Trips week 3

The lecture states there are 'no conceptual differences' between small and large molecules. What DOES differ?

CONCEPTUALLY identical: both must have a pharmacophore/bioactive face, both must overcome physiological barriers, both require a drug-target complex to produce a biological effect. PRACTICAL DIFFERENCES: (1) HOW they cross barriers — large molecules cannot use passive diffusion; (2) METABOLISM — small molecules are metabolized by CYP450; large molecules are degraded by proteases and filtered renally; (3) MANUFACTURING — chemical synthesis vs. biological production (cell culture); (4) ADMINISTRATION — mostly oral (SM) vs. parenteral injection (LM); (5) TARGET ACCESS — large molecules are restricted to extracellular/transmembrane targets.

What 4 physicochemical properties define large molecule drugs and how do they differ from small molecules?

Large molecules: (1) HIGH MOLECULAR WEIGHT (typically >1,000 Da; antibodies ~150,000 Da); (2) PRETTY LIPOPHILIC — contain hydrophobic protein cores; (3) PRETTY HYDROPHILIC — contain ionized groups, many H-bond donors/acceptors on surface; (4) FUNCTIONAL GROUPS — multiple amide bonds, charged residues, disulfide bonds. Unlike small molecules, they are BOTH lipophilic AND hydrophilic simultaneously, which is WHY they cannot cross lipid membranes by passive diffusion — they cannot achieve the simple lipophilic/hydrophilic balance required.

What makes peptides IDEAL starting points for drug design despite their liabilities?

IDEAL STARTING POINTS because: (1) Peptides are the NATURAL LIGANDS for most GPCRs, ion channels, and growth factor receptors — they are pre-validated pharmacophores. (2) They contain numerous STEREOGENIC CENTERS providing high selectivity potential. (3) They contain many functional groups (amines, carbonyls, hydroxyls) already constituting a pharmacophore for protein targets. (4) Solid-phase synthesis (Merrifield) enables rapid analog production. (5) Short plasma half-life is actually useful for rapidly modifying dose in dose-finding studies.

List the advantages AND disadvantages of peptide drugs from the lecture. Which 2 disadvantages are most critical for clinical development?

ADVANTAGES: natural ligands for GPCRs/channels; numerous stereocenters → selectivity; easily synthesized (SPPS); many pharmacophoric functional groups. DISADVANTAGES: (1) MEMBRANE IMPERMEABILITY — cannot cross intestinal mucosa → must be injected, poor patient compliance; (2) RAPID DEGRADATION — carboxypeptidases, aminopeptidases, and proteases give short plasma t½; (3) Limited CNS access (usually a BENEFIT for avoiding CNS toxicity); (4) Short half-life requires frequent dosing. The 2 most critical: MEMBRANE IMPERMEABILITY (forces parenteral administration) and RAPID PROTEOLYTIC DEGRADATION (requires formulation strategies or chemical modification).

Explain solid-phase peptide synthesis (SPPS). What is its key advantage over solution-phase synthesis?

In SPPS (developed by Merrifield): (1) The growing peptide chain is anchored to INSOLUBLE RESIN BEADS; (2) Amino acids are added sequentially in the C→N direction; (3) After each coupling, EXCESS REAGENTS are washed away (the peptide stays on the bead); (4) Protecting groups prevent unwanted side reactions; (5) Finally, the completed peptide is cleaved from the resin. KEY ADVANTAGE: ALL reactions occur in a SINGLE VESSEL — incomplete reactions, excess reagents, and byproducts are removed by simple FILTRATION/WASHING without isolation of intermediates. Dramatic reduction in purification steps, enabling high-throughput analog synthesis.

What is the difference between carboxypeptidase and aminopeptidase activity on a peptide, and why does this create a 'two-front attack'?

CARBOXYPEPTIDASE: hydrolyzes the peptide bond at the CARBOXY-TERMINAL (C-terminus) end → removes one amino acid at a time from the C-terminus. AMINOPEPTIDASE: hydrolyzes the peptide bond at the AMINO-TERMINUS (N-terminus) end → removes one amino acid at a time from the N-terminus. This creates a 'two-front attack' — proteolytic enzymes simultaneously degrade the peptide from BOTH ends, while internal proteases (serine, cysteine, aspartate, metalloproteases) cleave at specific internal sequences. Combined, these activities give peptides very short plasma half-lives (minutes).

