Monoclonal antibodies (mAbs) to ErbB-2/HER2 or to its sibling, the epidermal growth factor receptor (EGFR), prolong survival of cancer patients, especially when combined with cytotoxic therapies. However, low effectiveness of therapeutic mAbs and the evolution of patient resistance call for improvements. Here we test in animals pairs of anti-ErbB-2 mAbs and report that pairs comprising an antibody reactive with the dimerization site of ErbB-2 and an antibody recognizing another distinct epitope better inhibit ErbB-2-overexpressing tumors than other pairs or the respective individual mAbs. Because the superiority of antibody combinations extends to tumor cell cultures, we assume that nonimmunological mechanisms contribute to mAb synergy. One potential mechanism, namely the ability of mAb combinations to instigate ErbB-2 endocytosis, is demonstrated. Translation of these lessons to clinical applications may enhance patient response and delay acquisition of resistance.cancer ͉ growth factor ͉ immunotherapy ͉ signal transduction ͉ tyrosine kinase E rbB-2/HER2 is a member of the epidermal growth factor receptor (EGFR) family. When transactivated, ErbB-2/ HER2 stimulates several downstream signaling cascades, including the mitogen-activated protein kinase cascade (1). This ligand-less receptor is moderately expressed in normal adult tissues, where it regulates cell growth and differentiation. By contrast, amplification of the corresponding gene and consequent overexpression of the HER2/ErbB-2 protein have been reported in 20-30% of tumors of the breast (2-4) and ovary (4). In general, erbB-2 gene amplification associates with enhanced metastatic potential and poor prognosis. Because ErbB-2 is expressed at relatively low levels in normal tissues, it makes an attractive target for immunotherapy. This was originally demonstrated in animals by Greene et al. (5), who targeted Neu, the rodent form of ErbB-2, and later developed this into a widely used clinical strategy (6). The molecular mechanisms underlying the growth-inhibitory effects of anti-ErbB-2 monoclonal antibodies (mAbs) may involve indirect tumor cell cytotoxicity through immunological mechanisms, including antibodydependent cellular cytotoxicity (ADCC), complementdependent cytotoxicity (CDC), increased cancer cell apoptosis, as well as direct interference with signaling cascades (6).Clinical studies established that Trastuzumab (Herceptin), a humanized mAb directed against ErbB-2, is active against ErbB-2-overexpressing metastatic breast cancer, leading to its approval for clinical use (7). The objective response rates to Trastuzumab monotherapy is relatively low (Ϸ15%) and short lived (median duration, 9 months) (8). On the other hand, mAbs seem to display a synergistic effect when combined with chemotherapy, probably because of interruption of ErbB-2-driven survival pathway (9). Yet another strategy, relevant to pancreatic cancer, combines antibodies to EGFR and to ErbB-2 (10). The present study explores an alternative strategy to enhance the therapeutic acti...

Two isomers of methanocarba (MC) thymidine (T), one an effective antiherpes agent with the pseudosugar moiety locked in the North (N) hemisphere of the pseudorotational cycle (1a, N-MCT) and the other an inactive isomer locked in the antipodean South (S) conformation (1b, S-MCT) were used to determine whether kinases and polymerases discriminate between their substrates on the basis of sugar conformation. A combined solid-state and solution conformational analysis of both compounds, coupled with the direct measurement of mono-, di-, and triphosphate levels in control cells, cells infected with the Herpes simplex virus, or cells transfected with the corresponding viral kinase gene (HSV-tk), suggests that kinases prefer substrates that adopt the S sugar conformation. On the other hand, the cellular DNA polymerase(s) of a murine tumor cell line transfected with HSV-tk incorporated almost exclusively the triphosphate of the locked N conformer (N-MCTTP), notwithstanding the presence of higher triphosphate levels of the S-conformer (S-MCTTP).

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