PHA6125 Concepts of Computer-Aided Drug Design (CADD)

Comprehensive practice flashcards for PHA6125 Computer-Aided Drug Design (CADD), covering molecular mechanics, quantum mechanics, QSAR, pharmacophore modelling, similarity coefficients, and receptor modelling.

Across which stages of drug discovery is Computer-Aided Drug Design (CADD) generally performed?

Between the stages of target validation and hit finding, spanning from hit generation to lead optimization.

What is the specific definition of virtual screening in CADD?

The specific use of computers or software to filter out massive amounts of chemicals from a library to identify hits, excluding lead optimization.

How does virtual screening relate to traditional biological testing?

It serves as a complementary first step to in vitro and in vivo screening, reducing uncertainty in identifying promising molecules from a chemical library.

What group of professionals developed the complex algorithms used in modern CADD, and during which era?

Countless mathematicians and physicists starting from the $1900$'s.

What is considered a key requirement for modern CADD software?

Ease of use.

What are the two main players in Computer-Aided Drug Design?

1) The ligand (the prospect drug being developed), and 2) the drug target.

Which experimental testing type is primarily utilized during the Target Identification (Target ID) stage?

In vitro testing.

Which animal models are specifically cited for in vivo target validation?

Zebrafish, mice/rats, cats, and dogs.

Which three experimental modalities are employed during the Hit-to-Lead (H2L) screening stage?

In vitro, in vivo, and in silico testing.

What alternative name is given to the Optimization stage in drug discovery?

The "Battery Test".

What sequential stages comprise the discovery and development pipeline leading to a product?

Target selection, Target validation, Lead finding, Lead optimization, Preclinical development, Clinical development, and Regulatory approval.

Why are in silico and in vitro testing generally no longer needed in preclinical and clinical stages?

Because drug candidates have already been selected, and testing shifts to animal and human models in drug development.

What is the single exception where in vitro testing is required during the preclinical stage, and what tests are performed?

Antibiotic drugs, which require a culture test and determination of the minimum inhibitory concentration (MIC).

What is the primary financial and practical motivation for implementing CADD in pharmaceutical companies?

To lessen the enormous expenses and time consumed by conducting extensive in vitro and in vivo testing across millions of library compounds.

What two critical tasks must algorithms in virtual screening software be tested to perform?

Realistically docking drug molecules to the target protein/enzyme/channel and providing an accurate score (binding energy).

What provides the computational basis for selecting candidate compounds during virtual screening docking?

The binding score or "binding energy" representing the binding efficiency to a specific target protein.

What are the two major computational mechanics methods embedded in CADD software?

Molecular mechanics and quantum mechanics.

What perspective is molecular mechanics referred to as, and what type of computer hardware can run it?

"The Classical Perspective", which can run on a low-end laptop/computer.

How does molecular mechanics treat atomic nuclei and electrons?

It applies the laws of classical physics (Newtonian mechanics) to nuclei while entirely disregarding the quantum nature and movement of electrons.

What mechanical model is used by molecular mechanics software to mimic atomic movement?

The ball-and-spring model, which fits atoms into a structure of moving spheres.

What is the physical significance of calculated energies derived from molecular mechanics force fields?

They have no meaning as absolute quantities, but are useful for comparing different conformations of the same molecule.

What perspective is quantum mechanics referred to as, and what type of computer hardware does it require?

"The Electronic Perspective", which requires a high-end laptop/computer.

How does quantum mechanics handle the motion of electrons in its calculations?

It treats matter at a subatomic level, and the motion of electrons is explicitly included in the calculations.

Why are atomic nuclei assumed to be motionless in quantum mechanics calculations?

Because the motion of electrons is much faster in comparison to that of nuclei.

How are electron-electron interactions addressed in quantum mechanical calculations?

Electrons are treated as moving independently of each other, taking the influence of other electrons and nuclei as an average.

What does the Latin term "ab initio" translate to, and how does the ab initio calculation method treat electrons?

"From the beginning"; both core and valence electrons are explicitly calculated without relying on stored parameters or empirical data.

What are the primary operational characteristics and limitations of ab initio calculations?

It is the highly accurate "Gold standard", but it is extremely slow and computationally restricted to small molecules.

