Prospective evaluation of the prognostic value of circulating tumor DNA in patients with follicular lymphoma: A pilot study

7569 Background: Follicular lymphoma (FL) is the most frequently occurring indolent non-Hodgkin lymphoma, with generally favorable outcomes but a variable clinical course. Around 20% of patients suffer progression of disease within 24 months (POD24) of chemoimmunotherapy. In this prospective study, we examined the prognostic value of circulating tumor DNA (ctDNA), before and during treatment, to predict response and POD24 in FL patients. Methods: We collected 110 plasma samples from 39 patients diagnosed with FL, prospectively enrolled in 8 Spanish hospitals and treated with Chemotherapy-Rituximab regimen from April 2017 to November 2020 with a median follow-up of 41 months (m). Samples were collected before treatment (basal), mid-treatment (3m), at the end of treatment (EOT) (6m) and relapse or follow-up. We performed targeted deep sequencing in cell-free DNA and paired genomic DNA from 30 formalin-fixed paraffin-embedded tumoral (FFPET) samples with the same gene panel. Results: Before treatment, ctDNA levels, measured as haploid genome equivalents per milliliter of plasma (hGE/mL), were detected in 17/30 patients. Basal ctDNA levels were higher in patients without complete response (CR) than in CR patients [18 vs 14 log2(hGE/mL); Mann-Whitney P = 0.007] and in patients with POD24 compared to those not-POD24 [18 vs 13 log2(hGE/mL); P = 0.005]. None of the 13 patients with zero basal ctDNA levels, experienced POD24. Patients with at least one mutation detected in basal ctDNA had an inferior 24-months PFS than patients without alterations (64 vs 100%, log-rank test P = 0.027). In addition, basal ctDNA levels were detected in one patient with low FLIPI, but partial response and POD24, and by contrast, no ctDNA was detected in another patient with CR and no progression event but high FLIPI. From the alterations detected in FFPET samples, 66% were also identified in basal ctDNA. No alterations were detected in basal ctDNA from 7 patients with CR and 3 patients with watch-and-wait strategy. Dynamic analysis showed that ctDNA levels decreased after treatment for every patient (3 mo), but the reduction at EOT was higher in patients achieving CR than in non-CR patients (12 vs 3 log-reduction) and in not-POD24 patients compared to those with POD24 (10 vs 8). Furthermore, patients with at least one mutation detected in ctDNA at EOT had a shorter 24-months PFS than those without alterations (50 vs 90%). The sensitivity to detect alterations in ctDNA at EOT was 94% in patients not achieving a CR and no mutation was detected in patients with CR. Conclusions: In a real-life, prospective-based population, we showed that pre-treatment and dynamic molecular response measured by ctDNA, could be useful to stratify patients and predict response to treatment and early relapse in FL patients.