Abstract 430P
Background
ctDNA can be useful in mapping evolving mutations during NACRT to diagnosis residual disease and guide adaptive therapies in resistant patients (pts) without pCR.
Methods
Pilot study of 10 LARC pts enrolled on CTRIAL-IE 12-38 TRI-LARC (NCT02151019) trial and received NACRT. Tumour tissues(ts) with plasma samples were taken at baseline, on-treatment and post-operation (post-op). Ts DNA was sent for targeted sequencing with AgilentTM ClearSeq Panel. ctDNA was genotyped for mutations in NRAS, KRAS, EGFR, PIK3CA and BRAF genes.
Results
3 pts had pCR and 7 had non-pCR. Post-op ts were available in 6 non-pCR pts. Compared to baseline ts, 50% (3/6) post-op ts showed no residual mutations, followed by 17% (1/6) had alterations and 33% (2/6) had no changes. The alterations were ATRX gain with SMAD4 and KRAS losses. ctDNA detected more synchronous mutations than ts. Extra mutations in NRAS/KRAS were seen in 6/10 pts’ ctDNA taken at baseline, with MAF 0.83 [0.3-1.4]. Mutations in BRAFV600E and PIK3CA were seen in 1/10 and 5/10 pts' ctDNA, respectively. Concordance between ctDNA and ts is 40%. Post-operatively, non-pCR patients have ctDNA with median MAF 1.20±0.99 vs pCR pts 0.60±0 (p=0.74). Table: 430P
Genetic mutation landscape in ts at various timepoints. Tumour response according to Mandard Score. Among pts with non-pCR, mutations seen in baseline ts include NRAS/KRAS in 86% (6/7), FBXW7 in 29% (2/7), MAPK2K1 in 14% (1/7), SMAD4 in 14% (1/7), ATRX in 14% (1/7) and DDX3X in 14% (1/7) No BRAF mutation detected in any ts. NM=no mutations; NS=no sample available.
| Patients (Treatment Response) | 20 (TRG1) | 26 (TRG1) | 11 (TRG1) | 17 (TRG2) | 18 (TRG3) | 41 (TRG3) | 3 (TRG2) | 27 (TRG4) | 28 (TRG4) | 19 (TRG5) |
| Baseline ts | ||||||||||
| NRAS61 | ERBB4232 | APC1367 | APC1114 | NRAS61 | MAP2K157 | APC1367 | NRAS13 | FBXW7385 | APC876 | |
| PIK3CA 546 | APC1367 | TP53266 | APC1356 | FBXW7333 | KRAS12 | CHD7287 | KRAS12 | KRAS13 | ||
| SLCO1B1 489 | TP53248 | ATM3028 | APC1309 | TP53273 | ERBB3583 | TP53245 | TP53213 | |||
| KRAS12 | DDX3X66 | TP53199 | ||||||||
| ATRX494 | SMAD4496 | |||||||||
| Week 3 ts | ||||||||||
| NRAS61 | ERBB4232 | NS | NS | NM | MAP2K157 | NS | NS | FBXW7385 | NS | |
| SLCO1B1 489 | APC1367 | KRAS12 | ||||||||
| TP53248 | ||||||||||
| Post-op ts | ||||||||||
| pCR | pCR | pCR | NM | NM | NM | APC1367 | NRAS13 | FBXW7385 | NS | |
| ATRX1739 | CHD7287 | KRAS12 | ||||||||
| ERBB3583 | TP53245 |
Conclusions
Serial ctDNA can identify intratumoural heterogeneity and potentially elucidate driver mutations implicated in chemoradioresistance. We are analysing mutations of interest in ERBB, SMAD4, FBXW7 and ATRX genes.
Clinical trial identification
NCT02151019 (CTRIAL-IE 12-38 TRI-LARC clinical trial).
Editorial acknowledgement
Legal entity responsible for the study
The authors.
Funding
Northeast Cancer Research and Education Trust (NECRET), St Luke’s Institute of Cancer Research (SLICR) and the Ronnie Cox Award (Cancer Research Ireland).
Disclosure
All authors have declared no conflicts of interest.