancer patients. We and others have demonstrated that vitamin D analogs with little calcemic activities suppress the transcription of the parathyroid hormone‐related peptide (PTHrP) gene, a major humor responsible for cancer hypercalcemia, and thereby prevent the development of hypercalcemic syndrome. The present study was undertaken: to compare the therapeutic efficacy of a vitamin D analog, 22‐oxa‐1,25‐dihydroxyvitamin D3 (OCT), and a bisphosphonate (disodium 3‐amino‐1‐hydroxypropylidene‐1,1‐bisphosphonate pentahydrate [AHPrBP]), an inhibitor of osteoclastic bone resorption, on cancer‐induced hypercalcemia; and to see if the effect could be enhanced by combination treatment, using a nude mouse model implanted with a human pancreas carcinoma (FA‐6). After a single intravenous administration, OCT (5 μg/kg of body weight [BW]) was as effective as AHPrBP (10 mg/kg of BW) in lowering blood ionized calcium levels in tumor‐bearing nude mice, and their combination further enhanced the therapeutic effect. Although AHPrBP lost its efficacy after repeated injections, OCT was still effective after the third administration. The therapeutic effect of OCT in cancer hypercalcemia was observed in four other human tumors, including another pancreas carcinoma (PAN‐7), two squamous cell carcinomas of the lung (KCC‐C1 and LC‐6), and a squamous carcinoma of the pharynx (PHA‐1), all of which elaborated PTHrP into the circulation. Treatment with OCT resulted in a decrease in circulating PTHrP levels by approximately 50% in two representative models. However, the mechanism underlying the antihypercalcemic effect of OCT seemed complex, involving inhibition of PTHrP production, suppression of excessive bone resorption, and an antitumor activity. OCT also markedly inhibited the body weight loss with tumor growth, while AHPrBP, which exhibited a similar antihypercalcemic effect, was less effective than OCT in preventing cachexia. The anticachectic activity of their combination did not exceed that of OCT alone, suggesting a hypercalcemia‐dependent as well as an independent mechanism of cancer cachexia. It is concluded that OCT may be useful, either as a single agent or in combination with bisphosphonates, for the treatment of cancer‐associated hypercalcemia and cachexia.