General Guidelines/Summary for Compatibility (Solubility) and Chemical Stability

DAVID W. NEWTON, BS PHARM, PHD, FAPHA, PROFESSOR
Department of Biopharmaceutical Sciences
Bernard J. Dunn School of Pharmacy
Shenandoah University
Winchester, VA 22601 USA
e-mail: dnewton@su.edu, phone: 540-678-4333

Introduction

Compatibility, stability, and incompatibility are subjects of which a pharmacist must be thoroughly educated to protect patients from harm resulting from the administration of an improperly compounded or stored preparation. This paper will present the guidelines in an easy-to-use format categorized as follows:

1. Compatibility (Solubility) and Incompatibility (Precipitation) Guidelines for Salts of Organic Drugs
   1. Conjugate pairs of acids and bases
   2. Henderson-Hasselbalch equations for mono-functional acid and base conjugate pairs
   3. General principles
   4. Conditions favoring incompatibility or precipitation, the converse of which favor compatibility or solubility
   5. Facile identification of charges on organic drug ions in salts when lacking illustration and interpretation of chemical structures
   6. Ampholytes
   7. Zwitterions
   8. Soluble calcium salts mixed with divalent anions
   9. United States Pharmacopeia (USP) nomenclature policy and drug salts
   
   
   
2. Chemical Stability and Instability Guidelines
   1. Chemical symbols and their definitions
   2. Hydrolysis of esters, imides, imines, lactams, and lactones
   3. Auto-oxidation
   4. Photo-oxidation
   5. Non-enzymatic, heat catalyzed, covalent Maillard (my-ARE) condensation reactions
   6. Instabilities of monoclonal antibodies and peptide drugs
   7. Examples of common instability reactions and labile drugs and drug classes

A. Compatibility (Solubility) and Incompatibility (Precipitation) Guidelines for Salts of Organic Drugs

1. Conjugate pairs of acids and bases
   
   
   1. A large majority of drugs or active pharmaceutical ingredients (APIs) are salts of weak electrolytes. Weak electrolytes are conjugate forms, or Brönsted-Lowry acid and base forms, of the same compound that differ only by presence or absence, respectively, of protons. Acid forms incompletely dissociate protons in water, and base forms incompletely hydrolyze water to accept protons. The fractions of the acid and base forms depend on pH and pK_a values (see part A 2). Salt forms of weak electrolytes dissociate essentially completely into their respective ions in water and predominantly aqueous fluids.
      
      
   2. Chemical symbols and their definitions

      Symbol	Definition
      [ ]	Concentration, such as molar
      An-	Anionic conjugate base form of an HnA acida
      B	Non-ionized or free base form of a primary (RNH2), secondary (R2NH), or tertiary (R3N) aminea
      nCn+nAn-	Salt of an HnA acid, where C is a non-drug cation (e.g., potassium, sodium)
      H+	Hydrogen ion or proton
      HnA	Non-ionized acida
      HnBn+	Cationic conjugate acid or protonated form of an amine base, Ba
      HnBn+nXn-	Salt of a B base, where X is a non-drug anion (e.g., chloride, citrate, edisylate)
      H3O+	Hydronium or hydrated hydrogen ion
      n	Quantities and valences of ions in a salt
      OH-	Hydroxide or hydroxyl ion
      p	Negative logarithm10 or -log of some quantity
      pH	-log[H3O+]
      pKa	-log Ka, where Ka is an acid dissociation constant. pKa is the pH at which the thermodynamic activities, or concentrations practically, of acid and base forms of a weak electrolyte conjugate pair are equal.
      R4N+	Quaternary amine

