Oncogene (2000) 19, 6574 ± 6583
`ã 2000 Macmillan Publishers Ltd All rights reserved 0950 ± 9232/00 $15.00
`
`www.nature.com/onc
`
`From oncogene to drug: development of small molecule tyrosine kinase
`inhibitors as anti-tumor and anti-angiogenic agents
`
`Michael J Morin*,1
`
`1P®zer Global R&D, Groton, Connecticut, CT 06340, USA
`
`The con¯uence of two distinct but related activities in the
`past 10 years has dramatically accelerated e€orts
`towards the discovery and development of novel drugs
`to treat
`cancer. The ®rst
`is a rapidly emerging
`understanding that a number of distinct tyrosine kinases
`play roles in diverse but fundamentally important aspects
`of tumor progression (growth, survival, metastasis and
`angiogenesis). The second is the discovery that small
`molecule compounds have the capacity to potently and
`selectively inhibit the biochemical function of tyrosine
`kinases by competing for ATP binding at the enzyme
`catalytic site. These observations have been conjoined in
`major e€orts to bring forward into clinical development
`novel cancer drugs with the potential to provide both
`clinical ecacy and improved tolerability. The focus of
`this review is on the development of small molecule
`tyrosine kinase inhibitors, and does not extend to other
`approaches that could be applied to disrupt the same
`pathways in clinical
`tumors (receptor and/or ligand-
`competitive antibodies, intrabodies, antisense ribonucleo-
`tides, ribozymes, phosphatase inhibitors or SH2/SH3-
`directed agents). Selected tyrosine kinase inhibitors,
`known or believed to be in development
`in cancer
`treatment trials, are summarized as are some of the
`key issues that must be addressed if these compounds are
`to be developed into clinically useful cancer chemother-
`apeutic agents. Oncogene (2000) 19, 6574 ± 6583.
`
`Keywords: tyrosine kinase inhibitors; anti-tumor; anti-
`angiogenesis
`
`Origin of species ± brief overview of substrate-based
`inhibitors of protein tyrosine kinases
`
`Among all non-traditional (non-DNA-directed) cancer
`targets
`for which pharmacological
`intervention is
`feasible, there are none that have generated as much
`widespread interest, and have
`invoked as much
`resource investment in both the public and private
`sectors in the past 7 years, as have the tyrosine kinases.
`Several excellent recent reviews have described the
`functions of various
`tyrosine kinases
`in the key
`pathways that drive tumor progression,
`from ®rst
`genetic insult to disseminated disease (Hanahan and
`Weinberg, 2000; Hunter, 2000; Gibbs, 2000). Key
`among these are the receptor tyrosine kinases which
`initiate signal transduction in tumor cells or endothelial
`cells following the binding of the growth factors EGF,
`PDGF and VEGF. There are also several excellent
`reviews that provide detailed overviews of the work
`
`*Correspondence: MJ Morin
`
`accomplished to date to understand the molecular
`pharmacology of small molecule inhibitors of receptor
`tyrosine kinases (Sedlacek, 2000; Fry, 2000; Bridges,
`1999; Levitzki, 1999; Lawrence and Niu, 1998). With-
`out summarizing each of these important reviews, they
`provide an appropriate context for understanding the
`obstacles and triumphs that have led, very recently, to
`the ®rst reproducible, objective clinical responses in
`cancer patients treated with tyrosine kinase inhibitors.
`The catalytic function of protein tyrosine kinases
`involves the simple transfer of the gamma phosphate of
`ATP to hydroxyl group of a tyrosine residue of
`proteins (or peptides) encompassing a diversity of
`primary sequences and tertiary structures (Songyang
`and Cantley, 1998). Each of
`the substrates in the
`phosphotransfer reaction, the tyrosine hydroxy group
`and ATP, represent reasonable pharmacological start-
`ing points for the design of substrate analogs and
`competitive inhibitors of tyrosine kinases. A diverse set
`of pharmacophores, including natural products (laven-
`dustins and erbstatins) and synthetic tyrosine mimetics,
`have all been characterized on the basis of their ability
`to competitively inhibit
`tyrosine kinase
`function
`(Levitzki, 1999). These compounds tended to have
`poor potency (particularly in cells), to yield relatively
`¯at structure-activity relationships, and to be some-
`what non-speci®c in their kinase inhibition (Fry, 2000).
