  The Cherenkov Telescope Array (CTA) Observatory will be the next-generation
ground-based very-high-energy gamma-ray observatory, sensitive from 20 GeV up
to 300 TeV. The Large-Sized Telescope prototype (LST-1), currently in the
commissioning phase, was inaugurated in October 2018 on La Palma (Spain). It is
the first of four LST telescopes for CTA, to be built in La Palma. In 2021,
LST-1 performed observations of one of the Galactic PeVatron candidates, LHAASO
J2108+5157, recently discovered by the LHAASO collaboration. We present results
of our analysis of the LST-1 data, putting strong constraints on the emission
of the source in the multi-TeV band. We also present results of
multi-wavelength modeling using 12-years Fermi-LAT data and Target of
Opportunity observations with XMM-Newton. We test different scenarios for the
parent particles producing the high energy emission and put constraints on
their spectra.
