Drug Development Process

Introduction to the Drug Development Process

The drug development process is a systematic, multi-stage pathway that transforms an initial therapeutic idea into a safe, effective, and marketable medicine. It integrates scientific research, regulatory compliance, and clinical evaluation to ensure that the final product meets stringent standards of quality, safety, and efficacy.

This process generally involves the following broad phases:

1. Drug Discovery – Identification of a promising compound (new chemical entity, NCE) through screening, rational drug design, or natural sources.
2. Preclinical Research – Laboratory and animal studies to determine pharmacodynamics, pharmacokinetics, and toxicology.
3. Investigational New Drug (IND) Application – Regulatory submission to begin testing in humans.
4. Clinical Trials – Stepwise evaluation in human subjects (Phase I–IV) to assess safety, efficacy, dosage, and side effects.
5. Regulatory Review & Approval – Assessment by authorities such as the FDA, EMA, or CDSCO before marketing authorization.
6. Post-Marketing Surveillance (Phase IV) – Ongoing monitoring for long-term safety, rare adverse events, and real-world effectiveness.

The process is lengthy (10–15 years) and highly cost-intensive, with only a small fraction of compounds ultimately reaching the market.

Drug Discovery

Drug discovery is the first and most crucial step in the development of a new medicine. It involves identifying a potentially active compound (called a lead compound or new chemical entity – NCE) that can interact with a biological target to produce a desired therapeutic effect.

1. Sources of New Drug Molecules

New drug candidates can originate from multiple sources:

·        Natural Sources

• Plants: Example – Vincristine and Vinblastine from Catharanthus roseus.
• Microorganisms: Example – Penicillin from Penicillium notatum.
• Marine organisms: Bioactive peptides and alkaloids from sponges and corals.
• Animal sources: Hormones like insulin.

·        Synthetic Chemistry

• Rational chemical synthesis of new molecules.
• Example – Sulfonamides as antibacterial drugs.

·        Semi-Synthetic Compounds

• Modification of natural molecules to improve potency, reduce toxicity, or increase stability.
• Example – Ampicillin derived from natural Penicillin G.

·        Biotechnology / Recombinant DNA Technology

• Production of therapeutic proteins, monoclonal antibodies, and vaccines.
• Example – Recombinant human insulin.

·        Computer-Aided Drug Design (CADD)

• In silico techniques such as molecular docking, QSAR (Quantitative Structure–Activity Relationship), and structure-based drug design to identify lead candidates.

2. Identification of Drug Targets

A drug target is a specific molecule in the body, usually a protein (enzyme, receptor, ion channel) or nucleic acid, that interacts with the drug to produce a therapeutic effect.

• Target identification involves studying pathophysiology of diseases at the molecular level.
• Example:
• ACE inhibitors act on the angiotensin-converting enzyme in hypertension.
• Statins inhibit HMG-CoA reductase in hypercholesterolemia.

3. Lead Compound Identification and Optimization

• A lead compound is a chemical structure that shows desirable pharmacological activity but may still have limitations.
• The process involves:
• Screening libraries of chemical compounds.
• High-throughput screening (HTS): Automated testing of thousands of compounds for biological activity.
• Optimization: Structural modifications to improve efficacy, selectivity, pharmacokinetics, and safety.

4. Preclinical Candidate Selection

After optimization, the most promising compound(s) are chosen as preclinical candidates for further laboratory and animal testing.

• Criteria include:
• Potency and selectivity.
• Favorable pharmacokinetics (ADME profile).
• Acceptable safety margin.
• Potential for large-scale synthesis.

Preclinical Research

Once a lead compound is identified and optimized, it enters the preclinical research stage, where it is evaluated in laboratory and animal studies before being tested in humans. The goal is to establish the safety, pharmacological activity, and pharmacokinetic profile of the drug candidate.

This stage is mandatory before regulatory approval for clinical trials.

1. Objectives of Preclinical Research

• To determine pharmacodynamics (PD): How the drug affects the body (mechanism of action, therapeutic effect).
• To assess pharmacokinetics (PK): Absorption, distribution, metabolism, and excretion (ADME).
• To establish the toxicological profile (safety studies in animals).
• To decide the safe starting dose for human trials.
• To evaluate the formulation feasibility and stability of the drug.

