Circulating tumor DNA molecular response as a complementary biomarker to RECIST in immune checkpoint inhibitor therapy: current evidence and future directions

cytotoxic chemotherapy, encounters substantial limitations in the immunotherapy era.Pseudoprogression-radiographic tumor enlargement due to immune cell infiltration preceding genuine response-occurs in approximately 5-10% of patients treated with ICIs, often leading to premature treatment discontinuation [1]. Moreover, delayed responses and the insensitivity of imaging to detect early molecular changes render RECIST v1.1 ill-suited for rapid decision-making. Given that circulating tumor DNA (ctDNA) has a biological half-life of approximately 30 minutes and can reflect real-time tumor burden dynamics, its potential as an early, noninvasive surrogate endpoint has garnered considerable interest [2]. The central question is whether ctDNA molecular response (ctDNA-mR), defined variably as ctDNA clearance or a predefined percentage reduction in variant allele frequency, can reliably substitute for or augment RECISTbased assessments in ICI trials.Emerging prospective data support a meaningful concordance between ctDNA dynamics and radiographic response. The landmark phase 2 adaptive trial BR.36 (NCT04093167) enrolled 50 patients with treatment-naïve, PD-L1-positive (≥1%) advanced non-small cell lung cancer (NSCLC) receiving pembrolizumab monotherapy [3]. Using a tumor-informed personalized assay, the investigators demonstrated that maximal mutant allele fraction clearance by cycle 3 day 1 (approximately 6 weeks) achieved a sensitivity of 82% (90% confidence interval [CI], 52-97%) and a specificity of 75% (90% CI, 56.5-88.5%) for RECIST-defined best overall response. Notably, the median time to ctDNA-mR was 2.1 months (90% CI, 1.5-2.6), preceding conventional restaging by several weeks. Patients who attained ctDNA-mR experienced significantly prolonged progression-free survival (PFS; 5.03 vs. 2.6 months) and overall survival (OS; not reached vs. 7.23 months), underscoring the prognostic fidelity of early molecular response.These findings informed the ongoing randomized second stage of BR.36, in which patients with molecular progression are allocated to treatment intensification versus pembrolizumab continuation. Complementing this, a large pan-cancer study from Vall d'Hebron Institute of Oncology (VHIO) analyzed 1,455 longitudinal plasma samples across 24 cancer types in 202 patients receiving diverse immunotherapy modalities, including ICI combinations, bispecific antibodies, and T-cell engagers [4]. In this cohort, ctDNA-mR assessed as early as 3 weeks after treatment initiation correlated with improved PFS and OS. Patients achieving ctDNA molecular complete response (ctDNA-mCR; undetectable ctDNA at any timepoint) had a >3-fold increase in median PFS (6.4 vs. 1.9 months; P = 1.3 × 10⁻⁴) and a 5-year OS rate of 69%. Importantly, durable ctDNA-mCR maintained for ≥180 consecutive days (ctDNA-dmCR) was associated with 100% survival through the study period. Collectively, these data establish that ctDNA-mR is not merely a research curiosity but a clinically meaningful early endpoint with robust survival associations.Beyond concordance, ctDNA offers unique advantages that transcend the inherent constraints of anatomical imaging. First, ctDNA enables the early identification of primary resistance. In the BR.36 trial, ctDNA-mR was detectable by 7 weeks, whereas RECIST responses were typically not evaluable until 12 weeks [3]. In the VHIO cohort, only 7% of patients had clinically progressed by the pre-cycle 2 timepoint (approximately 3 weeks), yet ctDNA dynamics already stratified long-term outcomes, suggesting a window for early intervention in nonresponders [4]. Second, ctDNA dynamics effectively resolve the pseudoprogression dilemma. Among 45 patients with immune unconfirmed progressive disease (iuPD) per iRECIST who continued immunotherapy, those with decreasing ctDNA immediately prior to iuPD demonstrated a 1-year OS rate of 80%, compared with 39% in patients with increasing ctDNA [4]. This observation was corroborated in the validation cohort (67% vs. 16%). Furthermore, among patients with RECIST v1.1 progressive disease but concomitant decreasing ctDNA, continued immunotherapy was associated with significantly superior OS compared with patients harboring concordant radiographic and molecular progression.These findings align with the NADIM phase 2 trial in resectable stage IIIA NSCLC, where post-neoadjuvant ctDNA levels outperformed RECIST v1.1 assessments in predicting long-term survival (3-year OS, 81.9% in the intention-to-treat population) [5]. Thus, ctDNA provides actionable biological information that imaging alone cannot capture, particularly in scenarios of treatment ambiguity. Despite this promise, several critical barriers preclude the unqualified adoption of ctDNA-mR as a standalone surrogate endpoint for RECIST. Foremost is the lack of definitional uniformity. The BR.36 trial defined molecular response as maximal mutant allele fraction clearance at cycle 3 day 1 [3], whereas the VHIO study operationalized molecular response as ≥30% decrease in ctDNA at pre-cycle 2 relative to baseline, molecular clearance as any undetectable timepoint, and durable molecular clearance as sustained undetectability for ≥180 days [4]. In parallel, investigators have proposed alternative frameworks such as liquid biopsy-RECIST (LB-RECIST), which categorizes ctDNA changes into complete response, partial response, stable disease, and progression based on confidence interval overlap [6]. This heterogeneity complicates cross-trial comparisons and meta-analytic validation. A systematic review and metaanalysis of ctDNA clearance as a predictor of pathologic complete response (pCR) in patients receiving neoadjuvant ICIs highlighted this challenge: although pooled sensitivity was high (0.98; 95% CI, 0.86-1.00), specificity was only 0.53 (95% CI, 0.37-0.69), with substantial inter-study heterogeneity (I² ≈ 70%) [7]. The authors concluded that while the absence of ctDNA clearance reliably identifies patients unlik