Download Assuring quality of drugs by monitoring impurities

Advanced Drug Delivery Reviews 59 (2007) 3 – 11
www.elsevier.com/locate/addr
Assuring quality of drugs by monitoring impurities☆
Satinder (Sut) Ahuja ⁎
Ahuja Consulting, 1061 Rutledge Court, Calabash, NC 28467, USA
Received 14 January 2006; accepted 25 October 2006
Available online 16 November 2006
Abstract
To assure the quality of drugs, impurities must be monitored carefully. It is important to understand what constitutes an impurity and to identify
potential sources of such impurities. Selective analytical methods need to be developed to monitor them. It is generally desirable to profile
impurities to provide a yardstick for comparative purposes. New impurities may be observed as changes are made in the synthesis, formulation, or
production procedures, albeit for improving them. At times it is necessary to isolate and characterize an impurity when hyphenated methods do not
yield the structure or when confirmation is necessary with an authentic material. Availability of an authentic material can also allow toxicological
studies and provide a standard for routine monitoring of the drug product.
© 2007 Published by Elsevier B.V.
Keywords: Characterization; Chiral impurity; Degradation product; Drug product; Drug substance; Isolation; Profiling; Selective analytical methodologies;
Terbutaline
Contents
1.
2.
Introduction . . . . . . . . . . . . . . . . . . . . .
Terminology . . . . . . . . . . . . . . . . . . . .
2.1. Commonly used terms. . . . . . . . . . . .
2.1.1. Starting material(s) . . . . . . . . .
2.1.2. Intermediates . . . . . . . . . . . .
2.1.3. Penultimate intermediate . . . . . .
2.1.4. By-products . . . . . . . . . . . .
2.1.5. Transformation products . . . . . .
2.1.6. Interaction products . . . . . . . .
2.1.7. Related products . . . . . . . . . .
2.1.8. Degradation products . . . . . . . .
2.2. Compendial terminology . . . . . . . . . .
2.2.1. Foreign substances . . . . . . . . .
2.2.2. Toxic impurities . . . . . . . . . .
2.2.3. Concomitant components. . . . . .
2.2.4. Signal impurities . . . . . . . . . .
2.2.5. Ordinary impurities. . . . . . . . .
2.2.6. Organic volatile impurities (OVIs) .
2.3. ICH terminology . . . . . . . . . . . . . .
2.3.1. Organic impurities . . . . . . . . .
2.3.2. Inorganic impurities . . . . . . . .
2.3.3. Other materials . . . . . . . . . . .
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This review is part of the Advanced Drug Delivery Reviews theme issue on “Pharmaceutical Impurities: Analytical, Toxicological and Regulatory Perspectives".
⁎ Corresponding author. Tel.: +1 910 287 7565.
E-mail address: [email protected].
0169-409X/$ - see front matter © 2007 Published by Elsevier B.V.
doi:10.1016/j.addr.2006.10.003
4
S. Ahuja / Advanced Drug Delivery Reviews 59 (2007) 3–11
2.3.4. Residual solvents . . . . . . . . . . . . .
Comments on various terminologies . . . . . . . .
