edited ~- bv: MICHAEL R. SLABAUGH HELENJ. JAMES ~
Weber State College Ogden. Utah 84408
Drug Stabilization against Oxidative Degradation Michael J. Akers Eli Lilly and Company, Indianapolis, IN
Morphine, ascorbic acid, tetracycline, penicillin, epinephrine, chlorpromazine, heparin, prednisolone, and reserpine are but a few of a large number of chemically and therapeutically diverse active drug compounds that have one common characteristic-they are all subject to a chemical degradation process called oxidation. This article will describe the process of oxidation and ways in which pharmaceutical preparations can be protected from oxidative degradation.
where n is the number of equivalents of the reacting substance, F is the Faraday constant, and E and Eo are the EMF'S of the reversible electrochemical cell a t different and unit activities, respectively. Thus, substituting into eqn. (21,
The Process of Oxldailon (.1-21.
The process of oxidation involves the transfer of electrons and mutons. A chemical oaidation-reduction half-renction can he ekpressed by reduced form = oxidized form
+ n electrons
(1)
which is usually reversible or can be made to go in either direction. In an oxidative process there must be the presence of an electron donor and an electron acceptor. In any mixture of oxidation-reduction reagents. the substance that has the greatest tendency to lose e h r o n s is itself oxidized and thus causes reduction of other substances in the reacting svstem. The tendency to lose electrons is ascertained by gea"suring the electromotive force (EMF). When a a moles of reduced form A are converted to fl moles of oxidized form B, the change in free energy associated with the reaction is
where AG and AGO are free energy changes of the entire reaction and standard reaction, respectively, R is the gas constant, and T is the absolute temperature. Thermodynamically it can he shown that
Rearranging eqn. (4) gives E = E'
-2.303RT (B)* log nF (-4)-
Equation ( 5 ) , commonly called the Nemst equation, is used to compute the EMF of an oxidation-reduction system as a function of the standard EMF or standard potential, temperature, number of electrons transferred, and the activities of the reactants (reduced form) and products (oxidized form). The term Eo in the Nernst equation is called the standard electrode potential and is the measured EMF when the activities or concentrations of the reactants and products are equal (i.e., E = E"). The E o becomes the electrode potential of the oxidation process when electrons are transferred from the electrode, usually the reference calomel electrode. This can be expressed as follows:
This feahne presents applications of chemisby relevant to everyday life. The information presented might be used directly in class, posted on bulletin boards, or otherwise used to stimulate student involvement in ~ C t i V t i e Srelatedto chemishy. Conbibutions should be sent to feature editors.
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A ~ r i l1985
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Eo left = E Doxidation E Dright = E o reduction = E* calomel
+
E" cell = E" left E" right (6) The standard oxidation potential for the 1N calomel electrode is -0.2800volts at 2 5 T . However, if this electrode is used as a reference electrode to receive electrons from the electrode on the left-hand side, then it is heing reduced and the standard oxidation potential becomes a reduction potential by reversing the sign, i.e., E n right = +0.2800 V. Therefore, in a system employing a platinum electrode as the oxidation electrode and a calomel electrode as the reduction electrode, and the E" left = E o platinum = -0.200 V, then the standard oxidation potential of the whole electrochemical cell is In comparing two substances and their potential for oxidation, the substance with the higher standard oxidation notential will he the substance which will be oxidized, while the substance with the lower potential willaccept those electrons from the substance being oxidized and. conseauentlv. he reduced. The nrocess of oxidation mav involve a reaction between atmospheric oxvgen and many types of organic compounds. If this nn,cess is sDontaneous it is called autoxidation, a chain react& that beginn with the formation of a free radical due to the loss of a hydrogen atom (31
simultaneously result in an increase in the concentration of oxygen in solution. The lower temperature will lessen the probability of catalyzing a free radical formation, but if a free radical is formed, then more oxygen is available to propagate the reaction. Because temperature affects the standard oxidation potential, Eo (see eqn. (5)), the temperature must he defined and held constant when determining standard oxi.--L
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Hydrogen ion concentration also affects oxidation by a direct effect on the oxidation-reduction potential of the Nernst equation:
Equation (7) can he reduced t o
Thus, as pH is increased, the oxidation potential of the system increases. For example, a t pH 4.58 ascorbic acid has an E o of -0.136 V. Increasing the p H to 5.20 increases t h e E o to -0.115 V. The lowerine of nH retards the loss of a molecule of hvdrogen from the organic compound. Therefore, many oxyeen-sensitive comoounds. as lone as thev are still soluble, are Formulated a t lowa pH values t i increase their resistance t o oxidation. Trace metals which are oxidized by a one-electron transfer Drocess are most active in catalyzing the oxidation of drugs.
