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BasicPharmacology

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The author Degree in Veterinary Medicine and PhD in Pharmacology from the University of Zaragoza, Spain.

He worked as a full-time swine and bovine practitioner for eight years.

From 2004 to 2010, he worked as a senior researcher at CReSA (Center of Animal Health Research) working in the fields of epidemiology, pharmacology and immunology, focusing his work on bacterial and viral swine and bovine diseases.

From 2010 to now, he has been working as a professor at the University of Lérida (Spain). He has been actively collaborating in field trials with different pig and bovine production companies.

Throughout his career, he has published more than fifty papers in peer-reviewed journals.

In collaboration with Ana I. de Prado

She graduated in Veterinary Medicine from the University of León (Spain). She holds a Master’s degree in Management and Consultancy on Dairy Cattle Farms from the Autonomous University of Barcelona and a Master’s degree in Research in Medicine and Animal Health from the University of Santiago de Compostela (Spain).

After working as a practitioner and milk quality advisor, she joined the pharmaceutical industry in 2002, working for different companies as a marketing and technical advisor for vets and farmers, carrying out different projects and field trials.

She currently works for Ceva Animal Health as a Global Technical Manager in ruminants.

Lorenzo José FraiLe sauce

Basic Pharmacology

1. Aims and objectives .............................................................................................. 4

2. General concepts: what are we going to talk about? ......................................... 5

3. Pharmacokinetics: what does the animal do to the drug?

Pharmacokinetic parameters: .............................................................................. 8 3.1. Plasma clearance ............................................................................ 9 3.2. Volume of distribution .................................................................. 10 3.3. Half life ............................................................................................ 11

3.4. Area under the curve ..................................................................... 12 3.5. C

Maximum concentration .............................................................................. 15

4. Pharmacodynamics: what does the drug do to the animal?

Pharmacodynamic parameters: ........................................................................ 164.1. Minimum inhibitory concentration .............................................. 17 4.2. Minimum bactericidal concentration .......................................... 19

5. Clinical efficacy and its relationship with pharmacokinetic and

pharmacodynamic parameters ......................................................................... 215.1. What do we use PK-PD models for? ............................................ 21 5.2. Let’s use PK-PD: designing an administration schedule .......... 21 5.3. PK-PD and antimicrobials, their classification ........................... 24 5.4. If we use different antimicrobials at the same time ................ 26

6. Withdrawal period, a critical aspect to assure food safety ............................ 28

7. List of references ................................................................................................ 29

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Basic Pharmacology

1.

Basic pharmacologyAlthough every veterinary surgeon studies Pharmacology during his degree, over the years, the basis of this topic is often forgotten. And without noticing it, we are using pharmacology every day.

The principles of antimicrobial therapy are based on a three way therapeutic relationship among the bacteria that cause the infections, the sick animal and the drug which is administered to treat the infection. The choice of an antimicrobial treatment and the design of a rational dosage depend on the knowledge the clinician has about the disease but, mainly, on the pharmacokinetics and pharmacodynamics of the drugs he is going to use to fight it. PK/PD models are helpful to decide these administration schedules. PK/PD models also allow classifying antimicrobials and thus, understanding their action mechanisms. Finally, the withdrawal period is a critical aspect to assure food safety.

aims and objectives

Since veterinarians have to use antimicrobials and other drugs in the daily performance of their functions, the main aim of this review is to provide clinicians a solid grounding in the basic concepts of pharmacology.

Explaining these concepts from a practical point of view.

Defining the pharmacokinetic and pharmacodynamic parameters and their interpretation.

Understanding the importance and use of PK/PD models.

Considering what antimicrobial treatment involves in the daily practice.

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2.General concepts: what are we going to talk about?

The principles of antimicrobial therapy are based on a three way therapeutic relationship between the bacteria which causes the infection, the sick animal and the drug which is administered to treat the infection.

Pharmacokinetics

Toxic

ity

susceptibility

imm

une

response

infection

Pharmacodynamics

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The main objective of the antimicrobial therapy is to ensure that the infected area has enough drug concentration which remains effective and lasts for a sufficient period of time in order to achieve both clinical and bacteriological recovery, whilst minimising the appearance of undesired effects.

antimicrobial therapy

ensuring enough

drug concentration

remains effective

Main objective

Lasts for a sufficient period of time

Minimising undesired effects

examples of undesired effects:

Toxicity of the drug in the animal being treated.

Development of microbial resistance to the drug being administered.

in livestock, the presence of drug residues in edible tissues at levels higher than those allowed (Lees, 2002).