Name all 7 classes of proteases with their catalytic mechanism and a clinical example for each.

1) SERINE PROTEASE — covalent catalysis via Ser-OH nucleophile; e.g., AChE, trypsin, chymotrypsin. 2) CYSTEINE PROTEASE — Cys-SH nucleophile; e.g., cathepsins (tumor invasion), caspases (apoptosis). 3) THREONINE PROTEASE — Thr-OH; e.g., 20S proteasome core. 4) ASPARTATE PROTEASE — non-covalent acid-base; e.g., HIV protease, renin. 5) METALLOPROTEASE — zinc ion; e.g., ACE (angiotensin-converting enzyme), MMPs. 6) GLUTAMATE PROTEASE — Glu-COOH; e.g., Pseudomonas aeruginosa protease. 7) ASPARAGINE PEPTIDE LYASE — autocatalytic; e.g., rare; regulatory roles.

Describe the chemical mechanism of AChE. What is the role of the serine residue and why is this mechanistically identical to other serine proteases?

AChE (like all serine proteases) uses a CATALYTIC TRIAD (Ser, His, Asp): (1) His abstracts a proton from Ser-OH, making Ser more nucleophilic; (2) Activated Ser attacks the carbonyl of the substrate (acetylcholine's ester bond) → forms a COVALENT ACYL-ENZYME intermediate; (3) Water attacks the acyl-enzyme → regenerates the free enzyme + releases acetic acid + choline. Identical mechanism to trypsin/chymotrypsin but with DIFFERENT substrate specificity (AChE is specific for choline esters). Mechanistic identity means inhibitor strategies (transition-state analogs, covalent inhibitors) work similarly across serine proteases.

Distinguish REVERSIBLE from IRREVERSIBLE AChE inhibition clinically and mechanistically.

REVERSIBLE: inhibitor forms a non-covalent complex (donepezil, galantamine) OR a SLOWLY-REVERSIBLE carbamylated intermediate (physostigmine, neostigmine) that hydrolyzes over hours. AChE activity returns. Clinical use: Alzheimer's disease, myasthenia gravis. IRREVERSIBLE: organophosphates (VX nerve agent, sarin, echothiophate) PHOSPHORYLATE the active-site serine permanently. The phosphorylated enzyme undergoes 'AGING' (dealkylation) making regeneration impossible. Clinical: nerve agent poisoning. ANTIDOTE: pralidoxime (2-PAM) can reactivate the enzyme IF given BEFORE aging occurs.

What is the goal of peptidomimetic chemistry and what 5 elements of the peptide backbone must be mimicked?

GOAL: design small, CONFORMATIONALLY CONSTRAINED, non-peptidic organic molecules that possess the biological properties of a peptide but overcome its liabilities (proteolytic instability, membrane impermeability). Five backbone elements that must be mimicked: (1) BACKBONE GEOMETRY — correct Φ/Ψ angles or equivalent; (2) SIDE-CHAIN PHARMACOPHORE — spatial positioning of key H-bond donors/acceptors and hydrophobic groups; (3) CHIRALITY — correct orientation of stereogenic centers; (4) CONFORMATIONAL RIGIDITY — constraint into bioactive shape; (5) H-BONDING CAPABILITY — retain or mimic critical H-bonds to target.

Describe the angiotensin II → losartan conversion as an example of peptidomimetic design.

Angiotensin II is an 8-amino acid peptide that activates AT1 receptors to raise blood pressure. PEPTIDOMIMETIC APPROACH: (1) Identify the critical pharmacophoric elements (key side chains) from SAR of angiotensin analogs; (2) Map these groups in 3D space in the bioactive conformation; (3) Design a small, non-peptidic scaffold (biphenyl-tetrazole: losartan) that presents these groups in the correct geometry. Losartan BLOCKS (not activates) the AT1 receptor → antihypertensive. It is orally bioavailable, metabolically stable, and resistant to proteases — overcoming all major peptide liabilities.

What are retro-inverso peptides and how do they overcome proteolytic degradation while maintaining activity?

RETRO-INVERSO peptides are made from D-AMINO ACIDS arranged in a REVERSED sequence compared to the parent peptide. The 'retro' = sequence reversal; 'inverso' = inversion of stereochemistry at each Cα. Result: the SIDE-CHAIN TOPOLOGY (3D arrangement of pharmacophoric groups) is MAINTAINED (mimics the parent peptide's bioactive face) but the BACKBONE is in the 'wrong' orientation for protease recognition. Since proteases recognize specific L-amino acid sequences in the natural N→C direction, retro-inverso peptides are NOT substrates for most proteases → dramatically increased metabolic stability. Applications: vaccines, diagnostic agents, and therapeutic peptide mimics.

