Optimising Leads

Question 1

Physicochemical properties

Answer 1

• solubility = must be able to be absorbed in vivo
• lipophilicity = too polar won’t cross BBB and too lipophilic can have higher risk of toxicity or cross-reaction with other targets

logP =< 5 for best balance of properties

Question 2

Pharmacology

Answer 2

• selectivity = screen against closely related receptors
• mechanism of action studies = competitive vs non-competitive inhibition, agonist vs antagonist
• cross-species data = are key residues conserved?

Question 3

ADMET

Answer 3

• absorption
• distribution
• metabolism
• excretion
• toxicity

Question 4

Absorption

Answer 4

• model of intestinal absorption = human colon cancer line
• surrogate marker to estimate oral absorption
• cheaper and higher throughput than in vivo analysis

Question 5

Metabolism

Answer 5

• most in liver by cytP450 enzymes
• clearance through bile or kidneys
• can test by incubating in vitro with liver microsome and hepatocytes
• can determine time course of drug stability and measure intrinsic clearance

phase 1: oxidation (p450 alcohol dehydrogenase), reduction (NAD PH P450 reducaste), hydrolysis (esterases)

phase 2: conjugation with polar group

Question 6

Cytp450 Drug-Drug Interactions

Answer 6

• grapefruit juice has an inhibitor of cytP450
• so better when drug metabolised by multiple enzymes

Question 7

Oral bioavailability

Answer 7

amount of drug dose that reaches systemic circulation

Question 8

Oral dosing

Answer 8

• measure levels at different time points
• preclinical pharmacokinetic data to predict the human PK profile
• use information on potency at target and activity in in vivo models
• needs to be both safe and effective

Question 9

Toxicology

Answer 9

• identify potential adverse effects
• balance risk and benefit
• early lead optimisation looks for: cytotoxicity, genotoxicity and in vitro activity at receptors
• late lead optimisation uses in vivo studies to determine pathology and therapeutic index (legal requirement) in whole animals

Question 10

Therapeutic index

Answer 10

ratio between dose expected to elicit an adverse effect and dose required for benefit

Question 11

Genotoxic

Answer 11

reverse mutations allow cell growth in absence of histadine

Question 12

Animal models

Answer 12

• end point of lead optimisation is to show an effect in an animal model
• attempt to mimic human disease

Question 13

Why use animal models?

Answer 13

• cell data doesn’t always predict in vivo situation
• animals are multicellular
• pharmacokinetics will affect efficacy
• need to measure therapeutic index
• toxicology studies are a legal requirement
• dose prediction