2.4.1. Chiral impurities . . . . . . . . . . . . .
3. Identification and qualification thresholds of impurities . .
4. Sources of impurities . . . . . . . . . . . . . . . . . . .
4.1. Synthesis-related impurities . . . . . . . . . . . .
4.2. Formulation-related impurities . . . . . . . . . . .
4.3. Degradation-related impurities . . . . . . . . . . .
4.3.1. Kinetic studies . . . . . . . . . . . . . .
5. Selective analytical methodologies . . . . . . . . . . . .
5.1. Spectroscopic methods . . . . . . . . . . . . . . .
5.1.1. Infrared spectrophotometry . . . . . . . .
5.1.2. Nuclear magnetic resonance spectroscopy
5.1.3. Mass spectrometry . . . . . . . . . . . .
5.2. Separation methods. . . . . . . . . . . . . . . . .
5.3. Hyphenated methods . . . . . . . . . . . . . . . .
6. Impurity profiling . . . . . . . . . . . . . . . . . . . . .
6.1. Samples to be profiled . . . . . . . . . . . . . . .
6.2. Components seen in a profile . . . . . . . . . . .
7. Isolating impurities . . . . . . . . . . . . . . . . . . . .
8. Characterization of impurities . . . . . . . . . . . . . . .
9. A case study. . . . . . . . . . . . . . . . . . . . . . . .
9.1. HPLC methods . . . . . . . . . . . . . . . . . . .
9.1.1. Achiral impurities . . . . . . . . . . . . .
9.1.2. Chiral impurities . . . . . . . . . . . . .
10. Conclusions . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.4.
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1. Introduction
Webster's dictionary defines impurity as something that is
impure or makes something else impure. An impure substance
may be defined as follows: a substance of interest mixed or
impregnated with an extraneous or usually inferior substance.
These definitions can help generate a more concise definition of
an impurity: any material that affects the purity of the material
of interest, viz., an active pharmaceutical ingredient (API) or
drug substance [1–4]. The purity of a drug product is in turn
determined on the basis of the percentage of the labeled amount
of API found in it by a suitable analytical method. Later
discussion will also reveal that a drug product can have
impurities that need to be monitored even though they do not
affect the labeled content. The presence of some impurities may
not deleteriously impact on drug quality if they have therapeutic
efficacy that is similar to or greater than the drug substance
itself. Nevertheless, a drug substance can be considered as
compromised with respect to purity even if it contains an
impurity with superior pharmacological or toxicological
properties. Consequently, in order to ensure that an accurate
amount of the drug substance is being administered to the
patient, drug substance purity must be assessed independently
from these undesirable extraneous materials (e.g., inert, toxic, or
pharmacologically superior impurities).
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this discussion, they are all considered impurities. To better
acquaint the reader with advantages and limitations of the use of
various terms, a brief description of these terms is given below,
followed by some comments.
2.1. Commonly used terms
A number of terms have been commonly used in the
pharmaceutical industry to describe organic impurities:
•
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•
•
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•
Starting material(s)
Intermediates
Penultimate intermediate (Final intermediate)
By-products
Transformation products
Interaction products
Related products
Degradation products
Some of these terms denote potential sources of impurities,
e.g., intermediates; others tend to de-emphasize the negativity,
e.g., related products. Let us review them individually.
2.1.1. Starting material(s)
These are the materials that are used to begin the synthesis of
an API.
2. Terminology
A large number of terms have been used to describe the
materials that can affect purity of the API. For the purpose of
2.1.2. Intermediates
The compounds produced during synthesis of the desired
material are called intermediates, especially when they have
S. Ahuja / Advanced Drug Delivery Reviews 59 (2007) 3–11
been isolated and characterized. The most important criterion
is characterization, i.e., they cannot be just theorized
potential reaction products (see by-products below). The
theorized compounds are best designated as potential
intermediates.
2.1.3. Penultimate intermediate
As the name suggests, this is the last compound in the
synthesis chain prior to the production of the final desired
compound. It is more appropriate to call it Final Intermediate.
Sometimes confusion arises when the desired material is a salt
of a free base or a free acid. In the opinion of this author, it is
inappropriate to label the free acid or base as the penultimate
intermediate if the drug substance is a salt.
2.1.4. By-products
The unplanned compounds produced in the reaction are
generally called by-products. It may or may not be possible to
theorize all of them. Hence, they present a challenging problem
to the analytical chemist in that a methodology cannot be
optimally planned if it is not known what needs to be excluded
from evaluations.
2.1.5. Transformation products
This is a relatively nondescript term that relates to theorized
and non-theorized products that may be produced in the reaction,
which can include synthetic derivatives of by-products.
Transformation products are very similar to by-products, except
this term tends to connote that more is known about the reaction
products.
