Target Validation in Drug Discovery

Brian W. Metcalf, Susan Dillon

• Publisher: Academic Press
• Year: 2011
• eBook ISBN: 9780080465975
• Topic: Medicine
• Subtopic: Pharmacology

This work presents a comprehensive contemporary framework for approaching target validation in drug discovery. It begins with a detailed description of new enabling technologies, including aptamers, RNA interference, functional genomics, and proteomics. The next section looks at biologic drug development with in-depth discussion of lessons learned from such well-known cases as Erbitux, Herceptin, and Avastin. Additional targets known as "second generation" drugs, which can be identified when disease pathways are validated by biologics, present new possible small molecule therapeutics and serve as the focus of the final section of the book.

III

Validating targets of small molecule approaches

Outline

Chapter 9: EPIDERMAL GROWTH FACTOR RECEPTOR (EGFR) INHIBITOR FOR ONCOLOGY: DISCOVERY AND DEVELOPMENT OF ERLOTINIB

Chapter 10: PROGRESS IN ACHIEVING PROOF OF CONCEPT FOR p38 KINASE INHIBITORS

Chapter 11: IKK-2/NF-κB-DEPENDENT TRANSCRIPTION

Chapter 12: TNF SIGNALING PATHWAY INHIBITORS FOR INFLAMMATION-CCR2 ANTAGONISTS

Chapter 13: THE DISCOVERY OF ELTROMBOPAG, AN ORALLY BIOAVAILABLE TpoR AGONIST

Chapter 14: ORALLY BIOAVAILABLE GLYCOPROTEIN IIB/IIIA ANTAGONISTS: A NEGATIVE CASE STUDY

9

EPIDERMAL GROWTH FACTOR RECEPTOR (EGFR) INHIBITOR FOR ONCOLOGY: DISCOVERY AND DEVELOPMENT OF ERLOTINIB

KENNETH K. IWATA, Ph.D.^*, SHANNON E. BEARD, Ph.D.^† and JOHN D. HALEY^*, ^*OSI Pharmaceuticals, Farmingdale, New York; ^†OSI Pharmaceuticals, Melville, New York

Molecular-targeted drug therapies are based upon our understanding of tumor-cell biology to identify oncogenic targets and develop therapeutic approaches to these targets using small-molecular-weight inhibitors, antibodies, and antisense. Unlike traditional chemotherapeutics, which rely upon differential toxicities to tumor cells versus normal cells, molecular-targeted therapy approaches identify key molecular drivers of tumorigenicity and develop drug therapies against the target. The goal of molecular-targeted therapies is to develop agents with better efficacy and tolerability than current chemotherapeutic drugs. During the past two decades, the increased understanding of the involvement of growth factors, receptors, and tyrosine kinases in tumor biology has made these oncogenes the focus of new, targeted cancer-drug-discovery efforts. The extensive literature on epidermal growth factor receptor (EGFR) and its ligands in tumor cell lines and clinical specimens has made EGFR an important molecular target for drug discovery. The focus of this chapter is to provide a background on EGFR as a cancer target, the development of erlotinib (a potent selective inhibitor of EGFR tyrosine kinase), and preclinical data supporting erlotinib’s advancement into the clinic. The concluding section will present some of the issues faced by molecular-targeted therapies such as erlotinib. These will include the role of biomarkers in the identification of patients who might best respond to treatment and efforts to optimize the use of molecular-targeted therapies.

