TP53 Mutations In Relapsed/Refractory Multiple Myeloma (MM) Treated with Thalidomide (Thal) or Thalidomide Combination Therapy
Authors: Westerman David Alan , Zapparoli Giada , Hewitt Chelsee , Lynch Kevin , Mileshkin Linda , Link Emma , Dobrovic Alexander , Prince H. Miles 
Published: October 2019

Abstract
Abstract 4046

Background:
TP53, a tumour suppressor gene, has critical functions regulating normal cellular homeostasis and when inactivated can enhance oncogenesis and disease progression. TP53 deletion as detected by FISH is recognized to occur in approximately 10% of MM patients. Mutations in TP53 in MM are uncommon in de novo disease, 0–3%, while in advanced disease have been reported in up to 25% of patients. Currently the prognostic importance of TP53 mutations in the setting of IMiD therapy for MM is unknown. Our aim was to study the incidence and clinical relevance of TP53 mutations using high resolution melting (HRM) analysis and sequencing in patients with relapsed/refractory MM treated with thalidomide.

Patients and Methods:
Stored patient samples were obtained with ethics approval. All patients had been enrolled in either of two previously published multicentre phase II trials with thalidomide +/− IFNa-2B (MMThal) (Mileshkin L et al. Blood 2003) or celecoxib-thalidomide (CEThal) (Prince et al. Clin Cancer Res. 2005). Steroid treatment was not a part of either regimen. Bone marrow DNA was extracted from pre-treatment archived bone marrow aspirate samples and screened for TP53 mutations using HRM analysis targeting exons 5–9. Subsequent positive cases underwent confirmatory testing with sequencing of amplicons with aberrant melting patterns. The IARC TP53 database was used for sequence analysis interpretation.

Results:
55 (of 141 patients accrued) samples were available and have been analysed to date for TP53 mutations in exons 5–9. Median age is 65 years (range 36–85), 32 males. Median plasma cell differential on bone marrow aspirate smears was 16% (1-96). Median serum β2 microglobulin (β2m) was 3.3 (1.37-12.8), median Hb 107 g/L (60-145) while patients had received a median of 3 prior treatments (1-8) and 16% had refractory disease. 40/141 (28%) had received a prior autograft. Median follow up was 73 months and 46 months for MMTHAL and CETHAL, respectively, while PFS was 24% and 39% at 1 yr for the 2 trials, respectively.
8/55 (14.5%) had abnormal melts detected by HRM with 5 mutations confirmed by sequencing, and 3 awaiting sequence confirmation. Additionally two SNP's were detected. We believe the mutation rate is an under-representation given low tumour burden (<10% plasma cells on aspirate differentials) in 25% of all cases tested. Of the mutations demonstrated, 3 were in exon 8 and 2 in exon 6, whilst of the 3 requiring confirmation, 1 each were in exons 5, 7 & 8. The baseline data of the TP53 mutated patients were comparable to the non-mutated cohort with regards to age, Hb, stage of disease, and number of prior treatments. Median β2m was higher, 6.4 (3.1-72.7). Karyotyping of the TP53 mutated patients showed no mitoses in 4, normal in 1, while 1 had complex abnormalities seen in MDS. 2 patients were hypodiploid with loss of Chr 17. Four patients had stable disease (SD) to their prior line of therapy, 2 progressive disease (PD), and 1 a PR. Response to thalidomide based treatment in the mutated cohort included 1 CR, 3 PR, 2 SD and 2 PD. The progressive disease patients received thalidomide for only 3 and 26 days. Median PFS was 91 days.

Conclusions:
TP53 mutations were detected in the DNA binding domain (exons 5–9) in 14.5% (8/55) of our cohort studied to date. 4/8 patients with mutated TP53 had a CR/PR suggesting that thalidomide is a useful agent in this context, although further evaluation is required for confirmation of these findings, as would exploration of similar relationships in lenalidomide-treated patients. Both avenues of experimentation are ongoing.

Disclosures:
Westerman: Celgene Pty Ltd: Research Funding. Lynch:Celgene Pty Ltd: Employment, Equity Ownership. Prince:Celgene Pty Ltd: Honoraria, Research Funding.