Colorectal (CRC) and gastric (GC) cancers remain the top lethal cancers and targeted therapies in this setting are still very limited. Sialyl Lewis X (SLeX), a cancer-associated glycan highly expressed in both CRC and GC, plays a crucial role in cancer cell dissemination and metastasis. Thus, presenting a promising but still underexplored therapeutic target. In this work, we performed a high-throughput screening (HTS) approach to identify potential inhibitors of SLeX expression on cancer cells. Two libraries including a total of 7836 compounds were screened and monensin emerged as a promising SLeX inhibitor. Monensin promoted structural alterations in the secretory pathway, particularly at the Golgi apparatus, impacting protein O-glycosylation and secretion. RNAseq transcriptomic analysis uncovered significant alterations in Gene Ontology (GO) terms associated with protein misfolding, target to the membrane, as well as, epithelial cell-cell adhesion protein. In vitro studies showed that, upon treatment with monensin, SLeX-positive cancer cells showed reduced viability, concomitant with decreased motility and invasive capacities. Using in vivo xenograft models of chick embryo chorioallantoic membrane (CAM) and nude mice, revealed that monensin reduced tumor formation and invasion. Pre-clinical validation using gastric cancer patient-derived organoids (PDOs) and organoid xenotransplants in mice further underscored the clinical potential of monensin in suppressing the growth of SLeX- positive tumors. Overall, our findings set the ground for further evaluation of monensin as a novel therapeutic agent in GC and CRC in the clinical setting.

The authors have declared no competing interest.

This study was funded by (i) Fundo Europeu de Desenvolvimento Regional (FEDER) funds through the COMPETE 2020 - Operacional Programme for Competitiveness and Internationalization (POCI), Portugal 2020 (POCI-01-0145-FEDER-016585, POCI-01-0145-FEDER-007274). (ii) Norte Portugal Regional Programme (NORTE 2020), under the PORTUGAL 2020 Partnership Agreement, through the European Regional Development Fund (ERDF) project NORTE-01-0145-FEDER- 000029. (iii) Portuguese funds through Fundacao para a Ciencia e a Tecnologia (FCT)/Ministerio da Ciencia, Tecnologia e Inovacao through the research projects PTDC/MED-QUI/29780/2017 - POCI-01-0145-FEDER-029780, PTDC/MED-QUI/2335/2021 and EXPL/BTM-ORG/1450/2021. AFC, LSF and RA were supported by FCT PhD grants (UI/BD/150829/2021, 2021.05495.BD and 2020.05483.BD), CG (2022.04678.CEECIND), FP (2022.02109.CEECIND) and HOD (2022.00943.CEECIND).

I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained.

Yes

The details of the IRB/oversight body that provided approval or exemption for the research described are given below:

Ehtics commitee/IRB of Centro Hospitalar Universitario Sao Joao gave ethical approval of this work

I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals.

Yes

I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance).

Yes

I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable.

Yes

All data produced in the present study are available upon reasonable request to the authors