PMCCPMCCPMCC

Search tips
Search criteria 

Advanced

 
Logo of nihpaAbout Author manuscriptsSubmit a manuscriptHHS Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
 
Curr Opin Investig Drugs. Author manuscript; available in PMC 2011 February 1.
Published in final edited form as:
Curr Opin Investig Drugs. 2008 December; 9(12): 1264–1276.
PMCID: PMC3031872
NIHMSID: NIHMS266038

HER2 directed small molecule antagonists

Abstract

Inactivation of the HER2 tyrosine kinase holds significant promise as a cancer treatment hypothesis, making it a high value target for drug discovery. Screening and structure-based efforts have led to the development of several classes of ATP analog inhibitors of HER2 tyrosine kinase. These efforts have been further enhanced by detailed structural information regarding its sibling kinase EGFR and structural properties that can be exploited to confer activity and even selectivity towards HER2 kinase. Signaling and structural studies also suggest the critical involvement of the kinase inactive HER3 in the regulation of HER2 creating unique challenges in the efforts to inactivate HER2.

Key terms: ErbB2, HER2, EGFR, quinazoline, breast cancer, tyrosine kinase inhibitor

Introduction

The ErbB proteins are a four-member family of highly homologous receptor tyrosine kinases comprised of ErbB1 (EGFR, HER1), ErbB2 (HER2), ErbB3 (HER3), and ErbB4 (HER4). These proteins consist of a ligand-binding extracellular domain (ECD), a transmembrane domain, an intracellular tyrosine kinase (TK) domain, and a c-terminal signaling tail. An intracellular signal is generated through receptor dimerization and transphosphorylation of their c-terminal tails [1] (Figure 1). The differentiation of this family from a primordial ErbB gene has been associated with functional complimentarity and a necessity for cooperative activity in some of its members. Such cooperativity is exemplified by HER2 and HER3. HER2 has evolved into a catalytic driver, with robust kinase activity but no ligand-binding ability and little ability for self-regulation. On the other hand, HER3 has no significant kinase activity but is an optimal dimerization and regulatory partner for HER2 [2,3]. In fact, in the presence of ligand stimulation, the HER2-HER3 heterodimer is the most active signaling unit in this family [4]. EGFR, on the other hand, has maintained its bifunctional attributes and performs equally well as a catalytic or ligand-activated regulatory partner [4] [5].

Figure 1
Schematic representation of the HER2/3 heterodimer.

The reduced ability of HER2 to self-regulate suggests a potent oncogenic potential, and indeed overexpression of HER2 is seen in a number of human cancers, mostly breast cancers. The etiologic role of HER2 in tumorigenesis has been extensively studied in mouse transgenic models, confirming unequivocally the potent tranforming potential of the mouse HER2 homolog Neu when overexpressed or overactive [6]. The driving role of HER2 in tumorigenesis and the large number of cancer patients affected by this cancer subtype have made HER2 a high priority target for drug development for the past two decades.

Initial attempts to target HER2 in the 1980s focused on the development of monoclonal antibodies to interfere with functions residing within its ECD. These efforts have produced clinically active drugs, but they do not appear to effectively inactivate HER2 signaling and the molecular basis for their clinical activities remains undefined (reviewed in [7]). Furthermore, the HER2 ECD is redundant for its oncogenic function and is often proteolytically cleaved in tumors, indicating a potential limitation of ECD-targeting approaches [8,9]. However, the HER2 TK domain is essential for its transforming function [10] and targeting the catalytic TK function of HER2 presents the most compelling approach for the development of highly effective anti-cancer drugs.

Numerous HER-family selective TK inhibitors (TKIs) have been synthesized over the past decade and are listed in Table 1. The ongoing evolution of these TKIs started with investigations of EGFR, followed by pan-HER family inhibitors, followed only recently by HER-2 selective compounds. There is still much to be learned about the structure-activity relationships for these new HER-2 selective drugs. This review focuses on the key structural classes and discovery strategies for the HER family kinase inhibitors; a detailed analysis of preclinical models or clinical activity is outside the current scope. For clinical evaluation, the reader is directed to the references in Table 1 and to other recent reviews [1114].

Table 1
Selected HER family tyrosine kinase inhibitors

Structure and function of the TK catalytic site

The kinase domains of HER1, 2, and 4 are structurally similar to other kinases [15]. As shown schematically in Figure 2, the kinase domains contain an N-lobe comprised mostly of anti-parallel B-strands and a C-lobe comprised mostly of alpha-helices. The active site sits in the cleft between the N- and C-lobes, called the hinge region. Common features of the kinase active site include an ATP-binding pocket which is homologous among kinases, a more variable substrate binding site, and two regulatory regions called the Activation loop (located on the C-lobe) and the C-Helix (on the N-lobe). In the inactive conformation of the kinase domain, the C-helix, containing a catalytic glutamate residue, is pointed away from the active site. In addition, the Activation loop occludes the substrate binding site. Upon activation of the kinase, the C-helix rotates ~90 degrees to position the glutamate residue, and the Activation loop extends away from the C-helix, thereby exposing the substrate binding site. The small-molecule inhibitors described in this review contain a heterocyclic core that mimics the shape and hydrogen-bonding of ATP (Figure 3). Most TKIs bind to the active conformation, though there are therapeutically important examples of kinase inhibitors that bind to the inactive conformation [16,17] and/or gain selectivity through contacts with the substrate binding site [18].