The key questions which should be considered before applying any therapeutic regime are what drug should be used, the dose to be administered, the frequency of administration and how long the treatment should last.

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TreaTmenT success

The choice of an antimicrobial treatment and the design of a rational dosage depends on the available information of the microorganism that causes the disease (clinical experience or isolation), the effect of the drug on the microorganism (pharmacodynamics, susceptibilily), the effect of the drug on the animal being treated (toxicity), the presence of the drug in this particular animal (pharmacokinetics) combined with other considerations such as the appearance of antimicrobial resistance, animal welfare and the profitability of the treatment (Goodmand and Gillman, 2006).

other considerations

Microorganism information

Pharmacokinetics Toxicity

PharmacodynamicsTreatment success

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3.Pharmacokinetics: what does the animal do to the drug?

Pharmacokinetics (PK) is the science that describes, by means of mathematical concepts, the kinetics of these molecules when they are introduced in the organism and defines the pattern of ADME processes (absorption, distribution, metabolism and excretion) that control the presence of those substances (Rowland, 1995; Baggot, 2001, Martín-Jiménez, 2002).

absorption

Distribution

Metabolism

excretion

aDMe processes

The process of a substance entering the blood circulation.

The dispersion or dissemination of substances throughout the fluids and tissues of the organism.

The recognition by the organism that a foreign substance is present and the irreversible transformation of parent compounds into daughter metabolites that could be easily excreted.

The removal of the drugs from the organism.

WhaT do We use pharmacokineTic sTudies for?

We can use PK parameters to make predictions about the concentration of antimicrobials for specific doses and thus design the administration schedule.

This is possible because the kinetics of the drugs is linear:

The speed at which an antimicrobial is eliminated from the organism is directly proportional to the amount which remains within it.

Pharmacological products (especially antimicrobials) which fail to show a linear behaviour in their kinetics are not usually used (Prescott, 2000).

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Observe in this example the evolution of the concentrations of an antibiotic administered at a dose of 1.25 mg/kg of live weight by intravenous and intramuscular route in lactating cows.

We can appreciate that:

The antibiotic administered by IV route reaches the highest concentration in serum but this concentration is maintained for the shortest period of time.

The antibiotic administered by IM route reaches the maximum concentration later than the IV administered one.

The antibiotic concentration in milk is higher than the serum concentration at the same kinetic times for the IV and IM route of administration throughout the time.

The antibiotic administered by both administration routes appears approximately in milk at the same kinetic time and it disappears approximately at the same time too.

3.1. Plasma clearance (CL)

Plasma clearance is the most important pharmacokinetic parameter because it is one of the three which determine the maintenance dose (Toutain, 2004a):

Conc

entr

atio

n, μ

g/m

l

0 240 480 720 960 1,200 1,440

Minutes

0.04

0.1

1

IV serum IM serum IV milk IM milk

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3.2. Volume of distribution (Vd)

The volume of distribution is defined as the constant of proportionality between the amount of an antimicrobial in the organism and concentration in plasma.

Vd = Antimicrobial in the organism

Concentration in plasma

This parameter is not directly related to the distribution of the

drug in specific physiological compartments.

A high volume of distribution does not necessarily imply that the drug is widely spread throughout the organism, since it could show tropism for a specific organ.

The volume of distribution of a drug can be much higher than the total body water volume (Toutain, 2004b).

Maintenance dose: quantity of antimicrobial administered per unit of time.

CSS: steady blood Concentration of the drug (Steady State) that allows the desired effect.

– In the case of an antimicrobial, the clinical response may be an improvement of the symptoms

associated with the infection.

F: bioavailability. This parameter ranges from 0 to 1. The bioavailable dose of a product is the real amount

of drug that reaches the systemic circulation (it is therefore “available” to perform its action) when a

formulation is administered by extravascular route.

– That is to say, if a drug is administered orally and its bioavailability is 1, this means that it is

completely absorbed.

Plasma clearance: the volume of blood that is cleared of any substance per unit of time and per kilogram of live weight (its units are ml/min/kg).

in our study case, cL is the direct measurement of the ability of an animal to eliminate an antimicrobial.

