Physical Chemistry for the Chemical and Biological Sciences

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SECTION 9: Kinetics Chapter 12 in Chang Text
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Outline for Section 9 Outline for Section 9 – Part I Part I 9.1. Kinetics in Pharmaceutical Science 9.2. Rates of Reactions 9.3. Reaction Order: Zero-Order, 1 st Order
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9.1. Chemical Kinetics in Pharmaceutical Science Why do we, as pharmaceutical scientists, care about Chemical Kinetics and Kinetics in General ? We use rate equations and rate laws to define and describe processes involved in everything from …………. . “Making the Drug” to Formulation to Drug Administration in Animals
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Rates are important for the following: • rate of reaction to synthesize a drug molecule …..…. .rate of side reactions under specific conditions •rate of decomposition, degradation, biotransformation, and inactivation of a drug • rate of precipitation of a drug from a formulation under specific conditions ±of ±re lease of a drug from a delivery system ±uptake of drug into cells • rate of reaction between drug and biological target (or other molecules) ±c learance of drug from bloodstream • rate of elimination of drug from body
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Rate of Decomposition of Drug • characterization of the decomposition or stability of drugs is of critical importance • decomposition may proceed by hydrolysis, oxidation, isomerization, epimerization, photolysis etc. • many groups study the effect of ingredients of dosage forms or formulations and environmental factors (i.e. temperature, pH ) on the chemical and physical stability of drugs
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Examples: 1. Higuchi studied the decomposition of Chloramphenicol. Chloramphenicol decomposes via hydrolytic cleavage of the amide bond (shown above). • rate of degradation is low and independent of pH for pH range 2 – 7 •hydro lys is ± ±cata lyzed ±by presence of acids • maximum stability occurs at pH 6 at room temperature……….the half-life under these conditions is 3 years
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2. Stability of Doxorubicin (anti-cancer drug) • very difficult drug to study as it chelates with metal ions, self-associates in solution (DOX-DOX), absorbs to plastic and glass, undergoes oxidative and photolytic decomposition • Beijnen et al. studied the kinetics of degradation of DOX as a function of pH, ionic strength, temperature and concentration of drug. • decomposition followed first order kinetics at constant temperature • pH – rate profile demonstrated that maximum stability was achieved at about pH = 4.5
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Doxorubicin DOX is an anthracycline……includes chromophoric anthraquinone moiety and a charged sugar within structure • anthracyclines are generally light sensitive (many have investigated photodegradation of DOX when exposed to room light) • exposure to light, increase in pH and and adsorption to container known to result in DOX loss (decrease in concentration of intact DOX in solution) • kinetics of photodegradation of DOX has been studied in buffer, and biologically relevant media • incorporation of DOX into a delivery system, termed liposomes, has been found to reduce rate of photodegradation of drug
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DOXIL
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9a - SECTION 9: Kinetics Chapter 12 in Chang Text Outline...

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