Figure 2
Schematic representation of HER kinase domain conformations. The kinase domain contains a beta-strand rich N-lobe and a helix-rich C-lobe connected by a hinge region. The ATP and peptide substrates bind in the cleft between the two domains, and ATP forms ...
Figure 3
Hydrogen bonding between a nonhydrolyzable ATP analog and the EGFR kinase domain. The adenosine ring of ATP (cyan) forms two hydrogen bonds with the backbone of the hinge region of EGFR kinase domain (green). Tyrosine kinase inhibitors contain an ATP-mimetic ...

Selective inhibitors of the HER kinase family

The effort to identify small molecule inhibitors of HER family kinases began in the early 1990s with the identification of natural compounds, such as erbstatin, with activity against HER kinases. One of the first classes of synthetic compounds, called “tyrphostins,” was based on the structure of erbstatin and was designed to compete with the tyrosine substrate [19]. Synthesis of hundreds of these benzylidene malononitrile compounds yielded micromolar inhibitors with relative selectivity for HER kinases, including EGFR and HER2. Further studies identified compounds that even showed selectivity between EGFR and HER2 in vitro [20]. This is despite 80% homology in the kinase domains of EGFR and HER2. These EGFR and HER2 selective compounds led to the first observation that compounds showing EGFR or HER2 selectivity in several different in vitro assays did not appear to show such selectivity in cell based assays [20]. This paradoxical finding has been reproduced with all subsequent generations of HER TKIs (see below). Ultimately this class did not yield compounds with the potency or selectivity suitable for clinical development.

The field was revolutionized in the mid 1990s with the identification of a new generation of potent and selective classes of compounds. The best described of these classes are the 4-anilino quinazolines (Figure 4), which were simultaneously reported by Zeneca Pharmaceuticals and Parke-Davis Pharmaceuticals. Enzymological studies of the EGFR kinase suggested a ternary complex intermediate, in which ATP and the protein substrate bound simultaneously to the kinase, and in which the ATP γ-phosphate, tyrosyl hydroxyl, and the tyrosyl aromatic ring all interacted with the protein during catalysis [21]. Querying a three-dimensional structure database for compounds that mimic these three interactions identified 4-anilino-quinazolines as low nanomolar, ATP-competitive inhibitors of EGFR kinase [21]. Interestingly, while the aniline group was intended to mimic tyrosine, these compounds are noncompetitive with peptide substrate. High-throughput screening for inhibitors of EGFR kinase also identified 4-substituted quinazolines as highly potent and selective inhibitors of EGFR kinase [22]. Strategic substitutions of these bicyclic compounds increased potency to the picomolar range while maintaining selectivity [23]. A number of 4-anilinoquinazolines have been developed for clinical use including gefitinib [24], erlotinib [25], and lapatinib [26,27] (see Table 1).

Figure 4
Selected HER TKI compounds. Core structures for each compound class are shown in black; functional groups that vary between compounds within a class are shown in gray.

The structure-activity relationship between 4-anilinoquinazolines and HER kinases has been described (eg [28]). The quinazoline bicycle binds in the ATP binding site; N1 hydrogen bonds to the main chain NH of methionine in the hinge region, and N3 forms a water mediated hydrogen bond with the side chain of threonine 766 (in the active conformation of EGFR, see below) [29]. The 4-anilino group nestles in a hydrophobic pocket behind the ATP site, and substitutions on this ring play a significant role in kinase selectivity. Early studies suggested that small, hydrophobic substitutions at the 3 position increased affinity for EGFR [23,28], but large substitutions are also tolerated and are correlated with increased affinity for HER-2 [17,27,30,31]. The HER kinases prefer electron-rich substituents at the 6 and 7 position of the quinazoline ring, and ether substitutions are often found at these positions [28]. However, the SAR is quite flexible at this edge of the quinazoline, and these are common sites for manipulating the compound’s physical chemical properties and, ultimately, their activity in vivo (e.g. [32]).

The structural features of quinazoline binding to the EGFR kinase domain have been determined thus far for erlotinib [29], gefitinib [33], and lapatinib [17]. These compounds inhibit EGFR similarly, with IC50 values of 27 nM, 2 nM, and 11 nM for erlotinib, gefitinib, and lapatinib, respectfully [25] [24] [27]. In all three structures, the anilino-quinazolines bind at the ATP site, with N1 of the quinazoline bonding with the backbone carbonyl of a methionine residue in the hinge (Figure 3, ,4).4). As predicted [34], N3 forms a water-mediated hydrogen bond to a threonine side-chain, and the anilino group binds within a hydrophobic pocket [29]. The structures in complex with erlotinib and gefinitib show the kinase in the active conformation [29,33]. By contrast, the structure in complex with lapatinib shows EGFR kinase in the inactive conformation [17]. The bulky anilino substituent of lapatinib reaches deep into a back-pocket that is seen only in the inactive conformation (Figure 5). The compound appears enclosed by the protein, and the c-terminal tail of EGFR blocks the opening of the inhibitor binding site. As such, dissociation of lapatinib from EGFR likely requires a conformational change in the kinase; consistent with this prediction, lapatinib has a markedly slow off-rate in vitro and shows long-lived suppression of EGFR autophosphorylation in cells after washout [17].

Figure 5
X-ray structures of EGFR bound to erlotinib (left; [29]) and lapatinib (right; [17]). The erlotinib-bound protein adopts the active conformation, with a wide cleft between the N- and C-lobes (blue and green, respectively), and the activation loop (orange) ...

