Dissecting the In Vitro Efficacy of Octreotide and Cabergoline in GH- and GH/PRL-Secreting Pituitary Tumors.

CONTEXT: Cabergoline (CAB) is an off-label medical therapy for acromegaly, overshadowed by first-generation somatostatin receptor ligands, eg, octreotide (OCT). OBJECTIVE: This was a head-to-head comparison between OCT and CAB in inhibiting growth hormone (GH) secretion in primary cultures of GH- and GH/prolactin (PRL)-secreting tumors; we also investigated the role of somatostatin (SST) and dopamine type 2 (D2R) receptor expression. METHODS: We evaluated the antisecretory effect of OCT and CAB, together with receptor mRNA expression, in 23 tumor cultures obtained from acromegaly patients referred to the Erasmus Medical Center (Rotterdam, The Netherlands). GH concentrations in cell culture media were determined after 72-hour OCT and CAB treatment (10 nM). RESULTS: OCT showed a slightly higher efficacy compared with CAB (GH decrease -39.5% vs -32.5%, P = 0.079). The effect of the 2 drugs was superimposable in GH/PRL co-secreting tumors (-42.1% vs -44.8%), where SST1 and D2R had a higher expression compared with the pure GH-secreting tumors (P = 0.020 and P = 0.026). OCT was more effective than CAB in 8/23 cultures, while CAB was more effective than OCT in 3/23 (CAB+ group). In CAB+ tumors, SST1 expression was higher compared with the other groups (P = 0.034). At receiver operating characteristic (ROC) curve analysis, SST1 and D2R discriminated between GH and GH/PRL co-secretion (AUC 0.856, P = 0.013; AUC 0.822, P = 0.024). SST1 was the best predictor of CAB response (≥50% GH reduction, AUC 0.913, P = 0.006; 80% sensitivity, 94% specificity). CONCLUSION: OCT is 5% to 10% more effective than CAB in vitro. SST1 mRNA expression can represent a reliable marker of GH/PRL co-secreting tumors showing a preferential response to CAB treatment.

Octreotide and Pasireotide Combination Treatment in Somatotroph Tumor Cells: Predominant Role of SST2 in Mediating Ligand Effects.

First-generation somatostatin receptor ligands (fg-SRLs), such as octreotide (OCT), represent the first-line medical therapy in acromegaly. Fg-SRLs show a preferential binding affinity for somatostatin receptor subtype-2 (SST2), while the second-generation ligand, pasireotide (PAS), has high affinity for multiple SSTs (SST5 > SST2 > SST3 > SST1). Whether PAS acts via SST2 in somatotroph tumors, or through other SSTs (e.g., SST5), is a matter of debate. In this light, the combined treatment OCT+PAS could result in additive/synergistic effects. We evaluated the efficacy of OCT and PAS (alone and in combination) on growth hormone (GH) secretion in primary cultures from human somatotroph tumors, as well as on cell proliferation, intracellular signaling and receptor trafficking in the rat GH4C1 cell line. The results confirmed the superimposable efficacy of OCT and PAS in reducing GH secretion (primary cultures), cell proliferation, cAMP accumulation and intracellular [Ca2+] increase (GH4C1 cells), without any additive effect observed for OCT+PAS. In GH4C1 cells, co-incubation with a SST2-selective antagonist reversed the inhibitory effect of OCT and PAS on cell proliferation and cAMP accumulation, while both compounds resulted in a robust internalization of SST2 (but not SST5). In conclusion, OCT and PAS seem to act mainly through SST2 in somatotroph tumor cells in vitro, without inducing any additive/synergistic effect when tested in combination.

Discordant GH and IGF-1 Results in Treated Acromegaly: Impact of GH Cutoffs and Mean Values Assessment.

CONTEXT: Discordant growth hormone (GH) and insulin-like growth factor-1 (IGF-1) values are frequent in acromegaly. OBJECTIVE: To evaluate the impact of different GH cutoffs on discordance rate. To investigate whether the mean of consecutive GH measurements impacts discordance rate when matched to the last available IGF-1 value. DESIGN: Retrospective study. SETTING: Referral center for pituitary diseases. PATIENTS: Ninety acromegaly patients with at least 3 consecutive evaluations for GH and IGF-1 using the same assay in the same laboratory (median follow-up 13 years). INTERVENTIONS: Multimodal treatment of acromegaly. MAIN OUTCOME MEASURES: Single fasting GH (GHf) and IGF-1 (IGF-1f). Mean of 3 GH measurements (GHm), collected during consecutive routine patients' evaluations. RESULTS: At last evaluation GHf values were 1.99 ± 2.79 µg/L and age-adjusted IGF-1f was 0.86 ± 0.44 × upper limit of normality (mean ± SD). The discordance rate using GHf was 52.2% (cutoff 1 µg/L) and 35.6% (cutoff 2.5 µg/L) (P = 0.025). "High GH" discordance was more common for GHf <1.0 µg/L, while "high IGF-1" was predominant for GHf <2.5 µg/L (P < 0.0001). Using GHm mitigated the impact of GH cutoffs on discordance (GHm <1.0 µg/L: 43.3%; GHm <2.5 µg/L: 38.9%; P = 0.265). At receiver-operator characteristic curve (ROC) analysis, both GHf and GHm were poor predictors of IGF-1f normalization (area under the curve [AUC] = 0.611 and AUC = 0.645, respectively). The prevalence of disease-related comorbidities did not significantly differ between controlled, discordant, and active disease patients. DISCUSSION: GH/IGF-1 discordance strongly depends on GH cutoffs. The use of GHm lessen the impact of GH cutoffs. Measurement of fasting GH levels (both GHf and GHm) is a poor predictor of IGF-1f normalization in our cohort.

