Case Study 1 - ACE inhibitors Flashcards
Identifying the Pharmacophore
- It provides need-to-know information about the active conformation of the active drug
- Relative position in space of groups that interact with the receptor, e.g. H-bond acceptors and donors (generalised bonding pharmacophore)
- A 2- or 3-dimensional map of the site of association
- Knowledge of the binding site structure can be used to ‘reverse engineer’ the pharmacophore, by specifying the position of key functional groups.
The Active Conformation
- The active conformation must be identified in order to confirm the 3D pharmacophore
- Conformational analysis helps to identify all possible conformations and their activities (in silico)
- Conformational analysis is difficult for flexible molecules with rotatable bonds that have many degrees of freedom
- The active conformation must be compared to series of rigidified molecular architectures also
Rigidification
- Endogenous ligands (lead compounds) are often simple yet flexible and fit several targets due to many different active conformations
- Rigidify analogue molecule to limit conformations – impose conformational restraint
Simplification
- Naturally-occurring lead compounds are often complex and difficult to synthesize
- Simplifying the molecule makes the synthesis of analogues easier, quicker and cheaper
- Must retain the pharmacophore
- Simpler structures may fit the binding site easily with a concommitant increase in activity
Structure-Based Drug Design
- Crystallise target protein with bound lead compound and acquire structure by X-ray crystallography
- Identify binding interactions between ligand and target (in silico molecular modelling)
- Identify vacant sites for additional binding interactions (in silico)
- Design and ‘fit’ analogues into binding site (in silico)
- Choose lead compounds for synthesis
National Medal for Technology
- Bristol-Myers Squibb in 1998
- “For extending and enhancing human life through innovative pharmaceutical research and development.”
- Bristol-Myers Squibb developed angiotensin converting enzyme (ACE) inhibitors, used to treat hypertension in patients with cardiovascular disorders
- The development of captopril (and enalapril) is an excellent example rational drug design, in this case where the structure of the receptor is unknown
• Hypertension can be induced by:
• Disease of the CNS and peripheral nervous system
• Abnormalities of hormonal systems
• Abnormalities of kidneys and peripheral vasculature
- High arterial blood pressure causes hypertension
Hypertension types
• Primary
No obvious cause though family history, smoking,
alcoholism or obesity may predispose a patient
• Secondary
Well-defined condition that can be identified: renal disease; tumours; drug side-effects; pregnancy
• Isolated systolic hypertension
Occurs mostly in elderly patients (age >60 yo)
Systolic >160 mm Hg; Diastolic within normal limits
Associated with high incidence of strokes
Reflex Control of Blood Pressure
- Baroreceptor mechanism
- Pressure sensing areas throughout vasculature
- Alters the autonomic outflow
- Continuous monitoring, fast response
- Renin-angiotensin system
- Long term control of pressure
Renin
- Proteolytic enzyme
* Secreted from the juxtaglomerular cells (kidney)
Renin-Angiotensin System
- Renin is secreted into the blood in the kidneys by the juxtaglomerular apparatus
- Angiotensinogen is produced by the liver
- Renin cleaves angiotensinogen to give angiotensin I
- Angiotensin Converting Enzyme (ACE) cleaves angiotensin I to give angiotensin II during passage through the lungs
Renin-Angiotensin System
• Angiotensin II
- Angiotensin II constricts the renal efferent arteriole greater than the afferent arteriole
- Angiotensin II increases or maintains the glomerular filtration pressure
- High levels of aldosterone conserve salt and water and therefore increase blood volume
- Increased angiotensin II levels is often a physiological response to renal artery stenosis
Renin-Angiotensin System - what increases bp
- Renin release is under feedback control from Angiotensin II release
- Angiotensin II is a vasoconstrictor – INCREASED BLOOD PRESSURE
- Angiotensin II stimulates the adrenal cortex to release aldosterone, which in turn acts at the kidney to cause sodium and water retention – INCREASED BLOOD PRESSURE
Angiotensin Converting Enzyme
- Endothelial cell membrane-bound glycoprotein
- Mr 129 kDa, 26% polysaccharide, one Zn atom per in the active site of the enzyme
- Primary structure of ACE not determined until 1988, after the development of ACE inhibitors
