Selectively killing transformed cells through proteasome inhibition

The ubiquitin-proteasome system (UPS) is responsible for degrading a broad array of proteins involved in diverse cellular processes, including signal transduction, cell cycle regulation and cell survival, among numerous others. This system involves two principal processes: protein ubiquitination, mediated by ubiquitin activating, conjugating and ligating proteins, and protein degradation by the 20S component of the 26S proteasome mediated through the activity of peptidase, trypsin-like and chymotryptic-like enzymes.¹ Because the disposition of proteasome target proteins represents such an essential function for so many cellular regulatory events, it seems counter-intutitive that inhibition of the proteasome could be a viable anticancer strategy. Nevertheless, preclinical studies suggested that proteasome inhibitors might preferentially target transformed cells and display relatively little toxicity toward their normal counterparts.² These and similar findings supported the development of proteasome inhibitors as antineoplastic compounds, exemplified by bortezomib (Velcade), a reversible inhibitor of the 20S proteasome that has been approved for the treatment of patients with refractory multiple myeloma,³ and more recently, mantle cell lymphoma.⁴ The success of bortezomib, as well as the identification of several dose-limiting toxicities (e.g., neurotoxicity and thrombocytopenia), has prompted the search for even more effective proteasome inhibitors, and several second generation compounds of this class, including NPI-0052⁵ and PR-171 (carfilzomib)⁶ are currently undergoing clinical evaluation.

Despite the success of bortezomib in multiple myeloma and mantle cell lymphoma, the therapeutic range of activity of this and similar compounds remains rather narrow. For example, to date, the activity of proteasome inhibitors against epithelial tumors appears limited for reasons that are not entirely clear. In addition, the mechanism(s) by which proteasome inhibitors kill transformed cells remain(s) to be fully elucidated. For example, it has long been assumed that proteasome inhibitors kill myeloma cells so effectively because these cells are highly dependent upon NFκB activation for survival, and inhibition of the proteasome leads to accumulation of the NFκB-inhibitory protein IκBα, which inactivates NFκB.⁷ However, results of a very recent study raise certain questions about this assumption based on evidence that in multiple myeloma cells, bortezomib can actually increase rather than decrease NFκB activation.⁸ Moreover, results of studies in both epithelial⁹ and hematopoietic cells¹⁰ suggest that induction of oxidative injury (e.g., reactive oxygen species/ROS generation) may underlie proteasome inhibitor toxicity. Finally, the UPS is involved in DNA repair processes,¹¹ raising the possibility that proteasome inhibitors may act, at least in part, by promoting DNA damage.

Despite their limited therapeutic range, and uncertainties about their mechanism of action, the search for more effective as well as more selective proteasome inhibitors continues unabated. In a recent report in Cell Cycle, Kazio et al. described the activity of a new proteasome inhibitor, designanted PI-083, identified by in silico and experimental screening of the NCI’s chemical library to target compounds active against the chymotrypsin-like activity of the proteasome.¹² This agent exhibited several noteworthy characteristics, including rapid onset of activity against diverse epithelial neoplasms, including those of breast, ovarian, lung, prostate and myeloma cells. Notably, PI-083 was relatively non-toxic toward the normal counterparts of these transformed cells. In contrast, bortezomib displayed limited activity against epithelial tumors, and did not exhibit anti-tumor selectivity. Consistent with its in vitro actions, PI-083 was active in nude mouse xenograft lung and breast cancer model systems, whereas bortezomib was significantly less effective. Finally, PI-083 inhibited tumor but not normal liver chymotrypsin-like activity whereas bortezomib inhibited activity in both normal and transformed tissues, raising the possibility that this capacity might account for or contribute to PI-083 selectivity. The authors concluded that PI-083 warrants further attention as an antineoplastic agent, particularly in the setting of epithelial tumors.

Given the established activity of bortezomib in hematopoietic malignancies, the identification of a compound that, at least in preclinical studies, appears to have a more rapid onset of action, exhibit greater activity against epithelial tumors in vitro and in vivo, and which shows evidence of enhanced antitumor selectivity, is certainly noteworthy. Whether these desirable preclinical characteristics will translate into improved activity in patients remains to be established. One natural question is what is the basis for the enhanced preclinical selectivity of PI-083 compared with bortezomib? This issue will be hard to resolve until the mechanism of action of PI-083, and of proteasome inhibitors, is clearly identified. For example, selective toxicity toward transformed cells may reflect their enhanced dependence on an intact ubiquitin-proteasome system, or, alternatively, failure of PI-083 to inhibit proteasome activity in neoplastic cells. The results of the Kazio report suggest that the latter possibility may be particularly relevant in the case of PI-083.

Clearly, additional studies will be necessary to answer this and related questions. For example, to what extent does PI-083 inhibit NFκB activity, which is often critical to the survival of transformed cells?¹³ Do the selective actions of PI-083 reflect preferential induction of oxidative injury or DNA damage in neoplastic cells? Does the selective lethality of PI-083 toward tumor cells stem from enhanced proteasome inhibition, greater dependence upon an intact UPS, or a combination of these factors? Why does PI-083 display superior activity toward epithelial tumor cells than bortezomib, and will this capacity translate into improved activity against solid tumor malignancies in the clinical arena? Although much work will be needed to resolve these questions, the preclinical evidence presented here makes a strong case for further exploration of the therapeutic potential of PI-083 and analogous compounds, particularly in solid tumor malignancies.

Kazi A, Lawrence H, Guida WC, McLaughlin ML, Springett GM, Berndt N, et al. Discovery of a novel proteasome inhibitor selective for cancer cells over non-transformed cells. Cell Cycle. 2009;8:1940–51. doi: 10.4161/cc.8.12.8798.