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Proceedings of the First International Conference |
Royal Marsden Hospital, London and Surrey, United Kingdom
| ABSTRACT |
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| INTRODUCTION |
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BAY 439006 is a novel bi-aryl urea developed by BAYER Pharmaceutical Corporation and Onyx, which was designed as a small molecule inhibitor of the RAF proteins CRAF and BRAF in vitro (Fig. 2)
. Furthermore, subsequent characterization of this new drug using in vitro kinase assays revealed it to be a multitargeted inhibitor with activity against several other receptor tyrosine kinases such as vascular endothelial growth factor receptor (VEGFR)2, VEGFR-3, and platelet-derived growth factor receptor B(PDGFR-B) among others (Table 1)
, which are also involved in neovascularization and tumor progression. Laboratory studies with BAY 439006 demonstrated inhibition of the MAP kinase pathway in several tumor cell lines, including colon, pancreas, and breast, expressing mutant K-RAS and wild-type and/or mutant BRAF. Broad-spectrum antitumor activity has also been shown in corresponding tumor xenograft models, with confirmation of inhibition of ERK phosphorylation by immunohistochemical analysis (8)
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BAY 439006 has been the subject of a large, Phase II, multicenter, randomized discontinuation trial, which, although designed with power to detect significant antitumor activity in patients with advanced colorectal cancer, allowed recruitment of patients with a range of other tumor types including renal cell carcinoma. The randomized discontinuation design (10)
was selected to enable evaluation of the activity of this putative cytostatic agent, because it was initially anticipated that BAY 439006 may slow or even arrest tumor growth resulting in disease stabilization (rather than exerting true cytostatic effect) All of the patients received single-agent BAY 439006, 400 mg twice daily continuously, for a 12-week run-in period. At 12 weeks, a radiologic evaluation (computed tomography and/or magnetic resonance imaging) was performed, and patients with objective responses (
25% tumor shrinkage) continued with the drug indefinitely, until disease progression. Patients with stable disease after the 12-week run-in phase were randomized in a double-blind manner to either active drug or oral placebo. If after this stage any randomized patient progressed on the treatment, their randomization code was broken, and those receiving placebo were rechallenged with open-label BAY 439006. Patients were taken off study with discontinuation of BAY 439006 use in the event of disease progression while taking the active compound at any stage of the trial. Tumor evaluation occurred every 12 weeks after baseline imaging (0, 12, 24 weeks, and so on).
| CLINICAL RESULTS |
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| DISCUSSION |
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If current studies and the results from the trial described herein continue to support the therapeutic value of this kinase inhibitor in a relapse setting, it would be interesting to compare BAY 439006 against our current standard of care with immune modulators such as interferon or interleukin 2 as upfront treatment for newly diagnosed cases.
In addition to its use as a single agent, there is also interest in combining BAY 439006 with other drugs to maximize therapeutic potential. However, the issue of which agents are the most suitable for combination studies remains to be resolved. Possible candidates include interferon and interleukin 2, which have proven activity in renal cell carcinoma. However, biomarker data from the Phase I studies of BAY 439006 indicated that the drug blocked T-cell activation at the recommended dose, thus suggesting possible antagonism of these immunotherapy agents, resulting in a counterproductive combination.
Another logical choice for combination studies is bevacizumab, a monoclonal antibody directed against VEGF receptor, which has been shown to prolong survival in a small Phase III renal cell carcinoma trial (12) . Finally, the feasibility of combining BAY 439006 with chemotherapy has also been raised. Although renal cell carcinoma is well known to be a relatively chemoresistant tumor, parallel studies using BAY 439006 with carboplatin and paclitaxel (13) in malignant melanoma, another chemoresistant cancer, have demonstrated promising results, potentially paving the way for similar trials in renal patients.
