REVIEW
`
`Excipient–Drug Interactions in Parenteral Formulations
`
`MICHAEL J. AKERS
`
`Baxter Healthcare Corporation, Bloomington, Indiana 47402
`
`Received 10 December 2001; revised 12 February 2002; accepted 18 February 2002
`
`ABSTRACT: Excipients are added to parenteral formulations to enhance or maintain
`active ingredient solubility (solubilizers) and/or stability (buffers, antioxidants, chelat-
`ing agents, cryo- and lyoprotectants). Excipients also are important in parenteral formu-
`lations to assure safety (antimicrobial preservatives), minimize pain and irritation upon
`injection (tonicity agents), and control or prolong drug delivery (polymers). These are all
`examples of positive or synergistic interactions between excipients and drugs. However,
`excipients may also produce negative effects such as loss of drug solubility, activity, and/
`or stability. This review article will highlight documented interactions, both synergistic
`and antagonistic, between excipients and drugs in parenteral formulations. The reader
`will gain better understanding and appreciation of the implications of adding formu-
`lation ingredients to parenteral drug products. ß 2002 Wiley-Liss, Inc. and the American
`Pharmaceutical Association J Pharm Sci 91:2283–2300, 2002
`Keywords: parenteral; excipients; formulation; stabilizers; solubilizers; antimicro-
`bial preservatives; packaging
`
`INTRODUCTION
`
`Well-referenced and useful publications are avail-
`able listing every formulation component in all
`marketed parenteral drug products1–5 (Food and
`Drug Administration web sitea). The information
`in these publications has been invaluable to
`parenteral formulation scientists developing solu-
`ble, stable, resuspendable, manufacturable, and
`deliverable parenteral dosage forms. Formulation
`component precedence takes on high stature in
`the sterile product world because of significant
`toxicological and regulatory concerns. In other
`words, it is usually better to use a component
`that has a track record of relatively safe use in
`injectables and is likely not to raise concerns on
`the part of regulatory reviewers.
`
`awww.fda.gov/cder/drug/iig/default.htm
`
`Correspondence to: Michael J. Akers (Telephone: 812-355-
`7188; Fax: 812-332-3079; E-mail: michael_akers@baxter.com)
`
`Journal of Pharmaceutical Sciences, Vol. 91, 2283–2300 (2002)
`ß 2002 Wiley-Liss, Inc. and the American Pharmaceutical Association
`
`This review article will not cover all excipients
`used in parenteral formulations because the afore-
`mentioned publications already do so. What this
`review article presents are examples of synergistic
`and antagonist interactions that have been report-
`ed for excipients used in parenteral formulations.
`Although extensive, this review will not be exhaus-
`tive in the effort to cite all published references on
`parenteral drug–excipient interactions. Pharma-
`ceutical Excipients 20006 was a very helpful text in
`obtaining valuable information about drug–exci-
`pient interactions and compatibilities.
`When one studies stability and compatibility
`issues in parenteral drug formulation, the packa-
`ging system also must be considered. Potential
`interactions between excipients and rubber clo-
`sures in finished products are as much a concern
`as interactions between excipients and drugs.
`Therefore, some drug–sterile packaging interac-
`tions will be covered in this article.
`Although this article will focus on chemical and
`physical compatibilities of drugs and excipients
`used as injectable products, readers must also be
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`AKERS
`
`aware of the potential for any excipient and drug,
`either alone or in combination, when injected
`intravenously to cause certain problems. As
`Yalkowsky et al.7 pointed out, some formulation-
`related problems associated with intravenous
`drug delivery include hemolysis, precipitation,
`phlebitis, and pain. Therefore, as scientists de-
`velop sterile product formulations, not only must
`they be concerned with physical and chemical in-
`teractions that may occur in vitro, but they must
`also be concerned with the potential for formula-
`tion-related problems occurring in vivo.
`Drug–excipient interactions are studied in two
`basic ways. One is to perform traditional prefor-
`mulation studies using full factorial or Plackett
`Burman type of experimental designs. A good
`example of this approach for parenteral formu-
`lation development is a preformulation study
`published by Peswani and Lalla.8 Analytical
`methods such as differential scanning calorimetry
`isothermal microcalorimetry,10,11 and
`(DSC),9
`fourier transform infrared (FT-IR) spectroscopy12
`are excellent tools for predicting drug–excipient
`interactions. The other approach for studying
`drug–excipient interactions is to conduct both
`short-term and long-term stability studies on
`various formulations of the drug and measure
`both chemical stability (usually by chromatogra-
`phic techniques) and physical stability (e.g., by
`microscopic, electronic particle analysis, and cir-
`cular dichroism techniques).