`Attacking this
`reaction from the other
`side, by
`identifying compounds that mimic ATP, was originally
`thought
`to be even less tractable. As reviewed by
`Lawrence and Niu (1998), the theoretical obstacles
`were immense. First, the primary sequence of the ATP-
`binding pocket of all kinases is highly conserved, and
`therefore selectivity,
`if not speci®city, represents a
`signi®cant technical challenge. Secondly, the intracel-
`lular
`concentration of ATP can exceed 5 mM,
`particularly in tumor cells, while the Km for ATP in
`most kinase active sites is in the micromolar range,
`thus ensuring full-time saturation by ATP. ATP-
`competitive inhibitors would need to exhibit at least
`nanomolar inhibitory kinetic constants to e€ectively
`compete in this circumstance (Lawrence and Niu,
`1998). Finally,
`there are multiple non-kinase ATP-
`dependent enzymes important to normal physiology,
`and so an indiscriminant ATP mimetic would likely
`have
`toxicities
`that were pharmacologically
`and
`medically unacceptable.
`This theoretical
`logjam was broken in convincing
`fashion when the tyrosine kinase inhibitory activities of
`anilinoquinazolines were ®rst described in 1994 by
`three separate groups (Fry et al., 1994; Ward et al.,
`1994; Osherov and Levitzki, 1994). For example, the
`work of Fry et al. (1994) at Warner Lambert revealed
`that 4-anilinoquinazolines were potent (nM) inhibitors
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`of the EGFR tyrosine kinase with good cell activity
`and profound biochemical selectivity relative to other
`kinases within the tyrosine kinase family. Further
`elaboration of structure-activity relationships rich in
`new possibilities resulted in ATP-competitive inhibitors
`of the EGFR tyrosine kinase with Ki values in the
`single digit picomolar range. It is interesting to note
`that the Michaelis-Menten equation could not be used
`to derive the Ki values of these molecules. So avid was
`the binding of compound to the ATP site,
`the
`conventional approximation that total and free enzyme
`concentrations were equivalent did not apply under
`these conditions. These accomplishments, which may
`be among the most important in pharmacology for the
`last 10 years, were largely achieved by empirical
`screening and iterative medicinal chemistry. Even more
`new chemotypes may emerge as structure-based design
`becomes more commonly applied to the identi®cation
`of both active site- and allosteric site-directed inhibitors
`for an ever-widening slate of tyrosine kinase targets.
`While these early lead molecules had biopharmaceu-
`tical properties which were by-and-large incompatible
`with oral bioavailability and good duration of exposure
`in vivo, the results spurred on a number of groups,
`which have since identi®ed and developed tyrosine
`kinase inhibitors with signi®cant potential
`to treat
`clinical cancer.
`
`Selected development candidates ± updates
`
`PDGFR inhibitors: STI 571 and SU101
`
`STI 571 (CGP57148B) Among all of the candidates
`currently in clinical development, perhaps none has
`provided as much `proof of concept' for the clinical
`ecacy and tolerability of small molecule tyrosine
`kinase inhibitors as has STI 571. Originally disclosed by
`Novartis as a multitrophic tyrosine kinase inhibitor,
`STI 571 was described by Druker et al. (1996); and
`Druker and Lydon (2000) as having potent activity vs
`the translocation product bcr-abl,
`the transforming
`tyrosine kinase found in virtually all CML cells
`expressing the Philadelphia chromosome (Kurzrock et
`al., 1988; Kelliher et al., 1990). The inhibition of v-abl,
`bcr-abl and PDGFR autophosphorylation by the 2-
`phenylaminopyrimidine STI 571 (Figure 1) at nanomo-
`lar concentrations was found to translate to both in
`vivo anti-tumor activity, and to the inhibition of
`clonogenicity of blasts from CML patients (le Coutre
`et al., 1999; Druker et al., 1996). The results of a
`clinical trial
`in which STI 571 was administered to
`CML and ALL patients expressing bcr-abl
`in their
`leukemic blasts were most recently summarized in May
`2000 (Talpaz et al., 2000). STI 571 was used to treat 33
`acute leukemia patients, which included 21 myeloid
`blast crisis CML patients and 12 bcr-abl-positive ALL
`or
`lymphoid blast
`crisis CML patients. Clinical
`responses, as de®ned by a decrease in the percentage
`of patients achieving reduction in bone marrow blasts
`to 15% of pre-treatment levels, were observed in 55%
`of myeloid blast crisis patients, with complete responses
`in 22% of these patients. The response rates in patients
`with bcr-abl positive ALL and lymphoid blast crisis of
`CML were higher (82% with 55% complete responses),
`but all of
`the patients with lymphoid leukemias
`
`relapsed on drug between 45 and 81 days. Of 19
`responding patients, 10 experienced Grade 3 ± 4 neu-
`tropenia. This response rate, and the incidence of
`Grade 3 ± 4 toxicity, compares very favorably to the
`standard of care cytotoxic chemotherapies for CML.
