Advanced Drug Delivery Reviews 48 (2001) 115–136
`
`www.elsevier.com/ locate/drugdeliv
`
`Chemical reactivity in solid-state pharmaceuticals: formulation
`implications
`*
`b
`, Wei Xu , Ann W. Newman
`
`c
`
`Stephen R. Byrn
`
`a ,
`
`aPurdue University,West Lafayette, IN 47907,USA
`bMerck and Co.,West Point, PA,USA
`cSSCI, Inc.,3065 Kent Ave,West Lafayette, IN 47906,USA
`
`Received 18 October 2000; accepted 21 December 2000
`
`Abstract
`
`Solid-state reactions that occur in drug substances and formulations include solid-state phase transformations, dehydration/
`desolvation, and chemical reactions. Chemical reactivity is the focus of this chapter. Of particular interest are cases where
`the drug-substance may be unstable or react with excipients in the formulation. Water absorption can enhance molecular
`mobility of solids and lead to solid-state reactivity. Mobility can be measured using various methods including glass
`transition (T ) measurements, solid-state NMR, and X-ray crystallography. Solid-state reactions of drug substances can
`g
`include oxidation, cyclization, hydrolysis, and deamidation. Oxidation studies of vitamin A, peptides (DL-Ala-DL-Met,
`N-formyl-Met-Leu-Phe methyl ester, and Met-enkaphalin acetate salt), and steroids (hydrocortisone and prednisolone
`derivatives) are discussed. Cyclization reactions of crystalline and amorphous angiotensin-converting enzyme (ACE)
`inhibitors (spirapril hydrochloride, quinapril hydrochloride, and moexipril) are presented which investigate mobility and
`chemical reactivity. Examples of drug-excipient interactions, such as transacylation, the Maillard browning reaction, and acid
`base reactions are discussed for a variety of compounds including aspirin, fluoxitine, and ibuprofen. Once solid-state
`reactions are understood in a pharmaceutical system, the necessary steps can be taken to prevent reactivity and improve the
`stability of drug substances and products.
`2001 Elsevier Science B.V. All rights reserved.
`
`Contents
`
`1. Introduction ............................................................................................................................................................................
`2. Solid-state reactions ................................................................................................................................................................
`3. Mobility .................................................................................................................................................................................
`4. Solid-state reactions of drug substances ....................................................................................................................................
`4.1. Oxidation reactions ..........................................................................................................................................................
`4.1.1. Vitamin A .............................................................................................................................................................
`4.1.2. Peptides.................................................................................................................................................................
`4.1.3. Steroids .................................................................................................................................................................
`4.2. Cyclization and hydrolysis ................................................................................................................................................
`4.3. Deamidation and hydrolysis ..............................................................................................................................................
`5. Drug excipient interactions in formulations ...............................................................................................................................
`
`116
`116
`117
`118
`118
`118
`119
`121
`125
`127
`129
`
`*Corresponding author. Tel.: 11-764-494-1460; fax: 11-765-494-6545.
`E-mail address: sbyrn@pharmacy.purdue.edu (S.R. Byrn).
`
`0169-409X/ 01/ $ – see front matter
`PII: S0169-409X( 01 ) 00102-8
`
`2001 Elsevier Science B.V. All rights reserved.
`
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`5.1. Transacylation .................................................................................................................................................................
`5.2. Maillard reaction..............................................................................................................................................................
`5.3. Solid-state acid base reactions ...........................................................................................................................................
`6. Conclusions ............................................................................................................................................................................
`References ..................................................................................................................................................................................
`
`129
`130
`130
`134
`135
`
`1. Introduction
`
`Solid-state reactions can and do occur in drug
`substances and formulations. In formulations, solid-
`state reactions of the drug substance are of great
`interest. These can occur in cases where the drug
`substance is intrinsically chemically reactive or
`unstable. In such cases, the formulation can acceler-
`ate degradation in any or all of the following ways:
`
`• Acceleration due to interaction with excipients
`• Acceleration due to processing effects
`• Acceleration induced by excipients (but not in-
`volving chemical reactions with the excipient)
`
`Often, acceleration of reaction is due to the creation
`or presence of amorphous material. Thus, one of the
`best examples of
`these effects is the enhanced
`chemical reactivity of amorphous materials. In such
`cases processing or, possibly, simply interaction with
`the excipients can increase the amount of amorphous
`drug substance. This amorphous drug substance will
`then react due to its increased mobility and ability to
`interact with moisture.
`Direct reaction of excipients with the drug sub-
`stance can also occur, such as solid–solid acid base
`reactions. However, buffers or acids and bases are
`used to stabilize drug substances in formulations as
`well. In many cases these effects can be determined
`by mixing the drug substance with various excipi-
`ents. In other cases processing is needed to induce
`the reaction.
