Archive for the ‘ Pharmacology ’ Category

Antibiotic Resistance

Introduction

The generic purpose of an antibiotic is to prevent the growth and/or survival of invading organisms whilst causing minimal damage and toxicity to the host. The typical mechanism of antibiotic action involves targeting specific enzymes or substrates of the invading bacterial species. Antibiotics may be either bacteriocidal (i.e. kill bacteria, e.g. β-lactam antibiotics) or bacteriostatic (i.e. slow bacteria growth and reproduction e.g. tetracyclines). However, the majority of bacteria are actually killed by the host immune system, the administration of antibiotics is typically only to aid the immune system and thus speed up recovery.

Some antibiotics may be synergistic, that is, when used with other antibiotics the overall therapeutic effect is greater than the sum of their individual effects, this allows for reduced doses to be administered.

Antibiotics can have either target a narrow spectrum of bacteria (such as the penicillins and macrolides), or a broad spectrum of bacteria (such as the aminoglycosides, cephalosporins, quinolones and some synthetic penicillins).

The use of antibiotics in food animals is subject to EU legislation which governs maximum residue limits (MRLs), the maximum concentration of antibiotic allowed to be administered to the animal for it still to be fit for human consumption. Antibiotics must be prevented from entering the food chain in this manner, so MRLs have been categorised into 4 annexes:

  • Annex 1 – MRL has been fixed
  • Annex 2 – MRL not required
  • Annex 3 – Provisional MRL
  • Annex 4 – No MRL can be set

The use of annex 4 antibiotics (e.g. chloramphenicol) is prohibited in animals destined for human consumption, with use of annex 1 or 2 antibiotics being preferred.

Choice of Antibiotics

There are many factors to consider when selecting an appropriate antibiotic for use. The first question to ask should be, ‘Is the infection bacterial?’ Antibiotics should only be administered for bacterial infections as they are of no use against viral infections. It is also important to remember that antibiotics only treat the infection and do not act as anti-inflammatories or anti-pyretics.

The site of infection is another consideration, as whilst some antibiotics will be of great use for skin infections, they may not be as useful for respiratory tract infections. The species of bacteria must also be considered, this will firstly require the culture and identification of the invasive organism to determine the species. However, within a species there may be variations of antibiotic susceptibility so it is important to generate a resistance profile i.e. determine which antibiotics the bacteria is resistant to and thus avoid prescribing these antibiotics. It must then be decided whether to use a broad or wide spectrum of antibiotic and wether this antibiotic should be bacteriocidal or bacteriostatic.

Bacteriostatic antibiotics will require a duration of therapy which gives the host cellular and humoral immune responses enough time to eradicate the bacteria. Alternatively bacteriocidal antibiotics are used when the host immune system is considered to be ineffective against the invasive organism.

Other factors to consider include:

  • The distribution of the drug throughout the body
  • The cost for a course of the antibiotic
  • The toxicity posed to the host
  • Any underlying disease which may be affected by the use of the antibiotic
  • Whether the host is pregnant or juvenile
  • The practically of the drug i.e. the route of administration, dosage, frequency of dose and the duration of action etc.

The Cascade

One final consideration to be made when administering antibiotics for animals is ‘The Cascade,’ which is a regulation governing restrictions on the administering of veterinary medical products.

In animals, an antibiotic must be licensed for use with a particular species and disease. However if no such antibiotic exists, to avoid unacceptable suffering, the veterinary surgeon may prescribe another antibiotic in order of the cascade:

  1. A veterinary medicine licensed for use in another species or in the same species but for a different use. This is known as ‘off-label’ use.
  2. A medicine licensed for use in humans.
  3. An unlicensed medicine, created and prescribed as a ‘one-off’ by the veterinary surgeon.

Determination of Resistance

Two important factors to consider when attempting to determine whether a bacteria is resistant to a particular antibiotic are:

  • The Minimal Inhibitory Concentration (MIC) – The minimum concentration of a drug which prevents the growth of bacteria
  • The Minimum Bacteriocidal Concentration (MBC) – The concentration of a drug which causes a 99% reduction in a bacterial innoculum over a given period of time.