`As such, more de®nitive trials assessing the ecacy and
`safety of STI 571 are ongoing in CML.
`It is interesting to speculate as to the biochemical
`basis for both the ecacy and the toleration pro®le of
`STI 571. Two other tyrosine kinases potently inhibited
`by STI 571, c-kit and PDGFR, are both believed to play
`important roles in maintaining bone marrow stroma ±
`progenitor cell interactions (Ashman, 1999; Sungaran et
`al., 2000). Thus, inhibition of c-kit and PDGFR could
`also account for some of the compelling clinical activity
`of STI 571 in CML, as well as for its toxicity pro®le
`(neutropenia). Treatment of a c-kit expressing a human
`myeloid leukemia cell line, M-07e, with STI 571 before
`stimulation with kit ligand inhibited c-kit autopho-
`sphorylation, activation of mitogen-activated protein
`(MAP) kinase, and activation of Akt, with an IC50 of
`100 nM (Heinrich et al., 2000). STI 571 was even more
`potent in a human mast cell leukemia cell line (HMC-1)
`expressing an activated mutant form of c-kit. Similar
`results have also recently been reported in non-
`hematopoietic tumor cells (Wang et al., 2000). The
`ecacy and safety hypotheses for inhibition of c-abl in
`CML may perhaps only be addressed with a more
`selective abl
`tyrosine kinase inhibitor. Given the
`apparent therapeutic bene®t of STI 571, this may be
`largely an academic question, but one with important
`implications as one tries to rationalize the desired
`selectivity pro®les of tyrosine kinase inhibitors most
`likely to generate both ecacy and safety in humans.
`
`SU101 (le¯unomide; HWA 486) Le¯unomide was
`originally described and developed as an inhibitor of
`dihydroorotate dehydrogenase, a key enzyme in the de
`novo synthesis of pyrimidines, for use as an immuno-
`suppressive or
`anti-arthritic
`agent
`(Bartlett
`and
`Schleyerbach, 1985; Kuo et al., 1996). Le¯unomide
`has
`shown signi®cant activity as a treatment
`for
`rheumatoid arthritis (Smolen and Emery, 2000; Cohen
`et al., 2000b), and was launched by Aventis as Arava2
`in the US and elsewhere beginning in 1998. Extending
`the work of others (Mattar et al., 1993; Xu et al.,
`1995), Shawver and co-workers reported that micro-
`molar concentrations of
`le¯unomide inhibited the
`autophosphorylation of the tyrosine kinase receptors
`for PDGF and VEGF (Shawver et al., 1997). The
`compound was also e€ective at blocking mitogenesis
`stimulated by both PDGF and EGF, but exogenous
`uridine could not reverse the e€ect of le¯unomide on
`PDGF mitogenesis, suggesting that inhibition of the
`receptor
`tyrosine kinase, and not
`inhibition of
`pyrimidine pools, was a key pharmacological activity.
`The inhibition of EGF-induced mitogenesis by le¯u-
`nomide was reversed in part by uridine (Shawver et al.,
`1997), despite the fact that le¯unomide and close-in
`analogs also have inhibitory activity vs the EGFR
`tyrosine kinase (Ghosh et al., 1999).
