The Biology Behind Combining Cytotoxics and 17-Allylamino, 17-Demethoxygeldanamycin: Sequence and Tumor Biology Matters

A surprising outcome of the search for agents that might target signaling systems that drive cancer cell growth has been the empirical observation that many of these drugs appear to potentiate the effect of conventional therapeutic agents. For example, herceptin, in both preclinical (1) and clinical (2) circumstances, potentiates DNA-directed and microtubule-directed drugs. Certain anti-epidermal growth factor-receptor-directed monoclonal antibodies behave similarly (3). This phenomenon is observed with a variety of other “small molecule” protein kinase antagonists, including flavopiridol (4), UCN-01 (5), and Iressa (6), as well as other classes of agents targeting cell signaling functions, including proteosome inhibitors (7). The implications of these findings are that, in addition to being assessed for activity in their own right, strategies for development of these agents might reasonably include efforts to detect useful augmentation of chemotherapy. Despite this favorably expanded menu of development possibilities, this outcome actually creates a number of complications. First, except for a few noteworthy and admittedly incompletely understood examples (e.g., the capacity of UCN-01 to inhibit chk1, one regulator of the DNA damage checkpoint; Refs. 8 and 9), we do not understand the basis underlying the signaling molecules’ chemopotentiating effect. Although it is true in general terms that the pathways affected by the inhibitors feed into the regulation of cell survival pathways, e.g., through the activation of phosphatidylinositol 3 -kinase and akt-mediated phosphorylation of bad (10) or the down-regulation of cell survival gene expression (11), these general modifiers of the cell-death response lead to no clear roadmap for exploiting these development possibilities. The “subtargets” relevant to the signaling agents acting as potentiators of the chemotherapy effect may be inconsistently related to the activities of the agent on its “primary” target. The economics of “filling in the boxes” by testing all signaling agents versus all drugs in all tumors is overtly prohibitive. Second, certain aspects of the signaling agent action may lead to cell-cycle arrest, and, thus, theoretically antagonize the action of agents that may require some level of proliferative activity to maximally elicit cytotoxicity. In this issue of Clinical Cancer Research, Münster et al. (12) provide several important experiments that begin to clarify these issues with respect to 17-allyl-amino 17-demethoxygeldanamycin (17AAG; NSC330507). This agent entered into clinical trials sponsored by the National Cancer Institute, Rockville, MD, with an intent to clarify the potential of benzoquinoid ansamycins to act as useful antitumor agents. The prototypic compounds in the series, herbimycin and geldanamycin, were found in the 1980s to be agents that reversed the transformed phenotype of cells driven by v-src family members (13), and for a time herbimycin was considered a tyrosine kinase inhibitor, until several laboratories clarified that herbimycin and geldanamycin did not directly inhibit src family kinases but, apparently, caused their accelerated turnover in drug-treated cells, with actual decreased mass of a variety of tyrosine kinases including src, lck, erbB1, and erbB2, among others (14–17). A unifying mechanism that explained these results was provided by Whitesell et al. (18), who identified that derivatized geldanamycin analogues bound to the ubiquitously expressed the cellular chaperone molecule hsp90. The heat shock proteins include a variety of members that, as the name implies, are modulated in their expression not only by heat but by nutrient deprivation, oxidative, and other stressful circumstances where protein denaturation might occur (19). A unifying biochemical activity on the part of hsp90 is catalyzing the proper folding and maturation of a number of protein substrates, including many members of the tyrosine kinase family of cell signaling molecules (Fig. 1A). Without proper hsp90 function, the abnormal conformations of these hsp90 partners are ubiquitinated and targeted for proteosomal degradation. The benzoquinoid ansamycins, including herbimycin, geldanamycin, and 17AAG, bind to hsp90 and cause the displacement and degradation of the client proteins (20, 21). Indeed, elegant structural studies (22) have confirmed that the benzoquinoid ansamycins bind to the NH2-terminal domain of hsp90, and these studies provided an additional basis for constructing derivatives at the 17 position. An additional set of functions for hsp90 is illustrated in Fig. 1B, and emphasizes a distinct role as a “docking station” in the cytoplasm for a variety of important