Optimal Sunitinib Dosing Based on Platelet Count in an Older Patient With Gastrointestinal Stromal Tumor

Sunitinib, a second-line treatment for gastrointestinal stromal tumors (GIST), is commonly associated with adverse events (AEs) such as myelosuppression in older patients. However, an optimal dosing schedule has not been established. This study evaluated the therapeutic efficacy and toxicity management of sunitinib administration based on platelet count.

An 83-year-old man receiving imatinib for rectal GISTs was switched to sunitinib (initially 50 mg/day) due to imatinib failure. After eight days, he developed grade 2 anorexia and nausea, prompting a dose reduction to 37.5 mg/day. These AEs persisted, and sunitinib was discontinued 15 days after treatment initiation and later resumed at 25 mg/day after the symptoms improved. Thereafter, the sunitinib withdrawal period was adjusted based on neutrophil and platelet counts. As a result, the only non-hematologic AE of grade 2 or higher was hypothyroidism, with no serious AEs. He achieved a partial response according to Choi criteria but was switched to regorafenib after five cycles of sunitinib due to disease progression. Changes in neutrophil and platelet counts during the withdrawal period of each cycle of sunitinib therapy were predicted using a quadratic regression model and validated using leave-one-out cross-validation. The coefficient of determination (R²) of the neutrophil count prediction model during the withdrawal period was 0.28, and the validated R² for each cycle ranged from -26.34 to -0.11. In contrast, the platelet count model yielded an R² value of 0.86, with validated R² values ranging from 0.42 to 0.88.

Low-dose sunitinib monotherapy, with dosing intervals based on platelet counts, may be an effective treatment option for managing toxicity in older patients with imatinib-resistant GIST.