Clinical pharmacology of
corticosteroids in horses and
doping control
P. L. Toutain,
National Veterinary School, Toulouse, France
ICRAV 2012, September 15-22, 2012
Philadelphia, USA
1
The European Horse Scientific Liaison
Committee
The view expressed in this presentation are those of the author and do not commit
the official policy of the EHLSC
2
Risk analysis by the EHSLC
1. Risk assessment Science
Irrelevant Plasma concentration
Irrelevant Urine Concentration
2. Risk management Decision
(International ) screening limits
3. Risk communication Communication
Detection Times
3
Goals of the presentation
I. An overview on PK/PD of corticosteroids
– Cortisol as a biomarker of GGs administration
– Synthetic GC
• Potency, esters, routes of administration etc.
II. Principles to establish irrelevant plasma and
irrelevant urine concentrations
– For systemic action
– For local action
• Intra-articular administration
• inhalation
4
Glucocorticosteroids (GCs)
 Glucocorticosteroids (GCs) are broad and potent
anti-inflammatory drugs.
 They are extensively used in horses to mitigate or
suppress inflammation associated with a variety of
conditions especially joint and respiratory system
inflammation.
 GCs are not curative:
 GCs are only palliative symptomatic treatments and
chronic use of GCs can be, in fine , detrimental to the
horse raising a welfare issue.
 GCs possess many other pharmacological properties
of potential abuse in competing horses
5
Cortisol in horses:
an endogenous hormone and a
surrogate endpoint of the
duration of the GC effects
6
Natural & Synthetic glucocorticoids
• All GCs used in therapeutics are synthetic
derivatives of cortisol.
• Cortisol (hydrocortisone) is synthesized in the
adrenal cortex and it is the main corticosteroid
hormone in the horse.
7
Cortisol in horses:
an endogenous hormone and a
surrogate endpoint of the
duration of the GC effects
8
Plasma cortisol levels:
circadian rhythm & pulsatility
Possible confounding factor
with adrenal suppression
by synthetic GS
Toutain et al. Domestic.Anim.Endocrinol. 1988, 5:55
Plasma cortisol levels:
circadian rhythm & pulsatility
• the plasma cortisol profile follows a typical
circadian rhythm with an overall 24h mean
plasma concentration of 43.4± 6.2 ng/mL: the
minimal concentrations (27.9± 6.9ng/ml) were
observed early in the night at 21.2±0.59 h and
maximal plasma concentrations (58.9±
9.5ng/mL at 9.2±0.6h in the morning)
10
Plasma cortisol levels:
circadian rhythm & pulsatility
• The cortisol secretion is pulsatile with minuteto-minute variations in the plasma cortisol
concentrations making the interpretation of
snapshot plasma samples difficult.
11
Cortisol production rate during
rest and exercise
Plasma concentration profile
200
ng/ml
RIDE day
150
100
REST day
50
RIDE
0
9.5
16
0
8.5
Hour
(56km, 2 bouts)
Cortisol disposition & production rate
Rest vs. exercise
• The daily cortisol production
rate:
– Rest: about of 100 µg/kg or 5060mg/days
– Exercise: increased up to 6-fold
• the corresponding plasma
clearance is also increased (by
2.5-fold)
– plasma cortisol concentration is
a biased endpoint for the rate
cortisol secretion during the
exercise
13
DXM disposition: rest vs. exercise
• The 3-hour test exercise
included walking, trotting
and canter sequences
– the average speed was
calculated to be
approximately 12 km per
hour.
– Mean heart rate (about
120/min) for all horses to
standardise the test
exercise and for
comparability of horses.
14
DXM disposition: rest vs. exercise
Clearance: -25%
VSS:-17%
Half-life: no change
15
Urine cortisol concentrations (ng/mL)
during a rest or a ride day (56km)
300
300
ng / ml
Day
including a
RIDE
REST day
200
200
100
100
0
0
1
2
3
4
5
1- before the ride
3- 1st miction after the ride
5- up to 19.5h after the ride
6
1
2
3
4
RIDE
5
2- urine during the stop
4- up to 7.5h after the ride
6- after 19.5h post ride
6
Cortic 00A.16
Urine cortisol
Population investigations (n=254)
Popot & al EVJ 1997 29 220-229
Average
(geometric)
48ng/mL
maximum
388
Probability
exceeding
Urine
concentration
ng/ml
10-3
611
10-4
1025
10-5
1606
17
Urine cortisol
• International threshold: 1µg/ml or 1000ng/mL
18
Hydrocortisone sodium succinate
administration (1430µg/kg as cortisol)
Hydrocortisone
Hydrocortisone succinate
Rem1: Hydrocortisone production rate up to 25µg/kg/hour
Rem2: bioavailability of hydrocortisone from hydrocortisone succinate is not total
Rem3: Hydrocortisone succinate detectable at 24h
19
Plasma cortisol levels and
synthetic corticoid treatment
20
Adrenal suppression of cortisol secretion
by synthetic GCs
• Synthetic GC are able to
inhibit the production of
cortisol
– negative feedback on ACTH
secretion.
