Targeted Therapy

Question 1

Therapeutic Index

Answer 1

The therapeutic index (TI) is a pharmacological concept that represents the ratio of the dose of a drug that causes a therapeutic effect to the dose that causes toxicity.

It is calculated by the ratio of the median toxic dose (TD50) to the median effective dose (ED50).
Therapeutic index (TI) = TD50/ED50
➢ A larger therapeutic index suggests a wider margin of safety, meaning that the drug is less likely to cause toxic effects at therapeutic doses.
➢ When the therapeutic index exceeds a certain value, such as 10, it is generally considered that the drug has a good safety profile.

Question 2

Chemo vs Targeted Therapy

Answer 2

1. Chemotherapeutic agents do not discriminate between normal cells and cancer cells = Narrow therapeutic index
2. The majority of tumor cells have to be killed to achieve complete remission of the tumor, the application of high doses of chemotherapeutics is required at the expense of severe toxic side effects = Low selectivity
3. Most chemotherapeutic agents do not preferentially accumulate at the tumor site = Poor drug distribution

1) Targeted cancer therapies are drugs that block the growth and spread of cancer cells by interfering with specific molecules “molecular targets”(detected by companion diagnostic kit*) that are involved in the growth, progression, and spread of cancer.
2) Targeted therapies differ from standard chemotherapy in the following
three aspects:
2.1)Targeted therapies act on specific molecular targets that are associated with cancer, whereas standard chemotherapies act on all rapidly dividing normal and cancerous cells. Higher selectivity
2.2)Targeted therapies are deliberately chosen or designed to interact with their targets, whereas standard chemotherapies were identified because they kill rapidly dividing cells. Better drug distribution
2.3)Targeted therapies are often cytostatic (that is, they block tumor cell proliferation), whereas standard chemotherapy agents are cytotoxic (that is, they kill tumor cells). Larger therapeutic index

Question 3

What is a Biomarker?

Answer 3

The success of personalized medicine fundamentally depends on the development of biomarkers and companion diagnostics (CDx) that can be used to accurately identify the patient population suitable for the targeted therapies such as protein expression (HER2 overexpression-HercepTest for Trastuzumab/Herceptin), gene mutation (EGFR mutation-Therascreen PCR Kit for Gefitinib/Iressa), etc.

Question 4

Characteristics of a good target.

Answer 4

Characteristics of a good target.
* Highly expressed in cancer cells
* Selectively expressed in cancer cells
* Contributes mechanistically to cancer pathology
* Expression correlates with cancer stage or clinical outcome.

Question 5

Targeted Therapy Strategies

Answer 5

Small molecule drugs
* Advantage of low cost and high chemical stability (relative to biologic drugs)
* Drug discovery facilitated by high-throughput screening (HTS) techniques.
* Often cell permeable and can be used for intracellular targets.

Current Approaches: Therapeutic antibodies
* Advantage of high potential affinity/specificity
* High in vivo stability (very long half life)
* Amenable to engineering

Question 6

Hormone Therapy

Answer 6

Hormone therapies slow or stop the growth of hormone-sensitive tumors, which require certain hormones to grow.* e.g., Tamoxifen targeting ER positive BC

Hormones contribute to the progression of several cancers, notably breast, ovarian, and prostate.
- Treatment of breast cancer with Tamoxifen was the first bona fide targeted cancer therapy (1977). : Estrogen receptor antagonist, Remains standard therapy for breast cancer

Androgen deprivation for prostate cancer

Question 7

Neutralising Antibodies

Answer 7

• Ideal for cell surface receptors and
  extracellular ligands.
• Targets must be highly enriched in cancer cells, and mediate essential tumor-promoting pathways.

Question 8

Effector-mediated Cytotoxicity

Answer 8

• Takes advantage of endogenous function of antibodies.
• Antibody-dependent cell-mediated cytotoxicity
  (ADCC)
  – Activation of Fc receptor on effector cells
• Complement-dependent cytotoxicity (CDC)
  – Activation of C1 complex on effector cells
• Ideal for cancer. Less desirable for other indications.
• May be modulated by engineering or by post- translational modifications.
  – IgG1 strongly activates ADCC, IgG4 weakly activates ADCC

Question 9

Drug Conjugates

Answer 9

*Conferring specificity to cytotoxic drugs (i.e. higher tumor selectivity to drugs that are too toxic to be used on their own).
*Conferring cell killing power to monoclonal antibodies that are tumor-specific but not sufficiently cytotoxic.
*Ideally, a conjugate should be designed such that it remains non-toxic in circulation in vivo until it reaches its target
Brentuximab vedotin (Adcetris)
* Monomethyl auristatin E
o Highly toxic as free drug

Question 10

Molecular Targeted Radiotherapy

Answer 10

• Similar to external beam therapy
  – cancer cell destruction is achieved by means of radiation induced DNA
  damage.
• Similar to Chemotherapy
  – it is a systemic treatment that uses a molecule, in this case bearing a
  radiolabel, to deliver cytotoxic
  substance to disease sites.
• Has a “bystander” or crossfire effect
  – potential destruction of adjacent cells.
  – Very important since tumors are
  heterogeneous tissues.
• For primary and metastatic tumors
  – including malignant cell populations
  undetectable by diagnostic imaging.