`This review is organized according to major
`functions of parenteral excipients (solubilization,
`stabilization, preservation, and drug delivery aids).
`Several excipients serve more than one function
`[e.g., polyvinylpyrrolidone (PVP) as a complexing
`agent and as a freeze-drying bulking agent], so
`such excipients may be referenced in more than
`one segment of the article.
`Table 1 lists all the major pharmaceutical ex-
`cipients used in parenteral formulations. Table 2
`provides a listing of lesser-used excipients that
`are found in 1–2 commercial parenteral formula-
`tions. References1–5 provide much greater detail
`about the specifics of these excipients (e.g., con-
`centration) and the products in which they are
`components (e.g., brand names, manufacturer).
`
`Solubility Effects
`
`Many parenteral formulations require additives,
`either solvent or solute excipient, to increase and/
`or maintain solubility of the active ingredient in
`the solution. Sweetana and Akers13 summarized
`
`seven basic approaches for solubilization of paren-
`teral drugs as follows:
`
`1. Salt formation
`2. pH adjustment
`3. Use of co-solvents
`4. Use of surface-active agents
`5. Use of complexation agents
`6. Change formulation from solution to a
`dispersed system, oily solution, or a more
`complex formulation such as a microemul-
`sion or liposome
`7. ‘‘Heroic’’ approaches involving the use ofnon-
`commercially approved types and/or concen-
`trations of solvents or excipients
`
`This section will highlight some of the interac-
`tions between drugs and solubilizing agents,
`focusing on co-solvents, surfactants, suspending
`and emulsifying agents, complexation agents, and
`oils or lipids. Some examples of unpredicted inter-
`actions of excipients and drugs to enhance drug
`solubility are listed in Table 3.
`
`Co-Solvents
`
`There are approximately 20 different co-solvent
`agents used in approved parenteral products.
`However, the most commonly used co-solvents in
`parenteral formulations are ethanol, glycerin, pro-
`pylene glycol, sorbitol, polyethylene glycol (both
`300 and 400), dimethylacetamide, Cremophor EL,
`and N-methyl-2-pyrrolidone.
`Glycols are widely used solubilizing agents,
`but can cause some stability or compatibility
`problems. Glycerol is used not only as a co-solvent
`for improving solubility of poorly water-soluble
`drugs, but also as a tonicity-adjusting agent (e.g.,
`in insulin formulations). In freeze-dried formula-
`tions, glycerol can serve as a plasticizer, lowering
`the glass transition temperature of the product
`without the significant change in water content or
`activity.14 In certain formulations containing un-
`stable peptides, the presence of glycerol will in-
`crease the mobility of the freeze-dried formulation
`matrix, leading to peptide deamidation. Sorbitol
`has been reported to increase the degradation
`rate of penicillins in neutral and aqueous solu-
`tions.15 On a more positive note, propylene glycol
`will potentiate the antimicrobial activity of the
`parabens in the presence of nonionic surfactants
`and prevents the interaction of methylparaben
`and polysorbate 80.16
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`Table 1. A Listing of Major Excipients Used in
`Sterile Product Formulations (Both Commercial and
`Developmental)
`
`Solvent systems
`Co-solvents
`Propylene glycol
`Glycerin
`Ethanol
`Polyethylene glycol (300 and 400)
`Sorbitol
`Dimethylacetamide
`Cremophor EL
`Oils
`Sesame
`Soybean
`Corn
`Castor
`Cottonseed
`Peanut
`Arachis
`Ethyl oleate
`Isopropyl myristate
`Glycofurol
`Petrolatum
`Solubilization agents
`Co-solvents
`See above
`Surface-active agents
`Polyoxyethylene sorbitan monooleate (Tween 80)
`Sorbitan monooleate
`Polyoxyethylene sorbitan monolaurate (Tween 20)
`Lecithin
`Polyoxyethylene–polyoxypropylene copolymers
`(Pluronics1)
`Complexation agents
`Hydroxypropyl-b-cyclodextrin
`Sulfobutylether-b-cyclodextrin (Captisol1)
`Polyvinylpyrrolidone
`Amino acids (arginine, lysine, histidine)
`Stabilization agents
`Buffers
`Acetate
`Citrate
`Tartrate
`Phosphate
`Triethanolamine (TRIS)
`Antioxidants
`Ascorbic acid
`Acetylcysteine
`Sulfurous acid salts (bisulfite, metabisulfite)
`Monothioglyercol
`Chelating agents
`Ethylenediaminetetraacetic acid (EDTA)
`Sodium citrate
`Cryo- and lyoprotectants and bulking agents
`Mannitol
`Glycine
`Sucrose
`
`EXCIPIENT–DRUG INTERACTIONS
`
`2285
`
`Table 1.