• The duration of the AI effect
of a synthetic GC for a
systemic administration is
generally similar to the
duration of suppression of
endogenous cortisol
Cortisol
Synthetic GC
21
Adrenal suppression
Prednisolone vs. Dexamethasone (IV)
Prednisolone
(600µg/kg)
Short action
Dexamethasone
(50µg/kg)
Longer duration of action
Cortisol concentration (ng / ml)
Cortisol concentration (ng / ml)
100
80
80
60
60
40
40
20
20
0
0 0.25 0.5 1
2
4
8
12 24 48
0
0 0.16 0.5 1
Hours
2
3
4
6 24 48 72 96
Hours
Toutain et al., Am.J.Vet.Res 1985, 9: 1750
22
Adrenal suppression of prednisolone (600µg/kg)
succinate vs. Acetate (IM)
Succinate (IM)
Short action
Acetate (IM)
long action
Cortisol concentration (ng / ml)
100
80
60
40
20
0
0 0.25 0.5 1
2
4
8
12 24 48
days
Hours
Toutain et al., Am.J.Vet.Res 1985, 9: 1750
23
Duration of adrenal suppression (IV route)
Substances
Anti-inflammatory
potency
Mineralocorticoid
potency
Adrenal suppression
(h, days)
hydrocortisone
1
1
About 24h
Hydrocortisone
hemisuccinate
Prednisolone
0
0
A prodrug of cortisol
5
1
About 24h
Methylprednisolone
5
1
About 24h
isoflupredone
25
25
No data
Triamcinolone
5
0
No data
Triamcinolone
acetonide
Dexamethasone
30-50
0
24h
25
0
72-96h
Betamethasone
25
0
Likely similar to DXM
Flumethasone
120
0
Likely longer than for
DXM
24
Adrenal suppression of cortisol
secretion by synthetic GCs:
local administration
• For local administrations, the fraction that
gains access to the blood can be too low to
impact on the adrenal gland function.
25
Adrenal suppression for an intra-articular
methylprednisolone acetate (111mg in toto) administration
Autefage et al., Equine Vet J 1986
26
Adrenal suppression for an intra-articular
methylprednisolone administration
• For the IA route a negative effect on the adrenal
gland persisting for about 4 days after an
administration of 111mg in toto of
methylprednisolone acetate (100mg of
methylprednisolone in toto) was reported
(Autefage et al., 1986).
• In this trial the responsiveness of the adrenal
gland as judged by a series of ACTH tests was not
impaired showing that adrenal suppression
cannot be sensitive enough to be an appropriate
biomarker of the duration of GC articular effect
27
Adrenal suppression for an intra-articular
triamcinolone acetonide administration
• The same approach was used for triamcinolone acetonide
(TA) after an IV dose of 40µg/kg (Soma et al., 2011) with a
recovery of adrenal suppression beginning 19h following
administration (corresponding plasma concentration of
about 1ng/ml) and nearly complete at about 96h
(extrapolated corresponding plasma concentration of about
0.2pg/mL).
• Using the generic PK/PD model described by Toutain and
Lassourd (Toutain & Lassourd, 2002), and considering the
therapeutic TA dose (0.011-0.022mg/kg) (Plumb, 2011),
and the TA plasma clearance (7.46 mL/kg/min), an EPC of
1ng/mL and an IPC of 2-4pg/mL can be computed that is
consistent with the experimental data of Soma et al.2011.