In addition to the quinazolines, at least four other bicyclic compound classes have been identified as potent and selective inhibitors of HER kinase (Table 1, Figure 4). Though there is less published information on the chemical development of these classes compared to quinazoline, they appear to follow similar structure-activity relationships and to bind to EGFR analogously to the quinazolines. Pyridopyrimidines [35] and pyrrolopyrimidines [36] were both reported in the mid-1990s. Novartis has advanced the pyrrolopyrimide AEE-788 to clinical trials; this compound is described as an EGFR/VEGFR dual family inhibitor [37]. The crystal structure of AEE-788 bound to EGFR, shown in Figure 6, indicates that it binds analogously to gefitinib and erlotinib [33]. More recently, compounds with a pyrrolotriazine core have also been described; BMS-599626 is a clinical-stage example of this class [38]. Finally, expanding on the idea that the N3 of quinazoline makes a water-mediated hydrogen bond with EGFR kinase, investigators at Wyeth-Ayerst Research replaced this nitrogen with a nitrile group that could hydrogen-bond directly to the threonine side chain [34]. These cyanoquinolines have been developed as covalent inhibitors of HER kinases (see below), and the most advanced compound, HKI-272, is currently in clinical trials [39,40].

Figure 6
X-ray structures comparing the orientation of compounds that bind to the active (left) and inactive (right) conformations of EGFR kinase domain. Left: The pyrrolopyrimidine core of AEE788 (pink) overlays closely with the quinazoline core of gefitinib ...

Irreversible inhibitors

Irreversible inhibitors offer greater potency and durability of target inhibition compared to their reversible counterparts, and may also show a different pattern of disease resistance [41]. Investigators at Parke-Davis generated irreversible inhibitors by adding an alkylating group to the 6- or 7- position of quinazoline-based compounds [42]. These compounds were found to permanently and selectively inactivate HER kinases by covalently binding to a cysteine residue within the ATP pocket [42]. Solubilizing side chain modifications at the 7 position of the quinazoline yielded orally bioavailable compounds such as PD183805 (CI-1033) and CL-387,785 [43,44]. These irreversible inhibitors showed highly promising anti-tumor activity in mouse xenograft models, and potent and prolonged inactivation of their HER targets, promoting them into clinical development. Aveo Pharmaceuticals and Mitsubishi have an a related compound, MP-412 (AV-412) in early clinical development [45]. Analogously, alkylating substitutions at the 6 position in the pyridopyrimidine [46] and cyanoquinoline [34,40] scaffolds have yielded more potent and irreversible inhibitors of EGFR and HER2 without compromising selectivity. EKB-569 and HKI-272, from Wyeth-Ayerst, are irreversible cyanoquinolines currently in clinical development [30,39,41,47,48]. The recently published structure of HKI-272 bound to EGFR (Figure 6) shows the kinase in the inactive conformation, analogous to the structure of EGFR bound to lapatinib [17,49]. The structure suggests that binding to the inactive conformation might be a general feature of compounds with a bulky group on the aniline; furthermore, the structure indicates that alkylation of the cysteine forces modest changes in the orientation of the compound in the active site. Although the potential for nonselectivity raises safety concerns with the clinical use of irreversible inhibitors, such TKIs have thus far shown acceptable toxicity profiles in clinical studies [47,50,51]. The irreversible inhibitors have activities in clinical studies that are more modest compared to preclinical models, similar to many other classes of drugs.

Inhibitors selective for HER2

Although nearly all of the compounds listed in Table 1 inhibit EGFR and HER2 kinase activity, most of them favor EGFR over HER2. Designing HER2 selective inhibitors is made more challenging by the absence of a crystal structure of the HER2 kinase domain. Investigators at GlaxoSmithKline pursued a dual screening program to identify compounds equally active against EGFR and HER2 [27]. They found that addition of a bulky substituent, such as benzyl ether, to the 3-position of aniline increases potency against HER2 while maintaining activity against EGFR [27]. One such quinazoline derivative GW572016 (lapatinib) has been clinically developed for the treatment of HER2-amplified breast cancer [26]. In fact, addition of a bulky substituent consistently increases activity towards HER2. For instance, evolution of EKB-569 in the cyanoquinoline series yielded HKI-272 and HKI-357, which are equipotent for EGFR and HER2 and are currently in clinical testing [30]. Additionally, HER-2 activity in the pyrrolopyrimidine series is associated with addition of phenethylamine to an analogous exocyclic amine. While it is not yet known why bulky substitutions on the aniline increase binding to HER-2 kinase, it is noteworthy that the structures of lapatinib and HKI-272 both show EGFR in the inactive conformation [17,49]. It is possible that HER-2 prefers to bind inhibitors in the inactive over the active conformation, or that the HER-2 binding site has a larger hydrophobic pocket even in the active conformation. Detailed analysis awaits determination of the HER2 kinase domain structure.

Common toxicities of HER TKIs include skin rashes and diarrhea which could be mediated through EGFR [52]. Pfizer and OSI therefore pursued inhibitors of HER2 that were inactive against EGFR [31,53]. As with the GlaxoSmithKline and Wyeth-Ayerst studies, these investigators found that bulky substitutions at the 4-anilino position afforded HER2-selective inhibitors, including CP-654577 and the clinical candidate CP-724714. In a phase I study of CP-724714 diarrhea was not reported, but skin rashes and liver toxicities were observed [54]. So far, these are the only disclosed compounds that prefer HER2 over EGFR. It is not yet known why CP-724714 shows low activity towards EGFR, though it does lack a hydrogen-bond acceptor at the 7-position, which has previously been found to be important for binding to EGFR. Detailed structural studies such as x-ray crystallography and computational modeling will help to address the selectivity of compounds for HER-2 kinase over EGFR.