5
2.2. Compendial terminology
The United States Pharmacopoeia (USP) deals with
impurities in several sections:
Impurities in official articles
Ordinary impurities
Organic volatile impurities
The USP acknowledges that concepts about purity are
susceptible to change with time, and purity is intimately related
to the developments in analytical chemistry. What we consider
pure today may be considered impure at some future date if
methods are found that can resolve other components contained
in a particular compound. Inorganic, organic, or polymeric
components can all be considered impurities. The following
terms have been used by the USP to describe impurities:
▪
▪
▪
▪
▪
▪
Foreign substances
Toxic impurities
Concomitant components
Signal impurities
Ordinary impurities
Organic volatile impurities (OVIs)
2.2.1. Foreign substances
The materials that are introduced by contamination or
adulteration, not as a consequence of synthesis or preparation,
are labeled foreign substances, e.g., pesticides in oral
analgesics.
2.1.6. Interaction products
Interaction products is a slightly more comprehensive term
than the two described above (by-products and transformation
products); however, it is more difficult to evaluate in that it
considers interactions that could occur between various
involved chemicals — intentionally or unintentionally. Two
types of interaction products that can be commonly encountered
are drug substance–excipient interactions and drug substance–
container/closure interactions.
2.2.2. Toxic impurities
These impurities have significant undesirable biological
activity, even as minor components; and they require individual
identification and quantification by specific tests.
2.1.7. Related products
As mentioned before, the term related products tends to
suggest that the impurity is similar to the drug substance and
thus tends to play down the negativity frequently attached to
the term impurity. Clearly these products generally have
similar chemical structures as the API and may exhibit
potentially similar biological activity; however, as discussed
later, this by itself does not provide any guarantee to that
effect.
2.2.4. Signal impurities
These are distinguished from ordinary impurities discussed
below in that they require individual identification and
quantification by specific tests. These impurities include some
process-related impurities or degradation products that provide
key information about the process.
2.1.8. Degradation products
These are the compounds produced because of decomposition of the material of interest or active ingredient. This term can
also include those products produced from degradation of other
compounds that may be present as impurities in the drug
substance.
2.2.3. Concomitant components
Bulk pharmaceutical chemicals may contain concomitant
components, e.g., antibiotics that are mixtures and are
geometric and optical isomers (see Section 2.4.1).
2.2.5. Ordinary impurities
The species of impurities in bulk pharmaceutical chemicals
that are innocuous by virtue of having no significant undesirable
biological activity in the amounts present are called ordinary
impurities.
2.2.6. Organic volatile impurities (OVIs)
This term relates to residual solvents that may be found in the
drug substance. OVIs are generally solvents used in the
6
S. Ahuja / Advanced Drug Delivery Reviews 59 (2007) 3–11
synthesis or during formulation of the drug product. The
solvents have been classified as follows by ICH.
Class I (to be avoided): benzene, carbon tetrachloride, 1,2-dichloromethane, 1,1-dichloroethane, and 1,1,1-trichloroethane.
Class II (should be limited): acetonitrile, chloroform,
methylene chloride, 1,1,2-trichloroethane, 1,4-dioxane, and
pyridine.
Class III: low toxic potential and permitted daily exposure
(PDE) of 50 mg or more.
Class IV: solvents for which adequate toxic data are not
available.
2.4.1. Chiral impurities
Chiral impurities have the identical molecular formula and
the same connectivity between various atoms, and they differ
only in the arrangement of their atoms in three-dimensional
space. The differences in pharmacological/toxicological profiles have been observed with chiral impurities in vivo [4,5].
This suggests that chiral impurities should be monitored
carefully.
3. Identification and qualification thresholds of impurities
The International Conference on Harmonisation addresses
questions relating to impurities as follows [6]:
2.3. ICH terminology
2.3.1. Organic impurities
Starting materials
Process-related impurities
Intermediates
Degradation products.
2.3.2. Inorganic impurities
Salts
Catalysts
Ligands
Heavy metals or other residual metals.
2.3.3. Other materials
Filter aids
Charcoal.
Q1A (R) stability testing of new drug substances and
products
Q3A (R) impurities in drug substances
Q3B (R) impurities in drug products
Q3C impurities: residual solvents
Q6A specifications: test procedures and acceptance criteria
for new drug substances and new drug products; chemical
substances
ICH guidelines for the identification and qualification
threshold of impurities and degradation products are provided
in Table 1.