28
IC50 for cortisol suppression
Substances
Cortisol
Prednisolone
Methylprednisolone
Dexamethasone
IC50 (ng/mL)
8
1.2
0.52
0.17
• (Mager et al., 2003)
29
Synthetic derivatives of cortisol
30
Synthetic derivatives of cortisol
• A variety of GCs have been developed to
increase the potency of the antiinflammatory effect and to increase
selectivity by decreasing or even suppress
the mineralocorticoid effects of cortisol
31
The 2 main PD parameters:
ED50/EC50
Emax
1
Emax 1
1
2
2
Emax 2
EC501 EC502
Efficacy
Issue for vets
Potency
Issue for analysts
Potent drug=low LOQ
ECVPT Toulouse 2009 - 32
Efficay vs. Potency
• Potency and efficacy are the two most relevant PD
properties of any drugs including GC and these
two terms should not be confused: a more potent
drug is not necessarily a more efficacious drug.
• Efficacy is a PD parameter corresponding to a
maximal possible effect that can be obtained in a
given test system: it is qualified by a parameter
termed Emax (or Imax for an inhibitory effect).
• It is the most important parameter for clinicians
when the measured dependent variable has a
clinical meaning (lameness score, tidal volume for
respiratory function, lack of mineralocorticoid
activity for a GC, etc.).
33
Cortisone
Δ
GC filiation
Prednisone
C11
OOH
C11
OOH
C6
CH3
Methylprednisone
C11
OOH
Methylprednisolone
C16 & C17
C6
CH3
Cortisol
Δ
Prednisolone
PROPERTY
F
C16
C9
OH - Triamcinolone
CH3 - Dexamethasone
Fluoroprednisolone
(Predef 2X)
C6
C9
Triamcinolone acetonide
- Betamethasone
C16
CH3 - Flumethasone
ANTI-INFLAMMATORY - GLUCOCORTICOID
MINERALOCORTICOID
34
Structure activity relationship
• The introduction of an additional double bound
between C-1 and C-2 of cortisol in all synthetic
corticosteroids selectively increases glucocorticoid and
anti-inflammatory activity that are always parallel.
• This single modification, as in prednisolone, does not
alter mineralocorticoid activity, resulting in an
enhanced glucocorticoid/mineralocorticoid potency
ratio (see Table 1).
• Substances having a ketone radical at C-11 in lieu of a
-OH radical (cortisone, prednisone and
methylprednisone) are prodrugs (or metabolites) of
cortisol, prednisolone and methylprednisolone
respectively
35
Structure activity relationship
•
•
•
•
All halogenated GC are derivatives of prednisolone i.e of the 1-2
dehydrocortisol.
The addition of a halogen at the C-9 position (a fluorine for all GC but a
chloride for beclomethasone 17-monopropionate) enhances both
glucocorticoid and mineralocorticoid activity as in 9α-fluoroprednisolone or
isoflupredone. Isoflupredone lacks selectivity regarding mineralocorticoid
effects when isoflupredone is substituted at the C-16 by an -OH radical
(triamcinolone) or a -CH3 group (dexamethasone, betamethasone), the new
C-16 substituted compounds are practically devoid of any mineralocorticoid
activity while retaining their anti-inflammatory potency.
Tiamcinolone acetonide is a more lipophilic substance without
mineralocorticoid effect obtained by the introduction of an acetonide
between C-16 and C-17.
Flumethasone is a derative of dexamethasone with a supplementary
fluorine in position 6 and fluticasone can be viewed as a derivative of
dexamethasone with two supplementary fluorine in position 6 and
36
Prednisone
• Prednisone is used in horses because it is
inexpensive and tablet formulations are
convenient to administer.
• Prednisone is poorly active in horses to treat lung
conditions as recurrent airway obstruction (RAO)
by the oral route (Robinson et al., 2002).
• This is likely to be due to either poor oral
absorption of prednisone or a failure of the liver to
convert prednisone to the active prednisolone as
no prednisolone is detected in the plasma after an
oral administration of prednisone at 2.2mg/kg
(Peroni et al., 2002).
37
Isoflupredone (Predef 2X®)
• The addition of a fluorine at the C-9 position
considerably enhances both glucocorticoid and
mineralocorticoid activity as in 9αfluoroprednisolone (also named isoflupredone).
• Isoflupredone is marketed as an anti-inflammatory
drug (Predef 2X®) but it lacks selectivity regarding
mineralocorticoid effects and it was shown in
horses that isoflupredone acetate (30µg/kg/day for
14 days by the intramuscular route) was
associated with hypokaliema in the treatment of
RAO (Picandet et al., 2003).
38
Triamcinolone acetonide
• Tiamcinolone acetonide (TA) is a more
lipophilic substance obtained by the
introduction of an acetonide between C-16
and C-17.
