Disruption of the normal patterns of protein phosphorylation results in aberrant regulation of signal transduction and has been implicated in the etiology of a variety of major human diseases. The ability to modulate such signaling pathways selectively holds enormous therapeutic potential. Currently, the focus of effort in this aspect of drug development has been on protein kinases, particularly protein tyrosine kinases (PTKs); however, this ignores the other major component of phosphorylation-dependent regulation of signaling. Protein phosphorylation is a reversible process, in which the coordinated and competing activities of kinases and phosphatases are important for determining signaling outcome. Nevertheless, the protein tyrosine phosphatases (PTPs) remain a largely untapped resource for drug development. Since its discovery 25 years ago, PTP1B has become a highly validated therapeutic target. A major breakthrough was the definition of the role of PTP1B in down-regulating insulin and leptin signaling. In mice, targeted deletion of the PTPN1 gene, which encodes PTP1B, produced healthy animals that displayed increased insulin sensitivity and resistance to obesity induced by a high fat diet. Thus, inhibitors of PTP1B may promote signaling in insulin- and leptin-resistant states and offer a novel approach to treating diabetes and obesity. Nevertheless, the function of PTP1B is not restricted to metabolic regulation; it is over-expressed in breast tumors together with the PTK HER2. Mice expressing activated alleles of HER2 in mammary glands develop multiple mammary tumors and frequent metastases to the lung; however, when such mice were crossed with PTP1B-null mice, tumor development was delayed and the incidence of lung metastases was decreased. Conversely, targeted overexpression of PTP1B alone was sufficient to drive mammary tumorigenesis. These observations suggest that, in addition to its role as a negative regulator of insulin and leptin signaling, PTP1B plays a positive role in promoting signaling events associated with breast tumorigenesis. Therefore, inhibition of PTP1B function may represent a novel therapeutic strategy not only to address diabetes and obesity, but also mammary tumorigenesis and malignancy.
In light of these breakthroughs, there have been major programs in industry focused on developing small molecule inhibitors of PTP1B. These efforts, which followed standard procedures of targeting the active site of PTP1B, have been frustrated by technical challenges arising from the chemistry of PTP catalysis. Although it was possible to generate potent, specific and reversible inhibitors of PTP1B, such molecules were highly charged and thus of limited drug development potential. In contrast, DepYmed is overcoming these challenges by focusing on allosteric inhibitors that bind at unique sites remote from the catalytic center of the enzyme. PTP1B was purified originally from human placenta as a 37kDa catalytic domain, which has been the focus of attention to date for mechanistic analysis, as well as for drug screening. Nevertheless, PTP1B exists in vivo as a longer protein of ~50kDa, in which the C-terminal segment, which is deleted from the 37kDa protein, serves a regulatory function. We have demonstrated that an aminosterol natural product, MSI-1436/Trodusquemine, inhibited the full-length form of PTP1B preferentially in a reversible, selective manner. We have identified the binding sites for MSI-1436 in PTP1B and defined the mechanism of inhibition. By targeting the unique, C-terminal, non-catalytic segment that is unrelated to any other member of the PTP family, such allosteric inhibitors would have the potential to be highly specific for PTP1B. Furthermore, we have demonstrated that by targeting PTP1B, MSI-1436 attenuated HER2 signaling, resulting in extensive inhibition of tumor growth and abrogation of metastasis to the lung in HER2-positive animal models of breast cancer. Overall, these data establish that PTP1B is a bona fide target for therapeutic intervention in HER2-positive cancer and illustrate a novel mechanism for specific inhibition of PTP1B through which such intervention may be achieved.
Although the first drugs directed against protein tyrosine kinases (PTKs) represent breakthroughs in cancer therapy, challenges remain. For example, the humanized antibody Herceptin (Trastuzumab) targets the PTK HER2, which is amplified and/or overexpressed in ~25% of breast tumors, where it associated with poor prognosis. Herceptin is the treatment of choice for HER2-dependent cancer, although there are problems with de novo and acquired resistance. Similar problems have limited the success of other PTK-directed inhibitors. Therefore, it is anticipated that alternative therapies, to target simultaneously different signaling enzymes and processes, may be more effective than targeting individual PTKs alone. Currently, we are testing whether inclusion of MSI-1436 together with Herceptin, to target simultaneously the PTK and its downstream signaling, will prolong the time to development of resistance, or even overcome resistant states.