Kinetics of Drug Degradation and the Effect of pH on Drug Stability - Lab Report Example

Kinetics of Drug Degradation and the Effect of pH on Drug Stability Table of Contents I. Introduction 3 II. Methods 3 Flask 2 4 III. Results 4 A. Tables 4 B. Concentrations from Absorbance – Calculations 5 C. Graph One 5 D. Rates of Reaction- Calculations 6 E. Graph 2 6 IV. Discussion 7 V. Answers to Questions 7 Question 1 7 Question 2 8 Question 3 9 Question 4 10 References 12 I. Introduction The aim of the experiment was to make use of a technique in spectrophotometry to find out how a stable a drug is for different pH values, making use of iodine that is known to react with the product of degradation of a drug called “Drug X”. The reasoning is that any degradation in the drug for different values of pH will be reflected in decreases in the amount of iodine in the solution, to the point where all of the drug has undergone degradation, at which point the iodine amount in the setup will stabilize. Spectrophotometry is the method to measure how much of the iodine has not reacted in the degradation process. This iodine level reading then indicates the amount of iodine and therefore the amount of the drug that had been degraded and subject to the reaction with the used up iodine (Yoshioka and Stella 2000, pp. 61-69; Cohen et al. 1984; Brucat n.d.; Laude 2011; Stretton 2004). II. Methods The spectrophotometer was calibrated to 324 nm for wavelength, zeroed with the use of water, and set to absorbance recording. For the trial run, 250 mL capacity Flask 2 was used to hold 10 mL of the drug in aqueous solution making use of a pipette, together with 120 mL of water. 250 mL capacity Flask 1 was used to hold 10 mL of water, 50 mL of aqueous iodine solution, and 10 mL of H2SO4, 0.1 M, likewise making use of a pipette. All of Flask 1 was poured into Flask 2, and swirled, at the same time marking the start of the timer. Flask 2 contents were poured back into Flask 1. At the 30 second mark, the reading for absorbance is noted, and this reading is done for every 30 seconds up to the six minutes mark. The pH of the mixture is also noted. This is a run. The actual experiment is done in five iterations of this run, with the initial Flask 2 contents retained, while the Flask 1 acid and water levels were varied according to the table below (Yoshioka and Stella 2000, pp. 61-69; Cohen et al. 1984; Brucat n.d.; Laude 2011; Stretton 2004): Flask 1 Flask 2 Expt Volume I2 soln Volume Water Volume H2SO4 Volume drug Volume Water 1 50 mL 20 mL 0 mL 10 mL 120 mL 2 50 mL 19 mL 1 mL 10 mL 120 mL 3 50 mL 15 mL 5 mL 10 mL 120 mL 4 50 mL 10 mL 10 mL 10 mL 120 mL 5 50 Ml 0 mL 20 mL 10 mL 120 mL III. Results A. Tables Time Experiment 1 Experiment 2 Experiment 3 Experiment 4 Experiment 5 0 0.089 0.087 0.097 0.082 0.073 30 0.089 0.087 0.095 0.077 0.066 60 0.089 0.087 0.093 0.073 0.055 90 0.089 0.086 0.091 0.066 0.042 120 0.089 0.085 0.090 0.061 0.033 150 0.088 0.084 0.089 0.055 0.022 180 0.088 0.084 0.088 0.050 0.025 210 0.087 0.083 0.086 0.044 0.025 240 0.087 0.083 0.086 0.037 0.025 270 0.087 0.083 0.084 0.030 0.025 300 0.087 0.082 0.082 0.021 0.025 330 0.087 0.082 0.082 0.014 0.025 360 0.086 0.081 0.082 0.006 0.025 pH1 pH2 pH3 pH4 pH5 4.96 4.87 3.79 2.08 2.05 B. Concentrations from Absorbance – Calculations We use the formula c = A/85000 mol Iodine Concentrations Time Experiment 1 Experiment 2 Experiment 3 Experiment 4 Experiment 5 0 1.04706E-06 1.02353E-06 1.14118E-06 9.64706E-07 8.58824E-07 30 1.04706E-06 1.02353E-06 1.11765E-06 9.05882E-07 7.76471E-07 60 1.04706E-06 1.02353E-06 1.09412E-06 8.58824E-07 6.47059E-07 90 1.04706E-06 1.01176E-06 1.07059E-06 7.76471E-07 4.94118E-07 120 1.04706E-06 0.000001 1.05882E-06 7.17647E-07 3.88235E-07 150 1.03529E-06 9.88235E-07 1.04706E-06 6.47059E-07 2.58824E-07 180 1.03529E-06 9.88235E-07 1.03529E-06 5.88235E-07 2.94118E-07 210 1.02353E-06 9.76471E-07 1.01176E-06 5.17647E-07 2.94118E-07 240 1.02353E-06 9.76471E-07 1.01176E-06 4.35294E-07 2.94118E-07 270 1.02353E-06 9.76471E-07 9.88235E-07 3.52941E-07 2.94118E-07 300 1.02353E-06 9.64706E-07 9.64706E-07 2.47059E-07 2.94118E-07 330 1.02353E-06 9.64706E-07 9.64706E-07 1.64706E-07 2.94118E-07 360 1.01176E-06 9.52941E-07 9.64706E-07 7.05882E-08 2.94118E-07 C. Graph One Iodine concentrations (y-Axis) over time (30 sec intervals) D. Rates of Reaction- Calculations Using the data for the absorbance at different pH and time, the gradient is calculated with the use of LINEST and SLOPE to determine the rate of reaction. Both yield the same results. The derived rates of reaction