Aiming at the Wrong Target?Discussion of Abstracts

CRA3507 (Alberts) and 3508 (Goldberg)

Louis M. Weiner, MDLombardi Comprehensive Cancer CenterGeorgetown University Medical Center

Potential ConflictsRelevant to this Presentation

• Consultant, Stock Options– Merrimack Pharmaceuticals

• Research Support– Amgen

The Facts

• Cetuximab does not improve adjuvant therapy cure rates of resected Stage III colorectal cancer – Trends towards inferior outcomes– True for KRAS-wild type (Abstract CRA 3507) and

KRAS-mutant (Abstract 3508) tumors• No significant concerns raised by study design

or execution of the clinical trial– These results are definitive– Cetuximab toxicity may interfere with delivery of

chemotherapy• Not likely that results will improve if other clinical

trial designs are tested

This Should Have Worked!

• Cetuximab (and related antibodies (i.e., panitumumab) have modest activity in refractory colorectal cancer – Single-agent– Combined with

chemotherapy– KRAS mutations identify

patients who will not benefit • Minimally active

chemotherapy (i.e., 5-FU + LV) for metastatic colon cancer has benefit in the adjuvant setting

EGFR

How did we miss the target?

Cetuximab – Two Major Mechanisms of Action

1. Antibody-dependent Cellular Cytotoxicity 2. Signaling Perturbation

Possible Explanations and Implications

Tumor Cell

Killer CellNK, Macrophage, Neutrophil

Antibody

1. Antibody-Dependent Cellular Cytotoxicity (ADCC) not relevant

– True if EGFR effectively targeted– Not relevant if EGFR+ cells are not the correct targets for colon

cancer metastasis

Antigen

Fc Domain

Possible Explanations and Implications

1. ADCC is not relevant2. EGFR Signaling is complicated

– Robust EGFR resistance networks

EGFR: a central and heavily targeted pathway

Proliferation MetastasisAngiogenesisSurvival signaling

Shc

PI3-K

RafMEKK-1

MEKMKK-7

JNKERK

Ras

mTOR

Grb2

AKT

Sos-1

EGFR

“Downstream”Signaling proteins

Cancer-Relevantoutputs

Cell membrane

ErlotinibGefitinibLapatinib

CetuximabPanitumumab

Some Molecular Determinants of Clinical Benefit of Anti-EGFR Therapies

Determinant(s) Influence References

KRAS Mutation Negative J Clin Onc 25: 3230, 2007J Clin Onc 26: 374, 2008

BRAF Mutation, PTEN Loss

Negative J Clin Onc 27: 5924, 2009J Clin Onc 27: 2622, 2009

Amphiregulin or Epiregulin Expression

Positive J Clin Onc 25: 3230, 2007J Clin Onc 27: 5068, 2009

EGFR Gene Amplification

Positive J Clin Onc 27: 5924, 2009

The EGFR Signaling Network is Vast and Complicated

Making Sense of the ComplexityDefining the EGFR Network

638 Genes

Astsaturov et al, Science Signaling, In Press

CellNucleus

DNA

Exploring the EGFR functional network with siRNA-based genomics

Gene

siRNA

mRNA

ProteinProteinX638-element siRNA library created to target each gene in the EGFR signaling network

Astsaturov et al, Science Signaling, In Press

CellNucleus

DNA

Exploring the EGFR functional network with siRNA-based genomics

Gene

siRNA

mRNA

ProteinProtein

siRNA libraries target the expression of selected genes and permit study of the effects of targeted gene knockdown on cell function

XAstsaturov et al, Science Signaling, In Press

siRNA for 638 Genes Introduced into Cells

Lethal Phenotype?

“Hit”

Synthetic Lethal Screening of an EGFR Network-directed siRNA Library

Confirm, Validate and Map Hits

Cell Line Seeded into Multi-well Plate – Each Well is Precoated with

siRNA Against One Gene

Distinct Gene Knocked Down in Cells Growing

in Each Well

± EGFR inhibitor (IC30)

Astsaturov et al, Science Signaling, In Press

EGFR Network Determinants of Response to EGFR Inhibition

• 61 validated “hits” identify the EGFR “resistance space” in multiple cell lines

• Expected and unexpected mediators of resistance

• Knockdown of KRAS modestly sensitizes cells to EGFR inhibition

Astsaturov et al, Science Signaling, In Press

Possible Explanations and Implications

1. ADCC is not relevant2. EGFR Signaling is complicated

– Robust EGFR resistance networks 3. EGFR is not a relevant target in colon cancer

metastasis

Right Target, Wrong Setting?Epithelial-Mesenchymal Transition (EMT)

Adapted from Kalluri & Weinberg, J Clin Invest 119: 1420-8, 2009

EGFR SyndecanE-cadherin MUC1Cytokeratin DesmoplakinZO-1 1 (IV) collagenLaminin-1 miR200 familyEntactin

FTS binding protein FAP SnailFSP-1 SlugN-cadherin SIP1Vimentin -SMAFibronectin Twist-catenin GoosecoidO-cadherin LEF-1Syndecan-1 FOXC2miR10b miR21

Is EGFR an Appropriate Target when Cells Undergo EMT?

Epithelial EMT

EGFR pAKT

E-Cadherin PI3K

Vimentin IGF1R

Primary Tumor

Metastasis Established Metastasis

EGFR

EGFR

E-Cadherin

Vimentin

Druggable Targets

Relative Sensitivity to Cetuximab

High Low High

“EMT status may be a broadly applicable indicator of sensitivity to EGFR inhibitors.” (Barr et al, Clin Exp Metastasis (2008) 25:685–693)

Primary Tumor

Metastasis Established Metastasis

EGFR

Summary and Implications• Cetuximab therapy does not prevent metastasis

following resection of Stage III colon cancer – Likely to be true for panitumumab and other anti-

EGFR antibodies• EGFR-directed monoclonal antibodies should

not be used in Stage III colon cancer-directed adjuvant therapy regimens

• Numerous genes contribute to resistance to EGFR antagonism– May underlie resistance to cetuximab– Combination signaling inhibitor strategies are needed

• Cancer cell populations exhibit epithelial-mesenchymal transition (EMT)

• Colon cancer metastasis may not be dependent upon signaling through EGFR

• Targeting EMT-related processes may be a better approach to inhibit colon cancer metastasis

Summary and Implications

• Cancer cell populations exhibit epithelial-mesenchymal transition (EMT)

• Colon cancer metastasis may not be dependent upon signaling through EGFR

• Targeting EMT-related processes may be a better approach to inhibit colon cancer metastasis

A better understanding of colon cancer biology will inevitably lead to better treatments that– Target the right cells at the right time

– Effectively disrupt one or more targets that are required for the function of crucial signaling networks

Summary and Implications