HER2 inhibitors in preclinical studies

Since the earliest days of the development of HER family TKIs, the concept of family member selectivity has been complicated by discrepancies between in vitro and cell-based data. Although certain compounds clearly show much higher potency against purified EGFR kinase compared with purified HER2 kinase, these differences are much less apparent in cell based assays. The EGFR-selective quinazoline gefitinib shows equipotent growth inhibitory activity against EGFR-overexpressing and HER2 overexpressing tumor cells [55]. Both EGFR-mediated (EGF-stimulated) and HER2-HER3 mediated (neuregulin-stimulated) signal transduction is inhibited by gefitinib in cells [55]. These non-discriminatory cellular observations have been extensively reproduced with several of the quinazoline HER inhibitors including gefitinib, erlotinib, and AG1478 [5558]. This effect is due to the direct inhibition of HER2 kinase by these compounds, as demonstrated by elegant models established in EGFR-negative cells confirming the direct inhibition of HER2 kinase by erlotinib [58]. Virtually all HER TKIs, regardless of their in vitro selectivities, show growth inhibitory efficacy against HER2-driven tumor cell lines in xenograft models. It remains unclear why selectivity in cells and tumor models is much less than observed in vitro. It is possible that accumulation of compounds in cells raises the intracellular concentration above concentrations in biochemical assays. It is even more likely that purified kinase domains in in vitro reactions do not faithfully reflect the biochemical properties of their cellular counterparts, and protein orientation, position of the carboxy termini, and dimerization events in the cellular context may all be pharmacologically relevant parameters.

HER3; The other half of the HER2 target

HER TKIs are mostly cytostatic in cell culture models. In the specific case of HER2-amplified breast cancers, this falls short of expectations, since these tumors are known to be highly HER2-dependent, and transgene-inducible models show apoptotic tumor cell death when the HER2 transgene expression is withdrawn [59]. The reasons underlying this discrepancy are only now becoming apparent. Recent structural and biological insights into HER2 function suggest that the tumorigenic function of HER2 is critically dependent on its kinase-inactive dimerization partner HER3 [60]. While in the most simplistic model, HER3 can be thought of as merely a substrate for the kinase reaction, the HER2-HER3 interaction appears to be much more intricate. The recent analysis of the EGFR kinase structure reveals a unique mode of activation in this family whereby one partner in the kinase dimer performs a stimulatory function through an asymmetric c-lobe to n-lobe interaction with the other partner [5]. Asymmetric dimerization results in activation of the recipient partner, which in turn performs a catalytic function, transferring phosphates onto the carboxy tail of the stimulatory partner. While only the structure of the EGFR dimer has thus far been described, the highly homologous dimerization interfaces suggest that this mechanism applies to the whole family. As such, it is likely that HER2 and HER3 have evolved to optimize their catalytic and stimulatory partners, suggesting a previously unexplained evolutionary advantage to the loss of catalytic function in HER3.

The power of this highly evolved activation function became apparent when it was discovered that HER family TKIs are much less potent at inactivating the HER2-HER3 signaling complex compared with EGFR or HER2 homo- or hetero-dimeric signaling activities, significantly undermining their anti-tumor effects [61]. Feedback signaling can significantly induce HER3 expression and membrane localization, thereby buffering HER2-HER3 signaling against incomplete inactivation of HER2 catalytic function [61]. Therefore it appears that inactivation of the HER2-HER3 oncogenic signaling complex requires much more potent inhibitors that can completely inactivate HER2 catalytic function. Therefore, the modest clinical anti-tumor activities of current TKIs is entirely consistent with the fact that within their therapeutic index they can only partially inhibit HER2-HER3 signaling. While irreversible inhibitors can completely inactivate catalytic function in cell culture models, their off-target effects may limit their therapeutic indices, and it remains to be determined whether they can be administered to patients in high enough doses to fully inactivate tumor HER2.

Conclusions

Several reversible and irreversible small molecule HER2 inhibitors from structurally distinct classes have been developed and we will know the clinical activities of these classes of compounds within the next few years. But the current structural, biological, and preclinical studies have already provided the necessary hints that there is more to be done to fully reverse the robust tumorigenic powers of HER2. Combination therapies are one avenue that will be pursued in the near future. Allosteric inhibitors of HER2-HER3 transactivation represent another new strategy to target this resilient oncoprotein complex. Antibody-based strategies targeting the extracellular functions of HER2-HER3 signaling also continue to be pursued. The increasing interest in and the broad scope of endeavors to target HER2 reflect an ever increasing understanding of the value of this target in cancer therapeutics.

Acknowledgments

The authors would like to acknowledge Katherine Augustyn for Figure 2 and Daniel Gray for helpful discussion.

