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Introduction to Pharmacology

Contents:

1. Introducing Pharmacology

  • What is pharmacology?
  • A Brief history of development

2. Importance of Pharmacology in Medical Sciences

  • Pharmacology & related sciences
  • Subdivisions of Pharmacology
  • Pharmacokinetics & pharmacodynamics: the heart and soul of pharmacology!
  • Why should a Medic master pharmacology?

3. Drugs

  • Definition of drugs & classification
  • Routes of administration & drug preparations

4. Therapeutic activity Vs Side-effects

  • Selectivity and Specificity of drugs: an out-of-date outopia
  • Therapeutic window & side-effects

 

1. Introducing Pharmacology

 

1A. What is pharmacology?

The definition of pharmacology arises from its literal meaning. Most words in medical sciences have roots in ancient Greek language, and pharmacology is no exception:

  • pharmacon (φαρμακον; poison) and logos (λογos; talking). And since for ancient Greek philosophers the verb ‘to talk’ was also used for ‘to know about’, the latinized word of ‘pharmacology’ literally means ‘to know about drugs’.

 

In modern context, pharmacology is defined as the study of drugs; mainly in terms of their mechanism of action and their therapeutic or adverse effects, rather than their chemical structure (medicinal chemistry), dispensing (pharmacy) or prescribing suitability with regards to a particular diagnosis (medicine). A pharmacologist is the scientist that studies what a drug does in a particular system, model or molecular pathway, how the body reacts to it and determines its functional or physicochemical properties.

 

1B. A Brief history of development

Pharmacology as a discipline is relatively new (about 150 years old), since the science of pharmacology has sprung from the research of 19th-century chemists, biologists and medics. Due to its vital role in the development of drugs and the successful treatment of diseases, it was soon characterized as a separate scientific discipline.

Ancient civilizations like the Greeks, Egyptians, Chinese, Persians and Romans had extensive knowledge about the use of different medicines and where they could be found in nature. However, since the efficient study of drugs is vastly dependent on sufficient knowledge in other fields (i.e. successful extraction or synthesis, efficient purification methods, knowledge of its chemical structure, knowledge on the physiology of a particular disease state), pharmacology has developed only after advances in these fields (or simultaneously at best).

Since the early 18th century, the development of pharmacology mostly follows a specific pattern:

  • Successful isolation (or synthesis) of a drug depends on efficient purification.
  • Once the substance is purified, the focus is put on identifying its target in the body.
  • Identification of the target (usually a receptor) initiates research for identifying its endogenous ligand (the compound that the body uses to react with its ‘natural target’).
  • Identification of the endogenous ligand springs research in various neighboring fields. These include: 1) analysis of its chemical structure and comparison with that of the isolated drug, providing knowledge of how can we synthesize an improved version of the drug, 2) study of the diseased states produced by irregularities of the target or the production of the endogenous compound, providing knowledge on pathophysiology, 3) study of the molecular pathways linked to the target and how the endogenous ligand or the drug affects them, providing knowledge on the mechanisms responsible for the drug’s action.

 

The general goal of pharmacology is to understand the underlying molecular mechanisms responsible for the biological activity of drugs in order to enable their rational use and enhance their improvement. Recently, swift advances in a variety of scientific fields (molecular biology, genetics, chemical modeling, electrophysiology, physics, computing) have provided a platform for future pharmacological innovations. During the last decade, these scientific fields have developed sophisticated tools that are being used in modern pharmacology for greater advances in the prognosis, diagnosis and treatment of medical disorders.

     

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    2. Importance of Pharmacology in Medical Sciences

     

    2A. Pharmacology & related sciences

    In the biomedical world, there are a number of different disciplines that can overlap or even combine with others to structure their identity. There are six major disciplines that can be regarded as discrete sciences and combine together to produce more general sciences/professions, like pharmacy (preparation, storage and dispensing of drugs), medicine (diagnosis, treatment and prevention of diseases) and pharmaceutics (formulation and physical properties of pharmaceutical products).

    The Major-Six disciplines are:

    • biochemistry (study of chemical processes in living organisms),
    • biophysics (study of matter and energy in biological systems),
    • molecular biology (study of molecular processes of biological activity),
    • medicinal chemistry (study of design and synthesis of drugs),
    • genetics (study of structure and function of genes)and
    • pharmacology (study of drug activity).

     

    Of course, in modern biomedicine the rational study of any of the Major-Six premises substantial knowledge in all of them in order to make the most of its capabilities and applications (e.g. study of pharmacology presupposes some knowledge of biochemistry, molecular biology and genetics).