2. Types of Preclinical Studies

A. Pharmacological Studies

• Primary pharmacodynamics: Demonstration of desired therapeutic effect (e.g., antihypertensive effect in animal models).
• Secondary pharmacodynamics: Study of effects on other systems/organs to detect unintended pharmacological actions.
• Safety pharmacology: Focuses on vital functions such as cardiovascular, respiratory, and central nervous system activity.

B. Pharmacokinetic Studies (ADME Studies)

• Absorption: Rate and extent of drug absorption.
• Distribution: Tissue penetration, protein binding, blood-brain barrier crossing.
• Metabolism: Biotransformation pathways (e.g., liver enzymes such as CYP450).
• Excretion: Routes of elimination (renal, biliary, fecal, pulmonary).

C. Toxicological Studies

• Acute toxicity: Effects of a single high dose.
• Subacute/subchronic toxicity: Repeated dosing for weeks/months.
• Chronic toxicity: Long-term exposure studies (up to 6–12 months).
• Carcinogenicity: Potential to cause cancer.
• Mutagenicity/Genotoxicity: Ability to cause genetic mutations (e.g., Ames test).
• Reproductive and developmental toxicity: Effects on fertility, embryo, and fetus.
• Local tolerance tests: Irritation or damage at site of administration (e.g., intramuscular injection site).

3. Good Laboratory Practices (GLP)

• Preclinical studies must follow GLP standards to ensure data quality, reproducibility, and regulatory acceptance.
• GLP ensures:
• Proper documentation.
• Standard operating procedures (SOPs).
• Validation of instruments and methods.

4. Outcome of Preclinical Research

• Identification of the No Observed Adverse Effect Level (NOAEL).
• Determination of the maximum tolerated dose (MTD).
• Extrapolation of animal data to calculate the safe starting dose for first-in-human trials.
• Compilation of preclinical results into the Investigational New Drug (IND) application.

Investigational New Drug (IND) Application

After successful preclinical research, a pharmaceutical company must obtain approval from regulatory authorities to begin human testing. In the United States, this is done by submitting an Investigational New Drug (IND) application to the Food and Drug Administration (FDA).

The IND ensures that the investigational drug can be administered to humans under carefully controlled conditions while safeguarding safety, rights, and well-being of participants.

1. Objectives of IND Application

• To provide evidence that the drug is reasonably safe for initial human use.
• To demonstrate that there is a scientific rationale for testing the drug in humans.
• To describe the clinical trial plan, including design, dose, duration, and monitoring.
• To ensure compliance with ethical principles and Good Clinical Practice (GCP) standards.

2. Types of INDs

1. Commercial IND: Submitted by pharmaceutical companies to develop a drug for marketing approval.
2. Research/Investigator IND: Submitted by an individual researcher or institution for academic or non-commercial clinical studies.
3. Emergency Use IND: Allows use of an experimental drug in emergency situations (e.g., rare disease, outbreak) when no standard treatment exists.
4. Treatment IND (Expanded Access IND): Permits use of an investigational drug outside clinical trials for patients with serious or life-threatening conditions who cannot enroll in trials.

3. Contents of an IND Application

An IND is a comprehensive document that includes:

A. Preclinical Data

• Pharmacology and toxicology results from laboratory and animal studies.
• Data on mechanism of action, safety margins, and toxic dose levels.

B. Chemistry, Manufacturing, and Controls (CMC)

• Drug composition and active ingredient details.
• Methods of synthesis, purity, and stability data.
• Formulation details (tablet, capsule, injection, etc.).

C. Clinical Protocols and Investigator Information

• Detailed trial design (phase I, II, or III).
• Inclusion and exclusion criteria for participants.
• Proposed dose, frequency, and route of administration.
• Safety monitoring and reporting plan.
• Information about principal investigators (qualifications, training, facilities).

D. Regulatory and Administrative Documents

• Informed consent forms.
• Institutional Review Board (IRB)/Ethics Committee approvals.
• Investigator’s Brochure (IB) summarizing all relevant information.

4. FDA Review Process

• After submission, the FDA reviews the IND within 30 days.
• If the FDA does not raise objections, the sponsor may begin clinical trials.
• The FDA may place a clinical hold if there are safety concerns, deficiencies in the study design, or inadequate manufacturing controls.

5. Importance of IND

• Serves as a regulatory bridge between preclinical and clinical development.
• Ensures protection of human subjects before first-in-human studies.
• Provides a structured framework for ongoing communication with the FDA during drug development.