References

1. Ferguson KM. Structure-based view of epidermal growth factor receptor regulation. Annu Rev Biophys. 2008;37:353–373. [PMC free article] [PubMed]
2. Berger MB, Mendrola JM, Lemmon MA. ErbB3/HER3 does not homodimerize upon neuregulin binding at the cell surface. FEBS Lett. 2004;569(1–3):332–336. [PubMed]
3. Sierke SL, Cheng K, Kim HH, Koland JG. Biochemical characterization of the protein tyrosine kinase homology domain of the ErbB3 (HER3) receptor protein. Biochemical Journal. 1997;322(Pt 3):757–763. [PubMed]
4. Tzahar E, Waterman H, Chen X, Levkowitz G, Karunagaran D, Lavi S, Ratzkin BJ, Yarden Y. A hierarchical network of interreceptor interactions determines signal transduction by Neu differentiation factor/neuregulin and epidermal growth factor. Molecular and Cellular Biology. 1996;16:5276–5287. [PMC free article] [PubMed]
5. Zhang X, Gureasko J, Shen K, Cole PA, Kuriyan J. An allosteric mechanism for activation of the kinase domain of epidermal growth factor receptor. Cell. 2006;125(6):1137–1149. [PubMed]
6. Ursini-Siegel J, Schade B, Cardiff RD, Muller WJ. Insights from transgenic mouse models of ERBB2-induced breast cancer. Nat Rev Cancer. 2007;7(5):389–397. [PubMed]
7. Moasser MM. Targeting the function of the HER2 oncogene in human cancer therapeutics. Oncogene. 2007 [PMC free article] [PubMed]
8. Egeblad M, Mortensen OH, Jaattela M. Truncated ErbB2 receptor enhances ErbB1 signaling and induces reversible, ERK-independent loss of epithelial morphology. Int J Cancer. 2001;94(2):185–191. [PubMed]
9. Molina MA, Saez R, Ramsey EE, Garcia-Barchino MJ, Rojo F, Evans AJ, Albanell J, Keenan EJ, Lluch A, Garcia-Conde J, Baselga J, et al. NH(2)-terminal truncated HER-2 protein but not full-length receptor is associated with nodal metastasis in human breast cancer. Clin Cancer Res. 2002;8(2):347–353. [PubMed]
10. Weiner DB, Kokai Y, Wada T, Cohen JA, Williams WV, Greene MI. Linkage of tyrosine kinase activity with transforming ability of the p185neu oncoprotein. Oncogene. 1989;4(10):1175–1183. [PubMed]
11. Whenham N, D’Hondt V, Piccart MJ. HER2-positive breast cancer: from trastuzumab to innovatory anti-HER2 strategies. Clin Breast Cancer. 2008;8(1):38–49. [PubMed]
12. Ulhoa-Cintra A, Greenberg L, Geyer CE. The emerging role of lapatinib in HER2-positive breast cancer. Curr Oncol Rep. 2008;10(1):10–17. [PubMed]
13. Nanda R. Targeting the human epidermal growth factor receptor 2 (HER2) in the treatment of breast cancer: recent advances and future directions. Rev Recent Clin Trials. 2007;2(2):111–116. [PubMed]
14. O’Donovan N, Crown J. EGFR and HER-2 antagonists in breast cancer. Anticancer Res. 2007;27(3A):1285–1294. [PubMed]
15. Linggi B, Carpenter G. ErbB receptors: new insights on mechanisms and biology. Trends Cell Biol. 2006;16(12):649–656. [PubMed]
16. Schindler T, Bornmann W, Pellicena P, Miller WT, Clarkson B, Kuriyan J. Structural Mechanism for STI-571 Inhibition of Abelson Tyrosine Kinase. Science. 2000;289(5486):1938–1942. [PubMed]
17. Wood ER, Truesdale AT, McDonald OB, Yuan D, Hassell A, Dickerson SH, Ellis B, Pennisi C, Horne E, Lackey K, Alligood KJ, et al. A unique structure for epidermal growth factor receptor bound to GW572016 (Lapatinib): relationships among protein conformation, inhibitor off-rate, and receptor activity in tumor cells. Cancer Res. 2004;64(18):6652–6659. [PubMed]
18. Nagar B, Bornmann WG, Pellicena P, Schindler T, Veach DR, Miller WT, Clarkson B, Kuriyan J. Crystal Structures of the Kinase Domain of c-Abl in Complex with the Small Molecule Inhibitors PD173955 and Imatinib (STI-571) Cancer Res. 2002;62(15):4236–4243. [PubMed]
19. Gazit A, Osherov N, Posner I, Yaish P, Poradosu E, Gilon C, Levitzki A. Tyrphostins. 2. Heterocyclic and alpha-substituted benzylidenemalononitrile tyrphostins as potent inhibitors of EGF receptor and ErbB2/neu tyrosine kinases. J Med Chem. 1991;34(6):1896–1907. [PubMed]
20. Osherov N, Gazit A, Gilon C, Levitzki A. Selective inhibition of the epidermal growth factor and HER2/neu receptors by tyrphostins. J Biol Chem. 1993;268(15):11134–11142. [PubMed]
21. Ward WH, Cook PN, Slater AM, Davies DH, Holdgate GA, Green LR. Epidermal growth factor receptor tyrosine kinase. Investigation of catalytic mechanism, structure-based searching and discovery of a potent inhibitor. Biochem Pharmacol. 1994;48(4):659–666. [PubMed]
22. Fry DW, Kraker AJ, McMichael A, Ambroso LA, Nelson JM, Leopold WR, Connors RW, Bridges AJ. A specific inhibitor of the epidermal growth factor receptor tyrosine kinase. Science. 1994;265(5175):1093–1095. [PubMed]
