3/29/2023
Drug Design and Discovery
Lecture 1
Dr. Wesam Qayed, PhD
Associate professor, MedChem dep.
wesamqayed@aun.edu.eg
1) Identify target disease
Drug design and
development 2) Identify drug target
3) Establish testing procedures
4) Find a lead compound
5) Structure Activity Relationships (SAR)
6) Identify a pharmacophore
7) Drug design - optimising target interactions
Stages: 8) Drug design - optimising pharmacokinetic properties
9) Toxicological and safety tests
10) Chemical development and production
11) Patenting and regulatory affairs
12) Clinical trials
1
3/29/2023
1. Pharmacokinetics – drug design
Aims
• To improve pharmacokinetic properties of lead compound
• To optimise chemical and metabolic stability
(stomach acids / digestive enzymes / metabolic
enzymes)
• To optimise hydrophilic / hydrophobic balance
(solubility in blood / solubility in GIT / solubility
through cell membranes / access to CNS /
excretion rate)
1. Pharmacokinetics – drug design
• Drugs must be polar
- to be soluble in aqueous conditions
- to interact with molecular targets
• Drugs must be ‘fatty’
- to cross cell membranes
- to avoid rapid excretion
• Drugs must have both hydrophilic and lipophilic
characteristics
• Many drugs are weak bases with pKa’s 6-8
2
3/29/2023
1. Pharmacokinetics – drug design
1.1 Solubility and membrane permeability
1.2 Drug stability
1.3 Drug targeting
1.4 Reducing drug toxicity
1.5 Prodrugs
1.6 drug Sunergy
1.1 Solubility and membrane permeability
1.1.1. Vary alkyl substituents
1.1.2. ‘Masking’ or removing polar groups
1.1.3. Adding polar groups
1.1.4. Vary pKa
1.1.5. Bioisosteric replacement
3
3/29/2023
1.1 Solubility and membrane permeability
1.1.1 Vary alkyl substituents
Rationale:
• Varying the size of alkyl groups varies the hydrophilic /
hydrophobic balance of the structure
• Larger alkyl groups increase hydrophobicity
Disadvantage:
• May interfere with target binding for steric reasons
Methods:
• Often feasible to remove alkyl groups from heteroatoms
and replace with different alkyl groups
• Usually difficult to remove alkyl groups from the carbon
skeleton ?????
1.1.2 ‘Masking’ or removing polar groups
Rationale:
• Masking or removing polar groups decreases polarity
and increases hydrophobic character
Disadvantages:
• Polar group may be involved in target binding
• Unnecessary polar groups are likely to have been
removed already (simplification strategy)
• See also prodrugs
Methods:
4
3/29/2023
1.1.3 Adding polar groups
Rationale:
• Adding polar groups increases polarity and decreases
hydrophobic character
• Useful for targeting drugs vs. gut infections
• Useful for reducing CNS side effects
Antifungal agent with poor Systemic antifungal agent
solubility - skin infections only improved blood solubility
Disadvantage:
• May introduce unwanted side effects
1.1.4 Vary pKa
Rationale:
• Varying pKa alters percentage of drug which is ionized
• Alter pKa to obtain required ratio of ionized to unionized drug
Method:
• Vary alkyl substituents on amine nitrogens
• Vary aryl substituents to influence aromatic amines or
aromatic carboxylic acids
Disadvantage:
• May affect binding interactions
10
5
3/29/2023
1.1.5. Bioisosteric replacement
11
1.2 Drug stability
1.2.1 Steric Shields
1.2.2 ‘Electronic shielding’ of NH2
1.2.3 Stereoelectronic Effects
1.2.4 Bio-isosteres
1.2.5 Metabolic blockers
1.2.6 Remove / replace susceptible metabolic groups
1.2.7 Shifting susceptible metabolic groups
1.2.8 Introducing susceptible metabolic groups
12
6
3/29/2023
1.2 Drug stability
1.2.1 Steric Shields
Rationale:
• Used to increase chemical and metabolic stability
• Introduce bulky group as a shield
• Protects a susceptible functional group (e.g. ester) from
hydrolysis
• Hinders attack by nucleophiles or enzymes
Antirheumatic agent Terminal amide
D1927
Steric
Shield
Blocks hydrolysis of terminal amide
13
1.2.2 ‘Electronic shielding’ of NH2
Rationale:
• Used to stabilise labile functional groups (e.g. esters)
• Replace labile ester with more stable urethane or
amide
• Nitrogen feeds electrons into carbonyl group and
makes it less reactive
• Increases chemical and metabolic stability
14
7
3/29/2023
1.2.2 ‘Electronic shielding’ of NH2
Amides are more resistant to chemical
hydrolysis, due to the lone pair of the nitrogen
feeding its electrons into the carbonyl group and
making it less electrophilic
15
1.2.3 Stereoelectronic Effects
Rationale:
• Steric and electronic effects used in combination
• Increases chemical and metabolic stability
Local anaesthetic
(short duration) ortho Methyl groups act as steric shields &
hinder hydrolysis by esterases
Amide more stable than ester
(electronic effect)
See also: oxacillin and bethanechol
16
8
3/29/2023
1.2.4 Bio-isosteres
Rationale:
• Replace susceptible group with a different group without
affecting activity
• Bio-isostere shows improved pharmacokinetic properties
• Bio-isosteres are not necessarily isosteres
Examples:
• Amides and urethanes for esters (see earlier)
• Du122290 (dopamine antagonist)
17
1.2.5 Metabolic blockers
Rationale:
• Metabolism of drugs usually occur at specific sites. Introduce
groups at a susceptible site to block the reaction
• Increases metabolic stability and drug lifetime
18
9
3/29/2023
1.2.6 Remove / replace susceptible metabolic groups
Rationale:
• Metabolism of drugs usually occurs at specific groups.
