DESIGN OF A NOVEL ANXIOLYTIC AND ANTIDEPRESSANT AGENT

DESIGN OF A NOVEL ANXIOLYTIC DESIGN OF A NOVEL ANXIOLYTIC AND ANTIDEPRESSANT AGENTAND ANTIDEPRESSANT AGENT

Target - Serotonin Receptor Target - Serotonin Receptor Target - Serotonin Receptor Target - Serotonin Receptor Notes• Important receptor in central nervous system

• 7 types (5-HT1 - 5HT7) and 14 subtypes

• G-Protein-coupled receptors (except 5-HT3)

• Three subtypes of 5-HT2 receptor (5-HT2A 5-HT2B 5-HT2C)

• 5-HT2C receptor thought to be involved in anxiety

• mCPP is an agonist with some selectivity for 5-HT2C receptor

• mCPP causes anxiety in human and animal studies

• Antagonist with selectivity for 5-HT2C receptor may be useful in treating anxiety

HN N

Cl

meta-Chlorophenylpiperazine (mCPP)

Notes• Serotonin is a neurotransmitter• Abnormal levels of serotonin are related to various disorders

(e.g. anxiety, depression, migraine)• Indole ring system is present

Natural Ligand Natural Ligand Natural Ligand Natural Ligand

N

H

NH2

HO

Serotonin(5-Hydroxytryptamine)

Aims of Drug Design Aims of Drug Design Aims of Drug Design Aims of Drug Design • Selectivity for 5-HT2C receptor

• Selectivity over 5-HT2A is more important than over 5-HT2B

5-HT2B predominates in peripheral nervous system

5-HT2A and 5-HT2C receptors predominate in CNS• Resistance to drug metabolism• No effect on metabolic enzymes (drug-drug interactions)• Aqueous solubility• Non-sedating• No interaction with alcohol• Fast onset of action• High response rate • No withdrawal effects

Testing Procedures Testing Procedures Testing Procedures Testing Procedures In vitro tests

• Radioligand binding studies on 5-HT2A, 5-HT2B and 5-HT2C receptor subtypes

• Tests for activity against cytochrome P450 enzymes

In vivo testsAbility to block hypoactivity in rats caused by mCPP

Modelling studiesDrug design carried out on model binding sites

Lead Compound Lead Compound Lead Compound Lead Compound

• Produced by Lilly Pharmaceuticals• Serotonin antagonist• Insoluble in water

N

Me

CH2CH3

Me

HN

HN

O

CF3

UreaUrea

IndoleIndole

AromaticAromatic

From Lead Compound to SB 200646From Lead Compound to SB 200646From Lead Compound to SB 200646From Lead Compound to SB 200646

Notes• Substituents removed from indole ring (simplification)• Pyridine ring introduced (ring variation)• Pyridine ring more polar - increases water solubility• Urea link at optimum positions for both ring systems

• First selective 5-HT2B/2C antagonist

• 50-fold selectivity over 5-HT2A receptor

• Modest in vitro activity • Some oral activity in vivo

N

Me

CH2CH3

Me

HN

HN

O

CF3

3 5

N

Me

HN

HN

ON

SB 200646SB 200646PyridinePyridine

3 5

N

Me

HN

HN

ON

SB 200646SB 200646

Metabolically labile

From SB 200646 to SB 206553From SB 200646 to SB 206553From SB 200646 to SB 206553From SB 200646 to SB 206553

Notes• Rigidification limits number of possible conformations• Rigidify structure such that active conformation still allowed• Increased chance of active conformation being present• 10-fold increase of in vitro affinity

• 160-fold selectivity over 5-HT2A receptor

• 4-fold increase of in vivo activity

Tricyclic ring Tricyclic ring systemsystem

3 5

N

Me

HN

HN

ON

SB 200646 SB 206553

N

N

CH3

OHN

N

Rigidification

Extra ring

Conformation of SB 206553Overall structure is roughly planar


7. From SB 200646 to SB 2065537. From SB 200646 to SB 2065537. From SB 200646 to SB 2065537. From SB 200646 to SB 206553Metabolism of SB 206553Metabolism of SB 206553