Maintenance dose = CL x CssF

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In any case, in general terms, a high volume of distribution is a positive characteristic for any drug because it implies that it could be used to treat infections in many parts of the body. As an example, the following table shows the volume of distribution of known antimicrobials in bovine and the clinical consequences that it could have.

examples of volume of distribution

antimicrobial Volume of distribution (l/kg) comments

Gentamicin (aminoglycoside) 0.19Only feasible to treat extracellular

infections.Low penetration in lung tissue

Ampicillin (β-lactams) 0.44Higher tissue penetration

than aminoglycosides but less than macrolides

Tylosin (macrolide) 1.33 High penetration in lung tissue

Florfenicol (amphenicol) 0.76 Wide tissue distribution

Drug administration

Distributionequilibrium

concentration in plasma

3.3. Half-life (t½

)

The half-life is the time required, after the intravenous administration of a drug, for the concentration in plasma to be reduced to half once the distribution equilibrium has been reached.

is reduced to half

Timeline

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That implies that the calculation of this parameter can only be carried out when the decrease of the concentration in plasma is due solely to the elimination process (Toutain, 2004c). From a practical point of view, the relevance of this parameter is limited.

it is noteworthy to point out that, after the administration of

multiple doses of a drug, a steady state is reached after 3 to 5

half-lives have gone by.

So, for a drug with a half-life of 12 hours, a steady state will be reached after the second or third day of administration (30-60 hours).

For this reason, for drugs with a very long half life (> 24 hours), it could be interesting to use a loading dose to achieve the steady state as quick as possible.

Half-life of penicillin is short in bovine

Half-life of some sulphonamides is long in bovine

using a loading dose for these drugs could be

reasonable

The steady state will be reached quickly in a multiple dose

administration

Although it is not normally used under

field conditions

Let’s see some examples:

3.4. Area under the curve (AUC)

AUC is a direct measure of the drug exposure in an animal after the drug (antimicrobial in this case) administration and it is directly related to the administered dose and the clearance. Thus, the lower is the clearance the higher is the AUC:

CL= AUC= Administered doseAUC

Administered doseCL

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The prediction of the concentration in blood of a drug after its

administration can be calculated by means of pharmacokinetic

equations but this procedure does not give the concentrations present in the other tissues of the organism

at the same kinetic time.

Ctss: the concentration of the antimicrobial in tissue at a steady state.

Cpss: the concentration of the antimicrobial in plasma at a steady state.

We would be mistaken if we were to assume that concentration in tissue is the same as concentration in plasma at any kinetic time.

It is important to know the concentration in tissue because it is the concentration of antimicrobial in the target organ.

P= CtssCpss

The area under the curve is the area under the curve in a plot of concentration in plasma versus time.

Tissue kinetic study allows the calculation of the partition coefficient (P):

The value of P gives information about the degree of accumulation of the antimicrobials in the tissues of interest and it is very useful to evaluate the potential effectiveness of the antibacterial treatments in specific organs or tissues (sánchez-rubio, 1999).

From a practical perspective, the higher the P value, the better for a clinical point of view.

Conc

entr

atio

n

Time

IV administration

Oral administration

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Let’s see an example of this type of study: in these graphics are the results of mean concentration of ceftiofur and its metabolites in plasma, lochia and uterine tissues, after the subcutaneous administration of ceftiofur (hydrochloride) at a dosage of 1 mg/kg in dairy cows [mean ± SD (n= 4 cows)]. SD: standard deviation.

We can appreciate that:

The achieved concentration in plasma is notably higher than in the other tissues and the maximum serum concentration is obtained after 2 hours of the administration (before than in the other tissues).

Among the other tissues, the antibiotic concentration in endometrium is higher than in lochia and caruncles.

In caruncles, the antibiotic maximum concentration is reached later than in lochia but the antibiotic concentration is almost constant throughout time.

At 24 hours post-treatment, the lowest concentration is observed in lochia.

We must highlight that the tissue concentration of each tissue cannot be linked directly with the clinical efficacy of any antimicrobial treatment because it is necessary to know the MIC of the antimicrobial against the bacteria responsible of the infection.

Plasma Lochia

0 4 8 12 16 120 24

Conc

entr

atio

n, μ

g/m

l

0 4 8 12 16 120 24

Conc

entr

atio

n, μ

g/m

l

4 4

3 3

2 2

1 1

0 0

0 4 8 12 16 120 24

Conc

entr

atio

n, μ

g/g

Post-treatment time (hours)

Post-treatment time (hours)

0 4 8 12 16 120 24

Conc

entr

atio

n, μ

g/g

Post-treatment time (hours)

Post-treatment time (hours)

4 4

3 3

2 2

1 1

0 0

Caruncles Endometrium

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3.5. Maximum concentration (Cmax

)

It is the maximum concentration observed after the administration of a medicinal product to an animal by any route of administration.