   3. Acid↔base conjugate pairs. H_nA↔A^n- + nH⁺ and H_nBⁿ⁺↔B + nH⁺
   4. Base hydrolysis. B and A^- + H₂O ↔ BH⁺ and HA + OH^-
2. Henderson-Hasselbalch equations for mono-functional acid and base conjugate pairs
   1. pH = pK_a +log([base form]/[acid form]
         (1.) pH = pK_a+ log([A^-]/[HA])
         (2.) pH = pK_a+ log([B]/[HB⁺])
   2. percent of A^- form = 100/(1+10^[pKa-pH]) = (100-percent of HA form)
   3. percent of HA form = 100/(1+10^[pH-pKa]) = (100-percent of A^- form)
   4. percent of HB⁺ form = 100/(1+10^[pH-pKa) = (100-percent of B form)
   5. percent of B form = 100/(1+10^[pKa-pH]) = (100-percent of HB⁺ form)
3. General principles
   1. Precipitation is more likely to occur when opposite drug ions A^n- and HBⁿ⁺ contain aromatic rings, which can form induced dipole-induced dipole, London dispersion, or "hydrophobic" intermolecular forces that water cannot dissociate.
   2. Visible precipitation may neither appear instantly nor cease momentarily because it can take hours to weeks to reach equilibrium between dissolved and solid drug in some supersaturated solutions. The decline of supersaturated drug solute over time in particle-free filtrates or supernates usually correlates best with ln concentration versus time regression, or first order kinetics. Initially, larger crystals and aggregates form and precipitate followed more slowly by smaller masses as the supersaturated drug concentration declines over time.
   3. A compatible or incompatible outcome may depend on seemingly small differences in ranges of ingredient strength and pH in monographs for drug injections and other solutions in official compendia, such as the USP. Such differences occur in different lots by the same manufacturer and label-equivalent products from different manufacturers.
4. Conditions favoring incompatibility or precipitation, the converse of which favor compatibility or solubility
   1. The likelihood of B precipitating in solutions of H_nBⁿ⁺nX^n- salts increases with increasing H_nBⁿ⁺nX^n- concentration, pH, and nCⁿ⁺nA^n- concentration, and decreasing intrinsic solubility of B.
   2. The likelihood of H_nA precipitating in solutions of nCⁿ⁺nA^n- salts increases with increasing nCⁿ⁺nA^n- and H_nBⁿ⁺nX^n- concentrations, and decreasing pH and intrinsic solubility of H_nA.
5. Facile identification of charges on organic drug ions in salts when lacking illustration and interpretation of chemical structures
   1. Drug cations, H_nBⁿ⁺, in salts may be identified by the suffixes -ate or -ide in the name of the non-drug anion in the drug salt name (e.g., gentamicin sulfate, ondansetron hydrochloride).
   2. Drug quaternary amines, R₄N^+-, in salts may be identified by the suffix -ide in the non-drug ion name (e.g., neostigmine bromide, succinylcholine chloride); the suffixes -ium and -ate in the drug and non-drug ion names, respectively (e.g., atracurium besylate, bretylium tosylate); there are fewer such quaternary amines than drug anion esters in part d below that share these same suffixes; and the suffixes -ium and -ide in the names of the drug and non-drug ions, respectively (e.g., doxacurium chloride, ipratropium bromide).
   3. Drug anions, A^n-, in salts may be identified by the suffixes -ium, in the name of the non-drug cation (e.g., penicillin G potassium, phenytoin sodium).
   4. Drug anion esters, A^n-, in salts may be identified by the suffixes -ium and -ate, in, respectively, the names of the non-drug anion and ester (e.g., dexamethasone sodium phosphate, methylprednisolone sodium succinate).
6. Ampholytes can react either as an acid or base depending on pH. For example, dihydrogen phosphate, H₂PO₄^-, is an acid relative to monohydrogen phosphate, HPO₄^2-; monoprotonated hydroxyzine, HB⁺, is a base relative to diprotonated hydroxyzine, H₂B²⁺.
7. Zwitterions, which are best known as but not limited to amino acids (e.g., ⁺H₃NRCOO^-), are least water soluble at pH values equal to or within 0.1 unit of their isoelectric points, pI. For example, ampicillin has a pI of 4.9 from the mean of its -COOH pK_a of 2.5 and -CNH₃⁺ pK_a of 7.2. At the pI, the intramolecular attraction of the opposite ionic charges decreases external ionic attraction to water via ion-dipole intermolecular forces, thus, solubility is decreased.
8. Soluble calcium, Ca²⁺, salts mixed with divalent anions
   1. Calcium forms poorly soluble salts with most divalent inorganic anions. For example, the solubility of the CaHPO₄ is 0.3 mg/mL, but of the Ca(H₂PO₄^-)₂ salt is 18 mg/mL.
   2. Calcium may precipitate divalent organic anions. For example, calcium forms a precipitate when mixed with ceftriaxone sodium, the salt in Ceftriaxone for Injection USP, which is actually a disodium salt [see part 9 below]; thus, the valence of ceftriaxone is -2 from its two carboxylate, -COO^-, groups.
9. USP nomenclature policy excludes names of non-drug ions in article or monograph titles for salt forms of many drugs (i.e., it cannot be readily identified from the drug name that the drug species in solution will be an anion, A ^n-, or a cation, H_nBⁿ⁺).
   1. Drug salts formed in situ contain only the name of the therapeutic moiety. For example, Furosemide Injection USP consists of furosemide prepared with sodium hydroxide, NaOH, at a pH of 8.0 to 9.3. From the furosemide -COOH pK_a of 3 and the equation in part A 2 b, at least 99.999% of furosemide is in the anionic R-COO^- form at pH 8.0, and more at pH 9.3.
   2. Sterile drug powders for injection are named for the active drug or API moiety, but the labeling information reveals they are actually salts. For example, Ampicillin for Injection USP contains ampicillin sodium equivalent to the labeled strength of ampicillin, meaning ampicillin is an anion; ceftobiprole medocaril contains a mass of its sodium salt equivalent to the labeled strength of the named acid, meaning ceftobiprole is an anion; and Gentamicin Injection USP contains a mass of gentamicin sulfate equivalent to the labeled strength of gentamicin, meaning gentamicin is a polycation.