23. Rewcastle GW, Denny WA, Bridges AJ, Zhou H, Cody DR, McMichael A, Fry DW. Tyrosine kinase inhibitors. 5. Synthesis and structure-activity relationships for 4-[(phenylmethyl)amino]- and 4-(phenylamino)quinazolines as potent adenosine 5′-triphosphate binding site inhibitors of the tyrosine kinase domain of the epidermal growth factor receptor. J Med Chem. 1995;38(18):3482–3487. [PubMed]
24. Wakeling AE, Guy SP, Woodburn JR, Ashton SE, Curry BJ, Barker AJ, Gibson KH. ZD1839 (Iressa): an orally active inhibitor of epidermal growth factor signaling with potential for cancer therapy. Cancer Research. 2002;62(20):5749–5754. [PubMed]
25. Akita RW, Sliwkowski MX. Preclinical studies with Erlotinib (Tarceva) Seminars in Oncology. 2003;30(3 suppl 7):15–24. [PubMed]
26. Rusnak DW, Lackey K, Affleck K, Wood ER, Alligood KJ, Rhodes N, Keith BR, Murray DM, Knight WB, Mullin RJ, Gilmer TM. The effects of the novel, reversible epidermal growth factor receptor/ErbB-2 tyrosine kinase inhibitor, GW2016, on the growth of human normal and tumor-derived cell lines in vitro and in vivo. Mol Cancer Ther. 2001;1(2):85–94. [PubMed]
27. Rusnak DW, Affleck K, Cockerill SG, Stubberfield C, Harris R, Page M, Smith KJ, Guntrip SB, Carter MC, Shaw RJ, Jowett A, et al. The characterization of novel, dual ErbB-2/EGFR, tyrosine kinase inhibitors: potential therapy for cancer. Cancer Res. 2001;61(19):7196–7203. [PubMed]
28. Denny WA, Rewcastle GW, Bridges AJ, Fry DW, Kraker AJ. Structure-activity relationships for 4-anilinoquinazolines as potent inhibitors at the ATP binding site of the epidermal growth factor receptor in vitro. Clin Exp Pharmacol Physiol. 1996;23(5):424–427. [PubMed]
29. Stamos J, Sliwkowski MX, Eigenbrot C. Structure of the epidermal growth factor receptor kinase domain alone and in complex with a 4-anilinoquinazoline inhibitor. J Biol Chem. 2002;277(48):46265–46272. [PubMed]
30. Tsou HR, Overbeek-Klumpers EG, Hallett WA, Reich MF, Floyd MB, Johnson BD, Michalak RS, Nilakantan R, Discafani C, Golas J, Rabindran SK, et al. Optimization of 6,7-disubstituted-4-(arylamino)quinoline-3-carbonitriles as orally active, irreversible inhibitors of human epidermal growth factor receptor-2 kinase activity. J Med Chem. 2005;48(4):1107–1131. [PubMed]
31. Jani JP, Finn RS, Campbell M, Coleman KG, Connell RD, Currier N, Emerson EO, Floyd E, Harriman S, Kath JC, Morris J, et al. Discovery and pharmacologic characterization of CP-724,714, a selective ErbB2 tyrosine kinase inhibitor. Cancer Res. 2007;67(20):9887–9893. [PubMed]
32. Barker AJ, Gibson KH, Grundy W, Godfrey AA, Barlow JJ, Healy MP, Woodburn JR, Ashton SE, Curry BJ, Scarlett L, Henthorn L, et al. Studies leading to the identification of ZD1839 (IRESSA): an orally active, selective epidermal growth factor receptor tyrosine kinase inhibitor targeted to the treatment of cancer. Bioorg Med Chem Lett. 2001;11(14):1911–1914. [PubMed]
33. Yun CH, Boggon TJ, Li Y, Woo MS, Greulich H, Meyerson M, Eck MJ. Structures of lung cancer-derived EGFR mutants and inhibitor complexes: mechanism of activation and insights into differential inhibitor sensitivity. Cancer Cell. 2007;11(3):217–227. [PMC free article] [PubMed]
34. Wissner A, Berger DM, Boschelli DH, Floyd MB, Jr, Greenberger LM, Gruber BC, Johnson BD, Mamuya N, Nilakantan R, Reich MF, Shen R, et al. 4-Anilino-6,7-dialkoxyquinoline-3-carbonitrile inhibitors of epidermal growth factor receptor kinase and their bioisosteric relationship to the 4-anilino-6,7-dialkoxyquinazoline inhibitors. J Med Chem. 2000;43(17):3244–3256. [PubMed]
35. Thompson AM, Bridges AJ, Fry DW, Kraker AJ, Denny WA. Tyrosine kinase inhibitors. 7.7-Amino-4-(phenylamino)- and 7-amino-4-[(phenylmethyl)amino]pyrido[4,3-d]pyrimidines: a new class of inhibitors of the tyrosine kinase activity of the epidermal growth factor receptor. J Med Chem. 1995;38(19):3780–3788. [PubMed]
36. Traxler PM, Furet P, Mett H, Buchdunger E, Meyer T, Lydon N. 4-(Phenylamino)pyrrolopyrimidines: potent and selective, ATP site directed inhibitors of the EGF-receptor protein tyrosine kinase. J Med Chem. 1996;39(12):2285–2292. [PubMed]
37. Traxler P, Allegrini PR, Brandt R, Brueggen J, Cozens R, Fabbro D, Grosios K, Lane HA, McSheehy P, Mestan J, Meyer T, et al. AEE788: a dual family epidermal growth factor receptor/ErbB2 and vascular endothelial growth factor receptor tyrosine kinase inhibitor with antitumor and antiangiogenic activity. Cancer Res. 2004;64(14):4931–4941. [PubMed]
38. Wong TW, Lee FY, Yu C, Luo FR, Oppenheimer S, Zhang H, Smykla RA, Mastalerz H, Fink BE, Hunt JT, Gavai AV, et al. Preclinical antitumor activity of BMS-599626, a pan-HER kinase inhibitor that inhibits HER1/HER2 homodimer and heterodimer signaling. Clin Cancer Res. 2006;12(20 Pt 1):6186–6193. [PubMed]
