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Stability of Drug Preparations. Chapter 12. I. Introduction. A.Importance Stability is the guarantee of safety and effectiveness of any preparations B.Types of stability studies (1)chemical one: chemical degradation (2)physical one: physical appearance

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i introduction
I. Introduction

A.Importance

Stability is the guarantee of safety and effectiveness of any preparations

B.Types of stability studies

(1)chemical one: chemical degradation

(2)physical one: physical appearance

(3)biological one: microorganism pollution

(4)stability of bioavailability: in vivo

ii chemical kinetics and drug stability
II. Chemical kinetics and drug stability

A. Orders of reactions

-dC/dt=kCn

where -dC/dt is the rates of change for the reactants; k is the reaction rate constant; C is the concentration; n is the order of the reaction (n=0: zero-order; n=1: first-order; n=2: second-order…)

slide4
Rate Expressions for Zero-, First- and Second-Order Reactions

second-order

zero-order first-order a=b=c0 a≠b

Differential rate -dc/dt=k -dc/dt=kc -dc/dt=kc2 -dc/dt=kcacb

expression

Integrated rate k=(c0-c)/t k=(1/t)ln(c0/c) 1/c-1/c0=kt

expression

t1/2 c0/(2k) 0.693/k 1/(c0k)

t0.9 c0/(10k) 0.105/k 0.11/(c0k)

slide5
B. The Arrhenius equation

(1)showing the effect of temperature on the drug degradation rate

(2)integrated: k=Ae-Ea/RT

logarithmic: lgk=-Ea/(2.303RT)+lgA

rewritten as: ln(k1/k2)=(Ea/R)(1/T2-1/T1)

where Ea is activation energy (a constant and independent of temperature); 1 and 2 denote the two different temperature conditions; k is the constant of reaction rate; R is gas constant

slide6
(3)It is possible to conduct kinetic experiments at elevated temperature and obtain estimates of rate constants at lower temperatures by extrapolation of the Arrhenius plot (Accelerated stability testing)
iii routes by which pharmaceuticals degrade
III. Routes by which pharmaceuticals degrade

A.Chemical degradation routes

(1)hydrolysis

(2)oxidation

(3)dehydration

(4)isomerization

(5)incompatibilities

(6)others: hydration, decarboxylation, pyrolysis

slide8
(1)hydrolysis: esters (lactone) and amide (lactam)

methods for delayed hydrolysis:

adjusting pH

controlling water content

controlling T

reduce the solubility of drugs

solid forms

slide9
(2)oxidation: phenols, enols, unsaturated alcohol, arylamine

mechanism: reaction of free radical chains

induction: RH R• +H • (light, heat)

transmission: R • +O2 RO2 •

RO2 • +RH ROOH +R •

ROOH RO • + •OH (metal ion)

termination: RO2 • +x inactive product

RO2 • + RO2 • inactive product

slide10
methods for delayed oxidation:

reduce oxygen content

adjusting pH

reduce metal ion

lower T

avoid light

slide11
B. Physical degradation routes

(1)vaporization

(2)aging

(3)adsorption

(4)physical instability in heterogeneous systems (suspensions, emulsions, creams and ointments)

iv formulation and environmental factors that affect reaction rate
IV. Formulation and Environmental factors that affect reaction rate

A.pH—hydrolysis

(1)lgk versus pH profiles of different drugs

(specific acid-base catalysis)

slide13
1.0

F

HS

S-

0.5

0.0

2

3

4

5

6

pH

slide14
14

12

10

k,10-4s-1

8

6

4

5

6

7

8

9

10

11

pH

slide15
-3

25℃

-4

-5

-6

12

0

2

4

6

8

10

pH

slide16
-3.7

79.5℃

I=0.5

-3.9

lgk,s-1

-4.1

-4.3

-4.5

12

0

2

4

6

8

10

pH

slide17
-3.5

80℃

-4.0

lgk,s-1

-4.5

-5.0

-5.5

12

6

7

8

9

10

11

pH

slide18
-3

60℃

-4

lgk,s-1

-5

-6

2

4

6

8

10

12

14

0

pH

slide19
-3.5

35℃

I=0.5

-4.5

lgk,s-1

-5.5

-6.5

12

0

2

4

6

8

10

pH

slide20
-5

70℃

-6

lgk,s-1

-7

7

8

6

0

1

2

3

4

5

pH

slide21
-4.2

91.3℃

-4.4

lgk,s-1

-4.6

-4.8

0.5

0.7

0.9

1.1

1.3

1.5

1.7

1.9

pH

slide22
-2

25℃

-3

lgk,s-1

-4

-5

2

4

6

8

10

12

14

0

pH

slide23
(2) method: the optimum pH for stability—pHm

calculating:pHm=1/2pKw-1/2lgkOH-/KH+

through testing: a series of solutions with different pH values—accelerated testing—lgk~pH profiles—pHm

