The 2025 APCCC Diagnostics annual meeting featured a session on how to monitor metastatic prostate cancer and a presentation by Dr. Alexander Wyatt discussing when we should assess biology with ctDNA. Cell free DNA (cfDNA) is from apoptosing cells shedding cfDNA into the blood, with most normal cfDNA being from the blood lineage. The average person will have 5 – 10 ng of DNA in 1 mL of plasma, with a median  cfDNA fragment length of 167 bp (which is the unit of the nucleosome):

Dr. Wyatt then highlighted several properties of circulating tumor DNA (ctDNA):

• In a person with cancer, a fraction of cfDNA is tumor derived (ie. ctDNA), with ctDNA being slightly smaller than cfDNA
• ctDNA is identified by features absent in normal cfDNA and the germline
  • ie. DNA mutations and structural rearrangements
  • Lineage-specific methylation markers (methylation sequencing)
  • Lineage-specific nucleosome patterns (ie fragment’omics)
• ctDNA percentage is related to volume of active and proliferative cancer
  • Rapidly suppressed by effective anti-cancer therapy
  • Influenced by cancer subtype

Dr. Wyatt notes that work from Fonseca et al.¹ demonstrates that ctDNA fraction is strongly prognostic in the mCRPC setting. Specifically, high ‘baseline’ ctDNA is associated with poor progression free survival and overall survival across treatment contexts and lines of therapy:

Moreover, high ctDNA is independent of clinical prognostic factors and genomic alterations, in addition to low ctDNA appearing to derive benefit from most treatments (including androgen receptor pathway inhibitor switch and LuPSMA).

ctDNA percentage decreases rapidly with effective therapy in both the mHSPC and mCRPC setting, with deep declines in the first days and weeks after treatment initiation. Also, there is no evidence of a transient spike in ctDNA in responding patients. Work from Tolmeijer et al.² previously assessed early on-treatment changes in circulating tumor DNA fraction and response to enzalutamide or abiraterone in mCRPC. Among 33 patients, progression free survival and overall survival were shortest for patients with persistent ctDNA at 4 weeks (univariate HR 4.79, 95% CI, 2.62-8.77 and univariate HR 5.49, 95% CI, 2.76-10.91, respectively), independent of clinical prognostic factors:

ctDNA change had a positive predictive value of 88% and negative predictive value of 92% for identifying nondurable responses. Dr. Wyatt notes that early on treatment changes in ctDNA percentage is also associated with response in mCRPC across classes and all lines of therapy:

• First line mCRPC receiving androgen receptor pathway inhibitors: conversion from detected to undetected at C2D1 had no difference in overall survival from those with undetected alterations at baseline³
• Second line mCRPC receiving androgen receptor pathway inhibitors: ctDNA detection at C3D1 was associated with shorter progression free survival and overall survival in the IMbassador250⁴
• Second line mCRPC receiving ¹⁷⁷Lu-PSMA-617: Higher baseline and C2D1 ctDNA percentage with a short radiographic progression free survival and overall survival in PSMAFore (ASCO GU 2025). Additionally, ctDNA clearance provided information beyond PSA50 for radiographic progression free survival
• mCRPC receiving taxane chemotherapy or PARP inhibitors: in the FIRSTANA, PROSELICA phase 3 trials, as well as the TOPARP-A trial

Multiple international efforts are ongoing to develop ctDNA percentage as a response biomarker (ie. Friends of Cancer Research), either as a routine biomarker and/or as an intermediate endpoint in clinical trials. Additionally, the RECIST working group is evaluating the added value of ctDNA kinetics to imaging based assessments. Dr. Wyatt summarized six key challenges precluding the immediate uptake of ctDNA as a response biomarker:

1. An incomplete understanding of cfDNA and ctDNA biology
2. There is no established threshold for determining ctDNA response
3. There is large variation in assay design and level of detection
4. Optimal timing and frequency of blood collection is unclear
5. The scalability of ctDNA percentage testing is unknown
6. Some patients have undetected ctDNA prior to treatment initiation

The next steps for ctDNA are as follows:

• Focus on specific clinical scenarios and ctDNA thresholds
  • Large correlative datasets and meta-cohorts
  • Amend protocols to add extra blood collections
  • Discriminate stable disease
• Test biological confounders of ctDNA percentage change, but we need to be certain that change in ctDNA percentage is equivalent to tumor changes
• Simplify assay design: are highly sensitive and bespoke assays necessary?
• Consider designing randomized trials that intervene on the basis of ctDNA clearance and non clearance (de-escalation or treatment switch)

Dr. Wyatt concluded his presentation by discussing when we should assess biology with ctDNA with the following take-home points:

• Plasma ctDNA percentage is typically high in patients with clinically progressing metastatic prostate cancer
• Deep ctDNA percentage declines occur in the first weeks of effective treatment, irrespective of therapy class and line of therapy
• Early clearance of ctDNA percentage is associated with radiographic progression free survival and overall survival, providing more prognostic information than early PSA change
• ctDNA percentage kinetics offer promise as a routine response biomarker and an early endpoint in clinical trials
• We require more correlative data from ongoing trials to better define how to assess a clinically meaningful change in ctDNA percentage

Presented by: Alexander Wyatt, PhD, Vancouver Prostate Centre, Department of Urologic Sciences, University of British Columbia, Vancouver, Canada

https://www.urotoday.com/conference-highlights/apccc-diagnostics-2025/158677-apccc-diagnostics-2025-when-to-assess-biology-with-ctdna.html