Concurrent tumor lysis syndrome and 5-fluorouracil encephalopathy during the first cycle of zolbetuximab plus mFOLFOX6 for CLDN18.2-positive gastric cancer: a case report.

The safety data on zolbetuximab, a new CLDN18.2-targeting antibody for advanced gastric cancer (AGC), are still exiguous. We report herein a 75-year-old, male patient with CLDN18.2-positive, HER2-negative AGC with weight loss, massive nodal disease, and peritoneal carcinomatosis-related obstructive uropathy. First-line zolbetuximab plus mFOLFOX6 triggered grade 2 nausea, which was followed on day 3 by tumor lysis syndrome (TLS) and 5-fluorouracil (5-FU)-induced hyperammonemic encephalopathy. Discontinuation of 5-FU and supportive care consisting of hydration and rasburicase led to rapid clinical improvement. Chemotherapy, which was resumed after a dosage adjustment, achieved tumor shrinkage and resolved the hydronephrosis. To the best of our knowledge, the present study is the first to describe concurrent TLS and 5-FU-induced encephalopathy during the administration of a zolbetuximab-based regimen and highlights the need for proactive prophylaxis against TLS and for controlling nausea in AGC patients with a high tumor burden, baseline renal impairment, and cachexia.