B. Chemical Stability and Instability Guidelines

1. Chemical symbols and their definitions

   Symbol	Definition
   Ar	Aromatic ring-containing compound
   R	An aliphatic or aromatic group or radical

2. Hydrolysis of esters, imides, imines (azomethines or Schiff bases), lactams (cyclic amides), and lactones (cyclic esters)
   1. Hydrolysis rate constants, k; hydrolysis rates change exponentially in the direction temperature changes according to the Arrhenius equation, k=Ae^-(EA/RT). Therefore, stability times such as half life (t₅₀) and shelf life (e.g., t₉₀ , t₉₅.) change exponentially in the direction opposite the temperature change. For example, temperature changes of 10^o, 20^o, 30^o, and 40^o can theoretically cause changes of, respectively, 2- to 4-fold, 4- to 16-fold, 8- to 64-fold, and 16- to 256-fold in degradation rate constants; thus, in particular drug stability times or shelf lives, where the largest values occur with the largest energies of activation, E_A, of the particular hydrolysis reaction.
   2. Drugs are most labile to hydrolysis at 7<pH<5 (i.e., from general acid and base catalysis) and when dissolved concurrently with drug-specific catalysts (e.g., Cu²⁺ in non-purified water speeds hydrolysis of beta lactam antibiotics).
3. Auto-oxidation of hydroxyl groups on aromatic rings, Ar-OH, to therapeutically inactive and chemically reactive quinones, Ar=O, is catalyzed by ultraviolet (UV) illumination, pH>7 (alkaline), oxygen, and Cu²⁺, Fe²⁺, and Fe³⁺ ions. Oxidation rates are often less sensitive to temperature change than are hydrolysis rates.
4. Photo-oxidation catalyzed by UV illuminations of various drug classes to less active or inactive products
5. Non-enzymatic, heat catalyzed, covalent Maillard condensation reactions occur with non-ionized primary and secondary amines, RNH₂ and R₂NH, and aldehydes, RHC=O, and ketones, R₂C=O, including the aldose and ketose tautomers of reducing sugars, such as dextrose, fructose, and lactose.
6. Instabilities of monoclonal antibodies and peptide drugs
   1. Irreversible rapid denaturation of tertiary structural folding is likely when exposed to conditions prohibited in manufacturers' labeling (e.g., particular pH values, electrolytes, antimicrobial preservative chemicals such as benzyl alcohol and phenol derivatives, and extreme temperatures such as excessively warm or freezing.
   2. Adsorption may occur on some polymer filter membranes, glass and plastic containers, and plastic tubing. Manufacturers of drug products and devices should be consulted for appropriate usage.
   3. Irreversible aggregation and precipitation may occur with vigorous agitation or shaking instead of gentle rolling of vials to dissolve sterile powders.
7. Examples of common instability reactions and labile drugs and drug classes