`
`(Continued )
`
`Lactose
`Trehalose
`Dextran
`Povidone
`Sorbitol
`Competitive binding agents
`Serum albumin
`Heta-starch
`Tonicity-adjusting agents
`Sodium chloride
`Glycerin
`Mannitol
`Dextrose
`Antimicrobial preservative agents
`Phenol
`Meta-cresol
`Benzyl alcohol
`Parabens (methyl, propyl, butyl)
`Benzalkonium chloride
`Chlorobutanol
`Thimerosal
`Phenylmercuric salts (acetate, borate, nitrate)
`Delivery polymers
`See Table 6
`
`Co-solvents are known to cause hemolysis. In
`a study conducted by Fuet et al.17 comparing the
`hemolytic effects, both in vitro and in vivo,
`of a variety of co-solvents (ethanol, propylene gly-
`col, polyethylene glycol, dimethylisosorbide, and
`dimethylacetamide), complexing agents (nicotina-
`mide), and surfactants (Pluronic L64 and emul-
`phor EL-719), solutions most prone to elicit a
`hemolytic response were those containing propy-
`lene glycol, dimethylisosorbide, and nicotina-
`mide). However, these authors found that the
`hemolytic effects of propylene glycol can be alle-
`viated by the addition of either a tonicifying agent
`or polyethylene glycol 400.
`Cremophor EL (polyoxyl 35 castor oil) has
`been approved as a solvent in commercial in-
`jectable dosage forms containing paclitaxel,
`diazepam, propanidid, and alfaxalone. It is com-
`patible with many organic solvents and aqu-
`eous solutions. However, compounds containing
`phenolic hydroxyl groups may cause precipita-
`tion of Cremophor EL.
`
`Surfactants
`
`Surfactants serve a variety of very important
`functions in parenteral formulations. Among the
`most important are stabilizing proteins against
`aggregation. Tween 20 (polyoxyethylene sorbitan
`monolaurate) was shown to greatly reduce the
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`Table 2. Examples of Special or Uncommon Excipients Used in Injectable Drug
`Products
`
`Excipient
`
`Product
`
`Manufacturer
`
`Acacia
`Acetone sodium
`Aluminum monostearate
`Benzenesulfonic acid
`Benzyl benzoate
`Cyclodextrin (alpha)
`Diethanolamine
`Desoxycholate sodium
`Formaldehyde
`
`Tuberculin Old Test (ID)
`Talwin (IM)
`Solganal
`Tracrium
`Depo-testesterone
`Alprostadil
`Bactrim
`Fungizone
`Some vaccines
`
`Gelatin, hydrolyzed
`Gelatin, purified
`Hydroxypropyl-b-cyclodextrin
`Imidazole
`Monoethanolamine
`N,N-dimethylacetamide
`
`Polyoxyethylated fatty acid
`PEG 40 castor oil
`PEG 60 castor oil
`Sodium lauryl sulfate
`Sulfobutylether-b-cyclodextrin
`Triacetin
`
`Some vaccines
`Lupron Depot
`Itraconazole
`Kogenate
`Terramycin (IM)
`Vuman
`Busulfan
`AquaMephyton
`Monistat
`Prograf
`Proleukin
`Ziprasidone mesylate
`Prepidil Gel (ICV)
`
`Lederle
`Sanofi Winthrop
`Schering
`Glaxo Smith Kline
`Pharmacia
`Schwarz
`Roche
`Bristol Myers Squibb
`Lederle, Connaught,
`Merck
`Merck
`TAP
`Janssen
`Bayer
`Roerig
`Bristol Myers Squibb
`Orphan
`Merck
`Janssen
`Fujisawa
`Cetus
`Pfizer
`Pharmacia
`
`rate of formation of insoluble aggregates of re-
`combinant human factor XIII caused by both
`freeze thawing and agitation stresses18 (Fig. 1).
