Introduction
Substances that leach into drug products from their associated packaging system can potentially affect the suitability of the drug product for its intended use. Thus drug products may be tested for leachables to establish whether the leachables achieve levels in the drug product that are sufficiently high that they discernibly impact the product’s suitability for use.
Alternatively or additionally, the packaging system can be tested for extractables, as leachables and extractables are generally related. Such testing involves extraction of the packaging system (or its materials and components of construction) with one or more appropriate solvents and then testing the extracts for extracted substances. This combination of extract generation and extract testing is known as a controlled extraction study. If the purpose of the controlled extraction study is to produce an extractables profile for a packaging system that is similar to the leachables profile of a drug product stored in that packaging system, then the conditions of the extraction should mimic the conditions of clinical contact between the drug product and its packaging system. Relevant extraction conditions to consider in the design of the controlled extraction study include the chemical nature of the extraction solvent and the drug product, as it is well known that the chemical properties of the extracting medium and drug product have a marked effect on the extractables and leachables profiles.
One of the critical properties of an aqueous extraction medium or drug product is its pH. This is especially true for aqueous parenteral drug products, which are generally formulations whose pH can range from somewhat acidic (pH 3 or lower) to somewhat basic (pH as high as 10). Because extractables and leachables may be acidic or basic compounds, the pH of the extraction solvent and the drug product is a critical extraction design parameter, as the solubility, and thus the accumulation level, of an acidic or basic leachable (or extractable) will be pH-dependent. Such an effect of drug product or extraction solvent pH on the accumulation of leachables and extractables is well-documented in the literature [1-7]. For example, Jenke has documented the increase in the levels of substituted benzoic acids extracted from a polyolefin material as a function of the increasing pH of the aqueous extraction solvent [4]. Under the same extraction conditions, the accumulation levels of neutral or non-ionic extractables were unaffected by extraction solvent pH. This phenomenon occurs since the solubility of the dissociated form of an acid or base is greater than the solubility of the un-dissociated acid or base itself.
In addition to the solubility effect, the pH of the extraction solvent can affect an extractables or leachables profile if the specific extractable or leachable is affected by a process that is dependent on pH. For example, Jenke et al. have documented the pH-dependent accumulation behavior of a class of extractables that are prone to acid or base hydrolysis [8]. As the extraction time increased, the levels of these extractables decreased due to their hydrolysis; as the rate of hydrolysis was pH dependent, so too was the level of these extractables in the extracting solution. Additionally, it has been suggested that the accumulation of Bisphenol A leached from polycarbonate materials is pH dependent as the Bisphenol A arises at high pH due to depolymerization of the polymer [5]. Furthermore, the increased extraction of mono-(2-ethylhexyl) phthalate (MEHP) from di-(2-ethylhexyl) phthalate-plasticized polyvinyl chloride materials has been attributed to MEHP’s higher solubility at higher pH coupled with the base-catalyzed decomposition of DEHP [5].
Considering the thermodynamic aspects of the leaching and extraction processes, the equilibrium distribution of a leachable (or extractable) between its packaging system source and the drug product (or extraction medium) solution is reflected in a source/solution partition coefficient. To a first approximation, such a partition coefficient is related to the solubility of a leachable in the drug product or of an extractable in the extraction medium. Because solubility data are more readily available than are source/solution partition coefficients, the discussion in this manuscript will focus on solubility. Considering more specifically the effect of pH on the accumulation of acidic and basic extractables further, the relationship between the solution pH, the extractable’s acid dissociation constant (pKa), its effective solubility (Se) at a particular pH, and its intrinsic solubility (S0) is expressed as follows:
For an acid: log Se = log S0 +log [1+10(pH – pKa)]
For a base: log Se = log S0 +log [1+10(pKa – pH)]
This relationship between solubility and pH is illustrated in Figure 1 for an acid and base that have a pKa of 5.0 and an arbitrary S0 of 100. In essence, Figure 1 suggests that as the pH changes, the solubility will change without limit. In practical reality, the dissociated ion itself will not be infinitely soluble and thus the solubility versus pH plot eventually reaches a plateau whose value represents the intrinsic solubility of the dissociated ion. For example, consider the case of stearic acid. As the pH of the extraction medium increases, the solubility of this acid increases to such an extent that stearate ion concentrations in extracts with a pH greater than 7 can exceed 10 mg/L. However, if the extracting solution contains a source of an alkali metal (for example, Na+ of K+ as the counter-ions used in the buffer salts), the solubility product of the alkali metal–stearate salt will be exceeded and the salt will precipitate [8]. Additionally, it may be the case that the total pool of the extractable is limited, in which case the calculated solubility might exceed the total pool. In this circumstance, further increasing the pH of the extracting medium will not produce a further increase in the concentration of the acidic extractable in the extract because its pool has been depleted.