`The role of solid-state reactions in drug substances
`and formulations will be discussed here. Background
`information on solid-state reactions and mobility, as
`well as various examples of solid-state reactions in
`pharmaceutical applications will be presented.
`
`2. Solid-state reactions
`
`In order to understand more about the influence of
`formulations on solid state reactions it is worthwhile
`
`to review solid state reactions. Solid-state reactions
`in their broadest sense include solid-state phase
`transformations (polymorphic transformations), re-
`actions in which solvent of crystallization is lost or
`gained, and a broad range of solid state chemical
`reactions. Most of the emphasis of this chapter will
`be on chemical reactivity.
`It is necessary to establish criteria for solid state
`reactions in order
`to focus on true solid state
`reactions. This will avoid a liquid state reaction
`being identified as a solid state reaction. Morawetz
`suggested four criteria for determining whether a
`reaction is a true solid state reaction [1]. A fifth
`criterion can be added based on Paul and Curtin [2].
`A reaction occurs in the solid when:
`
`1. the liquid reaction does not occur or is much
`slower.
`2. pronounced differences are found in the reactivity
`of closely related compounds.
`3. different
`reaction products are formed in the
`liquid state.
`in different crystalline modi-
`4. the same reagent
`fications has different reactivity or leads to differ-
`ent reaction products.
`5. it occurs at a temperature below the eutectic point
`of a mixture of the starting material and products.
`
`the reaction is
`Once it has been established that
`occurring in the solid-state,
`the reaction can be
`understood in terms of a four step process [2]:
`
`1. Loosening of Molecules at the Reaction Site. It is
`reasonable to assume that molecular loosening
`achieves the mobility required to accomplish the
`next step.
`2. Molecular Change. This step is similar to the
`corresponding solution reaction where the bonds
`of the reactant are broken and the bonds of the
`product are formed.
`3. Solid Solution Formation. During the early stages
`of the reaction, a solid solution of the product in
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`117
`
`the site of
`is formed at
`the starting crystal
`reaction. However, after the concentration of the
`product reaches a certain point, the product will
`separate.
`4. Separation of Product. This step gives new
`crystals, either
`randomly oriented or with an
`orientation governed by the crystals of the starting
`material. This latter case is termed a topotactic
`reaction and will be discussed.
`
`Solid-state reactions begin at one or more nucleation
`sites and spread through the crystal. For desolvations
`and some thermal reactions, the reaction begins at a
`nucleation site and spreads through the crystal in a
`front that advances through the crystal.
`Nucleation sites (defects)
`for
`reaction are de-
`veloped during crystallization or can sometimes be
`produced by mechanical deformations such as prick-
`ing with a pin or cutting the crystal. Exposing the
`starting crystal
`to product crystals may produce
`nucleation sites. In other cases, nucleation is random
`and neither mechanical deformation nor exposure to
`product crystals nucleates the reaction. Obviously,
`this variability in nucleation and the random number
`of nucleation sites that are present in crystals can
`greatly complicate the kinetics of solid state re-
`actions.
`For solid gas reactions, diffusion of gas into the
`crystal itself requires molecular loosening. Thus, for
`solid–gas reactions simultaneous or sequential mo-
`lecular loosening and diffusion steps are involved.
`The chemical reaction is generally considered to
`follow the same mechanism as the solution reaction.
`As the reaction proceeds a solid solution of the
`product in the reactant crystal will be formed. After
`the limit of product solubility in the starting crystal
`lattice is reached, the product will either crystallize
`or continue to accumulate in an amorphous form.
`This step would not influence the apparent rate of the
`reaction as measured by the amount of product
`formed or by the disappearance of the reactant.
`However, if the rate is measured using the diffraction
`intensities of crystalline product, the rates may differ
`from those measured chemically.
`It is clear from the above discussion that the rates
`of solid state reactions depend on several factors,
`including nucleation and the molecular changes
`involved. It is important to realize that the crystal
`structure and crystal packing profoundly affect the
`
`molecular loosening and molecular change steps of a
`solid state reaction. The crystal packing determines
`the extent of molecular loosening required for the
`molecules to reorient sufficiently to undergo the
`required molecular changes. The crystal packing also
`determines the extent of molecular loosening re-
`quired for gases to diffuse to the reaction site in
`solid–gas reactions. Thus, determining the relevant
`crystal structures and investigating the molecular
`mobility in these structures can bring about a great
`deal of insight into solid state reactions.
`
`3. Mobility
`
`Understanding the mobility of groups in solids can
`lead to insight
`into the mobility of molecules in
`solution, the forces responsible for conformational
`interconversions, and the factors responsible for solid
`state reactions. It is clear that solid state degradations
`of pharmaceuticals are often related to molecular
`mobility [3–5]. In addition, Ahlneck and Zografi [6]
`have suggested that water absorption enhances the
`molecular mobility of pharmaceutical solids, perhaps
`explaining the enhanced chemical reactivity of these
`materials in the presence of water. Recent studies by
`Zografi’s laboratory and our laboratory confirm the
`relationship between molecular mobility and solid
`state reactivity as discussed in a later section.
