Hypercalcaemia of malignancy

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Abstract
Hypercalcaemia in malignancy is a major clinical problem. It contributes significantly to morbidity and mortality and can present difficult diagnostic and management dilemmas. Direct bony invasion by tumour cells rather than humorally mediated hypercalcaemia is probably the most common cause of malignant hypercalcaemia. Yet even in this situation the mechanism of bone resorption or the reason that the normal homeostatic mechanisms cannot cope with the calcium load are poorly understood. It is likely that the tumoral and paracrine factors produced by tumours which result in hypercalcaemia or in osteosclerotic bone metastases, are interposing themselves into the normal regulatory processes and deranging them. Humoral hypercalcaemia of malignancy is an important model for studying these questions, and it also provides some insight into the normal regulation of bone turnover. This review will examine the animal models and human syndromes of malignant hypercalcaemia and show now animal models, although helpful, fail to delineate the relative importance of the various potential humoral factors. A most interesting recent development in this area is the description of a new hormone, the parathyroid hormone-related peptide, which may explain many of the cases of humoral hypercalcaemia of malignancy. It is also a useful model with multiple sites of action within the bone and calcium homeostatic process. The active hormonal form of vitamin D3, 1,25-dihydroxyvitamin D3, may also be involved in a small proportion of cases, but again it is a useful model of some of the factors that may operate. Of considerable interest are the tumour derived factors, such as the transforming growth factors, and the cytokines, such as tumour necrosis factors, interleukins, and haemopoietic colony stimulating factors. Prostanoids are seldom of major importance, but may be important in certain tumour types. Osteosclerotic metastases, although seldom associated with hypercalcaemia, may provide insight into osteoblast regulating factors. Treatment of hypercalcaemia is discussed to show ways in which response to treatment may shed light on underlying pathophysiological mechanisms. Most effective treatments have many potential modes of action, and further study of the interactions of these agents and tumour types may help to unravel some of the enigmas in this human syndrome. The major advances in this complex problem involve the realisation of the necessity of multiple sites of action, including renal calcium handling as well as relative increases in bone resorption and/or intestinal calcium absorption. Various animal models and human examples indicate that rather than single factors operating alone, many humoral factors may act in concert to cause hypercalcaemia. Study of this problem has recently led to the discovery of a new calcium regulating hormone as well as to new roles for existing hormones and factors, aided by such powerful tools as molecular biology. Developments in assay systems for bone formation and bone resorption, renal calcium handling, and intestinal calcium absorption are essential if further advances are to be made.

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