`Le¯unomide/SU101 is clearly a tyrosine kinase
`inhibitor with multiple biochemical e€ects, and readily
`generates a predominant active metabolite (SU0020 or
`A771726; Figure 1)
`that has a complex inhibitory
`pro®le of its own (Hamilton et al., 1999). SU 101 was,
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`Figure 1 Structures of selected tyrosine kinase inhibitors in clinical development for cancer
`
`nonetheless, progressed into clinical trials by SUGEN
`(now part of Pharmacia). A Phase I study in cancer
`patients revealed that SU 101 was well-tolerated as a
`24 h continuous i.v. infusion at doses up to 443 mg/m2/
`wk. At this dose, the plasma concentration of the
`active metabolite was maintained at levels sucient to
`block both PDGFR and EGFR signaling, as well as
`pyrimidine biosynthesis (Eckhardt et al., 1999). Toxi-
`cities were
`relatively minor
`(Grade 1 ± 2 nausea,
`vomiting and fever
`in approximately 20% of all
`courses given). Surprisingly, hematopoietic toxicities
`and hemolysis, which had been noted in the preclinical
`experience with SU 101, were not seen in this Phase I
`population. One partial
`response was
`seen in 26
`patients receiving an average of two courses each; the
`responding patient received 13 courses (52 infusions) to
`treat an anaplastic astrocytoma, and had a notable
`(450%) reduction in one measurable lesion (Eckhardt
`et al., 1999). SU 101 has been reported to be in
`advanced trials for multiple solid tumor types, but
`recent disclosures (Garber, 2000) indicate that Phase
`III trials in at least one tumor type (glioblastoma) have
`been abandoned. The status of other trials (ongoing
`Phase II trials for ovarian and NSCLC; planned Phase
`III trials for prostate, colon and NSCLC) is uncertain
`at the present time.
`
`EGFR inhibitors: Iressa2 (ZD1839), OSI-774
`(CP-358,774) and CI-1033 (PD183805)
`
`Iressa2 (ZD1839) While STI 571 has provided no-
`table clinical proof-of-concept for the clinical ecacy
`and safety of
`tyrosine kinase inhibitors,
`the early
`
`clinical ®ndings with AstraZeneca's ZD1839 (Iressa2)
`have been equally compelling. The pharmacological
`characteristics of Iressa2 were ®rst described in 1996
`(Wakeling et al., 1996; Woodburn et al., 1997) as a
`potent and selective inhibitor of the EGFR tyrosine
`kinase. This quinazoline-based compound (Figure 1) is
`an ATP-competitive inhibitor of the EGFR tyrosine
`kinase (IC50 25 nM) with 50-fold selectivity relative to
`closely homologous erbB family members (IC50 for
`erbB2 1 ± 3 mM) and even greater selectivity for more
`divergent
`tyrosine kinases.
`It demonstrates good
`cellular potency (80 nM IC50 for inhibition of EGF-
`dependent mitogenesis) and robust, dose-dependent
`anti-tumor ecacy in a variety of human tumor
`xenografts (Woodburn et al., 1997). These results have
`been most recently extended to show that Iressa2 has
`in vivo ecacy in a diverse human tumor xenograft
`models both with (Ciardello et al., 2000) and without
`(Sirotnak et al., 2000) highly activated EGFR signaling
`pathways. Of equal interest are the observations that
`Iressa2 combines with standard cytotoxic agents
`(platinums, taxanes, topoisomerase I inhibitors, etc.)
`to produce additive or
`supra-additive anti-tumor
`ecacy in vivo without exacerbation of the toxicity of
`the co-administered cytotoxics. The ®ndings that tumor
`EGFR density does not predict ecacy when the
`compound is used in conjunction with cytotoxic agents
`have signi®cantly impacted the development strategy
`employed by AstraZeneca as Iressa2 moves towards
`pivotal clinical trials.
`trials with Iressa2 have been
`Multiple Phase I
`summarized, and the
`results
`revealed reasonable
`pharmacokinetics, good toleration and the ®rst signs
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`of clinical ecacy when used as a single agent in
`patients with advanced disease (Ferry et al., 2000;
`Baselga et al., 2000; Kelly et al., 2000). Following oral
`administration of a single dose (50 mg), maximum
`plasma drug concentrations (mean 45 ng/ml) occurred
`1 ± 5 h post-dose. The mean terminal t1/2 was 34 h.
