Quantitative Structure-Activity Relationships (QSAR)
Attempts to identify and quantitate physicochemical properties of a drug in relation to its biological activity or binding
Studies hydrophobic, electronic, and steric properties--either whole molecule or pieces
med chemist draws up an equation that quantifies the med chemist draws up an equation that quantifies the relationship & allows one to predict (to some extent) relationship & allows one to predict (to some extent) the biological activitythe biological activity
Quantitative Structure-Activity Relationships (QSAR)
dv fewer compounds may need to be made
oweverif compound does not “fit” the equation, then chemist knows they need to modify the equation
Log P (partition coefficient)
Hydrophobicity
P = [drug] in octanol / [drug] in water
• Vary log P & see how this affects the biological activity.
• Biological activity normally expressed as 1/C, where C = [drug] required to achieve a defined level of biological activity. The The more active drugs require lower concs. more active drugs require lower concs.
Log P (partition coefficient)
Hydrophobicity• Plot log 1/C vs. log P• • Typically over a small range of log P, e.g. 1-4, a
straight line is obtained e.ge.g. log 1/C = 0.75 log P + 2.30. log 1/C = 0.75 log P + 2.30
• If graph is extended to very high log P values, then get a parabolic curve. Reasons:
poorly soluble in aqueous phase trapped in fat depots• more susceptible to metabolism
Log P: Hydrophobicity
Log P
Parabolic curveParabolic curve::
log 1/C = - k1 (log P)2 + k2 log P + k3
•When P small, dominated by log P term
•When P large, log P squared dominates & so activity decreases
Log P
Note that one is not always measuring biological activity, sometimes binding!
*RELATIVELY FEW DRUGS EXIST WHOSE ACTIVITY IS RELATED TO LOG P ALONE!!!
--those that do are the general anesthetics--partition into cell membranes, & thereby affect membrane structure & nerve function
--no specific drug-receptor interactions
Log P Values: Uses
With these equations for anesthetics (ethers only), it is possible to predict activity if log P known (doesn’t work if structure very different)
ether chloroform halothane
0.98 1.97 2.3
(anesthetic activity increases in same order)
Drugs with Log P values close to 2 should be able to enter the CNS efficiently
e.g. barbiturates have log P values close to 2 also; want to make sure log P value is much lower if you don’t want possible CNS side effects
Example: decreased CNS side effects
P vs. •P measures drug’s overall hydrophobicity & measures drug’s transportability
measures the hydrophobicity of a specific region on the drug--hydrophobic bonding to a receptor
substituent hydrophobicity constant, substituent hydrophobicity constant,
• Possible to calculate the substituent hydrophobicity constant ()
• A measure of how hydrophobic relative to H
• Measure P experimentally for a standard compound with and without a substituent (X). Use this equation:
•
x = log Px - log PH
x = log Px - log PH
•H is for standard compound
•positive = substituent more hydrophobic than H
•negative = less hydrophobic than H
values for various substituents on aromatic rings
CH3 t-Bu OH CONH2 CF3 Cl Br F
0.52 1.68 -0.67 -1.49 1.16 0.71 0.86 0.14
Theoretical Log P for chlorobenzene
= log P for benzene + for Cl
= 2.13 + 0.71 = 2.84
values for various substituents on aromatic rings
CH3 t-Bu OH CONH2 CF3 Cl Br F
0.52 1.68 -0.67 -1.49 1.16 0.71 0.86 0.14
Theoretical Log P for meta-chlorobenzamide
= log P for benzene + for Cl + for CONH2
= 2.13 + 0.71 - 1.49 = 1.35= 1.35
Tables of
•See Table 2.5
•Many tables exist for all sorts of different structures.
•Note that values will be different when using different solvent systems.
•MOST QSAR equations have contribution from MOST QSAR equations have contribution from either P or either P or or both or both
Electronic Effects: The Hammett Constant
Hammett constant (1940) Measure e-withdrawing or e-donating effects (compared to benzoic acid & how affected its ionization)
Electronic Effects: The Hammett Constant
Electron Withdrawing Groups:
Equilibrium shifts to Right & Kx > Kbenzoic
Since x = log Kx – log Kbenzoic, then will be positive .
x = log (Kx/Kbenzoic)
Electronic Effects: The Hammett Constant
e-withdrawing groups stabilize the carboxylate ion: larger Kx, and have positive valuese.g. Cl, CN, CF3 e-donating groups (e.g. alkyl) equilibrium shifts left (favoring unprotonated): lower Kx and negative values
Hammett Constants
Hammett constant takes into account both takes into account both resonance and resonance and inductiveinductive effects; thus, the value depends on whether the effects; thus, the value depends on whether the substituent is substituent is para or metapara or meta substituted substituted--ortho not measured due to steric effects
In some positions only inductive effects effect & some both resonance & inductive effects play a part aliphatic electronic substituent constants are also available
Uses
Only one known example where just Hammett constants effectively predict activity (insecticides, diethyl phenyl phosphates.These drugs do not have to pass into or through a cell membrane to have activity).
Log (1/C) = 2.282 – 0.348
Steric Effects
much harder to quantitate
Examples are: Taft’s steric factor (Es) (~1956), an experimental value based on rate constants Molar refractivity (MR)--measure of the volume occupied by an atom or group--equation includes the MW, density, and the index of refraction-- Verloop steric parameter--computer program uses bond angles, van der Waals radii, bond lengths
Putting it all together
For a group of antihistamines,
Log (1/C) = 0.440 Es – 2.204 (n=30, s=0.307, r= 0.886)
Log (1/C) = 2.814 - 0.223 (n=30, s=0.519, r= 0.629)
Log (1/C) = 0.492 Es - 0.585 - 2.445 (n=30, s= .301, r= 0.889)
Hansch Analysis
• Proposed that drug action could be divided into 2 stages: 1) Transport & 2) Binding
• Log 1/C = k1P = k2P2 + k3 + k4Es + k5
Hansch Analysis
• Look at size and sign for each component of the equation.
• Values of r <<0.9 indicate equation not reliable
• Accuracy depends on using enough analogs, accuracy of data, & choice of parameters.
Craig Plots
• Plots of one parameter against another.
• For example, vs.
• Used to quickly decide which analogs to synthesize if the Hansch equation is known.
Hansch equations
log 1/C = 1.22 – 1.59 + 7.89 (n=22; s=0.238; r= 0.918
log 1/C = 0.398 + 1.089 + 1.03 Es + 4.541 (n=9; r= 0.955)
log Cb = 0.765 = 0.540 2 + 1.505
log 1/c = 1.78 – 0.12 + 1.674
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