`Maximum protection occurs at concentrations
`close to the critical micelle concentration of
`Tween 20, independent of initial protein concen-
`tration. In another report, Tween 20 at a 1% (w/v)
`concentration caused precipitation of a relatively
`hydrophobic protein (Humicola lanuginosa lipase)
`by inducing non-native aggregates.19
`Tween 80 is well known to protect proteins
`against surface-induced denaturation.20 Tween 80
`was demonstrated to reduce hemoglobin aggre-
`gation in solution by preventing the protein from
`reaching the air–liquid interface or the liquid–
`surface interfaces.21 Polyoxyethylene surfactants
`
`such as Tween 80 can form peroxide impurities
`after long-term storage. Knepp et al.22 concluded
`that Tween 80 and other nonionic polyether sur-
`factants undergo oxidation during bulk material
`storage and subsequent use and the resultant
`alkyl hydroperoxides formed can contribute to
`the degradation of proteins. In such formulations,
`they further reported that thiols such as cysteine,
`glutathione, and thioglycerol were most effective
`in stabilizing protein formulations containing
`peroxide-forming nonionic surfactants.
`The nonionic surfactant octoxynol 40 (ethoxy-
`lated alkyl phenol, Igepal CA897, GAF), was found
`to solubilize an otherwise insoluble complex of a
`nonsteroidal anti-inflammatory drug and a quar-
`ternary ammonium antimicrobial preservative
`
`Table 3. Examples of Esoteric Excipients Used as Solubilizers
`
`Generic Name
`
`Brand Name
`
`Manufacturer
`
`Excipient
`
`Cipro IV
`Ciprofloxacin
`Adriamycin RDF
`Doxorubicin HCl
`Ergonovine maleate
`Ergotrate Maleate
`Polyestradiol phosphate Estradurin
`Zomepirac
`Zomax
`
`Bayer
`Pharmacia
`Lilly
`Wyeth
`McNeil
`
`Lactic acid
`Methyl paraben
`Ethyl lactate
`Niacinamide
`Tromethamine105
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`EXCIPIENT–DRUG INTERACTIONS
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`products worldwide that contain cyclodextrins
`(see Table 2). However, based on the literature
`and scientific meeting presentations, there will be
`a higher number of
`cyclodextrin-containing
`injectable formulations in the future.
`The only reports of incompatibilities with cy-
`clodextrins involve certain antimicrobial preser-
`vatives, primarily parabens.26,27 One preliminary
`report described both sulfobutylether-b-cyclodex-
`trin and hydroxylpropyl-b-cyclodextrin accelerat-
`ing the degradation of an unidentified water-
`soluble drug to its insoluble degradant form.28
`The authors concluded that both the type and
`degree of substitution of the proximal hydroxyl
`groups in the cyclodextrin cavity will influence
`the potential for cyclodextrin additives to accel-
`erate chemical degradation of drugs. As cyclodex-
`trins become more prominent in injectable drug
`product development, there likely will be more
`reports of incompatibilities along with the ex-
`pected reports describing solubility and stability
`enhancements.
`Cyclodextrin-containing formulations (either
`0.1 M sulfobutylether-b-cyclodextrin or 0.1 M
`hydroxylpropyl-b-cyclodextrin) were shown to
`cause less damage to venous epithelial cells at
`the site of injection compared with formulations
`containing organic co-solvents.29 PVP (povidone)
`is a generally compatible polymeric excipient.