`Inter-subject variability in exposure was signi®cant
`following single and multiple administration (up to
`sevenfold at each dose level), but exposure increased
`proportionally with dose, with no apparent change in
`terminal t1/2 across the dose range tested (Kelly et al.,
`2000). In a larger dose-escalation trial, Ferry and
`collaborators administered Iressa2 at doses of 50 ±
`700 mg once daily, given orally for 14 days followed by
`14 days of observation (Ferry et al., 2000). In total, 64
`patients with advanced disease, who had each
`progressed while on prior chemotherapy, completed
`145 cycles. Cmax and AUC0-24h were proportional
`across
`the entire dose range (mean values 113 ±
`2255 ng/ml and 1.8 ± 38.5 mg.h/ml, respectively). As
`in single dose studies, Iressa2 showed a long terminal
`elimination half-life (mean of 46 h). Iressa2 was very
`well-tolerated in this study; the most common adverse
`events were diarrhea and acne-like skin rash (Grade 1 ±
`2). Acne-like skin rashes have emerged as a common,
`mechanism-based adverse event for EGFR inhibitors,
`but the speci®c toxicological e€ect in the skin is not yet
`well understood. Grade 3 ± 4 adverse events were
`shown to be rare with Iressa2 treatment, and were
`generally ascribed to disease progression. The dose-
`limiting toxicity, de®ned at the 700 mg dose level, was
`Grade 3 diarrhea (Ferry et al., 2000).
`A compelling level of ecacy was also revealed in
`these early trials (Ferry et al., 2000). Anti-tumor
`responses were most evident among the 16 NSCLC
`patients treated with Iressa2 ± two had an objective
`partial response, two patients had signi®cant regression
`of disease and two patients had stable disease. Similar
`pharmacokinetic and safety pro®les were noted in a
`separate study (Baselga et al., 2000), one that also
`revealed the potential
`for ecacy from Iressa2 in
`patients with advanced prostatic
`and head-neck
`cancers. These early results added importantly to the
`proof-of-concept that selective tyrosine kinase inhibi-
`tors could have signi®cant single agent ecacy, as
`measured by objective tumor regressions,
`in patients
`with advanced disease. The clinical observations have
`therefore recapitulated the pre-clinical data showing
`that Iressa2 increased apoptosis and regressions in
`human tumor xenograft models (Ciardello et al., 2000).
`The Iressa2 data indicate that the ecacy of these
`agents can be measured using more classically de®ned
`clinical endpoints. There will undoubtedly be signi®-
`cant value in the use of pharmacodynamic and
`surrogate endpoints to guide dose-intensi®cation or to
`pre-select patients for whom other tyrosine kinase
`inhibitors might represent the most promising treat-
`ment option. Pharmacodynamic endpoints have not
`played a major role in the early development of EGFR
`tyrosine kinase inhibitors, despite the fact that several
`reasonable options
`exist,
`including both invasive
`techniques (direct measurement of tumor-derived or
`normal tissue-derived EGFR phosphotyrosine, phos-
`phorylation of down-stream signaling molecules;
`apoptosis markers) and non-invasive techniques such
`as PET imaging of metabolically modulated tumors
`
`(Pollack et al., 1999; Goss et al., 2000; Allen et al.,
`2000). Given the overall safety and toleration pro®le of
`Iressa2, AstraZeneca has committed to an aggressive
`development strategy, which includes two large Phase
`III studies to assess the use of Iressa2 in combination
`with cis- or
`carbo-platinum plus
`a
`taxane or
`gemcitabine in ®rst-line therapy for NSCLC (trials 14
`and 17), as well as a Phase II trial (trial 16) to con®rm
`the single agent activity of Iressa2 in patients with
`advanced NSCLC (Kelly et al., 2000). It is important
`to note that these trials do not call for a prospective
`selection for patients with tumors with some pre-
`de®ned level of EGFR over-expression. All epithelial
`tumors express some EGFR, and in the disease target
`here, NSCLC,
`tumors often present with a high
`proportion of EGFR over-expression (up to 80 ± 90%
`in advanced disease). The strategy is also consistent
`with pre-clinical data suggesting that ecacy in drug
`combinations may not be determined in large part by
`the level of EGFR over-expression in tumors (Sirotnak
`et al., 2000). Results are expected from these pivotal
`trials in a late-2001 or early-2002 timeframe.