Drug Characterization in Drug Development

Definition

Drug characterization is the process of systematically gathering all physical, chemical, biological, pharmacological, and toxicological information about a new drug candidate before it is tested in humans.

👉 It is like the complete biography of a drug, from identity and stability to mechanism of action and safety.

1. Purpose of Drug Characterization

• To ensure quality, safety, and efficacy before human testing.
• To understand identity, purity, stability, mechanism of action.
• To guide formulation development and dose selection.
• To meet regulatory requirements (IND/NDA submissions).

2. Key Parameters

A. Physicochemical Properties

1. Chemical Structure & Identity → Molecular formula, MW, NMR, MS, IR.
2. Physical Form → Crystalline, amorphous, polymorphism.
3. Solubility Profile → Aqueous & pH-dependent solubility.
4. Partition Coefficient (Log P) → Lipophilicity, permeability.
5. pKa → Ionization, solubility, absorption.
6. Stability → Thermal, light, oxidative, hydrolytic stability.
7. Melting Point → Purity & identification.
8. Impurity Analysis → TLC, HPLC, GC.

B. Biological & Pharmacological Properties

1. Mechanism of Action → Receptor, enzyme, target.
2. In vitro Activity → IC₅₀, EC₅₀ (potency).
3. In vivo Pharmacodynamics → Dose-response in animal models.
4. Pharmacokinetics (ADME) → Absorption, distribution, metabolism, excretion.
5. Bioavailability → Fraction reaching systemic circulation.
6. Toxicology → Acute, subacute, chronic, genotoxicity, reproductive studies.

C. Manufacturing & Quality Control

• Source of drug substance.
• Purity profile (HPLC, GC).
• Residual solvents.
• Microbial limits (if biological origin).

3. Steps in Drug Characterization Process

1. Compound Discovery & Selection → Identify promising molecule.
2. Basic Physicochemical Testing → Solubility, stability, structure.
3. Preformulation Studies → Guide dosage form design.
4. Biological Testing → In vitro & in vivo studies.
5. Toxicology Studies → Safety profiling.
6. Regulatory Dossier Preparation → Data for IND/NDA.

4. Role Across Drug Development Timeline

Stage	Drug Characterization Activities
Discovery	Identify active compound, chemical & physical data
Preclinical	Full physicochemical, biological & toxicological profiling
IND Submission	CMC (Chemistry, Manufacturing, Control), stability, pharmacology, toxicology data
Clinical Trials	PK/PD monitoring, formulation refinement
NDA Submission	Full characterization package for approval

5. Summary

• Drug characterization = Identity + Properties + Activity + Safety.
• Ensures that the candidate drug is safe, stable, effective, and manufacturable.
• Plays a central role in preclinical studies, regulatory submissions, and successful drug approval.

📌 In short:
Drug characterization is the foundation of drug development, bridging discovery to human trials by ensuring the molecule is well-understood, controlled, and safe

Dosage Forms in Drug Development

🔹 Definition

The dosage form is the physical form in which a drug is manufactured and administered to safely, effectively, and conveniently deliver the active pharmaceutical ingredient (API).
It ensures stability, absorption, accurate dosing, and patient compliance.

1. Purpose of Dosage Form Development

• Ensure accurate dosing.
• Protect drug from degradation (light, oxygen, stomach acid).
• Mask unpleasant taste/odor.
• Control release rate (immediate, delayed, extended).
• Target specific sites (colon, lungs, bloodstream).
• Improve patient compliance & acceptability.

2. Classification of Dosage Forms

A. Solid Dosage Forms

• Tablets (coated/uncoated, chewable, sublingual).
• Capsules (hard gelatin, soft gelatin).
• Powders/Granules (for reconstitution).
• Lozenges/Troches (dissolve in mouth).

✅ Advantages: Stable, accurate dosing, portable.
❌ Limitations: Not for patients with swallowing difficulty.

B. Liquid Dosage Forms

• Solutions (API dissolved).
• Suspensions (API dispersed).
• Emulsions (oil-in-water / water-in-oil).
• Syrups & Elixirs (sweetened oral liquids).

✅ Advantages: Easy to swallow, rapid absorption.
❌ Limitations: Less stable, preservatives needed.