   Instability Reaction	Labile Drugs and Drug Classes
   Hydrolysis of esters	Aspirin, atropine, benzocaine, cefotaxime, famcyclovir, flumazenil, fosphenytoin, meperidine, oseltamivir phosphate
   Hydrolysis of imides	Barbiturates (pentobarbital, phenobarbital, thiamylal, thiopental)
   Hydrolysis of imines	Benzodiazepines (diazepam, lorazepam, midazolam), rifampin
   Hydrolysis of lactams	Benzodiazepines (diazepam, lorazepam), cephalosporins, penicillins
   Hydrolysis of lactones	Efavirenz, erythromycin, spironolactone, warfarin
   Auto-oxidation (catalyzed reaction with oxygen)	Acetaminophen, catecholamines (dobutamine, dopamine, epinephrine, isoproterenol), morphine, terbutaline
   Photo-oxidation or photolysis (catalyzed by ultraviolet illumination)	Catecholamines and other Ar-OH compounds, cimetidine, conjugated dienes (retinoids, unsaturated fatty acids), corticosteroids (dexamethasone, methylprednisolone) nifedipine, nitroprusside, phenothiazines (prochlorperazine, promethazine), sulfonamides (sulfacetamide, sulfamethoxazole), tetracyclines (doxycycline, minocycline)
   Maillard condensation of primary and secondary amines with aldehydes and ketones	Primary and secondary amine drugs: ampicillin, daptomycin, fluoxetine, oseltamivir, peptides, procainamide, proteins, sulfamethoxazole

C. Chronological Bibliography

1. Newton DW. Physicochemical determinants of incompatibility and instability in injectable drug solutions and admixtures. Am J Hosp Pharm 1978; 35(10): 1213-1222.
2. Newton DW. Introduction: Physicochemical determinants of incompatibility and instability of drug solutions and admixtures. In: Trissel LA. Handbook on Injectable Drugs. 2nd ed. Washington, DC: American Society of Hospital Pharmacists; 1980: xv-xxxi.
3. Newton DW, Narducci WA. Extemporaneous formulations. In: King RE, ed. Dispensing of Medication. 9th ed. Easton, PA: Mack; 1984: 258-288.
4. Connors KA, Amidon GL, Stella VJ. Chemical Stability of Pharmaceuticals. 2nd ed. New York, NY: John Wiley & Sons; 1986.
5. Lachman L, Deluca P, Akers MJ. Kinetic principles and stability testing. In: Lachman L, Lieberman HA, Kanig JL, eds. The Theory and Practice of Industrial Pharmacy. 3rd ed. Philadelphia, PA: Lea & Febiger; 1986: 760-803.
6. Newton DW, Miller KW. Estimating shelf-life of drugs in solution. Am J Hosp Pharm 1987; 44(7): 1633-1640. Previously unpublished corrections to page 1635 left column: paragraph 3, line 1 change "exponential" to "logarithmic;" paragraph 4, line 2 change "-E_a" to "-Ea/R."
7. Newton DW. The role of temperature in the life of a pharmaceutical preparation. PF 1999; 25: 7655-7661; 8627 corrections.
8. Newton DW. Three drug stability lives. IJPC 2000; 4(3): 190-193.
9. Gennaro AR. Organic Pharmaceutical Chemistry. In: Troy DB, ed. Remington: The Science and Practice of Pharmacy. 21st ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2005: 391-400.
10. O'Donnell PB, Bokser AD. Stability of Pharmaceutical Products. In: Troy DB, ed. Remington: The Science and Practice of Pharmacy. 21st ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2005: 1025-1036.
11. Newton DW, Driscoll DF. Calcium and phosphate compatibility: Revisited again. Am J Health Syst Pharm 2008; 65(1): 73-80.
12. Newton DW. Drug incompatibility chemistry. Am J Health Syst Pharm 2009; 66: 348-357; 1431 correction.
13. Newton DW. Crux of drug compatibility and incompatibility. Am J Health Syst Pharm 2010; 67(2): 108, 112.
14. United States Pharmacopeial Convention, Inc. United States Pharmacopeia 34-National Formulary 29. Rockville, MD: US Pharmacopeial Convention, Inc.; 2011: 659-662, 742-745.
15. Newton DW. Maillard reactions in pharmaceutical formulations and human health. IJPC 2011; 15(1): 32-40.
16. Trissel LA. Handbook on injectable drugs. 16th ed. Bethesda, MD: American Society of Health-System Pharmacists; 2011.
17. Lemke TL. Review of Organic Functional Groups. 5th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2012.