Figure 1. The Effect of pH on the Solubility of an Acidic or Basic Extractable; conditional solubility (Se) as a function of solution pH for an acidic or basic extractable with a pKa of 5.0 and an intrinsic solubility (S0) of 100 (arbitrary units). As the pH increases, the solubility of the acidic extractable increases, and as the pH decreases, the solubility of a basic extractable increases. As the solubility increases, the concentration of the extractable will increase until the total pool of the extractable in the material is exhausted or the solubility of the dissociated form of the extractable is exceeded.It is clear from this discussion that pH is an important property of an extracting medium which will affect the extractables profile obtained, both qualitatively (what extractables are present in the profile) and quantitatively (at what level is the extractable present in the extract). Thus when one is performing a controlled extraction study whose purpose is to forecast the leachables profile of a single drug product, the pH of the extraction solvent must match the pH of the drug product under consideration. However, in many cases the purpose of the controlled extraction study is to forecast the behavior of a number of drug products in a given packaging system or to forecast the behavior of a material in numerous applications. In these cases, the controlled extraction study must include multiple extracting solvents whose pH values span the application range. However, when the application range in pH is wide, or when it includes extreme pH values, there are several practical issues that must be addressed in order to ensure that the proper number and type of extraction solvents are utilized. Specifically, two questions are relevant:
- What is the proper range in pH values that provides the most complete extractables profile?
- Is there any intrinsic value in using extraction solvents with neutral pH values?
The purpose of this manuscript is to consider these and additional questions in the context of the chemical properties of commonly encountered acidic and basic extractables.
Discussion
When considering the impact of extraction solvent pH on a qualitative or quantitative extractables profile, a significant aspect to address is the acid/base properties of commonly encountered extractables. Thus, Tables 1 and 2 were compiled to include acidic or basic extractables that have been reported in the literature. Considering acidic extractables (Table 1), it is noted that some of the more commonly encountered organic extractables include weak organic acids such as acetic acid, formic acid, stearic acid, and fatty acids of variable molecular weight. In general, these acidic extractables possess pKa values in the relatively narrow range of 3.2 to 5.5. A somewhat shorter, albeit no less significant, list of basic organic extractables is listed in Table 2. Unlike the organic acid extractables, the basic organic extractables exhibit a fairly wide range in pKa values.
Table 1. Acidic Extractables and Their Associated Acid Dissociation Constants (pKa)
Table 2. Basic Extractables and Their Associated Acid Dissociation Constants (pKa)
Rather than individually considering the behavior of each single extractable listed in Tables 1 and 2, the effect of extraction solvent pH on the accumulation of acidic or basic extractables can be established by focusing on representative acidic and basic extractables. The representative extractables chosen are listed in Table 3 and meet two requirements:
- That the representative extractables bracket the behavior of the two groups of extractables listed in Tables 1 and 2, and
- That the representative extractables have published solubility data over a wide range of pH values.
