Delayed processing of CPT-collected blood preserves transcriptomic and genomic features of propagated circulating tumor cells

Technical Summary: Delayed Processing of CPT-Collected Blood Preserves Transcriptomic and Genomic Features of Propagated Circulating Tumor Cells

Problem Statement
Circulating tumor cells (CTCs) offer a minimally invasive window into metastatic disease biology, tumor evolution, and treatment response. However, their utility in routine clinical and research workflows is severely constrained by pre-analytical limitations. Specifically, the extreme rarity of CTCs and their fragility necessitate rapid specimen processing immediately following phlebotomy to maintain cell viability and molecular fidelity. This requirement creates a significant logistical bottleneck, restricting CTC research to specialized centers capable of immediate processing and preventing broader adoption in multicenter studies or diverse clinical settings. Current workflows often force a trade-off between capture efficiency, purity, viability, and molecular fidelity.

Methodology
The authors evaluated a workflow utilizing BD Vacutainer Cell Preparation Tubes (CPT) to support delayed processing of blood specimens intended for CTC enrichment and downstream propagation.

• Cohort: A pilot study involving four patients with metastatic solid tumors (specifically metastatic pancreatic cancer in the results section).
• Experimental Design: For each patient, paired CPT tubes were collected.
  • CPT A (Early Processing): Centrifuged and processed within 2 hours of phlebotomy.
  • CPT B (Delayed Processing): Stored at 4°C and processed 24 hours after collection.
• Sample Processing: Both tubes underwent centrifugation to separate the mononuclear cell fraction (containing white blood cells and CTCs) from red blood cells. The enriched fractions were propagated ex vivo using previously established culture methods.
• Molecular Analysis: Expanded cells were harvested for:
  • Bulk RNA Sequencing: Performed on Whole Blood Matched (WBM), CPT A, and CPT B samples to assess transcriptomic profiles, immune signatures, and pathway enrichment (e.g., Epithelial-Mesenchymal Transition [EMT], KRAS signaling).
  • Whole-Exome Sequencing (WES): Performed on CPT-derived material (specifically from Patient 107 and pooled Patient 114) to identify patient-specific oncogenic alterations and assess clonal heterogeneity.

Key Results

1. Transcriptomic Consistency: Unbiased hierarchical clustering demonstrated that paired CPT-derived samples (A and B) from the same patient clustered more closely with each other than with their matched WBM controls. This held true even when restricting analysis to EMT-associated gene sets.
2. Phenotypic Preservation: CPT-derived samples exhibited the expected CTC immunophenotype, characterized by higher KRT7 expression and significantly lower CD45 expression compared to WBM controls. Pathway analysis confirmed a relative depletion of innate and adaptive immune signatures and an enrichment of EMT and KRAS signaling pathways in the CPT samples.
3. Gene Expression Concordance: Direct comparison of CPT A and CPT B samples revealed strong intra-patient concordance in gene expression, indicating that a 24-hour delay in refrigerated storage did not grossly alter the transcriptional landscape of the harvested CTCs.
4. Genomic Preservation: Whole-exome sequencing identified distinct, patient-specific oncogenic alterations (e.g., in genome maintenance, Hedgehog signaling, and receptor tyrosine kinases). In Patient 107, the paired CPT A and CPT B samples shared 97.1% of detected mutations, with only minor unique variants in each condition. This suggests that delayed processing preserves tumor-informative genomic features and clonal heterogeneity without introducing significant bias.

Significance and Claims
The paper claims that a CPT-based workflow enables the delayed processing of blood specimens (up to 24 hours at 4°C) while retaining the transcriptomic and genomic features necessary for propagated CTC analysis.

• Logistical Decoupling: The primary significance is the potential to decouple specimen collection from immediate access to specialized laboratory infrastructure. This could facilitate centralized processing models and expand the accessibility of CTC-based research to multicenter studies and broader clinical settings.
• Biological Validity: The study provides evidence that delayed processing does not compromise the biological signal, as evidenced by the preservation of tumor-associated pathways (EMT, KRAS) and patient-specific mutational profiles.
• Feasibility: The authors conclude that this workflow is feasible for propagating and studying CTCs, thereby addressing a key barrier to broader CTC research.

Limitations Noted by Authors
The authors maintain a modest tone regarding their claims, acknowledging several limitations:

• Sample Size: The pilot nature of the study (four patients) limits statistical power and generalizability across tumor types and disease stages.
• Pre-analytical Variables: Larger studies are needed to define the acceptable limits of transport variables (time, temperature, handling).
• Bulk Sequencing: The reliance on bulk RNA sequencing may mask biologically meaningful subpopulations; single-cell sequencing would be required to definitively attribute signals to CTCs versus residual immune cells.
• Clinical Correlation: The study establishes molecular preservation but does not yet link CPT-derived CTC cultures to clinical outcomes, such as metastatic burden, treatment response, or drug sensitivity.