Selpercatinib Treatment of RET-Mutated Thyroid Cancers Is Associated With Gastrointestinal Adverse Effects

CONTEXT: Metastatic medullary thyroid carcinoma (MTC) and radioactive iodine–refractory differentiated thyroid carcinoma (RAI-R DTC) have poor prognosis and limited treatment options. Selpercatinib (LOXO-292), a selective kinase inhibitor targeting the RET gene, has shown a 69% to 79% objective response rate in this cohort with benefits in other tumors including lung cancer harboring the same oncogenic driver. Published reports describe only 17% of patients experiencing gastrointestinal (GI) adverse effects (AEs), which is in contrast to our local experience. OBJECTIVE: Here we characterize the AEs and correlate them with radiological and histopathological findings. METHODS: Sequential patients enrolled in LIBRETTO-001 at Royal North Shore Hospital, Sydney, Australia, with available imaging (n = 22) were recruited. Patients had regular visits with AEs documented and computed tomography (CT) scans every 3 months. CT at screening, at time of GI AE, and at most recent follow-up were reviewed and scored. Endoscopic examination was performed in 5 patients. RESULTS: Of 22 patients in this cohort, the majority had somatic RET alterations (n = 18), most commonly p.Met918Thr (n = 14). Ten patients (50%) developed GI AEs. Dose reduction was required in 8 of the 10 patients, but none discontinued therapy. The majority had stable disease (n = 17). Gastric and small-bowel edema was evident in symptomatic patients after a median time of 67 weeks’ treatment. Histological correlation in 5 patients revealed mucosal edema correlating with radiological evidence of congestion and edema. CONCLUSION: GI AEs with selpercatinib may be more common than previously described. Most are self-limiting but often require dose adjustments. Histological evidence of mucosal edema observed in conjunction with the radiological findings of congestion and wall thickening suggest bowel-wall edema is a predominant mechanism of abdominal pain in these patients.