C. Semi-Solid Dosage Forms

• Ointments (greasy, occlusive).
• Creams (O/W or W/O emulsions).
• Gels (water-based, non-greasy).
• Pastes (thick, protective).

✅ Advantages: Localized action, patient-friendly.
❌ Limitations: Messy, limited systemic use.

D. Parenteral Dosage Forms

• Injections (IV, IM, SC, ID).
• Infusions (large-volume sterile solutions).
• Implants (long-acting under skin).

✅ Advantages: Rapid onset, 100% bioavailability.
❌ Limitations: Requires sterility, skilled administration.

E. Inhalation Dosage Forms

• Metered Dose Inhalers (MDIs).
• Dry Powder Inhalers (DPIs).
• Nebulizers.

✅ Advantages: Direct lung delivery, fast action.
❌ Limitations: Device dependency, technique sensitive.

F. Novel Drug Delivery Systems

• Transdermal patches (continuous skin delivery).
• Microneedles (painless skin penetration).
• Nanoparticles & Liposomes (targeted delivery).
• Ocular inserts (sustained eye delivery).

✅ Advantages: Targeted, sustained release, better bioavailability.
❌ Limitations: Costly, technical complexity.

3. Factors Influencing Dosage Form Selection

• Drug properties – solubility, stability, pKa, log P.
• Route of administration – oral, parenteral, topical, inhalation.
• Target site – systemic or local.
• Patient factors – age, compliance, swallowing ability.
• Release profile – immediate, controlled, sustained.

4. Role of Dosage Forms in Drug Development Timeline

Stage	Dosage Form Role
Preclinical	Prototype forms (solutions/suspensions) for animal studies.
Phase I	Simple & safe dosage forms for safety/tolerability.
Phase II	Optimized formulations to test efficacy.
Phase III	Market-ready dosage form, validated for stability & manufacturing.
Post-Approval	Lifecycle management (new strengths, flavors, controlled-release forms).

💡 Quick memory tip:
👉 “Dosage form = vehicle of the drug. Without the right vehicle, even the best drug won’t reach the destination.”

📘 Declaration of Helsinki –

🔹 What is the Declaration of Helsinki?

The Declaration of Helsinki is an international set of ethical principles for medical research involving human participants, including research on identifiable human data or biological materials.

It was developed by the World Medical Association (WMA) and first adopted in 1964 (Helsinki, Finland).

It is considered the gold standard for ethics in human research.

📌 Purpose

To protect:

• The rights
• Health
• Safety
• Dignity
  of participants in clinical research.

📅 Major Revisions

The declaration has undergone many revisions:

• 1964 – First adoption
• 1975 – Major revision (Tokyo)
• 1983, 1989, 1996 – Updates
• 2000 – Biggest revision (Seoul)
• 2008 (Seoul)
• 2013 (Fortaleza, Brazil) – Latest version

🧭 Key Principles (Detailed)

✔️ 1. Ethical Principles and Oversight

• Research must follow ethical standards consistent with human rights.
• Must be reviewed and approved by an Independent Ethics Committee (IEC)/IRB.

✔️ 2. Risk–Benefit Assessment

Research is allowed only if:

• Benefits outweigh risks
• Risks are minimized
• Constant monitoring for safety

✔️ 3. Vulnerable Populations

Extra protection must be given to:

• Pregnant women
• Children
• Prisoners
• Mentally disabled persons
• Economically or socially disadvantaged groups

Research involving them is justified only if necessary.

✔️ 4. Scientific Requirements

Research must be:

• Scientifically sound
• Based on proper pre-clinical and laboratory data
• Registered in a publicly accessible registry (e.g., CTRI, clinicaltrials.gov)

✔️ 5. Informed Consent

A key highlight of Helsinki:

Participants must receive:

• Purpose of study
• Procedures
• Risks
• Benefits
• Right to withdraw anytime

Consent must be:

• Voluntary
• In writing
• Given preferably by subject or legal representative

✔️ 6. Privacy and Confidentiality

Personal data must be:

• Protected
• Not disclosed without permission
• Coded/anonymized

✔️ 7. Use of Placebo

Placebo is allowed only when:

• No proven therapy exists, OR
• Needed for scientific validity and
• Patients will not face additional serious harm

✔️ 8. Post-Trial Access

Participants should have access to:

• The best proven interventions after the trial ends
  especially if they benefitted from it.

