Response to intravenous bisphosphonate therapy in hypercalcaemic patients with and without bone metastases: the role of parathyroid hormone-related protein.

Plasma parathyroid hormone related-protein (PTHrP) may inhibit the calcium-lowering effect of bisphosphonate therapy. In this prospective study we examined the relationship between plasma PTHrP levels, renal tubular markers of calcium reabsorption, and the effectiveness of intravenous bisphosphonate therapy (IVBPT) in lowering serum calcium in patients with hypercalcaemia of malignancy (HM), with and without bone metastases. Thirty-five symptomatic hypercalcaemic patients (17 without and 18 with bone metastases) were treated with IVBPT (pamidronate 30-60 mg or BM21.0955 2-6 mg). Normocalcaemia was achieved in 24/35 (71%) patients with a mean fall in serum calcium of 0.85 mmol l-1 (range 0.11-1.93, P < 0.001). In the 35 patients studied, serum calcium levels reached a nadir between days 3 and 7, and this was accompanied by a small but significant reduction in plasma PTHrP levels (median reduction 0.77 pmol l-1, P = 0.007). Patients who responded to bisphosphonate therapy by becoming normocalcaemic had significantly lower basal plasma PTHrP levels, mean 4.06 vs 8.2 pmol l-1 (P < 0.04). A significant reduction in urinary calcium excretion was seen (mean 106 mumol l-1, P < 0.02) in patients with bone metastases, and urinary cAMP (mean 170 mmol l-1, P < 0.01) fell in all patients. Patients without demonstrable bone metastases had significantly higher plasma PTHrP levels (P < 0.002), required more doses of IVBPT, and had a poorer reduction in serum calcium compared with patients with bone metastases, only one of whom required more than one dose. We conclude that circulating PTHrP has an important role in increasing renal tubular reabsorption of calcium in HM, thus reducing the effectiveness of bisphosphonate therapy, particularly in patients with humoral HM and no bone metastases.