`
`OSI-774 (CP-358,774) CP-358,774 is also a potent
`and selective quinazoline-based inhibitor of the EGFR
`function (Figure 1). This compound is a reversible,
`ATP-competitive inhibitor (IC50 of 2 nM) of the EGFR
`tyrosine kinase, with greater than 500-fold selectivity
`against other tyrosine kinases, such as the closely
`related erbB2 kinase, as well as v-src, c-abl and the
`insulin and IGF-1 receptors, (Moyer et al., 1997). CP-
`358,774 inhibits the autophosphorylation of the EGF
`receptor in a variety of EGFR over-expressing tumor
`cells (IC50=20 nM), and produces cell cycle arrest and
`apoptosis in multiple cell types (Moyer et al., 1997;
`Barbacci et al., 1997; Iwata et al., 1997). In vivo, CP-
`358,774 e€ectively inhibits EGFR-speci®c
`tyrosine
`phosphorylation in human tumor xenografts (ED50 of
`10 mg/kg p.o. when given as a single dose) with
`signi®cant duration of action; daily dosing produces
`substantial growth inhibition and regressions in human
`tumor xenografts (Pollack et al., 1999). Moreover, the
`dose-response for tumor growth inhibition shows good
`agreement with the dose-response for inhibition of
`EGFR-phosphotyrosine in tumors from treated ani-
`mals. As with Iressa2, CP-358,774 was
`found to
`generate additive anti-tumor activity when used in
`combination with cis-platinum and other cytotoxic
`agents, without exacerbating the toxicities of the other
`chemotherapeutants (Pollack et al., 1999).
`Clinical studies with CP-358,774 have revealed that
`the agent is well-tolerated at oral doses that achieve
`plasma concentrations projected to be required for
`anti-tumor ecacy in humans (400 ± 500 ng/ml). In one
`study, escalating doses were administered orally once
`every week (Karp et al., 1999). Eighteen patients with
`advanced solid tumors were treated at ®ve doses (100 ±
`1000 mg)
`for a maximum period of 24 weeks.
`Toxicities were observed only at doses higher than
`200 mg/week, and included mild fatigue, Grade 2
`maculopapular (acneiform) rash, Grade 2 nausea, and
`Grade 2 diarrhea. Like Iressa2, CP-358,774 exhibited
`intra- and inter-subject variability in exposure, but
`dose-proportional increases in exposure were observed
`throughout
`the 100 ± 1000 mg weekly dose range.
`During the ®rst 24 h following a single dose, the Cavg
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`(0.9 ± 4.8 mg/ml for 100 ± 1000 mg doses, respectively)
`was
`some
`two-
`to 10-fold above
`the projected
`ecacious plasma concentration. No maximally toler-
`ated dose or dose-limiting toxicity was discerned in this
`study. In a second Phase I study (Siu et al., 1999),
`patients were given CP-358,774 tablets in a variety of
`dose schedules, culminating in daily dosing at
`the
`maximally tolerated dose. The target Cavg of 400 ±
`500 ng/ml was achievable at doses at and above
`100 mg/day on a well-tolerated schedule (Cavg values
`following continuous daily dosing at the 50, 100 and
`200 mg/day levels were 432, 973 and 2120 ng/ml,
`respectively). Dose-limiting diarrhea was encountered
`at
`the 200 mg/day level. An intermediate dose of
`150 mg/day was subsequently de®ned as the maximally
`tolerated dose (two of three patients had Grade 1
`diarrhea with loperamide support).
`Siu and co-workers also made e€orts to understand
`the `characteristic' Grade 1 ± 2 acneiform rash seen in
`patients treated with CP-358,774, which was limited to
`regions of the upper body where adolescent acne is
`usually manifest (face, back and scalp). Histopathology
`of
`skin biopsies showed subepidermal neutrophilic
`in®ltration and epidermal hyperproliferation (Siu et
`al., 1999). While the precise cytopathic basis for the
`acneiform rash has not yet been determined,
`the
`consistent clinical observations with three di€erent
`agents targeting EGFR function (CP-358,774, Iressa2
`and Imclone's C-225 antibody) suggest that this is a
`mechanism-based ®nding (Siu et al., 1999; Ferry et al.,
`2000; Cohen et al., 2000b). Skin changes are consis-
`tently noted in preclinical studies with rodents exposed
`to CP-358,774 for extended dosing periods, and these
`toxicological results are analogous to the skin changes
`seen in the waved-2 mouse, which has a mutated and
`marginally functional EGFR tyrosine kinase (Luetteke
`et al., 1994).