What is PEGylation, how does it improve the PK of protein/peptide drugs, and what is the quantitative improvement seen with IFN-α?

PEGYLATION = covalent attachment of POLYETHYLENE GLYCOL (PEG) polymer chains to a protein or peptide. Improvements: (1) HALF-LIFE — dramatically extended by reducing renal filtration (PEG increases hydrodynamic radius above glomerular filtration threshold) and reducing proteolytic degradation; (2) IMMUNOGENICITY — reduced (PEG shields antigenic epitopes); (3) SOLUBILITY — improved. QUANTITATIVE: IFN-α without PEG: t½ 6–9 h, clearance 6,000 mL/h. PEG-IFN-α (peginterferon alfa-2a): t½ 72–96 h, clearance 60–100 mL/h. PEGylation reduced clearance ~100-fold and extended t½ ~10-fold, allowing once-weekly instead of three-times-weekly dosing.

Describe the 3 structural components of an ADC and explain the function of each.

1) MONOCLONAL ANTIBODY (mAb): provides TUMOR SELECTIVITY by binding to a surface antigen highly expressed on cancer cells but not (or minimally) on normal cells. It targets the ADC to the tumor. 2) CYTOTOXIC WARHEAD (small molecule, typically 300–1,000 Da): the cancer-killing payload. Must have SUBNANOMOLAR IC50 (highly potent) because only a small amount reaches each cancer cell. Examples: auristatins (microtubule inhibitors), maytansinoids, calicheamicins. 3) SYNTHETIC LINKER: connects antibody to warhead; controls WHERE and WHEN the warhead is released (stable in circulation → cleavable inside tumor cells by lysosomal enzymes or pH/reduction).

Why must the ADC warhead have subnanomolar (pM–nM range) potency when a simple small-molecule drug might only need micromolar potency?

Because the AMOUNT of warhead delivered per cancer cell is extremely small: an ADC typically carries 2–8 drug molecules per antibody (DAR = drug-antibody ratio). The antibody binds the target antigen, is internalized, and releases the warhead inside the cell. The intracellular warhead concentration achieved is orders of magnitude LOWER than what could be achieved by direct administration of a small molecule. Therefore, the warhead must be intrinsically potent enough to kill cells at the low concentrations actually achieved after ADC delivery.

What is the fundamental difference between traditional pharmacology and chemical inducers of proximity (CIPs)?

TRADITIONAL PHARMACOLOGY: drug binds target → inhibits/activates it STOICHIOMETRICALLY (1 drug molecule occupies 1 target copy at all times); requires sustained target occupancy; only works on 'ligandable' binding pockets; affects CATALYTIC function only. CHEMICAL INDUCERS OF PROXIMITY: drug brings two proteins into proximity → triggers PROXIMITY-DEPENDENT POST-TRANSLATIONAL MODIFICATION (ubiquitination, deubiquitination, phosphorylation); process is CATALYTIC (1 CIP → multiple events); can affect SCAFFOLDING functions; can target proteins WITHOUT ligandable active sites; substoichiometric activity possible.

What are the 3 required design components of a PROTAC and why is each critical?

1) TARGET WARHEAD: binds the protein of interest (POI). Does NOT need to bind the catalytic site — any surface-accessible site works. Must bind with sufficient affinity to form a stable ternary complex. 2) E3 LIGASE LIGAND: recruits the E3 ubiquitin ligase (most commonly CRBN or VHL). The E3-POI combination must be COMPATIBLE — not all E3/POI pairs form productive ternary complexes (some have steric clashes or unfavorable geometry). 3) LINKER: connects the two warheads. Length, rigidity, and chemistry determine whether the E3 and POI can achieve the correct GEOMETRY for ubiquitin transfer. CORRECT EXIT VECTORS from both warheads are critical — the linker attachment points must orient both warheads toward each other, not away.

PROTAC mechanism: explain the full 5-step catalytic cycle and where the 'catalytic advantage' arises.