Observe the maximum concentration of an antibiotic after intramuscular administration in the following graphic:

rouTes of adminisTraTion

The treatment of a typical infection process must last between 3 and 7 days in order to reach the therapeutic objective. From a practical point of view, we must highlight that the pharmacokinetic profile observed depends not only of the route of administration but also of the particular medicinal product used. Thus, two medicinal products that contain the same drug, at the same dose and administered by the same route (intramuscular) could have different pharmacokinetic profiles.

The most usual routes of drug administration in bovine are intramuscular, subcutaneous and oral with the exception of adult animals in dairy where the oral route is hardly used. In the case of oral route, it is normally carried out by adding the product to drinking water and/or feed and it is usual in feedlots.

useful to characterise the pharmacokinetic

properties

intravenous routes in cattle

oralFEEDLOT

operations

iM/scDAIRY

operations

impractical for health treatment

routes of

administration

Conc

entr

atio

n, (μ

g/m

l)

Time (hours)0

10

0

20

30

40

50

1 2 3 4

cmax

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4.Pharmacodynamics: what does the drug do to the animal?

The target of antimicrobial therapy is the microorganism responsible for the infectious disease. Its elimination from the organism is therefore critical to cure the disease. However, these drugs do not completely eliminate a pathogen without the help of the immune system.

in fact, the fundamental objective of antimicrobial therapy is to help the natural defence mechanisms to eliminate the infectious agent (Prescott, 2000).

Pharmacodynamics describes the relationship between the time-course of the concentration of an antimicrobial in the organism and the intensity and duration of its pharmacological effects.

antimicrobial Pharmaco-dynamics

(PD)

Time course of the concentration

intensity and duration of the pharmacological effect

In this particular case, the effect which is usually measured rather than being the destruction of the microorganism is the inhibition of the bacterial growth.

Growth inhibition measurement is carried out by means of in vitro microbiological techniques which allow clinicians to know the susceptibility of the microorganism to one or several antimicrobials.

From a practical point of view, we should have access to the susceptibility pattern, to the maximum amount of antimicrobials possible, of the microorganisms which are to be treated.

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4.1. Minimum inhibitory concentration (MIC)

The MIC is the lowest antimicrobial concentration that inhibits in vitro the growth of the target bacteria in specific conditions of incubation (Sánchez-Rubio, 1999; Mckellar, 2004).

These conditions are not the same as those in which bacteria grow in vivo (blood, extracellular fluid, intracellular environment, urine, milk or in the presence of pus or debris). This allows the easy understanding of the fact that the data obtained in vitro do not have to be a perfect reflection of what happens in vivo.

in spite of its limitations, the Mic is the most used pharmacodynamic parameter when it comes to antimicrobials (Mouton, 2005).

Not all the strains of a same bacterium have the same MIC. For this reason, two criteria are used to evaluate the susceptibility of a bacterial species to an antimicrobial. Namely, the MIC

50 and the

MIC90

, which are defined as the lowest concentrations of antimicrobial that inhibit growth of 50% and 90% of the total population of the target bacteria.

From a practical point of view, taking into account the MIC90

is more useful because this parameter covers most of the pharmacodynamic variations observed at population level. In the next table we can observe the comparative MIC and MPC1 values for 285 M. haemolytica strains collected from cattle.

Qualitativetechniques

Quantitativetechniques

susceptible

intermediate

resistant

Mic

MBc

Microorganism susceptibility

1. Mutant prevention concentration (MPC): the lowest drug concentration that blocks the growth of mutant bacterial sub-populations.

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What can we observe?

The calculation of MIC50

and MIC90

allows the comparison among antimicrobials for clinical efficacy. The calculation of MPC

50 and MPC

90 allows similar comparison but related to the

generation of antimicrobial resistance.

The lower the MIC value, the better from a clinical point of view. However, considering pharmacokinetic parameters, it is also necessary to associate directly the MIC value (pharmacodynamic parameter) with clinical efficacy.

Enrofloxacin and ceftiofur had the lowest MIC and MPC values whereas the highest values were seen for tilmicosin.

The rank order (from the lowest to the highest) of MIC90

values was ceftiofur > enrofloxacin > florfenicol = tulathromycin > tilmicocosin.

The rank order of potency (from the lowest to the highest) based on MPC90

values was enrofloxacin > ceftiofur > tulathromycin> florfenicol>tilmicosin.

Applying MPC principles may serve to optimize therapy and reduce resistance selection.