As can be seen from the data in Table 2, ICH treats the
degradation products slightly differently than impurities even
though for all intents and purposes the degradation products are
impurities.
2.3.4. Residual solvents
Organic and inorganic liquids used during production and/or
crystallization.
4. Sources of impurities
2.4. Comments on various terminologies
4.1. Synthesis-related impurities
The impurities that may be present in the starting material(s)
can potentially be carried into the active ingredient of
interest. And the impurities that relate to the solvents used
during synthesis and the inert ingredients (excipients) used
for formulation must also be considered potential impurities that
may be found in API or drug product. Inorganic impurities
may also be found in compendial articles. These impurities may
be as simple as common salt or other compounds that are
controlled, such as heavy metals, arsenic, etc., which can
be introduced during various synthetic steps. Potential reaction
by-products, degradation products, and drug substance–excipient interactions must also be evaluated. All of these impurities have the potential of being present in the final drug
product.
Of the various terminologies described above, the International Conference on Harmonisation (ICH) provides a simple
classification to adequately address various impurities that may
be present in pharmaceutical products. However, all of these
terminologies fail to adequately highlight that enantiomeric
(chiral) impurities might warrant additional considerations.
Impurities in a drug substance or a new chemical entity
(NCE) originate mainly during the synthetic process from raw
materials, solvents, intermediates, and by-products. The raw
materials are generally manufactured to much lower purity
requirements than a drug substance. Hence, it is easy to
understand why they can contain a number of components that
can in turn affect the purity of the drug substance.
Similarly, solvents used in the synthesis are likely to contain
a number of impurities that may range from trace levels to
significant amounts that can react with various chemicals used
Discussed below are three important sources of impurities.
Table 1
Thresholds for reporting impurities
Maximum
daily dose
Reporting
threshold
Identification
threshold
Qualification
threshold
Less or equal
to 2 g/day
>2 g/day
0.05%
0.10% or 1.0 mg/day
(whichever is lower)
0.05%
0.15% or 1.0 mg/day
(whichever is lower)
0.05%
0.03%
S. Ahuja / Advanced Drug Delivery Reviews 59 (2007) 3–11
Table 2
Threshold for reporting degradation products in a new drug product
7
4.3. Degradation-related impurities
Maximum daily dose
Threshold
1g
>1 g
0.1%
0.05%
in the synthesis to produce other impurities. Intermediates are
also not generally held to the purity level of the drug
substance—hence the remarks made for the raw materials
apply. It is not reasonably possible to theorize all by-products;
as a result, any such products that may be produced in the
synthesis would be hard to monitor. The “pot reactions,” i.e.,
when the intermediates are not isolated, are convenient,
economical, and timesaving; however, they raise havoc in
terms of the generation of impurities because a number of
reactions can occur simultaneously. Incidentally, this problem
of numerous reactions occurring simultaneously can be also
encountered in single reactions where intermediate is isolated.
The final intermediate is generally controlled in the
pharmaceutical synthesis by conducting regulatory impurity
testing. This typically entails residual solvents (that are not used
in further downstream processing) or process impurities (in
cases where they conclusively demonstrate that these moieties
are not also degradation products). It is important to remember
that this step is the last major source of potential impurities,
therefore, it is very desirable that the methods used for analysis
at this stage be rigorous. It should be remembered that base-tosalt or acid-to-salt conversions could also generate new
impurities. Furthermore, thermally labile compounds can
undergo decomposition if any further processing involves
heating.
4.2. Formulation-related impurities
A number of impurities in a drug product can arise out of
interactions with excipients used to formulate a drug product.
Furthermore, in the process of formulation, a drug substance is
subjected to a variety of conditions that can lead to its
degradation or other deleterious reactions. For example, if heat
is used for drying or for other reasons, it can facilitate
degradation of thermally labile drug substances.
Solutions and suspensions are potentially prone to degradation that is due to hydrolysis or solvolysis (see kinetic studies
discussed below). These reactions can also occur in the dosage
form in a solid state, such as in the case of capsules and tablets,
when water or another solvent has been used for granulation.
