Option D: Drugs and Medicines (HL)

D.8 Drug action


D.8.1 Describe the importance of geometrical isomerism in drug action.
Stereoisomers are compounds with the same molecular and structural formula, but a different arrangement of atoms in space. Geometric isomers are a type of stereoisomers. In general, geometric isomers occur in species with a bond with restricted rotation. However, in some geometric isomers, when there are different atoms bonded to the same molecule, like in cisplatin, the phenomenon may occur.

The trans- and the cis- form of a drug can exhibit different properties. One geometric isomer may have the desired pharmacological effect, while the other may be less effective or have adverse side effects.

Example: Cisplatin is a drug used to treat testicular and ovarian cancer. The medicine exhibits geometric isomerism. However, only the cis- form of the drug is effective—the trans- form has no effects whatsoever. Only cisplatin can react with two guanine molecules in the DNA. Remember that cisplatin is square planar! 

 

                                      Cisplatin                                           Transplatin

 
D.8.2 Discuss the importance of chirality in drug action.
Optical isomers are stereoisomers chiral, or asymmetric, around one of the atoms. Body is a very precise machine, so the chemicals inside often exist as one of the enantiomers.

Only one enantiomer of drugs exhibiting optical isomerism sometimes has the desired pharmacologic effect. The other enantiomer may have weaker effect or severe adverse side effects. The pharmaceutical companies are likely to manufacture only the desired enantiomer because producing the ineffective enantiomer is financially inefficient, or to avoid the adverse effects of the harmful enantiomer.

The atom around which a molecule is chiral has four different atoms attached to it! Do not confuse optical isomerism with geo-isomerism!

Example: Thalidomide – one enantiomer alleviates symptoms of the pregnancy morning sickness, the other enantiomer causes deformities in the limbs of fetus

Ibuprofen - one enantiomer is effective, while the other is not
Naproxen – one enantiomer is a pain reliever, the other one is a liver toxin

Other examples include amphetamine and taxol. We use asterisk (*) to denote the chiral atom.

D.8.3 Explain the importance of the beta-lactam ring action of penicillin.
The beta-lactam ring has an unusual square structure, where each bond is 90̊. Due to its irregularity, the beta lactam ring is very reactive. The ring opens and covalently bonds to the enzyme transpeptidase, responsible for the forming of bacterial cell walls. If bacterium is unable to build its cell wall, it bursts, disintegrates and dies.

D.8.4 Explain the increased potency of diamorphine (heroin) compared to morphine.
Diamorphine (heroin) and morphine have the same functional groups, except for two. While morphine contains two alcohol groups, diamorphine is synthetically modified, so that the two alcohol groups are replaced by esters. 


Due to the difference in functional groups, diamorphine is much less polar than morphine. As a result of this, heroin can pass the blood-brain barrier much quicker in a much larger concentration. The lower polarity of heroin facilitates the drug’s transportation in the non-polar environment of the central nervous system (CNS)—remember, likes dissolve likes!

Because heroin requires a secondary reaction in brain, which hydrolyzes the ester links, another derivative of morphine, 6-monoacetylmorphine, is even more effective than heroin. 6-monoacetylmorphine has one alcohol and one ester group.

D9 Drug Design


D.9.1 Discuss the use of a compound library in drug design.

Compound library is a collection of a very large number of related compounds produced by combinatorial synthesis techniques. Each chemical in the library has associated information about its biological activity, such as the effect on enzymes and ability to bind to certain receptor sites. Researchers can use this information to find a compound, which fits well the properties of the researched disease (e.g. polarity, stereoisomerism). The process of creating compound libraries is lengthy and expensive, so firms must pay royalties to access the libraries. As opposed to synthesizing and individually evaluating different compounds for biological properties, this approach allows money and time to be saved.

D.9.2 Explain the use of combinatorial and parallel chemistry to synthesize new drugs.

Combinatorial chemistry uses a large number of starting reactants to create a variety of different compounds. These compounds are then tested for biological activity, such as the effect on enzymes and the ability to bond to receptor sites. This information is then stored in combinatorial libraries. Combinatorial chemistry is very repetitive and lengthy, so it uses a lot of computer mechanics.

Combinatorial synthesis utilizes solid phase chemistry. Solid phase chemistry is the following process:
1)      Very small resin beads are made—these provide a surface for the attachment and reaction of other molecules.
2)      Different molecules are attached to the solid beads. This is called the “mix and split” method.
3)      The products are cleaved from the beads by filtration and washing the beads.
Solid phase chemistry can be fully automated and use robotics, which saves time and money.

