Chiral drugs

Chirality is derived from the Greek word χειρ (kheir) that stands for "hand". An object is said to be chiral if the object and its mirror image are non-superimposable, just like our right and left hand. Now you must be wondering what we mean by ‘non-superimposable’. When the mirror image of the object is placed over the original object and they do not overlap, as shown in the figure below, then the object and its image are said to be non-superimposable .

Molecular chirality was discovered by Louis Pasteur back in 1848, when he successfully separated the two isomers of sodium ammonium tartarate. He observed that the two isomeric crystals were non-superimposable mirror images of each other, they had the same physical properties, but differed in their ability to rotate plane polarized light. This property was termed as optical activity.

In non-polarized light, electric and magnetic fields randomly orient in all planes. When the non-polarized light is made to pass through a polarizing filter, oscillations get oriented in only one particular direction, and is called plane polarized light.

As stated, chiral molecules are optically active, which implies that when a beam of plane-polarized light passes through a chiral molecule, it interacts with the molecule in such a way that the angle of the plane of oscillation rotates.

A pair of enantiomers always rotate plane-polarized light to an equal but opposite degree. Example: If a pure sample of (S)-carvone rotates plane-polarized light by +10°(clockwise), then a sample of (R)-carvone (in the exact same concentration and under the same experimental conditions) will rotate the plane polarized light by (–)10° (counterclockwise).

Most biological compounds are chiral.

The rule of thumb is: chiral carbon centers are carbon atoms that are attached to four different substituents, that are placed at the corners of a tetrahedron. Chiral carbon atoms are also referred to as ‘stereogenic carbons’ or ‘asymmetrical carbon atoms’.

Compound 1 has a chiral carbon center, because it is attached to four different groups (W, X, Y and Z). Compound 2 is the mirror image of compound 1. As per our definition of chirality, compounds 1 and 2 should be non-superimposable. How do we confirm this? Let’s rotate compound 1 around one axis of rotation, and see if any of the products of rotation lead to compound 2. If not, then compound 1 is chiral. So let’s go ahead and sequentially rotate compound 1 about the C-X axis in a clockwise direction. The various products of rotation are shown below.

So as you can see, none of the above structures is the same as the mirror image, compound 2. Thus we can be convinced that compound 1 is chiral.

Before we move on, let’s try to identify if the carbon atom in the following three compounds is a chiral center or not.

A chiral compound and its mirror image are referred to as “enantiomers”. So, we can define *enantiomers as being two stereoisomers that contain asymmetric carbon atoms, and are related to each other as non-superimposable mirror images. *

Enantiomers differ only in their optical activity i.e. the direction in which they rotate plane polarized light. If an enantiomer rotates polarized light to the right or in a clockwise direction, it is said to be the (+) or the dextrorotatory isomer. On the other hand, if the plane polarized light is rotated to the left or in a counter-clockwise direction, the stereoisomer is called as the (−) or the levorotatory isomer. Enantiomers have the same physical properties, except for the direction in which they rotate plane polarized light. Since these isomers exhibit different optical activity, they are sometimes also referred to as optical isomers.

Configuration of each chiral carbon of the two enantiomers is systematically assigned using the R/S system. This method of naming the enantiomers was developed by three chemists: R.S. Cahn, C. Ingold, and V. Prelog and is also often called the Cahn-Ingold-Prelog rules of naming enantiomers.

PS: There is no correlation between (+) and (-) , and the R & S stereochemical labels.. An ‘R’ stereoisomer can either be (+) or (-). Optical activity of R and S stereoisomers has to be measured experimentally with a polarimeter.

Before we get into the discussion of how to assign a configuration (R or S) to a chiral carbon enantiomer, let us be clear of how the spatial arrangement of the tetrahedral carbon, with respect to the plane of the molecule, is depicted on paper.

Step 1: The four groups attached to the chiral carbon atom have to be arranged in the relative order of priority. The group with the highest atomic number is given the highest priority #1 and the group with the lowest atomic number is given the lowest priority i.e. #4. Let’s clarify this with an example.

Step 2: Make sure that the lowest priority group (#4) is pointing away from you i.e. it is attached to the hashed wedge in the 2D spatial arrangement.

Step 3: Now draw a curved arrow from the highest priority (#1) substituent to the second lowest priority (#3) substituent. If the arrow turns in a counterclockwise direction, the configuration at the stereocenter is labeled S ("Sinister" → Latin= "left"). If, however, the arrow turns clockwise, then the stereocenter is labeled R ("Rectus" → Latin= "right").

As illustrated above, 1a is an ‘S’ isomer while 1b is the ‘R’ enantiomer.