Biological and Biochemical Basis of the Differential Efficacy of First and Second Generation Somatostatin Receptor Ligands in Neuroendocrine Neoplasms.

Endogenous somatostatin shows anti-secretory effects in both physiological and pathological settings, as well as inhibitory activity on cell growth. Since somatostatin is not suitable for clinical practice, researchers developed synthetic somatostatin receptor ligands (SRLs) to overcome this limitation. Currently, SRLs represent pivotal tools in the treatment algorithm of neuroendocrine tumors (NETs). Octreotide and lanreotide are the first-generation SRLs developed and show a preferential binding affinity to somatostatin receptor (SST) subtype 2, while pasireotide, which is a second-generation SRL, has high affinity for multiple SSTs (SST5 > SST2 > SST3 > SST1). A number of studies demonstrated that first-generation and second-generation SRLs show distinct functional properties, besides the mere receptor affinity. Therefore, the aim of the present review is to critically review the current evidence on the biological effects of SRLs in pituitary adenomas and neuroendocrine tumors, by mainly focusing on the differences between first-generation and second-generation ligands.

Cell specific interaction of pasireotide: review of preclinical studies in somatotroph and corticotroph pituitary cells.

BACKGROUND: Pasireotide is a second-generation somatostatin (SRIF) receptor ligand (SRL), approved for medical treatment of acromegaly and Cushing's disease (CD). The molecule is a stable cyclohexapeptide synthetized based on SRIF structure. Differently from first-generation SRLs (e.g. octreotide), preferentially binding somatostatin receptor (SST) subtype 2 (SST2), pasireotide has high affinity for multiple SSTs (SST5 > SST2 > SST3 > SST1). Interestingly, early preclinical studies demonstrated that pasireotide shows distinct functional properties compared to SRIF and first-generation SRLs when binding SSTs. METHODS: We aimed to highlight the differential receptor-targeted action of pasireotide in the treatment of somatotroph and corticotroph adenomas, throughout the critical revision of preclinical studies carried out on acromegaly and CD models. RESULTS: Different authors demonstrated that the antisecretory effect of pasireotide in somatotroph adenoma cell cultures is comparable to that of the SST2-preferential agonist octreotide. Some reports even show a direct correlation between SST2 mRNA expression and GH reduction after pasireotide treatment, thus laying for a predominant role of SST2 in driving pasireotide efficacy in somatotropinomas in vitro. On the other hand, the inhibitory effect of pasireotide on ACTH secretion in corticotropinoma cells seems to be mainly mediated by SST5. Indeed, most reports show a higher potency and efficacy of pasireotide compared to SST2 preferential agonists, while functional studies confirm the pivotal role of SST5 targeting in corticotroph cells. CONCLUSIONS: The analysis of preclinical studies carried out in somatotroph and corticoph adenomas points out that pasireotide shows a cell-specific activity, exerting its biological effects via different SSTs in the different adenoma histotypes.

Long-term optical stimulation of channelrhodopsin-expressing neurons to study network plasticity.

Neuronal plasticity produces changes in excitability, synaptic transmission, and network architecture in response to external stimuli. Network adaptation to environmental conditions takes place in time scales ranging from few seconds to days, and modulates the entire network dynamics. To study the network response to defined long-term experimental protocols, we setup a system that combines optical and electrophysiological tools embedded in a cell incubator. Primary hippocampal neurons transduced with lentiviruses expressing channelrhodopsin-2/H134R were subjected to various photostimulation protocols in a time window in the order of days. To monitor the effects of light-induced gating of network activity, stimulated transduced neurons were simultaneously recorded using multi-electrode arrays (MEAs). The developed experimental model allows discerning short-term, long-lasting, and adaptive plasticity responses of the same neuronal network to distinct stimulation frequencies applied over different temporal windows.