How does the semi-empirical method simplify quantum mechanical calculations?

It computes valence electrons only, while core electrons are frozen or ignored, using approximations to achieve faster calculations on larger molecules.

What is Ligand-Based Drug Design (LBDD)?

Drug design performed in the absence of structural data for the drug target, basing selection entirely on existing active hits, leads, or similar molecules.

What is Structure-Based Drug Design (SBDD)?

Drug design performed in the presence of complete 3D structural knowledge of the drug target or binding site, measuring interactions directly.

What is de novo drug design?

Designing truly novel drugs with novel mechanisms atom-by-atom directly into a known protein binding site when no active ligands currently exist.

How do the four drug design quadrants map based on protein and ligand structural knowledge?

Structure-Based Design (Protein Known, Ligand Known), Ligand-Based Design (Protein Unknown, Ligand Known), De Novo Design (Protein Known, Ligand Unknown), and Library Design (Protein Unknown, Ligand Unknown).

Which computational techniques are employed when both the target protein and ligand structures are known?

Molecular docking, scoring, and virtual screening.

Which computational techniques are applied when the target protein structure is unknown but active ligands are known?

Pharmacophore modelling, $2\text{D}/3\text{D}$ similarity searches, quantitative structure-activity relationship (QSAR), and virtual screening.

Which screening and synthetic methods are utilized in Library Design when both target and ligand are unknown?

High-Throughput Screening (HTS), combinatorial chemistry (Combichem), and virtual screening.

What are molecular "grids" in computational drug design?

Pre-constructed $3\text{D}$ lattices placed around a molecule or inside a binding site used to measure the influence of molecular properties on surrounding space as fields.

What are the two most commonly measured molecular fields in grid-based modeling?

Steric fields and electrostatic fields.

How does a probe atom operate within a molecular grid?

It is positioned at each grid point and moved through the contours and crevices of the structure to measure steric and electrostatic interactions.

How is the overall shape of a molecule defined during grid field calculations?

By tabulating steric and electrostatic fields at each grid point and selecting a particular steric energy value.

What physical scenario do steric effects represent in grid analysis?

Regions where molecules are compact and close to one another, providing no open path for probe atoms to enter.

What do electrostatic fields detect as the probe atom moves along a molecule in a grid?

Regions of positive, negative, or neutral charge, identifying electron-donating groups (e.g., alkyl groups) and electron-withdrawing groups (e.g., halogens).

Which probe molecule is utilized in grid simulations to measure a drug's hydrophobicity?

Water molecules.

Which intermolecular interactions are typically evaluated at grid points within an enzyme binding site?

Ionic bonding, van der Waals interactions, and hydrogen bonding.

How do grids accelerate the calculation of binding strengths for multiple docked molecules?

By rapidly identifying which atoms or groups of the docked ligands coincide with pre-calculated grid points.

What is a standard ligand in CADD benchmarking?

A well-characterized reference molecule with known biological activity that serves as a benchmark for evaluating binding affinity and performance of new candidates.

What are three prominent software packages used to construct chemical structures to a professional standard?

ChemDraw, ChemWindow, and IsisDraw.

What type of string code from PubChem can be entered into programs to generate $3\text{D}$ structures?

A SMILES string.

What chemical and spectral properties can software like ChemDraw Ultra predict directly from a drawn structure?

Molecular formula, exact mass, molecular weight, elemental analysis, $\text{Log } P$, molar refractivity, boiling/freezing points, heat of formation, and predicted $^{13}\text{C}$ and $^{1}\text{H}$ NMR spectra.

Why must newly drawn molecular structures undergo energy minimization?

Because initial free-style drawing often creates unrealistic models with unfavorable, non-ideal bond lengths, bond angles, or torsion angles (high energy).

What type of computational program performs automatic energy minimization?

A molecular mechanics program.

What is an energy minimum in the context of molecular geometry optimization?

A stable conformation where further structural modifications result in only slight changes in energy ($\Delta E$).

What geometry change occurs to the catechol ring of apomorphine after energy minimization in Chem3D?

Its nonplanar, deformed aromatic ring with uneven carbon-carbon bond lengths ($1.420\text{ Å}$ and $1.579\text{ Å}$) is corrected into the desired stable planar conformation.

