The proteasome, which has been described variously as the cellular trash disposal or an inventory control system, degrades proteins that are no longer needed, and does so with three proteases.

When those proteases are inhibited, proteins begin to accumulate in the cell. That's bad news, and sooner or later, unable to deal with the clutter, the cell will commit suicide via its apoptotic pathway.

In 2003, Millennium Pharmaceutical Inc.'s Velcade (bortezomib) became the first proteasome inhibitor to win FDA approval; Velcade is currently approved for the treatment of multiple myeloma in patients who have received at least one prior therapy.

Last year at the annual meeting of the American Society for Hematology, San Diego, Calif.-based Nereus Pharmaceuticals Inc., presented data on its proteasome inhibitor, NPI-0052. Together with researchers from Harvard Medical School's Dana-Farber Cancer Institute, scientists from Nereus reported that the compound was active in multiple myeloma cells resistant to Velcade, as well as dexamethasone and doxorubicin, but its mechanism of action was unclear at that time. (See BioWorld Today, Dec. 10, 2004.)

Now, in the November 2005 issue of Cancer Cell, Nereus and the Harvard researchers have published data on that mechanism, as well as in vivo data showing NPI-0052's promise as a multiple myeloma treatment.

Discussing the pros and cons of publishing in the preclinical phase, Nereus CEO Kobi Sethna told BioWorld Today that as long as patents are in place, "there is no hard-and-fast rule" about when is the best time to publish. Once patents have been filed, information on compounds is in the public domain, and time of publishing depends on an interplay of the science, the company and academic collaborators.

In the case of NPI-0052, "people have been talking about this compound for two years, so it's no secret that we are moving forward. So there is no value to holding back now - when you have a compound that looks unique in a preclinical setting and a prestigious journal like Cancer Cell accepts your paper, it behooves you to take advantage of that."

The data published in Cancer Cell showed that in vitro, NPI-0052 had comparable effects to Velcade when they were given separately; given together, Velcade and NPI-0052 had synergistic effects. Both bortezomib and NPI-0052 are proteasome inhibitors of "the same class, but they are distinct in their impact on proteasome activities and in their triggering of apoptotic pathways," first author Dharminder Chauhan, senior scientist at the Dana Farber Cancer Institute, told BioWorld Today.

Given that both compounds act via similar but distinct mechanisms, the researchers hope that giving them simultaneously might have synergistic effects in cell culture - a hope that was borne out in in vitro studies: More than 50 percent of multiple myeloma cells died when treated with a combination regimen of low doses of both compounds, neither of which were sufficient by themselves to kill the cells.

The three proteases have no real names of their own, instead being described as trypsin-, chymotrypsin-, and caspase-like, respectively. At high concentrations in vitro, both bortezomib and NPI-0052 inhibited all three proteases, with NPI-0052 being more effective against chymotrypsin- and trypsin-like proteases, and bortezomib being better at inhibiting caspase-like protease. In animal studies, however, only NPI-0052 inhibited the trypsin-like protease - bortezomib actually enhanced trypsin-like protease activity.

Chauhan said he believes that the differential effect on different proteases might be what accounts for their synergistic effect. If only one or two of the three proteases are inhibited, the uninhibited ones might simply compensate for their colleagues. Chauhan said that the ideal situation from a clinical point of view is a "total breakdown" of proteasome activity, leading to sufficient protein accumulation to trigger apoptotic pathways.

In animal studies with implanted human tumor cells, NPI-0052 was as effective as bortezomib at prolonging survival and reducing the rate of cancer recurrences. The scientists are working on testing a combination regimen of both drugs in vivo, but Chauhan pointed out that "it is not a simple matter of adding the two." They are testing multiple combinations of relative doses and timing of the two compounds in a panel of mice; such testing "will form the basis for a good study in humans," he said.

Nereus plans to file an investigational new drug application by the end of the year, with trials at several centers, including Dana-Farber, starting in early 2006.