I INTRODUCTION

Tyrosine kinases are among the earliest molecular targets identified for cancer-drug discovery (Gschwind et al., 2004). Determination that the DNA sequence for a virally associated oncogene v-sis has a proto-oncogene homologue c-sis, which encodes for the growth factor PDGFRβ (reviewed Heldin and Westermark, 1990), provided evidence that growth factors and growth factor receptors can drive tumorigenesis. The observation that growth factors, growth factor receptors, and kinases play a major role in oncogenesis, along with advances in molecular biology and automated high-throughput drug-screening technologies, has revolutionized approaches to small-molecule cancer-drug discovery resulting in the emergence of molecular targeted therapeutics. Unlike traditional chemotherapeutic drug discovery, which relies upon differential toxicities of normal cells versus tumor cells, targeted drug discovery identifies key molecular mediators of tumorigenicity against which small-molecule inhibitors are developed to generate cytostatic and/or cytotoxic effects on tumors with minimal negative effects on normal tissue and functions. Targeted approaches to anti-cancer therapy have the potential to further reduce toxicity and provide a better quality of life for the patient. EGFR and its ligands have long been observed to be elevated in many tumor cell lines (Arteaga, 2001; Salomon et al., 1995) and in tumor biopsy specimens and associated with poor clinical prognosis (Table 9.1). These observations made EGFR an ideal early target for molecularly targeted drug discovery.

TABLE 9.1

EGFR Over-Expression and Clinical Prognosis

Tumor Type	Cases/Year^a	Percent^b^,^c Over-expression	Related to Poor Prognosis	References
Prostate	232,090	40–70	Yes	Kim et al. 2001
				Scher et al. 1995
Breast	212,930	35–70	Yes	Nicholson et al. 2001
NSCLC	172,570	50–90	Yes	Hirsch et al. 2003
				Selvaggi et al. 2004
Colon	104,950	45–80	Yes	Resnik et al. 2004
				Nicholson et al. 2001
				O’Dwyer et al. 2002
Bladder	63,210	30–90	Yes	Neal et al. 1990
Pancreatic	32,180	30–50	Yes	Xiong et al. 2002
				Papouchado et al. 2005
Head & Neck	29,370	70–100	Yes	Pomerantz and Grandis 2004
Ovarian	22,220	35–60	Yes	Scambia et al. 1992

^aAmerican Cancer Society 2005.

^bCiardiello, F. et al. Expert Opin. Investig. Drugs (2002) 11: 755–768.

^cSalomon, D. S. et al. Crit. Rev. Oncol. Hematol. (1995) 19: 183–232.

II EPIDERMAL GROWTH FACTOR RECEPTOR AND LIGANDS

Epidermal growth factor (EGF) is a highly conserved small molecular weight 6000 Dalton polypeptide that was first identified and purified by Stanley Cohen (reviewed Carpenter and Cohen, 1990).

EGFR is a 170-kDa transmembrane receptor tyrosine kinase. The EGFR gene encodes a 1186 amino acid polypeptide that is highly glycosylated in the extracellular ligand binding portion of the receptor. The extracellular domain consists of 621 amino acids including two cysteine-rich domains, to which N-linked carbohydrate is attached. The transmembrane domain contains 23 amino acids and the cytoplasmic domain, containing intrinsic tyrosine kinase activity, comprises of 542 amino acids.

Seven ligands for EGFR—EGF, TGFa, amphiregulin, betacellulin, epigen, epiregulin and heparin binding EGF-like growth factor (HB-EGF)—have been identified (Burgess et al., 2003). Crystallographic data suggest that the extracellular regions of EGFR function as a negative regulators of intrinsic kinase activity, which is relieved by interaction with ligand (Burgess et al., 2003, Ferguson et al., 2003, Schlessinger 2002). Binding of EGFR with ligand results in a conformational change, followed by receptor dimerization and activation of the tyrosine kinase domain. As shown in Figure 9.1, upon receptor activation, the EGFR transphosphorylates the tyrosine residues on the adjacent EGFR dimer.

FIGURE 9.1 A model of EGFR activation. The left figure shows EGFR with the extracellular region of EGFR functioning as negative regulator of activity. The middle figure shows EGF bound to EGFR relieving the negative interaction resulting in EGFR dimerization and receptor tyrosine kinase activation. The right figure shows some of the activities resulting from activated EGFR.

More than 12 tyrosines in the cytoplasmic domain of EGFR have been shown to be ...

Table of contents

• Cover image
• Title page
• Table of Contents
• PREFACE
• CONTRIBUTORS
• I: Pharmaceutical biotechnology for target validation
• II: Target validation for biopharmaceutical drug discovery
• III: Validating targets of small molecule approaches
• INDEX