Table 3. Extractables Included in Figure 2, Effect of pH on Solubility
Figure 2 contains plots of mass solubility for the representative extractables as a function of solvent pH. The mass solubility axis has been standardized by expressing the solubility in a relative sense, where the point of reference is the maximum solubility reported for the individual extractable. This allows the data for the individual extractables, which exhibit a wide range of absolute solubilities, to be presented in a single plot.
Figure 2.The Effect of pH on the Reported Solubility of Selected Extractables. Because DEP is a non-ionic extractable, its solubility is unaffected by pH. The solubility of the acidic extractables (AA, SA, and MEHP, Figure 2a) increases with increasing pH and the solubility of the basic extractables (SAM, DBA, TDA, BTA, Figure 2b) increases with decreasing pH, depending on their specific pKa values. A zone of divergence is created between the pH values where the weakest acid (SA) and the weakest base (BTA) achieve their maximum solubilities. If a set of extraction solvents seeks to capture essentially all possible acidic or basic extractables at their likely highest concentration, then the set of extraction solvents must have a pH that spans the pHs that define the zone of divergence.It is often desirable to ascertain the “worst case” leachables profile for several drug products that may be packaged in the same packaging system, where “worst case” means the greatest number of leachables at their highest concentrations. If it can be concluded, based on a toxicological assessment of this worst case leachables profile, that these leachables represent an acceptable safety risk, then all drugs products covered by the worst case can be concluded to be safe in the packaging system of interest. Similarly, if a packaging system’s “worst case” extractables profile can be ascertained, and that extractables profile can be toxicologically established to have minimal and acceptable safety impact, then all drugs products packaged in the system are expected to be safe from a leachables perspective. If a key differentiating characteristic between the drug products is their pH, then Figure 2 can be used to answer specific questions about the proper pH values for extraction solvents.
Question: What is the proper range in pH values that provides the most complete extractables profile for typical drug products and for typical extractables?
Answer: The pH range of roughly 2 to 10 defines a “zone of divergence,” where the zone of divergence is that pH range in which the solubilities of the representative extractables are changing as a function of pH and within which the maximum solubility for the extractables is achieved. Utilization of extraction solvents at the two pH extremes of the zone (pH 2 and pH 10) would individually produce extractables profiles that (a) include the greatest number of acidic or basic extractables, and (b) include these extractables at their highest concentrations. Furthermore, aqueous parenteral drug products are generally formulated with a pH value in this range. Thus if the purpose of the controlled extraction study was to establish a material’s “worst case” extractables profile (all possible extractables at their highest levels), this purpose would be achieved by performing the extractions at both pH 2 and 10. The pH 2 extract would represent the worst case for basic extractables while the pH 10 extract would represent the worst case for acidic extracts. Either pH would be representative for non-ionic extractables. Considering the representative extractables only, generating extracts at pH values outside the zone of divergence would not materially change the extractables profiles versus what they were at the endpoints of the zone.
Question: What is the proper range in pH values that provides the most complete extractables profile for specific drug products with a particular pH range and for typical extractables?
Answer: The answer to question 1 notwithstanding, if the purpose of the extraction study is to address a set of drug products whose pH range is smaller than the zone of divergence, then the proper pH range to use is the product range and not the pH extremes represented by the zone of divergence. Using pH values outside the product range will produce extractables profiles that over-estimate and thus exaggerate the situation being evaluated.
Question: Can a single extraction solvent with a single pH be representative for either a single drug product or multiple drug products?
Answer: Although a drug product may be thought of as having a single pH, in fact all drug products have a range of acceptable pH values. As the solubilities of the representative extractables are strongly impacted by the pH of the extraction solvent, it is clear that the extractables profile at two individual pH values would be different, both in terms of the specific extractables that might be detected and the measured concentrations of the detected extractables. The magnitude of the difference in extractables profiles at two pH values, for example, pH a and pH b, will depend on the magnitude of the difference between a and b and where a and b fall on the pH scale. The larger the difference between pH a and pH b, the greater the difference in the extractables profiles at pH a and b and the less likely that a single extraction solvent with a single pH will properly represent a single drug product, let alone a family of drug products. Furthermore, the more extractables that have a pKa near to or between a and b, the greater will be the difference in the extractables profiles at pH a and b and the less likely that a single extraction solvent with a single pH will properly represent a single drug product, let alone a family of drug products.