Also of importance are:

  • Pharmacodynamics – The effect of a drug on a pathogen i.e. the MIC and MBC
  • Pharmacokinetics – The effect of the body on a drug i.e. the absorption, distribution, metabolism and excretion of the drug

Breakpoints are predetermined concentrations of an antibiotic, which determine whether a bacteria is deemed susceptible or resistant to that antibiotic. If this concentration of antibiotic causes a reduction in bacterial numbers or prevents further growth then that strain of bacteria is defined as susceptible to the antibiotic in question. If the bacteria does not at least show signs of prevented growth, the bacteria is defined as resistant to the antibiotic.

Disc Diffusion

One method of determining antibody resistance is disc diffusion. A species of bacteria is isolated and cultured on growth medium. After an incubation period that allows for visible colonies to be observed on the growth medium, diffusion discs are placed around the growth plate. These discs are impregnated with different antibiotics. After a short incubation period, susceptible bacteria around the discs will have been killed, indicated by a clearing in the colonies. The diameter of the clearing can be used to determine how susceptible the species is to an antibiotic. If there is no clearing (or a very small clearing) this indicates resistance to the antibiotic which impregnated the disc.

Mechanisms of Antibiotic Resistance

Intrinsic Resistance

Intrinsic resistance is that which is not acquired, inherent resistance, such as a natural low permeability to antibiotics due to a bacterial envelope. This type of resistance is characteristic to almost all representatives of a species. An example of this type of resistance is observed in Pseudomonas species. Their resistance to antibiotics is owed to the low permeability of their bacterial envelope to certain antibiotics and the presence of a multi-drug efflux pump.

This efflux pump occurs naturally in bacteria and usually removes waste products from the cell, such as bile, fatty acids and organic solvents. However a mutation in some species also permits the efflux of antibiotics from the cell thus preventing them from exerting their action upon the bacteria. This trait is also acquirable however, such as the efflux pump gene responsible for tetracycline resistance.

Acquired resistance

Antibiotic resistance genes found on plasmids, can be transferred between individual bacteria, hence non-resistant bacteria can easily acquire resistance. Acquired resistance can also occur due to mutations. Beneficial mutations can then be transmitted around the bacterial population. The biggest resistance problems occur in Gram-negative organisms due to the large numbers of plasmids found within the population, another resistance problem occurs in Staphylococcus species i.e. MRSA and MRSP.

Targets of Antibiotics

Antibiotics can work in a number of ways:

  • Inhibit cell wall synthesis e.g. penicillins, cephalosporins, carbapenems, glycopeptides
  • Inhibit DNA synthesis e.g. fluoroquinolones
  • Inhibit RNA synthesis e.g. rifampicin
  • Inhibit folic acid synthesis e.g. sulfonamides, trimethoprim
  • Inhibit protein synthesis e.g. macrolides, chloramphenicol, tetracycline, aminoglycosides

However, bacteria have a number of mechanisms to resist the effects of antibiotics:

  • Decrease permeability to the antibiotic
  • Inactive the antibody by chemically altering it with enzymes
  • Efflux of the antibody via efflux pumps in the membrane
  • Alter the target of the antibiotic, so it no longer has an effect
  • Bypass steps in metabolism which the antibiotic targets

How do Antibiotics Work?

β-Lactams

β-lactams can be both natural and semi-synthetic. They work by inhibiting enzymes associated with the synthesis of peptidoglycans, so essentially inhibit cell-wall synthesis. Such antibiotics include; penicillin and its derivatives, cephalosporins, monobactams and carbapenems.

However over use of these antibiotics has selected for bacteria which have developed resistance, in the form of β-lactamase – an enzyme which hydrolyses β-lactams. The hydrolysation of β-lactam antibiotics will alter their conformation and thus they will no longer have an antimicrobial effect.

To combat this, β-lactamase inhibitors are now used alongside β-lactam antibiotics. Although β-lactamase inhibitors do not act as antibiotics alone, coupled with β-lactams, they can effectively target bacteria. β-lactamase inhibitors, inhibit the enzyme responsible for hydrolyzing β-lactam antibiotics, thus when coupled with β-lactams, the antibiotics are free to act upon their targets again – enzymes associated with cell wall synthesis. An example of this is Synulox, which contains the β-lactam amoxicillin and the β-lactamase inhibitor clavularic acid.

Bacterial β-lactamase enzymes work by hydrolyzing the ring-bond of the β -lactam which denatures their structure thus preventing them from exerting their antibiotic action.

