MON-104 - Therapeutic Response and Exploratory Clinicopathological Correlates of Preoperative Lanreotide Autogel in TSH-Secreting Pituitary Tumors

Objective: Lanreotide Autogel (LAN-ATG), a long-acting somatostatin analog, is used for preoperative control of thyrotoxicosis in thyrotropin (TSH)-secreting pituitary tumors. However, clinicopathological correlates of therapeutic response and the optimal preoperative treatment duration remain unestablished. This single-center case series aimed to evaluate the effects of preoperative LAN-ATG on thyroid function, tumor volume, surgical outcomes, and to explore potential clinicopathological correlates of response.

Methods: We retrospectively analyzed seven Japanese adults with TSH-secreting pituitary tumors who received preoperative LAN-ATG (approved for this indication in Japan) every four weeks before undergoing transsphenoidal tumor resection. Thyroid hormone profiles and tumor volumes were assessed after initiation of LAN-ATG. Associations between the number of LAN-ATG injections and biochemical and radiological responses were examined. Potential predictors, including TSH suppression rates on the octreotide suppression test (OST) and semi-quantitative immunohistochemistry (IHC) scores for somatostatin receptor (SSTR) 2A and SSTR5, were descriptively evaluated in relation to therapeutic response.

Results: Six of the seven patients presented with macroadenomas (median diameter 19.1 mm, range 7.3 to 28.0 mm). None had concomitant acromegaly or hyperprolactinemia. LAN-ATG was well tolerated (median 4 injections preoperatively). Both thyroid hormone levels and tumor volume decreased after LAN-ATG treatment. A single LAN-ATG injection normalized free T4 levels in all patients, and euthyroidism was maintained until surgery. Tumor reduction greater than 20% (median reduction 24.1%, range 13.7 to 46.3%) was observed in four patients. The number of injections was not clearly associated with the changes in TSH, free T3, free T4, or tumor reduction. Patients exhibiting low TSH suppression on OST nevertheless achieved biochemical improvement and tumor reduction with LAN-ATG. All tumors showed high SSTR2A expression, while SSTR5 expression varied and was not clearly associated with therapeutic response. No perioperative thyroid storm occurred, and all patients achieved remission. New-onset postoperative hypopituitarism developed in two patients with macroadenomas.

Conclusion: Preoperative LAN-ATG rapidly improved thyrotoxicosis in TSH-secreting pituitary tumors and induced tumor shrinkage in a subset. In this descriptive series, no clear trend was observed between therapeutic response and number of injections, OST results, or SSTR5 expression, whereas uniformly high SSTR2A expression may support the clinical utility of LAN-ATG.

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