Not only can the water used in the formulation contribute its
own impurities, it can also provide a ripe situation for
hydrolysis and metal catalysis. Similar reactions are possible
in other solvents that may be used.
Oxidation is possible for easily oxidized materials if no
precautions are taken. Similarly, light-sensitive materials
can undergo photochemical reactions. Details are provided
in Chapter 6 of reference [1] regarding how various excipients can contribute to degradation and the resulting
impurities.
A number of impurities can be produced because of API
degradation or other interactions on storage. Therefore, it is very
important to conduct stability studies to predict, evaluate, and
ensure drug product safety [7]. Stability studies include
evaluation of stability of API, preformulation studies to evaluate
compatibility of API with the excipients to determine its stability
in the formulation matrix, accelerated stability evaluations of the
test or final drug product, stability evaluation via kinetic studies
and projection of expiration date, routine stability studies of drug
products in marketed, sample or dispensed package under
various conditions of temperature light, and humidity.
The stability studies under various exaggerated conditions of
temperature, humidity, and light can help us determine what
potential impurities can be produced by degradation reactions
(for details see Chapter 8 of reference [1]). It is important to
establish a viable stability program to evaluate impurities. A
good stability program integrates well the scientific considerations with regulatory requirements. The importance of kinetic
studies in monitoring and evaluating impurities is discussed
below.
4.3.1. Kinetic studies
Most of the degradation reactions of pharmaceuticals occur
at finite rates and are chemical in nature. These reactions are
affected by conditions such as solvent, concentration of
reactants, temperature, pH of the medium, radiation energy,
and the presence of catalysts. The order of the reaction is
described by the manner in which the reaction rate depends on
the concentration of reactant. The degradation of most
pharmaceuticals can be classified as zero order, first order, or
pseudo-first order, even though they may degrade by complicated mechanisms, and the true expression may be of higher
order or be complex and noninteger.
An understanding of the limitations of experimentally
obtained heat of activation values is critical in stability
predictions. For example, the apparent heat of activation of a
pH value where two or more mechanisms of degradation are
involved is not necessarily constant with temperature. Also, the
ion product of water, pKw, is temperature-dependent, and −ΔHa
is approximately 12 kcal, a frequently overlooked factor that must
be considered when calculating hydroxide concentration. Therefore, it is necessary to obtain the heat of activation for all
bimolecular rate constants involved in a rate–pH profile to predict
degradation rates at all pH values for various temperatures.
It is incumbent upon the chemist to perform some kinetic
studies to predict stability of a drug substance and to evaluate
degradation products. However, it is also important to recognize
the limitations of such predictions. The importance of kinetic
studies and the effect of various additives on the reaction rates
are discussed at some length in Chapter 7 of reference [1].
5. Selective analytical methodologies
Development of a new drug mandates that meaningful and
reliable analytical data be generated at various steps of the new
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S. Ahuja / Advanced Drug Delivery Reviews 59 (2007) 3–11
drug development [7]. Ensuring the safety of a new pharmaceutical compound or drug requires that it meet the established
purity standards as a chemical entity or when admixed with
animal feeds for toxicity studies or pharmaceutical excipients for
human use. Furthermore, it should exhibit excellent stability
throughout its shelf life. These requirements demand that the
analytical methodology that is used be sensitive enough to measure low levels of impurities. This has led to analytical methods
that are suitable for determination of trace/ultratrace levels, i.e.,
sub-microgram quantities of various chemical entities [8–12].
A variety of methods are available for monitoring impurities.
The primary criterion is the ability to differentiate between the
compounds of interest. This requirement reduces the availability of methods primarily to spectroscopic and separation
methods or a combination thereof.
5.1. Spectroscopic methods
The following spectroscopic methods can be used:
▪
▪
▪
▪
Ultraviolet (UV)
Infrared (IR)
Nuclear magnetic resonance (NMR)
Mass spectrometry (MS)
UV at a single wavelength provides minimal selectivity of
analysis; however, with the availability of diode array detectors
(DAD), it is now possible to get sufficient simultaneous
information at various wavelengths to ensure greater selectivity.
