Growth of colon cancer cells, both in vitro and in vivo, has been shown to be inhibited by the active hormonal form of vitamin D, 1,25-dihydroxyvitamin D3. Unfortunately, doses of 1,25-dihydroxyvitamin D3 required to effectively treat colon tumors in vivo also stimulate the vitamin's classic effects on calcium homeostasis, resulting in the development of life-threatening hypercalcemia. Our preliminary data indicate, contrary to accepted dogma, very little 1,25-dihydroxyvitamin D3 administered either orally or injected subcutaneously actually reaches the colon to alter the transcription of target genes. We have approached these problems by synthesizing a pro-drug form of 1,25-dihydroxyvitamin D3. The pro-drug is capable of delivering high amounts of the hormone directly to the colon without substantially increasing blood concentrations of 1,25-dihydroxyvitamin D3, thus reducing the risk of a hypercalcemic response. This was achieved by conjugating a glucuronide to the hormone in a ?-linkage that renders it biologically inactive and resistant to mammalian digestive enzymes. Upon reaching the lower intestinal tract, however, ?-glucuronidase enzymes produced by resident bacterial populations cleave off the glucuronide and release the active hormone. The current proposal will determine whether using the glucuronide pro-drug system to target delivery of 1,25-dihydroxyvitamin D3 to the lower intestinal tract can slow or prevent the development of colorectal tumors without inducing systemic hypercalcemic toxicity. Chronic inflammation contributes to the development of carcinogenesis;people suffering from ulcerative colitis and Crohn's disease have an increased risk of developing colon cancer.
Aim 1 will determine the efficacy of the vitamin D glucuronide pro-drug in preventing or limiting the formation and size of colon tumors induced by azoxymethane/dextran sodium sulfate in a mouse model of inflammation-associated carcinogenesis. Mutations in the adenomatous polyposis coli (Apc) gene, originally linked to familial forms of colon cancer, have also been identified in the preponderance of human colon tumors.
Aim 2 will determine the effects of the glucuronide in preventing or limiting the formation and size of colon tumors in ApcMin/+ mice that carry a mutation in the mouse Apc gene and serve as a model of sporadic human colon cancer. Prior studies using 1,25-dihydroxyvitamin D3 or its analogs in the treatment of colon cancer have been limited by the development of hypercalcemia and sub-therapeutic amounts reaching the colon. The strength of the current proposal is the ability to target delivery of high concentrations of 1,25- dihydroxyvitamin D3 directly to the colon so its effects on colon carcinogenesis can be assessed in the absence of hypercalcemia.
A pro-drug form of vitamin D will be used to deliver the active vitamin D metabolite to the colon to assess its ability to inhibit the growth of tumors in to different animal models of colon cancer that mimic human disease. The natural hormonal form of vitamin D, while effective against colon cancer cells, cannot be used because it reaches all tissues of the body and causes a life-threatening rise in blood calcium concentrations. Delivering vitamin D to the colon avoids this side-effect and should enhance its effectiveness against colon tumors.