Drug Mic/MPc distribution values (μg/ml)

≤ 0.008 0.016 0.031 0.063 0.125 0.25 0.5 1 2 4 8 ≥16 ≥32 Mic50

/Mic90

MIC distribution

Ceftiofura 17 22 2 0.016/0.016

Enrofloxacin 31 114 16 39 85 0.016/0.125

Florfenicol 5 120 160 2/2

Tilmicosin 3 64 56 82 38 42 2/8

Tulathromycin 2 15 53 98 117 1/2

MPC distribution MPC50

/MPC90

Ceftiofura 2 1 19 15 4 1/2

Enrofloxacin 4 31 59 59 49 65 18 0.25/1

Florfenicol 8 64 142 55 16 4/8

Tilmicosin 1 60 58 87 79 16/≥32

Tulathromycin 3 61 138 77 6 4/8

a Testing against 41 isolates.

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4.2. Minimum bactericidal concentration (MBC)

The minimum bactericidal concentration is defined as the lowest concentration of antimicrobial able to reduce the initial bacterial population by 99.99% after 24 hours of incubation at 37 ºC with a standard amount of inoculums.

MBc

The Mic provides a good idea of the bactericidal activity.

The analysis of the susceptibility of a bacterium to an antimicrobial carried out in vitro can underestimate the activity in vivo due to the post-antimicrobial effect (Pae) and post-antimicrobial leukocyte enhancement effect (PaLe).

The Mic is used as a key pharmacodynamic parameter

in antibiotherapy.

It has been demonstrated that the MIC and MBC for bactericidal antimicrobials

are very similar.

The techniques to quantify the MBC are very complicated.

It reduces the initial bacterial population by 99.99%

The post-antimicrobial effect (Pae) can be defined as the growth suppression of a microorganism as a consequence of being exposed to an antimicrobial, whilst the drug is no longer at the infected area.

It can be quantified as the difference between the times needed by two populations of the same bacteria, one having never been exposed to an antimicrobial while the other having already been exposed, to multiply their population by 10.

The intensity of the effect depends on the microorganism, the drug, the concentration reached and the exposure time.

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This effect is greater in vivo than in vitro (Mouton, 2005).

This effect can be very important since it can explain the efficacy shown by concentration-dependant antimicrobials when administered at very long dosing intervals.

Greatest Pae

Gram-negative bacteria Gram-positive bacteria Gram-positive and gram-negative bacteria

Macrolides β-lactams Carbapenems

Fluoroquinolones –

Aminoglycosides –

The post-antimicrobial leukocyte enhancement effect (PaLe) can be defined as the highest susceptibility to phagocytosis shown by bacteria after exposure to an antimicrobial.

The drugs with a greater PAE effect tend to have a greater PALE.

Obviously, this effect is not taken into account in in vitro studies, but may be significant when it comes to explaining the efficacy of some administration schedules in the treatment of certain diseases.

For instance, there are new medicinal products that are administered in one single administration (quinolones or macrolides): the efficacy of these products could be explained not only according to their persistence in the body for long periods of time (one week for example in the case of macrolides) but also according to their PALE effect on the bacteria. Thus, this effect helps to explain why one antimicrobial could be efficacious to control a bacterial infection for a longer period of time although there are no detectable concentrations in the target tissue after its administration.

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5.clinical efficacy and its relationship with pharmacokinetic and pharmacodynamic parameters

5.1. What do we use PK/PD models for?

A PK/PD model is a mathematical description providing clinically relevant information about the relationship between the pharmacokinetics of a drug and its pharmacological effect.

Nowadays, both microbiologists and specialists in infectious diseases believe that PK/PD models can be an appropriate tool for the design of optimum administration schedules. Clinical efficacy studies cannot be substituted by these models, although they should be complementary when it comes to designing an administration schedule.

5.2. Let’s use PK/PD: designing an administration schedule

Although the most useful pharmacokinetic parameters when establishing an administration schedule are the AUC, the maximum drug concentration achieved (C

max) and the time at which

the concentrations in plasma exceed a previously defined pharmacodynamic threshold, the most used pharmacodynamic parameter in the case of antimicrobials, as has already been explained in detail, is the MIC.

The relationship between the concentration of the antimicrobial in various tissues of the animal under treatment and the effect on the target bacteria is not simple.

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The efficacy of the treatment being applied could depend on:

Reaching a concentration in plasma several times higher than the MIC of the pathogen (Cmax

/MIC).

Maintaining a concentration in plasma above the MIC for a prolonged period (T>MIC) (Craig, 1998).

For some drugs and pathogens, a combination of the exposure time to the antimicrobial and the concentration reached (AUC/MIC).

if these objectives are not reached, the probability of the appearance of resistant strains after the treatment increases (Blondeau, 2001).

The PK/PD parameters which have been studied in depth are AUC/MIC, Cmax

/MIC and T>MIC.

