253 Summary HGF/SF-MET signaling is deregulated in many human solid cancers (1). As MET is mutated, overexpressed (at mRNA and protein level), and known to orchestrate invasive growth in HNSCC (2-4), it is an interesting target for therapy (5, 6). Although numerous MET targeted therapies have been developed (7, 8), major survival benefits have not yet been achieved (8, 9). This, together with the high costs that are associated with the treatment of cancer patients, hinders the use of MET-based targeted therapies in clinical practice (10, 11). The absence of clinical benefit of MET targeted therapies may be explained by the lack of reliable CDx (9), defined as “a medical device, often an in vitro device, which provides information that is essential for the safe and effective use of a corresponding drug or biological product” (12). The development of CDx for MET targeted therapies is difficult because of several reasons (9). Some are due to technicalities, such as the lack of reliable antibodies and optimal scoring methods (7, 9, 13), while other reasons may be due to biology, such as ectodomain shedding (9). Thus, one of today’s challenges is the design of valid CDx that identify those patients eligible for treatment with targeted therapies directed against MET. Consequently, the primary aim of this thesis was to take the initial steps in the development of CDx for MET targeted therapies for patients diagnosed with OSCC. In chapter 3, the reliability of five commercial C-terminal MET antibodies was tested using the Rimm Lab Algorithm for antibody validation (14). Two antibodies met the criteria of the algorithm and were used to explore C-terminal MET immunoreactivity across a selection of OSCC WTSs. The antibody that was most sensitive in the detection of membranous MET – being clone D1C2 – was used to examine the association between MET immunoreactivity and survival in an OSCC and HPV negative oropharyngeal SCC patient cohort represented on a TMA. The results illustrated that MET immunoreactivity is either uniform (negative or positive) across cancers or differs between a cancer’s center and periphery (variable). Ultimately, it was shown that D1C2 uniform staining was associated with poor overall and disease-free survival of patients lacking signs of vasoinvasive growth. While examining the reliability of the C-terminal MET antibodies presented in chapter 3, specific degradation patterns were observed under reducing conditions. On the one hand, these patterns were explained by stress related caspase cleavage of MET (15, 16). On the other hand, they were explained by MET ECD shedding and RIPping (17-20). Since deletion of the ectodomain unleashes the aggressive nature of MET oncogene product (21), it was investigated whether proteolysis of MET occurs 9
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