Antibody Drug Conjugates (ADCs) and the Evolving Biopharmaceutical Industry

Introduction to Antibody Drug Conjugates (ADCs)

• Dr. Larry Winkers' Background:
  • Former graduate from MacHam (1993).
  • Industry veteran, retired as VP and Global Head of Pharmacokinetics and Drug Metabolism.
  • Currently doing consulting work in the biopharmaceutical industry.

The Evolving Biopharmaceutical Industry

• Relevance of the Class:
  • The lecture discusses how the course fits into the evolving environment of the biopharmaceutical industry.
• Focus on Antibody Drug Conjugates (ADCs):
  • ADCs as hybrid molecules combining antibodies and small molecules.
  • Significant mergers and acquisitions activity in the ADC space (approximately $100 billion$ spent recently).
  • ADCs are gaining prominence in the industry.

The Golden Era of Biopharmaceutical Research

• Favorable Time to Enter the Industry:
  • Great insights into biology due to advanced analytics and data curation.
  • Better understanding of disease biology, genetics, and pathophysiology.
  • Intersection of different fields leading to better and bigger ideas.
  • Increased potential for positive impact on human health.
• Global Health Mindset:
  • Growing focus on diseases impacting regions beyond North America.
• Unmet Medical Needs:
  • Significant opportunities in addressing unmet medical needs.
  • Innovative research is still rewarding, enabling companies to be profitable.

Evolution of Modality Independent Strategy

• Traditional Approach (Small Molecule Focus):
  • Historically, companies focused solely on small molecule drugs (pills) for every disease.
  • High failure rates, large doses, and individual variability were common.
  • Limited toolset: "If the only tool you have is a hammer, every problem looks like a nail."
• Modern Modality-Independent Approach:
  • Companies have diverse pipelines with various types of molecules.
  • Focus is on understanding the disease and selecting the optimal molecule type for intervention.
  • Technology portfolio allows for choosing the best molecule for a specific disease target.
• Examples of Molecule Types:
  • Viruses, nanoparticles, antibodies, bispecific antibodies, fusion proteins, small molecules, oligonucleotides, and antibody-drug conjugates.

Fundamental Requirements for Any Therapeutic

• Efficacy:
  • The drug must work as intended (pharmacodynamics).
• Safety:
  • The drug must be delivered safely at the intended regimen.
• Pharmacodynamics vs. Pharmacokinetics:
  • Pharmacodynamics: what the drug does to the body.
  • Pharmacokinetics: what the body does to the drug (drug disposition).
• Goal of Therapeutics (Especially in Cancer):
  • To "bend the curve" on Kaplan-Meier plots, indicating improved survival rates and progression-free survival.
  • Buying time for patients while scientific understanding and treatments advance.

Antibody Drug Conjugates (ADCs) Explained

• ADC Structure:
  • An antibody (heavy chain and light chain) linked to a drug (cytotoxic payload) via a tether (linker).
  • Various types of linkers exist.
• ADC Mechanism of Action (Idealized):
  • Antibody targets a receptor on a tumor cell.
  • ADC is internalized via endocytosis (early endosome, late endosome).
  • Vesicle fuses with lysosome, where the drug is released.
  • The released payload exits the lysosome and kills the cell.
• Rationale for ADCs:
  • To deliver a highly toxic payload selectively to cancer cells.
  • Increase the therapeutic index of cytotoxic drugs, making them safer and more effective.

Historical Context: The Magic Bullet Theory

• Paul Ehrlich's Vision:
  • Over 100 years ago, Paul Ehrlich proposed the idea of directing cancer-killing molecules specifically to cancer drugs (the "magic bullet" theory).
• Early Attempts (1950s-1960s):
  • Initial efforts used mouse antibodies, which were highly immunogenic.
  • Immunogenicity led to rapid clearance and release of the toxin, resulting in failure.
• Evolution of Antibodies:
  • Shift from mouse antibodies to chimeric, humanized, and fully human monoclonal antibodies.
• Advantages of Modern Antibodies:
  • Long half-life (e.g., IgG1 with a 21-day half-life, enabling once-a-month dosing).
  • Exquisite selectivity.
  • Low immunogenicity (e.g., Repatha with < $1\%$ immunogenicity in a 50,000-patient trial).
• Advancements in Payloads:
  • Early attempts used existing cytotoxic drugs, which were not potent enough.
  • Next-generation payloads are necessary to fully exploit the magic bullet concept.

Key Considerations in ADC Development

• Target Antigen:
  • Tumor specificity: The antigen should be uniquely expressed on the tumor to avoid toxicity to healthy tissues.
  • Patient stratification: Identifying patients who will benefit from the drug and avoid harm.
  • Patient selection is important since ADCs are toxic.
• Advantage Over Antibody Alone:
  • The ADC should provide a significant improvement in anti-tumor activity compared to the antibody alone.
  • Example: Comparing an antibody with some ADCC activity to the same antibody conjugated to a potent warhead.
• Impact on Binding Affinity:
  • Drug-Antibody Ratio (DAR): Number of drug molecules attached to each antibody.
  • Attachment of drugs should not compromise the antibody's affinity for its target.
  • DAR optimization: Aiming for an optimal DAR (e.g., DAR 4) while considering the distribution of drug attachment sites.
• Potency of Payloads:
  • Traditional cytotoxic drugs (e.g., methotrexate, vinblastine, doxorubicin) are often not potent enough for ADCs.
  • More potent payloads are needed to maximize the therapeutic effect.