β-lactamases can be either chromosomally derived (typically in Gram-negative bacteria) or  their genes can be found on plasmids and thus spread throughout a population. Extended spectrum β-lactamases are encoded for on plasmids (plasmids; TEM, SHU and CTX-M). These extended spectrum β-lactamases can catalyse a broad spectrum of β-lactam antibiotics.

Other mechanisms for penicillin (and derived antibiotics) resistance include; alterations in the penicillin binding protein found in bacterial cell membranes. Alterations in this protein reduces the bacterial affinity for penicillin. Similarly, the acquisition of a novel penicillin binding protein (mecA) also reduces affinity for penicillin. This gene is found in MRSA and is what makes it so resistant to penicillin its derivatives.

Aminoglycosides

Aminoglycosides irreversibly bind to the 30S ribosome and freeze the initiation complex. This effectively inhibits protein synthesis within the bacterium. Aminoglycosides have a broad spectrum but do not target anaerobes. They can also have a synergistic effect when used with β-lactams. They do however pose a toxicity risk; nephrotoxicity (toxic to the kidneys) and ototoxicity (toxic to the ear, specifically the cochlea or auditory nerve).

The mechanisms for resistance against aminoglycosides include; alteration of the bacterial ribosomes which prevent aminoglycosides from binding, decreased permeability to the antibiotics or the inactivation of aminoglycosides by mechanisms such as acetylation, phosphorylation and adenylation. These mechanisms are facilitated by aminoglycoside enzymes found in the bacteria.

Tetracyclines

Tetracyclines are bacteriostatic and similarly to aminoglycosides, they bind to the 30S ribosome (however, they bind irreversibly). They also inhibit RNA from binding to the 70S ribosome and thus inhibit protein synthesis. They have a broad spectrum of targets, but there are also many tetracycline resistant strains of bacteria.

Fluoroquinolones

Fluoroquinolones inhibit DNA synthesis, they do this by targeting the DNA enzymes gyrase and topoisomerase. They too have a broad spectrum, but bacterial plasma-mediated QNR genes provide resistance against the antibiotic. QNR genes offer low level protection for gyrase and topoisomerase from fluoroquinolones, however, the presence of QNR genes increases the mutation rate of gyrase and topoisomerase genes. Mutation of these genes can further increase resistance to fluoroquinolones.

Trimethoprim

Trimethoprim inhibits the bacterial dihydrofolate reductase (DHFR) enzyme which therefore inhibits folic acid synthesis. Again, trimethoprim has a broad spectrum, it is often used in combination with other antibiotics.

Resistance to trimethoprim occurs if the DHFR enzyme becomes less susceptible to the antibiotic e.g. through mutation, or if the metabolic step in folic acid production which requires DHFR is skipped altogether thus removing the target for trimethoprim completely.

Antibiotic Resistance

Resistance has always existed, even before antibiotic use. However the use of medical antibiotics have increased the prevalence of resistance. It is important to note though, that antibiotics do not cause resistance. They do however select for resistance already prevalent in a population.

Opioid Analgesics (Morphine) & Equine Colic (Butorphanol)

Introduction

The primary effect of opioids is to temporarily remove pain when used at therapeutic levels; this is done by binding to opioid receptors found primarily in the central nervous system (some receptors are found in the gastrointestinal tract). When larger doses are given, opioids can induce beneficial and non-beneficial pharmacological effects such as sedation, respiratory depression or constipation. The term given to non-synthetic opioids is opiates; opiates are derived from the naturally occurring opium alkaloids found in the resin of the opium poppy.

The main uses of opioids include:

  • Treatment of acute pain (e.g. post-operative)
  • Palliative care to alleviate serve chronic pain (e.g. cancer)
  • Surgical premedication regimes (due to their calming, sedative action – also reduces the amount of post pain relief required)
  • Neuroleptanalgesia (a state of quiescence, altered awareness, and analgesia produced by a combination of an opioid analgesic and a neuroleptic – a tranquilliser). And neuroleptanaesthesia (a form of anaesthesia achieved by the administration of a neuroleptic agent, a narcotic analgesic, and nitrous oxide with oxygen. Induction of anaesthesia is slow, but consciousness returns quickly after the inhalation of nitrous oxide is stopped)
  • Restraint
  • Antitussive (the alleviating or suppressing coughing)

Pharmacology

The body naturally releases endogenous opioid peptides or endorphins which bind to opioid receptors in the body. There are three primary receptor types, each with different functional responses and these are:

  • μ (mu) – Responsible for supraspinal (above the spine) analgesia, respiratory depression, euphoria and physical dependence of opioids (misuse and abuse of opioids)
  • κ (kappa) – Responsible for spinal analgesia, miosis (pupil constriction of the eye) and sedation
  • δ (delta) – Responsible for hallucinations and dysphoria (agitation and anxiety)

Exogenous opioids (synthetic or natural) mimic the body’s own endogenous opioids and are therefore able to bind to the above receptors – resulting in a response specific to the receptor they bound.  Opioids are able to either stimulate or depress the receptors, meaning opioid drugs can be classes as; agonists, antagonists or both – agonists-antagonists. Agonists bind to receptors and induce pharmacological responses whereas antagonists bind to receptors and do not produce a response, this makes them able to counteract the effect of other drugs or endogenous compounds.

Agonists are used for the primary reasons listed earlier, mainly analgesia. Examples of opioid agonists are:

  • Morphine
  • Pethidine
  • Methadone
  • Fentanyl
  • Etorphine.

Antagonists are primarily used to reverse the effects of agonists i.e. analgesia. They do this by binding to μ and κ opioid receptors, which together are responsible for analgesia. Examples of opioid antagonists include:

  • Naloxone (Narcan)

Agonists-Antagonists have both agonistic and antagonistic properties. This means they are able to antagonise the pure agonists (e.g. morphine) at μ and κ opioid receptors but they also have their own milder agonistic effects. The agonist effect is sufficient enough to be used as analgesics. Examples of opioid agonists-antagonists include:

  • Butorphanol
  • Pentazocine (Fortral)
  • Nalorphine
  • Diprenorphine (Revivon)
  • Buprenorphine (Temgesic)

The principal usage of opioids in medication is for analgesia. Analgesia is the loss of pain perception. Opioids effect both the physical and psychological perception of pain, physically blocking or raising the threshold of pain stimulation and removing the association of pain with fear. Associated with analgesia is sedation which is not considered hazardous, respiratory depression (which can also be associated with opioid analgesia) however can be a distressing side effect. A list of unwanted opioid effects includes:

  • Sedation
  • Excitement
  • Respiratory depression
  • Cough suppression
  • Nausea
  • Vomiting
  • Constipation

Opioid Selection

There are many opioids available for use, each with different properties, when selecting an opioid it is important to consider its potency, how quickly it acts (speed of onset) and how long it lasts (duration). The best analgesics are those which have a mild potency, rapid onset and a long duration of effect. When combining an opioid with a neuroleptic for neuroleptanalgesia, the desired properties of the opioid are slightly different; strong potency, rapid onset and brief period of duration.

As opioids can have an effect on the gastrointestinal system, (as opioid receptors are also found in the gastrointestinal tract) if they are to be given orally then they must have low lipid solubility.

Another point to consider is whether to use an agonist or an agonist-antagonist as both are able to produce analgesia. The main consideration is that pure agonists are more reliable and predictable than agonist-antagonists, but the agonist-antagonists produce fewer side effects such as vomiting, sedation and respiratory depression. Also as agonist-antagonists have antagonistic effects, any further use of analgesics may be compromised.

Below is a comparison of the potency of certain opioids relative to morphine, the most potent being Etorphine. Etorphine (or Immobilon) is extremely powerful and typically only used to immobilise large mammals (e.g. elephants). Due to its potency it can prove lethal to man.

Drug Relative Potency
Meperidine 0.1
Morphine 1
Butorphanol 1-2
Hydromorphone 10
Alfentanil 10-25
Fentanyl 75-125
Remifentanil 250
Sufentanil 500-1,000
Etorphine 1,000-3,000

Opioids are often used as part of a pre-medication routine i.e. before surgery as a pre-emptive form of analgesia. This is because once pain has been established (i.e. during surgery) pain relief drugs prove less effective. As a result larger doses would be needed to prevent the pain which increases the onset of associated side effects e.g. respiratory depression.

Examples of Opioids and their Properties:

Morphine

Morphine (agonist) is considered the standard opioid with all other forms of analgesia being compared against it. It is the most potent natural analgesic, more potent derivatives have been artificial synthesised. Morphine produces a mixture of stimulant and depressant actions depending on the size of the dose as well as the species and absence or presence of pain.