N

N

CH3

OHN

N

Metabolically labile

N

N

H

OHN

N

N-Demethylation

Metabolite is active but non-selective

Variation permitted

Analogues of SB 206553Analogues of SB 206553Analogues of SB 206553Analogues of SB 206553

N

HN

N

O

N

Me

Substitution bad

Substitution bad

Notes• Methyl group replacable with Et, Pr or iPr

• Slightly increased selectivity for 5-HT2C over 5-HT2A

• Hydrophobic pocket available for N-alkyl group

• Slightly bigger for 5-HT2C receptor

Analogues of SB 206553Analogues of SB 206553Analogues of SB 206553Analogues of SB 206553

• No methyl group• Loss of

selectivity

•Retains Me group •More stable to metabolism•Good in vitro affinity and selectivity•Poor oral activity •Poorly absorbed from gut or rapidly metabolized•Administered by IV injection

Ring variation strategyRing variation strategy

Pyrrole

N

HN

N

O

N

MeSB206553

No methyl group

ThipoheneN

S

OHN

N

FuranN O

Me

OHN

N

3D QSAR studies of SB 206553 and 3D QSAR studies of SB 206553 and analoguesanalogues 3D QSAR studies of SB 206553 and 3D QSAR studies of SB 206553 and analoguesanalogues

Notes• Structures overlaid using urea group• Dark and light blue = areas accessed by compounds having 5-

HT2C activity

• Light blue = disallowed area for 5-HT2A activity

• Light blue area = region of N-methyl group of SB 206553

Molecular modelling studies of SB Molecular modelling studies of SB 206553 and analogues206553 and analoguesMolecular modelling studies of SB Molecular modelling studies of SB 206553 and analogues206553 and analogues

Notes• Model receptor binding site created• SB 206553 docked into binding site• Carbonyl oxygen of urea group is crucial for activity• Positioned to form hydrogen bond to Ser-312

- Ser-312 present in 5-HT2 receptors but not 5-HT1 receptors

- binding to Ser-312 thought to be important for selectivity for 5-HT2 receptors over 5-HT1 receptors

• Carbonyl oxygen interacts with Ser-312 and Ser-315• Aromatic rings positioned into hydrophobic pockets


N

N

Me

ONH

N

Molecular modelling studies of SB Molecular modelling studies of SB 206553 and analogues206553 and analogues Molecular modelling studies of SB Molecular modelling studies of SB 206553 and analogues206553 and analogues

O

HO

H

Ser- 138Ser- 141

Phe-223

Trp-324

Phe-327

Phe-328

Phe-220

Phe-224

Val-212

Val-608

Hydrophobic pocket

Hydrophobic pocket

N

N

Me

OHN

N


Notes•Val-608 and Val-212 are present in the hydrophobic pocket occupied by the N-methyl group

•Bulkier leucine groups are present in the model binding site for the 5-HT2A receptor

•Hydrophobic pocket is smaller for the 5-HT2A receptor

•More difficult for N-methyl group of SB 206553 to fit the 5-HT2A receptor

•May account for selectivity of SB 206553

From SB 206553 to SB 221284From SB 206553 to SB 221284 From SB 206553 to SB 221284From SB 206553 to SB 221284

Aim•To replace the metabolically labile N-methyl group with a metabolically stable group•The new group must bind to the hydrophobic pocket for selectivity

SB 206553

N

N

CH3

OHN

N

N

X

Y6

5

OHN

N

Indolines

Notes•Indole ring is removed - simplification•Indoline structures are synthesized with varying substituents •Substituent X needs to bind to the hydrophobic pocket•Electron-withdrawing substituent at position 6 is preferred •Thioether or ether at position 5 is beneficial


Notes•Best balance of affinity vs selectivity•Potent inhibitor of 5-HT2B and 5-HT2C receptors•Good selectivity over 5-HT2A receptor•Inhibits cytochrome P450 enzymes•Pyridine N is responsible for inhibiting cyt P450 enzymes •Selectivity increases for SEt, SnPr or OiPr, but affinity falls

