The document discusses pharmacokinetics of multiple dosing. It defines dosage regimen as how a drug is taken when treatment duration is longer than a single dose's therapeutic effect. The objectives of dosage regimen design are to achieve target drug concentrations. Multiple dosing is used when treatment duration is larger than a single dose's effect, leading to drug accumulation. Loading and maintenance doses are discussed to quickly reach the desired plasma concentration and maintain it.

1. PHARMACOKINETICS OF MULTIPLE
DOSING
UNDER THE GUIDENCE OF
Dr.SATYABRATA BHANJA
M. Pharm, Ph.D.
Department of Pharmaceutics
Mallareddy College Of Pharmacy
PRESENTED BY :
K.V.SIVA PRASAD
M.Pharm(Pharmaceutics).
256213886018
Mallareddy College of pharmacy
30 September 2014 1

2. CONTENTS
• Dosage regimen
• Objectives of dose regimen
• Therapeutic drug monitoring
• Multiple dosage regimen
• Multiple dosing with respect to I.V.
• Multiple dosing with respect to Oral route.
• Concept of loading dose, maintenance dose.
• Drug accumulation
 References
2

3. Dosage regimen
 It is defined as the manner in which a drug is taken.
For certain analgesics, hypnotics, anti-emetics etc. a
single dose may provide an effective treatment. But the
duration of most illness is longer than the therapeutic
effect produced by a single dose. In such cases drugs
are required to be taken on a repetitive basis over a
period of time.
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4. Objective of dosage regimen
 The overall objective of dosage regimen design is to achieve
a target drug concentration at the receptor site
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5. Examine patient, collect data, and make diagnosis
Define therapeutic objective
Choose Drug and Dosage Regimen
Evaluate how
well objective
has been
achieved
Modify Regimen
Or
Change Drug
Modify
objective
Stop
Therapy
Continue
Regimen
Steps in the initiation and management of the drug
therapy
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6. Therapeutic Drug Monitoring
 The success of Drug therapy is highly dependent on
Choice of Drug and Drug Product
Design of the Dosage Regimen
 While the choice of Drug and Drug product is based on
Patient's characteristics
Pharmacokinetic of Drug
 Every patient have different Drug absorption, distribution, and
elimination as well as different pathophysiological condition, so
for the improvement in the clinical effectiveness of the drug
THERAPEUTIC DRUG MONITORING are done.
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7.  It specializes in the measurement
of medication levels in blood. Its main focus is on
drugs with a narrow therapeutic range, i.e. drugs
that can easily be under- or overdosed.
 Therapeutic drug monitoring is important as
Insufficient levels of drug in the plasma will lead
to under treatment or resistance, and excessive
levels can lead to toxicity and tissue damage.
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8. Indications for TDM
include:
 There is narrow therapeutic window.
 There are potential patient compliance problems.
 The drug dose cannot be optimized by clinical
observation alone.
 Knowledge of the drug level influences
management.
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9. Sampling and drug analysis
 Usually, plasma or serum is used for drug assays.
 Drug assay methods should have adequate sensitivity,
be specific for the drug (or metabolite) to be measured
and have appropriate accuracy and precision.
 Automated immunoassay methods, High performance
liquid chromatography (HPLC) and Gas liquid
chromatography (GLC) (e.g. amiodarone, perhexiline)
can be used.
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10. Examples of drugs analyzed by therapeutic drug
monitoring :
10

11. BBDNITM , Lucknow
11
Function of Therapeutic Drug Monitoring
Selection of Drug
Dosage Regimen Design
Evaluation of Patient Response
To Determine need for measuring serum drug conc.
Monitoring serum drug concentration
Assay of drug concentration in biological fluid
Pharmacokinetic evaluation of drug concentration
Recommend special requirement.

12. Multiple dosage regimen
 When the duration of treatment of disease is smaller than
the therapeutic activity of drug, single dose are given e.g.
Aspirin
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13. When the duration of treatment of disease is larger than
the therapeutic effect of drug, Multiple dosage regimen
are given e.g. antibiotics
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14. Multiple dosing with respect to
oral route
 When an oral multiple dosing regimen is followed,
plasma conc. will increase, reach a maximum and
begin to decline. A 2nd dose will be administered
before all of the absorbed drug from 1st dose is
eliminated.
 Consequently plasma conc. resulting from 2nd dose
will be higher than from 1st dose. This increase in conc.
with dose will continue to occur until a steady state is
reach at which rate of drug entry into the body = rate
of exit
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15. Plasma drug Conc v/s time
15 15
Plasma drug conc.
time
C max
C min

16. Multiple dosing with respect to I.V.
 On repeated drug administration, the plasma conc.
will be added upon for each dose interval giving a
plateau or steady state with the plasma conc.
fluctuating between a minimum and maximum
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17. Plasma drug concentration v/s time
17 17
Css max
Steady state
Css min
time
Plsama drug conc.