In addition to the clinical program, the scientific development of BAY 439006 must also continue, because it has yet to be fully determined how this drug is exerting antitumor activity in renal cell carcinoma. Although the compound was initially designed as a RAF inhibitor (CRAF and BRAF), preliminary studies to date have not demonstrated BRAF mutations in renal cell carcinoma (14)
unlike in malignant melanoma, which has been found to exhibit BRAF mutations in
70% of cases (15)
. Alternative targets for the drug in renal cell carcinoma have been postulated, the most frequent being VEGFR2 and VEGFR3, which appear to play an important role in the angiogenesis of this disease (16)
and are inhibited by BAY 439006. Studies with the VEGF-receptor tyrosine kinase inhibitor PK787 demonstrated antitumoral and antiangiogenic activity in murine renal cell carcinoma models (17)
. Preliminary data suggest that this drug can be administered safely, and the efficacy data appear promising. Additional support for VEGF pathway inhibition can be derived from a randomized Phase II trial comparing the anti-VEGF antibody bevacizumab with placebo. This study involved 116 patients with metastatic renal cell carcinoma and revealed that this drug can significantly prolong the time to progression of disease, although no significant differences in overall survival were seen between groups (18)
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Another potential mode of action of BAY 439006 in renal cell carcinoma may be through PDGFR, which is also inhibited by the drug (Table 1)
. Platelet-derived growth factors exert their biological activity by binding to three different tyrosine kinase receptor isoforms, in particular the PDGFR
, which is associated with growth stimulation (19)
. The expression of this receptor and its ligands was studied in human renal cell carcinoma and found to be significantly higher in grade 3 and 4 tumors, and higher receptor expression correlated with tumor progression (19)
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Another possible candidate is c-kit, with a BAY 439006 IC50 of 40 to 80 nmol/L. However, in a study that analyzed gene expression profiles in 15 renal cell carcinomas (conventional, papillary, and chromophobe) using high-density oligonucleotide arrays, although the oncogene kit was up-regulated, this was found specifically in chromophobe renal tumors and not in conventional renal cell carcinoma, the dominant histologic subtype (20) .
Interestingly neither protein kinase C nor epidermal growth factor receptor (EGFR) appear to be inhibited by BAY 439006 according to the pharmaceutical data available. However, it has been suggested in the literature that inhibitors of both these signaling system components may have a role in the treatment of renal cell carcinoma. This hypothesis has been tested with varying results. A Phase II trial of bryostatin, a protein kinase C inhibitor, involving 32 patients with advanced renal cell carcinoma produced a low proportion of objective responses (21) . Nevertheless, prolonged stable disease or partial remission in 25% of patients was seen, suggesting that there may be therapeutic potential.
EGFR and its ligands epidermal growth factor and transforming growth factor
are overexpressed in human renal cell carcinoma, compared with normal renal tissue (22)
, and EGFR signaling mechanisms have been associated with development and progression of renal cell carcinoma metastases. Conversely, blockade of EGFR signaling decreases proliferation of renal carcinoma cells in vitro (23)
. Furthermore, it has been demonstrated that down-regulation of EGFR signaling by a novel oral EGFR tyrosine kinase inhibitor PKI 166 not only markedly inhibits cell proliferation in vitro but can retard the growth of human renal carcinoma xenografts implanted into nude mice (23)
when the drug is used alone or in combination with Taxol. However, clinical studies have not corroborated these data. A multicenter Phase II trial of 55 patients with metastatic renal cell carcinoma treated with the anti-EGFR antibody C225 (Cetuximab) failed to demonstrate any responses (24)
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The data described herein suggest that there may be other important targets to be considered, and additional research into the mutual interactions among intracellular signaling pathways is needed to help elucidate the mode of action of BAY 439006, other potential kinase inhibitors attractive for clinical development, the choice of agents for selection in combination studies, and possible identification of sensitivity markers, which will ultimately allow improved patient selection for treatment.
| OPEN DISCUSSION |
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Dr. Tim Eisen: There is this discrepancy in the bevacizumab experience and certainly other antibody treatments. They may work by a similar pathway, but they are different drugs. I wonder, Jim, whether you have a comment about that in renal cell. What is the importance of the method of targeting a pathway?
Dr. James Yang: I think we know a lot more about the targets, but we dont know the whole availability and pharmacokinetics.
Dr. Michael Atkins: Are there any data on VEGF levels in this patient population? Is there any evidence that there is compensatory increase in VEGF as you inhibit with this agent or with any of the other agents as you make the cells hypoxic? Is there an increase in circulating VEGF levels that would be a justification for adding a VEGF binding agent, VEGF TRAP or bevacizumab, in combination with BAY 439006?
Dr. Eisen: Im not aware of any such data.
Dr. Walter Stadler: Ive heard some rumors to that effect, but I havent seen the data.
| FOOTNOTES |
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Requests for reprints: Tim Eisen, Royal Marsden Hospital, Institute of Cancer Research, Downs Road, Sutton, Surrey SM2 5PT, United Kingdom. E-mail: tim.eisen{at}icr.ac.uk
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