- .
RH-R.+H where R is an organic molecule. The free radical R- then reacts with molecular oxygen to form a peroxy free radical
-
R. + o2 ROO. The chain reaction is propagated by the peroxy free radical reacting with another organic molecule, ROO. + RH
-
ROOH + R.
Thus, the chain reaction continues until R H is consumed or
R:R is formed. Oxidation may occur in the absence of oxygen. In fact, most biological oxidations occur in the absence of oxygen (4). Many organic molecules (e.g., organic acids involved in glucose metabolism and the Krehs citric acid cycle) are oxidized not by interacting with oxygen, hut by the removal of hydrogen atoms which are transferred to another molecule. For example, pyruvate (breakdown product of glucose metabolism), fatty acids, and amino acids are oxidatively degraded to produce carbon dioxide and hydrogen atoms. The important thing to realize is that oxidation of one molecule involves the reduction of another molecule. Several important organic molecules serve an important purpose of receiving hydrogen atoms during cell metaholism. In other words, they are oxidizing agents in the place of oxygen. These special molecules are termed coenzymes, the most impurtant of which are nicotinamide INAI)). tlavin mononucleotide ~ - adenine ~ ~ dinurlcotide ~ ~ ~ (FMN), and flavin adenine dinuclekide (FAD). Coenzymes accent hvdroeen atoms from various intermediates in the rei is cyhe, then, in turn, transfer the atoms to other coenzymes or to oxygen itself. As these reduced coenzymes oxidize, energy is released in the form of ATP during the respiratory chain. ~~~~~
Factors Affecting Oxidation Of Drugs Autoxidation is catalyzed by temperature, hydrogen ion concentration, trace metals, peroxides, and light. While the soluhilitv of oxveen in water is less a t higher temneratures, the rate of oxidation will increase as temperature is increased (i.e.. . ~,k = A,,e-E/RT). However. it must he recognized that storage of solutions bfoxygen-sknsitive drugs a t low temperatures (e.g., 5 ' 0 , while decreasing the rate of reaction, will
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Journal of Chemical Education
Metals can also react directly with oxygen and with hydroneroxides to form free radicals which could initiate the chain ieaction of autoxidation. Copper and iron are the most active catalvsts. For examnle. 0.0002 M comer has been shown to increase the rate oI ascorbic acid G d a t i o n by a factor of 10.000 over that in the absence of copper (5). he absorption of a photon of light by a molecule will lead to a photochemical reaction. The smaller the wavelength of light the greater is the energy produced when light is absorbed and the more rapid the photochemical reaction. Light will expediently catalyze the initiation of oxidation hy generating a free radical, RH+hv-R. (10) where h is Planck's constant and u is the light ray frequency or v = c/X, where c is the velocity of light and Xis the wavelength region. Methods to Protect Drugs - from Oxldatlve Degradatlon From the principles discussed above a number of methods have been introduced which are effective, in part, in minimizing or retarding oxidative degradation of drugs (6). 1) Protection from light. Drugs sensitive to oxidation are packaged
in amber containers andlor in secondary outer packaging that prevent light from penetrating the container and coming in contact with the drug. Light protection also is necessary during manufacturing of the drug product. This is accomplished primarily by keeping the product in covered Light-resistanteontainers and minimizing light exposure during transfers and fillings. Drugs sensitive to light degradation (photolysis) or light-catalyzed oxidation will be packaged in containers with labeling statements warning the user to avoid exposure of the drug to any light source (natural or artificial). 2) Elimination of heauy metal contamination. The most efficient means of eliminating heavy metal contamination in drug products (heavy metals being present in water, formulation ingredients, and packaging systems) is to incorporate additives called sequestering or chelating agents. The most common sequestering agent is elhylenediaminetetraacetie acid (EDTA). These agents serve to bind multivalent ions because of the presence of free pairs of electron? on oxygen and nitrogen atoms
Antioxidants Used In Pharrnacevtlcal Products
Water-Soluble Antioxidams A. Sulf~ro~s Acid Sans 1. Scdium Bisulfite 2. Scdium Sulfie
B. Ascorbic Acid Isomers 1. L-AscorbicAcid 2. E-Ascorbic Acid (Erymorblc Acid)
C. Thlol Derivatives 1. Acetylcysteine 2. Cysteine 3. Thioglycerol 4. Thiogly~~lli~ Acid 5. ThiolaCtiC Acid 6. Thiourea 7. Dilhi~lhreitol
8. Glutathione Water-Insoluble Antioxidants 1. Pronvi Gallate 4. Ascorbyl Palmitate 5. Nordihydroguaiaretlc Acid
6. Alpha-Tocopherol