    • DID YOU KNOW: ..that in the US and the EU the Pharmacology University-degrees are considered to be the fastest-developed and most-grown degrees within Biomedical Sciences during the last decade?

     

    2B. Subdivisions of Pharmacology

    Another consequence of the rapid advance in most of the Major-Six is the development of subdivisions in order to focus their training and research in discrete topics. For example, in pharmacology, some of the subdivisions that can be often found as discrete degrees or research areas are:

    • Neuropharmacology = Looking at the molecular mechanisms of drug action for PNS & CNS diseases, like Alzheimers, Parkinsons, neuropathic pain, dementia, memory loss, etc.
    • Psychopharmacology = Looking at the effects of drugs on the psychological state and mental health for neural diseases like mania, phsychosis, anxiety, depression, addiction, drug abuse, panic, phobias etc.
    • Neuropsychopharmacology = A connecting division between the two above, looking at the molecular mechanisms of drug action for disease that affect the psychological state and mental health.
    • Pharmacogenetics = Looking at drugs that interact with the genetic material or affectin its behaviour.
    • Pharmacoepidimiology = Researching drugs implicated in epidimeology, virology etc.
    • Pharmacognosy = Focusing on natural drugs extracted from different plants.
    • Clinical pharmacology = The study of drugs' clinical use, in a strict clinical setting.
    • Behavioural pharmacology = Focusing on drugs that affect the mood and behaviour, utilizing observational means of reserch (i.e. observing how an organism reacts under different conditions with and without a drug).
    • Environmental pharmacology = Looking at drugs affecting the ecosytem by their toxicity through the human or veterian use.
    • Toxicology = Study of toxic effects of drugs and poisons in humans (e.g. venoms, pesticides, carcinogenics).

    For obvious reasons, the boundaries between these rapidly-developed areas of research are not evident and refined. Their scope of study and research objectives should be viewed as a continuous and overlapping research across the spectrum of pharmacology, such as the colours overlap across the light spectrum. Also, their major differences are not based on the diseases they implicate, but rather on the angle of research they use to approach a specific disease (e.g. molecular, pathological, psychological, behavioural, genetic).

     

    2C. Pharmacokinetics & pharmacodynamics: the heart and soul of pharmacology!

    From the time that a drug enters the body to the point of its excretion, pharmacology looks at every aspect of the relationship ‘drug-body’:

    • how the body handles the drug (pharmacokinetics) and
    • how the drug affects the body (pharmacodynamics).

     

    Pharmacokinetics involve the administration of the drug in the body (and the various barriers of the latter to its diffusion – i.e. the blood brain barrier), absorption of the drug from the tissues, metabolism of drug by the body (as a defense mechanism for an external unknown substance), distribution of the drug to the tissues (and its site of action) and excretion of the drug (removal from the body – i.e. sweat, urine, feces). You can see that all the aspects of pharmacokinetics involve actions of the body to the drug.

    Pharmacodynamics involve the ‘soul’ of pharmacology, what the drug does to the body once it reaches its target, what biological effects does the drug produce and how these effects change the body from a diseased state to a healthy one. Using a useful metaphor for pharmacokinetics and pharmacodynamics, it can be seen as the missions to the moon: the launch of the rocket, the travel in space, the landing to the moon, the later departure from the moon  and the landing on earth they all represent ‘pharmacokinetics’. What the landing spacecraft does in the moon once it arrives there represents ‘pharmacodynamics’.

    On Fastbleep you can find analytical texts that explain in detail these two main parts of pharmacology. 

     

    2D. Why should a Medic master pharmacology?

    All the aspects of medicinal science have their own importance to the therapeutic outcome: accurate diagnosis, correct prescribing, effective treatment and a thorough follow-up are all part of the job of a good doctor. However, deep knowledge of pharmacology marks the difference between a good doctor and an excellent doctor.

    Mastering pharmacology means that you know the major differences in various similar drugs, you are aware of the contraindications which are vital for tailoring a prescription to each patient’s own needs, you acknowledge all the drug-drug interactions between a patients’ simultaneous treatments, you are able to identify a drug’s side-effect and choose a different drug therapy for the same disease, etc.

    In order for a doctor to be capable and skillful for completing the above, he/she has to master the knowledge of where it acts, how it acts and what are its disadvantages and advantages compared to another similar drug.

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    3. Drugs

     

    3A. Definition of drugs & classification

    Drug can be defined as ANY substance that causes a biological effect, either therapeutic or toxic. The classification of drugs exists in different forms.

