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Example research essay topic: Organic Chemistry Anti Inflammatory - 2,348 words

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What would you think if I gave you a drug and said this is going to help you but I want you to know ahead of time that 50 % of the dose is not affected, in fact the drug is 50 % not pure. The idea of that would make you feel you want another drug. The reality of that is that a large # of drugs we give are 50 % impure and the reason is b / c so many drugs we give are racemic mixtures, a 50: 50 mixture of enantiomer's. Most drugs that demonstrate optical activity only one isomer is pharma logically active, so what that means is that we are giving a product where 50 % of it is inactive. There are some important considerations you have to take into account when you have drugs that are optically active and what we would like to do today is take the things we have gone thru this semester where we have really ignored stereochemistry thru that whole process and talk about the times in which you need to consider aspects of stereochemistry. This has become a big issue now the FDA has pushed for drug companies to be able to characterize both isomers when giving a racemic mixture, also to force manufactures to only market the active isomer and not going with the standard practice of giving a drug which is only 50 % pure.

There are also some drugs on the market as racemic mixtures that now compounds are being introduced as enantiomer's and therefore this results in different doses because now you are giving a drug which is 100 % active. Stereochemistry has long been recognized, Louis Pasteur was probably the 1 st to state the significance of it, he said most natural organic products, the essential products of life are asymmetric This established perhaps the only well marketed line of demarcation that can at present be drawn between the chemistry of dead matter and the chemistry of living matter. What he was trying to say is that basically the chemistry of life is highly dependent on geometric form and as a consequence stereochemistry is essential. If you look at the drugs on the market over 60 % are optically active. In 1980, at least 398 drugs that were marketed as racemic mixtures.

We now want to review some of the nomenclature that you never thought you would see again since organic chemistry. Remember when you talk about stereochemistry you are talking about a carbon atom which has 4 different ligands. In this case when you have four different ligands this is asymmetric and often referred to as a chiral carbon. This results in 2 mirror images that can not be superimposed. On pg. 152 what is shown is an example of L (+) alanine and D (-) alanine and no matter how many times you try to twist and turn that molecule you can never superimpose the left hand side on top of the right hand side. You cannot superimpose optically active isomers, you recall from organic chemistry that when you have this type of situation that such a compound will rotate plane polarized light and one isomer will rotate in one direction and the other isomer in the other.

A compound that rotates it to the right is dextro rotatory and that is signified using the term d or (+). A substance that turns it to the left is levorotatory and is signified as L or (-). You also remember when you have a pair of nonsuperimposable images like this there are different words used to describe them. Sometimes they are called optical isomers, optical antipodes, most commonly in the drug literature they are referred to as enantiomer's. What is extremely important to recognize is that enantiomer's are chemically identical. , they have the same melting point, kp, same solubility, same lipophilic ity.

From a chemical point of view they are identical the only difference is there geometric configuration, that is the only place they are not identical. The other thing you will recognize is that the rotation of plane polarized light does not indicate the configuration. The absolute configuration is generally determined by X-ray crystallography and you recall that the designations for this based on the actual geometric configurations is You also recall that if you have a pure enantiomer its thermodynamically unstable and will tend to break down to give you a 50: 50 mixture of 2 enantiomer's. When you have such a mixture its referred to as a racemic mixture and you generally designate it as d, L or (+), (-).

A good example of that would be propranolol which is designated using as d, L-propranolol, therefore when you see that you know that this drug is a racemic mixture, that is the form in which it is marketed. If it is marketed as a pure isomer then it would only have d, propranolol or l, propranolol. Now there are some compounds that have more than one chiral center and when that happens and you rotate only one of these chiral centers you will end up with a compound that is not a mirror image of the non-inverted compound and this process is referred to as epimerization and results in the formation of diastereomers. What is it to recognize is that diastereomers are different than enantiomer's they are not only geometrically different but also chemically distinguishable. In the middle of pg. 132 it just shows you 4 cinchona alkaloids that are diastereomers and 2 of which are marketed as drugs quinidine and quinine.

Quinidine is widely used in the treatment of arrhythmias and quinine is used in the treatment of malaria. Therefore here you have compounds that display different activity and have different physical chemical properties. This table below is just a review for you of nomenclature that you went thru in organic chemistry. d-/l- is the rightward or leftward rotation of plane polarized light D-/L- is the rightward or leftward arrangement of molecule and has to come from x-ray crystallography data. Limited for amino acids R-/S- is the rightward / leftward rotation; so you can have a drug that is levorotatory in how it turns plane polarized light but actually has an absolute configuration that is right ward, therefore you would get this configuration R-/S- R- (-) - is levorotatory but with absolute configuration R Pharmacodynamic Significance of Stereoisomers What is shown on pg. 133 is two enantiomer's which have a significant different pharmacologic effect in terms of taste. This is d-carbon (caraway) and on the right is l-carbon (spearmint), The only difference here is the geometric configuration.