present in the molecule, which serve as donor groups for eationic metals. EDTA contains six such binding sites and is very effective in tying up iron and copper ions so that they cannot catalyze the oxidation process according to eqn. (9). 3) Avoidance of high temperatures. Temperatures above ambient (30%) should be avoided during storage and use of onygensensitive drugs. Higher temperatures cause an acceleration in the deeradation of antioxidants oresent in the formulation. Once antir,x~~dnnuarecun*umed thedrug will readily oxidize. ~ i g h e r 1QmprmItrresalso wjll naturally accelerate the rate of reaction ol any thermod)namir pmrecs such as drug drgradatiun. 4) Use of acidic solutions. Equation (8)shows that the lower the DH of a solution. the less the oxidation-reductionootential.. and., ihus, the more stable the drug to oxidation. some pharmaceutical solutions are formulated at pH's less than 3.5 in order to take advantage of the oxidative stability. However, while oxidation may not occur as readily in acidic solution, one must he certain that acid-catalyzed hydrolysis of the drug will not occur either. 5 ) Use " / i n e r t R n W S A common practice in the manufacture uf pharmareurieol products contn~ningoxygen-sensitive drugs is to use inert gases such as nitrogen or carbon dioxide to displace oxygen insolutwn and in themr spaceahwe thesolution 1~0th in the mixing rank and in the final package. Theoretically, a catnlytirsllv pmduced freedrue. radical uould have nouxveen .present with which to react. fn practice, however, inert gas purging and overlaying does not completely remove oxygen so
that if free radical catalysis were allowed to take place, enough oxygen would he present to propagate the reaction. 6)
Use of antioxidants. A widespread practice in the pharmaceutical and food industry is to incorporate antioxidants into the formulation to aid in protecting the product from oxidative degradation. Antioxidants work by one of three different mechanisms: a) preferentially undergoing oxidative degradation in place of the drug because of the higher standard oxidation potential ( E o )of the antimidant; b) serving as an acceptor of the free radical and inhibiting the free radical chain reaction process; c) retarding the formation of free radicals. Water-soluble antioxidants such as sodium hisulfite and ascorbicacid follow mechanism (a), water-insolubleantioxidants such as propyl gallate and butylated hydroxy toluene act aecording to mechanism (b), and metal-sequestering agents are examples of retardants of free radicals. A list of pharmaceutically acceptable antioxidants is presented in the table. It must be recognized that antioxidants are most effective in stabilizing oxygen-sensitive drug formulations when they are oxidized in place of the drug yet are not oxidized so rapidly that they are consumed too quickly.
I ) Selection of correct packaging. AU of the above precautions can he taken to assist in minimizing or eliminating the oxidative process, yet, if the product is not properly packaged, such precautions will be significantly compromised. A primary example is the packaging of drug produds in ruhber-stoppered vials. The vials could be amher-colored. the formulation eomonsed of antioxidants and sequestering Gents, the solution at an acidic pH, and the product stored at relatively low temperature. However, if the eantainer-closure interface is not a hermetic seal, none of the above precautions will he able to overcome the package flaw.
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To summarize, the process of oxidation is a major route of d r u g degradation in vitro for pharmaceutical preparations. An understanding of the basic principles of oxidation-reduction reactions is extremelv i m.~ o r t a n in t deciding w h a t measures can he taken to protect drug products from &dative dearadation. Several methods are available for minimizine d r i g oxidation, but no method alone is sufficient to eliminate completely its potential.
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Literature Cited (1) Martin. A. N.,
Swsrbriek.J.,and Cammmata, A,. "Physical Pharmacy." 2"d ed.. Lea &Fcbigcr.Phiiadelphia.1969.p. 283. (21 k ~ a n , L . . D e L u e aP. , P., and Akera. M. J.,"KmeticPlinciplaandStab~tyTa~: m "Thmniand P m c k of Indu%trial Phanoacy: 3rd d., (Editors: Laehman etal.), Lea & Febiger, Philadelphis, in preaa. (31 Uri, N., '"Physico-ChemicalAspects of Autaridation: in "Autaxidation and Antimidants." (Ediloc Lundbprg, W. 0.1, lntereeiena,NelxYork, 1961,p p 5 M 8 . (4) Watson. J.D.."Molwulsr Biology oftheGene." 2nd ed.. W.A. Benjamin, Menlopark. CA. 1970,pp. 42-51. (5) Connors, K. A.,Amid0". G. L.,andK-on, L.,"ChemidStabili~ofPharmaeeutieala." John Wiley & Sans, New York, 1979,p So. I81 Akers, M. J., "Antioxidants in Pharmaautical Pmducta," J Porentor. Sei. Tech.,36. 222 (19a2).
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Number 4
April 1985
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