`However, it can form molecular adducts (a posi-
`tive reaction with respect to iodine therapy
`topically) and will complex with some preserva-
`tives such as thimerosal.6 Lecithin is a commonly
`used emulsifying and stabilizing agent in intra-
`muscular and intravenous injections, primarily
`the intravenous fatty or lipid emulsions used in
`parenteral nutrition. Lecithin also is a component
`of some liposomal formulations. Polaxamers (e.g.,
`Poloxamer 188, BP) are nonionic polyoxyethy-
`lene–polyoxypropylene copolymers used as emul-
`sifying agents in intravenous fat emulsions. They
`have also been used in several patented protein
`formulations as stabilizers and sustained release
`injectables in development as solubilizing and
`stabilizing agents.30 Polaxamers, like the poly-
`sorbates, can form peroxide impurities over time
`and are incompatible with antimicrobial preser-
`vatives such as phenol and paraben.
`
`Oils/Lipids
`
`Many commercially available parenteral products
`contain lipophilic or oleaginous solvents. Exam-
`ples of
`injectable lipid solvents include ethyl
`
`Figure 1. Recovery of native rFXIII (A) and forma-
`tion of soluble (B) and insoluble (C) aggregates after 10
`freeze–thaw cycles of 1 mg/mL (*), 5 mg/mL (*), and
`10 mg/mL (!) as a function of Tween 20. (From
`Krielgaard et al., J Pharm Sci, 87, 1593–1603, ß 1998
`John Wiley & Sons, Inc., reproduced with permission.)
`
`mixture in an ophthalmic formulation.23 This is a
`rather unique drug–excipient
`interaction in
`which the interaction of the excipient involves
`not only a drug, but also a drug–preservative
`combination that is otherwise incompatible.
`
`Complexing and Dispersing Agents
`
`Cyclodextrins have emerged as very effective ad-
`ditive compounds for solubilizing hydrophobic
`drugs. In the parenteral dosage form area, modi-
`fied cyclodextrins, such as hydroxylpropyl-b-
`cyclodextrin and sulfobutylether-b-cyclodextrin
`have been reported to solubilize and stabilize
`many injectable drugs, including dexamethasone,
`estradiol,
`interleukin-2, and other proteins
`and peptides24 without apparent compatability
`problems.25 There are still only a few approved
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`oleate, isopropyl myrsistate, glycofurol, and fixed
`vegetable oils (e.g., peanut, corn, cottonseed,
`sesame, soybean, castor, arachis). There are
`several oily injectable solutions and suspensions
`used as sustained-release formulations (Clo-
`pixol1, Haldol Decanoate1, Deca-Durabolin1,
`Modecate1, Depixol1, and others).31 Oils are
`generally compatible with lipophilic drugs and
`excipients. However, formulation scientists must
`be aware of the potential for oils to be absorbed by
`rubber closure materials.32,33 Oils can contain
`unacceptable impurities (e.g., gossypol in cotton-
`seed oil, saturated fatty materials, unsaponifi-
`able materials, other organic residuals), but the
`United States Pharmacopeia and other compen-
`dia specify limits on these impurities.
`Highly purified sesame oil was found to
`improve the long-term stability of lidocaine.34
`Lidocaine in sesame oil that was not purified was
`degraded and formed crystals because of oxida-
`tion products such as hydroperoxides and impu-
`rities such as mono- and di-glycerides, free fatty
`acids, plant sterols, and the colorants chlorophyll
`and carotene).
`Soybean oil is the preferred oil in parenteral
`fatty emulsions. Soybean oil emulsions have been
`studied extensively and have been found to be
`incompatible with calcium chloride, calcium glu-
`conate, magnesium chloride, phenytoin sodium,
`tetracycline hydrochloride, and potentially many
`other drug substances and intravenous infusion
`solutions.35
`Petrolatum is a commonly used ointment base
`for topical ophthalmic ointments. There are no
`known incompatibilities with petrolatum.36
`
`Stability Effects
`
`This section highlights both classical and more
`recent publications that report on both positive
`and negative effects of excipients on drug stability
`in parenteral formulations. Examples of some
`esoteric effects (the stabilization of the drug by
`the excipient was not predicted or expected) of
`excipients stabilizing certain drugs are given in
`Table 4.