Antimicrobial drugs have been classified as concentration-dependent where increasing concentrations at the locus of infection improves bacterial kill, or time-dependent where exceeding the MIC for a prolonged percentage of the inter-dosing interval (T>CMI) correlates with improved efficacy.

Ratios of 100-125 for AUC0-24

/MIC and 10 for Cmax

/MIC have been recommended to achieve high clinical efficacy for concentration-dependent antimicrobial drugs like enrofloxacin and marbofloxacin, and exceeding MIC by 1–5 multiples for between 40 and 100% of the inter-dosing interval is appropriate for most time-dependent agents, like amoxicillin or classical macrolides (Mckellar, 2004).

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In PK/PD models, average pharmacokinetic parameters and MIC90

of the population of animals and bacteria are used respectively to compare the PK/PD parameters obtained with the threshold values and, in this way, to predict clinical efficacy.

Observe in this figure the pharmacokinetic profile of an antimicrobial after intramuscular administration and different PK/PD parameters.

For example, in the particular case of β-lactams, the most appropriate dosage regime is the one which maintains a css

above the Mic for most of the dosing interval obviously, it is better for the whole interval

but that which avoids the appearance of resistant strains in the population.

This objective can be reached if the Css is higher than the product 5 x MIC.

For this reason, the dosage regime can be crucial in selecting resistant strains.

Conc

entr

atio

n

Time

AminoglycosidesFluoroquinoles

Fluoroquinolesβ-lactams

PAE

MICT>MIC

AUC>MIC

β-lactams

Threshold values that could be associated with clinical efficacy:

• AUC0-24

/MIC ≥ 125.

• Cmax

/MIC ≥ 10.

• T >MIC= 40-100%.

Cmax

/MIC

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5.3. PK/PD and antimicrobials, their classification

In general, the antimicrobials which are normally used in cattle medicine can be divided into three groups, taking into account the PK/PD parameters and their correlation with the clinical efficacy of the treatments (Mckellar, 2004):

antimicrobial classification depending on PK/PD models

concentration-dependent antimicrobials Time-dependent antimicrobials co-dependent antimicrobials

Concentration-dependent antimicrobials are those whose

effect on microorganisms depends on their concentration:

• Aminoglycosides.

• Fluoroquinolones.

Time-dependent antimicrobials are those whose effect on microorganisms

depends on the exposure time to the drug rather than on its concentration,

as long as it exceeds the MIC:

• ββ-lactams.

• Classical macrolides.

Co-dependent antimicrobials are those whose effect on

microorganisms depends equally on their concentration as

on the exposure time:

• New macrolides (azalides).

• Fluoroquinolones for anaerobic germs.

In their case, the Cmax

/MIC ratio is the PK/PD index which best

correlates with clinical efficacy.

In their case, the T>MIC is the PK/PD index which best correlates

with clinical efficacy.

In their case, the PK/PD index which best correlates with clinical efficacy

is usually the AUC/MIC ratio.

This is an antimicrobial and bacteria-specific classification.

For example, marbofloxacin presents:

A concentration-dependent activity for some gram-negative microorganisms (e.g. Pseudomonas spp.).

A co-dependent activity for other gram-negative germs (e.g. Pasteurella multocida).

A time-dependent activity for gram-positive germs (e.g. Staphylococci).

This example clearly highlights the fact that, in order to be clinically effective in treatments, not only a good knowledge of the drug administered, but also of the microorganism which needs to be eliminated, is necessary in order to design the administration schedule.

The information provided in the previous table helps to understand the different administration schedules of commonly used in bovine medicine antibiotics. Thus, there are medicinal products that require multiple administrations or use a long-acting presentation (ß-lactams, for instance), whereas other ones are administered in one-shot presentations (quinolones for example). The rationale behind this difference is based on how the PK/PD parameters can be achieved in order to be clinically effective.

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Although it is not possible to generalise in all cases, let’s see the PK/PD parameters in relation to their capacity to predict the therapeutic success of the antimicrobials:

PK/PD parameters associated with clinical success using antimicrobials

Parameter antimicrobial

Cmax

/MIC ratio Aminoglycosides and fluoroquinolones.

AUC/MIC ratioAminoglycosides, fluoroquinolones, new macrolides (e.g. tildipirosin)

and tetracyclines.

T>MICPenicillins, cephalosporins, classical macrolides (e.g. tilmicosin),

pleuromutilins (e.g. tiamulin) and lincosamides.

Finally, another way of classifying antimicrobials is by looking at their bacteriostatic or bactericidal effect.

