Lecture 19 - Targeted Therapies

What is a targeted therapy?

A therapy directed toward one specific molecular target

• often oncogenes or tumor supressors

What are the two approaches of targeted therapy?

1. Inhibit or activate a specific molecule or pathway

2. Target a drug to cells expressing particular molecules

When are targeted therapies most successful?

When they are combined with standard therapies

What are the three principles of targeted therapy

1. Target the driver mutation that is most central to the cancer

   • take away the tumor’s greatest advantage

2. increasing specificity results in increased discrimination of tumor vs. normal cell

   • decreases side effects

3. Customize treatment for each patient

   • the best chance of success and minimizes toxicity

What molecules are commonly targeted in targeted therapy?

Receptors (GF, Hormones) primarily, Signalling transducers (downstream of receptors), Mediators of Angiogenesis (HIF1a, VEGF), Mediators of immune inhibition

What are Small Molecule Inhibitors

A category of targeted therapy that bind to a specific site on a protein via defined molecular interactions

Three modes of action of SMI

1. Interfacial Inhibition

2. Orthosteric Inhibition

3. Allosteric Regulation

Interfacial Inhibition

The drug can bind to dimer (ligand-receptor) protein in both it’s active and inactive state, locking the protein into it’s inactive state

What “undruggable” molecules/DNA binding proteins can interfacial inhibition potentially target

mutated p53 or KRAS genes without targeting healthy genes

Orthosteric Inhibtion

The drug binds to normal ligand binding sites, resulting in direct ligand competition

• Example: creating a drug that binds to the estrogen hormone, therefore it can’t bind to ER

Allosteric Regulation

Create a drug that binds to a distant binding site on the receptor, resulting in a protein conformation change of the receptor or changes the affinity for the ligand

What are Blocking Antibodies?

Antibodies that bind to ligands or receptors and change the binding of ligand to the receptor

• similar to SMI, but just antibodies

What is cancer gene therapy?

A therapy that transfers genetic material to the cancer cell to change it’s genes

List the four mechanisms of action for cancer gene therapy

1. Gene Repair

2. Suicide Gene Delivery

3. Gene Silencing

4. Gene Reinstatement

Mechanism 1 - Gene repair

Transfer genetic material to cancer cell that corrects the mutations in the cancer cell (like CRISPR)

• change mutation on p53 gene to make it normal again

Mechanism 2 - Suicide Gene delivery

Deliver an enzyme to the tumor cell which can then activate a pro-drug only in the cancer cell

Mechanism 3 - Gene Silencing (used to target Oncogenes)

Delivering a drug/gene that inhibits oncogenes only present in cancer cells

• no effect on healthy cells!

Potential use for radio/chemo sensitizers

Mechanism 4 - Gene Reinstatement (Used to target tumor suppressors)

Deliver a wild-type tumor suppressor gene back to the cell to reinstate control over proliferation and survival

• if the gene is delivered to a healthy cell, who cares? there is no effect

What are three gene delivery methods to use for cancer gene therapy?

1. Liposomes

   • easy entry with the plasma membrane via fusion

2. Nanocarriers

   • passive uptake of DNA coated/encapsulated particles

   • can then magnetize particles to activate it

3. Viral delivery

   • most efficient method to deliver gene to a cell, nature already does it

What does viral delivery need to have to deliver gene properly?

It needs to have the ability to properly target the cancer cell’s membrane, in addition to having the proper genetic material for efficacy and safety

Two types of viruses that can be used to deliver gene

Retrovirus —> integrates new gene into the host cell genome

Adenovirus —> gene remains outside the human genome, so it can’t affect it

What is one issue we run into with cancer gene therapy using viral delivery?

immune compatibility; will the body create immunity to the virus capsule after exposure?

What is oncolytic tumor virus therapy?

Creating a productive and lytic virus that infects and kills tumor cells

Benefits of Oncolytic Tumor Virus Therapy

• It is safe: viruses are self-limiting, and they will get destroyed by the immune system

• can target metastasis: viruses can spread throughout the body

• genetically manipulative: we can edit the viral genome to improve targeting and efficacy or increase the immune system

How does oncolytic tumor virus therapy selectively target cancer cells?

cancer cells already have mutations in cell cycle control pathways so the viral genes involved in the dysregulation of p53 are redundant in cancer cells, leading to a specific expression of viral genes so immune cells can target them

• normal cells have effect control of their pathways and cell cycle, so if a virus activated it, the virus would activate p53 and undergo apoptosis

What Targeted Therapies Are Currently Available?

EGFR inhibitors, HER2 inhibitors, BCR-ABL inhibitors, and mTOR inhibitors

What two mechanisms do EGFR inhibtors use?

1. Antibody Receptor Inhibition: antibody blocks EGFR receptor so ligand can’t bind

2. Intracellular Tyrosine Kinase Inhibition

   • block intercellular phosphorylation/activation from happening

What cancer mutations would lead to insensitivity to EGFR inhibtors

mutations down stream from EGFR, like RAS

EGFR Inhibitor Drug Example

Cetuximab

How do HER2/neu inhibtors work?

blocks ligands, internalizes receptor, inhibits HER2 signalling —> apoptosis induction and cell cycle arrest

HER2/neu Inhibitor Drug Example

Herceptin (trastuzumab)

Alternative: SMI of tyrosine kinase = lapatinib

What is BCR-ABL

It is a fusion protein not expressed in normal cells, so it is an easy thing to target

How do BCR-ABL inhibitors work?

they are small-molecule inhibitors of tyrosine kinases. they bind to the activation pocket of the kinase and prevent conversion of ATP —> ADP

What is a con/negative thing about BCR-ABL inhibitors (not a side effect to patient)

The BCR-ABL fusion protein only has to have a single mutation for inhibitor drug not to work

BCR-ABL inhibitor drug example

Imatinib (Gleevac)

and if fusion protein mutates used dasatinib (leads to allosteric inhibition)

How do mTOR inhibitors work?

Cross-links FKBP12 and permanently inhibits formation of mTOR complex

What cell signalling pathway is mTOR part of?

PI3K

mTOR inhibitor drug example

Rapamycin (a SMI) or it’s rapalogs

What targeted therapy drugs are under development

1. Inhibitors of Apoptosis proteins

2. Metabolic modulators

3. epigenetic regulators

Limitations of Targeted Therapies

Target therapies have high specificities, so they can’t kill heterogenous cancers

Cancers develop resistance easily to them

Apply to a limited subset of the population, needing intense screening

Are targeted therapies often combined?

Yup