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Intratumor heterogeneity is a key factor contributing to therapeutic failure and, hence, cancer lethality. Heterogeneous tumors show partial therapy responses, allowing for the emergence of drug-resistant clones that often express high levels of the receptor tyrosine kinase AXL. In melanoma, AXL-high cells are resistant to MAPK pathway inhibitors, whereas AXL-low cells are sensitive to these inhibitors, rationalizing a differential therapeutic approach. We developed an antibody-drug conjugate, AXL-107-MMAE, comprising a human AXL antibody linked to the microtubule-disrupting agent monomethyl auristatin E. We found that AXL-107-MMAE, as a single agent, displayed potent in vivo anti-tumor activity in patient-derived xenografts, including melanoma, lung, pancreas and cervical cancer. By eliminating distinct populations in heterogeneous melanoma cell pools, AXL-107-MMAE and MAPK pathway inhibitors cooperatively inhibited tumor growth. Furthermore, by inducing AXL transcription, BRAF/MEK inhibitors potentiated the efficacy of AXL-107-MMAE. These findings provide proof of concept for the premise that rationalized combinatorial targeting of distinct populations in heterogeneous tumors may improve therapeutic effect, and merit clinical validation of AXL-107-MMAE in both treatment-naive and drug-resistant cancers in mono- or combination therapy.

Summary CD38 is a multifunctional cell surface protein that has receptor as well as enzyme functions. The protein is generally expressed at low levels on various hematological and solid tissues, while plasma cells express particularly high levels of CD38. The protein is also expressed in a subset of hematological tumors, and shows especially broad and high expression levels in plasma cell tumors such as multiple myeloma (MM). Together, this triggered the development of various therapeutic CD38 antibodies, including daratumumab, isatuximab, and MOR202. Daratumumab binds a unique CD38 epitope and showed strong anti‐tumor activity in preclinical models. The antibody engages diverse mechanisms of action, including complement‐dependent cytotoxicity, antibody‐dependent cellular cytotoxicity, antibody‐dependent cellular phagocytosis, programmed cell death, modulation of enzymatic activity, and immunomodulatory activity. CD38‐targeting antibodies have a favorable toxicity profile in patients, and early clinical data show a marked activity in MM, while studies in other hematological malignancies are ongoing. Daratumumab has single agent activity and a limited toxicity profile, allowing favorable combination therapies with existing as well as emerging therapies, which are currently evaluated in the clinic. Finally, CD38 antibodies may have a role in the treatment of diseases beyond hematological malignancies, including solid tumors and antibody‐mediated autoimmune diseases.

Gefitinib has a modest activity in second-line treatment of advanced esophageal cancer. However, the patient outcome was significantly better in female patients and in patients demonstrating high EGFR expression or SCC histology. The selection of esophageal cancer patients for future studies with EGFR-TKIs based on the level of EGFR expression in their tumors or SCC histology should be considered.

