In this study we report the identification, characterization and in vivo
testing of a novel reversible cathepsin inhibitor, VBY-825, in a mouse model of pancreatic islet cancer. VBY-825 was identified from an extensive structure-based drug design program, and found to have high potency for inhibition of cathepsins B, L, S and V, and to a lesser extent cathepsins F and K. We have previously shown that cathepsins B, C, H, L, S and X/Z are upregulated during the different stages of RT2 tumorigenesis [8
], whereas the remaining cathepsin family members are either not expressed, or are expressed at low levels that do not change. Thus, cathepsins B, L and S are the potential targets for inhibition by VBY-825 in this cancer model.
VBY-825 treatment in an intervention trial resulted in a significant decrease in tumor growth and tumor incidence, which was comparable to the reduction observed previously with the irreversible inhibitor JPM-OEt in a similar trial design [8
]. The reduced tumor growth could potentially be explained by the trend towards a combined decrease in cell proliferation and increase in apoptosis we observed in the VBY-825 treated tumors. However, while both proliferation and cell death were somewhat perturbed following VBY-825 treatment, the effects were not as pronounced as we had previously observed with the pan-cathepsin inhibitor JPM-OEt [8
]. Similarly, we did not observe a significant difference in tumor invasion or angiogenesis in VBY-825 treated mice, in contrast to the substantial reductions in both processes that we had previously found following JPM-OEt treatment [8
]. Thus, while both JPM-OEt and VBY-825 have a similar potency in reducing tumor growth and tumor number, there are some differences in their observed effects on the hallmark capabilities of cancer.
There are several possible explanations that may account for some of the differences between the VBY-825 and JPM-OEt inhibitors in the RT2 model. First, JPM-OEt is a pan-cathepsin inhibitor, whereas VBY-825 has a more selective inhibitory profile, with the expected targets being restricted to cathepsins B, L and S in this model. Therefore, it is possible that the remaining upregulated cathepsins (H and X), which we know to be important for tumor angiogenesis and invasion respectively [Ref. 13
, and Gocheva et al, manuscript in preparation] are not inhibited, and are sufficient to promote these tumorigenic processes when cathepsins B, L and S are inhibited by VBY-825. A second possibility is that reversible inhibition of these specific cathepsins is not as effective as irreversible inhibition in reducing tumor angiogenesis and invasion, although this is unlikely given the high plasma levels of the compound throughout the duration of dosing. Finally, it is possible that the defined timepoint of the analyses after 3.5 weeks of treatment may not reveal important but transitory effects occurring earlier in the therapeutic regimen. Analysis of these parameters at earlier timepoints following the initiation of treatment at 10 weeks could be informative in determining whether the biological effects of VBY-825 treatment are more pronounced initially. For example, it is possible there is an initial wave of increased apoptosis and decreased cell proliferation, which debulks the tumor mass, but then is attenuated by the time they are assessed histologically at the 13.5-week endpoint.
An additional consideration is the route of administration. VBY-825 was delivered via subcutaneous injection, whereas JPM-OEt was administered by intraperitoneal injection. Reinheckel and colleagues recently showed that JPM-OEt effectively inhibited cathepsin activity in the organs close to the injection site in the peritoneal cavity (pancreas, liver and kidney) but was less effective in the distally located lungs [24
]. Thus examining the efficacy of intraperitoneal administration of VBY-825 in the RT2 pancreatic cancer model could be informative. Additional strategies to improve the efficacy of VBY-825 in preclinical trials include combining this inhibitor with other anti-cancer agents, such as chemotherapy. Indeed, we previously found that the addition of cyclophosphamide to the cathepsin inhibitor JPM-OEt significantly enhanced its anti-tumor efficacy, and increased overall survival in a RT2 regression trial [11
], motivating further investigation of VBY-825 in combination with other drugs.
In conclusion, we have identified and characterized a novel, reversible and selective cathepsin inhibitor that significantly reduces tumor incidence and tumor growth in a mouse model of pancreatic islet cancer. In addition, VBY-825 treatment was non-toxic to animals over the course of this preclinical trial, as we have previously reported for JPM-OEt [4
], further encouraging the potential application of cathepsin inhibitors as potent anti-cancer therapeutics. Indeed, the range of cysteine cathepsin inhibitors that have been developed and tested in animal models has significantly expanded in the past several years. In addition to the identification of selective small molecule inhibitors such as VBY-825, there have also been programs to develop antibody and peptide-based inhibitors [39
]. Another exciting advance is the ongoing evaluation of cathepsin K-selective small molecule inhibitors in early clinical trials for patients with bone metastases [46
]. It will thus be very interesting to establish whether the potential benefits of inhibiting cathepsins, as indicated from pre-clinical studies such as this one, will translate into a successful therapeutic strategy in the clinic.