Focus on Payloads: The Key to Improved Efficacy

• Payload Selection Matters:
  • Antibody (trastuzumab) can be the same, but different warheads can significantly impact efficacy.
  • Example comparing trastuzumab-DM1 to a new HER2 ADC with a different warhead shows major differences in survival rates.
• Mechanical Considerations for Payloads:
  • Handle for attachment: The cytotoxic drug needs a functional group (e.g., free thiol, amine) for linker attachment.
  • Attachment location: The attachment point should not interfere with the drug's mechanism of action.
  • Cellular uptake: The drug needs to be able to enter cells, considering potential transporter effects.
  • Lysosomal Stability: The drug needs to survive degradation within the lysosome.

Bystander Effect: Enhancing Efficacy

• Bystander Effect Defined:
  • The ability of the released cytotoxic payload to cross the membrane and kill neighboring cancer cells.
  • Important for heterogeneous tumors ( tumors with varying antigen expression).
• Experiment to Illustrate Bystander Effect:
  • Tumor growth curves in mice treated with control antibodies (non-tumor-targeting) conjugated to payloads.
  • The non-specific ADCs show some anti-cancer effects due to payload release and bystander killing.
• Molecular Properties Affecting Bystander Effect:
  • Membrane permeability of the payload is crucial.
  • Charge state: Charged molecules (e.g., with free carboxylic acids) may not cross cell membranes effectively.

Examples of Payloads and Their Properties

• Maytansinoids (MMAE vs. MMAF):
  • MMAE is more potent than MMAF due to differences in charge and membrane permeability.
  • MMAE is used in several marketed ADCs, while MMAF has not yielded efficacious drugs.
  • MMAF contains a carboxylic acid and therefore has poor cell permeability.
• Camptothecins:
  • Mechanism of action: Bind to topoisomerase I, inhibiting cell replication.
  • Lactone vs. Carboxylate form: The lactone form is the active form, while the carboxylate form is inactive.
  • Optimization of camptothecins: Introducing substituents (e.g., chlorine) to shift the equilibrium towards the active lactone form.

Transporters and Drug Resistance

• Role of Transporters:
  • Transporters (e.g., MDR1/PGP) can pump drugs out of cells, leading to drug resistance.
• Screening for Transporter Substrates:
  • Payloads should be screened to ensure they are not substrates for MDR1/PGP.
• Experimental Evidence:
  • Cells with PGP knocked out are much more sensitive to cytotoxic agents compared to cells with active PGP.

Factors Influencing Drug Metabolism

• Interpatient Variability:
  • Patient heterogeneity should be investigated. Does intercellular transport always happen?
• Multiple Factors Affect Metabolism:
  • Environmental Factors: Alcohol consumption (affects CYP2E1 activity).
  • Age: Changes in P450 enzyme expression (e.g., CYP3A7 in infants).
  • Disease: Rheumatoid arthritis (suppresses CYP3A4 expression).
  • Genetics: Polymorphisms in drug-metabolizing enzymes.
• Impact of Genetic Polymorphisms:
  • Poor vs. extensive metabolizers: Genetic variations can affect enzyme activity.
• Enzyme Saturation:
  • Drugs with metabolism via a few enzymes will show greater levels of AUC when one enzyme is blocked.
• UGT1A1 and SN38:
  • SN38 (active metabolite of irinotecan) is glucuronidated by UGT1A1.
  • Polymorphisms in UGT1A1 (e.g., *6, *28) lead to reduced activity.
  • UGT1A1 deficiency causes debilitating diarrhea due to SN38 accumulation (toxicity of payloads).

Lysosomal Stability and Enzyme Activity

• Lysosomal Stability:
  • ADCs must survive the harsh lysosomal environment (pH 4.5).
• Enzyme Activity in the Lysosome:
  • Lysosomal enzymes are highly active at pH 4.5.
• Importance of pH-Dependent Stability Testing:
  • Molecules stable at pH 7.4 may be rapidly degraded in the lysosome.

Potential Issues During Development

• Case Study: Covalent Inhibitor ADC:
  • A potent covalent inhibitor (kills cancer cells in cell lines) was ineffective as an ADC.
  • Capthesins in the lysosome degraded the molecule before it could exit and exert its effect.
  • Payload was completely catabolized.
• Key Screens for ADC Development:
  • Permeability, attachment location, multiple enzymes, MDR1 not a substrate, survival of lysosome.
• Why does the lysosome work in the ADC and not in simple cell lines?:
  • ADC requires transfer through a vesicle and ultimately the lysosome.
  • Cell lines skip all of those steps.

Concluding Remarks

• Perfect Timing to Enter the Industry:
  • Encouragement for students to pursue careers in the biopharmaceutical industry.
• Advice for a Successful Career:
  • Be curious: Maintain a lifelong curiosity and a desire to learn.
  • Think big: Don't limit yourself to a specific activity; see yourself as a scientist first.
  • Share ideas: Don't be possessive of your ideas; sharing leads to collaboration.
  • Challenge dogma: Bring scientific diversity and challenge existing beliefs.
  • Be modality agnostic: Adapt to different therapeutic modalities and stay open to new possibilities.