✔️ 9. Compensation for Injury

Participants must be compensated for:

• Research-related injuries
• Disability
• Death

(India has strict compensation rules based on this principle.)

✔️ 10. Publication Ethics

• Both positive and negative results must be published.
• No data fabrication or selective reporting.
• Conflicts of interest must be declared.

🩺 Importance in Clinical Research

The Declaration of Helsinki is used to guide:

• Clinical trials
• Ethics committees
• Regulatory guidelines (like Schedule Y in India)
• Informed consent process
• Patient safety regulations

In short

“The Declaration of Helsinki (1964), developed by the World Medical Association, provides ethical guidelines for medical research involving human subjects. It emphasizes informed consent, independent ethical review, protection of vulnerable populations, scientific validity, risk–benefit assessment, use of placebo only when justified, confidentiality, compensation for injuries, and post-trial access. It is revised periodically, with the latest revision in 2013, and forms the basis for ethical regulations worldwide.”

✅ Therapeutic Index 

Definition

Therapeutic Index (TI) is the ratio between a drug’s toxic dose and its effective dose.
It indicates how safe a drug is.

Formula

\text{Therapeutic Index (TI)} = \frac{TD_{50}}{ED_{50}}

• TD₅₀ = dose that produces toxicity in 50% of the population
• ED₅₀ = dose that produces desired effect in 50% of the population

✅ Meaning

• Higher TI = safer drug (wide safety margin)
• Lower TI = more dangerous drug (narrow safety margin)

🔥 Examples

Drugs with High TI (safer)

• Penicillin
• Benzodiazepines (diazepam)
• SSRIs

Drugs with Low TI (narrow) → require close monitoring

• Warfarin
• Digoxin
• Lithium
• Phenytoin
• Theophylline

🧪 Therapeutic Window vs Therapeutic Index

Concept	Meaning
Therapeutic Index	Ratio (numerical) of toxic dose to effective dose
Therapeutic Window	Actual plasma concentration range where the drug is safe and effective

Example: Lithium’s therapeutic window = 0.6–1.2 mEq/L (narrow)

📌 For exams, remember:

• TI tells us safety margin.
• Low TI = monitor plasma levels.u
• Used in drug dosing, clinical trials, and toxicology.

✅ ED₅₀ (Effective Dose 50)

Definition:
The dose of a drug that produces the desired therapeutic effect in 50% of the population.

Meaning:

• Measures the potency of a drug.
• Lower ED₅₀ = more potent drug (because small dose produces effect).

Example:
If 10 mg of a drug relieves pain in 50% of subjects → ED₅₀ = 10 mg.

✅ LD₅₀ (Lethal Dose 50)

Definition:
The dose of a drug that causes death in 50% of test animals (usually animals, not humans).

Meaning:

• Measures acute toxicity.
• Higher LD₅₀ = safer drug.

Example:
If 200 mg/kg kills 50% of test animals → LD₅₀ = 200 mg/kg.

🔥 Relationship with Therapeutic Index (TI)

TI = LD₅₀ / ED₅₀
A larger TI means a safer drug.

📌 Quick Differences (Memory Aid)

Term	Full Form	Meaning	Used for
ED₅₀	Effective Dose 50	Produces effect in 50%	Potency
LD₅₀	Lethal Dose 50	Kills 50%	Toxicity

Human Equivalent Dose (HED) is the converted drug dose for humans based on an animal study.
Because animals and humans have different body sizes and metabolic rates, doses cannot be compared directly by mg/kg. Instead, conversion is done using body surface area (BSA).

✅ What is Human Equivalent Dose (HED)?

HED is the estimated dose in humans that would give a similar biological effect as in animals.

It is used in:

• Preclinical drug testing
• Toxicology studies
• Translating animal experiment doses to safe human starting doses

The conversion is based on Km factors, which are constants representing the ratio of body weight to body surface area for each species.

