Next-Generation AGT Inhibitors: Unveiling Bioactivity of Natural Products as Allosteric Modulators

D.ssa Rosa Merlo 
IBBR - CNR - Italy

October 07, 2026 (11:30-12:30)
Webinar Link: https://teams.microsoft.com/meet/321702670085630?p=PWU2JIj70v2QZwBvew

Abstract: O6-methylguanine DNA methyltransferase (MGMT) is a DNA repair protein evolutionarily conserved across the three domains of life. Known as a "suicide" enzyme, MGMT directly repairs cytotoxic O6-methylguanine lesions, becoming irreversibly inactivated. This activity plays a double-edged role: while maintaining genome stability, MGMT overexpression in glioblastoma confers resistance to alkylating treatments like temozolomide (TMZ) by repairing lesions that would otherwise trigger cancer cell death. To counteract this effect, classic pseudosubstrates, including O6-benzylguanine (O6-BG), mimic the substrate and react within the active site, thereby preventing MGMT from repairing the DNA damage intentionally induced by TMZ. However, O6-BG’s systemic action exacerbates TMZ-induced myelosuppression and triggers protein degradation and compensatory resynthesis1. Here, we investigated non-catalytic MGMT inhibitors capable of modulating protein activity through ligand-induced conformational changes. We found that β-carotene induced a concentration-dependent decrease in MGMT activity via non-covalent interaction, as demonstrated by mass spectrometry and Biolayer interferometry analysis. This interaction occurs within a hydrophobic pocket distinct from the catalytic site, minimally impacting protein stability. Building on this finding, we screened structurally related retinoids sharing the hydrophobic β-ionone ring and identified all-trans retinol as the most effective inhibitor, impairing MGMT-DNA binding. Notably, MGMT homologues from Mycobacterium tuberculosis and Saccharolobus solfataricus2 showed lower sensitivity to inhibition, suggesting pocket specificity. Furthermore, all-trans retinoic acid transcriptionally downregulates MGMT expression in glioblastoma, supporting that retinoids interfere with chemoresistance through different pathways3. Overall, our results reveal a druggable, non-catalytic hydrophobic pocket in MGMT that can be selectively targeted by all-trans retinoid scaffolds. By stabilising MGMT in an inactive conformation without triggering its degradation, this allosteric binding mode may avoid the compensatory resynthesis associated with classic inhibitors, offering a promising strategy to restore sensitivity to alkylating chemotherapeutic agents