SAT-LB076 T2-Signal Intensity, SST Receptor Expression and First-Generation Somatostatin Analogues Efficacy Predict Hormone and Tumor Responses to Pasireotide in Acromegaly

Background: Previous studies indicate that PAS-LAR can achieve control of insulin-like growth factor I (IGF-I) levels and may reduce tumor size, however a subset of acromegaly patients responds poorly. T2-signal intensity, somatostatin receptor (SST) subtype 2 and 5 expression, and the response to first-generation somatostatin receptor ligands (SRLs) are recognized predictors of therapy response. Valid prediction of the response to PAS-LAR can alter treatment stratification. Aim: To analyze T2-signal intensity and SST receptor expression in relation to the hormone and tumor response during PAS-LAR treatment, and to determine to what extent this equals SRLs responsiveness. Methods: We included 45 patients initially receiving SRLs, followed by a combination therapy including pegvisomant, and finally treated with PAS-LAR. The hormone response to PAS-LAR was evaluated using IGF-I (x ULN) levels at 24 weeks. T2-weighted MRI signal intensity of the adenoma was visually assessed and quantified by region of interest measurement. A tumor volume change of ≥25% from baseline was considered significant. SST receptor expression in adenomas was evaluated in 13 out of 45 patients using a validated immunoreactivity score (IRS). The clinical characteristics and the hormone and tumor response to PAS-LAR were assessed using multivariable regressions. Results: Patients with the lowest percentage IGF-I (x ULN) reduction during SRLs also showed weak IGF-I (x ULN) control during PAS-LAR and, however, more significant tumor shrinkage (r=0.41, p=0.006; p=0.036). Lower IGF-I (x ULN) levels during PAS-LAR were associated with higher T2-signal intensity and less significant tumor shrinkage (ß=-0.29, p=0.045; ß=0.34, p=0.035). With regards to tumor response, adenoma volume at baseline was associated with higher random GH levels at diagnosis and greater absolute tumor shrinkage during PAS-LAR (ß=69, p=0.0018; ß=1.05, p=0.020). Significant tumor shrinkage was associated with female patients, higher IGF-I (x ULN) levels during PAS-LAR and borderline significant with non-hypointense adenomas at baseline (OR=6.35, 95% CI=1.42-36.4; OR=13.2, 95% CI=2.14-129.1; OR=5.97, 95% CI=0.91-65.5 respectively). Lower IGF-I (x ULN) levels after PAS-LAR correlated with higher (r=-0.68, p=0.011; r=-0.52, p=0.083), while significant tumor shrinkage correlated with lower SST(2) levels as well as SST(2)/SST(5) ratio expression (p=0.040; p=0.024). Conclusions: Patients not responding to somatostatin analogs with particularly large adenomas, low SST(2) receptor expression and higher T2-signal intensity are more prone to show tumor shrinkage during PAS-LAR than patients with high SST(2) receptor expression and T2-hypointense adenomas. Surprisingly, tumor shrinkage is not accompanied by lower IGF-I (x ULN) levels, which are associated with a high SST(2) receptor expression and a higher T2-signal intensity. Unless otherwise noted, all abstracts presented at ENDO are embargoed until the date and time of presentation. For oral presentations, the abstracts are embargoed until the session begins. Abstracts presented at a news conference are embargoed until the date and time of the news conference. The Endocrine Society reserves the right to lift the embargo on specific abstracts that are selected for promotion prior to or during ENDO.