`from ongoing Phase II
`Early ecacy readouts
`clinical trials with CP-358,774 have been compelling.
`The agent appears to have a broad potential to treat a
`variety of human solid tumors,
`including NSCLC,
`breast, ovarian and squamous head and neck tumors
`(Bonomi et al., 2000; Allen et al., 2000; Siu et al., 2000;
`Hammond et al., 2000). For example, in 34 NSCLC
`patients who had failed prior chemotherapy, daily oral
`doses of 150 mg CP-358,774 were well-tolerated, with a
`maculopapular
`(acneiform)
`rash being the most
`common adverse event reported. In 56 total patients
`evaluable for tumor response,
`there have been six
`partial responses in the lung and/or liver at 8 weeks
`and several patients with stable disease (Bonomi et al.,
`2000).
`In 71 patients with refractory squamous
`carcinomas of the head and neck, CP-358,774 was
`again found to cause a reversible acneiform rash and
`Grade 1 ± 2 diarrhea. Of 78 patients evaluable for
`response,
`there have been at
`least eight con®rmed
`partial responses and 23 patients with stable disease
`(Siu et al., 2000). These preliminary results indicate
`that CP-358,774 is generally well-tolerated and demon-
`strates evidence of single agent anti-tumor activity in
`patients with recurrent head and neck cancer, as well
`as in treatment-refractory NSCLC.
`Due to signi®cant interests in both CP-358,774 and
`CI-1033, P®zer was directed to divest one of these two
`agents as a condition of their acquisition of Warner
`Lambert in 2000. As such, Oncogene Science (OSIP)
`
`has taken over complete responsibility for the devel-
`opment of CP-358,774, which is now formally referred
`to as OSI-774.
`
`CI-1033 (PD183805) As described above, the selec-
`tive and reversible inhibitors of the EGFR tyrosine
`kinase appear to o€er the promise of
`therapeutic
`ecacy coupled to reasonable
`tolerability.
`It
`is
`important to note, however, that the therapeutic index
`of neither Iressa2 nor CP-358,774 has yet to be fully
`elaborated, and that there may be signi®cant proximity
`between the maximally tolerated doses and the
`ecacious doses
`for both agents. Moreover,
`the
`ecacy of neither agent has yet to be established in a
`blinded, placebo controlled study. As
`such,
`there
`continues to be an opportunity to discover and develop
`distinctly di€erent EGFR tyrosine kinase inhibitors
`with even greater potential for ecacy and a broader
`spectrum of activity. CI-1033 is one such distinctly
`di€erent development candidate. As recently reviewed
`by David Fry of
`the
`former Warner Lambert
`organization, signaling through the erbB family of
`tyrosine kinase receptors often involves complex cross-
`talk among the members of that receptor family (Fry,
`2000). The four family members (EGFR or erbB;
`erbB2, erbB3 and erbB4) are known to intensify their
`kinase-dependent transforming signals via the forma-
`tion of heterodimers with each other (Tzahar et al.,
`1996). There is, therefore, a compelling rationale to
`consider the potential utility of nonspeci®c but selective
`inhibitors that e€ectively block the function of the erbB
`family but do not inhibit more structurally diverse
`tyrosine kinases.
`to consider
`There
`is also a strong rationale
`irreversible tyrosine kinase inhibitors. The reversible
`inhibitors have apparently generated clinical ecacy
`with dosing regimens designed to maintain plasma
`concentrations at fairly high levels for extended periods
`of
`time. The optimal dosing paradigm for an
`irreversible inhibitor would be less likely to require
`prolonged exposure. Moreover, the `absolute ®nality'
`(Fry, 2000) of the irreversible inhibitors could con-
`ceivably provide signi®cant advantages in terms of
`antitumor ecacy. To be balanced, a multi-tropic and
`irreversible inhibitor would also have the potential to
`generate a toxicity pro®le that was di€erent and,
`perhaps, without advantages
`relative to the more
`selective, reversible inhibitors. Preclinical data suggest
`that irreversible EGFR tyrosine kinase inhibitors can
`generate
`signi®cant
`ecacy with good toleration
`(Vincent et al., 1999), but the ultimate utility of these
`agents can only be determined in clinical trials.