In this study, we have characterized a panel of NSCLC cell lines with differential sensitivity to gefitinib for activating mutations in egfr, pik3ca, and k-ras, and basal protein expression levels of PTEN. The egfr mutant NSCLC cell line H1650 as well as the egfr wild type cell lines H292 and A431 were highly sensitive to gefitinib treatment, indicating that other factors determine gefitinibsensitivity in egfr wild type cells. Activating k-ras mutations were specifically detected in gefitinib-resistant cells, suggesting that the occurrence of k-ras mutations is correlated with resistance to EGFR antagonists. No pik3ca mutations were detected within the panel of cell lines, and PTEN protein expression levels did not correlate with gefitinib sensitivity. Gefitinib effectively blocked Akt and Erk phosphorylation in two gefitinib-sensitive NSCLC cell lines, further supporting our previous findings that persistent activity of the PI3K/Akt and/or Ras/Erk pathways is associated with gefitinib-resistance of NSCLC cell lines. Gefitinib-resistant NSCLC cell lines, showing EGFR-independent activity of the PI3K/Akt or Ras/Erk pathways, were treated with gefitinib in combination with specific inhibitors of mTOR, P13K, Ras, and MEK. Additive cytotoxicity was observed in A549 cells co-treated with gefitinib and the MEK inhibitor U0126 or the farnesyl transferase inhibitor SCH66336 and in H460 cells treated with gefitinib and the PI3K inhibitor LY294002, but not in H460 cells treated with gefitinib and rapamycin. These data suggest that combination treatment of NSCLC cells with gefitinib and specific inhibitors of the PI3K/Akt and Ras/Erk pathways may provide a successful strategy. ' 2005 Wiley-Liss, Inc.Key words: EGFR; Erk; Akt/PKB; mTOR; SCH66336; rapamycin The EGFR II (ErbB1, HER1) is the prototypic member of the ErbB family of RTKs, which further consists of ErbB2-4 (HER2-4). The ErbB receptors share a similar protein structure, consisting of an extracellular ligand binding domain, a single transmembrane domain and an intracellular C-terminal domain with tyrosine kinase activity. Upon specific binding of EGF-like ligands to the extracellular domain, ErbB receptors dimerize, either as homodimers or as heterodimers, and undergo autophosphorylation at specific tyrosine residues within the intracellular domain. These phosphorylated tyrosines serve as docking sites for adapter molecules such as Grb2 and the p85 subunit of PI3K, which activate downstream signalling pathways. These pathways, including the Ras/MAPK and Akt/mTOR kinase cascades, in turn, regulate transcription factors and other proteins involved in cell proliferation, survival, motility and differentiation 1 (Fig. 1). Aberrantly high EGFR activity is common in several tumor types, including NSCLC, and correlates with a more aggressive disease, resistance to chemotherapy and poor patient prognosis.Monoclonal antibodies to the extracellular domain of EGFR, such as cetuximab, and EGFR tyrosine kinase inhibitors, including gefitinib and erlotinib, specifically block E...

Kahalalide F (KF) is a novel marine-derived antitumor agent that is currently undergoing phase II clinical trials. The mechanism of action of KF is not well understood. In line with previous reports, we show that KF caused rapid and potent cytotoxicity in the breast cancer cell lines SKBR3 and BT474, characterized by cytoplasmic swelling and DNA clumping. Several markers of caspase-dependent apoptosis, such as phosphatidyl-serine externalization, cytochrome c release, and caspase-3 and poly-(ADP-ribose) polymerase cleavage were negative after KF exposure. Inhibitors of caspases or cathepsins failed to protect against KF cytotoxicity. Altogether, these data indicate that KF-induced cell death is a necrosis-like process. The sensitivity to KF in a panel of human tumor cell lines derived from breast (SKBR3, BT474, and MCF7), vulval (A431), non-small-cell lung (H460, A549, SW1573, and H292), and hepatic (Skhep1, HepG2, and Hep3B) carcinomas positively correlated with ErbB3 (HER3) protein levels. A KF-resistant subline of colon carcinoma cells, HT29/KF, expressed significantly reduced levels of all ErbB receptors, but short-term KF exposure of sensitive cell lines such as SKBR3 selectively induced down-regulation of ErbB3. On the other hand, stable transfection of an ErbB3-expressing plasmid increased the KF sensitivity of H460 cells, the most resistant cell line in our panel. Finally, we found that KF efficiently inhibited the phosphatidylinositol 3-kinase (PI3K)-Akt signaling pathway in sensitive cell lines and that ectopic expression of a constitutively active Akt mutant reduced KF cytotoxicity in this cell line. In summary, our results identify ErbB3 and the downstream PI3K-Akt pathway as important determinants of the cytotoxic activity of KF in vitro.

The growth and proliferation of cells are usually tightly regulated processes that are activated by stimuli from their environment. Epidermal growth factor (EGF)-related peptides represent a class of molecules that can trigger cell proliferation, among several cellular processes, such as differentiation, migration, and survival. Binding of EGF-like peptides to the EGF receptor (EGFR) at the cell surface leads to a cascade of intracellular reactions that transduce signals to the nucleus, resulting in particular gene expression patterns. However, in many tumor cells, the regulation of EGFR activity is lost, due to increased or aberrant expression of the receptor or its ligands, and this contributes to many processes important for tumor growth, including cell proliferation, survival, angiogenesis, invasion, and metastasis. Many strategies have been developed that specifically target the EGFR and inhibit its activity. Of these, small-molecule tyrosine kinase inhibitors represent one of the most promising classes of anticancer agents. Here, we describe the status of small-molecule EGFR tyrosine kinase inhibitors in preclinical and clinical development.

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