Inhibits cytochrome

P450 enzymes

N

SMe

CF3

OHN

N

SB221284

Fits hydrophobic pocket

Metabolically stable

N

SMe

CF3

OHN

N

SB221284

Electron-withdrawing

Fits hydrophobic pocket

Metabolically stable

N

SMe

CF3

OHN

N

SB221284

Notes

• SB 221284 is docked into the model binding site for the 5-HT2C receptor

• Pyridine and indoline rings are positioned in hydrophobic pockets

• Hydrogen bonding interactions take place with serine residues

• S-Methyl group fits the hydrophobic pocket

• CF3 group orientates the thiomethyl group into the correct conformation

• CF3 acts as a conformational blocker

Modelling Studies on SB 221284Modelling Studies on SB 221284Modelling Studies on SB 221284Modelling Studies on SB 221284

Modelling Studies on SB 221284Modelling Studies on SB 221284Modelling Studies on SB 221284Modelling Studies on SB 221284

O

HO

H

Ser- 138Ser- 141

Phe-223

Trp-324

Phe-327

Phe-328

Phe-220

Phe-224

Val-212

Val-608N

S

CF3

OHN

N

Me

Hydrophobic pocket

Hydrophobic pocket

N

S

CF3

OHN

N

Me

NoteVacant areas are available in the hydrophobic pocket accommodating the pyridine ring

3D-QSAR Studies on SB 2212843D-QSAR Studies on SB 2212843D-QSAR Studies on SB 2212843D-QSAR Studies on SB 221284Notes

• 55 Analogues are synthesized• Analogues are docked into the model receptor • Receptor-ligand complex is minimized• Ligands are removed and subjected to CoMFA analysis• Predicted affinity versus actual affinity demonstrates a good

relationship 9.00

8.50

8.00

7.50

7.00

6.50

6.006.00 6.50 7.00 7.50 8.00 8.50 9.00

Actual pKi

Pre

dic

ted

p

K i

3D-QSAR Studies on SB 2212843D-QSAR Studies on SB 2212843D-QSAR Studies on SB 2212843D-QSAR Studies on SB 221284

Notes

• Steric fields are more important than electrostatic fields• Dark blue represents beneficial areas• Light blue represents detrimental areas• Large number of detrimental areas round the indoline ring• Suggests a tight binding pocket• Little scope for modification

Electronrich

Notes• Aromatic ring is best placed at position 5• Increased binding interactions with hydrophobic pocket • Slightly increased affinity and selectivity• Aromatic ring acts as a steric shield for pyridine • 100-fold decrease in cytochrome P450 inhibition• Level of cytP450 enzyme inhibition is still unacceptable • Low oral activity• Electron-rich aromatic ring is possibly susceptible to

metabolism

N

SMe

CF3

SB 221284

OHN

N

Inhibits cytochrome

P450 enzymes


I

N

OMe

CF3

OHN

N

5

Structure II• Pyridine ring is more polar leading to increased water

solubility• 10-fold increase in affinity due to additional binding

interactions

• Lower selectivity between 5-HT2C and 5-HT2A receptors

Structure III• Selectivity is recovered by adding a methyl substituent

• Slightly increased affinity for the 5-HT2C receptor

• Moderate oral activity• Slight drop in cytochrome P450 inhibition

I

N

OMe

CF3

OHN

N

5

Electronrich

Vary ring and rigidification

II R=HIII R=Me

4'

N

OMe

CF3

OHN

N

N

R

5


Steric clash

Notes• Methyl group acts as a conformational blocker• Forces rings to be orthogonal

• Orthogonal rings favored by 5-HT2C receptor but not 5-HT2A receptor


N

OMe

CF3

OHN

N

N

Me H

N

HN

N

O

CF3

OMe

NCH3 H

OrthogonalringsStructure III

Notes• Pyridine nitrogen is responsible for inhibiting cytochrome

P450 enzymes• Pyridine is replaced by an aromatic ring (ring variation)• Poor water solubility• Water solubility is improved by a pyridine substituent (R)• Substituent is best at position 3 (variation of substituents)


Ring variation

R 3

IV

4

N

OMe

CF3

OHN

N

OMe

CF3

OHN

N

N

Me H

Structure III

4’

