Somatostatin 5 receptor expression in prolactinomas: Is there a role for Pasireotide in the management of prolactinomas?

The majority of prolactinomas are treated with dopamine agonists (DA) with excellent response, however DA-resistance occurs in 10% of prolactinomas. Somatostatin (SST) receptors have been increasingly studied in prolactinomas. There are five SST receptor subtypes and a significant number of prolactinomas show expression of SST₅ and SST₁ mRNA. The somatostatin analog (SSA) pasireotide, which has 40-fold greater binding affinity to SST₅ compared to first-generation SSAs, shows promising results in case reports of DA-resistant prolactinomas. This two-center retrospective cohort study investigated the expression patterns of dopamine 2 (D2R), SST₂ and SST₅ receptors in surgical specimen of 34 patients with prolactinomas, 22 of which were DA-resistant. In vitro effects of cabergoline, octreotide and pasireotide on prolactin production was also examined in cultured prolactinoma cells. Receptor expression was scored using the immunoreactivity score (IRS). 31/34(91%) patients used DA preoperatively; 22/34(64.7%) were DA-resistant. Receptor expression in the cases was 97.1% for D2R, 70.6% for SST₅ and 41.2% for SST₂. In the majority of SST₂ positive cases SST₂ expression was very low. In in vitro studies comparing the effects of octreotide, pasireotide, and cabergoline on prolactin secretion, octreotide was the least potent drug and cabergoline was the most potent. SST₅ and D2R expression was highest in prolactinomas showing the highest response to pasireotide and cabergoline in vitro (median D2R IRS 1.0 vs 8.0 for < 50% vs. > 50% inhibition by cabergoline and median SST₅ IRS 3.5 avs. 12.0 for < 50% vs. > 50% inhibition by pasireotide). In a subgroup, pasireotide inhibited prolactin secretion with comparable potency to cabergoline. Targeting SST₅ with pasireotide may be a potential treatment modality for further clinical investigation in the treatment of a subset of DA resistant or intolerant prolactinomas.