5.1.1. Infrared spectrophotometry
Infrared spectrophotometry provides specific information on
some functional groups that may allow quantification and
selectivity. However, low-level detectability is frequently a
problem that may require more involved approaches to
circumvent the problem.
5.1.2. Nuclear magnetic resonance spectroscopy
Nuclear magnetic resonance spectroscopy provides fairly
detailed structural information on a molecule and is a very
useful method for characterization of impurities; however, it has
limited use as a quantitative method because of cost and time
considerations.
5.1.3. Mass spectrometry
Mass spectrometry provides excellent structural information, and, based on the resolution of the instrument, it may
provide an effective tool for differentiating molecules with
small differences in molecular weight. However, it has limited
use as a quantitative technique because of cost and time
considerations.
In summary, IR, NMR, and MS are excellent techniques for
characterization of impurities that have been isolated by any of
the techniques discussed above. UV has been found to be
especially useful for analyzing most samples with high-pressure
liquid chromatography. This combination is commonly used in
pharmaceutical analysis.
5.2. Separation methods
The following separation methods can be used:
▪
▪
▪
▪
▪
Thin-layer chromatography (TLC)
Gas chromatography (GC)
High-pressure liquid chromatography (HPLC)
Capillary electrophoresis (CE)
Supercritical fluid chromatography (SFC)
A brief account of the above-listed methods is given here to
provide a quick review of their potential use [10].
Except for CE, all these techniques are chromatographic
methods. CE is an electrophoretic method that is frequently
lumped with the chromatographic methods because it shares
many of the common requirements of chromatography.
However, it is not strictly a two-phase separation system — a
primary requirement in chromatography. Hyphenated methods
such as GC–MS, LC–MS, GC–LC–MS, LC–MS–MS, etc. are
all discussed later in this chapter.
A broad range of compounds can be resolved using TLC by
utilizing a variety of different plates and mobile phases. The
primary difficulties related to this method are limited resolution,
detection, and ease of quantification. The greatest advantages
are the ease of use and low cost.
Gas chromatography is a very useful technique for quantification. It can provide the desired resolution, selectivity, and
ease of quantification. However, the primary limitation is that
the sample must be volatile or has to be made volatile by
derivatization. This technique is very useful for organic volatile
impurities.
High-pressure liquid chromatography is frequently casually
referred to as high-performance liquid chromatography today.
Both of these terms can be abbreviated as HPLC, and they are
used interchangeably by chromatographers. This is a useful
technique with applications that have been significantly
extended for the pharmaceutical chemist by the use of a variety
of detectors such as fluorescence, electrometric, MS, etc.
Capillary electrophoresis is a useful technique when very
low quantities of samples are available and high resolution is
required. The primary difficulty is assuring reproducibility of
the injected samples.
Supercritical fluid chromatography offers some of the
advantages of GC in terms of detection and HPLC in terms of
separations, in that volatility of the sample is not of paramount
importance. This technique is still evolving, and its greatest
application has been found in the extraction of samples.
5.3. Hyphenated methods
The following hyphenated methods can be used effectively
to monitor impurities [3]:
▪
▪
▪
▪
GC–MS
LC–MS
LC–DAD–MS
LC–NMR
S. Ahuja / Advanced Drug Delivery Reviews 59 (2007) 3–11
▪ LC–DAD–NMR–MS
▪ LC–MS–MS
Of course, these methods are not always available or
applicable — a detailed discussion is included in Chapters 4
and 9 of reference [1] as to why it is not always possible to use
these methods. In case it is necessary to procure authentic
material for purposes of structure confirmation, synthesis or
isolation methods should be utilized.
6. Impurity profiling
Ideally an impurity profile should show all impurities in a
single format to allow monitoring of any variation in the profile
because of planned or unplanned changes in synthesis,
formulation, or stability, etc. The driving forces for studying
an impurity profile are
Quality considerations
Regulatory (FDA) requirements
It is the belief of this author that quality considerations
should be the driving force for profiling.