Differences between the species can be observed e.g. in the dog, the cortex is depressed and little excitement is produced. In the cat, very small doses are able to induce excitement and in the horse morphine will not produce excitement if no pain is present (effect is less predictable in horses however). Despite this morphine is safe to use in all species as long as the correct dosage is used, the presence of excitement tends to increase with dose.

The duration of morphine is about 4 hours in all species, it is eventually metabolised by the liver. It is normally injected subcutaneously at a dose of around 0.1mg/Kg (in dogs and cats).

Use of morphine can either stimulate the medulla (which is followed by depression of the medulla) or directly depress the medulla.

Morphine has a number of effects on the gastrointestinal tract. Initially it may invoke vomiting and defaecation which is followed by constipation. Constipation is due to local effects on the small/large intestinal opioid receptors. Segmental tone of the intestines increases, along with sphincter tone but the action peristalsis decreases. This increases the time taken for intestinal contents to pass.

Other areas affected by morphine include:

  • The chemoreceptor trigger zone (CTZ) of the medulla is stimulated by morphine – this induced vomiting.
  • The occulomotor centre is stimulated which is responsible for producing miosis.
  • The cough centre is depressed – reducing coughing but making post-operation mucus accumulation a possible problem.
  • The vagal centre is stimulated, which increases gastrointestinal activity and is responsible for the initial defaecation. If a large dose is administered bradycardia may be induced (slowed heart rate <60bpm) by myocardium depression.
  • The respiratory centre is easily depressed by morphine, even with a low dose. This is due to a reduced response to elevated CO2 levels. The mechanisms involved in the regulation of respiratory rhythm are also affected – contributing to the overall depression of the respiratory centre.

Butorphanol

Butorphanol (agonist-antagonist) is a widely used sedative and analgesic in dogs, cats and horses – combined with tranquillisers for sedation. It is around 1-2 times as potent as morphine, but it does have a slightly shorter duration of action at around 2-3 hours (Morphine – 4 hours). It has a much less profound effect on the respiratory system as the dose increases compared to morphine.

One major use of Butorphanol is for intravenous administration in the horse (0.1mg/Kg) to alleviate abdominal pain associated with torsion, impaction, intussusception (intestinal prolapse) and spasmodic colic.

Equine Colic

The term colic can encompass all forms of gastrointestinal conditions which cause pain as well as other causes of abdominal pain not involving the gastrointestinal tract. Some examples are:

  • Spasmodic colic – Increased peristaltic contraction
  • Impactive colic – Caused by irritation to the lining of the bowel or ileum due to diet or ingestion of large amounts of sand/ dirt
  • Obstructive colic – Obstruction of the bowel by large food masses
  • Flatulent colic – Build of intestinal gases causing distension and pain
  • Parasitic colic – Intestinal pain from parasites such as roundworm or tapeworm
  • Idiopathic colic – From another cause which remains unknown

There are also many diagnostic tests for equine colic:

  • Increased heart rate with decreased circulating volume
  • Distinctive behavioural signs
  • Auscultation – Listening to internal body sounds
  • Abdominocentesis – The extraction of fluid from the peritoneum which can be useful in assessing the state of the intestines
  • Nasogastric Intubation – Insertion of a tube from the nose to the stomach which can be used to drain excess liquid from the stomach – for therapeutic reasons and for diagnosis
  • Rectal/Faecal examination

There are also many drugs/treatments available to treat the symptoms:

  • Analgesics
  • Spasmolytics
  • Lubricants/laxatives
  • Antizymotics – Used against disease producing organisms e.g. bacteria
  • Anthelmintics – Used against parasites
  • Fluid therapy

The major analgesics used against colic are α2-agonists (xylazine, romifidine and detomidine), opioids (butorphanol) and NSAIDs (flunixin).

Butorphanol is usually used alongside small doses of xylazine, romifidine and detomidine. This is because it has minimal effects on the cardiovascular system (which is not true for xylazine, romifidine and detomidine). Both butorphanol and the α2-agonists have a duration of around 2-3 hours and they both reduce intestinal motility/activity.

Non-Steroidal Anti-inflammatory Drugs (NSAIDs)

Introduction

NSAIDs are non-narcotic analgesics (An analgesic reduces or removes the sensation of pain), they are also anti-pyretic (fever) and anti-inflammatory. These effects are produced by the inhibition of the fatty acid cyclooxygenase (COX) which inhibits prostaglandin synthesis.

