R8,9- Drgs used in the treatment of asthma and copd : B2 agonists

What are the types of adrenoreceptors?

Alpha (α - receptors)

• GPCR receptors

• Second messenger depends on subtype

• Two subtypes – α1 and α2 – further divided

Beta (β - receptors)

• GPCR receptors

• Second messenger is always cyclic AMP

• Three subtypes – β1, β2 ( airways) and β3

What are the 2 natural ligands for adrenoreceptors?

Adrenaline-hormone

• secondary amine

• equally potent/active at both subtypes

• produced from noradrenaline in the adrenal glands

Noradrenaline-neurotransmitter

• primary amine

• more potent at receptors

These are catecholamines :

• Both OH are together ( catechol)

• alkylamine

How are the natural ligands made ?

• Made by biosynthesis

1. Tyrosine hydroxylase works on L-tyrosine to produce levodopa

2. DOPA decarboxylase removes a carbonyl group to give dopamine (NT)

3.dopamine b-hydroxylase changes dopamine into noradrenaline

4. N-methyltransferase transfers a methyl group to N to give adrenaline ( only one isomer so the reaction goes one way)

Note:

• Tyrosine hydroxylase is the rate-limiting step ( if no TH, then nothing works)

• Negative feedback loop from noradrenaline -the body knows how much to make; only made if needed

What are the 2 pathways for natural ligands to be metabolised?

• Two mechanisms - same metabolite

• Metabolite is water-soluble and excreted in urine

• Both reactions involve enzymes: COMT( transfers methyl group to catechol) ,MAO ( oxidises amine)

• ALDEHYDE - CARBOXYLIC ACID ( vanillymandelic acid), so it is water-soluble and can be excreted

• The reaction can use any enzyme in any order

How do you determine how natural ligands ( na, a) bind?

Determined through:

• mutagenesis – deletion of an amino acid (replaced with alanine to see whether binding still occurs)

• molecular modelling

• structure-activity relationships

• no crystal structure

• Receptor is a GPCR -7 transmembrane domains

• Binding occurs at TM 3, TM 5 and TM 6

• Structure-activity relationships reveal essential and useful binding interactions

What is the SAR for adrenoreceptors- b alcohol ?

• Stereochemistry matters – indicates it has a role in binding through H-bonding

• R is more active than S

• BUT dopamine also binds when oh removed So this binding interaction is not ESSENTIAL but IMPORTANT

What is the SAR for adrenoreceptors- amine?

• Protonated at physiological pH (99%)

• Replace N with C – large drop in activity – ESSENTIAL

• Substitution pattern matters ,Primary and secondary amines – good activity – Natural ligands,Tertiary and quaternary – low activity

What is the SAR for adrenoreceptor -phenols?

• Both are IMPORTANT, not essential

Analogues with fewer phenols show low activity, e.g. tyramine

Can be changed to other H-bonding groups at the meta position

to Sulphonamides, extended alcohols; important for drug design approach

• also allows for hydrogen bonding - better for b receptors

What is the SAR for adrenoreceptors - alkyl substitution?

• Decreases activity at both α and β receptors; possible steric effect reducing β-hydroxyl binding, BUT Introducing a methyl group does increase α2 selectivity

• Putting a substitution at the a position decreases activity, but gives some selectivity

summary sar

• B- alcohol is important, as R form

• Essential for amine - protonated at ph 1,1 better than 3,4

• phenols important - can be replaced by an H-bonding group at the meta position

• Alkyl substitution on alpha causes reduced activity; more selectivity alpha

How do natural ligands bind in adrenoreceptor?

Physiological pH:

• amine is charged

• phenols are not

Key binding interactions:

1.Asp113 – charged nitrogen – ionic ( strongest)

2.Phe290 – aromatic ring – π-π stacking (VDW)

3.Ser207, 204 – phenols – H- bonding

4.Asn293 – β–hydroxyl – H- bonding

How to ensure b-selectivity - 1?

N-alkyl substitution

• Noradrenaline has selectivity to α receptors

• Adrenaline works as well at both

Difference in structure – a methyl group

• Alkyl groups can affect selectivity

• Increasing the size of the alkyl group – increased β-selectivity

Isoprenaline has full β-selectivity,isopropyl group,tertiary butyl group (Enhances further)

How to ensure b-selectivity - 2?