• Remove susceptible group or replace it with metabolically
stable group [e.g. modification of tolbutamide ]
Unsusceptible
group
Susceptible
group
TOLBUTAMIDE
Rapidly excreted - short lifetime
19
1.2.7 Shifting susceptible metabolic groups
Rationale:
• Used if the metabolically susceptible group is important
for binding
• Shift its position to make it unrecognisable to metabolic
enzyme
• Must still be recognisable to target
20
10
3/29/2023
1.2 Drug stability
1.2.1 Steric Shields
1.2.2 ‘Electronic shielding’ of NH2
1.2.3 Stereoelectronic Effects
1.2.4 Bio-isosteres
1.2.5 Metabolic blockers
1.2.6 Remove / replace susceptible metabolic groups
1.2.7 Shifting susceptible metabolic groups
1.2.8 Introducing susceptible metabolic groups
21
1.2.8 Introducing susceptible metabolic groups
Rationale:
• Used to decrease metabolic stability and drug lifetime
• Used for drugs which ‘linger’ too long in the body and
cause side effects
• Add groups known to be susceptible to Phase I or Phase II
metabolic reactions
Example:
Anti-arthritic agents
22
11
3/29/2023
1.2.8 Introducing susceptible metabolic groups
Example:
Anti-asthmatic agents
• Cromakalim produces cardiovascular side effects if it reaches blood
supply
• Add metabolic instability such that compound rapidly metabolised in
blood
• UK143220 - ester quickly hydrolysed by esterases to inactive acid
• UK 157147- phenol quickly conjugated and eliminated
23
Introducing chemically susceptible groups
Rationale
• Used to decrease drug lifetime
• Avoids reliance on metabolic enzymes and individual variations
Example Atracurium - i.v. neuromuscular blocking agent
• Stable at acid pH, unstable at blood pH (slightly alkaline)
• Self destructs by Hoffmann elimination and has short lifetime
• Allows anaesthetist to control dose levels accurately
• Quick recovery times after surgery
24
12
3/29/2023
1.3 Drug targeting
1.3.1 Linking a biosynthetic building block
1.3.2 Linking drugs to monoclonal antibodies
1.3.3 Targeting gut infections
1.3.4 Targeting peripheral regions over CNS
25
1.3 Drug targeting
1.3.1 Linking a biosynthetic building block
Rationale:
• Drug ‘smuggled’ into cell by carrier proteins for natural building block
(e.g. amino acids or nucleic acid bases)
• Increases selectivity of drugs to target cells and reduces toxicity to other
cells
Example:
Anticancer drugs
Non selective alkylating agent
Toxic Uracil Mustard
26
13
3/29/2023
1.3.2 Linking drugs to monoclonal antibodies
Example:
Anticancer agents
Rationale:
• Identify an antigen which is overexpressed on a cancer cell
• Clone a monoclonal antibody for the antigen
• Attach a drug or poison (e.g. ricin) to the monoclonal antibody
• Antibody carries the drug to the cancer cell
• Drug is released at the cancer cell
27
1.3.3 Targeting gut infections
Rationale:
• Design the antibacterial agent to be highly polar or ionised
• Agent will be too polar to cross the gut wall
• Agent will be concentrated at the site of infection
• Example - highly ionised sulfonamides
28
14
3/29/2023
1.3.4 Targeting peripheral regions over CNS
Rationale
• Increase polarity of the drug
• Drug is less likely to cross the blood brain barrier
1.3.5 Targeting membrane tethers
MitoQ is an agent undergoing clinical trials which contains
an antioxidant prodrug linked to a hydrophobic
triphenylphosphine moiety.
29
1.4 Reducing drug toxicity
Rationale:
• Toxicity is often due to specific functional groups
• Remove or replace functional groups known to be toxic e.g.
