(P 185) LONGITUDINAL MUTATIONAL ANALYSIS OF TP53 IN CIRCULATING TUMOR DNA IN THE PLASMA OF PATIENTS WITH LIPOSARCOMA IN A PHASE I STUDY OF BRIGIMADLIN (BI 907828), AN MDM2-P53 ANTAGONIST

Objective: The TP53 gene is the most frequently mutated gene in human cancer, is defined as a tumor suppressor gene and its gene product p53 plays a crucial role in preventing cancer formation. Inactivation of p53 can occur due to TP53 mutations or downregulation of wild-type p53 by its primary negative regulator, MDM2. Targeting the MDM2–p53 interaction can restore p53 function. Brigimadlin (BI 907828), a highly potent, orally available MDM2–p53 antagonist, is being evaluated in an ongoing Phase Ia/b study (NCT03449381) in patients with advanced solid tumors, including dedifferentiated liposarcoma (DDLPS). Brigimadlin has demonstrated early signs of preliminary efficacy and a manageable safety profile (LoRusso et al., Cancer Discovery 2023; LoRusso et al., ASCO 2023). Mutations in TP53 are considered as a potential mechanism of resistance to MDM2 antagonists. Here we present data from a longitudinal mutational analysis of TP53 using circulating tumor DNA (ctDNA), with the objective to assess if mutations in TP53 are associated with possible acquired resistance to brigimadlin.

Methods: Collection of blood samples for ctDNA analyses was optional in Phase Ia and mandatory in Phase Ib; samples were taken longitudinally from patients at baseline and every cycle until the time of the last available sample (LAS). ctDNA was purified from plasma and analyzed using tumor-specific next generation sequencing (NGS; custom-made 8-gene panel including TP53 using KAPA HyperCap technology) to identify tumor-derived somatic mutations that may be relevant to identify potential resistance mechanisms to brigimadlin. The limit of detection was determined to be 0.5% mutant allele frequency; common polymorphisms were filtered out.

Results: At data cut-off (March 28, 2023), 69 patients with DDLPS had been enrolled to receive brigimadlin Q3W, of whom 65 were evaluable for response according to RECIST. Plasma samples for ctDNA analysis were available from 21/65 (32%) patients. Baseline samples from 16 (25%) and the LAS from 17 (26%) patients were analyzed. The median interval between baseline sampling and LAS was 128 days (range, 43–631). As expected, the vast majority of patients (15/16; 94%) had no detectable mutations in TP53 at baseline; the one aberration detected (K373fs) is not likely to be a loss-of-function (LoF) mutation. At LAS, no (0) mutations were found in 7 (40%) samples, 1 mutation in 4 (24%), 2 mutations in 4 (24%), 5 mutations in 1 (6%), and 6 mutations in 1 sample (6%). No sample had more than 6 mutations.
The most frequent mutations found were R175H, R273H, R248Q/W and R280G (across 5 of 33 samples analyzed [15%]), all of which are known TP53 hotspot mutations and are LoF. No clear association of induction of mutations could be identified for any brigimadlin dose, dosing schedule, or treatment duration. Only 7/17 (41%) patients still showed wild-type TP53 status at LAS. Signs of efficacy were observed in patients with acquired TP53 LoF mutations, suggesting that these have no effect on efficacy. Analysis of TP53 mutational status at LAS in conjunction with correlative clinical efficacy will be presented.

Conclusion: This mutation analysis represents one of the most robust assessments of longitudinal ctDNA by NGS for TP53 from an MDM2–p53 antagonist trial in liposarcoma. While acquired mutations in TP53 are very common, these data suggest that brigimadlin does not systematically lead to broad acquisition of resistance by inducing alterations in TP53.