are given below: pH Gradient 4.96 -1.01271E-10 4.87 -2.04697E-10 3.79 -4.97738E-10 2.08 -2.4693E-09 2.05 -1.45874E-09 E. Graph 2 The plot of pH versus rate of reaction is given below: IV. Discussion The rate of decline of iodine concentrations is a function of the acidity of the solution, as can be seen in Graph 1. Where the pH is most acidic, which is experiment 5 at a pH value of 2.05, the rate of decline is fastest. This corresponds to the fastest rate of degradation for Drug X. Graph 2 is to be interpreted in the same manner. Where the slope is least negative corresponds to a rate of reaction that is slowest, and also corresponds to the pH value that is least acidic. This is the rightmost point on Graph 2. The finding is that there is a direct relationship between the acidity and the rate of degradation of Drug X. Sources of error include timing errors for measurements, as well as errors in the calibration of the measurement device. There may also be measurement errors for the different solutions (Yoshioka and Stella 2000, pp. 61-69; Cohen et al. 1984). V. Answers to Questions Question 1 This is a zero order reaction. The literature tells us that there are several ways to determine the orders of reactions, based on the available data, for instance, and on the nature of the available data. There are methods that look at initial rates for instance, in order to derive the order of the reaction (Brucat n.d.). There are methods that look at the rate constant and the units that are tied to that, as well as methods that look at the overall rate equation, and methods that look at balancing the reaction equation in order to derive the order of the reaction from the balanced equation (Laude 2011; Stretton 2004). In the literature there is a technique for determining the order of the reaction from changes in the observed variable through time. In the original data, the changes in the absorbance data for experiment 3 shows some variability. To test whether this is a zero order reaction, the changes in the absorbance data through the different data sets, as a function of time, must be a constant. In this case, the data says that such changes over time do aggregate towards one value, indicating that this is a zero order reaction. This is true for instance for experiment 3, while there are some variability in the resulting ratios for experiments 4 and 5. For experiments 1 and 2, while there are no changes for a large portion of the data, the remaining data yield ratio outputs that congregate around one value. Therefore on balance one can say that this is indeed a zero order reaction (Brucat n.d.; Laude 2011; Stretton 2004). Question 2 Temperature is considered as one of the key factors that impact the stability of drugs in the literature. The academic literature recognizes this fundamental precept in the Arrhenius equation, which relates temperature to the rate constant. In general, temperature is considered to have significant impact in the dynamics of processes that go into equilibrium. This is also an offshoot of the general observation that where reactions occur at higher temperatures, those reactions proceed at a faster rate in comparison to the same reactions that occur in lower temperatures. The way drugs react to changes in temperature are factored into degradation equations, with different drugs and substances in general having their own unique temperature-dependent reactions relating to degradation. The sensitivity of different drugs and substances to changes in temperatures are tied to the value of the activation energy E in Arrhenius plots, where E is expressed in kcal/mol. The higher the value of the activation energy E, then the higher the change in the reactivity of the substance or the degradation rate of the substance with changes in temperature. The literature tells us too that the determination of the rate constant is tied to how the rate constant varies with changes in temperature, and this is used in pre-formulation studies to determine for instance how well the drug stands up to temperature changes in terms of maintaining its stability or withstanding degradation. Data for E for instance can enable manufacturers to predict how stable a drug is over a range of temperatures. The Arrhenius plot can also yield rates of degradation for a drug via linear regression. In other words, temperature impacts drug stability in a fundamental way (Yoshioka and Stella 2000, pp. 61-69). Question 3 In this experiment one can see that pH alters the rate of reaction of iodine with the degradation product for Drug