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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5320_Библиотеки_им_академика_М_И_Перельмана
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Pharmaceutical Dosage Forms and Drug Delivery
7.3.3.2 Acid– Base Catalysis
+
+ ion
specic or general catalysis
+ ions.
rate, and proton transfer from an acid to the solvent or from the solvent to a base is the rate- limiting step.
7.3.3.3 pH– Rate Profile
k
obs
) on
the yx-
isoelectric point
FIGURE 7.5
acidic (b), or both acidic and basic conditions (c and d).
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Chemical Kinetics and Stability
pH of optimal stabilityk
7.3.4 Cosolvent and Additives
PEG, propyleneglycol (PG), and ethanol. In add-
of the reaction medium, such as dielectric constant, ionic strength, surface tension, and viscosity, all
7.3.4.1 Drug– Excipient Interactions
degradation.
Buffer salts
2PO
4
4
excipient compatibility studies
strategies.
7.3.4.2 Catalysis
catalysts.
but not the equilibrium concentration of reactants and products in the reaction. As seen in Figure 7.6,
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Pharmaceutical Dosage Forms and Drug Delivery
FIGURE 7.6 x
(on the y
FIGURE 7.7
a catalyst inuences the speed but not the extent of a reaction
7.4 Drug Degradation Pathways
7.4.1 Hydrolysis
Figure 7.7).
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Chemical Kinetics and Stability
7.4.1.1 Ester Hydrolysis
in Figures 7.8 and 7.9
7.4.1.2 Amide Hydrolysis
FIGURE 7.8
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160
Pharmaceutical Dosage Forms and Drug Delivery
FIGURE 7.9
7.4.1.3 Control of Drug Hydrolysis
-
ition, desiccants
shelf life.
nonaqueous cosolvents
less soluble
Complexation
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Chemical Kinetics and Stability
7.4.2 Oxidation
161
Figure 7.10
*
Figure 7.10 indicates antibonding molecular orbitals.
2
1
2, 2s2, 2p
x
, 2p
y
, 2p
1
z
2
2
2
2
*1
*1
2, 1s*2, 2s2, 2s*2, 2p
2, 1s*2, 2s2, 2s*2, 2p
x
2
x
, 2p
, 2p
y
y
, 2p
2
, 2p
z
z
, 2p
2
, 2p
, 2p
x
*2
,2p
x
*0
, 2p
y
z
*0
*0
, 2p
y
z
FIGURE 7.10
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162
Pharmaceutical Dosage Forms and Drug Delivery
lag time in their detection,
3+
/ Fe
and Cu+ / Cu
2+
2+
Fenton’s systems.
radical end products.
7.4.2.1 Control of Drug Oxidation
• Inhibiting the initiation and/ or propagation phases
• Promoting chain termination
1. Inhibitors of initiation: Compounds that prevent the initiation phase of the free radical- mediated
2. Free radical terminators
3. Reducing agents
hc
==
λ
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Chemical Kinetics and Stability
163
7.4.3 Photolysis
E h
ν
(7.68)
Ehc8m/ sec); ν is the freλ
photosensitization
quencher
Type I photooxidation, also called an electron transfer
Type II photooxidation
reactive since it has a similar spin state as ground- state organic molecules.
7.4.3.1 Control of Photodegradation of Drugs
Control strategies to prevent drug photodegradation can include using amber- colored glass containers
-
dine tablets.
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164
7.5 Stabilization Strategies
Pharmaceutical Dosage Forms and Drug Delivery
Ka, salt/ counterion, propen
•
•
microenvironment in the drug product that disfavors the disproportionation of the drug.
• -
Review Questions
7.1
Figures 7.17.3.
i
ii First- order reaction
iii Second- order reaction
A t, inde-
B
C
D -
E
F -
+→ +
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Chemical Kinetics and Stability
t (and
I t (and half-
7.2
A
B Arrhenius equation
C
D
E
7.3
A
B Arrhenius equation
C
D
E
7.4
A
B
C
D
A Calculate k.
B
7.6 For a second- order reaction,
Determine the rate constant, k, for the reaction and the half- life.
7.7
k1 and k2 -
R. mol).
7.8
7.9
o
7.10
COOC H KOH CHCOOK CHOH
25 25 25 25
- 6 s- 1
at 120 o
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