Phenol Groups

• If not present, activity drops significantly at β-receptors molecule

α-Substitution

• Increases selectivity for α2

Extension

• Can introduce phenol groups - significantly increases β-receptor selectivity

• Suggests additional H-bond interaction in hydrophobic pocket

• Point of variation for drug design

What do we want when designing a drug- an asthma β2 agonist?

• Selective to B2

• long duration of action - not metabolised by COMT,mao

• no toxicity , severe side effects

Why is adrenaline not used?

• Used in emergencies

• Fast acting

• Not selective

• Cardiovascular side effects

• Short duration of activity-Metabolised by COMT and MAO

Require a synthetic analogue

Selectivity

Enhanced duration of activity

What were some drugs considered for asthma to increase selectivity?

Isoprenaline

-Initially used, not selective between β subtypes, Cardiovascular side effects

Isoetharine

-Increased selectivity by substitution at the α position,However has a short duration of action

How can we enhance duration of action?

COMT:

• Responsible for metabolising adrenaline and noradrenaline

• Introduces a methyl group onto the meta hydroxyl, Ether removes activity- metabolised

COMT inhibitors have been developed and are used in CNS conditions ( help regulate dopamine levels )

What is the structure of COMT?

What is the suggest mechanism for comt - metabolism?

1. A lone pair grabs a proton from the oxygen

2. Oxygen takes a methyl; methyl is transferred

• Lys144 in COMT accepts the proton from the hydroxyl, and the methyl group from the

• AdoMet is transferred to the hydroxyl group

How do you reduce comt metabolism ?

Change the meta phenol that is methylated by COMT, BUT Need to retain activity through ensuring hydrogen bonding – need a bioisostere same binding but gets rid of pharmacokinetic problems

What is the first bioisostere- enhance duration ?

soterenol

• Initial lead drug

• Long-acting and selective

• Sulfonamide used to replace phenol, still able to H-bond, not able to methylate - no comt

Not used clinically due to the side effects

What is the second bioisostere- enhance duration ?

Salbutamol ( albuterol)

• A more subtle change – a group shift (OH moved carbon over, added a methyl), Phenol replaced by a hydroxymethyl group

• Still able to H-bond; additional small change to N substituent – isopropyl to tert-butyl

• Same potency as isoprenaline 2000x more selective to β2

• Increased duration of action of 4 hours ( does not cover night)

• Not metabolised by COMT

• Slower metabolism to an inactive sulfate derivative ( longer)

Salbutamol is a racemic mixture

R form is 68x more effective – has been marketed as levalbuterol (sal is cheaper)

How else can you enhance the activity?

• Modify the alkyl group further: extension strategy

• Aims to exploit a secondary binding site: hydrophobic pocket, potential H-bonding

Examples: salmeterol ( phenol ether , additional h bonding ) - more active due to enhanced lipophilicity,stays better in lung tissues

formoterol ( has a formamide group - increased lipophilicity ,due to both chnages)

What is another way to enhance activity?

Increase lipophilicity:

• Increased residence time at the receptor and in the vicinity – nonspecific

  Longer duration of activity

  Achieved by introducing alkyl chains and a benzene ring

Example : salmeterol

What is another way to enhance activity

Additional theory

Salmeterol binds at an exosite ( another site not part of the β2 binding site), which keeps it in proximity:

• TMD 4 AA. 149-158

• acts as an anchor

• Pivots on and off ( released from the binding site from the exosite)

May explain the importance of the ether oxygen

Example- Salmeterol

How else can you enhance duration of action further, activity?

Vilanterol

• Designed to be used once daily

• Halogens enhance lipophilicity further

• Introduced in 2013

summary

Duration of action is enhanced :

• Reducing metabolism: COMT inhibitors - bioisostere

• Change phenol to sulfonamide

• Group shift: add a CH2, move the OH carbon over, isopropyl to tertiary butyl

Enhance activity

• extension strategy

• add in a formamide

• increase lipophilicity - introduce alkyl chains , benzene rings

• exosite

• halogens to increase lipophilicity an duration