elsaid - growth factor receptors

human epidermal receptors (HER)

• 4 structurally related receptors:

  • ErbB1 (EGFR, Her1)

  • ErbB2 (HER 2; Neu)

  • ErbB3

  • ErbB4

• extracellular domain containing:

  • ligand binding domain

  • dimerization domain

• intracellular domain is a tyrosine kinase domain

  • EXCEPT for ErbB3

ErbB1 activation

aka EGFR or HER1

• ligand binds —> extended (open) conformation available for receptor dimerization

  • dimerization w/ another ErbB1 (homodimer) or with a different receptor (heterodimer)

• dimerization —> tyrosine kinase activation and transphosphorylation

ErbB2 activation

aka HER2

• NO ligand binding domain —> activated by binding to one another

• extended (open) conformation available for receptor dimerization

  • dimerization w/ another ErbB1 (homodimer) or with a different receptor (heterodimer)

• dimerization —> tyrosine kinase activation and transphosphorylation

ErbB3

• ligand binds —> extended (open) conformation available for receptor dimerization

• NO tyrosine kinase

  • can only produce a heterodimer (bc there needs to be a tyrosine kinase for signaling — 2 ErbB3s will have no tyrosine kinase)

• dimerization (hetero) —> tyrosine kinase activation and transphosphorylation

ErbB4

• ligand binds —> extended (open) conformation available for receptor dimerization

  • dimerization w/ another ErbB1 (homodimer) or with a different receptor (heterodimer)

• dimerization —> tyrosine kinase activation and transphosphorylation

ligands and ErbB receptors

there are multiple ligands that can bind to one ErbB receptor

• ex. EGF, EPG, TFG𝜶, and AR are ligands that can bind to ErbB1 (HER1)

so, inhibiting ligands will NOT work —> inhibit the receptor directly

signaling pathways as a result of HER activation

1. PI3K/AKT Pathway

   • activates the mTOR pathway that results in cell survival and inhibits apoptosis pathway

2. MEK/ERK pathway

   • results in cell proliferation

3. PLC gamma and PKC pathway

   • results in cell proliferation

pharmacological approaches to inhibit signaling through HER family

1. monoclonal antibodies against the ligand binding domains of the HER family or other extracellular domains

2. small molecules (tyrosine kinase inhibitors; TKI) that binds to the ATP pocket in the intracellular kinase domain and prevents transphosphorylation of the intracellular domain

monoclonal antibodies against HER MOA

binding of the mab to the extracellular domain results in blocking the ligand from binding —> prevents dimerization

tyrosine kinase inhibitor (TKI) MOA

• TKI binds to the ATP pocket in the intracellular kinase domain and prevents transphosphorylation of the intracellular domain

• the ligand still binds to the extracellular domain and dimerization takes place

  • HOWEVER in the presence of TKI, NO signal pathway activation will take place

EGFR inhibitors:

• first generation TKI against EGFR

• eroltinib

• gefitinib

EGFR inhibitors:

• second generation irreversible TKI

afatinib

EGFR inhibitors:

• monoclonal antibodies against EGFR

• cetuximab

• panitumumab

Trastuzumab (Herceptin)

• a monoclonal antibody against the extracellular domain of HER2

• mechanistically, Trastuzumab can lead to:

  • blocking dimerization

  • activation of antibody-dependent cell-mediated cytotoxicity (ADCC)

  • endocytosis and receptor degradation

mechanisms of resistance towards Trastuzumab

1. steric effects:

   1. the Trastuzumab binding site is removed by the cancer cell

   2. heterodimerization with HER 1, 3 or 4

2. alternative elevations of other receptor tyrosine kinases:

   1. HER2 is NOT the main driver of proliferation. Other receptors e.g. insulin growth factor-1 receptor (IGF-1R) or C-MET are overexpressed

3. intracellular alterations:

   1. loss of function in PTEN (inhibitor of the PI3K/AKT pathway)

Emtansine

• is released from lysosomes following drug-HER2 endocytosis and lysosome mediated degradation