Antimicrobials with a bacteriostatic effect are those which only inhibit bacterial growth, in such a way that the clinical and bacteriological overcome of the infection depends on the animal’s immune system (e.g. tetracyclines and “classical” macrolides).

Bactericidal antimicrobials are able to destroy the target bacteria (e.g. β-lactams and fluoroquinolones).

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5.4. If we use different antimicrobials at the same time

The ideal situation is to select one antimicrobial to treat a bacterial infection in order to be clinical efficacious and reduce the probability of generating antimicrobial resistance. However, in some cases, we need to combine different antimicrobials.

We must take into account that some combinations could be antagonic, synergistic or indifferent if the antimicrobials are acting on the same bacteria.

antagonic combination: the combined effect of two antimicrobials is lower than the effect of the antimicrobials administered separately. This combination must be avoided.

Mainly bacteriostatic

Amphenicols Macrolides2

PleuromutilinsLincosamidesTetracyclines

SulphonamidesDiaminopiridines

FlorfenicolTilmicosin Tiamulin

LincomycinDoxicycline

SulphametazineTrimethoprim

Mainly time-dependent bactericidal

Penicillins Cephalosporins

AmoxicillinCeftiofur

Mainly concentration-dependent bactericidal

with very significant postantimicrobial action

AminoglycosidesFluoroquinolones

StreptomycinEnrofloxacin

acTion

antimicrobial ββ-lactams Aminoglycosides Polypeptidic Quinolones

Macrolides

Antagonic combination

Pleuromutilins

Tetracyclines

Sulphonamides

Diaminopiridines(Trimethoprim)

Amphenicols

Aminoglycosides and polypeptidic combination is also antagonic.

2. This is true for all macrolides. New macrolides must be classified in a case by case scenario.

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synergistic combination: the combined effect of two antimicrobials is greater than the effect of those antimicrobials administered separately. These combinations are usually available in the pharmaceutical market.

NA: non-applicable. Sulphonamides and trimethoprim combination is also synergistic.

indifferent combination: the combined effect of two antimicrobials is similar to the observed effect for each antimicrobial administered separately. This combination should be avoided too.

Any other kind of antimicrobial combination is indifferent. For instance:

Quinolones and all other bactericidal antimicrobials.

Any combination of bacteriostatic antimicrobials, with the exception of sulphonamides and trimethoprim.

antimicrobial β-lactams Aminoglycosides Polypeptidic

β-lactams NA Synergistic Synergistic

Aminolycosides Synergistic NA Antagonic

Polypeptidic Synergistic Antagonic NA

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Basic Pharmacology

6.Withdrawal period, a critical aspect to ensure food safety

One critical point that must be ensured when using antimicrobials in cattle is the presence of drug residues in edible tissues at lower levels than those established by the European legislation (maximum residue limits-MRL) for each particular tissue.

This goal is achieved if the animal is sacrificed after waiting for a period of time (withdrawal period) that is calculated in a way that guarantees the antimicrobial concentration in all the edible tissues is below a threshold value (MRL).

For some antimicrobials, the goal is to have a total concentration of parent drug plus metabolite/s below a threshold value. The European legislation establishes for each case whether it must be taken into account only the parent drug or a marker residue that normally includes one or several metabolites.

In the particular case of residues in milk, different analytical methods that allow assuring the presence of these substances below the established limits at tank level are very common.

Treatment Withdrawal period Food production

Co

nc

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AbsorptionDistribution

0

MRL

Depletion phase (elimination only)

Ct < MRL

Ct: concentration in tissues.

Basic Pharmacology

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7.List of references

Baggot J.D., The physiological basis of veterinary clinical pharmacology. Blackwell Sciencie Ltd., Oxford, 2001.

Blondeau, et al. Mutant prevention concentrations of fluorquinolones for clinical isolates of Streptococcus pneumoniae.

Antimicrobial Agents Chemotherapy 2001; 45: 433-438.

Craig W.A. Pharmacokinetic/pharmacodynamic parameters: Rationale for antibacterial dosing of mice and men. Clinical

infectious diseases 1998; 26: 1-12.

Goodman & Gilman’s. The Pharmacological Basis of Therapeutics. Authors: Brunton, Laurence; Lazo, John ; Parker, Keith.

McGraw-Hill education 2006; 11th Edition.

Lees, et al. Principios de antibioterapia. En Farmacología y terapéutica veterinaria. Botana L.M, Landoni F., Martín-

Jiménez, T. Ed. Mcgraw-Hill interamericana 2002.

Martín-Jiménez, T. Farmacocinetica I: absorción y distribución. En Farmacología y terapéutica veterinaria. Botana L.M,

Landoni F., Martín-Jiménez, T. Ed. Mcgraw-Hill interamericana 2002.