`Homology modeling of ATP binding to the pocket
`of EGFR suggested that the thiol of cys773 would be a
`key potential site for attack by a rationally designed
`irreversible ATP-mimetic. One compound containing
`an acrylamide functionality at the six position of the 4-
`anilinoquinazoline nucleus (Figure 1) was found to
`have a profoundly rapid onset and long-lasting
`inhibition of both EGFR and erbB2 in tumor cells,
`and to be selective relative to non-erbB tyrosine kinases
`(Fry et al., 1998). When compared to very closely
`related reversible analogs (in which the acrylamide
`double bond was reduced), the 6-substituted irrever-
`sible analogs were more potent
`in vitro and had
`signi®cantly greater ecacy in vivo. Further improve-
`
`6578
`
`Oncogene
`
`5 of 10
`
`BI Exhibit 1093
`
`

`

`Tyrosine kinase inhibitors in cancer treatment trials
`MJ Morin
`
`6579
`
`ments (addition of substitutions which also improved
`water-solubility) led to the elaboration of PD 183805/
`CI-1033 (Figure 1). Like its predecessors, this com-
`pound has excellent (low nM) potency against erbB2
`and EGFR in both enzyme- and cell-based assays
`(Sherwood et al., 1999). Consistent with a predicted
`advantage relative to reversible inhibitors, CI-1033
`potently inhibits human tumor xenografts when dosed
`as infrequently as once per week, and a single dose
`eliminated the level of EGFR phosphorylation in
`tumors for longer than 72 h (Vincent et al., 1999).
`Like CP-358,774, CI-1033 combines well
`in drug
`combinations with cytotoxic agents. Given 24 h after
`gemcitabine, CI-1033 produced a signi®cant increase in
`the apoptotic fraction in tumors over treatment with
`either drug alone (Nelson and Fry, 2000). CI-1033 also
`e€ectively decreased the clonogenicity of human tumor
`cells taken from patients (Medina et al., 2000), with
`notable responses seen in breast (67%), NSCLC (60%)
`and ovarian cancer specimens. CI-1033 Phase I clinical
`trials have
`recently been initiated, but data on
`pharmacokinetics or safety have not yet been disclosed.
`
`Small molecule tyrosine kinase inhibitors targeting
`angiogenesis pathways
`
`There are multiple tyrosine kinase receptors which
`appear to have key roles in the generation of new
`tumor blood vessels and, as such, represent reasonable
`targets for cancer chemotherapy (for excellent recent
`reviews, see Cherrington et al., 2000; Randal, 2000;
`Thompson et al., 1999; Hamby and Showalter, 1999).
`Included among the key tyrosine kinase targets that
`have generated the most interest in the scienti®c and
`patent (Connell, 2000) literature are PDGFR, VEGFR,
`FGFR and tie-2. The key development candidates
`targeting PDGFR, STI 571 and SU101, were described
`above, though neither compound is likely to reveal the
`clinical utility of PDGFR-directed inhibition of
`angiogenesis due to their multiple mechanisms of
`action. Agents that selectively target FGFR and tie-2
`are not known to be in development, though several
`drugs targeting VEGFR have inhibitory activity vs
`FGFR. As such, the focus of the remainder of this
`overview will be on the clinical candidates targeting
`VEGFR. Two high anity receptors for VEGF have
`been identi®ed and characterized on human endothelial
`cells, ¯t-1 and KDR. KDR appears to be expressed
`primarily on activated endothelial cells and is thought
`to be more of a key driver of mitogenic responses
`commonly found in neovascularizing tumors, while ¯t-
`1 is expressed on multiple other cell types (Plate et al.,
`1994; Wedge et al., 2000a). For the purposes of this
`review, the terms KDR and VEGFR will be used
`interchangeably, unless otherwise speci®ed.
`
`SU 5416 and SU 6668 The former SUGEN organiza-
`tion (now part of Pharmacia) has clearly set the early
`pace in the race to identify and develop inhibitors of
`the VEGFR tyrosine kinase. E€orts towards this end
`have initially focused on the indolin-2-one pharmaco-
`phore (Figure 1). Among the earliest compounds of
`this class was SU 5416, which was found to be a potent
`inhibitor of the kinase a