Real-World Determinants of Olanzapine Prescribing for Antiemetic Prophylaxis in Adults Receiving Highly-Emetogenic Chemotherapy.

Background: Since 2017, guidelines recommend a four-drug prophylactic antiemetic regimen-5HT3 receptor antagonist (5HT3RA), NK1 receptor antagonist (NK1RA), dexamethasone, and olanzapine-for patients receiving highly emetogenic chemotherapy (HEC). This study examines prophylactic olanzapine prescribing in patients initiating HEC and associated sociodemographic, clinical, and contextual factors. Methods: A retrospective cohort study was conducted using electronic health record data from an academic cancer center and affiliated community practices across the state of North Carolina, joined with geocoded community factors. Adults ≥21 years initiating cisplatin, carmustine, dacarbazine, mechlorethamine, streptozocin, or cyclophosphamide plus anthracycline (AC) in 2022 were included. Antiemetic orders placed before HEC administration were classified as three-drug (5HT3RA, NK1RA, dexamethasone) or four-drug (plus olanzapine). Late olanzapine use (within 30 days post-HEC) was also assessed. Multivariable logistic regression evaluated factors associated with regimen type. Results: Among 878 patients, all received orders for the three-drug regimen, but only 300 (34.2%) had olanzapine ordered within 30 days before first HEC. After adjusting for covariates, olanzapine prescribing was less likely in patients with breast (OR = 0.13, 95%CI = 0.07-0.23), gastrointestinal (OR = 0.20, 95%CI = 0.04-0.94), gynecological (OR = 0.14, 95%CI = 0.03-0.69), and head and neck (OR = 0.17, 95%CI = 0.05-0.66) compared to hematological cancers, in those receiving cisplatin (OR = 0.20, 95%CI = 0.06-0.70) and carmustine (OR = 0.02, 95%CI = 0.00-0.48) compared to AC, and in community (OR = 0.40, 95%CI = 0.27-0.61) versus academic settings. Prescribing was also lower among patients residing in low- (OR = 0.49, 95%CI = 0.25-0.93) or lower-middle income (OR = 0.50, 95%CI = 0.32-0.78) areas, but higher among those living >6 miles from treatment facilities (OR = 1.95, 95%CI = 1.08-3.58). Among patients without prophylactic olanzapine, 12.5% were prescribed it within 1 to 30 days post-HEC. Conclusions: Despite guideline recommendations, prophylactic olanzapine use in HEC remains suboptimal, influenced by clinical and non-clinical contextual factors. Targeted efforts at information dissemination and updated prescribing systems are needed to promote equitable, evidence-based supportive care.