Combinatorial chemistry also uses the mix and split method.  The advantages of the combinatorial chemistry are:
1)      Mass testing and screening for biological activity
2)      High efficiency

Parallel synthesis carries out the chemistry to create a single product. Parallel chemistry can produce much smaller and more focused libraries.

D.9.3 Describe how computers are used in drug design.

Three-dimensional molecules can be created in silico, by computer software. An active ingredient can be made so that the chemical structure of the drug matches the shape of the compound, which the drug is supposed to affect. For example, an alteration in this compound may stop the spread of disease—this is the case of the beta-lactam ring in penicillin. Computer models can also assess the biological and other effects of compounds without the need to synthesize the chemicals in reality.

D.9.4 Discuss how the polarity of a molecule can be modified to increase its aqueous solubility and how this facilitates its distribution around the body.

The polarity of molecules can be increased to make them more water soluble. Because ions are the most water soluble, manufacturers try to make drugs ionic. Many drugs that contain amine group are reacted with hydrochloric acid and are administered as hydrochloride salt. Many drugs that contain carboxylic acid group are reacted with a base (e.g. NaOH) and are administered as their sodium or calcium salt.

Soluble aspirin – makes anion with NaOH; loses H+, so Na+ is attracted to the negative site where H+ was lost

Prozac (fluoxetine hydrochloride) – makes cation with HCl; H+ from HCl bonds to nitrogen in Prozac

Most drugs are distributed around the body through blood—they dissolve in the blood plasma. Blood plasma is mostly water, a polar solvent. Making a molecule more polar, i.e. transforming it into its ionic form makes the chemical more soluble. This facilitates the distribution of the drug around the human body, i.e. increases the drug’s bioavailability.

D.9.5 Describe the use of chiral auxiliaries to form the desired enantiomer.
A chiral auxiliary is a compound, much like a catalyst, which is used to convert non-chiral molecule into just the desired enantiomer. This is important because sometimes only one enantiomer has the desired pharmacological effect. The working of chiral auxiliaries is described below:
1)      Chiral auxiliary attaches to a non-chiral reactant.
2)      This creates the stereochemical conditions necessary to force the reaction to follow a certain path.
3)      In this path, only one enantiomer is formed.
4)      Chiral auxiliary is taken off the product and recycled.
5)      The enantiomer is ready and chiral auxiliary can be used again.
Chiral auxiliaries are used to synthesize Taxol (paclitaxel), an anti-cancer drug, which is normally found in the bark of a Pacific yew tree. Without chiral auxiliaries, enormous quantities of trees would have to be cut down to meet the demand for Taxol.


D.10.1 Describe the effects of lysergic acid diethylamide (LSD), mescaline, psilocybin and tetrahydrocannabinol.
Drug
Effects
Lysergic acid diethylamide
·         Flashbacks
·         Changes in visual and sound perception – hallucinations; good or bad trips
·         Desire to laugh
·         Hypertension, dilated pupils, change in body temperature & hearth rate
·         Psychological dependence
Mescaline
·         Subjective hallucinations
·         Anxiety, static tremors, psychic disturbances
·         Abdominal pain – muscle ache
Psilocybin
·         Subjective hallucinations milder than with mescaline
·         Change in mood – pleasant or apprehensive
·         Inappropriate laughter, dizziness
·         Muscle weakness is common
Tetrahydrocannabinol
·         Appetite stimulation
·         Lethargy, sluggishness
·         Anxiety and irritability
·         Psychological dependence

D.10.1 Discuss the structural similarities and differences between LSD, mescaline and psilocybin.

Should compare all three to the indole ring.
Drug
Functional Groups
Lysergic acid diethylamide
·         Benzene ring
·         Alkene (2)
·         Secondary amine
·         Tertiary amine
·         Tertiary amide
Mescaline
·         Benzene ring
·         Primary amine
·         Ether (3)
Psilocybin
·         Benzene ring
·         Secondary amine
·         Tertiary amine ion
·         Phosphate

All three drugs contain benzene ring, LSD and psolocybin also contain cyclopentene ring (?) with secondary amine, so they have the indole part. Mescaline misses the secondary amine.

D.10.3 Discuss the arguments for and against the legalization of cannabis.
Arguments for:
Arguments against:
·         Relieves symptoms of Parkinson’s disease
·         Stepping stone drug – can lead to the use of harder drugs
·         Source of additional tax revenue
·         Increased risk of lung cancer
·         No more or less harming than tobacco or alcohol.
·         Risk of driving or manipulation with machinery under influence, which increases the chance of an injury.

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