(3)general acid-base catalysis

PBS, ABS

method: change the type or reduce the concentration

slide24
B. solvent—hydrolysis

lgk=lgk∞-k’ZAZB/ε

where k is the reaction rate constant, k’is a constant, εis the dielectric constant,k∞ is the reaction rate constant when ε∞,ZA and ZB is the electric charge of the two ions of A and B, respectively

slide25
C. ion strength

lgk=lgk0+1.02ZAZBI1/2

where k is the reaction rate constant, k0 is the reaction rate constant when I=0, ZA、ZB is the electric charge of two ions,respectively,I is the ion strength

slide26
D. Surfactants

enhance or decrease the stability , determined by the results of testing

E. Other excipients

determined by the results of compatibility testing in order to choose correctly

slide27
F. Temperature

In general, the higher T is, the faster the reaction rate is

Arrhenius equation

G. Light—oxidation, photodegradation

Avoid light during preparation and storage package is very important

slide28
H. Air (oxygen)—oxidation

inert gas (N2, CO2)

vacuum-packed

reducing agents

adding antioxidants blockers of oxidation

synergists

(note: pH value range in which antioxidants are suitable to application)

p272

slide29
I. Metal ions—initiate oxidation reactions

employ raw materials and excipients with higher purities

do not use metal instruments

use chelating agents (EDTA, citric acid, and tartaric acid)

J. Humidity (water)—major determinant of drug product in solid dosage forms

lower RH% during preparation

put drying agents in the package

slide30
K. Package materials

glass, plastics, aluminum foil etc

“package evaluation”

v stability and degradation kinetics of solid drug preparations
V. Stability and degradation kinetics of solid drug preparations

A. Properties of stability of solid drug preparations

(1)degradation slowly

(2)be not uniform

(3)difference between exterior and interior

(4)multi-phase systems

(5)obtain a balance [Van’t Hoff equation: lnK=-ΔH/(RT)+α]

(6)effect of crystal form

slide32
B. Chemical degradation kinetics

(1)nucleation theory

(2)liquid-layer theory

(3)topochemical reactions

vi stability testing in the pharmaceutical industry
VI.Stability testing in the pharmaceutical industry

A. Impact factor testing (Stress testing)

high T (60℃, 40 ℃)

high H (25 ℃, 75±5%, 90±5%) 10d

strong light (4500±500lx)

slide34
B. Accelerated testing

done more frequently and for a shorter duration

(1)in general, three batches, with package, 40±2 ℃, RH75±5%, 6m(3m for clinical testing and 6m for production)

(2)specific preparations with various testing conditions

(3)obtain “tentative” expiry date (shelf time)

slide35
C. Long-term testing

(1)in general, three batches, with package, 25±2 ℃, RH60±10%, 6m for clinical testing, 12m for production and go on

(2) specific preparations with various testing conditions

(3)obtain “definitive” expiry date

slide37
F. Classical isothermal method--done in research

(1)pre-testing to determine Ts and sampling time; determine analysis methods

(2)put samples at predetermined Ts, take a sample at predetermined times (t), and determine the drug concentrations

(3)obtain profiles of C ~t, and determine the reaction order (lgC~t: linearity, first-order)

(4)according to the equation: k=(1/t)ln(C0/C), obtain k at different Ts

(5)according to Arrhenius equation:

lgk=-Ea/(2.303RT)+lgA, obtain profiles of lgk~T

(6)calculate t0.9, k25 ℃, Ea , lgA

slide38
G. Stability testing in new medicine development

(1)raw materials

(2)stability in formulation and preparation process study

(3)stability of package materials

(4)accelerated and long-term testing of preparations

(5)stability after marketing

(6)stability testing for any change in formulation, preparation process or package

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