Question 11

CAR-T

Answer 11

• Adoptive Cell Transfer (ACT)
  – Engineering patient’s own immune cells to target and kill cancer cells.
• Chimeric Antigen Receptor (CAR) T-cells
  – T cells are engineered to express CARs that can recognize tumor cells
• Promising clinical trials, particularly in acute lymphoblastic leukemia (ALL) – CD19+ malignant B cells

1. Extract T cells – collect T cells from patient
2. Reprogramming T cells – viral vector inserts CAR gene into T cells.
3. Manufacturing CAR T cells – CAR T cells are proliferated in vitro to a specific clinically relevant quantity and quality.
4. Patient Pre-conditioning – Patients are treated with chemo to lower white blood cell count to more readily accept T cells.
5. Treatment – Patient is given CAR T-cells.

Question 12

Nanomedicine

Answer 12

*A wide range of nanomaterials with their own pros and cons
*Functionalize with a wide range of therapeutics and targeting mechanisms
*Capable of making multi-functional complexes (Swiss Army knives of cancer therapy
*Improve tumor-specific drug delivery and impair general systemic distribution

• Enhanced Permeation and Retention (EPR) effect –
• Interendothelial gap junctions are larger between tumor and blood vessels than normal tissue.
• Impaired lymphatic drainage prevent NP clearance from tumors.
• Targeting component– Not a homing missile. More like a lock and key enhancing binding affinity.

Question 13

Other Kinases to target

Answer 13

Chronic myelogenous Leukaemia : bcr-abl2
t(9;22) 90-95% of cases
2bcr-abl fusion protein is produced, which results in a constitutively active abl kinase

Translocation results in “Philadelphia chromosome”
Results in fusion of BCR (multi-domain signaling protein) and Abl (non-receptor tyrosine kinase )

Imatinib (Gleevec, STI-571)
Indications: CML, gastrointestinal stromal tumors(GISTs)

Mechanism: competitive inhibitor of tyrosine kinases

Question 14

Gene Expression Modulators

Answer 14

Gene expression modulators modify the function of proteins that play a role in controlling gene expression. e.g., panobinostat, a HDAC inhibitor targeting HDAC to suppress expression of some oncogenes

Question 15

Apoptosis Inhibitors

Answer 15

Apoptosis inducers cause cancer cells to undergo a process of controlled cell death called apoptosis. e.g., Bcl-2 antisense (G3139) GENTA/Aventis in Phase II and III

Question 16

Angiogenesis inhibitors

Answer 16

Angiogenesis inhibitors block the growth of new blood vessels to tumors (a process called tumor angiogenesis). e.g., Bevacizumab (Avastin)

Avastin (Bevacizumab - TKI inhibitor) –Targeted Antibody Therapy
Duration of survival increased in a Phase III study of previously untreated patients with metastatic colorectal cancer: Chemo +/- Avastin
VEGF anti-agogenesis treatment

Question 17

Immunotherapies

Answer 17

Immunotherapies trigger the immune system to destroy cancer cells. Some immunotherapies are monoclonal antibodies that recognize specific molecules on the surface of cancer cells. e.g., KEYTRUDA® (pembrolizumab), PD-1 inhibitor approved for any unresectable or metastatic solid tumor with certain genetic anomalies.

Question 18

Conjugate Monoclonal antibodies

Answer 18

Monoclonal antibodies that deliver toxic molecules, an antibody-drug conjugate (ADC), can cause the death of cancer cells specifically. Once the antibody has bound to its target cell, the toxic molecule that is linked to the antibody—such as a radioactive substance or a poisonous chemical—is taken up by the cell, ultimately killing that cell. The toxin will not affect cells that lack the target for the antibody—i.e., the vast majority of cells in the body. e.g., Trastuzumab emtamsine (Kadcyla), HER2 overexpression Breast Cancer

Question 19

CD20

Answer 19

CD20
Selectively expressed by B-cells
Rituximab (Rituxan) – chimeric mAb
Indications: lymphoma, leukemia
Also used for autoimmune disease
Mechanism: ADCC, CDC
Ofatumumab (Arzerra) – human mAb
Indications: leukemia
Mechanism: ADCC, CDC
Tositumomab (Bexxar) – radiolabeled murine mAb
Indications: lymphoma
Mechanism: ADCC, CDC, radiotherapy (131I)
Obinutuzumab (Gazyva) – humanized mAb
Indications: leukemia
Mechanism: ADCC, direct cell death (DCD)

Question 20

CD30

Answer 20

CD30
Expressed by activated lymphocytes
Brentuximab vedotin (Adcetris)
Chimeric mAb drug conjugate
Indications: lymphoma
Mechanism: ADC-CD30 internalization of drug conjugate.*

Question 21

CD52

Answer 21

CD52
Expressed by mature lymphocytes
Alemtuzumab (Campath)
Humanized mAb
Indications: leukemia
Under investigation for autoimmune disease
Mechanism: ADCC, CDC