• TA is not a prodrug of triamcinolone and
can be used for local administration as for
IA administration in horses to treat
osteoarthritis or as inhalation formulation.
39
Triamcinolone acetonide is 5-20 folds more
potent than triamcinolone
40
Relative potency of corticosteroids
Substances
glucocorticoïd
action
Minéralocorticoïd
action
Duration action
Cortisol
1
1
12h
Prednisolone
5
0.8
24h
Méthylprednisolone
5
0.8
24h
Isoflupredone
25
25
48-72h
Triamcinolone
5
0
24h
Triamcinolone
acétonide
30
0
48-72h
Dexamethasone
25
0
48-72h
betamethasone
25
0
48-72h
Flumethasone
120
0
48-72h
Order of potency of the different corticoids is now internationally accepted and
cannot be reconsidered even if there are some discrepancies in the literature
41
Most GCs are administered as esters
substance
Hydrosoluble
Non hydrosoluble
(IV)
(depot)
Phosphate
Acetate
Succinate
Butyrate
……….
………
42
Most GCs are administered as esters
• Most GC are administered as esters,
• Esterification considerably alters the disposition
and duration of the GC action.
– Esterification with a monoacid (like acetic acid) at C21 gives non-hydrosoluble drugs that can be used as
long-acting formulations when administered by the
intramuscular, subcutaneous or IA routes.
– Esterification of the same parental drug by a diacid
(such as succinic acid) can give a hydrosoluble ester
enabling a salt to be formed (e.g. a sodium
succinate). Phosphate esters are also hydrosoluble.
43
Ester are prodrugs
• Practically all esters except beclomethasone
17-monoprpionate and fluticasone propionate
are inactive prodrugs and have to be
hydrolyzed to release their active moiety
because an OH radical at C-21 is necessary for
the binding of corticoids to their cellular
receptors.
44
Hydrolysis by esterases
• Hydrolysis by esterases may occur either in
different body fluids such as blood or synovial
fluid (as for methylprednisolone acetate for
which the half-life is about 1h in synovial fluid)
or mainly in liver (as for succinate), meaning
that for a local administration, the judicious
selection of an appropriate ester is in order
45
All esters used for systemic
administration are prodrugs
Prednisolone & prednisolone acetate
Hydrolysis
Prednisolone
Prednisolone acetate
A prodrug
46
Prednisolone succinate vs. prednisolone acetate
both are prodrugs of prednisolone (0.6mg/kg)
Prednisolone Acetate IM
Prednisolone succinate (IV,IM)
Time Days
Time H
47
Prednisolone succinate vs. prednisolone
acetate
• illustrates the differences between the
disposition of prednisolone after
administration of prednisolone succinate by
the IV and IM routes and after prednisolone
acetate administration by the IM route.
48
PK parameters of GCs: IV route
Substances
CL
(mL/kg/min)
2.28
Vd
(mL/Kg)
229
HL
(h)
1.55
2.56
600
10.1
3.91
561
1.65
Methylprednisolone
15
3600
2.85
Dexamethasone
7.33
2060
10.7
Triamcinolone
acetonide
8.1
5300
12
Hydrocortisone
(tritiated cortisol)
Hydrocortisone
(after HC
succinate,1g)
Prednisolone
F%
IM:92
49
Corticosteroid esters and local routes
of administration
50
Intra-articular administration
51
Heathrow EHLSC 2008
Musculoskeletal injury following local
corticosteroid injection in Thoroughbred racehorses
• Veterinary records for 1911 thoroughbred
• Thoroughbred racehorses receiving local
corticosteroid injection (LCI) suffer
musculoskeletal injury MSIs at approximately 4.5
times the rate of horses not receiving treatment,
and for horses receiving multiple LCI the rate is
approximately twice that of horses receiving
single LCIs.
Whitton, C., et al 2012Faculty of Veterinary Science, University of
Melbourne, Victoria, 3030, Australia.
53
What joint?
• It seems impossible to
investigate all joints
• The largest and the most
reproducible joint (in
term of administration
accuracy) should be
selected?
54
Volume of synovia and albumin turnover
Joint
Volume mL
Albumine
elimination HL (min)
Hock (tarsal)
39.8
330
Radiocarpal
12.6
146
Intercarpal
14.9
210
Foreleg fetlock
(metacarpo- joint)
12
173
55
Accuracy of IA administration:
blind vs. ultrasound-guided injection
56
Tarsal (Hock) joint
• The largest joint
(40 mL)
• Communicate with
the proximal
intertarsal joint
57
Disposition of methylprednisolone in the
synovial fluid
• Disappearance of MPA within 6 days but a persistence of its
active moiety for 5 to 39 days depending on the horse. The
terminal half-life of MP was between 81 and 261h:
58
Methylprednisolone acetate (MPA): IA
• Disappearance of MPA within 6 days but a persistence
of its active moiety for 5 to 39 days depending on the
horse.
