9566 Background: HIV-protease inhibitors like Ritonavir produces regressions in HIV-associated Kaposi‘s sarcoma, reportedly due to proteasome inhibition. We have here assessed the effect of Ritonavir and the proteasome inhibitor Bortezomib on the viability and proteasome activity of the soft tissue sarcoma cell lines RD (rhabdomyosarcoma) and A-673 (ewing‘s sarcoma) in vitro. Methods: Cytotoxixity was assessed by MTS-test. To directly dissect proteasome activity in viable sarcoma cells, we used a recently developed cell-permeable affinity label that for the first time allows to assess the individual activity profile of the different catalytically active proteasomal subunits in living cells (Berkers et al., Nature Methods, May 2005). Results: Both types of cells were resistant towards Bortezomib and Ritonavir monotherapy at therapeutic concentrations (20 nM for Bortezomib, 20μM for Ritonavir). Ritonavir induced cytotoxicity in sarcoma cell lines with an IC50 of 40μM. The combination of clinically meaningful concentrations of Ritonavir (20μM) with Bortezomib (10–20 nM) induced robust synergistic cytotoxicity in both sarcoma cell lines. Using affinity labelling of proteasome activity, we directly demonstrate that a) Botezomib abrogates beta5, but also beta1-proteosomal activity in sarcoma cells and b) that Ritonavir at 20μM does not affect proteasom activity. Conclusions: We here present the first analysis of active proteasomal subunits in living human sarcoma cell lines. Ritonavir and Velcade induce synergistic cytotoxicity at clinically meaningful concentrations in vitro, which is not mediated through a direct effect of Ritonavir on proteasome activity. Based on our data, the synergistic combination of Ritonavir and Bortezomib might warrant further clinical testing in soft tissue sarcoma in vivo. No significant financial relationships to disclose.
An in vitro analysis of medial structures and a medial soft tissue reconstruction in a constrained condylar total knee arthroplasty
