Abstract Introduction: Metastatic renal cell carcinoma (mRCC) is routinely treated with sunitinib, a tyrosine kinase inhibitor (TKI) of VEGF signalisation. Although disease eventually progresses in most mRCC patients, length of progression-free survival (PFS) is ranging from few weeks up to more than 24 months. Patients with initial resistance to sunitinib could be redirected to other therapeutical options, as there are several other TKIs available for use in second and third line. However, without a reliable biomarker no result can be predicted. MicroRNAs (miRNAs) belong to class of short non-coding RNAs and could serve as biomarkers of therapy response due to their unique character and presence in tissues and body fluids. Their biomarker potential has been discussed concerning many diseases including mRCC, but current knowledge is very weak, has several discrepancies and is acquired on relatively small cohorts. Material and method: Candidate microRNAs have been chosen based on global expression profiling using Affymetrix GeneChip 4.0 in 47 samples of FFPE mRCC tissue of patients treated with sunitinib (good response n=25, PFS longer than 17 months; poor response n=22, PFS shorter than 9 months). Validation was performed using qRT-PCR TaqMan assays on an independent cohort of 136 FFPE samples from mRCC patients treated with sunitinib with variable length of PFS. Local ethical committees at all involved centres approved the study protocol. The study was performed according to the World Medical Association Declaration of Helsinki and all patients signed an informed consent. Results and discussion: Of all tested miRNAs, expression of miR-4668-5p and miR-376b was the most significantly deregulated in non-responding patients with high statistical significance (p>0,005) and combined AUC higher than 0,8. Although other independent validations are necessary, microRNAs presented here seem to be very promising as tools for therapy personalization. Conclusion: We have successfully validated miR-4668-5p and miR-376b to have predictive potential in estimation of therapeutical response to sunitinib. Further functional analyses could shed more light on their involvement in development of therapy resistance. This work was supported by Ministry of Health of the Czech Republic, grant nr. NV18-03-00554. All rights reserved. Citation Format: Julia Kovacova, Jaroslav Juracek, Alexandr Poprach, Tomas Buchler, Ondrej Fiala, Lenka Radova, Marek Svoboda, Alena Kopkova, Marek Vecera, Ondrej Slaby. miR-376b and miR-4668 predict therapeutic response to sunitinib in metastatic renal cell carcinoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 1800.