6.1. Samples to be profiled
Impurity profiling should be done for the following samples:
▪ Active ingredient
▪ Process check (synthesis or formulation)
▪ Final product.
6.2. Components seen in a profile
Ideally, an impurity profile should show the following:
▪
▪
▪
▪
▪
▪
▪
▪
▪
▪
▪
▪
▪
9
Accelerated solvent extraction
Supercritical fluid extraction
Column chromatography
Flash chromatography
Thin-layer chromatography
Gas chromatography
High-pressure liquid chromatography
Capillary electrophoresis
Supercritical fluid chromatography.
Isolation should be initiated based on simple extraction or
partition methods. It may be possible to extract impurities
selectively on the basis of acidity, basicity, or neutrality. The
extraction process usually involves liquid–liquid extraction,
where one phase is an aqueous solution and the other is an
organic phase that is nonpolar. By appropriate adjustment of the
pH of the aqueous solution, one can extract acidic, basic, or
neutral impurities. Further separations can be made by
chromatographic methods. Frequently, the isolation methods
tend to be the same methods that are used for analysis.
8. Characterization of impurities
The characterization of impurities is generally achieved by
the following means:
▪
▪
▪
▪
▪
Matching retention data
UV
IR
NMR
MS
Once an impurity has been detected, it becomes necessary to
estimate its content. Detectability frequently means that a given
component provides a signal at least twice that of background
noise or the baseline. For quantification of impurity, the mul-
Synthesis-related impurities
Formulation-related impurities
Degradation products
Interaction products.
7. Isolating impurities
It is often necessary to isolate impurities because the
instrumental methods mentioned above are not available or
further confirmation is needed. For example, when hyphenated
methods such as LC–MS are not suitable or do not provide
unambiguous characterization, it may be necessary to isolate
impurities for further confirmation of structure or for conducting toxicity studies. Of course, after the structure has been
established, these impurities can be synthesized by a suitable
route.
The following methods have been used for isolation of
impurities:
▪ Solid-phase extraction
▪ Liquid–liquid extraction
Fig. 1. Resolution of potential degradation products. 1 = 3,5-dihydroxyacetophenone, 2 = 3,5-dihydroxybenzaldehyde, 3 = 2-t-butyl-4,6,8-trihodroxy-tetrahydroisoquinoline, 4 = terbutaline, 5 = 3,5-dihydroxy-ù-t-butylaminoacetophenone, 6 = 3,5-dihydroxybenzoic acid, ethyl ester. Copyright (© 1998) from
Impurities Evaluation of Pharmaceuticals by Satinder Ahuja. Reproduced by
permission of Routledge/Taylor and Francis Group, LLC.
10
S. Ahuja / Advanced Drug Delivery Reviews 59 (2007) 3–11
legends in Figs. 1 and 2) that must be resolved. HPLC was
clearly indicated as the preferred methodology of choice, based
on physicochemical properties of terbutaline (for more details,
see reference [1]).
9.1.1. Achiral impurities
All of the potential impurities were classified into four
groups to assist the method development:
Fig. 2. Resolution of potential dibenzyloxyphenyl impurities. 1 = terbutaline,
2 = solvent, 3 = solvent, 4 = α-[(t-butylamino)methyl]-3,5-dibenzyloxybenzyl
alcohol, 5 = α-methyl-3,5-dibenzyloxybenzyl alcohol, 6 = 3,5-dibenzyloxyacetophenone, 7 = α-[(benzyl-t-butylamino)methyl]-3,5-dibenzyloxybenzyl alcohol, 8 = 3,5-dibenzyloxy-2,6-dibromoacetophenone, 9 = 3,5-dibenzyloxy-1′bromoacetophenone, 10 = 3,5-dibenzyloxy-2,6,α-tribromacetophenone, 11 =
1′-benzyl-t-butylamino-3,5-dibenzyloxyacetophenone. Copyright (© 1998)
from Impurities Evaluation of Pharmaceuticals by Satinder Ahuja. Reproduced
by permission of Routledge/Taylor and Francis Group, LLC.
tiple is set much higher. Initial estimations are generally done
against the parent compound because in most cases the
authentic sample of impurity is not available. When the
authentic sample is available, it is important that it be used
for estimations. If the estimations indicate that a given impurity
content is greater than 0.1%, it must be characterized as per the
FDA and ICH requirements.
