Inhibition of cancer cell mitosis by reducing the availability of phosphate.

The addition of phosphate groups is an essential requirement for the proper functioning of cyclin and cyclin dependent kinase which control various stages in the mitotic division of cancer cells. Thus limiting the availability of phosphate is likely to interfere with the metabolism of rapidly growing malignant cells. The human hormone glucagon and the anti metabolite mithramycin reduce serum phosphate by increasing phosphaturia and are both very effective in treating Paget's disease of bone, a precancerous condition. In this disorder large doses of glucagon given intravenously relieve bone pain and cause serum phosphate and alkaline phosphatase as well as urine hydroxyproline to fall, indicating a marked reduction in bone turnover. A constant iv infusion of glucagon was given to each of three patients all of whom had secondary malignant bone deposits. Two of the patients had primary prostate cancer and one had a squamous cell lung tumour. All three patients had relief of bone pain and a fall in serum alkaline phosphatase. Serum acid phosphatase also fell in the two patients with prostate cancer. It is proposed that the marked drop in serum phosphate due to glucagon causes intracellular phosphate to fall. This in turn disrupts the addition and removal of phosphate groups essential for the proper functioning of cyclin and cyclin dependent kinase. These two proteins control the transition from G1 to S (DNA synthesis phase) and G2 to M (mitotic phase) in the dividing cycle of malignant cells. Depriving a tumour of an essential ingredient used in phosphorylation reactions will disrupt its growth. It is also proposed that, by the same mechanism, glucagon induced hypophosphataemia renders malignant cells more sensitive to established chemotherapeutic agents and radiation waves. If this hypothesis proves to be correct, lowering intracellular phosphate may become an useful tool in cancer therapy. However extensive studies are necessary to determine whether mitosis in cancer cells can be advantageously disrupted by glucagon induced hypophosphataemia.