18. Drug Accumulation -
When the drug is administered at a fixed dose and a
fixed dosing interval, “accumulation occur because
drug from previous dose has not been remove.”
Accumulation of drug depend upon the dosing
interval and elimination half life and is independent
of the dose size.
Accumulation index = 1
1 – e- KE τ
τ= Dosing interval
KE = Elimination half life
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19. In drug accumulation,
Time for 90% steady state plasma conc. – 3.3 times
of elimination half life.
Time for 99% steady state plasma conc. – 6.6 times
of elimination half life
2X0
1X0
Xo +½X0
0τ Dosing interval in hr ( τ = t½)
5τ
Amount of drug in the body
X0 X0 X0 X0 X0
Upper
Asymptote,
Xss,max
Steady state
Lower
asymptote,
½Xo Xss,min

20.  e. g. of Drug Accumulation
 For a avg. adult , rate of metabolism of ethanol is 10
gm/hr
 45 ml of whiskey contain 14 gm of ethanol.
 If drink 45 ml of whiskey every hr, will accumulate 4
gm ethanol per hr and develop coma in 48 hr.
 However, can drink 30ml whiskey ( 9 gm ethanol)
every hr.
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21. 21
After single rapid IV injection,
DB = D0e-k t
If the τ is the dosing interval, then amount of drug
remaining in the body after several hr,
DB= D0e-k τ
The fraction of the Dose remaining in the body
f = DB/D0 = e -k τ

22. 22
Problem of missed Dose -
Plasma drug conc. at t hr after the nth dose-
Cp = D0 e ( 1 – e )/ VD -k t -nk τ ( 1 – e ) ……1
Conc. contributed by missing dose is
-k τ
Cp’ = D0 e - k t m is s/ VD……………………………2
So missing dose, (eq 1 – eq 2)is
Cp = D0(1 – e -nk τ )(e -k t - e -k tmiss
)/VD( 1 – e -kt
)

23. 23
 The drug may not reach steady state
Short IV infusion Elimination period
Another short IV
infusion
Conc. after one or more short IV infusion
Cp = D ( 1 – e ) / tinf VD k -k t

24. 24
Where , D / tinf = Rate of infusion .i.e. R
D = Size of infusion dose
tinf = infusion period
VD = volume of distribution
k = elimination rate constant
Drug conc. post IV infusion is
Cp = Cstop e-kt
Where, Cstop = Conc. when infusion stop
t = time elapsed since infusion stopped

25. 25
The plasma conc. at any time during oral multiple
dose regimen,
Cp =
F.kaDo
VD(k – ka)
-nka τ
1 - e
1 - e
-ka τ
e-kat 1 – e
1 - e
-nk τ
-k τ
e-k t
Where, n= no. of doses
τ = dosing interval
F = fraction of dose absorbed

26. 26
FOR MULTIPLE ORAL DOSE
∞
Cav = F D0 / ClT τ
∞ -k tp - kτ
Cmax = F Do e / VD ( 1 – e )
Cmin = ka F D0 e / VD ∞ - kτ ( ka – k )(1 – e- k τ )

27. Max. Conc. & min. Conc. during multiple dosing
27
Css,max = C0 / 1 – e
-kτ
Css,min = Css,max . e
-kτ
Ratio of Cmax / Cmin is known as fluctuation,
Greater the ratio, greater the fluctuation.
Css,av = F X0 / ClT .τ.

28. An initial or first dose intended to be therapeutic is
called as priming or loading dose
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D0,L = Css, av VD / F
For 1 compartment model,
LD =
For 2 compartment model,
VD ×D(P)target LD =
F
VD(ß) ×D(P)target
F

29. 29
∞
OBJECTIVE- to achieve desired plasma conc., Cav , as
quickly as possible.
DImax =1.44 t½ × log TW
Route of Drug administration ( MD /DI)=
D(P)target × Cl
F
While, TW=
MSC
MEC

30.  A maintenance dose is given to maintain Cav
∞ and
steady state so that the therapeutic effect is also
maintained
 The ratio of loading dose to maintenance dose
 X0, L / X0 , is called as Dose ratio
 When, τ = t½ , dose ratio should be equal to 2
 τ > t½ , dose ratio should be smaller than 2
 τ < t½ dose ratio should be greater than 2
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31. Dose ratio > 2
(τ < t ½)
Dose ratio = 2
(τ = t ½)
MSC
31
Maintenance Doses X0
Dosing interval in hr
Plasma Drug Conc.
MEC
Dose ratio < 2,
Loading dose
X0 X0 Xo Xo X0
(τ > t ½)

32. Loading dose
Time( in hr)
Plasma conc.(μg/ml)
MSC
MEC
Therapeutic
range
Maintenance
dose
Sustained
release
Prolonged
release
Convention
al dose
Plot showing the effect of loading dose and maintenance dose.

33. REFERENCES
Shargel Leon, Andrew Yu B.C., “Applied
biopharmaceutics and pharmacokinetics” , 5th edition,
published by Mc Graw Hill ,page no. 185 .207,613-625.
 Gibaldi M., “Biopharmaceutics And Clinical
Pharmacokinetics” 4th edition, Pulished by Pharma Med
Press,Page no.345.
Notari Robert E., “Biopharmaceutics & clinical
pharmacokinetics.”, Fourth Edition, published by
Marcell Dekker, page no.351-397.
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