Mourtada, F. | Search bioRxiv

bioRxiv2025

FOLR1-targeted Actinium-225-based Alpha-particle Therapy Eliminates Ovarian Cancer

Despite the advancement in therapies, ovarian cancer treatment is challenging due to poor prognosis and high relapse associated with acquired resistance. Targeting overexpression of FOLR1 in ovarian cancers has proven to be an attractive strategy. The recent FDA approval of FOLR1 targeted antibody drug conjugate has shown promising results albeit resistance with repeated use appears inevitable. Emerging targeted alpha-particle therapies, particularly Actinium-225 (225Ac), for treating refractory cancers have opened avenues for improved therapeutic options. The success of alpha-particle therapy relies on tumor specific delivery of the alpha emitters. Herein we describe the first example of FOLR1-targeted 225Ac alpha-particle therapy for treatment of ovarian cancer. Longitudinal PET imaging demonstrated high tumor-specific uptake of FOLR1 in SKOV3 xenografts. FOLR1-targeted 225Ac demonstrated high therapeutic efficacy achieving marked tumor regression, 80% survival and 40% complete response. The therapy resulted in tumor specific double stranded DNA damage, and no obvious toxicity was observed in normal tissues. Estimated human dosimetry showed high absorbed dose for tumor and minimal absorbed dose for healthy tissues establishing its safety. In totality, FOLR1-targeted 225Ac alpha-particle therapy is an efficacious and safe treatment with high feasibility for clinical translation to fight against ovarian cancer.

bioengineering↗

bioRxiv2025

Revealing the unexpected interplay between the Proteasome Activator PA200 and the immunoproteasome

The proteasome activator PA200 binds to the catalytic core of the proteasome, the 20S, and activates its proteolytic activities. The cellular function of PA200 is poorly understood and appears to be cell type and differentiation specific. Recent evidence suggests that PA200 not only binds to the standard 20S (s20S) proteasome but also to the specialized immunoproteasome (i20S) which plays a key role in anti-viral and anti-tumor immunity. We here investigated the interaction of PA200 and the immunoproteasome in detail. We show the very first cryo-EM structures of the singly- and doubly-capped i20S-PA200 complexes that revealed no major difference regarding the first binding event of PA200 to the i20S vs. the s20S. However, first PA200 binding triggered a subtle and long range allosteric bending of the i20S barrel which was not seen in the s20S-PA200 complexes. This resulted in major structural rearrangements in the opposite unbound ring - the displacement of atoms up to 5.4 [A] and the increase in its outer diameter - thereby increasing the occupancy of the second PA200 binding site. Mass photometry confirmed higher occupancy of PA200 to the i20S versus the s20S. Binding of PA200 to the i20S enhanced proteasomal activation compared to the s20S. Co-expression of PA200 and the i20S in cells and tissues, however, is restricted but their interaction is favored upon co-expression. The expression of PA200 and the catalytic subunits of the i20S is differentially regulated depending on the cellular context. Our data also suggest that PA200 has the potential to regulate i20S gene expression whereas the i20S has no effects on PA200 expression. Overall, this work sheds new light on the interaction of PA200 with the i20S from a structural, mechanistic and cellular point of view. Importantly, we identify PA200 as a key regulator of the i20S whenever PA200 and the catalytic subunits of the i20S are co-expressed in the same cell.

biochemistry↗