Antitumor activity of the growth hormone receptor antagonist pegvisomant against human meningiomas in nude mice

Object. The authors have previously demonstrated that modulation of the growth hormone (GH)/insulin-like growth factor-I (IGF-I) axis can significantly affect meningioma growth in vitro. These studies were performed to evaluate the efficacy of GH receptor blockade in vivo. Methods. Primary cultures from 15 meningioma tumors obtained in humans were xenografted into athymic mice. Approximately 1.5 million cells from each of the lj tumors were implanted into the flanks of two female mice, one pair for each tumor. One animal from each of the lj pairs was then treated with the GH receptor antagonist pegvisomant and the other with vehicle alone for 8 weeks. The tumor volume was measured using digital calipers three times per week. The mean tumor volume at the initiation of injections was 284 +/- 18.8 mm(3) in the vehicle group and 291.1 +/- 20 mm(3) in the pegvisomant group. After 8 weeks of treatment, the mean volume of tumors in the pegvisomant group was 198.3 +/- 18.9 mm(3) compared with 350.1 +/- 23.5 mm(3) for the vehicle group (p < 0.001). The serum IGF-I concentration in the vehicle group was 319 +/- 12.9 <mu>g/L compared with 257 +/- 9.7 in the pegvisomant group (p < 0.02). A small but significant decrease was observed in circulating IGF binding protein (IGFBP)-3 levels, whereas slight increases occurred with respect to serum IGFBP-1 and IGFBP-4 levels. In the placebo group the tumor weight was 0.092 +/- 0.01 g compared with 0.057 +/- 0.01 g in the pegvisomant group (p < 0.02). The IGF-I and IGF-II concentrations were measured in the tumors by using a tissue extraction method. These human-specific immunoassays demonstrated that there was no autocrine production of IGF-I in any of the tumors, either in the pegvisomant or vehicle group. The IGF-II levels were highly variable (0-38.2 ng/g tissue) and did not differ significant ly between treatment groups. Conclusion. In an in vivo tumor model, downregulation of the GH/IGF-I axis significantly reduces meningioma growth and, in some instances, causes tumor regression. Because the concentrations of IGF-II in tumor did not vary with pegvisomant treatment and there was no autocrine IGF-I production by the tumors, the mechanism of the antitumor effect is most likely a decrease of IGF-I in the circulation and/or surrounding host tissues. Because the authors have previously demonstrated that the CH receptor is ubiquitously expressed in meningiomas, direct blockade of the GH receptor on the tumors may also be contributing to inhibitory actions.