Real-world tissue test failure rates are between 14-34%, and there is well-documented spatial and temporal heterogeneity of FGFR alterations in mUC, indicating archival tissue may not represent the clinically dominant tumor population at the time of metastatic disease. Plasma circulating tumor DNA (ctDNA) offers a minimally invasive alternative that provides a real-time snapshot of the cancer drawn from a simple blood sample. With this study, we aimed to prospectively address how ctDNA compares to standard tissue testing and whether ctDNA is capable of identifying additional patients with FGFR alterations.
Across 12 Canadian centers, we enrolled 208 patients with mUC undergoing standard-of-care clinical FGFR tissue testing for erdafitinib eligibility. All patients underwent additional ctDNA testing, which was performed using a custom mUC-specific assay covering FGFR1-4 exons and common fusion breakpoints.
To allow for fair comparison between tissue and ctDNA, we first excluded the 38% (78/208) of samples that had low levels of ctDNA in their baseline blood collection (which occurred in part because blood could be drawn while the disease was still responding to a line of systemic therapy). In evaluable baseline samples, FGFR alteration frequency was 26% using either tissue or ctDNA, highlighting the general reliability of either analyte. Among 125 patients with baseline-detected ctDNA and paired tissue results, FGFR status was concordant in 90%, and ctDNA had an 84% sensitivity for tissue-detected alterations, while also identifying 7 additional cases where tissue testing reported wild-type. One of those patients received erdafitinib on the basis of ctDNA alone and remained on treatment for 33 months, a durable response that would not have been possible under a tissue-only testing paradigm. In the other direction, 5 FGFR3 fusions detected in tissue were absent from ctDNA, despite adequate levels of circulating tumor DNA to detect the fusion event, raising the possibility that archival tissue did not reflect the biology of the dominant metastatic clone(s) in those patients. In 2 additional tissue-ctDNA discordant cases, a technical explanation emerged where somatic intronic variants adjacent to the FGFR3 S249C hotspot disrupted primer binding and produced a false-negative tissue result unrelated to tumor biology.
A key advantage of blood-based testing is that repeat blood samples are practical to obtain. Indeed, in our study, serial plasma collections post-baseline further clarified FGFR status in mUC, including in patients who had no ctDNA detected in their first blood draw. At progression on erdafitinib, ctDNA re-identified the original driving alteration in every evaluable patient while also capturing acquired resistance mechanisms, including FGFR3 gatekeeper and molecular brake mutations and, in one case, on-treatment emergence of an FGFR3-TACC3 fusion, a resistance-associated event that has not been previously described. This fusion would technically qualify as a targetable alteration, and its emergence under therapy hints at a potential context-dependent value of stratifying patients with this alteration.
Our findings support the validity of ctDNA-based FGFR screening and the integration of ctDNA genotyping as a complementary upfront blood test for mUC, which will expand FGFR therapy to patients without sufficient tissue for testing. However, reflex tissue testing remains necessary for patients without sufficient levels of ctDNA for FGFR genotyping at the time of blood draw (such as those patients where the disease is already responding to ongoing treatment). This proposed ctDNA-first workflow would improve overall sensitivity for detecting actionable alterations compared to tissue testing alone and may alleviate time and costs associated with the tissue retrieval process from pathology laboratories.
We will continue to collect samples at progression on erdafitinib from our enrolled patients to learn more about erdafitinib resistance. Resistance mechanisms are particularly relevant considering increased erdafitinib use in non-muscle invasive bladder cancer and current testing of second-generation FGFR3-specific inhibitors that are active against erdafitinib-resistant cancers harbouring FGFR3 alterations.
Written by: David C. Müller,1,2 Andrew J. Murtha,1 Jack V.W. Bacon,1 Maria Stephenson,1 Connor Wells,3 Carlos Vasquez-Rios,1 Kimia Rostin,1 Lisa Rebane,3 Elena Schönlau,3 Jussi Nikkola,1 Nimira Alimohamed,4 Naveen S. Basappa,5 Daygen Finch,6 Jenny J. Ko,7 Jean-Michel Lavoie,8 Lucia Nappi,1,3 Krista Noonan,9 Michael Ong,10 Guliz Ozgun,3 Sunil Parimi,8 Maryam Soleimani,3 Srikala S. Sridhar,11 Paul Toren,12 Eric Winquist,13 Cecily Q. Bernales,1 Melissa Koudjanian,1 Jaskirat Atwal,1 Emily Fung,1 Laiba Khan,3 Bryndan Eigl,3 Dalia Othman,1,3 Tarek A. Bismar,14 Gang Wang,1,15 Reem Merza,16 Andreas I. Papadakis,16 Alan Spatz,16 Christian Kollmannsberger,3 Corinne Maurice-Dror,3 Matti Annala,17 Kim N. Chi,3 Gillian Vandekerkhove,1,3 Alexander W. Wyatt,1,18 Bernhard J. Eigl,1,3
- Vancouver Prostate Centre, Department of Urologic Sciences, University of British Columbia, Vancouver, BC, Canada.
- Department of Urology, University Hospital Basel, University of Basel, Basel, Switzerland.
- Department of Medical Oncology, BC Cancer, Vancouver, BC, Canada.
- Department of Medical Oncology, Arthur J.E. Child Comprehensive Cancer Centre, Calgary, AB, Canada.
- Department of Medical Oncology, Cross Cancer Institute, University of Alberta, Edmonton, AB, Canada.
- Department of Medical Oncology, BC Cancer, Kelowna, BC, Canada.
- Department of Medical Oncology, BC Cancer, Abbotsford, BC, Canada.
- Department of Medical Oncology, BC Cancer, Victoria, BC, Canada.
- Department of Medical Oncology, BC Cancer, Surrey, BC, Canada.
- The Ottawa Hospital-Research Institute, Ottawa, ON, Canada.
- Princess Margaret Cancer Centre, Toronto, ON, Canada.
- CHU de Québec-Université Laval, Laval University, Quebec City, QC, Canada.
- Verspeeten Family Cancer Centre at London Health Sciences Centre, London, ON, Canada.
- Departments of Pathology and Laboratory Medicine, Oncology, Biochemistry and Molecular Biology, University of Calgary, Cumming School of Medicine, Calgary, AB, Canada.
- Department of Pathology and Laboratory Medicine, BC Cancer, Vancouver, BC, Canada.
- Lady Davis Institute for Medical Research, Jewish General Hospital, Montreal, QC, Canada.
- Faculty of Medicine and Health Technology, Tampere University and Tays Cancer Centre, Tampere, Finland.
- Department of Basic and Translational Research, BC Cancer Research Centre, Vancouver, BC, Canada.