• The terminal half-life of MP was between 81 and 261h:
• the reason for the persistence of MP was most likely
due to a precipitation of MPA that adhers to the
synovial membrane
• This likely explains why MPA is not well tolerated in
horses (McIlwraith, 2010) and that other GC are now
preferred for IA administration, namely betamethasone
esters and triamcinolone acetonide
59
Origin of the sustained release of MP
after a MPA administration
• the reason for the persistence of
MP was most likely due to a
precipitation of MPA that adhers
to the synovial membrane and
most likely acts as a foreign body.
• This likely explains why MPA is
not well tolerated in horses
(McIlwraith, 2010) and that other
GC are now preferred for IA
administration, namely
betamethasone esters and
triamcinolone acetonide
60
Triamcinolone acetonide
• TA is available as a sterile crystalline
suspension.
• This formulation is practically insoluble in
water and provides a depot effect with
constant release of the active agent from the
injection site over a long period of time.
• Apparently TA is well tolerated by the joint
61
Triamcinolone acetonide
• after 10 days and Kay et al (Kay et al., 2008)
reported that the reduction in LPS-induced
lameness persisted for at least 10 days after
TA injection, confirming that a synovial
concentration of 1ng/mL is an effective local
concentration of TA
62
Triamcinolone acetonide
Mean synovial Triamcinolone acetonide (TA) concentration (ng/mL)
vs. time (h) in the metacarpophalangeal joint after an
administration of TA at a dose of 9mg in toto (16µg/kg
Data were fitted with a biexponential equation
to give the terminal half-life of TA in synovial
fluid as 20.2h with an overall mean residence
time of 13.8h.
(redrawn from raw data of Kay et al. (Kay et al., 2008)
63
GC Inhalation
64
Relative anti-inflammatory (AI) potency of topical GC
used for inhalation (1=dexamethasone)
Substances
Beclomethasone
dipropionate
AI Potency
(1=dexamethasone)
0.5
Beclomethasone 17monopropionate
13
Beclomethasone
0.5
Fluticasone propionate
18
Budesonide
9
Triamcinolone acetonide
50
Comments
A prodrug of
beclomathasone
monopropionate
Active moiety of
beclomethasone
dipropionate
Metabolite of
beclomethasone
monopropionate
Active substance
65
Many devices: are they equivalent?
In horses, the pulmonary bioavailability is rather a function of the delivery device
used for inhalation than a property of the drug.
Cortic 00A.66
Disposition of GG after inhalation
Inhalation
Atmosphere
User
safety
Swallowing
Digestive tract
Possible Adrenal
suppression
Lung
disposition
Possible Adrenal
suppression
Yes for beclomethasone
No for fluticasone
67
Inhalation treatment:
an user safety issue?
• Nebulized medication that is released into the
atmosphere from the nebulizer or exhaled by
the patient becomes a form of "secondhand"
exposure that may affect health-care
providers and others in the vicinity of the
treatment
68
Systemic disposition of inhaled GS
• Beclomethasone dipropionate (a prodrug of
beclomethasone 17-monopropionate) and fluticasone
propionate are the two local GC used most in the horse.
• A variety of inhaler devices are used and they deposit GC in
both the lungs and the oropharynx.
• A critical issue for this modality of administration is to
accurately know the delivered dose and also the particle
size distribution.
• The pulmonary bioavailability is rather a function of the
delivery device used for inhalation than a property of the
drug. In clinical investigations, it is important to provide
precise details about the inhaler device and how it was
used because not all devices are equivalent.
69
Fluticasone propionate
• For example swallowed fluticasone propionate is extensively
metabolized by a hepatic first-pass effect and the plasma profile
after inhalation reflects only the deposition in, and absorption
through the lungs with a bioavailability of about 17% in man
(Winkler et al., 2004).
• This also explains that fluticasone propionate in horse (2mg,q 12h)
can decrease neutrophilia in the airways and respiratory effort
without suppressing adrenal gland function (Giguere et al., 2002).