`
`Table 4. Esoteric Examples of Excipient Stabilizers
`
`Drug or Brand Name
`
`Stabilizing Agent
`
`Dexamethasone acetate
`Cardiotec
`Albumin
`Stelazine/compazine
`
`Creatine or creatinine
`Gamma cyclodextrin
`Sodium caprylate
`Sodium saccharine
`
`Buffers
`
`Buffer components in parenteral formulations
`can cause stability problems. Phosphate buffer,
`particularly the dibasic phosphate anion, serves
`as a nucleophile that can attack electrophilic
`centers like ester or amide carbonyl groups or
`polarized carbon-nitrogen double bonds.37
`Hasegawa et al.38–42 published several articles
`describing the use of pharmaceutical phosphate
`buffer solutions in the presence of calcium and/or
`aluminum by the use of EDTA in pH ranges of 5 to
`9, carboxylic acids such as citric acid and maleic
`acid in acidic to neutral solutions, and pyropho-
`sphate and lysine hydrochloride in alkaline solu-
`tions. Of course, the best approach is to eliminate
`metal contaminants in solutions or additives by
`techniques such as ion exchange, but often this is
`not practical.
`Zhu et al.43 presented a poster describing a
`study in which an unstable drug needed to be
`buffer stabilized at pH 3. Four buffer systems
`(citrate, glycinate, maleate, and tartrate) were
`studied using the same pH; only one buffer
`(glycinate) did not catalyze drug degradation
`(see Fig. 2). The authors did not speculate on
`why glycinate buffer did not catalyze the degra-
`dation of the drug although low concentrations
`(0.1 M) of all buffers would be acceptable. In a
`similar study, Nakamura et al.44 found that
`
`Figure 2. Rate of hydrolysis of GW280430 (0.2 mg/
`mL) as a function of buffer type and concentration at
`608C. (From Zhu, Merserve, & Floyd, Drug Dev Ind
`Pharm, 28, 135–142, ß 2002 Marcel Dekker, Inc.,
`reproduced with permission.)
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`Table 5. Stability Data for Minodronic Acid in Different Buffer Systems44
`
`EXCIPIENT–DRUG INTERACTIONS
`
`2289
`
`Buffer
`
`Glycine HCl
`
`Citrate
`
`Succinate
`
`Acetate
`
`Tartrate
`
`Lactate
`
`Maleate
`
`pH
`
`3.0
`5.1
`3.0
`5.0
`3.1
`5.0
`3.1
`5.0
`3.0
`5.0
`3.1
`4.4
`3.1
`5.0
`
`Potency (%) After
`4 Weeks at 608C
`
`Number of Particles >2 mm in 100 mM
`Buffer After 4 Weeks at 608C
`
`78.1
`95.5
`99.5
`101.8
`82.4
`94.4
`74.8
`88.5
`93.4
`95.7
`86.0
`93.9
`87.3
`80.1
`
`1000–9999
`> 10,000
`0–99
`0–99
`1000–9999
`> 10,000
`100–999
`1000–9999
`0–99
`0–99
`100–999
`1000–9999
`1000–9999
`> 10,000
`
`citrate and tartrate buffers maintained both
`chemical and physical (fewer particles) stability
`of minodronic acid in a parenteral aqueous solu-
`tion whereas glycine, succinate, acetate, lactate,
`and maleate buffers did not (see Table 5). Li et al.45
`compared the stability of tezacitabine in three
`buffer systems (phosphate, glycine, and TRIS)
`and found phosphate to be superior. Higher con-
`centrations of phosphate also improved drug stabi-
`lity (see Fig. 3a and b).
`
`Tris buffer, when used in a peptide formulation
`and stored at 708C, will degrade to liberate
`formaldehyde.46 Although this was not seen at
`lower temperatures, formulators need to be aware
`of this possible instability when using this com-
`mon biological buffer.
`Tris buffer will form a stable complex with N-
`nitrosourea anticancer agents and retard the de-
`gradation of these agents.47 However, Tris buffer
`will degrade 5-fluorouracil, causing the formation
`
`Figure 3.
`(a) Effect of buffer on stability of FMdC solution at 10 mg/mL. (b) Effect of
`phosphate buffer concentration on stability of FMdC solution at 10 mg/mL. (From Li,
`Chang, Pan, & Jones, AAPS Pharm Sci, (S) 3, ß AAPS Denver, reproduced with
`permission.)
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`2290
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`AKERS
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`of two degradation products that can cause
`serious-to-lethal cardiotoxicities.48
`Sometimes the cationic species in a buffer sys-
`tem matters. Sarciaux et al.49 found that a sodium
`phosphate buffer system consistently resulted in
`more turbid reconstituted solutions of bovine
`immunoglobulin (IgG) than a potassium phos-
`phate buffer system at the same concentration.
