Antiproliferative effects of somatostatin analogs in pituitary adenomas

The antisecretory effects of somatostatin (SRIF) and its analogs are widely recognised and provide the basis for treatment of hormonal hypersecretion in pituitary adenomas, especially in the settings of acromegaly. Evidence for an antiproliferative effect of these compounds has also been provided. This review focuses on the mechanisms transducing the antiproliferative effects of SRIF and its analogs on pituitary adenomas, and on the clinical consequences on tumor volume of pharmacological treatment of pituitary adenomas with these drugs.

Advances in medical therapy for pituitary disease: treating patients with growth hormone excess and deficiency

Two recent developments in the medical treatment of patients with pituitary disease are discussed. Conventional treatment for patients with acromegaly has been surgery and/or radiotherapy. Dopamine agonist therapy may be useful. Somatostatin is a naturally occurring neuropeptide that inhibits growth hormone (GH) secretion. The development of long-acting preparations has resulted in a considerable advance in the medical treatment of acromegaly, though some caution remains about the long-term side-effects of such therapy (e.g. cholelithiasis) and its cost/benefit analysis. Although further epidemiological studies are required, hypopituitarism appears to be associated with an increased mortality rate, and this has largely been attributed to GH deficiency. With the widespread availability of recombinant GH (thereby circumventing any risks from cadaveric GH) many adult GH-deficient patients are now being treated. Beneficial effects on body composition, nitrogen and calcium balance and bone mass have already emerged, although studies on 'well being' have been conflicting. The debate continues concerning the minimum effective dose and who exactly should be treated, in view of the costs incurred. The endocrinologists' use of long-acting somatostatin analogues and recombinant GH will undoubtedly increase over the next few years. Whilst it is clear that many patients with both acromegaly and hypopituitarism will benefit, long-term controlled trials are still required to establish firmly the benefit in terms of major morbidity.

Long-term in-vitro treatment of human growth hormone (GH)-secreting pituitary adenoma cells with octreotide causes accumulation of intracellular GH and GH mRNA levels

OBJECTIVE

We studied the effects of long-term in-vitro exposure of human GH secreting pituitary adenoma cells to octreotide on GH release, intracellular GH concentrations and GH messenger ribonucleic acid (mRNA) levels.

DESIGN

Human GH-secreting pituitary adenoma cells were cultured for periods from 4 days up to 3 weeks without or with octreotide (10 nM) and/or bromocriptine (10 nM). The effects of these drugs were measured on GH release, intracellular GH concentrations and intracellular GH mRNA levels.

PATIENTS

Thirteen patients with GH-secreting pituitary adenomas were studied. Twelve patients were untreated, one had been pretreated with octreotide (12 weeks, 3 x 100 micrograms daily).

MEASUREMENTS

GH, PRL, alpha-subunit and IGF-I concentrations in plasma, media and cell extracts were determined by immunoradiometric or radioimmuno-assays. GH mRNA levels were determined by automatic quantification of grain numbers in individual adenoma cells.

RESULTS

Incubation of the adenoma cells for 4 days with 10 nM octreotide induced a dose-dependent inhibition of GH release and a parallel increase (increase varying between 124 and 617% of control) in the intracellular GH levels was observed in six of seven adenomas. In addition, bromocriptine, when effective in inhibiting GH release by the adenomas, also induced an increase in intracellular GH levels. Even after 3 weeks of exposure to 10 nM octreotide in vitro there was a statistically significant increase in intracellular GH levels (between 191 and 923% of control). Withdrawal of octreotide after 6 days of incubation resulted in a lowering of intracellular GH levels to control values, showing that the octreotide-induced increase in intracellular GH is reversible. In a 96-hour incubation with 10 nM octreotide, GH mRNA levels were increased in two, and slightly decreased in one of the three adenomas tested. This effect was time dependent in that there was no significant effect of 10 nM octreotide on GH mRNA levels in a 24-hour incubation.

CONCLUSIONS

(1) Long-term in-vitro exposure of GH-adenoma cells to octreotide causes an increase in intracellular GH levels in the majority of the adenomas, probably because of an increase in GH mRNA levels in the adenoma cells; and (2) this considerable increase in intracellular GH levels may be one of the explanations for the relatively poor effect of octreotide on tumour shrinkage in patients with GH-secreting pituitary adenomas.

Presurgical octreotide treatment in acromegaly

The aim of this study was to determine the role of octreotide administration in acromegalic patients as a preparation for selective adenomectomy using transsphenoidal route. Octreotide was administered for 3 to 6 weeks before surgery in 12 patients and for 4 to 39 months in 25 patients. The clinical response was judged as excellent or good in 10 of 12 patients from group I and in 23 of 25 patients from group II. Marked reduction (ie, greater than 50% of initial values) in serum growth hormone (GH) levels was seen in all patients, with levels to less than 5 micrograms/L in 68% of patients and less than 2 micrograms/L in 27%. Insulin-like growth factor 1 (IGF-1) levels decreased to within normal limits in half the cases. During long-term treatment, an escape phenomenon could be seen. Varying degrees of tumor shrinkage were seen in more than 50% of cases. During surgery, with regard to the relative ease or difficulty in removing the tumor, the consistency of the tumor and the separation of normal from pathological tissue, no significant difference was observed between patients given octreotide and those from control series. Morphological changes in adenomatous tissue were rather small. The surgical outcome was similar in the pretreated series as in the control series, except in enclosed adenomas, which showed a tendency to a higher success rate. Since octreotide improves both the clinical condition and hormonal parameters and induces varying degrees of tumor shrinkage, it is potentially useful as an adjunct to surgery. Morphological data suggest that octreotide exercises a functional inhibitory effect on GH release.