Strategies to Enhance Epidermal Growth Factor Inhibition: Targeting the Mevalonate Pathway JimDimitroulakos, 1,3 IanA.Lorimer, 1,3 andGlenwoodGoss 2,3 Abstract Mevalonate metabolites play an essential role in transducing epidermal growth factor (EGF) receptor(EGFR)^mediatedsignaling,asseveralofthesemetabolitesarerequiredforthefunction of this receptor and the components of its signaling cascades.Thus, the depletion of mevalonate metabolites may have a significant effect on EGFR function. Lovastatin is a specific and potent inhibitor of 3-hydroxy-3-methylglutaryl CoA reductase, the rate-limiting enzyme of the mevalo- nate pathway.Targeting 3-hydroxy-3-methylglutaryl CoA reductase using lovastatin induces a potenttumor-specificapoptoticresponseinavarietyoftumortypesattherapeuticallyachievable levels of this drug.The effects of lovastatin on EGFR function and the potential combination effects with EGFR tyrosine kinase inhibitors, such as gefitinib, were evaluated. Lovastatin treat- ment inhibited EGF-induced EGFR autophosphorylation and its downstream signaling cascades by 24 hours. Combining lovastatin and gefitinib showed enhanced inhibition and cooperative cytotoxicity in a variety of cell lines that included all eight squamous cell carcinomas, four non ^ small cell lung carcinoma, and four colon carcinoma cell lines tested. Isobologram analyses confirmed that this combination was synergistic, inducing a potent apoptotic response. A phase Istudyhasshownthesafetyandpotentialclinicalbenefitofhigh-doselovastatininpatientswith recurrent squamous cell carcinoma.The use of lovastatin, which is metabolized by CYP3A4, is contraindicatedwithdrugs,suchasgefitinibanderlotinib,whicharealsometabolizedbyCYP3A4 due to greatly enhanced toxicity. Rosuvastatin, a relatively novel potent mevalonate pathway inhibitor thatis not metabolized significantly by CYP3A4, is a more appropriate statintocombine with either erlotinib or gefitinib.The combination of erlotinib and rosuvastatinhas been proposed foraphaseI/IIstudyinadvancednon^smallcelllungcarcinoma. The ErbB family of receptor proteins [ErbB1/epidermal growth factor (EGF) receptor (EGFR), ErbB2, ErbB3, and ErbB4] plays a key role in the growth, differentiation, migration, and cell survival of epithelial tissues (1, 2). These cell membrane receptors are functionally divided into three regions: an extracellular ligand-binding region, an intracellular region with tyrosine kinase activity and regulatory functions, and a region that spans the cell membrane and anchors the receptor to the cell (3). In the inactive state, each ErbB receptor exists as a monomer. Despite their high degree of structural homology, the ErbB receptors differ in their ligand specificities. EGFR, ErbB3, and ErbB4 are activated by a large family of ligands, with each receptor having differences in its ligand specificity (4). ErbB2 has no ligand but preexists in a conformation that allows it to heterodimerize with other ErbB family members (4). These ErbB2-containing heterodimers form the highest affinity binding sites for their respective ligands (4). Ligand binding to the EGFR promotes either homodimerization (EGFR/EGFR) or heterodimerization (EGFR/ErbB2), activating (via autophos- phorylation of specific residues) the receptor tyrosine kinase (5). This biochemical trigger starts a series of downstream signaling cascades that regulates the effects of these receptors on cell proliferation and cell survival. These effects are mediated by a complex series of signaling mechanisms, such as engagement of the mitogen-activated protein kinase and phosphatidylino- sitol 3-kinase pathways (1, 6). The EGFR as aTherapeuticTarget As stated, the EGFR is a key regulator of growth, differenti- ation, and survival of epithelial cells, but it is also involved in the development and progression of cancers derived from these tissues (1, 6). In malignant cells, this receptor and its downstream signaling pathways often are deregulated, leading to cell hyperproliferation, enhanced cell survival, and increased metastatic potential (1, 6). High EGFR expression has been associated with advanced tumor stage, with resistance to standard therapies (chemotherapy and radiation) and, in some tumors, with poor patient prognosis (7). Consequently, EGFR has been proposed as a rational target for antitumor strategies. More than 200 studies have analyzed relapse-free-interval or Authors’ Affiliations: 1 Centre for CancerTherapeutics and 2 Medical Oncology, Ottawa Hospital Regional Cancer Centre and Ottawa Health Research Institute; and 3 FacultyofMedicine,UniversityofOttawa,Ottawa,Ontario,Canada Received1/13/06;accepted4/19/06. Grant support: Canadian Institute of Health Research, Cancer Research Society, andCanadianCancerSociety. Presented at the Third Cambridge Conference on Novel Agents in theTreatment of Lung Cancer: Advances in EGFR-Targeted Agents, September 23-24, 2005 in Cambridge,Massachusetts. Requests for reprints: JimDimitroulakos,CentreforCancerTherapeutics,Ottawa Regional Cancer Centre, 503 Smyth Road,Third Floor, Ottawa, Ontario, Canada K1H1C4. Phone: 613-737-7700, ext. 70335; Fax: 613-247-3524; E-mail: jdimitroulakos@ohri.ca. F 2006AmericanAssociationforCancerResearch. doi:10.1158/1078-0432.CCR-06-0089 www.aacrjournals.org Clin Cancer Res 2006;12(14 Suppl) July 15, 2006 4426s Downloaded from http://aacrjournals.org/clincancerres/article-pdf/12/14/4426s/1965465/4426s.pdf by guest on 12 June 2022