What are the three molecular properties commonly predicted in silico once a $3\text{D}$ model is built and minimized?

Steric energy, charge distribution, and spectroscopic transitions.

What does steric energy measure in an in silico molecular model?

The strain within a molecule caused by unfavorable atom-to-atom distances, bond distortions, or torsional stress.

What is the key restriction when using calculated steric energy to evaluate compounds?

It can only be used to compare conformations of atoms within the same molecule; it cannot compare atoms between different molecules.

Why is mapping charge distribution across a drug candidate critical?

It maps atomic partial charges, which is essential for predicting electrostatic interactions, hydrogen bonding, and active-site recognition.

Why must explicit water molecules be removed from computational models prior to calculating docking scores?

Water has a high dielectric constant that disturbs and masks true partial charges and electrostatic interactions between the drug and receptor.

What are Molecular Electrostatic Potentials (MEPs) and how are they calculated?

Whole-molecule potential maps calculated using quantum mechanics by considering molecular orbitals.

What do the contour lines in an MEP map visually identify?

Electron-rich and electron-poor areas, representing possible hydrogen bonding regions around each molecule.

What does HOMO stand for, and what is its role in chemical reactivity?

Highest Occupied Molecular Orbital; it is the electron-rich area of the molecule that acts as an electron donor.

What does LUMO stand for, and what is its role in chemical reactivity?

Lowest Unoccupied Molecular Orbital; it is the electron-deficient area of the molecule that acts as an electron acceptor.

In the ketanserin complex model, which group functions as the electron-deficient LUMO?

The fluorobenzoyl system.

In the indole/fluorobenzoyl complex, in which direction does electron transfer occur between frontier orbitals?

Electrons shift from the HOMO of the indole donor to the LUMO of the fluorobenzoyl acceptor.

What spectroscopic profiles can CADD software mathematically estimate beforehand?

Infrared (IR), Ultraviolet-Visible (UV-Vis), Mass Spectrometry (MS), and Nuclear Magnetic Resonance (NMR) spectra.

Why is calculating the theoretical IR wavenumber useful when designing novel $\beta$-lactam analogues?

A predicted shift away from the characteristic carbonyl peak indicates absence of the active $\beta$-lactam pharmacophore, predicting a lack of antibacterial activity.

What is the core premise of the Similarity Property Principle (SPP)?

"Similar compounds should have similar properties" (i.e., compounds that look alike probably have the same biological effect).

Why do commercial pharmaceutical companies intentionally avoid developing molecules that are too structurally similar to marketed drugs?

To avoid patent infringement and intellectual property (IP) disputes during registration and commercialization.

What is "SAR discontinuity"?

The phenomenon where small chemical modifications lead to significant changes in biological activity, representing a major limitation of the SPP.

What is an "activity cliff" in medicinal chemistry?

A phenomenon where a very slight structural change between two similar compounds causes a night-and-day difference in their biological activity.

What shared structural core do carbamazepine (CBZ), amitriptyline, and loratadine possess, and what are their respective pharmacological uses?

They share a tricyclic core: CBZ is an anticonvulsant, amitriptyline is a TCA antidepressant, and loratadine is an $H_1$ antihistamine.

Why is molecular similarity considered inherently subjective ("in the eye of the beholder")?

Different chemists evaluate structural features differently based on intuition, and no absolute mathematical standard exists for overall likeness.

Name three commonly used similarity coefficients in cheminformatics.

Tanimoto coefficient ($T_c$), Tversky coefficient ($T_v$), and Dice coefficient ($D_c$).

What Tanimoto coefficient ($T_c$) threshold has been propagated in literature for over $15$ years to indicate similar bioactivity?

A $T_c$ value of at least $0.85$ ($85\%$ similarity).

What is the mathematical formula for calculating the Tanimoto coefficient ($TC$)?

$$TC = \frac{A}{A + B + C}$$

In the Tanimoto formula $TC = \frac{A}{A + B + C}$, what do the variables $A$, $B$, and $C$ represent?

$A$ = bits/features present in both compounds; $B$ = features present only in compound 1; $C$ = features present only in compound 2.

