(P 168) EFFICACY OF LOW-DOSE IMATINIB IN A PATIENT WITH A GIST WITH KIT EXON 11 W557_K558 DELETION

Objective: Gastrointestinal stromal tumors (GISTs) are rare cancers arising from the Cajal stromal cells in the gastrointestinal tract. The most common driver mutation in these cancers is the KIT mutation. Although imatinib is effective against this mutation, the efficacy of low-dose imatinib remains unknown. This report presents a case of a response to low-dose imatinib in a patient with GIST with KIT exon 11 W557_K558 deletion.

Methods: An 82-year-old man developed perineal pain in June 2023. Computed tomography (CT) and magnetic resonance imaging revealed a mass measuring > 7 cm extending from the rectum to the prostate. Submucosal tumor biopsy revealed CD34-positive, CD117-positive, DOG1-positive, desmin-negative, and S100-negative spindle-shaped cells. Consequently, a diagnosis of GIST was made. The c-kit gene mutation analysis detected exon 11 W557_K558 deletion.

Results: As the rectal tumor had invaded the adjacent organs, the patient was started on imatinib at 400 mg/day in late October 2023 to preserve organ function. Due to non-hematological toxicity, the dose of imatinib was reduced to 300 mg/day after 5 days of treatment and to 200 mg/day after 10 days, and 200 mg was continued thereafter. In mid-November, the non-hematological toxicity resolved and the patient’s perineal pain improved. CT scans in January and April of 2024 showed that the rectal GIST shrank. However, the blood imatinib concentration from January to March 2024 remained at approximately 650 ng/mL and decreased to 553 ng/mL in April 2024.

Conclusion: Response rates to imatinib for GISTs depend on genetic mutations. GIST with KIT exon 11 W557_K558 deletion is associated with a high risk of recurrence; however, in vitro data showed that low-dose imatinib was effective in such cases. In conclusion, low-dose imatinib is a treatment option for patients with GIST with KIT exon 11 W557_K558 deletion who are intolerant to high-dose imatinib.