DESIGN OF A NOVEL ANXIOLYTIC AND ANTIDEPRESSANT AGENT

1
DESIGN OF A NOVEL ANXIOLYTIC AND ANTIDEPRESSANT
AGENT
2
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

3

• Notes
• Serotonin is a neurotransmitter
• Abnormal levels of serotonin are related to
  various disorders
• (e.g. anxiety, depression, migraine)
• Indole ring system is present

4
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

5
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 tests Ability to block hypoactivity in
rats caused by mCPP
Modelling studies Drug design carried out on
model binding sites
6
Lead Compound
7
From 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

8
From SB 200646 to SB 206553
Tricyclic ring system

• 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

9
From SB 200646 to SB 206553
Conformation of SB 206553 Overall structure is
roughly planar
10
7. From SB 200646 to SB 206553
11

• 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

12
Analogues of SB 206553
Ring variation strategy
13
3D QSAR studies of SB 206553 and analogues

• 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

14
Molecular modelling studies of SB 206553 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

15
Molecular modelling studies of SB 206553 and
analogues
16
Molecular modelling studies of SB 206553 and
analogues
17
Molecular modelling studies of SB 206553 and
analogues

• 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

18
From 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

• 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

19
From SB 206553 to SB 221284

• 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

20
Modelling Studies on SB 221284

• 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

21
Modelling Studies on SB 221284
Note Vacant areas are available in the
hydrophobic pocket accommodating the pyridine
ring
22
3D-QSAR Studies on SB 221284

• Notes
• 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

23
3D-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

24
From SB 221284 to SB 228357

• 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

25
From SB 221284 to SB 228357

• 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

26
From SB 221284 to SB 228357
Structure III

• 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

27
From SB 221284 to SB 228357
Structure 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)

28
From SB 221284 to SB 228357
Ring variation
Structure 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)

29
From SB 221284 to SB 228357
Ring variation
Structure III
30
From SB 221284 to SB 228357
Electron rich

• 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

31
From SB 221284 to SB 228357

• 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

32
From SB 221284 to SB 228357
33
From SB 228357 to SB 243213

• 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

34
15. From SB 228357 to SB 243213
SB 228357
Structure IX
35
From SB 228357 to SB 243213
SB 228357
Structure IX
36
From SB 228357 to SB 243213

• 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

37
From SB 228357 to SB 243213

• 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

38
From SB 228357 to SB 243213
Modelling studies for SB 243213
Pyridine ring substituent well inserted into
hydrophobic pocket
39
From SB 228357 to SB 243213
Binding interactions for SB 243213