Mckellar, Q.A. et al. Pharmacokinetic/pharmacodynamic relationship of antimicrobial drugs used in veterinary medicine.

Journal Veterinary Pharmacology Therapeutics 2004; 27: 503-514.

Mouton J.W., Vinks A.A. Pharmacokinetic/pharmacodynamic modelling of antibacterials in vitro and in vivo using

bacterial growth and kill kinetics. The minimum inhibitory concentration versus stationary concentration. Clinical

pharmacokinetics 2005; 44(2): 201-210.

Prescott, et al. Antimicrobial therapy in veterinary medicine. 3. ed. Iowa State University Press, Ames, Iowa 2000.

Rowland, M., Tozer, T., Clinical pharmacokinetics. Concepts and applications. Williams and Wilkins, Media PA 1995.

Sánchez-Rubio, A., Sánchez Recio, MM. Basis of anti-infective therapy. Pharmacokinetic-pharmacodynamic criteria and

methodology for dual dosage individualisation. Clinical Pharmacokinetics 1999; 37(4): 289-304.

Toutain, et al. Plasma clearance. Journal Veterinary Pharmacology Therapeutics 2004a; 27: 414-425.

Toutain, et al. Volumes of distribution. Journal Veterinary Pharmacology Therapeutics 2004b ; 27: 441-453.

Toutain, et al. Plasma terminal half-life. Journal Veterinary Pharmacology Therapeutics 2004c; 27: 427-439.

Glossary

aDMe: absorption, distribution, metabolism, excretion.

antagonic combination: the combined effect of two antimicrobials is lower than the effect of the antimicrobials administered separately.

area under the curve (auc): the area under the curve in a plot of concentration in plasma versus time.

Bactericidal antimicrobials: those which are able to destroy the target bacteria.

Bacteriostatic antimicrobials: those which only inhibit bacterial growth.

co-dependent antimicrobials: those whose effect on microorganisms depends equally on their concentration as on the exposure time.

concentration-dependent antimicrobials: those whose effect on microorganisms depends on their concentration.

Half-life (t½

): time required for the concentration in plasma to be reduced to half once the distribution equilibrium has been reached.

indifferent combination: the combined effect of two antimicrobials is similar to the observed effect for each antimicrobial administered separately.

Maximum concentration (cmax

): maximum concentration observed after the administration of a drug to an animal by any route of administration.

Maximum residue limits (MrL): maximum concentration of residue accepted by the European legislation in a food product obtained from an animal that has received a veterinary medicine.

Minimum bactericidal concentration (MBc): lowest concentration of antimicrobial able to reduce the initial bacterial population by 99.99% after 24 hours of incubation at 37 ºC with a standard amount of inoculums.

Minimum inhibitory concentration (Mic): lowest antimicrobial concentration that inhibits in vitro the growth of the target bacteria in specific conditions of incubation.

Mic50

: lowest concentration of antimicrobial that inhibit growth of 50% of the total population of the target bacteria.

Mic90

: lowest concentration of antimicrobial that inhibit growth of 90% of the total population of the target bacteria.

Mutant prevention concentration (MPc): lowest drug concentration that blocks the growth of mutant bacterial sub-populations.

Partition coefficient (P): ratio of concentrations in tissue and plasma at steady state.

Pharmacodynamics (PD): science that describes the relationship between the time-course of the concentration of a drug in the organism and the intensity and duration of its pharmacological effects.

Pharmacokinetics (PK): science that describes the kinetics of drug molecules when they are introduced in the organism and defines the pattern of ADME processes.

Plasma clearance (cL): volume of blood that is cleared of drug per unit of time and per kilogram of live weight.

Post-antimicrobial effect (Pae): growth suppression of a microorganism as a consequence of being exposed to an antimicrobial, whilst the drug is no longer at the infected area.

Post-antimicrobial leukocyte enhancement effect (PaLe): highest susceptibility to phagocytosis shown by bacteria after exposure to an antimicrobial.

synergistic combination: the combined effect of two antimicrobials is greater than the effect of those antimicrobials administered separately.

Time-dependent antimicrobials: those whose effect on microorganisms depends on the exposure time.

Volume of distribution (Vd): constant of proportionality

between the concentration in plasma and the amount of a drug in the organism.

Withdrawal period: interval between the last administration of a veterinary medicinal product to animals under normal conditions of use and the production of foodstuff from such animals to ensure that such foodstuffs do not contain residues in quantities in excess of the maximum residue limits laid down.

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