Step 1: PROTAC binds the Target protein (POI). Step 2: The POI-PROTAC binary complex recruits the E3 UBIQUITIN LIGASE → forms a TERNARY COMPLEX (E3:PROTAC:POI). Step 3: The E3 ligase UBIQUITINATES the POI (transfers ubiquitin chains). Step 4: Poly-ubiquitinated POI is recognized by the 26S PROTEASOME → POI is DEGRADED. Step 5: PROTAC is RELEASED intact and can engage ANOTHER POI molecule → repeat. CATALYTIC ADVANTAGE: the PROTAC is not consumed in the reaction. One PROTAC molecule can degrade MULTIPLE POI copies in sequence. This is fundamentally different from an inhibitor that must occupy a POI copy continuously.

Compare PROTAC to Molecular Glue — what structural difference accounts for their different properties?

PROTAC: HETEROBIFUNCTIONAL — has a distinct warhead for POI, a distinct E3 ligand, and a LINKER connecting them. Large MW (typically >700 Da). Less cell permeable. More rationally designable once warheads are known. MOLECULAR GLUE: MONOFUNCTIONAL — a single small molecule that binds EITHER the E3 ligase OR the POI and creates a new surface that recruits the other partner (no linker). Small MW → better cell permeability and oral bioavailability. Rationally designing molecular glues is DIFFICULT (mechanism often discovered serendipitously — thalidomide, lenalidomide). Both achieve proximity-dependent ubiquitination but through different mechanisms.

What is a DUBTAC and how does it achieve the OPPOSITE of a PROTAC?

DUBTAC (De-Ubiquitinase Targeting Chimera) recruits a DE-UBIQUITINASE (DUB) enzyme to REMOVE ubiquitin chains from a protein that is being degraded by the cell. Mechanism: DUBTAC (warhead for target POI + ligand for DUB) → forms ternary complex (DUB:DUBTAC:POI) → DUB removes ubiquitin from POI → POI is no longer degraded by the proteasome → POI PROTEIN LEVEL STABILIZED/INCREASED. Clinical utility: diseases where a beneficial protein is BEING INAPPROPRIATELY DEGRADED — DUBTAC could restore normal protein levels. Opposite of PROTAC: PROTAC degrades a pathological protein; DUBTAC stabilizes a beneficial protein.

Give the key statistics for monoclonal antibody approvals from the lecture.

~700 mAbs approved GLOBALLY; ~109 new mAbs approved per year globally; 165 mAbs approved by the FDA (~20% of all approved drugs); approval/success rate for novel mAbs: 14–23%; IgG1 is the most widely used subtype for cancer therapeutics. In 2024, 15 new antibody-based biologics and 1 CAR-T-based antibody were approved by the FDA.

What are the top 4 selling drugs in 2025 and what do they have in common?

1) KEYTRUDA (pembrolizumab, Merck) — PD-1 checkpoint inhibitor, oncology. 2) OZEMPIC/WEGOVY (semaglutide, Novo Nordisk) — GLP-1 receptor agonist, T2D/obesity. 3) MOUNJARO/ZEPBOUND (tirzepatide, Eli Lilly) — GLP-1/GIP dual agonist, T2D/obesity. 4) DUPIXENT (dupilumab, Sanofi/Regeneron) — IL-4/IL-13 inhibitor, immunology. Common theme: ALL are BIOLOGICS (large molecules) — 3 of 4 are monoclonal antibodies or peptide-based; 2 of 4 target metabolic disease (GLP-1 pathway); all require parenteral or subcutaneous administration.

What is siRNA and what is its mechanism of action for gene silencing?

siRNA (small interfering RNA) is a short (21–23 nucleotide) DOUBLE-STRANDED RNA molecule that triggers gene silencing via the RNA interference (RNAi) pathway. Mechanism: (1) siRNA is introduced into the cell; (2) The RISC (RNA-Induced Silencing Complex) is loaded with the antisense strand of siRNA; (3) RISC uses the antisense strand to find complementary mRNA sequences; (4) RISC CLEAVES the target mRNA → prevents translation → gene product is not made. Unlike gene therapy, siRNA does NOT alter DNA — it works at the post-transcriptional level and the effect is TRANSIENT (requires repeated dosing). Example: ONPATTRO (patisiran) for hereditary transthyretin amyloidosis.