− aromatic nitro groups
− aromatic amines
− bromoarenes
− hydrazines
− polyhalogenated groups
− hydroxylamines
• Vary substituents
• Vary position of substituents
30
15
3/29/2023
1.4 Reducing drug toxicity
Example - varying substituents
• Fluconazole (Diflucan) - antifungal agent
Substituents varied
Less toxic
31
1.5 Prodrugs
Definition:
Inactive compounds which are converted to active
compounds in the body.
Uses:
• Improving membrane permeability
• Prolonging activity
• Masking toxicity and side effects
• Varying water solubility
• Drug targeting
• Improving chemical stability
• ‘Sleeping agents’
32
16
3/29/2023
1.5.1 Prodrugs to improve membrane permeability
1.5.1.1 Esters
• Used to mask polar and ionisable carboxylic acids
• Hydrolysed in blood by esterases
• Used when a carboxylic acid is required for target binding
• Leaving group (alcohol) should ideally be non toxic
Example:
Enalapril for enalaprilate (antihypertensive)
33
1.5.1.2 N-Methylation of amines
• Used to reduce polarity of amines
• Demethylated in liver
Example:
Hexobarbitone
34
17
3/29/2023
1.5.1.3 Trojan Horse Strategy
• Prodrug designed to mimic biosynthetic building block
• Transported across cell membranes by carrier proteins
Example: Levodopa for dopamine
35
1.5.1 Prodrugs to improve membrane permeability
Blood Brain
supply cells
H2N COOH
H 2N COOH
L-Dopa Enzyme
H2N
Dopamine
BLOOD BRAIN
BARRIER
36
18
3/29/2023
1.5.2 Prodrugs to prolong activity
1.5.2.1 Mask polar groups
• Reduces rate of excretion
Example:
Azathioprine for 6-mercaptopurine
37
1.5.2 Prodrugs to prolong activity
1.5.2.2 Add hydrophobic groups
• Drug concentrated in fat tissue
• Slow removal of hydrophobic group
• Slow release into blood supply
Example:
Cycloguanil pamoate (antimalarial)
38
19
3/29/2023
1.5.2 Prodrugs to prolong activity
1.5.2.2 Add hydrophobic groups
Example:
Hydrophobic esters of fluphenazine (antipsychotic)
39
1.5.3 Prodrugs to mask toxicity and side effects
• Mask groups responsible for toxicity/side effects
• Used when groups are important for activity
Example:
Aspirin for salicylic acid
40
20
3/29/2023
1.5.3 Prodrugs to mask toxicity and side effects
Example:
Cyclophosphoramide for phosphoramide mustard
(anticancer agent)
Phosphoramidase
(liver)
41
1.5.3 Prodrugs to mask toxicity and side effects
Example
Antiviral drugs
42
21
3/29/2023
Example:
LDZ for diazepam
a) Aminopeptidase
b) Cyclisation
LDZ Diazepam
• Avoids drowsy side effects of diazepam
43
1.5.4 Prodrugs to lower water solubility
• Used to reduce solubility of foul tasting orally active drugs
• Less soluble on tongue
• Less revolting taste
Example:
Palmitate ester of chloramphenicol (antibiotic)
Palmitate ester
Esterase
Chloramphenicol
44
22
3/29/2023
1.5.5 Prodrugs to increase water solubility
• Often used for i.v. drugs
• Allows higher concentration and smaller dose volume
• May decrease pain at site of injection
Example:
Succinate ester of chloramphenicol (antibiotic)
Succinate ester
Esterase
Chloramphenicol
45
Example:
Lysine ester of oestrone
• Lysine ester of oestrone is better absorbed orally than oestrone
• Increased water solubility prevents formation of fat globules in gut
• Better interaction with the gut wall
• Hydrolysis in blood releases oestrone and a non toxic amino acid
46
23
3/29/2023
1.5.6 Prodrugs used to target drugs
Example:
Hexamine
47
1.5.7 Prodrugs to increase chemical stability
Example
Hetacillin for ampicillin
48
24
3/29/2023
1.5.8 Prodrugs activated by external influences
-sleeping agents
Example: Photodynamic therapy - Foscan
49
1.6 Drug alliances - synergism
Definition:
Drugs which have a benefical effect on the activity or
pharmacokinetic properties of another drug
1.6.1 Sentry Drugs
1.6.2 Localising drugs to a target area
1.6.3 Increasing absorption
50
25
3/29/2023
1.6.1 Sentry Drugs
Definition
A drug that is added to ‘protect’ another drug
Example
Carbidopa
51
1.6.2 Localising drugs to a target area
Example: Adrenaline and procaine (local anaesthetic)
• Adrenaline constricts blood vessels at the injection area
• Procaine is localised at the injection area
1.6.3 Increasing absorption
Example: Metoclopramide
52
26