`The authors believed that this effect was not
`attributable to a pH shift sometimes seen during
`freezing as a result of crystallization of sodium
`phosphate. This is because sodium chloride-
`containing formulations also showed substan-
`tially higher levels of aggregation compared
`with potassium chloride-containing formulations.
`Bovine IgG will denature at
`the ice/freeze-
`concentrate interface that is irreversible after
`freeze-drying and reconstitution. This ice/freeze-
`concentrate interfacial denaturation is dependent
`on the amount or percentage of protein residing
`at the ice/freeze-concentrate interface and the
`surface area of the freeze-dried solid. Formula-
`tions containing sodium salts showed a higher
`surface area of dried solids than formulations
`containing potassium salts. The higher the surface
`area, the more drug is exposed to the interfacial
`area, resulting in a higher degree of denatura-
`tion and resultant aggregation. When phosphate
`buffers are frozen, selective precipitation of the
`less-soluble buffer component (dibasic sodium
`phosphate) and subsequent pH shift may induce
`protein denaturation.50 In the case of monomeric
`and tetrameric b-galactosidase, both sodium and
`potassium phosphate buffers caused significant
`secondary structural perturbations, greater for
`
`sodium phosphate samples. The addition of suc-
`rose was able to minimize this freeze-dried
`denaturation in phosphate buffers, even if there
`remained large-scale changes in solution pH
`during freezing.
`Histidine was used as a buffer system in an
`experimental formulation containing humanized
`IgG2 monoclonal antibody.51 Histidine under-
`went oxidation and the oxidation products caused
`a significant loss of potency of the monoclonal
`antibody. The antibody also degraded in citrate
`buffer, although not as much as in histidine
`buffer. Histidine buffer oxidizes in the presence
`of peroxides and the source of peroxides in
`these formulations presumably originated from
`Tween 80 also present. Nitrogen overlay inhibited
`the histidine buffer oxidation and enhanced anti-
`body potency.
`
`Antioxidants
`
`Ascorbic acid has been reported to be incompa-
`tible with certain drugs such as penicillin G.52
`However, this is not a direct incompatability of
`two organic molecules, but rather an incompabil-
`ity caused by the pH effects of ascorbic acid,53 as
`specified by Stella.37
`Sodium bisulfite and sodium metabisulfite
`are strongly nucleophilic antioxidants capable of
`catalyzing drug degradation.37 The well-known
`interaction of bisulfite and epinephrine leads to
`degradation of epinephrine. (Scheme 1)
`Epinephrine reacts with bisulfite to form the
`sulfonic acid derivative.54 However, the addition of
`sodium borate complexes the parahydroxybenzyl
`
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`Scheme 1.
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`derivatives and prevents the reaction between
`epinephrine and bisulfite.55 Zinc, copper, and iron
`also will not catalyze any reaction between epine-
`phrine and bisulfite.56 Interestingly, aluminum
`(III) will catalyze epinephrine degradation via a
`complexation effect not seen with these other
`metal ions.56
`Sodium bisulfite also has been reported to
`dehalogenate uracil-type molecules57 and cause
`ester hydrolysis.58 Bisulfite will react with phy-
`sostigmine in aqueous solutions rapidly and
`reversibly, which is dependent both on the total
`bisulfite added and pH.59 Bisulfite will attack
`physostigmine on the optically active carbon-
`10a,60 which is reversed if the mixture is diluted
`and pH adjusted to values greater than pH 6. This
`pH dependency suggested that
`the reaction
`
`2
`reacting with the
`and HSO3
`involved both SO3
`protonated form of physostigmine.
`Sodium metabisulfite inactivates cisplatin in
`solution61 and is incompatible in ophthalmic solu-
`tions containing phenylmercuric acetate, espe-
`cially when autoclaved.62 If dextrose and sodium
`metabisulfite are combined in aqueous solution,
`metabisulfite stability declines.63
`Ascorbic acid is a frequently used antioxidant
`in parenteral formulations. Formulators must be
`aware that ascorbic acid generally is incompatible
`with alkaline solutes, heavy metals, phenylephr-
`ine hydrochloride, pyrilamine maleate, salicyla-
`mide, sodium nitrite, sodium salicylate, and
`theobromine salicylate.64 Kerwin et al.65 found
`that ascorbate ion in sufficient concentrations
`reacted with oxygen producing superoxide that in
`turn caused chemical modification and aggrega-
`tion of recombinant deoxy hemoglobin. Lower
`levels of ascorbate and oxygen and lower solution
`pH combined to eliminate this problem.
