Initial results from CLIMATE, a prospective cohort study assessing the clinical utility of miR-371a-3p (miR-371) as a marker of minimal residual disease (MRD) in clinical stage 1 testicular germ cell tumour (TGCT): ANZUP 1906.

586 Background: Active surveillance (AS) remains a preferred option for clinical stage 1 (CS1) TGCT. Biomarkers to predict recurrence are limited by poor accuracy. Circulating miR-371 has high sensitivity and specificity for TGCT, and as a marker of MRD post-orchiectomy (orch), is a promising biomarker for predicting recurrence. Initial results from CLIMATE examine the discriminatory accuracy of baseline post-orch miR-371 in predicting recurrence in patients with CS1 TGCT undergoing AS. Methods: Patients from 12 sites in Australia and New Zealand, aged ≥18y, with histologically confirmed CS1 TGCT, no evidence of metastases and planned for AS, were enrolled ≤6w post-orch. Plasma and serum were collected at baseline (≤6w post-orch) and every 3mo for 24mo and at recurrence. Clinical and outcome data were recorded in iTestis, Australia’s TGCT registry. miR-371 was assessed using a qPCR assay based upon published methodology (Murray et al.) and deemed detectable if the mean of triplicate qPCR Ct was ≤40. Results: CLIMATE enrolled 200 patients from 2021 to 2025. 196 patients had assays run on baseline samples prior to data cutoff and were included in this analysis. Orch histology included 117 (60%) pure seminoma and 79 (40%) non-seminoma. With median follow-up (F/U) 18.9mo, there were 40 recurrences. miR-371 was detected at baseline in 42 (21%) plasma and 34 (17%) serum samples. Assays using plasma performed better than serum (AUC 0.77 v 0.69) and are reported hereafter. In predicting recurrence, baseline miR-371 demonstrated positive predictive value (PPV) 62%, negative predictive value (NPV) 91%. Detectable baseline miR-371 was associated with significantly poorer Recurrence Free Survival (RFS) compared to undetectable miR-371 (HR 10.28, p4cm (AUC 0.86 v 0.57, p=0.02) and Boorman risk group (AUC 0.86 v 0.61, p=0.03). For non-seminoma, with 30 (38%) recurrences and median F/U 20.8mo, miR-371 had superior discriminatory accuracy for recurrence compared to presence of lymphovascular invasion (AUC 0.73 v 0.58, p=0.04), or embryonal carcinoma (AUC 0.73 v 0.53, p24mo F/U supported high discriminatory accuracy of miR-371 (AUC 0.80, PPV 90%, NPV 67%) and significant difference in RFS (HR 8.59, p