Hit to lead activities 2

What is lead optimisation

Refine chemical structure of confirmed hit to improve drug like characteristics

What can be done in lead optimisation

Can do analogue synthesis - lots of compounds with similar structures - look at SAR to find optimum

Assess potency, bioavailability , selectivity, stability

What does analogue synthesis involve

Work with what you already have eg fragments

Work with what you know is likely to lead to lead compound

Develop a pharmacophore to work out what interactions are essential

What five features does the pharmacophore contain

Hydrogen bond donor

Hydrogen bond acceptor

Cationic centre

Hydrophobic linker

Aromatic substituent

Describe the dual D2/5HT antagonist

Have a known ligand for serotonin receptors and have dopamine which is more polar due to oh

Can fuse them forms a dual antagonist which has poor in vivo efficacy and has poor bioavailability and solubility

Then can modify whilst retaining pharmacophore - do this by adding halogen add a nitrogen and sulphur

Ended up forming ziprasidone

In optimisation cycle what should be assessed before designing and synthesising

Look at potency, selectivity, effacy physical chemical properties, pk , kinetics metabolism before design and synthesis

What factors to consider in lead optimisation

Make sure you have a good lead with a good log p molecular weight potency and it's functional

Chemical tractability

Polarity - add polar groups but don't want to be too polar as will get eliminated by urine quickly

Consider heteroaromatic groups - improves physical chemical properties

Add bioisosteres - replace hydrogen with fluorine makes it more metabolically stable

Make structure more simple

Describe the criteria for lipsinkis rule of 5

Molecule weight no more than 500 da

Less than 5 hydrogen bond donors

Less than 10 hydrogen bond acceptors

Log p has to be less than 5

Why is lipsinkis rule of five even used

Good guideline increases likelihood of drug crossing the membrane to reach target

Downsides to lipsinkis rule of 5?

Doesn't look at metabolic stability

Doesn't look at off target effects and toxicity

If there's too many hydrogen bond donors - able to interact with water, harder for desolation to occur - means drug less likely to cross membrane - donors influence more than acceptors

Higher log p than 5 - more lipophilic - means less aqueous solubility means poor absorption - interact with proteins so higher plasma protein binding

Also interact with cyp enzymes - means more interactions and off target effects - more likely to be metabolised quickly - becomes hydrophilic and removed by urine

Molecular weight over 500- harder to modify, might be less selectivity, off target toxicity and can affect absorption.

Give an example of lipophilicity being balanced in lead optimisation

Olanzipine vs clozapine

Phenyl group more polar in olzanzipine due to adding sulphur - given less frequently , means has a lower log p , less lipophilic so less likely to be metabolised quickly, less likely to interact with cyp enzymes so less interactions less off target toxicity

Compared to clozapine

How is lipophilicity promiscuous

Can increase affinity to target but also to off targets since hydrophobic interactions are less specific than polar interactions

Why consider lipophilicity in a drug candidate

Lipsinkis rule say that log p partition coefficient between water and octanol should be under 5 therefore for an ideal drug candidate, a drug shouldn't be too lipophilic or too hydrophilic - if too lipophilic - increases promiscuity - means more likely to have off target effects cyp interactions increased protein binding which means increased elimination when becoming hydrophilic to remove

Lipophilicity increases affinity to target BUT will also increase affinity to off targets - hydrophobic interactions are less specific to polar interactions

Too hydrophilic and too many polar groups means elimination may be fast - need to achieve a balance and might not cross the membrane

Involved in lead optimisation - refining confirmed hit to achieve drug like characteristics because important factor in achieving bioavailability absorption etc

What do you assume in in vitro and in vivo studies before doing human studies

In vitro assays can predict in vivo effect

Chemicals used in animals in vitro can have same effect on humans

And use of high doses in animals valuable for predicting how they'd affect humans

However can't always be certain that a chemical won't be toxic to humans despite studying in vitro

What five toxic effects could there be

Idiosyncratic reactions - unexpected

Formation of reactive metabolites

Interact with other substances

Activates receptors like herg which can cause fatal arrthymias prolong t wave

Mechanism based pharmacology

How can you avoid the problems of toxicity

By using high doses - less sensitive to toxicity and smaller

So less likely to have off target effects and more inclined to bind to target

What happens if a patient takes too much salbutamol

Normally causes airways to dilate by activating b2 receptors in lung

But if take too much then there's more in systemic circulation and can affect b2 receptors in heart - causing palpitations

How does paracetamol normally get metabolised

Normally metabolised through phase 2 metabolism through glucorinidation

Can it metabolise any other way and what toxic effects can this have

Can metabolise by phase 1 oxidation - forming n acetyl 4 benzoquinone imine which can either react with glutathione and get excreted by urine

But if take too much N acetyl 4 benzoquinine imine can react with protein and have toxic effect can damage hepatic cells causing dead

What three ways can you avoid this toxicity

Avoid functional groups that can show reactive metabolites

Test presence of reactive groups can be done by mass spec - look at binding to proteins and glutathione

Do the Ames test to test mutagenicity - gets genetically modified bacteria that won't grow without presence of histidine , expose it to the chemical

If chemical is mutagenic - will cause him bacteria to grow

Can do this in presence of liver enzymes to find mutagenic metabolites

What two ways can we prevent off target toxicity

Do lots of screening earlier in the project, look at similar targets and lots of assays for activity

Look at safety of potency effect it has on another receptor compared to primary receptor

What happens if herg gets activated

Can cause t wave to be delayed

Fatal arrhythmias

Or longs electrical impulses regulating heart beat

Why is hERG important?

Some drugs may have herg pharmacophore - can do silica studies to try and eliminate herg pharmacophore

Give four examples of drugs with herg pharmacophore

Terfenadine, astemizole. Greptafloxacin, sertindole

What can reduce herg activity by over 10x

Changing lipophilic aromatic group to a polar one eg change cl to oh

What is the main primary route of clearance

Metabolism

What is the main primary metabolic enzyme

Cyp450

What effect can compounds that can inhibit or induce cyps have

Can interact with other compounds

Give an example of drugs inhibiting cyp450 and the effect it had

Eg terfenadine as an otc antihistamine - when co admisntered with erythromycin or ketoconazole can cause cardiac arrhythmias

Due to inhibition of hepatic p450 enzymes

What can pk of lead compounds in animal help identify

Can predict pk in humans and screen pk properties based on effects of animals eg animals

What properties are assessed for pk and what is done to optimise pk

Look at half life cmax, tmax, volume of distribution, clearance

Use onset of action, duration of action , bioavailability to optimise pk

How do we select a pre clinical candidate

First have target, do in vitro screening, then have lead compound, optimise the lead to select the candidate

Then look at toxicology, clinically develop

Then select candidate

Prioritise safety and efficacy

What is the 6 criteria's of preclinical candidates

Shelf life of 2 years

Scale up to 100g

Select half life and dose

No toxicity in animal studies

Metabolism and pk studies of two species

Preferred crystalline form since more stable