Resistance to Somatostatin Analogs in Acromegaly

Somatostatin analogues (SA) currently represent the medical treatment option for most patients with acromegaly and neuroendocrine tumors. In the setting of acromegaly, SA are indicated both as first-line treatment and as adjuvant treatment after unsuccessful surgery. Resistance to SA is still not fully defined but is generally considered to occur in the patients without control of GH and IGF-I excess with high dosages of the drugs. Tumor shrinkage is also a relevant clinical outcome of SA treatment. This review examines the current state of our understanding of these treatments and provides a definition of SA resistance. Somatostatin analogs (SA) are widely used in acromegaly, either as first-line or adjuvant treatment after surgery. First-line treatment with these drugs is generally used in the patients with macroadenomas or in those with clinical conditions so severe as to prevent unsafe reactions during anesthesia. Generally, the response to SA takes into account both control of GH and IGF-I excess, with consequent improvement of clinical symptoms directly related to GH and IGF-I excess, and tumor shrinkage. This latter effect is more prominent in the patients treated first-line and bearing large macroadenomas, but it is also observed in patients with microadenomas, even with little clinical implication. Predictors of response are patients' gender, age, initial GH and IGF-I levels, and tumor mass, as well as adequate expression of somatostatin receptor types 2 and 5, those with the highest affinity for octreotide and lanreotide. Only sporadic cases of somatostatin receptor gene mutation or impaired signaling pathways have been described in GH-secreting tumors so far. The response to SA also depends on treatment duration and dosage of the drug used, so that a definition of resistance based on short-term treatments using low doses of long-acting SA is limited. Current data suggest that response to these drugs is better analyzed taking together biochemical and tumoral effects because only the absence of both responses might be considered as a poor response or resistance. This latter evidence seems to occur in 25% of treated patients after 12 months of currently available long-acting SA.