This document discusses multiple dosage regimens including loading doses, bioavailability, and repetitive intravenous injections. It defines loading dose as the initial dose needed to achieve the target concentration based on volume of distribution and bioavailability. It describes bioavailability as the fraction of the administered dose that reaches systemic circulation, and distinguishes between absolute and relative bioavailability. It also defines bioequivalence as formulations having absorption rates and extents that are not statistically different. Finally, it examines repetitive intravenous injections and how the drug amount remaining in the body decreases exponentially with each dose based on the elimination constant and dosing interval.

1. Multiple Dosage Regimens
Submitted by,
Chandon Chakma
PHA-16038
Session:2015-16
MBSTU
Submitted to,
A.H.M. Mazbah Uddin
Lecturer,
Department of Pharmacy
MBSTU
Course: Biopharmaceutics & Pharmacokinetics II

2. Loading Dose
Loading Dose
Amount of drug in the body
Bioavailability (F) x Volume of distribution (Vd)
=
 Amount of drug in the body = Target conc. (C) x Volume of d
Loading Dose =
C x Vd
F x S

3. Bioavailability
Bioavailable dose
Administered dose
F =
 Measurement of the relative amount & rate at which ,
the drug from administered dosage form , reaches the
systemic circulation & becomes available at the site of
action.
 Bioavailable fraction (F) , refers to the fraction of
administered dose that enters the systemic circulation.

4. Types of Bioavailability
Absolute Bioavailability :-
If the systemic availability of a drug administered orally is
determined by doing its comparison with I.V. administration
, it is known as absolute bioavailability.
F=
AUCExtravascular
Dose Extravascular
Dose Intravenous
AUCIntravenous

5. Types of Bioavailability
Relative Bioavailability :-
If the systemic availability of a drug administered orally is
determined by doing its comparison with that of an oral
standard of the same drug , it is known as a relative
bioavailability.
Frel =
AUCExtravascular 1
Dose Extravascular 1
Dose Extravascular 2
AUC Extravascular 2

6. Bioequivalence
Operationalized as :
 Compared exposure in terms of AUC and Cmax of the plasma Conce-
-ntration vs time curve
 Conclude bioequivalence if the confidence for the ration between
the formulations lies between 0.8 and 1.25 for both AUC and Cmax.
FDA definition :
Pharmaceutical equivalents whose rate and extent of ab-
sorption are not statistically different when administered
to patients or subjects at the same molar dose under sim-
ilar experimental conditions.

7. -kt
 The maximum amount of drug in the body following a
single rapid IV injection is equal to the dose of the drug.
 For a one-compartment open model , the drug will be
eliminated according to first-order kinetics.
DB = Doe
 If 𝜏 is equal to the dosage interval , then the amount of
drug remaining in the body after several hours can be
determined with :
DB = Doe
-k𝜏
Repetitive Intravenous Injections

8. Repetitive Intravenous Injections
 The fraction (𝒇) of the dose remaining in the body is
related to the elimination constant and the dosage interval
(𝜏 )as follows :
𝒇 =
𝑫𝑩
𝑫𝑶
= 𝒆−𝒌𝒕
 If 𝜏 is large , 𝒇 will be smaller because DB (the
amount of drug remaining in the body) is smaller.

9. Repetitive Intravenous Injections
 If the dose Do = 100mg is given by rapid injection every 𝑡1
2
,then 𝜏 = 𝑡1
2
.
 After the 1𝑠𝑡
𝑡1
2
the DB = 500mg.
 After the 2𝑛𝑑 𝑡1
2
the DB = 150mg.
 After the 2𝑛𝑑
𝑡1
2
the DB = 75mg.
 After the 3𝑟𝑑 𝑡1
2
the DB = 175mg and so on.
You will reach equilibrium at which :
Dmax = 200mg and Dmin = 100mg