39. Wissner A, Overbeek E, Reich MF, Floyd MB, Johnson BD, Mamuya N, Rosfjord EC, Discafani C, Davis R, Shi X, Rabindran SK, et al. Synthesis and structure-activity relationships of 6,7-disubstituted 4-anilinoquinoline-3-carbonitriles. The design of an orally active, irreversible inhibitor of the tyrosine kinase activity of the epidermal growth factor receptor (EGFR) and the human epidermal growth factor receptor-2 (HER-2) J Med Chem. 2003;46(1):49–63. [PubMed]
40. Rabindran SK, Discafani CM, Rosfjord EC, Baxter M, Floyd MB, Golas J, Hallett WA, Johnson BD, Nilakantan R, Overbeek E, Reich MF, et al. Antitumor activity of HKI-272, an orally active, irreversible inhibitor of the HER-2 tyrosine kinase. Cancer Res. 2004;64(11):3958–3965. [PubMed]
41. Kwak EL, Sordella R, Bell DW, Godin-Heymann N, Okimoto RA, Brannigan BW, Harris PL, Driscoll DR, Fidias P, Lynch TJ, Rabindran SK, et al. Irreversible inhibitors of the EGF receptor may circumvent acquired resistance to gefitinib. Proceedings of the National Academy of Sciences USA. 2005;102(21):7665–7670. [PubMed]
42. Fry DW, Bridges AJ, Denny WA, Doherty A, Greis KD, Hicks JL, Hook KE, Keller PR, Leopold WR, Loo JA, McNamara DJ, et al. Specific, irreversible inactivation of the epidermal growth factor receptor and erbB2, by a new class of tyrosine kinase inhibitor. Proc Natl Acad Sci U S A. 1998;95(20):12022–12027. [PubMed]
43. Smaill JB, Rewcastle GW, Loo JA, Greis KD, Chan OH, Reyner EL, Lipka E, Showalter HD, Vincent PW, Elliott WL, Denny WA. Tyrosine kinase inhibitors. 17. Irreversible inhibitors of the epidermal growth factor receptor: 4-(phenylamino)quinazoline-and 4-(phenylamino)pyrido[3,2-d]pyrimidine-6-acrylamides bearing additional solubilizing functions. J Med Chem. 2000;43(7):1380–1397. [PubMed]
44. Discafani CM, Carroll ML, Floyd MB, Jr, Hollander IJ, Husain Z, Johnson BD, Kitchen D, May MK, Malo MS, Minnick AA, Jr, Nilakantan R, et al. Irreversible inhibition of epidermal growth factor receptor tyrosine kinase with in vivo activity by N-[4-[(3-bromophenyl)amino]-6-quinazolinyl]-2-butynamide (CL-387,785) Biochem Pharmacol. 1999;57(8):917–925. [PubMed]
45. Suzuki T, Fujii A, Ohya J, Amano Y, Kitano Y, Abe D, Nakamura H. Pharmacological characterization of MP-412 (AV-412), a dual epidermal growth factor receptor and ErbB2 tyrosine kinase inhibitor. Cancer Sci. 2007;98(12):1977–1984. [PubMed]
46. Smaill JB, Palmer BD, Rewcastle GW, Denny WA, McNamara DJ, Dobrusin EM, Bridges AJ, Zhou H, Showalter HD, Winters RT, Leopold WR, et al. Tyrosine kinase inhibitors. 15.4-(Phenylamino)quinazoline and 4-(phenylamino)pyrido[d]pyrimidine acrylamides as irreversible inhibitors of the ATP binding site of the epidermal growth factor receptor. J Med Chem. 1999;42(10):1803–1815. [PubMed]
47. Wong KK, Fracasso PM, Bukowski RM, Munster PN, Lynch T, Abbas R, Quinn SE, Zacharchuk C, Burris H. HKI-272, an irreversible pan erbB receptor tyrosine kinase inhibitor: Preliminary phase 1 results in patients with solid tumors; ASCO Annual Meeting Proceedings; 2006. p. 3018.
48. Wissner A, Mansour TS. The development of HKI-272 and related compounds for the treatment of cancer. Arch Pharm (Weinheim) 2008;341(8):465–477. [PubMed]
49. Yun CH, Mengwasser KE, Toms AV, Woo MS, Greulich H, Wong KK, Meyerson M, Eck MJ. The T790M mutation in EGFR kinase causes drug resistance by increasing the affinity for ATP. Proc Natl Acad Sci U S A. 2008;105(6):2070–2075. [PubMed]
50. Agus DB, Terlizzi E, Stopfer P, Amelsberg A, Gordon MS. A phase I dose escalation study of BIBW 2992, an irreversible dual EGFR/HER2 receptor tyrosine kinase inhibitor, in a continuous schedule in patients with advanced solid tumours. ASCO Annual Meeting Proceedings. 2006;24:2074.
51. Zinner RG, Nemunaitis J, Eiseman I, Shin HJC, Olson SC, Christensen J, Huang X, Lenehan PF, Donato NJ, Shin DM. Phase I Clinical and Pharmacodynamic Evaluation of Oral CI-1033 in Patients with Refractory Cancer. Clin Cancer Res. 2007;13(10):3006–3014. [PubMed]
52. Fish-Steagall A, Searcy P, Sipples R. Clinical experience with anti-EGFR therapy. Semin Oncol Nurs. 2006;22(1 Suppl 1):10–19. [PubMed]
53. Barbacci EG, Pustilnik LR, Rossi AM, Emerson E, Miller PE, Boscoe BP, Cox ED, Iwata KK, Jani JP, Provoncha K, Kath JC, et al. The biological and biochemical effects of CP-654577, a selective erbB2 kinase inhibitor, on human breast cancer cells. Cancer Res. 2003;63(15):4450–4459. [PubMed]
54. Munster PN, Britten CD, Mita M, Gelmon K, Minton SE, Moulder S, Slamon DJ, Guo F, Letrent SP, Denis L, Tolcher AW. First Study of the Safety, Tolerability, and Pharmacokinetics of CP-724,714 in Patients with Advanced Malignant Solid HER2-Expressing Tumors. Clin Cancer Res. 2007;13(4):1238–1245. [PubMed]
