Effect of SMS 201-995, a long-acting somatostatin analogue, on the secretion and morphology of a pituitary growth hormone cell adenoma.

The present study reports the effects of SMS 201-995, a long-acting somatostatin analogue, on blood GH levels, glucose tolerance and tumour morphology in a 36-year-old, previously untreated acromegalic woman. Treatment (50 micrograms s.c., 8-hourly) resulted in marked suppression of GH concentration and an improvement in glucose tolerance. After 10 d of treatment, the tumour was removed by transsphenoidal surgery and studied by histology, immunohistochemistry, transmission electron microscopy and morphometry. Histologically, the tumour was an acidophilic adenoma which contained immunoreactive GH in many adenoma cells. By electron microscopy, the tumour was composed of densely granulated somatotrophs containing numerous large secretory granules and many lysosomes showing crinophagy. No cell necrosis or vascular impairment were evident. Using morphometry, the tumour was compared with 10 densely granulated somatotroph adenomas, removed from acromegalic patients not treated with somatostatin. The nuclear and cytoplasmic areas of the adenoma subjected to SMS 201-995 treatment were smaller, and the lysosomes occupied more of the cytoplasmic volume than those of controls. The nuclear/cytoplasmic ratio, cytoplasmic volume densities of endoplasmic reticulum, Golgi apparatus, mitochondria, secretory granules and secretory granule diameters were within the range of control adenomas. In vitro, treated adenoma cells secreted GH and retained responsiveness to both GRH stimulation and somatostatin suppression. The morphologic findings after SMS 201-995 treatment, are consistent with suppression of GH release. There is no evidence that somatostatin has any direct cytotoxic or vasotoxic effects. It appears that SMS 201-995 represents a potent and promising drug in the medical treatment of acromegaly, however, more work is needed to elucidate the mechanism of somatostatin suppression and to provide evidence for adenoma shrinkage.