✅ Formula to Convert Animal Dose → Human Equivalent Dose

HED (mg/kg) = Animal dose (mg/kg) × (Animal Km ÷ Human Km)

Where:

• Km (human adult) = 37
• Common Km values:

Species	Km
Mouse	3
Rat	6
Hamster	5
Rabbit	12
Dog	20
Monkey	12
Human (Child 20 kg)	25
Human (Adult 60–70 kg)	37

📌 Example 1: Rat → Human Dose Conversion

Rat dose = 100 mg/kg

Km (rat) = 6
Km (human) = 37

HED = 100 × (6 / 37)
HED = 16.2 mg/kg

📌 Example 2: Mouse → Human Dose Conversion

Mouse dose = 200 mg/kg
Km (mouse) = 3
Km (human) = 37

HED = 200 × (3 / 37)
HED ≈ 16.2 mg/kg

📌 Reverse: Human Dose → Animal Dose

Animal dose = HED × (Human Km / Animal Km)

Example: Human dose = 10 mg/kg
Convert to dog dose:

Dog Km = 20
Human Km = 37

Dog dose = 10 × (37 / 20) = 18.5 mg/kg

⭐ Why Use BSA and Km Instead of Body Weight?

Because metabolic rate (drug clearance, toxicity threshold) correlates better with body surface area than weight.
Small animals have higher metabolism, so they need higher mg/kg doses.

⭐ Shortcut Table (Approximate Multipliers)

From Animal → Human	Multiply by…
Mouse → Human	0.081
Rat → Human	0.162
Dog → Human	0.540
Monkey → Human	0.324

In clinical trials, a Proof of Concept (PoC) is an early-stage study designed to show that a new drug actually works in humans for the intended disease or condition.

It is the stage where researchers answer the key question:

“Does this drug show a real therapeutic effect in patients?”

✅ Where PoC Fits in Clinical Trial Phases

PoC is usually established during Phase IIa trials.

Phases Overview

• Phase I → Safety, dosage, pharmacokinetics in healthy volunteers
• Phase IIa (PoC) → Early efficacy in small group of patients
• Phase IIb → Dose optimization
• Phase III → Large-scale efficacy & safety
• Phase IV → Post-marketing studies

⭐ Definition (Easy and Exam-Friendly)

Proof of Concept in clinical trials is the early clinical evidence that a drug produces the expected biological effect and improves the condition in a small patient population.

🎯 Objectives of PoC Studies

1.     Confirm early efficacy

2.     Verify dose-response relationship

3.     Assess short-term safety in patients

4.     Determine optimal dosage range

5.     Decide whether to continue or stop drug development

This is the go/no-go decision point in drug development.

🧪 Characteristics of PoC Trials

• Small sample size (20–100 patients)
• Short duration
• Conducted in actual patients, not healthy volunteers
• Uses biomarkers, surrogate endpoints, or early clinical outcomes
• May use adaptive design

🩺 Example (Simple)

A new drug for asthma shows improvement in FEV1 (lung function) in 40 patients over 4 weeks → PoC achieved.

🧬 PoC vs Phase I vs Phase II

Feature	Phase I	PoC (Phase IIa)	Phase IIb
Volunteers	Healthy	Patients	Patients
Focus	Safety	Early efficacy	Dose optimization
Outcome	Tolerability	Does it work?	Best dose

📌 Why is PoC Important in Clinical Research?

• Saves time and cost
• Prevents unnecessary Phase III trials
• Reduces risk for sponsors
• Helps justify funding
• Identifies failures early

Maximum Tolerated Dose (MTD) is a key concept in drug development, toxicology, and oncology trials.

✅ Definition (Simple & Exam-Friendly)

Maximum Tolerated Dose (MTD) is the highest drug dose that does not cause unacceptable or dose-limiting toxicity (DLT) in patients or animals.

It is the dose where:

• Side effects occur
• BUT are still tolerable
• And do not require stopping treatment

⭐ Where MTD Is Used

• Preclinical toxicology studies (animal LD50 → MTD determination)
• Phase I clinical trials (especially cancer trials)
• Safety evaluation for new drugs
• Dose selection for Phase II and III studies

🎯 Purpose of MTD

To find the safe upper limit of dosing before the side effects become too harmful.

🧪 Characteristics of MTD

• Identified during dose-escalation studies
• Based on adverse events classified as Dose-Limiting Toxicities (DLTs)
• Helps determine recommended Phase II dose (RP2D)

📌 How MTD is Determined in Clinical Trials

Phase I dose-escalation design is used:

3+3 Dose-Escalation Method (Classical method)

1.     Start with low dose in 3 patients

2.     If 0/3 show dose-limiting toxicity → escalate

3.     If 1/3 show DLT → add 3 more patients

4.     If ≥2/3 or ≥2/6 show DLT → stop

5.     The previous safe dose = MTD

📌 Criteria for Dose-Limiting Toxicity (DLT)