55. Moasser MM, Basso A, Averbuch SD, Rosen N. The tyrosine kinase inhibitor ZD1839 (“Iressa”) inhibits HER2-driven signaling and suppresses the growth of HER2-overexpressing tumor cells. Cancer Res. 2001;61(19):7184–7188. [PubMed]
56. Hirata A, Hosoi F, Miyagawa M, Ueda S, Naito S, Fujii T, Kuwano M, Ono M. HER2 overexpression increases sensitivity to gefitinib, an epidermal growth factor receptor tyrosine kinase inhibitor, through inhibition of HER2/HER3 heterodimer formation in lung cancer cells. Cancer Res. 2005;65(10):4253–4260. [PubMed]
57. Emlet DR, Schwartz R, Brown KA, Pollice AA, Smith CA, Shackney SE. HER2 expression as a potential marker for response to therapy targeted to the EGFR. Br J Cancer. 2006;94(8):1144–1153. [PMC free article] [PubMed]
58. Schaefer G, Shao L, Totpal K, Akita RW. Erlotinib directly inhibits HER2 kinase activation and downstream signaling events in intact cells lacking epidermal growth factor receptor expression. Cancer Res. 2007;67(3):1228–1238. [PubMed]
59. Moody SE, Sarkisian CJ, Hahn KT, Gunther EJ, Pickup S, Dugan KD, Innocent N, Cardiff RD, Schnall MD, Chodosh LA. Conditional activation of Neu in the mammary epithelium of transgenic mice results in reversible pulmonary metastasis. Cancer Cell. 2002;2(6):451–461. [PubMed]
60. Holbro T, Beerli RR, Maurer F, Koziczak M, Barbas CF, III, Hynes NE. The ErbB2/ErbB3 heterodimer functions as an oncogenic unit: ErbB2 requires ErbB3 to drive breast tumor cell proliferation. Proceedings of the National Academy of Sciences USA. 2003;100(15):8933–8938. [PubMed]
61. Sergina NV, Rausch M, Wang D, Blair J, Hann B, Shokat KM, Moasser MM. Escape from HER family tyrosine kinase inhibitor therapy by the kinase inactive HER3. Nature. 2007;445:437–441. [PMC free article] [PubMed]
62. Moyer JD, Barbacci EG, Iwata KK, Arnold L, Boman B, Cunningham A, DiOrio C, Doty J, Morin MJ, Moyer MP, Neveu M, et al. Induction of apoptosis and cell cycle arrest by CP-358,774, an inhibitor of epidermal growth factor receptor tyrosine kinase. Cancer Res. 1997;57(21):4838–4848. [PubMed]
63. Jani JP, Barbacci EG, Bhattacharya S, Boos C, Campbell M, Clark T, Coleman K, Connell R, Cosker T. Discovery and development of CP-724714, a selective HER2 receptor tyrosine kinase inhibitor. Proceedings of the American Society of Clinical Oncology. 2004;22:3122.
64. Jani JP, Barbacci EG, Bhattacharya S, Moyer JD. CP-724714, a novel erbB2 receptor tyrosine kinase inhibitor for cancer therapy. Proceedings of the American Assocation for Cancer Research. 2004;45:4637.
65. Levitzki A, Gazit A. Tyrosine kinase inhibition: an approach to drug development. Science. 1995;267(5205):1782–1788. [PubMed]
66. Miknis G, Wallace E, Lyssikatos J, Lee P, Zhao Q, Hans J, Topalov G, Buckmelter A, Tarlton G, Ren L, Tullis J, et al. ARRY-334543, A potent, orally active small molecule inhibitor of EGFR and ErbB-2. Proceedings of the American Assocation for Cancer Research. 2005;24(1):3399.
67. Pheneger T, Woessner R, Lyssikatos J, Miknis G, Anderson D, Winski S, Lee PA. In vivo anti-tumor activity of arry-334543, a small molecule inhibitor of EGFR and ErbB-2. Proc AACR-NCI-EORTC Conference. 2005:A247.
68. Slichenmyer WJ, Elliott WL, Fry DW. CI-1033, a pan-erbB tyrosine kinase inhibitor. [Review] [19 refs] Seminars in Oncology. 2001;28(5 Suppl 16):80–85. [PubMed]
69. Rabindran SK. Antitumor activity of HER-2 inhibitors. Cancer Lett. 2005;227(1):9–23. [PubMed]
70. Torrance CJ, Jackson PE, Montgomery E, Kinzler KW, Vogelstein B, Wissner A, Nunes M, Frost P, Discafani CM. Combinatorial chemoprevention of intestinal neoplasia. NatMed. 2000;6(9):1024–1028. [PubMed]
71. Solca F, Baum A, Guth B, Colbatzky F, Blech S, Amelsberg A, Himmelsbach F. BIBW 2992, an irreversible dual EGFR/HER2 receptor tyrosine kinase inhibitor for cancer therapy. Proc AACR-NCI-EORTC Conference. 2006:A244.
72. Traxler P, Bold G, Frei J, Lang M, Lydon N, Mett H, Buchdunger E, Meyer T, Mueller M, Furet P. Use of a pharmacophore model for the design of EGF-R tyrosine kinase inhibitors: 4-(phenylamino)pyrazolo[3,4-d]pyrimidines. J Med Chem. 1997;40 (22):3601–3616. [PubMed]
73. Brandt R, Wong AM, Hynes NE. Mammary glands reconstituted with Neu/ErbB2 transformed HC11 cells provide a novel orthotopic tumor model for testing anti-cancer agents. Oncogene. 2001;20(39):5459–5465. [PubMed]
74. Traxler P, Bold G, Buchdunger E, Caravatti G, Furet P, Manley P, O’Reilly T, Wood J, Zimmermann J. Tyrosine kinase inhibitors: from rational design to clinical trials. Med Res Rev. 2001;21(6):499–512. [PubMed]