2010;11:21C28. inhibitors have been characterized in lung and colorectal cancer. In lung cancer, molecular determinants were presaged by the realization that a specific clinically-definedsubpopulation (Asian, female, never-smokers, adenocarcinomas) responded best to TKIs. Subsequently, EGFR mutations associated with TKI sensitivity (exon 19 and L858R) or resistance (T790M) were identified[4]. In colorectal cancer, KRAS mutations were found to be associated with cetuximab resistance[5]. In both lung and colorectal cancers, EGFR copy number predicts response to cetuximab somewhat, but the predictive value is not high. Although not yet in clinical use, preclinical data has also implicatedresistance mechanisms such as VEGF signaling, AKT/mTOR pathway Triptonide activation, and oncogenic shift to other receptor tyrosine kinases such as ERBB2, ERBB3, MET or IGF-1R, via overexpression or increased ligand availability[6]. In contrast, our understanding of mechanisms underpinning resistance to EGFR-targeted therapy is usually comparatively poor in HNSCC. Molecular determinants are not well defined. The most predictive factor for cetuximab sensitivity in HNSCC is a clinical obtaining C the development of a skin rash during treatment[1]. EGFR copy number is not predictive of response. Activating EGFR mutations are very rare, as are KRAS and BRAF mutations. Unlike in some other cancers such as GBM, the EGFRvIII variant does not predict response. Some promising insights have been reported recently, however. Preclinical data have demonstrated that increased expression of the ligand heparin-binding EGF-like growth factor (HB-EGF) occurs during the development of resistance in HNSCC cell lines, and that plasma Mouse monoclonal antibody to PA28 gamma. The 26S proteasome is a multicatalytic proteinase complex with a highly ordered structurecomposed of 2 complexes, a 20S core and a 19S regulator. The 20S core is composed of 4rings of 28 non-identical subunits; 2 rings are composed of 7 alpha subunits and 2 rings arecomposed of 7 beta subunits. The 19S regulator is composed of a base, which contains 6ATPase subunits and 2 non-ATPase subunits, and a lid, which contains up to 10 non-ATPasesubunits. Proteasomes are distributed throughout eukaryotic cells at a high concentration andcleave peptides in an ATP/ubiquitin-dependent process in a non-lysosomal pathway. Anessential function of a modified proteasome, the immunoproteasome, is the processing of class IMHC peptides. The immunoproteasome contains an alternate regulator, referred to as the 11Sregulator or PA28, that replaces the 19S regulator. Three subunits (alpha, beta and gamma) ofthe 11S regulator have been identified. This gene encodes the gamma subunit of the 11Sregulator. Six gamma subunits combine to form a homohexameric ring. Two transcript variantsencoding different isoforms have been identified. [provided by RefSeq, Jul 2008] HB-EGF levels are elevated in recurrent tumors[7]. There is also evidence that head and neck tumors can evade EGFR inhibition by undergoing epithelial-to-mesenchymal transition, thereby losing EGFR dependency. Recently, frequent deletion of the gene, encoding protein tyrosine phosphatase receptor S, was described in HNSCC[8]. A comprehensive genome-wide analysis of copy number alteration in HNSCC identified recurrent, intragenic microdeletions at the gene locus in 26% of tumors. The focal nature of these deletions argues that Triptonide is the target of copy number alteration at chromosome 19p13. These deletions result in loss of protein expression of PTPRS, a membrane-bound phosphatase that dephosphorylates EGFR. Depletion of PTPRS leads to increased levels of phosphorylated EGFRand increasedEGFR signaling. Interestingly, loss of PTPRS, and consequently increased EGFR phosphorylation, renderscancer cells significantly more resistant to EGFR inhibitors. In fact, in normally TKI-sensitive HNSCC and lung cancer cells, knockdown of PTPRS is sufficient to induce erlotinib resistance. PTPRS seems to play a similar role modulating cetuximab resistance in HNSCC cells. Interestingly, clinical outcome is also dramatically influenced by PTPRS status. Patients with lung adenocarcinomas harboring activating EGFR mutations loss, is able to help drive EGFR pathway activation, and modulate sensitivity to EGFR inhibitors. With additional clinical investigation, these findings may open the door to the possibility of status serving as a biomarker for drug resistance, analogous to EGFR or KRAS resistance mutations in lung and colorectal cancer. This might aid in triaging patients to EGFR inhibitors or conventional chemotherapy. TKI trials, limited to sensitive EGFR mutations in lung cancer, have achieved impressive response rates of 50-70%. 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