Therefore an extraction solvent with a single pH will be somewhat representative for a single drug product. It is less likely, however, that a single pH would be representative for multiple drug products exhibiting a wide range in pH values.
Question: Can a single extraction solvent with a single pH reasonably be a worst case for a single drug product or multiple drug products?
Answer: This question is similar to Question 3 but differs via the distinction between “representative” and "worst case”. In Question 3, the challenge was to find a single pH that produces an extractables profile that mirrors the leachables profile of a drug product or drug products over a range of pH values. In this Question 4, the challenge is to find a single pH that produces an extractables profile that is worse than the leachables profile for a drug product or drug products over a range of pH values. Whereas the answer to Question 3 was “generally there is no such single pH”, the answer to Question 4 is that “at the pH extremes there could be”. Consider, for example, a drug product or set of drug products with a pH range between 2 and 4. For these drug products, the worst case extractables profile would be produced at the lowest pH of the range, as few acidic extractables exhibit greatly increasing solubility as pH increases from 2 to 4 and basic extractables are such that their solubilities increase as pH decreases. Conversely, consider a drug product or set of drug products with a pH range greater than 9. For these drug products, the worst case extractables profile would be produced at the highest pH of the range, as most acidic extractables have achieved their greatest solubility at pH values of 9 or lower and few basic extractables have their solubilities increasing greatly as pH decreases to 9.
The pH range between 4 and 9 is that range over which the solubilities of a large number of ionic extractables are strongly affected by solution pH. Thus within this range there is no single pH value that can be established as producing the worst case extractables profile for an solution whose pH is within this range.
Question: Even though an extraction solvent with a single pH may not be representative of or a worst case for a single drug product or a group of drug products, is there a single extraction solvent pH that would produce an extractables profile that contained all the ionic extractables in measurable quantities?
Answer: In general, it is likely that there is no single pH at which all the potential extractables would be readily measurable. At pH values less than roughly 6, the levels of stearic acid (and other acidic extractables with similar pKa values) would likely be sufficiently low that the stearic acid would be present in the extract at trace levels. Similarly, at a pH greater than 6, the levels of 2,4-toluenediamine and 2-benzothazolamine would likely be sufficiently low that they would be present in the extract at trace level. Thus a controlled extraction study designed to qualitatively establish all of a material’s potential extractables would need to include multiple extraction solvents at different pH values.
In a specific extraction study, it may be the case that the members of an extractables profile are such that the profile does not consist of chemically diverse compounds and therefore that a single extraction pH is adequate for establishing the qualitative profile.
Question: If I perform an extraction study using extraction solvents at the pH extremes that you have suggested (pH 2 and pH 10), do I also have to use an extraction solvent at an intermediate, near neutral pH as well?
Answer: None of the targeted extractables reach and then retreat from a solubility maximum in the pH range of roughly 6 to 8. Thus if the purpose of the controlled extraction study was to establish a material’s “worst case” extractables profile (all possible extractables at their highest levels) and the study included extraction solvents whose pH values were beyond the range of 6 to 8, then performing an extraction with a solvent whose pH was within this range would add no value it terms of revealing “new” extractables or producing higher concentrations for the already-measured extractables. For example, if the relevant range in drug product pH values was between 4.5 and 8.5, then there would be no value gained in generating extractables profiles using three solvents, one at pH 4.5, one at pH 8.5 and one at a pH value between these two extremes versus generating profiles only at the pH extremes. The extractables profile obtained at the intermediate pH would be less extreme (fewer extractables at lower levels) than the profiles established at the pH extremes. As the pH extremes of the extraction solvents diverge, the above statements remain valid. Thus if the purpose of the extraction study is to establish the worst case extractables profiles for any drug product with a pH between 2 and 10, then there is little or no value in performing an extraction at a pH between these extremes.
