WK4- Modern Chemotherapy - ADCS

What is trastuzumab (Herceptin)?

A monoclonal antibody that targets the Her2 receptor on breast cancer cells. About 20% of breast cancers are Her2-positive.

How does trastuzumab work?

Binds to extracellular domain of Her2 → prevents dimerisation → G1 cell cycle arrest → reduced proliferation → reduced angiogenesis → may activate immune system (ADCC).

What is a biological drug?

A large molecule made by living cells (e.g., monoclonal antibody). Highly specific for its target.

How does a biological drug differ from classical chemotherapy?

Biologic = large molecule, made by living cells, high specificity, fewer off-target side effects. Chemotherapy = small molecule, chemically synthesised, low specificity (kills all dividing cells), more side effects.

What is an Antibody-Drug Conjugate (ADC)?

An ADC combines an antibody (targeting) with a cytotoxic payload (killing) via a linker. The antibody delivers the drug specifically to cancer cells.

What are the 4 components of an ADC?

1) Antibody (targeting), 2) Linker (connects drug to antibody), 3) Payload (cytotoxic drug), 4) Conjugation method (attachment chemistry).

What are the 4 steps of ADC mechanism?

1) Bind to receptor on cancer cell, 2) Receptor-mediated endocytosis, 3) ADC in endosome → lysosome, 4) Linker cleaved → drug released → cell death.

What are the two amino acids used to attach linkers to antibodies?

Lysine and cysteine.

What is the difference between lysine and cysteine conjugation?

Lysine = many sites (DAR variable), attached via NHS ester. Cysteine = fewer sites (hinge region), more controlled DAR, attached via maleimide.

What are the 4 types of linkers?

1) Acid-labile (hydrazone), 2) Reducible (disulfide), 3) Enzyme-cleavable (peptide), 4) Non-cleavable (thioether).

What is an acid-labile linker?

Hydrazone linker. Stable at blood pH (7.4). Breaks in acidic lysosome (pH ~4.5-5). Example: Mylotarg.

What is a reducible linker?

Disulfide linker (S-S). Breaks by glutathione inside cells (high intracellular glutathione, low in blood).

What is an enzyme-cleavable linker?

Peptide linker (e.g., valine-citrulline). Cut by cathepsin B (lysosomal enzyme). Example: Adcetris.

What is a non-cleavable linker?

Thioether linker. Very stable. Drug only released after complete antibody degradation in lysosome. Little bystander effect. Example: Kadcyla.

What is the bystander effect?

When a cleavable linker releases a membrane-permeable drug that can diffuse out and kill neighbouring cancer cells (even if they don't express the target antigen).

What are the three main payloads used in approved ADCs?

Maytansine (DM1), Calicheamicin, MMAE (monomethylauristatin E).

What is DM1?

A maytansine derivative. Microtubule inhibitor. Payload in Kadcyla (trastuzumab emtansine).

What is calicheamicin?

An enediyne that causes DNA double-strand breaks. Payload in Mylotarg (gemtuzumab ozogamicin).

What is MMAE?

Monomethylauristatin E. A synthetic analogue of dolastatin. Microtubule inhibitor (200x more potent than vincristine). Payload in Adcetris (brentuximab vedotin).

What are the key features of Mylotarg (gemtuzumab ozogamicin)?

Target: CD33. Payload: Calicheamicin. Linker: Acid-labile (hydrazone). Cancer: Acute myeloid leukaemia (AML). First ADC approved.

What are the key features of Kadcyla (trastuzumab emtansine)?

Target: Her2. Payload: DM1 (maytansine). Linker: Non-cleavable (thioether). Cancer: Her2-positive breast cancer. Little bystander effect.

What are the key features of Adcetris (brentuximab vedotin)?

Target: CD30. Payload: MMAE. Linker: Cleavable (peptide – valine-citrulline). Cancer: Lymphoma. Bystander effect.

What does Fab stand for and what does it do?

Fragment antigen-binding. The arms of the Y-shaped antibody that bind to the target antigen.

What does Fc stand for and what does it do?

Fragment crystallisable. The stem of the antibody that interacts with immune cells (e.g., NK cells, macrophages).

What is the drug-to-antibody ratio (DAR)?

The average number of drug molecules attached to each antibody. Typically 2-4 for approved ADCs.

What was the first ADC approved?

Mylotarg (gemtuzumab ozogamicin) in 2000. Withdrawn in 2010, reintroduced in 2017 at lower dose.

Why was Mylotarg withdrawn?

Increased patient death and no benefit over standard chemotherapy at the original dose. Reintroduced at lower dose.

What is the advantage of a non-cleavable linker?

More stable in blood, less premature release, fewer off-target side effects. Disadvantage: little bystander effect.

What is the advantage of a cleavable linker?

Allows bystander effect – drug can kill neighbouring cancer cells. Disadvantage: potential for premature release in blood.

What is the structure of an antibody?

Y-shaped protein with two heavy chains and two light chains. Fab regions at the tips (antigen binding). Fc region at the stem (immune cell binding).

What is the difference between Fab and Fc?

Fab = binds antigen (target). Fc = binds immune cells (calls for backup).

What is meant by "ultrapotent" payload?

The drug must be extremely toxic because only 2-4 molecules are delivered per antibody. Active at pM to low nM concentrations.

Complete the sentence: "Mylotarg uses a(n) ___________ linker (hydrazone) that breaks in the ___________ ."

acid-labile / acidic lysosome

Complete the sentence: "Kadcyla uses a(n) ___________ linker (thioether) which results in little ___________ effect."

non-cleavable / bystander

Complete the sentence: "Adcetris uses a(n) ___________ linker (peptide) that is cut by ___________ in the lysosome."

cleavable / cathepsin B

What is the hinge region of an antibody?

The flexible area between the Fab and Fc regions. Where papain cuts. Where cysteine conjugation often occurs (Adcetris).

What is maleimide used for in ADC chemistry?

Maleimide reacts with cysteine thiols (-SH) to attach linkers to the hinge region of antibodies.

What is NHS ester used for in ADC chemistry?

NHS ester reacts with lysine amines (-NH₂) to attach linkers to antibodies.

What is a self-immolative group?

A chemical group that automatically falls apart after linker cleavage, releasing the free drug. Found in MC-VC-PABC linker (Adcetris).

What is the target of Mylotarg?

CD33 (expressed on AML cells).

What is the target of Kadcyla?

Her2 (expressed on ~20% of breast cancers).

What is the target of Adcetris?

CD30 (expressed on lymphoma cells).

What is the mechanism of DM1?

Microtubule inhibitor – prevents tubulin polymerisation, blocks mitosis.

What is the mechanism of MMAE?

Microtubule inhibitor – similar to vinca alkaloids, 200x more potent than vincristine.

What is the mechanism of calicheamicin?

Causes DNA double-strand breaks via enediyne chemistry.

What happens if the linker is cleavable?

Drug is released in lysosome and can diffuse out to kill neighbouring cells (bystander effect).

What happens if the linker is non-cleavable?

Drug is released only after complete antibody digestion. The lysine or linker remains attached. Little bystander effect.

Why is a high drug-to-antibody ratio (DAR) not always better?

Too many hydrophobic drugs can cause ADC aggregation and poor solubility. DAR 2-4 is optimal.