9. A case study
A case study is presented below relating to monitoring
impurities in terbutaline sulfate (it is sold as a racemate).
▪ Dihydroxyphenyl compounds with t-butylamino side chain
▪ Cyclized dihydroxyphenyl compounds with basic N in the
ring
▪ Dibenzyloxyphenyl compounds with no t-butylamino side
chain
▪ Dihydroxyphenyl compounds with no t-butylamino side
chain
Two HPLC methods were developed to resolve all achiral
impurities with the same C-8 column with 3-μm particle size
[1].
System 1 (suitable for degradation products and less likely
synthetic impurities): 0.005 mol 1-octanesulfonic acid in
water:tetrahydrofuran:methanol (75:11:14).
System 2 (suitable for dibenzyloxyphenyl compounds;
starting material and intermediates): water:tetrahydrofuran:
acetonitrile:acetic acid:triethylamine (500:465:35:5:2).
In summary, System 2 was designed primarily for quality
control of API. Since no impurities were found in the API with
System 2, the quality of drug product for QC and stability
studies can be monitored using System 1 only.
9.1.2. Chiral impurities
The L-isomer of terbutaline is 3000 times more potent as a
relaxant of tracheal smooth muscle than the D-isomer [4].
1. The isomers can be resolved on AGP column with 0.003 M
tetrapropyl–ammonium bromide solution adjusted to pH
7.0.
2. Capillary electrophoresis can be used to resolve enantiomers
with a background electrolyte that contains β-cyclodextrin
or heptakis (2,6-di-O-methyl)-β-cyclodextrin.
9.1. HPLC methods
10. Conclusions
The first step in this process was to review all potential
sources of impurities in terbutaline.
Synthesis: starting materials, solvents used, intermediates,
theorize potential by-products.
Formulation: solvents used, potential interaction products,
any potential degradation products.
Stability: potential degradation products or reaction products
that may be produced because of thermal, hydrolytic, oxidation,
or photochemical reactions.
A careful assessment revealed that there could be 13
potential impurities in terbutaline (for chemical names, see
To assure quality of drug substances and drug products, it is
important to give a careful consideration as to what constitutes
impurities for a given case and proceed carefully to design a
program to achieve the desired results. It is believed that the
discussion included herein would be helpful in developing such
a program.
References
[1] S. Ahuja, Impurities Evaluations of Pharmaceuticals, Dekker, New York,
1998.
S. Ahuja / Advanced Drug Delivery Reviews 59 (2007) 3–11
[2] S. Gorog, Identification and Determination of Impurities in Drugs,
Elsevier, Amsterdam, 2000.
[3] S. Ahuja, K. Alsante, Handbook of Isolation and Characterization of
Impurities in Pharmaceuticals, Academic, San Diego, CA, 2003.
[4] S. Ahuja, Chiral Separations by Chromatography, Oxford, New York,
2000.
[5] S. Ahuja, Chiral Separations by Liquid Chromatography, ACS, Washington, DC, 1991.
[6] ICH Web site (http://www.ich.org).
[7] J.A. Mollica, S. Ahuja, J. Cohen, J. Pharm. Sci. 67 (1978) 443.
[8] S. Ahuja, Chromatography of Pharmaceuticals. Natural, Synthetic and
[9]
[10]
[11]
[12]
11
Recombinant Products, ACS Symposium Series #512, ACS, Washington,
DC, 1992.
S. Ahuja, Trace and Ultratrace Analysis by HPLC, Wiley, New York,
1992.
S. Ahuja, Chromatography and Separation Science, Academic, San Diego,
CA, 2003.
S. Ahuja, S. Scypinsky, Handbook of Modern Pharmaceutical Analysis,
Academic, New York, NY, 2001.
S. Ahuja, M. Dong, Handbook of Pharmaceutical Analysis by HPLC,
Academic, San Diego, CA, 2005.