Question 22

Bispecific Antibody

Answer 22

Bispecific antibody : Blinatumomab
First-approved bispecific antibody drug
Crosslinks CD3 and CD19 to target an immune response against CD19-positive tumor cells
Approved in December, 2014 for the treatment of Relapsed or Refractory B-Cell Precursor Acute Lymphoblastic Leukemia

Question 23

CTLA-4

Answer 23

CTLA-4(CD152)
Inhibitory receptor on Th cells
Ipilimumab (Yervoy)
Human mAb
Indications: metastatic melanoma
Mechanism: Neutralizing
Acts as immunotherapy by activating T cell response and immune surveillance

Question 24

PDL1

Answer 24

Programmed cell death protein 1 (PD-1)
Expressed by T cells
Engagement with PD-L1 or PD-L2 inhibits T cell activation
Many tumors evade the immune response by expressing PD-L1/2
Nivolumab
Pembrolizumab
Atezolizumab

Question 25

Nuclear Proteins

Answer 25

Cyclin-dependent kinase 4 & 6 Interact with cyclin D to drive G1 and G1/S transition Mutation/overexpression of CDK4 and 6 cause deregulated cell cycle progression in a number of cancers Palbociclib is a small molecule inhibitor of CDK4 and 6 Approved for HER2-negative breast cancer

Question 26

Does Dual Therapy Work?

Answer 26

Pathological Complete Response: Trastuzumab(triggers immune system to target HER2 cells) (52.5%,); Lapatinib(TKI inhibitor) (53.2%); Combo (62.0%), p-value (combo vs trastuzumab) 0.095 (not significant) : Dual therapy does not necessarily improve therapy for overall HER2+ breast cancer population. Lapatinib : an oral, small molecule, dual tyrosine kinase inhibitor of ErbB-2 and ErbB-1, for use in combination with capecitabine Trastuzimab : In combination with paclitaxel

Question 27

HER2

Answer 27

HER2 overexpression - HER2 overexpression has been confirmed to be a good biomarker predicting likelihood of responding to HER2-targeted therapies - Furthermore, HER2-targeted therapy similarly was found to improve outcomes in HER2-positive gastric cancer and other cancer types such as colon cancer, bladder cancer, and biliary cancer.

Question 28

EGFR

Answer 28

EGFR mutations in lung cancer can result in more stable active conformations through the favorable repositioning of the N-lobe (magenta) of the acceptor kinase. Common EGFR mutations in NSCLC include exon 19 deletions and the L858R point mutation in exon 21 First-generation EGFR-TKIs Gefitinib (Iressa) - no IVD until Therascreen PCR kit in 2015 Erlotinib (Tarceva) Second-generation EGFR TKI Afatinib (Gilotrif) Third-generation EGFR TKI osimertinib (Tagriso) Despite initial responses, resistance to first- and second-generation TKIs can develop, often through the emergence of secondary mutations in the EGFR gene, such as T790M. The 3rd generation TKI, osimertinib (Tagrisso), were developed to overcome resistance, particularly the T790M mutation.

Question 29

MET targeted therapies

Answer 29

1. Small Molecule Kinase Inhibitors: These compounds interfere with the intracellular signaling cascade by inhibiting the phosphorylation of tyrosine residues on MET, thereby hindering downstream signaling. 2. Competitive Analogs of HGF: HGF is the natural ligand that activates MET. Competitive analogs of HGF are designed to mimic HGF and compete with it for binding to the MET receptor. 3. α-MET Antibodies: Monoclonal antibodies targeting the extracellular domain of the MET receptor are referred to as α-MET antibodies. These antibodies can bind to the MET receptor and block its activation, preventing the binding of HGF and inhibiting downstream signaling. 4. α-HGF Antibodies: Antibodies targeting HGF itself are designed to prevent HGF from binding to and activating the MET receptor. By neutralizing HGF, these antibodies indirectly inhibit MET signaling. 5. MET Decoys: MET decoys are soluble forms of the MET receptor or its extracellular domain. These decoys are designed to sequester HGF in the extracellular space, preventing it from binding to the cell surface MET receptor and triggering downstream signaling. This approach aims to neutralize HGF activity.

Question 30

Target Limitations

Answer 30

Limitations : Targeted therapies are only effective if tumors rely on targeted molecules Example: “triple negative breast cancer”. Do not express/rely on estrogen receptor (ER), progesterone receptor (PR), or HER2. Do not typically respond to tamoxifen or Herceptin, etc. By design, targeted therapies are selective for specific proteins or pathways. It is unlikely that any specific therapy will be effective for 100% of the cells in a given tumor Typically requires traditional chemotherapeutics as co-therapies Drug Resistance Mutation at drug binding site Downstream pathway mutation and/or activation Feedback upregulation of target Parallel pathway activation Pharmacological resistance “Approximately 50% of patients with EGFR mutant lung cancers who develop acquired resistance to EGFR TKIs have a second-site mutation T790M”. Good news is that Osimertinib, a third-generation drug, can be used in patients with EGFR T790 M. Toxicity : hypertension, hyperthyroidism Neutralisation Patients often develop immune responses directed toward biologic drugs Host antibodies can neutralize therapeutic mAbs or interferon