Distinguish mRNA-based therapeutics from traditional protein biologics and from gene therapy.

mRNA THERAPEUTICS vs. PROTEIN BIOLOGICS: instead of manufacturing the protein externally (complex, expensive biologics manufacturing), you deliver the mRNA INSTRUCTION — the patient's own cells produce the protein. This enables rapid production, personalization, and avoids the immunogenicity of foreign proteins. mRNA THERAPEUTICS vs. GENE THERAPY: mRNA does NOT enter the nucleus and does NOT integrate into the genome → NO risk of insertional mutagenesis. However, mRNA is TRANSIENT (cells eventually degrade it) → effect is not permanent (may be advantage for vaccines, disadvantage for long-term protein replacement).

What are the key challenges still facing mRNA-based therapeutics?

1) DURABILITY — mRNA is naturally degraded; effect is transient; requires repeated dosing or modified nucleosides to extend half-life. 2) IMMUNOGENICITY — innate immune system recognizes foreign RNA → inflammatory side effects; modified nucleosides (pseudouridine, 1-methylpseudouridine) reduce this. 3) DELIVERY — mRNA must reach the target cells intact; currently mostly delivered to LIVER via LNPs; non-liver tissue targeting is challenging. 4) PERSONALIZATION — personalized cancer mRNA vaccines require rapid manufacturing for each patient. 5) STABILITY — requires cold chain (-20°C to -70°C) storage.

What are the advantages of AAV gene therapy and what are its key limitations?

ADVANTAGES: (1) Minimal pathogenicity in humans; (2) Can establish LONG-TERM gene expression (months to years) in non-dividing cells (neurons, photoreceptors, muscle) — unlike retroviral vectors; (3) Multiple serotypes (AAV1–13) with different tissue tropisms enable targeting; (4) Non-integrating (mostly episomal) → low insertional mutagenesis risk. LIMITATIONS: (1) LIMITED CARGO CAPACITY (~4.7 kb insert limit) — cannot deliver large genes (e.g., dystrophin requires microdystrophin truncation); (2) PRE-EXISTING IMMUNITY — many people have antibodies to AAV capsid from natural exposure; (3) MANUFACTURING COMPLEXITY — difficult, expensive, batch-to-batch variability; (4) ONE-TIME TREATMENT only (anti-capsid immune response prevents re-dosing).

Explain CRISPR/Cas9 gene editing: mechanism, advantages over previous gene therapy, and key concerns.

MECHANISM: A guide RNA (gRNA) is designed to match the target DNA sequence. The gRNA directs the Cas9 NUCLEASE to that site → Cas9 cuts BOTH strands of DNA. The cell's repair machinery then: (1) NON-HOMOLOGOUS END JOINING (NHEJ) — error-prone, creates insertions/deletions (INDELs) → disrupts gene (knockout); or (2) HOMOLOGY-DIRECTED REPAIR (HDR) — if a DNA template is provided, the correct sequence is inserted (knock-in, correction). ADVANTAGES: precise, programmable, corrects disease-causing mutations, works in somatic cells. KEY CONCERNS: (1) OFF-TARGET editing — Cas9 may cut at similar but unintended genomic sites → mutagenesis risk; (2) DELIVERY to target tissues in vivo; (3) IMMUNE RESPONSE to Cas9 protein; (4) Ethical concerns about germline editing.

Compare ex vivo and in vivo gene therapy — when is each preferred?

EX VIVO: Cells (blood, bone marrow, stem cells) are REMOVED from the patient → genetically corrected in the LABORATORY (viral vector transduction or CRISPR editing) → quality-controlled, expanded → RE-INFUSED. PREFERRED when: target cells are accessible (hematopoietic cells, T cells); quality control of modified cells before administration is critical; off-target editing can be screened before re-infusion. Examples: CAR-T, stem cell gene therapy for sickle cell (Casgevy). IN VIVO: gene therapy delivered DIRECTLY INTO the patient (via AAV vector injection or LNP injection). PREFERRED when: target cells are NOT accessible ex vivo (neurons, photoreceptors, liver hepatocytes in situ); systemic delivery is needed. Examples: onasemnogene (Zolgensma) for SMA.

What is CAR-T cell therapy and what are its 3 key clinical challenges?

CAR-T (Chimeric Antigen Receptor T-cell) therapy: (1) Patient's T cells are COLLECTED (leukapheresis); (2) T cells are ENGINEERED in the lab to express a synthetic CAR — a fusion of an extracellular antibody fragment (targeting the tumor antigen, e.g., CD19) linked to intracellular T-cell signaling domains; (3) Engineered CAR-T cells are EXPANDED; (4) INFUSED back into the patient where they find and KILL cancer cells expressing the target antigen via cytotoxicity. 3 KEY CHALLENGES: (1) CYTOKINE RELEASE SYNDROME (CRS) — massive immune activation from rapid cancer cell killing → life-threatening inflammatory storm; (2) NEUROTOXICITY (ICANS) — neurological side effects; (3) MANUFACTURING COMPLEXITY — autologous (patient-specific) production is time-consuming and expensive; not yet effective for most SOLID TUMORS.

