Circulating tumor DNA and RNA as an exploratory biomarker to evaluate GT0918 in a phase I/II clinical trial in mCRPC patients

Abstract

Background: Metastatic castration resistant prostate cancer (mCRPC) is a complex disease with distinct molecular features in relation to genomic instability and selective treatment pressure. Circulating tumor DNA and RNA fragments (ctDNA & ctRNA) found in blood offers the potential of disease diagnosis, monitoring and resistance mechanism interrogation by detecting genomic alterations from tumor. We explored ctDNA & ctRNA-based biomarkers from patient blood to assess their associations with clinical response of GT0918, a potent AR antagonist, in a Phase 1/2 clincal study in mCRPC who progressed after abiraterone or enzalutamide and docetaxol, or cannot tolerate either or both therapies. Methods: We performed a retrospective analysis of blood samples from mCRPC patients collected at baseline, on- and after study during the trial. A highly sensitive ctDNA- & ctRNA-based NGS assay was developed to detect mutation, copy number gain, fusion and splicing variants. Statistical analyses were performed in R. Results: 20 blood samples were collected at multiple time points from 8 patients. CtDNA-based variants are detected in all of patients. The most frequent mutations are TP53 (55.0%) and AR (30.0%). Combined mutation rates in PTEN-PI3K-AKT and DNA damage repair pathways (BRCA1/BRCA2/ATM) are both 35.0%. Importantly, AR hotspot mutations (W742C, T878A, and S889G) and amplifications are detected in 4 subjects. AR splicing variants (AR-V3, AR-V7) were found in 3 patients by ctRNA assay. Interestingly, one AR-V3+ patient became negative during the treatment accompanied by a decrease of other molecular biomarkers including prostate-specific SPOP mutation and cfDNA yield. In contrast, another patient who was AR-V3+ at C4D1, had constantly high AR amplification and increasing cfDNA yields over treatment. Last, as a hallmark of prostate cancer, TMPRSS2-ERG fusion was also detected in 2 patients. Conclusions: This is a preliminary study to explore genomic alterations in mCRPC in response to GT0918 treatment. As a non-invasive assay, the ctDNA & ctRNA-based assay was highly sensitive and provided useful molecular insights for monitoring treatment effect and deciphering drug sensitivity & resistance mechanisms.