• For a 4 week fluticasone treatment, with a dose of 1980µg q12h for
the first two weeks, followed by a dose of 1100µg per 24h for 1
week and finally 1100µg q for 48h, there was no adrenal
suppression in horse (Couetil et al., 2005).
• The lack of adrenal suppression indicates that this endpoint will be
not be useable to screen horses under fluticasone propionate
treatment.
70
Beclomathasone 17-monopropionate
• Beclomethasone propionate (0.5-1.5mg twice daily):
after inhalation, the fraction of beclomethasone
dipropionate (BDP) gaining access to lung is
immediately metabolized by lung esterases to its active
metabolite, beclomethasone 17-monopropionate
(BMP).
• The fraction of BDP that is swallowed is also
transformed into BMP in the small intestine and the
bioavailability of BMP (about 20% in man) is sufficient
to partially suppress adrenal function in horses by 3550% within 24h for a BDP dose >1mg,BID)(Rush et al.,
1999).
71
Budesonide
• For budesonide, the first-pass inactivation is
89% in man i.e. between fluticasone
propionate (99%) and BDP (80%).
• The mean absorption times which reflect the
duration of lung retention in man are 5-7h for
fluticasone propionate, 2.9h for TA and 1h for
budesonide
72
Inhaled Corticosteroids (ICS)
• Inhaled corticosteroids (ICS) are now
considered the first-line therapy in treating
asthma and are approved for chronic use in
children as young as 12 months of age.
73
Pharmacodynamic of inhaled GCs
• DXM (0.1mg/kg s.i.d) reduces airway obstruction
by day 3 of administration and the maximal
response is obtained by day 7 (Rush et al., 2000).
•
• After the discontinuation of BDP administration,
the return to pretreatment values was also slowly
observed over 3 to 7 days (Rush et al., 2000) and
after fluticasone propionate, the improvement
persisted for 2 weeks when the horses were
placed in a low-dust environment (Couetil et al.,
2005)
74
Pharmacodynamic of inhaled GS
• Such a progressive establishment of clinical
effects and return to pretreatment status
makes it difficult or even impossible to
establish any PK/PD relationship useful for
drug monitoring.
• In addition, the dose-effect relationship is flat
and difficult to establish.
75
Inhalation
• A systemic route is used for severe conditions and a TA
dose of 90µg/kg can relieve airway obstruction for 4
weeks (Rush 2001) (Rush, 2001).
• The pulmonary route offers the advantage of
delivering the drug selectively where it is needed
allowing relatively low doses to be used thus
minimizing adverse systemic effects.
• In addition specific GC have been developed for
inhalation in order to be very potent (to reduce the
administered volume), and to have a long pulmonary
residence time while the systemic elimination remains
rapid.
76
Beclomethasone dipropionate
• Beclomethasone dipropionate is partly
hydrolyzed at position C-21 in the lungs,
resulting in its activation to beclomethasone17-monopropionate thus beclomethasone
dipropionate is a prodrugs that require
activation to exert its anti-inflammatory
activity by binding to pulmonary
glucocorticoid receptors.
77
Determination of a relevant and
an irrelevant plasma
concentration for GC: the use of
PK/PD concepts and their limits
78
Fluticasone propionate
• Environmental management is the most
important factor in the treatment of RAO but
early GC treatment can help accelerate the
recovery of a horse with severe RAO (Couetil
et al., 2005).
• GCs are only a symptomatic treatment and
RAO-affected horses cannot be cured if they
are maintained in a dusty environment
79
How to address the question of irrelevant
concentrations for GCs
80
An example:
The dexamethasone
systemic effect
81
The PK/PD approach to
determine irrelevant plasma or
urine drug concentrations
Steps :
1: effective plasma concentration (EPC)
2: Irrelevant plasma concentration (IPC)
3: Irrelevant urine concentration (IUC)
82
Risk assessment:
determination of IPC and IUC
83
Computation of an Effective
Plasma Concentration (EPC)
Effective _ Dose
EPC 
Clearance
84
Step 1 : example of Dexamethasone
• Standard dose: equivalent to 40µg/kg/24 h
• Plasma clearance: 440 mL/kg/h or about 10000
mL/kg/24 h (Soma et al JVPT 2005)
40 000ng.kg-1.24h-1
= 4 ng/mL
EPC =
-1
-1
1000 mL.kg .24h
Note 1: from PK/PD experiment, 1 to 4 ng/mL
85
Step 2: computation of
irrelevant plasma
concentrations
• An IPC can be deduced from EPC by
applying a safety factor (SF) to EPC:
EPC
IPC =
SF
!
