Endocrine Abstracts

Selpercatinib is a receptor tyrosine kinase RET inhibitor for the treatment of cancers such as RET-mutant medullary thyroid cancer, demonstrating partial response and low incidence of serious adverse events. Vandetanib, a tyrosine kinase inhibitor, has shown promising results with an increase in progression-free survival and prolonged lifetime, but it causes adverse events such as hypertension, diarrhea, rash, or long QT interval. Most of them are can be even disabling for the patient, leading to discontinuation of the medication, but do not necessarily suppose a severe health risk. In this report, we describe a case of an advanced medullary thyroid carcinoma who could not stand Vandetanib therapy and who needed a change of its therapy. A 47 years-old woman asked the Digestive System Unit complaining of chronic diarrhea that worsened the last week. They performed a complete study including a blood test, abdominal ultrasound, gastroscopy, and colonoscopy. The last of them revealed an 8–10 cm polypus near to the ileocecal valve with a malignancy appearance that could not be resected by endoscopy. After that a whole-body CT scan was performed as a tumor extension study, identifying a right thyroid lobe enlargement (73 × 50 × 45 mm) with endothoracic extension that caused tracheal displacement to the left. It also evidenced several cervical, mediastinal and hilar lymphadenopathies, suggestive of metastatic spread. Eco-guided fine-needle aspiration of the right thyroid lobe suggested a medullary thyroid carcinoma (Category V of Bethesda). Endocrinology Unit performed a blood test and a 24-hour urinary fractionated metanephrines determination to rule out primary hyperparathyroidism and pheochromocytoma respectively, that were negative. One month later, a subtotal thyroidectomy and a wide lymphadenectomy was performed. Because of the incomplete resection and the tracheal compression, the colonic polypus resection needed to be delayed. A (18)F-FDG PET/TC revealed two bone metastatic lesions (T4bN1bM1). The patient started Vandetanib 300 mg diary, but six months later she suffered severe hypomagnesemia (0.5 mg/dl) that needed intravenous replacement. Also, she developed a rash and long QT interval. The hypomagnesemia persisted despite a Vandetanib dose reduction and oral magnesium replacement, because of that we decided to change ITK therapy to Selpercatinib (after positive determination of RET mutation). Three months later, she has no adverse events with the new RET inhibitor therapy. Could Selpercatinib be a better therapeutic option than Vandetanib for our patients in the next future? further studies will be necessary to answer this question.