Israel Medical Association Journal

The introduction of novel targeted therapies into the clinic in recent years has had a considerable impact on the management of several neoplastic diseases – such as gastrointestinal stromal tumors, hepatocellular carcinomas and renal cell carcinomas – considered until recently refractory to systemic therapies. We describe here two such novel biological agents, sunitinib and sorafenib, as a paradigm of the successful clinical application of new concepts. Sunitinib and sorafenib are small molecule tyrosine kinase inhibitors that target vascular endothelial growth factor receptor, platelet-derived growth factor receptor, C-Kit and others. Both agents are administered orally; sunitinib is typically given in cycles for 4 consecutive weeks with 2 weeks off, while sorafenib is given continually. Side effects occur in most patients, similar for both agents; they may affect several systems and organs but are mostly mild and easily manageable, rarely requiring discontinuation of the drug. However, these toxicities require prompt attention and intervention. The most frequently observed effects are hypertension, nausea, anorexia, asthenia and cutaneous manifestations; cardiac abnormalities may include congestive failure. Sunitinib, and markedly less frequently sorafenib, may cause thyroid gland dysfunction, mainly hypothyroidism. Antitumor activity has been shown for renal cell carcinoma in pivotal trials, for sunitinib as first-line treatment and for sorafenib in previously treated patients as second-line. Sunitinib is now approved as second-line therapy for patients with GIST[1] refractory to imatinib; sorafenib has resulted in a significant prolongation in median survival in patients with hepatocellular carcinoma. Ongoing clinical trials will further define the spectrum of these agents' antitumor activity, their role in combination with other drugs, as well as their optimal dose and schedule of administration.

Polyubiquitylation of cellular proteins has long been recognized as a prelude to a degradative fate in proteasomes. In recent years, however, ubiquitin conjugation has emerged as a regulatory strategy of considerable versatility. Most notably, monoubiquitylation is attributed an intimate role in trafficking of membrane proteins between various cellular compartments. Diverse classes of transmembrane proteins from across the eukaryotic spectrum (e.g., epidermal growth factor-receptor and other receptor tyrosine kinases) become modified with monoubiquitin molecules. Monoubiquitylation of substrates, in turn, regulates both their endocytosis at the plasma membrane and sorting in endosomes for delivery to lysosomes or vacuoles. A mechanistic rationale lies in the identification of a growing list of ubiquitin-binding domains carried by a variety of endocytic adaptor proteins. Thus, ubiquitin-conjugated membrane proteins may form extensive contacts with the endocytic machinery. Further, ubiquitin-binding adaptors and other endocytic components are, likewise, often monoubiquitylated. In this case, ubiquitin conjugation may serve to enhance intermolecular avidity in cargo-bound endocytic complexes, or alternatively, to mediate timely inactivation of ubiquitin-binding adaptors. Interestingly, the ubiquitin/endocytosis interface is appropriated by pathogenic organisms, for instance, during budding of viruses from host-infected cells. Moreover, compromised ubiquitin-mediated transport of certain signaling receptors is associated with disease states, including oncogenic transformation.