How are we supposed to proceed if the lowest priority group is not pointing away from us? In such a case you will need to select any one of the two groups that are present in the plane of the molecule, rotate the molecule about that axis and step wise move the other three groups around until the lowest priority group (#4) is pointing away from you. Let’s understand this through the following example. Let’s sequentially rotate the molecule about the C-X bond in a clockwise direction until hydrogen points away from us.

Priority number is assigned to atoms that are attached directly to the stereogenic carbon, so what happens if there are two substituents with the same rank? In that case, simply proceed along the two substituent chains until you find a point of difference. Let’s again understand this through an example. Consider the following molecule

Here, the 4 atoms attached directly to the chiral carbon (stereocenter) are chlorine (Cl) and 3 carbon atoms (C, C and C) respectively. Clearly based on atomic numbers, Cl atom gets the highest priority (#1), but the other three atoms are tied (since they are all carbon atoms). So in such a case, we move along each substituent chain and see what atoms are directly attached to the three carbon atoms respectively

As you can notice, each carbon atom is directly attached to 3 other atoms (out of which two are hydrogens), so the deal breaker is the third atom (hydrogen vs carbon vs oxygen). Thus, we have been able to assign relative priorities to the four groups attached to the chiral carbon atom by moving along the substituent chains.

The importance of chiral drugs in the drug development space cannot be understated. In pharmaceutical industries, 56% of the drugs currently in use are chiral molecules and 88% of the last ones are marketed as racemates (or racemic mixtures), consisting of an equimolar mixture of two enantiomers.

Although the enantiomers of chiral drugs have the same chemical connectivity of atoms; they exhibit marked differences in their pharmacology, toxicology, pharmacokinetics, metabolism etc. Therefore, when chiral drugs are synthesized, as much effort goes towards the rigorous separation of the two enantiomers. This ensures that only the biologically active enantiomer is present in the final drug preparation.

The enantiomers of a chiral drug differ in their interactions with enzymes, proteins, receptors and other chiral molecules too including chiral catalysts. These differences in interactions, in turn, lead to differences in the biological activities of the two enantiomers, such as their pharmacology, pharmacokinetics, metabolism, toxicity, immune response etc. *Surprisingly, biological systems can recognize the two enantiomers as two very different substances. *

Recognition of chiral drugs by specific drug receptors is explained by a three-point interaction of the drug with the receptor site, as proposed by Easson and Stedman. The difference between the interaction of the two enantiomers of a chiral drug with its receptor is illustrated below.

Easson-Stedman’s illustration of hypothetical interaction between the two enantiomers of a racemic drug with a receptor at the drug binding sites: The three substituents A, B, C of the active enantiomer (left) can interact with three binding sites a, b, c of a receptor by forming three contacts Aa, Bb and Cc, whereas the inactive enantiomer (right) cannot because the contact is insufficient.

In this case, one enantiomer is biologically active while the other enantiomer is not. The substituents of the active enantiomer drug labeled A, B, and C must interact with the corresponding regions of the binding site labeled a, b, and c of the receptor in order to have a proper alignment Aa, Bb, Cc. In this case, this fitting interaction produces an active biological effect. In contrast, the inactive enantiomer cannot bind in the same way with its receptor; thus, there is no active response. The attachment of an enantiomer to the chiral receptor is analogous to a hand fitting into a glove. Indeed, a right hand can only fit into a right hand glove. Similarly, only a particular enantiomer that has a complementary shape to the receptor site can fit into a receptor site. The other enantiomer will not fit, like a right hand will never fit into a left glove.

Some drugs are marketed solely as a pure single enantiomer (that is; the drug preparation has no contamination with the other enantiomer). Enantiomeric excess (ee) is a measurement of the degree of purity of any chiral sample. It reflects the degree to which a sample contains one enantiomer in excess over the other. A racemic mixture has an ee of 0% (both enantiomers are present in a 1:1 ratio), while a completely pure enantiomer has an ee of 100%. As an example, if a sample contains 70% of R isomer and 30% of S isomer, then it will have an enantiomeric excess of 40%. This can be rationalized as a mixture of 40% pure R with 60% (30% R and 30% S) of a racemic mixture.

Enantiomer ratio is extremely important because while one enantiomer is beneficial to the body, the other enantiomer can be highly toxic to the body. A well-known example of enantiomer related toxicity is the R- and S-enantiomers of thalidomide.

The R-enantiomer is an effective sedative, which has a soothing effect that relieves anxiety and makes the patient drowsy; while, the S-enantiomer is known to cause teratogenic birth defects. A teratogenic fetus is one with deficient, redundant, misplaced or grossly misshapen parts. In fact, S-Thalidomide was shown to be responsible for over 2000 cases of serious birth defects in children born to women who took the racemic mixture during pregnancy.

Hopefully now you have an appreciation of the concept of chirality. It’s amazing how two molecules that look so similar on paper can have strikingly different biological activities and thus strikingly different effects on the human body!