    Major classifications include two:

    1. according to their effect (e.g. antiepileptics, antihypertensives, hypnotics) and
    2. according to their mechanism of action (e.g. beta-blockers, alpha2-agonists).

    "Indications" are a minor classification of drugs (e.g. for epilepsy there are barbiturates and benzodiazepines, for asthma there are bronchodilators, corticosteroids, cromolyns and leukotriene-receptor antagonists).

     

    3B. Routes of administration & drug preparations

    It has been mentioned above that drug administration is part of pharmacokinetics. There are a number of different routes of administration that a drug can enter the body.

    To classify the different routes we divide into two main sections: topical (the drug acts at the site of application and might enter the systemic circulation) and systemic (the drug is delivered on purpose into the general blood circulation).

    Also, there can be a number of different preparations for the same drug so that, either to make use of different physicochemical advantages (i.e. for the oral route; tablets, capsules and  syrups, have different physicochemical characteristics)  or to make use of a different route of administration (i.e. morphine exists in tablets, capsules, solutions, suppositories, injections and transdermal patches) - See table below.

     

    Relationship of each route of administration and the different forms of drug preparations

    Choosing a particular preparation of a drug is based on the acknowledgment of the advantages & disadvantages of each one related to the disease to be treated and the particular needs of the patient.

    It is known that the body has a number of defense mechanisms in order to protect itself from harmful substances. These mainly are the metabolism of drugs by the liver (the ‘first-pass effect’) - where the blood is ‘screened’ by the liver for unknown substances - and the ‘blood brain barrier’ (BBB) which protects the brain from active substances reaching the brain. Depending on factors such as the physicochemical properties of the drug (molecular weight, lipophilicity, ionization constant, molecular stability etc) and its pharmacological properties, the doctor decides which particular preparation of a drug is appropriate.

    • Example: Why is insulin administered only by injection and not by a tablet?
    • Answer: The reason is that insulin is not effective when taken by mouth because it is degraded in the stomach by its high acidity. Therefore, the physicochemical properties of insulin underline the reasons for its parental preparation.

    When a tablet is given, the drug must overcome a number of ‘barriers’ before reaching its target:

    •  It has to withstand stomachs’ acidic environment (pH 1-3) and be dissolved in solution in order to be absorbed (usually in the intestine) into the bloodstream [most small proteins cannot be given into oral form because they break down into amino acids in the stomach]
    •  The absorption of the drug from the intestinal walls and across membranes into the bloodstream will depend on the drug’s lipophilicity, its size and its molecular weight,   
    •  All blood from the gut passes through the liver via the portal system and drugs undergo extended metabolism (chemical changes) that can alter the drug’s biological activity or render it inactive [bypassing this ‘first pass effect’ can be achieved by different routes of administration]   
    •  Drugs that enter the circulation may bind to serum proteins and thus reduce the percentage of the free drug in the blood that can reach the target,
    • Free drug in the blood must cross membranes in order to reach its target [some drugs are deposited in the adipose tissue - fat - because of their high lipophilicity and therefore not enough of the drug reaches other tissues], 
    • Drugs that target the brain have to pass through the blood brain barrier, which consists of tightly packed membranes with gates that control which substances pass through.

     

    • Examples: Benzyl penicillin, insulin and lincomycin are examples of drugs destroyed in the stomach. Lignocaine is metabolized in the liver so that not enough of the active drug reaches the target.

     

    In some cases, a drug is given in an inactive or less active form (called a “pro-drug”) and it is metabolized by the liver into an active form, thus using the first pass effect to our advantage. Examples: Valaciclovir is an antiviral pro-drug that converts into the active acyclovir. Heroin is an opiate pro-drug that is converted to morphine by the liver. Prednisone is a cortico-steroid that is activated by the liver to prednisolone.

    If you think about it, for a drug to be effective it must in one way or another be delivered unchanged or in a form and concentration that is effective to a target. The target might be a metabolic system, a cell, an enzyme or a specific tissue, or a bacterium. How the drug is delivered to its target depends on the chemical characteristics of the drug and the available product formulations (as discussed earlier in the article). Where a drug can be formulated for a range of routes of administration, it is up to the doctor to decide which form will be most effective and convenient for the patient (e.g. some elderly patients find it difficult to swallow and therefore large tablets should be excluded if possible when prescribing).

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    4. Therapeutic activity Vs Side-effects

     

    4A. Selectivity and Specificity of drugs: an out-of-date outopia

    When a drug has a therapeutic effect through its activity on a particular tissue or molecular target, it is only logical that any other "secondary" activity of the drug will produce a biological effect that is not intented. This effect is called a side-effect (or adverse reaction).

