Tumor products and the hypercalcemia of malignancy

Normalization of hypercalcemia associated with a decrease in renal calcium reabsorption in Leydig cell tumor-bearing rats treated with WR-2721

The Rice-500 Leydig cell tumor (LCT) in Fischer rats is a model of humoral hypercalcemia of malignancy (HHM). In this model, the elevation of plasma calcium (Ca) does not result merely from an increased bone resorption, but also from an enhanced tubular Ca reabsorption (TRCa). We investigated the hypocalcemic response to WR-2721 [S-2,2-(3-aminopropylamino)-, ethylphosphorothioic acid] in LCT-bearing Fischer rats. WR-2721 is a potent inhibitor of normal and aberrant parathyroid hormone (PTH) secretion. Moreover, it exerts a PTH-independent inhibitory effect on TRCa. In hypercalcemic LCT-bearing rats WR-2721 induced a fall in plasma Ca from 3.24 +/- 0.12 to 2.66 +/- 0.23 mmol/liter within 2 h after one single injection of 0.7 mmol/kg body wt. The decrement in plasma Ca was associated with a marked increase in urinary Ca excretion, indicating an inhibition of TRCa. The elevated urine cyclic AMP of LCT-bearing rats, however, was not altered by WR-2721 treatment. These results suggest that in this HHM model, WR-2721 can normalize calcemia through its PTH-independent inhibitory effect on TRCa. WR-2721 could therefore be an effective drug to treat human hypercalcemia of malignancy, particularly in those tumors wherein a markedly enhanced renal Ca reabsorption contributes to the elevation of the plasma Ca level.

Effects of DNA and prostaglandin synthesis inhibitors on the stimulation of bone resorption by epidermal growth factor in fetal rat long-bone cultures

We examined two inhibitors of DNA synthesis, hydroxyurea (HU) and aphidicholin (APC), and two inhibitors of prostaglandin cyclooxygenase, indomethacin and flufenamic acid, for their effects on the resorptive responses of fetal rat long-bone cultures to epidermal growth factor (EGF) and parathyroid hormone (PTH). As we have previously found, HU decreased unstimulated 45Ca release but had little effect on the resorptive response to PTH. HU also did not block resorption stimulated by EGF. Addition of the cyclooxygenase inhibitor, indomethacin, did not alter the resorptive responses of unstimulated or PTH-treated cultures in either the presence or absence of HU or the resorptive response of bones cultured with EGF alone. However, indomethacin completely blocked the resorptive response to EGF of bones that were cultured with HU. The effects of indomethacin on EGF-mediated resorption in HU-treated cultures appeared to be related to an inhibition of prostaglandin synthesis since flufenamic acid had similar effects. However, the effects of HU on the resorptive response to EGF may not have resulted solely from its inhibitory action on DNA synthesis since APC, in the absence of cyclooxygenase inhibitors, completely blocked EGF-mediated resorption without significantly affecting the response to PTH. These results demonstrate that the mechanisms regulating PTH- and EGF-mediated resorption in fetal rat long-bone cultures differ, and imply that a component of EGF-mediated resorption in these cultures is dependent on sustained DNA synthesis.

Quantitation of the renal effect of calcitonin in the hypercalcaemia of malignancy

The renal effect of calcitonin (independent of changes due to variations in sodium excretion) was quantitated in malignancy associated hypercalcaemia. There was considerable inter- and intra-patient variability in response but this was unaffected by tumour type. Humoral hypercalcaemia tended to be resistant to control. In responsive patients the renal effects of calcitonin unlike those described in bone did not diminish with time.