Because NSAIDs are non-narcotic they do not cause any largely noticeable effects on the CNS (central nervous system) function. This makes them ineffective against normal nociceptive tests, these are test designed to test pain responses in living organisms and they are specifically used in the testing of new analgesic drugs. Methods usually involve the applying of pressure to a specific point of the organism. NSAIDs only raise the pain threshold when pressure is applied to a swollen and inflamed joint (this is known as analgesia via peripheral mechanisms). Therefore NSAIDs are considered anti-inflammatory agents with a mild central analgesic effect (associated with anti-pyretic action). NSAIDs are therefore primarily used in the treatment of acute or chronic conditions producing mild-moderate pain, especially involving the musculo-skeletal system. Principal utilisation occurs in the horse and dog.

Another benefit of NSAIDs non-narcotic function is that, by having an unnoticeable effect on the CNS they can therefore be used as a pre-med drug, before general anaesthetic, without fear of overloading the CNS. The use of an analgesic before the introduction of pain means that a lower dose will be required when pain is inflicted – thus reducing the chances of side effects associated with high doses. However, the type of NSAID must be selected for carefully as there may be a possibility of renal damage/toxicity. There are only a couple of NSAIDs which are believed to be renal safe.

Prostaglandins and Inflammation

NSAIDs work by inhibiting prostaglandin synthesis by targeting the COX enzyme. Prostaglandins activate the inflammatory response giving the production of pain and fever, they are produced when leukocytes reach a site of damaged tissue in an attempt to minimise tissue destruction.

Prostaglandins are involved in several other organs such as the gastrointestinal tract (inhibit acid synthesis and increase secretion of protective mucus), increase blood flow in kidneys, and leukotrienes which promote constriction of bronchi associated with asthma.

Question:

What is/are the precursor(s) of prostaglandins?

The answer may be found in the ‘Mechanisms of NSAID Action’ section.

Cyclooxygenase Enzymes (COX)

COX forms two isoforms, COX-1 and COX-2:

  • COX-1 is often thought of as being the ‘good’ COX; this is due to its involvement in tissue homeostasis. It is required to keep the body ‘normal’ – primarily the synthesis of prostaglandins responsible for protection of the stomach lining. (Constitutive physiological).
  • COX-2 therefore is thought of as the ‘bad’ COX; this is because it is produced during inflammation, by the inflammatory cells which have been activated by cytokines. (Inducible physiological).
  • There is some evidence for a COX-3, a possible variation of COX-1 which is also associated with the inflammatory response. It has been found in the CNS and is affected by paracetamol.

Mechanism of NSAID Action

The primary action of NSAIDs is the inhibition of the COX enzyme, by inhibiting this enzyme the production of prostaglandins are also inhibited. The COX enzyme synthesised prostaglandins from fatty acids such as arachidonic acid.

Most NSAIDs inhibit both major forms of the COX enzyme, however all are still considered toxic. Newer drugs which are believed to be COX-2 specific (thereby not affecting the COX-1 enzyme and allowing prostaglandins associated with normal function to continue normal operation) are relatively safer in chronic use. There are fewer side effects which is what makes them be suited to prolonged periods of use. Examples: Merck’s rofecoxib and etoricoxib, Pfizer’s celecoxib and valdecoxib.

The NSAIDs selective for COX-2 are now however under scrutiny, due to reports of cardiovascular toxicity. These include strokes and myocardial infarctions. This has resulted in the withdrawal of certain COX-2 selective drugs such as rofecoxib. Further studies are suggesting that the cardiovascular toxicity of these COX-2 selective NSAIDs may actually not be much greater than ‘trusted’ NSAIDs such as ibuprofen. However, their toxicity still remains a point of research and discussion.

Other Actions of NSAIDs

Besides from the inhibition of the COX enzyme, other actions include:

  • Inhibit superoxides (toxic) and free radicals
  • Inhibit Bradykinin production (A Peptide which dilates blood vessels, lowering blood pressure)
  • Stabilises lysosomes
  • Inhibits metalloproteinases (Proteolytic enzymes whose catalytic mechanism involves a metal)
  • Antagonises interleukin-1 (fever inducer and controlling factor of lymphocytes) and tumour necrosis factor (TNF – cytokine involved in the induction of inflammation and apoptosis, dysfunction of this factor is believed to be involved with the production of cancers.)