Question: My drug product (or group of drugs products) has a wide pH range. Is it necessary to use an extraction solvent whose pH is in between the pH extremes of the drug product or can I just use extraction solvents at the pH extremes?
Answer: This question is similar to Question 6. If the purpose of the controlled extraction study was to establish a material’s “worst case” extractables profile (all possible extractables at their highest levels) and the study included extraction solvents at the extremes of the pH range, then performing an extraction with a solvent whose pH was within this range would add no value it terms of revealing “new” extractables or producing higher concentrations for the already-measured extractables. The extractables profile obtained at the intermediate pH would be less extreme (fewer extractables at lower levels) than the profiles established at the pH extremes.
Question: I have established that my extractables profile does not include any ionic extractables. Do I need to worry about the pH of my extraction solvent for these non-ionic extractables?
Answer: The accumulation of a non-ionic extractable, represented by diethyl phthalate (DEP) in Figure 2, is unaffected by pH and thus an extraction solvent at any pH is appropriate for qualitatively and quantitatively delineating the non-ionic members of an extractables profile. The only exception to this statement is those extractables which are unstable and whose decomposition is pH dependent. While the pH of the extraction solvent will not affect the solubility of such extractables, the concentration of an unstable extractable in an extraction solvent could depend on the pH of the extraction solvent.
Question: Is there any pH at which the extractables profile would always be “least” case (fewest extractables at their lowest levels)?
Answer: Examination of Figure 2 clearly illustrates that in the general case of an unspecified extractables profile, it is not possible to specify a single pH that provides the “least case” extractables profile, as there is considerable overlap in the pKa’s of the seven model extractables that are shown in this figure. For example, examination of Figure 2a suggests that for the acidic extractables shown, a pH of 4 (or lower) would produce such a least case. However, examination of Figure 2b suggests that for the basic extractables, the least case would be achieved at a pH of 9 or greater. Thus for an extractables profile that could include both unspecified acidic and basic extractables, it is not possible to unilaterally establish a single extraction pH that would produce the least case profile.
In considering the answers provided to the above questions, it is noted that these answers were given in the context of an unspecified extractables profile whose ionic and non-ionic constituents are unknown.
Clearly, if one has ascertained the individual members of the extractables profile and then wants to design extraction studies and establish the proper extraction solvent pH values to address the levels of these known extractables, one can use the knowledge to draw conclusions specific for their circumstance. One can envision specific circumstances where the generalizations made previously would not be applicable.
If it is the case that one could provide more applicable answers to the above questions if one only knew what the extractables were, then it is reasonable to ask, “Is there some easy test that can performed that will at least tell me whether I have acidic or basic extractables (or both)?” The answer is maybe. Measuring the pH of a water extract and performing an acidity/alkalinity titration on that extract would provide the answer to the question of “what is the acid and base character of my extractables?” For example, obtaining an extract whose pH was low would surely indicate that the extract contained acidic extractables. The shape of the acid/base titration curve would establish the pKa's of the acidic and basic extractables and the amount of the acidic or basic extractables. The reason that answer was maybe was that the titration would have to be performed with sufficient sensitivity that it could deal with what are usually relatively dilute extracts. Furthermore, the more ionic extractables there are in an extractables profile, the more complex the titration curve will be, leading
Conclusions
Considering the two questions posed in the Introduction, the following answers are provided:
- The proper pH range for simulating extracting solvents used in controlled extraction studies designed to forecast drug product leachables spans the pH range of the relevant drug products. Nevertheless, relatively complete extractables profiles, reflecting all relevant extractables and establishing their highest possible levels as drug product leachables, are obtained at pH extremes of 2 and 10.