DLTs vary but commonly include:

• Grade 3 or 4 nausea/vomiting
• Severe neutropenia
• Organ toxicity (liver, kidney, heart)
• Life-threatening adverse effects

📌 MTD vs LD50 vs NOAEL (Important MCQ Differences)

Term	Meaning	Context
MTD	Highest tolerable dose without severe toxicity	Clinical trials, toxicology
LD50	Dose causing death in 50% of animals	Toxicology, animals
NOAEL	Highest dose with no observed adverse effects	Preclinical safety
LOAEL	Lowest dose where adverse effects appear	Preclinical safety

🧪 Example (Easy)

In a Phase I cancer trial:

• Dose 1 → no toxicity
• Dose 2 → mild toxicity
• Dose 3 → 2 out of 3 patients show severe toxicity

➡️ Dose 2 is the MTD

 

Lead Selection and Lead Optimization – Drug Development Process

Lead selection and optimization are crucial steps in early drug discovery where potential chemical compounds are identified, evaluated, and improved to become viable drug candidates.

1. Lead Selection

Lead selection involves choosing promising chemical entities from a pool of “hits” obtained through screening (HTS, virtual screening, natural products, etc.).
A lead is a compound with desirable biological activity but requires further improvement.

Steps in Lead Selection

1.     Hit Identification

• Hits arise from high-throughput screening, structure-based design, ligand-based design, combinatorial chemistry, or serendipitous findings.

2.     Primary Screening

• Compounds are tested for activity against a target (enzyme, receptor, protein).
• Preliminary potency (IC₅₀ / EC₅₀) is determined.

3.     Secondary Assays

• More specific tests to confirm selectivity and mechanism of action.
• Eliminates false positives.

4.     Early ADME/Tox Evaluation

• Solubility
• Permeability (Caco-2)
• Metabolic stability (liver microsomes)
• Cytotoxicity
• Helps filter out compounds with poor pharmacokinetics.

5.     Selection Criteria for Lead Molecules

• Adequate potency (micromolar to nanomolar range)
• Selective for target
• Acceptable physicochemical properties (Lipinski’s Rule of 5)
• Early safety signals acceptable
• Novel structure (chemical diversity)

Outcome: A lead compound with confirmed activity and acceptable early drug-like properties.

2. Lead Optimization

Lead optimization is a systematic modification of the selected lead to improve its pharmacological and pharmaceutical properties.

Objectives of Lead Optimization

• Increase potency
• Improve selectivity
• Enhance ADME properties
• Reduce toxicity
• Improve stability and solubility
• Achieve suitable PK/PD profile
• Maximize therapeutic index

Approaches Used in Lead Optimization

1. Structure–Activity Relationship (SAR) Studies

• Correlate chemical structure with biological activity.
• Modify functional groups to identify essential pharmacophores.

2. Medicinal Chemistry Modifications

• Bioisosteric replacement
• Rigidification/flexibilization
• Addition/removal of substituents
• Stereochemical optimization
• Prodrug design
• Optimization of lipophilicity (LogP)

3. Computational Approaches

• Molecular modeling
• Docking studies
• QSAR (quantitative structure–activity relationship)
• Pharmacophore modeling

4. ADME Optimization

• Improve absorption (enhanced solubility/permeability)
• Reduce first-pass metabolism
• Improve metabolic stability
• Reduce plasma protein binding if needed
• Enhance half-life

5. Safety and Toxicity Optimization

• Reduce hERG inhibition (cardiotoxicity)
• Minimize off-target interactions
• Decrease formation of reactive metabolites

3. Lead Candidate (Preclinical Candidate) Selection

After several rounds of optimization, a single compound is selected as the drug candidate.

Criteria for Candidate Selection

• High potency and specificity
• Favorable PK/PD
• Acceptable toxicity profile
• Good manufacturability
• Chemical stability
• Patentability
• Meets regulatory expectations for preclinical studies

Outcome: A development candidate that proceeds to preclinical testing (toxicology, pharmacokinetics, formulation development).

Conclusion

Lead selection and optimization form the foundation of successful drug discovery.

• Lead selection identifies promising molecules.
• Lead optimization refines these molecules into safe, effective, and drug-like candidates.

These steps use a combination of medicinal chemistry, computational tools, biological assays, and ADME/Tox studies to produce a compound ready for preclinical development.