What is the calculated Tanimoto coefficient between aspirin and salicylic acid using the eight-fingerprint table?

$TC = \frac{3}{3 + 2 + 0} = 0.6$ (or $60\%$).

What is the theoretical maximum value of the Tanimoto coefficient, and when is it achieved?

$1.00$ (or $100\%$), achieved only when comparing a compound to itself or an identical structure.

What is the purpose of performing structure overlays in molecular modeling?

To align two structures and identify regions of overlap, common pharmacologic features, and potential structure-activity relationships.

Which common pharmacophoric groups overlap in the $2\text{D}$ overlay of cocaine and procaine that confer local anesthetic activity?

The aromatic ring, the ester group, and the amino group.

What metric is used by overlay software to evaluate structural alignment fit?

Root Mean Square Distance (RMSD).

What range of RMSD indicates high structural similarity between overlaid molecules?

A lower RMSD between $0$ and $1.50$.

Why is molecular overlay fitting routinely conducted on a "rigid basis"?

Locking conformations prevents single-bond rotations, drastically reducing calculation time and computational demand.

What is the primary drawback of assuming molecular rigidity during overlay procedures?

It is less realistic because molecules in solution constantly rotate and vibrate.

How do the biological activities of the structural isomers quinine and quinidine illustrate the limits of the SPP?

Despite near-identical structures, quinine is an antimalarial drug, whereas quinidine is an antiarrhythmic drug.

How do cisplatin and transplatin demonstrate that structural similarity does not guarantee identical pharmacological effects?

They differ only in the geometric orientation of amine and chloride ligands, yet only cisplatin possesses active anticancer chemotherapeutic properties.

What is the "Golden Rule of Selection" when screening structurally similar compounds?

It is not enough that compounds look similar; their biological activity must be explicitly confirmed by testing.

What is the ultimate goal of quantifying chemical similarity in CADD?

To extrapolate calculated similarity metrics into meaningful predictions of physicochemical properties and biological activity.

What is Quantitative Structure-Activity Relationship ($2\text{D}$ QSAR)?

A computational dry-lab method that converts quantified physicochemical properties of molecules into mathematical equations or plots to predict biological activity.

What three criteria must candidate compounds satisfy to be included in a QSAR study?

They must be structurally related, act at the same biological target, and share the same mechanism of action.

What does it imply if an analogue or screening compound falls outside a derived QSAR equation/plot?

It implies that the compound is inactive.

How does QSAR accelerate synthetic chemistry workflows?

It directs synthetic efforts exclusively toward analogues predicted to have improved activity, cutting down the total number of compounds synthesized.

What is the acceptable correlation threshold ($R^2$) for a viable QSAR regression equation?

$R^2 > 0.8$ (with values of $0.98$ or $0.99$ preferred in robust models).

Why must Standard Deviation (SD) be kept low in QSAR datasets?

A high SD indicates scattered, distant data points, making the resulting QSAR regression equation unreliable for predicting activity.

Which physical parameter is commonly used to quantify molecular hydrophobicity in QSAR studies?

The partition coefficient ($P$), or more commonly its logarithm ($\log P$).

What mathematical expression defines the partition coefficient ($P$)?

$$P = \frac{\text{Concentration of drug in octanol}}{\text{Concentration of drug in aqueous solution}}$$

What characteristic curve shape does a plot of $\log(1/C)$ versus $\log P$ typically follow?

A parabolic or bell-shaped curve with an optimum peak at $\log P^0$.

Why does biological activity decline once a drug exceeds its optimal lipophilicity ($\log P^0$)?

Excessively hydrophobic drugs become poorly soluble in blood and watery fluids, precipitate, get trapped in fat depots, or cannot be transported effectively.

For which specific class of pharmacological agents is biological activity correlated with $\log P$ alone?

Anesthetics.

What is the general rule of thumb for the ideal $\log P$ value of drugs targeting the central nervous system (CNS)?

A $\log P$ value of approximately $2$.

Why are newer cardiac drugs like sulmazole designed with a lower $\log P$ of $1.17$?

To avoid crossing the blood-brain barrier and prevent CNS side effects, such as the yellow/bright vision disturbances seen with high $\log P$ cardiac drugs like digoxin.