`Edetic acid and its salts are used as metal
`chelating agents to aid in stabilization of drugs
`sensitive to metal-catalyzed oxidation and/or
`photolysis. They also can serve to enhance anti-
`microbial activity of formulations. Edetate salts
`are incompatible with zinc insulin, thimersosal,
`amphotericin, and hydralazine hydrochloride.6
`
`Bulking Agents and Lyoprotectants
`
`Several mechanisms have been proposed to ex-
`plain why excipients serve as cryo- or lyo protec-
`tants. The most widely accepted mechanism to
`explain the action of cryoprotection is the pre-
`ferential exclusion mechanism.66 Excipients that
`will stabilize proteins against the effects of freez-
`
`EXCIPIENT–DRUG INTERACTIONS
`
`2291
`
`ing do so by not associating with the surface of
`the protein. Such excipients actually increase the
`surface tension of water and induce preferential
`hydration of the protein. Examples of solutes that
`serve as cryoprotectants by this mechanism in-
`clude amino acids, polyols, sugars, and polyethy-
`lene glycol.
`For lyoprotection, that is, stabilization of pro-
`teins during the drying stages of freeze drying
`and during storage in the dry state, two mechan-
`isms have been generally accepted. One is the
`water-substitute hypothesis67 and the other is
`the vitrification hypothesis.68 Both are legitimate
`theories, but both also have exceptions,
`i.e.,
`neither fully explain the stabilization of proteins
`by excipients during dehydration and dry sto-
`rage.69 The water-substitute hypothesis states
`that a good stabilizer is one that hydrogen bonds
`to the protein just as water would do were it
`present and, therefore, serves as a water sub-
`stitute. Sugars are good water substitutesb which
`is why many freeze-dried protein formulations
`contain sucrose or trehalose.
`The vitrification hypothesis states that excipi-
`ents that remain amorphous (glass formers) form
`a glassy matrix with the protein with the matrix
`serving as a stabilizer. Acceptance in this hypoth-
`esis requires formulators to determine glass
`transition temperatures of formulations to be
`freeze dried and to develop freeze-dry cycles that
`maintain drying temperatures below the glass
`transition temperature. Studies have been pub-
`lished showing that excipient stabilizers that
`crystallized during storage caused degradation
`(typically aggregation and loss of potency) of the
`protein.70–72
`Freeze-dried formulations typically contain
`one or more of the following bulking agents:
`mannitol, lactose, sucrose, trehalose, dextran 40,
`and povidone. These excipients may also serve as
`cryo- and/or lyoprotectants in protein formula-
`tions. Fakes et al.73 studied these bulking agents
`for moisture sorption behavior before and after
`freeze drying. Moisture uptake certainly can affect
`drug stability in the freeze-dried state, particu-
`larly with proteins. They reported the following
`observations for each excipient:
`
`bIt may at first appear contradictory that sugars can serve
`both as cryoprotectants because of being excluded from the
`surface of the protein and as lyoprotectants that hydrogen
`bond to the protein. However, keep in mind that the excluded
`solute concept involves a frozen aqueous system whereas the
`water-substitute concept occurs in a dry system.
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`Mannitol
`Crystalline and nonhygroscopic both before
`and after freeze drying
`Total moisture contents of 0.1 to 0.3% w/w
`between 10 and 60% relative humidity (RH)
`Lactose
`Crystalline and nonhygroscopic before lyo-
`philization
`Moisture content 0.86% before lyophilization
`Amorphous after lyophilization with moisture
`content of 1.6%
`Absored moisture rapidly upon exposure to
`high RH
`Converted to crystalline form at 55% RH after
`absorption of 10% moisture
`Desorption of all moisture sorbed by the amor-
`phous form
`Sucrose
`Crystalline and nonhygroscopic before lyophi-
`lization
`Moisture content 0.15% before lyophilization
`Amorphous after lyoph