• targets (inhibits) the microtubules

• ~100x more potent compared to vinca alkaloids

  • NOT used alone b/c that would be very toxic

Ado-Trastuzumab emtansine

• Trastuzumab-Emtansine conjugate

• indicated for HER2-positive metastatic breast cancer who have progressed on Trastuzumab

• structure = 2 molecules of Emtansine per 1 molecule of Tastuzumab joined by a linker

Lapatinib

small molecule that inhibits HER1/HER2 at at 1:1 ratio

• good in case HER2 dimerizes w/ HER1 since:

  • Trastuzumab is specific for HER-2

  • Afatinib, Gefitinib, Erlotinib, Cetuximab, Panitumamab are specific for HER-1

BCR-ABL

• a fusion protein found in most pts with CML (Chronic Myelogenous Leukemia) and some patients with AML and ALL

• result of chromosomal translocation in hematopoietic stem cells

  • ABL gene (kinase) from chromosome 9

  • BCR gene from chromosome 22

• chromosomal (9, 22) translocation is called Philadelphia chromosome

  • disease is called Ph+ disease

• BCR-ABL is an intracellular kinase enzyme that activates PI3K/AKT and MAPK pathways —> cell proliferation and enhanced survival

Imatinib (Gleevac)

• binds to the ATP binding pocket of the inactive conformation of BCR-ABL

• shifts equilibrium —> active conformation switch to inactive to restore equilibrum

• net result: reduction of the availability of the active conformation available to phosphorylate substrates

imatinib resistance

point mutations (over 90 identified)

• mutation in the binding site of Imatinib resulting in loss of binding affinity

• mutations at other sites that result in inability of the enzyme to assume the inactive conformation (necessary for Imatinib binding)

T315I

Threonine to Isoleucine at position 315 results in loss of critical hydrogen bond at the TKI binding site

• isoleucin lacks hydroxy group in side chain —> NO H-bond

2nd generation BCR-ABL inhibitors

Nilotinib and Desatinib

• higher binding affinities compared to Imatinib

• do NOT inhibit T315I BCR-ABL in clinically-relevant doses

3rd generation BCR-ABL inhibitor

Ponatinib:

• higher binding affinities compared to Imatinib

• can inhibit T315I BCR-ABL

angiogenesis

• formation of new blood vessels from the endothelium of existing vessels

• fundamental to tumor growth, progression, and metastasis

• tumors initially has an avascular phase followed by an angiogenic switch

angiogenic switch

an imbalance between proangiogenic and antiangiogenic signals (favoring proangiogenic)

proangiogenic signals

• VEGF: vascular endothelial growth factor

• FGF: fibroblast growth factor

• PDGFB: platelet-derived growth factor beta

• EGF: epidermal growth factor

VEGF receptors (VEGFR)

VEGFR-1, VEGFR-2, VEGFR-3

• VEGFR-2 is the most important for angiogensis

6 ligands:

• VEGF-A, VEGF-B, VEGF-C, VEGF-D, VEGF-E, and PIGF

each receptor subtype has its own ligands based on binding affinity

• VEGF-A is common between VEGFR-1 and -2

binding of ligands in the extracellular ligand binding domain results in dimerization and autophorylation of the intracellualr tyrosine kianse domain and initiation of intracellular signaling cascade

• result: endothelial cell survivial, proliferation, migration, and invasion

anti-angiogenic agents

NOT cytotoxic (doesn’t inhibit growth/kill cell) and potentiates chemotherapy

• extracellular — targets VEGF-A

  • Bevacizumab

  • Aflibercept

• intracellular — targets receptor tyrosine kinase

  • Sorafenib

  • Sunitinib

  • Axitinib

  • Pazopanib

Bevacizumab

monoclonal antibody that targets VEGF-A

Afliberacept

VEGF-trap (fusion protein of VEGF-A binding domain in VEGFR-1 and VEGFR-2 and human IgG1 Fc) that targets VEGF-A

small molecule TKIs

Sorafenib, Sunitinib, Axitinib, and Pazopanib

• Sorafenib, Sunitinib, Pazopanib are multi-targeted TKIs — they inhibt angiogenesis pathways and inhibit specific tumor growth pathways and thus have the ability to reduce tumor growth and reduce angiogensis