IC50

This is the half maximal inhibitory concentration (IC50). It is a measure of the effectiveness of a compound (typically a drug candidate) in inhibiting biological or biochemical function. This quantitative measure indicates how much of a particular drug is needed to inhibit a given biological process (or component of a process, i.e. an enzyme, cell, cell receptor or microorganism) by half. It is commonly used as a measure of antagonist drug potency in pharmacological research.

This can be related to inhibition of COX enzymes, we can use it to find out how many times a dose required to inhibit COX-1 we need to administer in order to inhibit COX-2, i.e. COX-2/COX-1,  a lower ratio is better as it shows that the drug has a higher selectivity for COX-2 the ‘bad’ COX associated with inflammatory responses.

COX-1 Selective               Drugs                       Amount times dose required to inhibit COX-2

  • Aspirin                                                                  170
  • Piroxicam                                                             250

Less Selective COX-1 Drugs

  • Paracetamol                                                            7
  • Ibuprofen                                                                15

Equipotent

  • Naproxen                                                                  1

COX-2 Selective Drugs

  • Meloxicam                                                               <1

The Anti-Pyretic Effect of NSAIDs

NSAIDs do not affect normal body temperature only when pyrexia (fever) is present, do they alter temperature. Bacterial or endogenous (substances from within the body) pyrogens can act directly on the hypothalamus. Heat temperature is regulated in the hypothalamus by controlling temperature regulating peripheral mechanisms such as vasoconstriction/dilation, sweating, shivering and metabolic activity.

Pyrogens activate hypothalamic COX, increasing prostaglandin concentration. The effect of this is that the set body temperature (Average 37oC) is increased (Pyrexia is considered >38oC). NSAIDs counter this by inhibiting prostaglandin synthesis by the inhibition of COX enzymes. By doing this they have effectively blocked the action of the pyrogens on the CNS and return the raised set body temperature back down to normal levels (37oC).

Commonly Used NSAIDs

Some of the most commonly found NSAIDs and their properties:

Aspirin (Acetylsalicylic acid)

Aspirin is a potent anti-inflammatory drug with mild central analgesic and antipyretic actions. It is administered orally and readily absorbed from the stomach and small intestine, an acid drug is well absorbed in an acidic environment. It is metabolised by tissue /plasma esterases. Aspirin may also be used in low doses, daily to prevent platelet aggregation.

In a healthy body, thromboxane and prostacyclin (eicosanoids – fatty acid signalling molecules) are balanced. Aspirin however disrupts this balance in the favour of prostacyclin, inhibiting aggregation.

Aspirin irreversibly binds to platelet COX, however as platelets are produced every few days, the condition is not permanent, and this is why chronic dosing may be necessary.

Aspirin is toxic in cats due to their lack of the enzyme UDP-glucuronyl transferase, therefore when giving aspirin to cats the maximum stated dose is 25mg/kg daily (Compared to 25mg/kg 3-4 times a day in dogs) Toxicity effects may still appear even at lower doses e.g. vomiting, abdominal pain, anorexia and gastric ulceration.

Paracetamol

Paracetamol is a weak anti-inflammatory drug; however it does have a more potent central analgesia and anti-pyretic effects than aspirin (See IC50 table to see a smaller dose is required of paracetamol to inhibit COX-2, the COX enzyme responsible for inflammatory responses).

Due to paracetamol being well tolerated, producing less gastric irritation than aspirin and having much fewer side effects than aspirin, it has become a predominant household analgesic. Acute overdoses can cause fatal hepatic damage; early symptoms include anorexia, vomiting, diarrhoea and abdominal pain. It is the reactive metabolites of paracetamol latching onto -SH groups that cause hepatic toxicity.

The dog is more resistant to paracetamol toxicity than cats, an oral dose is recommended every 6 hours of 25-30mg/kg.

Phenylbutazone

Phenylbutazone is the most widely used NSAID in equine medicine; however it is extremely toxic in humans. It has a long  t1/2 (half-life) of 70 hours and produces severe gastric ulceration and agranulocytosis. It can be administered orally and by intravenous injection. Due to the acidity of the drug, it is readily absorbed from the stomach/duodenum. Phenylbutazone metabolites are weak acids and therefore preferably excreted in alkaline urine. Training horses may have acidic urine, and so it is recommended not to take Phenylbutazone within 8 days of competition.