X. This means that in general, pH has an effect on how fast or how slow a drug degrades. In this experiment, the higher acidity results in a faster reaction rate and therefore a faster degradation rate. Moreover, in the literature, data on the stability of drugs over a range of temperatures are stated for a given range of pH values as well. This implies that the pH of the solution alters the impact of temperature on drug stability. Stated another way, the effects of temperature on drug stability are dependent on the pH of the solution. It can be said that differing pH values will have differing temperature-related degradation reactions for specific drugs. In the case of Drug X, the higher the acidity, the higher the rate of reaction. The experiment did not include testing for the effects of temperature on degradation rate, but this can be arranged in an extension of this original experiment (Yoshioka and Stella 2000, pp. 61-69). It can be imagined that pH related degradation reactions for different drugs factor into prescriptions for modes of administration or administration routes. Where the drug for instance degrades sharply with higher acidity, as can be found in the human stomach for instance, then oral administration may result in the drug being degraded to a level where it ceases to be potent. The drug may be coated in a buffer, for instance, to make sure that it does not degrade in the highly acidic environment of the stomach, and reaches its target with no loss in efficacy. That, or other routes of administration may be considered, such as via IV, or via intravenous injections, or as skin patches, to preserve their integrity and to prevent pH-related degradation effects (Yoshioka and Stella 2000, pp. 61-69; Cohen et al. 1984). Question 4 Some drugs are sensitive to light due to the way the components undergo photochemically-induced reactions in the presence of light. There is a certain level of light that when present leads to the drug components degrading, and the rate of degradation is dependent on the drug characteristics and the strength of the light that reaches the drug components. In the formulation process, the sensitivity of the drug components to light is determined. Formulation exercises also include packaging considerations, so that photosensitive drugs are placed in glass containers that have color. Ultraviolet radiation is a special concern owing to the intensity of that spectrum and the way that light effects rapid degradation in many kinds of drugs. Amber colored glass containers for instance are used to protect drugs from UV light, together with containers that are made of glass and are colored green-yellow. It is said that there are many other variables that come into play when considering the impact of light on drug degradation and efficacy/potency. Those include not just the nature of the light that reaches the drugs and for how long, but also such extraneous characteristics as the shape and size of the container, its color, and the components of the container materials themselves. In the drug formulation process, the impact of light on drug stability is considered in light stability phases of stability tests, after which the proper precautions in formulation, packaging and also storage are undertaken and prescribed. Such light stability tests also factor into determining the shelf lives of drugs (Tonnesen 2010, pp. 173-176). References Brucat, PJ (n.d.). Determining Reaction Order. UFL.edu. [Online] Available from: http://www.chem.ufl.edu/~itl/4411/lectures/lec_l.html [Accessed 29 January 2013] Cohen, B. et al. (1984). Effects of pH and Temperature on the Stability and Decomposition of N,N’N”-Triethylenethiophosphoramide in Urine and Buffer. Cancer Research 44. [Online] Available from: http://cancerres.aacrjournals.org/content/44/10/4312.full.pdf [Accessed 29 January 2013] Laude, David (2011). How to Determine Orders of Reaction. UTexas.edu. [Online] Available from: http://laude.cm.utexas.edu/courses/ch302/others/order.pdf [Accessed 29 January 2013] Stretton, Tom (2004). Experimental Determination of Reaction Order. UCDSB. [Online] Available from: http://www2.ucdsb.on.ca/tiss/stretton/CHEM2/rate08.htm [Accessed 29 January 2013] Tonnesen, Hanne Hjorth. (2010). Photostability of Drugs and Drug Formulations, Second Edition. [Online] Available from: Google Books. http:/books.google.com [Accessed 29 January 2013] Yoshioka, S. and Stella, V. (2000). Stability of Drugs and Dosage Forms. [Online] Available from: Google Books. http:/books.google.com [Accessed 29 January 2013] Read More