Oncology is the third largest pharmaceutical market, behind the cardiovascular and CNS therapy areas, and is currently experiencing strong growth. Worth an estimated $35 billion in 2003, analysts predict that the sector will grow to $60 billion by 2010, yielding a compound annual growth rate of 8% over this period. Major changes are however occurring in the oncology market. Cytotoxics, a cornerstone of oncology, are reaching patent expiry and the last cytotoxic genericization is expected in 2011 (Xeloda). In addition to commercial restraints imposed by genericization, cytotoxic agents are suboptimal because of the frequency and debilitating nature of adverse effects. Thus targeted therapeutics represent one of the most exciting new developments in the field of oncology (see the DataMonitor report Innovative Cancer Therapies: Targeted therapy, a clinical and commercial revolution).

In contrast to the cytotoxics, cytostatic agents slow the rate of tumor progression. Cytostatic agents have traditionally been viewed as those that directly target the proliferation of cancer cells however angiogenesis inhibitors are now also viewed by some as cytostatic agents, slowing tumor growth by restricting the blood supply. The Journal of Cellular Biochemistry study highlighted here reports on work by Praecis Pharmaceuticals investigating methionine aminopeptidases (MetAP) as a target for cytostatic therapy.

MetAP is the enzyme responsible for the removal of the initiator NH2-terminal methionine from newly synthesized proteins thereby facilitating their intracellular translocation from the ribosome. This process is the most frequently occurring protein modification and contributes to multiple cellular processes including cell proliferation. Strategies for blocking this process have therefore been investigated as candidate anti-cancer treatments. Two different isoforms of MetAP are known to exist, MetAP-1 and MetAP-2 (also known as eukaryotic initiation factor-2-associated 67-kDa protein; p67). MetAP-2 expression correlates with cell growth, and nondividing often show less immunodetectable levels of this enzyme. In addition to playing a role in the regulation of tumor cell proliferation, MetAP-2 also appears to be associated with metastasis, being a target of the metastasis-associated protein, S100A4 (Endo et al, 2002), and also angiogenesis. In angiogenesis, considered a relevant pathogenic event in cancer, MetAP-2 has been identified as the molecular target of the angiostatic agents fumagillin and ovalicin as well as TNP-470 (TAP-470; AGM-1470), a derivative of fumagilin that Takeda/TAP Pharmaceuticals (a collaboration of Takeda & Abbott) took into phase II trials. These compounds selectively and covalently bind MetAP-2 and block its aminopeptidase activity.

Having identified MetAP-2 as the target angiostatic agents a number of companies embarked on the rational development of selective enzyme inhibitors. Praecis is one such company, having developed PPI-2458 an irreversible inhibitor MwetAP-2 inhibitor that has shown promising activity in preclinical studies, PRAECIS is enrolling patients in a phase 1 clinical trial of PPI-2458. PRAECIS also has an agreement with the NCI to ultimately expand the clinical development of PPI-2458. Abbott has developed A-357300, benefiting from the identification of the crystal structures of MetAP-2. A-357300 is a reversible inhibitor that induces cytostasis by cell cycle arrest at the G1 phase selectively in endothelial cells and in a subset of tumor cells. A-357300 also inhibits angiogenesis and shows potent antitumor efficacy.

Eukaryotic cells express both MetAP-1 and MetAP-2. Although MetAP-1 and MetAP-2 in general share an enzyme activity and might compensate for each other, MetAP-2 is more efficient than MetAP-1 at catalyzing methionine removal from peptide sequences of some proteins such as cyclophilin A, 14-3-3 gamma and glyceraldehyde-3-phosphate dehydrogenase. Thus the level of redundancy in this system is unclear and it has not yet been demonstrated whether inhibition of both MetAP-1 and MetAP-2 is required for optimal control of cell proliferation. One of the aims of the Journal of Cellular Biochemistry study highlighted here was to determine the relative contribution of MetAP-1 and MetAP-2 to proliferation and to identify the level of redundancy.

In their study Bernier et al developed siRNA duplexes that targeted either MetAP-1 and MetAP-2 and determined their effect on the proliferation of HUVEC (endothelial) and A549 (lung cancer cell line) cells. One of the findings of this study is that there are crucial differences in the regulation of the two cell types by MetAP. When MetAP-1 and MetAP-2 siRNA were administered alone, either treatment caused a dramatic reduction in the proliferation of HUVEC cells, reducing growth by as much as 80%. In stark contrast, A549 cell proliferation was only reduced by 20-30%. The Praecis group than proceeded to inhibit MetAP-2 using fumagillin or PPI-2458 in cells that had already been treated with siRNA directed to MetAP-1. The effects of inhibiting both isoforms in this way was dramatic; the proliferation of HUVEC cells was completely inhibited while the proliferation of A549 cells was reduced from 80-90% of control values (in cells treated with MetAP-1 siRNA alone) to about 10% of control values when both MetAP isoforms were inhibited. This near-complete block of proliferation following combinatorial treatment was associated with minimal cytotoxicity in both cell lines consistent with a cytostatic mechanism of action.

This important study further supports the concept that MetAP-2 inhibition represents an approach to the reduction of angiogenesis. The study also demonstrated for the first time that MetAP-1 inhibition may produce a similar anti-angiogenic response and moreover the combined inhibition of MetAP-1 and -2 may lead to reduced proliferation of tumor cells as well as the tumor vasculature thus producing increased efficacy. The effect of MetAP inhibition requires further investigation in other tumor cell lines and eventually the comparative efficacy of single versus dual isoform inhibition needs to be evaluated in vivo however these data may point to the development of dual MetAP-1/MetAP-2 inhibitors. Whether this leads to prohibitive toxicity due to effects on non-cancerous cells remains to be determined.

As a final point, if dual MetAP-1 /MetAP-2 inhibition does turn out to be a preferred approach to cancer, where does this leave selective MetAP-2 inhibitors? In a separate paper (Bernier et al, 2004) Praecis researchers have demonstrated that PPI-2458 inhibits the growth of both human fibroblast-like synoviocytes derived from rheumatoid arthritis patients and human endothelial cells. Both of these cell types are thought to be important in the progression of joint destruction in rheumatoid arthritis patients. PPI-2458 also significantly lessened joint swelling at well-tolerated doses when administered therapeutically after the onset of chronic disease in a rodent model of rheumatoid arthritis suggesting that PPI-2458 or other MetAP-2 inhibitors represent potential treatments for rheumatoid arthritis. PRAECIS is currently seeking a partner for the development of PPI-2458 as an approach in cancer and inflammatory disease.