How to select SF?
86
Safety factor: default value=500
• I proposed a default SF = 500
500 = 50 x 10
Interindividual horse variability
Transform an
EPC into an IPC
for a given
horse
PK variability
PD variability
3.3
3.3
87
DXM: computation of an irrelevant
plasma concentration (IPC)
EPC 4000 pg / mL
IPC 

 8 pg / mL
SF
500
50 X 10
by rounding up : 10 pg/mL
88
Relationship between the CV% and the
validity of the default uncertainty factor
(Log-normal distribution)
For man and a variety of
substances, the number of
subjects not covered by a
factor of 3.16 is about of
1/117 for PK and 1/52 for
PD
The probability of the same
individual falling outside the
range for both PK and PD is
about 1/6172
Probability of false positive for our thresholds (as cortisol): 1/10000
ECVPT Toulouse 2009 - 89
Duration of action of DXM on cortisol
plasma level
• DXM solution
– IV, 50µg/kg
• Duration of action:72h
• Plasma concentration at
72h
(Soma et al JVPT 2005)
Step 3: determination of irrelevant
urine concentration (IUC)
IUC = IPC x RSS
Plasma
steady state urine to plasma
concentration ratio
concentration
Urine
10
Rss
urine
1
Plasma
Pseudo-equilibrium state
(time)
91
Dexamethasone IV
Individual Data (n=12)
Dexamethasone (ng/mL)
1000
O
Plasma
O
Urine
100
Rss≈10
10
1
0.1
0
10
20
30
40
50
60
Time (h)
92
Step 3: determination of irrelevant
urine concentration (IUC)
IUC  IPC  Rss  10 pg / mL 10  100 pg / mL
Risk analysis for DXM
1. Risk assessment
IPC=10pg/mL
IUC=100pg/mL
1. Risk management
Selection of a screening limit
Science + other considerations
differences expected between organizations:
harmonization
2. Risk communication
MonteCarlo-Orlando06 - 94
The 3 main steps of a Risk analysis
1. Risk assessment
2. Risk management
3. Risk communication
Large differences expected
between organization with many possible options to
communicate with stakeholders
Exchanges of information
MonteCarlo-Orlando06 - 95
Determination of a relevant and
an irrelevant plasma
concentration for GC for a local
route of administration and a
local effect
Marker plasma concentration (MPC)
EPC
Plasma concentrations
as a driving force
controlling biophase
concentrations
MPC
Plasma concentrations
as marker
of biophase exposure
Biophase
Effect
ELBC
Biophase
Effect
97
The case where plasma concentration is not
the driving force controlling effect
• For any drug administered locally to
develop a local effect, plasma drug
concentrations is not the driving force
controlling drug concentrations at the
biophase (i.e.the site of action).
• Plasma concentration It is only a marker of
a local exposure
98
Two preliminary definitions
1. Effective local biophase concentration
(ELBC)
•
is the biophase drug concentration for which
effects are observed (typically reported as a
EC50)
2. Marker Plasma Concentration (MPC)
•
MPC is the plasma concentration corresponding
to the ELBC
99
The MPC vs. EPC
• At variance of EPC, MPC is not a pharmacodynamic
parameter expressing substance potency
• It is an exposure variable determined by the
administered pharmaceutical form and route of
administration
– A single EPC vs. several possible MPCs
– With EPC , we can control drug effect
– With MPC , we can help vets to ensure good veterinary
practices
100
Step1: define an ELBC
• To select an ELBC, it is reasonable to assume
that the ELBC is of the same order of
magnitude as the plasma EPC or an EC50
obtained from some relevant in vitro test
system,
–.