- Multiple substrates are processed by the enzyme
Snake Venom and ACE Inhibitors
• Range of peptidic ACE inhibitors isolated from the Brazilian Arrowhead viper Bothrops jararaca
Snake Venom and ACE Inhibitors
•Most potent was Teprotide (SQ20881): pGlu-Trp-Pro-Arg-Pro-Gln-Ile-Pro-Pro
ACE and Carboxypeptidase A
- Carboxypeptidase A is a monopeptidase
- ACE is a dipeptidase
• Both enzymes are exopeptidases: they cleave residues from the ends of a protein
• Both enzymes require a free C-terminal CO2H
• Neither enzyme will hydrolyse an imino-acyl bond
i.e. will not hydrolyse an amide/peptide bond where the NHR component is proline
• Neither enzyme will hydrolyse a peptide with has a C-terminal dicarboxylic acid, e.g. Asp or Glu
• The active site of both enzymes contains zinc
The ACE Analogy
- Proline chosen as the C-terminus residue following results with BPP5a and Teprotide
- Zn site and the carboxyl binding site are further apart in ACE as compared to carboxypeptidase A
- Two hydrophobic pockets are required for strong binding to the substrate in ACE
- The non-scissile amide bond should be hydrogen- bonded to the enzyme
Captopril
Captopril 25 mg/day
- reduction in systolic b.p. -21 mm Hg
- reduction in diastolic b.p. -9 mm Hg
-Captopril is a tightly-binding competitive ACE inhibitor It is not active against other peptidases at concentrations <10-3 M, i.e. it is specific for ACE
• Marketed in the UK by Squibb as Capoten
• Rapid onset of response but T1/2 only 2 hours
• Side effects specific to captopril thiol group
Neutropenia; rash; nephrotic-range proteinuria; taste disturbances
Enalapril
• Enalapril is a pro-drug: the diacid, enalaprilat,
is formed by metabolic processing in the liver
• Diacid has lower IC50 than captopril with fewer side effects (no thiol)
• Marketed in the UK by MSD as the maleate
Me-Too Drugs
Ramipril -Indicated for use as prophylaxis
against adverse CVS events in susceptible patients over 55 yo
Perindopril
Trandolapril
Chain extension
R: -CH3
Pharmacological activity
Analgesic (morphine)
Chain extension
R: -CH2CH3
Pharmacological activity
Opioid agonist activity decreased
Chain extension
R: -CH2CH2CH3
Pharmacological activity
Opioid antagonist activity increased
Chain extension
R: -CH2CH2CH2CH3
Pharmacological activity
Inactive as opioid agonist or antagonist
Chain extension
R: -CH2CH2CH2CH2CH3 AND
-CH2CH2CH2CH2CH2CH3
Pharmacological activity
Opioid antagonist activity increased
Chain extension
R: -CH2CH2-benzene ring
Pharmacological activity
14X potency of morphine
Rigidification may increase
- May increase activity (more chance of the desired active conformation)
- May increase selectivity (less chance of any other undesired active conformations)
- However, the target molecule may now be more complex and more difficult to synthesise
• The pharmacophore defines
• The pharmacophore defines the essential groups that are involved in binding to the desired receptor
• Simple structures may be more..
- Simple structures may be more selective and less toxic if excess, unnecessary functional groups are removed
- Oversimplification can result in decreased selectivity and activity, and may lead to unwanted side effects
Structure-Based Drug Design is based upon..
• Based upon interactions between the lead compound and the target binding site
what is blood pressure
• Blood pressure is a function of the amount of blood pumped by the heart (cardiac output) and the resistance to blood flow by peripheral vasculature (total peripheral resistance)
• Secretion of renin is increased by:
- A fall in Na+ concentration
- A fall in renal perfusion pressure
- B-adrenoceptor agonists
- Prostacyclin
• Secretion of renin is inhibited by:
- Angiotensin II (feedback control)
* Atrial natriuretic peptide
• ACE creates
angiotensin II
• ACE inactivates
bradykinin
• Inhibition of ACE
should lower blood pressure
• ACE has a high affinity for
peptides with an aromatic amino acid, e.g. Phe or Tyr, in the antipenultimate position
• Side effects of ACE inhibitors
Hypotension; cough; hyperkalemia; renal failure; fetal anomalies; angioedema
• 1st orally-active ACE inhibitor (1978)
Captopril
Blood pressure (mm Hg) - normal
Systolic <130
Diastolic <85
Blood pressure (mm Hg) - high normal
Systolic 130 - 139
Diastolic 85 - 89
Blood pressure (mm Hg) - Mild hypertension:
Systolic 140 - 159
Diastolic 90 - 99
Blood pressure (mm Hg) - Moderate hypertension:
Systolic 161 - 179
Diastolic 100 - 109
Blood pressure (mm Hg) - Severe hypertension:
Systolic 180 - 209
Diastolic 110 - 119