Half-life in dogs of Phenylbutazone has been recorded at 3-8 hours (however this may vary dependent on the dose). Phenylbutazone can inhibit the synthesis of prostaglandin in inflammatory exudates for 12-24 hours, with the response lasting for up to three days after the final dose in the course.

Signs of Phenylbutazone toxicity include inappetance and depression with weight loss and oedema. The oedema (fluid retention) is due to the decreased NaCl excretion.

Dosing of Phenylbutazone should not exceed:

  • Day 1 – 4.4 mg/kg twice a day
  • Day 2-4 – 2.2 mg/kg twice a day
  • Day >5 – 2.2 mg/kg daily

Phenylbutazone is also administered in combination with other drugs for the treatment of musculoskeletal disorders e.g. Tomanol – Phenylbutazone and Isopyrin

Meclofenamic Acid (Arquel)

Meclofenamic acid is a potent anti-inflammatory, anti-pyretic analgesic. It is more potent than aspirin but similar in effect. As well as inhibiting COX enzymes it has found to be a prostaglandin antagonist, interacting with prostaglandin receptors. It therefore prevents the action of prostaglandin already present possibly exerting a more rapid reduction of inflammation.

Half-life is 6-8 hours; therapeutic levels are maintained with daily doses.

Unlike other NSAIDs onset of Meclofenamic acid is relatively slow, taking 36-96 hours for effects to begin

Prolonged administration in the horse may lead to anorexia, depression, ulceration of buccal mucosa, gastric haemorrhage or diarrhoea

Naproxen

Naproxen is a propionic acid derivative (like ibuprofen or Ketoprofen) with a tendency for reduced frequency of serious side effects at therapeutic doses. In the horse, half-life of the drug is 5 hours; double daily doses have been proven effective in soft-tissue inflammatory conditions such as myositis. In the dog however, half-life is much longer at around 70 hours, it is therefore recommended to avoid this NSAIDs due to excessive toxicity

Ketoprofen

Ketoprofen is a potent COX inhibitor which is also able to stabilise lysosomal membranes and is a Bradykinin antagonist. It is also reported to be an inhibitor of the lipoxygenase enzyme system (iron-containing enzymes which catalyse the dioxygenation [incorporation of two oxygen atoms] of some polyunsaturated fatty acids). It is around 50x more potent than Phenylbutazone, however this is not accompanied by an equivalent increase in toxicity. It is also suggested that it may be cartilage sparing, neither accelerating chondrocyte damage nor reducing proteoglycan production.

Carprofen

Carprofen is a potent anti-inflammatory drug, but is a weak inhibitor of COX. Its mode of action is not yet known but it significantly inhibits neutrophil migration. Due to weak inhibition of COX, toxicity of Carprofen its toxicity tends to be low.

Piroxicam

Piroxicam is a potent and long lasting anti-inflammatory drug. Its half-life of around 60 hours enables lower dosing (alternate days). A higher frequency of dosing will produce standard NSAID toxic effects (see below, section ‘Side Effects of NSAIDs’)

Meloxicam

Meloxicam is a similar drug, but with a shorter half-life (30-40 hours). It is thought to have greater potency for COX-2 than COX-1 therefore side-effects may be less. It is also thought to be chondroprotective (The slowing of degradation of articular cartilage)

Flunixin

Flunixin is a potent versatile anti-inflammatory drug with a short half-life in all species (2-8 hours). However its duration of action is relatively long (24-36 hours)

When to Use NSAIDs

  • Mild to moderate inflammatory lesions and associated pain
  • Acute inflammation and pain
  • Joint inflammation and pain
  • Suppression of pulmonary oedema
  • Endotoxaemia
  • Anti-thrombic

Side Effects of NSAIDs

  • Gastric irritation and ulceration – This is a main side effect of chronic NSAID use, it occurs because when NSAIDs inhibit the COX-1 enzyme, COX-1 synthesises prostaglandins associated with inhibiting acid synthesis and increasing secretion of protective mucus. SO by inhibiting COX-1, the stomach becomes unprotected from the gastric acid causing irritation and possibly ulceration in chronic use.
  • You can protect the gut by administering proton pump inhibitors (Losec), prostaglandin analogues (Misoprostol) or H2 receptor antagonists (Zantac)
  • Vomiting and diarrhoea
  • Hepatotoxicity
  • Renal papillary necrosis, chronic nephritis
  • Bone marrow disturbance
  • Skin rashes
  • Respiratory distress