    A general notion among pharmacologists was that side-effects were produced solely because of the non-specificity of a drug's action and so the main aim of the pharmaceutical industry and the pharmacological science should be to produce and develop drugs that are more selective on the desired target or tissue.

    • Remember: Selective is a drug that acts on a particular target and not another. Specific is a drug that has a particular effect and not another. A selective drug does NOT mean that is specific and vice versa. For example, a drug binds on a particular receptor-target (so its selective), but that target may be expressed in different tissues and thus may exert different biological effects (so no-specific). In that sense, it is very rare (close to impossible) to find a drug that is specific and selective at the same time. This is why selectivity/specificity has been the holy grail of pharmacologists in previous generations.

     

        Nevertheless, new ideas have promoted "multi-functionality" in drug development, a strategy that takes advantage of the variability of the targets and their expression in different tissues and keeps away from selectivity and specificity. "Multiple Receptor Selectivity" is a new term that has been introduced to distinguish the intentional design of drugs to be multi-selectivity, with the unintentional non-selective drugs existed.

        • An example of multiple-receptor selectivity is found in opioid receptors (types: MOP, DOP and KOP) where the old notion was that a strong opioid analgesic should be highly selective to the MOP receptor to have maximum analgesia. Recent advances in opioid pharmacology has shown that when activating the MOP receptor and blocking the DOP receptor, there is strong analgesia with reduced opioid side-effects (tolerance, dependance, constipation). Intentional design of new drugs that will do both these two actions are considered an example of multiple-receptor selectivity.

         

        Important: The concept of selectivity should not be regarded as outdated or without scientific essence in terms of pursuing it in research. The development of side-effects due to the activity of drugs in "secondary" targets is quite real indeed. Equally, it has to be acknowledged that the complexity of biological systems and the relationship between 'therapy-target-drug' is constantly changing in a dynamic manner. Appreciating this concept of constant change brings forward new strategies that seek to explore and take advantage of this variability in drug targets and drug structures so that a therapeutic effect and a reduction in side-effects is achieved by creating multifunctional drugs.

         

        4B. Therapeutic window & side-effects

        To understand the meaning of the "therapeutic window", one has to realise that ALL substances in the universe may cause side-effects (or toxic effects) if their DOSE is adjusted accordingly. Additionally, if the DOSE of any drug is too low, there will be no therapeutic effect. This dose-effect relationship of drugs is the main study of Pharmacodynamics.

        The logarithmic sigmoidal relationship of a drug's action (see picture below) shows that there is only a particular "dosing window" that can produce a therapeutic effect when gradually increasing the dose of a drug (below that window there is no effect, above that window there are toxic effects).

         

        • Example 1: Administration of oral broad-spectrum antibiotics may alter the microflora in the gastrointestinal tract and produce symptoms of diarrhoea or thrush. This is because these antibiotics will be active against a wide range of microorganisms, including the symbiotic ones in the gut. organism (i.e. tetracyclins). 
        • Example 2: Aspirin is an antiinflammatory analgesic (inhibiting cyclooxygenase) and also a preventive of strokes due to its anticoaggulant activity on platelets (inhibiting thromboxane which binds platelets to form clots). However, its inhibition of the protective cyclooxygenase-2 (which produces prostaglandins for the protection of the stomach walls from its acid) may cause ulcers.

           

          Illustration of an example of therapeutic window of a drug

          Sigmoidal curve representing the response of a drug with increasing doses. Doses that produce a resp

          Side-effects are not only part of a standard practical & theoretical approach. Apart from variations in the dose of drug and its non-selective "unwanted" effects, there are other issues that may produce a side-effect which is not expected. These issues are similar to those responsible for drug variability among a population:

          • Pharmacokinetics can differ (based on weight, metabolism, body structure. This is why medics subscribe higher doses to super-overweight patients).
          • Pharmacodynamics can differ (based on variation of expression of targets in some humans, i.e. some people do not produce specific enzymes)
          • Age (differences in organ efficiency, system maturation etc)
          • Gender (differences in expression of targets)
          • Pregnancy or breast feeding
          • Concurrent drugs (drug-drug interactions can affect the activity/metabolism of drugs)
          • Concurrent diseases (a disease state may affect the activity of a drug for another disorder, i.e. renal disfunction may affect excretion of drugs for Alzheimer's)
          • Genetics (polymorphism and genetic variability can affect target expression)
          • Allergies/hypersensitivities/intolerance (people have allergies and hypersensitivities to different substances, some people are intolerant to even low doses of a particular drug)
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