Now how can you have compounds that are mirror images of one another and yet have such different taste? It relates to their ability to interact with receptors. For example if we have a chiral carbon and we have our 4 substituents and it interacts with the receptor designated here as 3 points on the receptor designated as B C D and these are the points of interaction on the molecule. If now we look at the mirror image of that compound and try to rearrange this, we can never rearrange it in a way where we get more than 2 sites of interaction. We are only going to be able to combine 2 sites at once. So now interaction with the receptor is significantly different because of this slight geometric change.

Thats why geometry is important because in order to exert its effect most drugs have to interact with a macromolecule and so the geometry of the drug becomes very important in that interaction and a slight change may move from a 3 point interaction to a 2 point interaction which can have a big impact on the effect you exert. It may take a drug from being pharma logically active to being inactive, an example of that can be seen with Ketamine. At the top of pg. 134 ketamine is shown and its used widely in veterinary medicine as a local anesthetic. If you look at S- (+) -ketamine its far more potent than R- (-) -ketamine in terms of its anesthetic ability, one of the problems with R (-) - ketamine is in terms of its spontaneous motor activity and post-emergent distress. This graph gives you some measure, it shows the percent reduction in median frequency vs plasma concentration for these two isomers, the R and S isomer. You will notice that the EC 50 for the S isomer is much lower than for the R isomer indicating its much more potent.

You will also notice that the Emax is substantially higher for the S ketamine than the R ketamine. So here just small different geometric configurations and you get significant differences in the pharma logical effect. The table below just provides you with a number of examples of this, where you can see a wide range of differences. You can see drugs where there is a marked difference in the activity of different isomers. A good example of that would be terbutaline, if you look at the minus plus isomer in terms of its ability to act as a bronchodilator, you get trachea relaxation. The difference in activity of these two isomers is 3000: 1, so in essence the plus (+) isomer is inactive at any given dose.

Then you have a compound like propranolol if you look at the S to R in terms of its ability to block tachycardia, so its beta receptor blocking activity, you see the difference is about 100: 1. Pindolol is something similar its isomers are 200: 1. In contrast if you look at something like methadone and you look at ability to cause respiratory depression you see its isomers only have a difference of about 3: 1. So you get a wide range of differences in isomers. You can have some isomers that display no difference in terms of one another in terms of pharma logical activity, some which make the difference between active and inactive and all of these are the result of what is the impact of this geometric difference, can it still fit in the receptor pocket, can it still bind as tightly to the receptor. Some drugs will go into the pocket ok but wont bind as tightly and thats where you see small differences between isomers.

Some the geometric configuration is enough that it cant even get into the receptor and thats where you see large differences in pharma logic effect. So there is a couple of situations that can be observed. 1) You can have enanitiomers that have identical efficacy and toxicity. 2) You can have enantiomer's that have the same therapeutic and toxic effects, but differ in the magnitude, so in the first case they are qualitatively and quantitatively the same and in the second case they are qualitatively the same but quantitatively different, they have the same pharma logic effect but different magnitude. 3) You may have a situation where one enantiomer may posses all of the pharmacologic activity while the other is essentially inactive. 4) You may also have a situation where both are pharmacologic ally active but they have qualitatively different effects. For example if you look at Ibuprofen, a very commonly used over the counter nonsteroidal anti-inflammatory drug, it is optically active, present as R and S. S-Ibuprofen inhibits platelet thromboxane production twice as effectively as does the raceface.

The R form remixes to the S-form and does not cause G. I. lesions, that seems to be related to the S-Ibuprofen. One of the things that seems to be unique about the non-steroidal anti-inflammatory agents is that you can get race metic conversions in vivo. Therefore even if you give the pure isomer you could end up with the alternative and that occurs thru the below means on pg 135. If you give R-Ibuprofen it can undergo metabolism to form an acyl CoA conjugate which then several things can happen.

One it can undergo hydrolysis to give you back R-IBU or in the presence of racemose you can end up with the conversion of S-IBU as a CoA conjugate and then in the presence of hydrolase you can give rise to S-IBU. So if you give somebody R-IBU as a pure elastomer you will find that they will exhibit concentrations of S-IBU because of this conversion that takes place. This is not a reaction that commonly occurs but what this does do is wart efforts to try to give a drug that might avoid some of the toxicities. The way this was discovered is there was some early data that suggested that the gastric lesions were actually coming from the S-IBU and that R-IBU did not cause it, so when studies were done it was a surprise to find even though you gave R you ended up with S and its because of this conversion and this complicates matters when you give a pure enantiomer, even though you give a pure enantiomer it does not mean you are not going to expose the patient to the other enantiomer, there are these unusual reactions that take place. The other type of reaction you may see is a mixed agonist activity and a good example of that is labetalol, which is a combined alpha beta blocker and there are different levels of activity in terms of the alpha or beta receptors with the different isomers.

If we look at pg. 136 at the top we can see what is being compared here is the RR, SS, RS, SR isomers. What you will notice is that if you look at the alpha 1 activity the SR isomer is the most potent. If you look at the beta 1 activity you can see that the RR isomer is the most potent and if we look at beta 2 activity the RR isomer is also the most potent there. So here we have multiple activities of the drug with different potencies with different isomers. Bibliography: Stereochemistry Considerations What would you...


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Research essay sample on Organic Chemistry Anti Inflammatory

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