The Pharmacodynamics of drugs

From our main last discussion on the pharmacokinetics of drugs, we where able to understand that pharmacokinetics means, the processes involved in a drug's absorption, distribution, metabolism and the excretion of its toxic or non toxic metabolites from the systemic circulation.

We were able to understand that pharmacokinetics simply means how the body handles a drug. Today, we will succinctly discuss the about Pharmacodynamics and it's associated terminologies. Let's do this , shall we!

We will briefly discuss about some concepts that are indispensable before going into the real deal about pharmacodynamics. Just to remind and bring you up to speed, the general mechanism of drug actions is their binding to biological targets. Their binding leads to conformational change in the receptors which in turn leads to a biological response.

The action of the drug could either be full or partial and this brings us to the two very concept in Pharmacodynamics - Emax and EC50.

The concept of Emax, this is the maximal effect a drug can produce on any parameter that is being measured. Another one is EC50, which is the concentration of the drug which produces half of the maximum effects at a very steady state.

Pharmacodynamics is the near opposite of pharmacokinetics as it relates to how drugs we ingest affect our biological system. Pharmacodynamics is simply the study of the physiological, mololecular and biochemical effects of a pharmaceutical drug on the body. The concept of Pharmacodynamics places major emphasis on the dose-response relationship.

To further clarify the above, Pharmacodynamics is all about the relationship that exists between a drug's concentration and its effects on the site of action. Drugs generally bind to receptors before they exert either their physiologic, biochemical or pharmaceutical actions. You cannot talk about drug actions without full understanding of the concept of drug-receptor interactions. Pharmacodynamics is anchored on three main points and they include:

•Receptor binding and sensitivity

•Post receptors effects and

•The chemical interactions of the drugs.

The three above are the main focal point on which Pharmacodynamics of drugs stands on. We will pick each point and elaborate more on them for a better understanding of the concept of Pharmacodynamics. You are sure to learn a lot here. Let's begin with the first point.

Receptor binding and sensitivity

Receptors are glycoproteins that are located in the cell membranes of biological tissues. They could be intracellular (within the cell membrane) or extracellular (outside the membrane). Those that are found outside the membrane of the cell sometimes transverse across the membrane into the cell. They recognize ligands (drugs) and bind to them. Just like I pointed out earlier, drugs bind to receptors in order to exert theirs pharmaceutical actions.

Now the pharmacological response as a result of the binding in turn depends on whether the drug binds to its target or not. In the same vein, the drug's effects effects after successfully binding to its active site depends on the drug's concentration at the receptor site. What this simply means is that, the concentration of a drug at the receptor site influences the effects of the drug.

The binding of a drugs or ligand to a receptor is purely governed by the law of mass action which states that:

The rate of the chemical reaction is directly proportional to the product of the activities or concentrations of the reactants

The higher the concentration of the drug, the greater the effects. But repeated usage can lead to a condition known as tolerance. Tolerance is simply a situation where there is a decreased response of the body to a particular type of drug due to repeated usage. Been tolerant to a drug means that you have either abused the drug or overused it. Drugs that are commonly associated with tolerance are the opiods and alcohols.

We will continue next time.