The enzyme fibroblast activation protein alpha (FAPα), herein abbreviated FAP, is a serine protease that possesses dipeptidyl-peptidase activity specific for N-terminal Xaa-Pro sequences. In addition to the dipeptidyl peptidase activity FAP also possesses collagenolytic activity capable of degrading gelatin and type I collagen. FAP is a type II transmembrane serine protease which is expressed as a homodimer. The 95-kDa protein exhibits 48% amino acid identity with DPIV and displays structural similarity to other members of the dipeptidyl peptidase family including DP8 and DP9. Unlike DPIV, FAP has also been reported to possess endopeptidase activity (Aertgeerts, J. Biol. Chem., 2005, 280, 19441). FAP overexpression has been shown to potentiate tumour growth (Cheng, Mol. Cancer. Ther., 2005, 4, 351), and this potentiation is dependent upon its enzymatic activity. Natural substrates for FAP have not yet been identified. FAP is not expressed by normal mature somatic tissues and benign epithelial tumours but is expressed on stromal fibroblasts in more than 90% of carcinomas including breast, colon, ovarian, bladder and pancreas (Garin-Chesa et al., Proc. Natl. Acad. Sci., 1990, 87: 7235) as determined by immunohistochemistry, making this a very specific target for potential anti-tumour agents. Expression of FAP has also been correlated with invasive potential of melanoma cells (Aoyama et al., Proc. Natl. Acad. Sci., 1990, 87: 8296) and its presence has been associated with metastasis of colorectal tumours (Iwasa et al., Cancer Letts., 2003, 199: 91). Its presence has been shown to increase tumour formation in animal models (Cheng et al., Cancer Res., 2002, 62: 4767).
The catalytic action of FAP expressed in stromal fibroblasts in carcinomas has been utilised in the conversion of prodrugs of cytotoxic or cytostatic drugs to active drugs as a potential treatment for cancer (EP 1333033).
In addition to its activity in tumour tissue FAP is also expressed in organs undergoing non-malignant remodelling, for example liver fibrosis (Levy et al., Hepatol., 1999, 29: 1768). In patients with fibrosis as a result of alcohol abuse or viral challenge, the cells responsible for the excessive scarring and constriction of the liver express large quantities of FAP. There is evidence to suggest that FAP plays an important role in the formation of the fibrotic scar.
Normal tissue is normally FAP negative, however, it is found transiently in healing wounds (Ramirez-Montagut et al., Oncogene, 2004, 23, 5435) and it is possible FAP plays a role in tissue remodelling and repair. Other potential indications for FAP inhibitors include blood cell disorders such as anemia and chemotherapy-induced neutopenia (Rosenblum et al., Curr. Opin. Chem. Biol., 2003, 7, 496).
The only FAP inhibitor reported to date is ValboroPro (PT-100, Talobostat) which has been reported to show potent anti-tumour activity in mice, slowing growth of syngeneic tumours and causing regression and rejection of tumours after oral administration (Adams et al., Cancer Res., 2004, 64: 5471). This compound is currently undergoing clinical trials as a treatment for cancer (McIntyre, Drugs of the Future, 2004, 29, 882). However, this compound, which is a non-specific inhibitor of dipeptidyl peptidases, has been shown to be a potent inhibitor of DPIV (Coutts et. al., J. Med. Chem., 1996, 39, 2087). It has been demonstrated (Cheng, Mol. Cancer Ther., 2005, 4, 351) that FAP-driven tumour growth can be attenuated using ValboroPro; this effect is unlikely to be due to DPIV inhibition suggesting that FAP plays an important role in the promotion of tumour growth. ValboroPro has also been shown to stimulate the growth of hematopoietic progenitorcells and to accelerate neutrophil and erythrocyte regeneration, this regeneration was observed in DPIV knock-out mice (Jones, Blood, 2003, 102, 150).
Inhibitors of DPIV are at present in clinical trials for treatment of type II diabetes. Of the many reported inhibitors of DPIV one class of compound reported are the aminoacyl pyrrolidine nitriles (Evans, IDrugs, 2002, 5, 577). However, these fall into two distinct classes; those with an unsubstituted amino group at the N-terminus (WO 9515309, WO 0181337, WO 0181304), and the N-allylglycine derivatives (WO 0196295). No data has been reported for the activity of these compounds as FAP inhibitors. There are no reports of DPIV inhibitors with an N-acyl or N-carbamoyl N-terminus.
Whilst inhibitors of prolyl ologopeptidases (which have endopeptidase activity) are known, they appear not to have been reported as showing any activity against cancer in clinical testing.
According to an aspect, the present invention aims at providing active, selective FAP inhibitors, preferably without DPIV inhibition. Preferably, the compounds are easy to synthesize. More preferably, they are orally available.