5

V

N

OMe

CF3

OHN

N

Me


Ring variation

N

OMe

CF3

OHN

N

N

Me H

Structure III

4’

5

Notes• Pyridine nitrogen is responsible for inhibiting cytochrome

P450 enzymes• Pyridine is replaced by an aromatic ring (ring variation)• Poor water solubility• Water solubility is improved by a pyridine substituent (R)• Substituent is best at position 3 (variation of substituents)

• Improved selectivity

• Higher 5-HT2C affinity

• Potent oral activity• Still inhibits cytochrome P450

enzymes


V

N

OMe

CF3

OHN

N

Me

Ring variation

N

OMe

CF3

OHN

N

N

Me H

Structure III

4’

5

Notes• Methyl substituent moved to the ortho position• Acts as a conformational blocker • Aromatic and pyridine rings cannot be co-planar

• Increased selectivity for the 5-HT2C over the 5-HT2A receptor

• Short duration of action • Electron-rich aromatic ring is susceptible to metabolism


V

N

OMe

CF3

OHN

N

Me

3

Electron rich

Vary substituents

N

OMe

CF3

OHN

N

H

Me

Structure VI

Notes• Analogues made with electron-withdrawing substituents on

the aromatic ring• Substitution at 2 and 6 is bad - ring is twisted out of plane

with the urea group• Substitution at positions 4 and 5 is acceptable


Electron rich

N

OMe

CF3

OHN

N

H

Me

Structure VI

Bad

Bad

2

6

4

5 N

OMe

CF3

OHN

N

R

Structure VIIIStructure VIII

• Increased duration of action (6 hours)

• Modeling studies suggest extra space available in hydrophobic pocket

• Further extension possible


Electron rich

N

OMe

CF3

OHN

N

H

Me

Structure VI

N

OMe

CF3

OHN

N

F

SB 228357

Notes• Substitution pattern is altered to para• Linker oxygen atom is inserted • Pushes pyridine further into the hydrophobic pocket• Poor oral activity due possibly to reduced solubility


N

OMe

CF3

OHN

N

F

SB 228357

N

OMe

CF3

OHN

ON

Structure IXLinker

Original pyridine restored to increase water solubility

15. From SB 228357 to SB 24321315. From SB 228357 to SB 24321315. From SB 228357 to SB 24321315. From SB 228357 to SB 243213

N

OMe

CF3

OHN

N

F

SB 228357

N

OMe

CF3

OHN

ON

Structure IXLinker

N

OMe

CF3

OHN

NO

N

Structure X


N

OMe

CF3

OHN

N

F

SB 228357

N

OMe

CF3

OHN

ON

Structure IXLinker

N

OMe

CF3

OHN

NO

N

Structure X

N

OMe

CF3

OHN

NO

N

Me Structure XI

Notes• ortho Methyl group acts as a conformational blocker• Increases torsion angle between the pyridine rings

• Increased selectivity and affinity for 5-HT2C receptor

• 80-fold selectivity over 5-HT2B receptor

• Low cyctochrome P450 activity• Good in vivo activity


N

OMe

CF3

OHN

NO

N

Me Structure XI

Notes• Methoxy group is replaced with a methyl group• Less liable to metabolism• Good profile of affinity, activity and selectivity• Negligible cytochrome P450 activity• Better aqueous solubility than SB 228357• Entered phase I clinical trials as a non-sedating

antidepressant / anxiolytic


N

OMe

CF3

OHN

NO

N

Me Structure XI

N

Me

CF3

OHN

NO

N

Me SB 243213

Modelling studies for SB 243213Modelling studies for SB 243213

Pyridine ring substituent well inserted into hydrophobic pocket


O

HO

H

Ser- 138Ser- 141

Phe-223

Trp-324

Phe-327

Phe-328

Phe-220

Phe-224

Val-212

Val-608N

Me

CF3

OHN

NO

N

Me

Binding interactions for SB 243213Binding interactions for SB 243213


DESIGN OF A NOVEL ANXIOLYTIC AND ANTIDEPRESSANT AGENT

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