- Use of an extraction solvent with a neutral pH between 6 and 8 adds little or no value in terms of establishing the worst case extractables profile (greatest number of extractables at their highest levels). This is the case as it is rare that an extractable achieves a concentration in an extract in the pH region of 6 to 8 which is higher than the concentration achieved at lower and higher pH.
Combining points 1 and 2 produces a third conclusion: - Simulating the “leaching power” of drug products that span a specific pH range requires the use of extraction solvents whose pH values closely match the extreme pH values of the drug products. Expanding the pH range of the simulating solvents much beyond the pH range of the drug products produces an extractables profile that unnecessarily over-exaggerates the drug products’ leachables profile. Furthermore, using a third simulation solvent whose pH is between the pH extremes serves no useful purpose as the extractables profile generated at the intermediate pH will not be as extensive as the extractables profiles generated at the pH extremes.
While these conclusions may be valid for many pharmaceutical drug products and their associated packaging systems, they are especially meaningful to parenteral drug products, which are typically aqueous formulations whose pH may vary greatly (generally between 3 and 10). In considering these conclusions it is important to understand that they are relevant to the thermodynamic mass transfer aspects of extraction and leaching. However, it is well known that an extractables or leachables profile can be the result of other chemical processes. For example, the extractables profile can be affected by the acid- or base-mediated reaction of the extractables themselves or of their source materials. For instance, the accumulation of extractables known as “cyclic esters” is pH dependent as the cyclic esters are prone to acid- and base-hydrolysis [7]. Additionally, it is possible that the kinetics of extraction and leaching could be changed at high or low pH due to a pH-mediated change in the physical characteristics of the material or system being characterized. If the primary process involved in producing extractables or leachables is extraction or leaching, then the generalizations noted above are relevant and applicable. If the process of producing extractables or leachables includes pH-dependent reactions and or diffusion kinetics, then the generalizations noted above are less relevant, depending on the relative importance and pH-dependence of the individual processes (solubility/ partitioning, reactivity, or diffusion).
References
- D.R. Jenke; E.K. Chess; D.C. Zietlow; and B.E. Rabinow. Model for estimating the accumulation of solutes leaching from polymeric containers into parenteral solutions. Int. J. Pharm. 1992; 78: 115-122.
- D. Jenke; D. Zietlow; and S. Sadain. Leaching of organic acids from irradiated EVA plastics as a function of solution pH and polarity. PDA J. Pharm. Sci. Technol. 2004; 58(1): 24-31.
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- D. Jenke; J. Castner; T. Egert; T. Feinberg; A. Hendricker; C. Houston; D.G. Hunt; M. Lynch; A. Shaw; K. Nicholas; D.L. Norwood; D. Paskiet; M. Ruberto; E.J. Smith; and F. Holcomb. Extractables characterization of five materials of construction representative of packaging systems used for parenteral and ophthalmic drug products. PDA J. Pharm. Sci. Technol. 2013; 76(5): 448-511.
- A. Teasdale; M. Jahn; S. Bailey; A. Feilden; G. Taylor; M. Corcoran; R. Malick; D. Jenke; and L. Nagao. Controlled extraction studies applied to polyvinylchloride and polyethylene materials: conclusions from the ELSIE controlled extraction pilot study. In press.
- L.A. Cruz; M.P. Jenke; R.A. Kenley; M.J. Chen; and D.R. Jenke. Influence of solute degradation on the accumulation of solutes migrating into solution from polymeric parenteral containers. Pharm. Res. 1990; 9: 967-972.
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Dr. Dennis Jenke is a Baxter Distinguished Scientist at Baxter Healthcare Corporation. He has published extensively in the areas of analytical chemistry, environmental science, and material/solution compatibility, serves as an expert reviewer for pharmaceutical and analytical journals, is a member of professional and standard-setting organizations whose charter is to establish best demonstrated practices for material/solution compatibility, and is a frequent speaker on the subject of material/solution compatibility. He authored the book, Compatibility of Pharmaceutical Solutions and Contact Materials; Safety Considerations Associated with Extractables and Leachables.