Impact of Frontline Ivosidenib on Volumetric Growth Patterns in Isocitrate Dehydrogenase–mutant Astrocytic and Oligodendroglial Tumors

Purpose: Isocitrate dehydrogenase (IDH)-mutant gliomas are usually treated with radiotherapy and chemotherapy, which increases the risk for neurocognitive sequelae during patients’ most productive years. We report our experience using off-label first-in-class mutant IDH1 inhibitor ivosidenib and its impact on tumor volume in IDH-mutant gliomas. Experimental Design: We retrospectively analyzed patients ages ≥18 years with radiation/chemotherapy-naïve, mutant IDH1, nonenhancing, radiographically active, grade 2/3 gliomas, and ≥2 pretreatment and ≥2 on-treatment ivosidenib MRIs. T2/ FLAIR-based tumor volumes, growth rates, and progression-free survival (PFS) were analyzed. log-linear mixed-effect modeling of growth curves adjusted for grade, histology, and age was performed. Results: We analyzed 116 MRIs of 12 patients [10 males, median age 46 years (range: 26–60)]: 8 astrocytomas (50% grade 3) and 4 grade 2 oligodendrogliomas. Median on-drug follow-up was 13.2 months [interquartile range (IQR): 9.7–22.2]. Tolerability was 100%. A total of 50% of patients experienced ≥20% tumor volume reduction on-treatment and absolute growth rate was lower during treatment (—1.2 ± 10.6 cc/year) than before treatment (8.0 ± 7.7 cc/ year; P ≤ 0.05). log-linear models in the Stable group (n ¼ 9) showed significant growth before treatment (53%/year; P ¼ 0.013), and volume reduction (—34%/year; P ¼ 0.037) after 5 months on treatment. After treatment, volume curves were significantly lower than before treatment (after/before treatment ratio 0.5; P < 0.01). Median time-to-best response was 11.2 (IQR: 1.7–33.4) months, and 16.8 (IQR: 2.6–33.5) months in patients on drug for ≥1 year. PFS at 9 months was 75%. Conclusions: Ivosidenib was well tolerated and induced a high volumetric response rate. Responders had significant reduction in tumor growth rates and volume reductions observed after a 5-month delay. Thus, ivosidenib appears useful to control tumor growth and delay more toxic therapies in IDH-mutant nonenhancing indolently growing gliomas.