Ziconotide (Prialt) is a peptide drug for pain. What are its structural features and why must it be administered intrathecally?

Ziconotide: synthetic equivalent of ω-conotoxin MVIIA from the cone snail Conus magnus. It is a 25-AMINO ACID polybasic peptide with 3 DISULFIDE BONDS (making it conformationally rigid and resistant to some proteolysis). Mechanism: N-type voltage-sensitive CALCIUM CHANNEL ANTAGONIST → blocks neuronal calcium entry → reduces pain signaling. INTRATHECAL ADMINISTRATION (directly into CSF): because (1) 25 amino acids → too large to cross the blood-brain barrier or GI mucosa; (2) Systemic (IV or oral) administration would require doses too high to achieve analgesic CNS concentrations without causing peripheral side effects; (3) Intrathecal delivery places drug directly at spinal cord pain pathways.

What are the delivery limitations of gene editing tools? List at least 4 delivery methods and their key limitations.

1) AAV (viral): limited cargo capacity (~4.7 kb); immune response to capsid prevents redosing; manufacturing challenges. 2) LENTIVIRAL VECTORS (viral): risk of INSERTIONAL MUTAGENESIS (integrate into genome); limited tropism. 3) LIPID NANOPARTICLES (LNPs, non-viral): low bioavailability to non-liver tissues; lack of target cell specificity; possible instability. 4) ELECTROPORATION (physical): physical CELL DAMAGE from electrical pulse; alteration of cellular homeostasis; used mainly ex vivo. 5) ADENOVIRUS (viral): STRONG IMMUNE RESPONSE; mostly transient transgene expression; efficient but one-time use. 6) VIRUS-LIKE PARTICLES (non-viral): lower transduction efficiency; cargo limitations.

What are the 4 types of biological products and give one example of each?

1) BLOOD DERIVATIVES/WHOLE BLOOD/BLOOD COMPONENTS — plasma, clotting factors (Factor VIII for hemophilia). 2) VACCINES — Gardasil 9 (HPV), COVID-19 mRNA vaccines (Comirnaty). 3) ALLERGENIC EXTRACTS — allergy immunotherapy extracts. 4) THERAPEUTIC PROTEINS — insulin (Humulin R), monoclonal antibodies (Herceptin/trastuzumab), fusion proteins (Enbrel/etanercept), ADCs (Adcetris/brentuximab vedotin).

What is the difference between a BIOLOGIC, a BIOSIMILAR, and a GENERIC drug?

BIOLOGIC: original large-molecule drug produced by a biological system (cells, organisms). BIOSIMILAR: a biologic drug produced by a DIFFERENT manufacturer that demonstrates 'no clinically meaningful differences' from the reference biologic. NOT identical (cannot be exactly reproduced due to complex biological manufacturing) — hence 'biosimilar' not 'biobrand.' Requires analytical, non-clinical, and some clinical data. GENERIC: a small-molecule drug that is chemically IDENTICAL to the original ('same active ingredient, same dose, same route'). Produced by chemical synthesis, which is perfectly reproducible. Generics are interchangeable; biosimilars may or may not be designated interchangeable.

The lecture mentions peptide conjugates — what approaches are used and what properties of peptides make them attractive for conjugation?

Properties making peptides attractive conjugation partners: FACILE SYNTHESIS (SPPS allows rapid attachment of functional groups); HIGH SPECIFICITY AND AFFINITY for biological targets; NARROW PHARMACOKINETIC DISTRIBUTION (targeted delivery); SHORT HALF-LIFE (useful for controlling exposure duration). Conjugation approaches: (1) ADC-like DRUG CONJUGATES — cytotoxic payload linked to a targeting peptide; (2) RADIOLABELED PEPTIDES — peptide targets tumor, radioactive isotope delivers local radiation (peptide receptor radionuclide therapy, PRRT); (3) IMAGING AGENTS — peptide-chelator-metal complexes for PET/SPECT; (4) VACCINE ADJUVANT CONJUGATES.