METABOLISM and STABILITY OF THE COMPOUNDS/DRUGS

ü  Drug metabolism means the necessary changes in the drug molecules which are essential for the easy excretion from the body. 

ü  Drug metabolism can result in toxication or detoxication - the activation or deactivation of the chemical. While both occur, the major metabolites of most drugs are detoxication products.

ü  Metabolism of the drug is nothing but biotransformation of it.  We already learnt that drug’s are chemically and enzymatically modified to more water soluble compounds (called metabolites).

ü  In general – drug metabolism reactions are divided into two type of reactions →                         Phase I reaction & Phase II reaction

After the drug is absorbed by the GI tract, it is taken up by the part of the bloodstream called the hepatic portal system. Most of the drugs are absorbed into this system except for the lipids which are absorbed into the lymphatic system and then delivered into the blood by the thoracic duct into the superior vena cava. 

The hepatic portal system is designed to take digested foodstuff into the liver where it can be processed, in some cases it is stored before being distributed and it is possible that this may happen to the drug and the drug would be metabolized before reaching the rest of the body. Such drugs that metabolized by the liver are said to have a high hepatic first pass. Hence drugs with a very high hepatic first pass cannot be given orally.

The Blood Brain Barrier:  The capillaries in the CNS are different they have pores which are sealed by the connective tissue and hence only small molecules can cross the blood brain barrier and the substances that can cross over have to be very lipophilic in nature. The blood-brain barrier (BBB) is the protective mechanism of the CNS and is not present everywhere in the brain. This is sometimes useful as it avoids some drugs from crossing the CNS and causing deleterious effects.

Phase I reaction (FIRST PASS METABOLISM) : Here generally oxidation, reduction and hydrolysis type of reactions occur.  Types of enzymes involved in such reactions include cytochrome P450 (CYP) superfamily, flavin-containing mono-oxygenases (FMO), monoamine oxidases, dehydrogenases (for alcohol or aldehyde), reductases, esterases, amidases and epoxide hydrolases.

Such type of metabolism occurs before a drug reaches the systemic circulation.  Typically metabolism occurs in the gut and / or liver before reaching the systemic circulation.

Two thirds of drugs cleared by metabolism are metabolized in part by the cytochrome P450 enzymes with CYP3A4 accounting for almost 50% CYP activity.

One third of the top 200 prescribed drugs which undergo drug metabolism are substrates for metabolic clearance mediated by enzymes other than CYPs. 

Phase II reaction (SECOND PASS METABOLISM) : Here generally addition (or conjugation) of highly polar groups to the drug or drug metabolites occur.   Types of reactions involved include glucuronidation, sulphation, methylation, N-acetylation and glutathione conjugation.

It is not necessary that Phase I reaction occurs first and then Phase II reaction.  If suitable functionalities are present on the drug molecule then Phase II reaction can occur directly.

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Chemical Stability of the compound:  As discussed earlier, compounds/drugs can be degraded by both chemicals and enzymes in the body. 

Let us first talk about how non-enzymatic degradation process of the compound is measured in a in vitro process.

Test compound is incubated with buffer of interest at different pH (commonly pH2, pH6 and pH10).  Some buffers which are also investigated include simulated intestinal fluid (SIF) and simulated gastric fluid (SGF). 

At different time intervals the concentration of test compound is quantified by LC-MS. 

Plasma Stability of the compound: 

In addition to hepatic metabolism, compounds are also subjected to degradation/ modification by enzymes in plasma.  Instability of compound/drug in plasma generally show poor in vivo efficacy.  Investigation of plasma stability should be performed early in the discovery process in order to assess potential degradation and/or protein binding issues.

In general, esters, amides, lactones, lactams, carbamides, sulphonamides, and peptic mimetics tend to more susceptible to hydrolysis in plasma.

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A solution of test compound in plasma is prepared and incubated for a predetermined time period. Aliquots are removed at pre-defined time points and analyzed by LC/MS/MS. The peak area for the parent compound is compared to the time zero sample in order to assess the amount of compound still available.

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