101
Triamcinolone acetonide
• Kay et al., 2008 reported that the reduction in LPSinduced lameness persisted for at least 10 days
after TA injection, suggesting that a synovial
concentration of 1ng/mL is an effective local
concentration of TA thus ELBC=1000pg/mL
102
Step 2: Select an articulation and a
dose fitting good veterinary practices
• Very different veterinary practices
• Require harmonisation
• For triamcinolone acetonide, dose is around
10 mg per joint
103
Step 3: perform a PK study of the substance
disposition in the synovial fluid
2. Synovial drug concentration
1-Agreed
Dose
2-ELBC
Time of efficacy
1. Agreed ELBC
e.g 1000 pg/mL
Plasma concentrations
Time when effect vanish
3-MPC
ELBC: effective local biophase concentration
MPC: irrelevent marker plasma concentration
Time
104
Step 4: computation of an MPC for
triamcinolone acetonide
Such a low plasma concentration is not detectable with the current analytical techniques
(LOD of about 10pg/ml) and the plasma concentration cannot be used to monitor an IA
administration of TA
This is due to the high TA plasma clearance (8mL/kg/min)
105
Step 5:Transformation of an MPC
to an irrelevant MPC (IMPC) for all
horses
MPC MPC
IMPC 

SF
10
To take into account interanimal variability
106
Step 5: computation of an IMPC for
triamcinolone acetonide
Such a low plasma concentration is not detectable with
the current analytical techniques (LOD of about
10pg/ml) and the plasma concentration cannot be used
to monitor an IA administration of TA
107
Step 6: Irrelevant urine concentration
(IUC) for triamcinolone acetonide
Even for urine, it is impossible to control properly an IA of TA i.e.
to guarantee to actually be able to detect urine concentrations
corresponding to the irrelevant synovial TA concentrations.
ISL=500 pg/mL
108
What is the synovial concentration of
TA that can be controlled with a LOD
of 10 pg/mL in urine?
• A: about 8.8ng/ml i.e. at day 7 i.e. much
above the effective synovial concentration
• It would be necessary to follow TA up to 13
days after a IA administration of 9mg in toto
of TA to guarantee a lack of effect
109
ISL for TA: 500 pg/mL
ISL
urine
500pg/mL
Synovial fluid
Plasma
500ng/mL
50pg/mL
(ELBC=1ng/mL)
TA dispostion in synovial fluid
110
MPC vs. EPC & IPC
• An MPC is not a PD parameter but a variable
associated with a given formulation, for a given
dose, a given joint etc.
• An IMPC cannot be used to derive a universal SL
to monitor drug efficacy;
• A given MPC/IMPC can only be used to determine
a relevant detection time for that formulation in
order to promote good veterinary practices but
not to surely select an SL
111
Multiplicity of MPCs/IMPCs for DXM
DXM administration
Same total dose
Solution
Rapid
Suspension
Slow
MPC1>MPC2
Plasma
MPC 1
Plasma
MPC 2
112
EPC vs MPC
• For example, the EPC of dexamethasone is about
1-4 ng/mL whatever the ester and whatever the
route of administration as long as we are looking
at some systemic effect.
• By contrast, the dexamethasone MPC for an intraarticular administration will be different for a
phosphate and an acetate formulation, for a
solution or a suspension of the same
isonicotinate ester or for an intra-articular vs.
inhalation of the same TA formulation, etc.
113
Step 6: risk management
• As in the transformation of an EPC into an IPC, it is
necessary to apply a safety (uncertainty) factor to the MPC.
• It should be realized here that the MPC determined is a
non-effective marker plasma concentration (or
alternatively, as the lower limit of efficacious
concentrations).
• Therefore it can be suggested that a default uncertainty
(safety) factor of 10 is appropriate and not 500 as in the
transformation of an EPC into an IPC, because only the
variability of the PK and the PD has to be considered in this
case.
• As for the IPC, we may coin the name of Irrelevant MPC or
IMPC for this value that will be given to the risk manager to
finally select a screening limit.
114
Control of medication
LOD
SL
115
Control of medication
LOD
SL2
SL1
116
To support good
veterinary practices
Control of medication
LOD
SL
SL2
SL1
117
Multiplicity of MPCs and a single SL:
• For glucocorticoids (GCS) administrable by
both local and systemic routes, the selection
of a single screening limit (SL) for medication
control will be ineluctably a compromise
between different possible values derived
either from an irrelevant plasma
concentration (IPC) or one of the several
known irrelevant marker plasma
concentrations (IMPC).
118
MPC for inhalation
• Use a pragmatic approach
• Selection by consensus of a DT
• Select as SL plasma/urine concentration
corresponding to the DT (HK approach)
119
Conclusions
1. GCs are well known for their systemic effect
2. GCs are not very well documented (PK & PD) after a local
administration (IA, Inhalation)
3. Computation of IPC and IUC for systemic effect is easy
4. Computation of IPC and IUC is more challenging for local
administration(concept of MPC)
5. It exist a single IPC for systemic effect but a multiplicity of
possible IMPCs for local administration
6. Selection of a single screening limit will be a compromise
7. Analytical technics are unlikely able to control all kind of
local administration
120