`Further study of the relationship between molecu-
`lar mobility and solid state reactivity requires the
`development of new approaches to determining
`molecular mobility, especially in mixtures such as
`pharmaceutical dosage forms where single-crystal
`X-ray methods cannot be used.
`Solid state NMR offers several attractive ap-
`proaches to the study of the molecular mobility of
`solids. These include:
`
`1. Determination of the activation energies from T1
`relaxation of individual carbon atoms using vari-
`able temperature solid-state NMR.
`2. Study of processes which result in peak coalesc-
`ence of solid-state NMR resonances using vari-
`able temperature solid-state NMR.
`3. Use of interrupted decoupling to detect methylene
`and possible methine groups with unusual mobili-
`ty.
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`4. Comparison of solid-state MAS spectra measured
`with and without cross polarization.
`
`X-ray crystallography offers another approach to the
`measurement of mobility in the solid-state. Trueb-
`lood and Dunitz have used variable temperature
`X-ray crystallography to measure the force constants
`for vibration of methyl groups [7–10].
`It is interesting to explore the possible relationship
`between studies of molecular mobility by solid-state
`NMR and X-ray crystallography. A priori there is no
`reason to expect that the force constants for methyl
`vibration (or any other motion) determined by X-ray
`crystallography and the activation energy for methyl
`rotation determined by solid state NMR would be
`correlated. Variable temperature solid state NMR is
`thought to measure activation energies for the rota-
`tion of methyl groups about their C axes. (However,
`3
`many of the barriers obtained for methyl rotation
`from solid state NMR are significantly lower than
`those expected based on molecular mechanics calcu-
`lations of methyl rotation barriers.) In contrast, X-ray
`crystallography measures the vibrational motion of
`the individual atoms. One would expect
`that vi-
`bration (as measured by X-ray crystallography) and
`rotation of methyl groups (as determined by solid-
`state NMR) in solids may not always be correlated.
`On the other hand,
`if the crystal packing in the
`vicinity of two methyl groups in the same molecule
`is different, one might expect that the methyl group
`that is not as tightly packed might exhibit a lower
`force constant for vibration and also a lower activa-
`tion energy for rotation. In our laboratory we have
`compared the relative barriers to methyl rotation of
`the multiple methyl groups
`in ibuprofen and
`phenacetin measured by solid state NMR to the force
`constants measured by variable temperature X-ray
`crystallography. These studies
`indicate that
`the
`methyl group with the greatest barrier to motion by
`solid state NMR also has the greatest force constant
`for vibration by X-ray crystallography.
`Water is known to enhance the mobility of amor-
`phous solids [6]. This is due to the plasticization that
`results when water is absorbed. In addition, water
`can be absorbed into amorphous regions in otherwise
`crystalline materials. Such solids that may contain a
`small or undetectable amount of amorphous material
`are expected to show enhanced reactivity. Once
`
`degradation begins in these small amorphous regions
`it can proceed throughout
`the crystal by forming
`eutectic melts of the products and reactants. Ahlneck
`and Zografi have pointed out that the effect of water
`is amplified in these cases because the small amounts
`of amorphous material can contain a relatively large
`amount of water; yet the total water content of the
`solid will be low [6]. Further studies of the mobility
`of solids in the presence and absence of water are
`needed in order to determine whether a predictive
`technique can be developed which will form the
`basis for selection of the most stable crystal form
`prior to stability studies.
`
`4. Solid-state reactions of drug substances
`
`The origin of solid-state reactions in a drug
`substance can be explained by various factors as
`discussed above. Solid-state reactions can include
`oxidation, cyclization, hydrolysis, and deamidation.
`Excipients present in formulated products may not be
`directly involved in the degradation mechanism, but
`may add parameters, such as water, which contribute
`to the solid-state reaction. Examples of degradation
`of drug substances alone and in formulations will be
`presented.
`
`4.1. Oxidation reactions
`
`4.1.1. Vitamin A
`Solid-state oxidation reactions have been known
`for many years. Early studies suggested that these
`reactions were free radical processes. For example,
`Finkel’shtein and co-workers postulated the mecha-
`nism for
`the reaction of beta carotene (Fig. 1)
`because 2,6 di-tert-butyl-4-methyl phenol (BHT) and
`other antioxidants inhibit this reaction and the rate
`depends on oxygen pressure and temperature [11].
`Crystalline esters of vitamin A (including the hemi
`succinate,
`the nicotinate, and the 3,4,5-trimethox-
`ybenzoate) decompose by both polymerization and
`oxidation pathways [12,13].Vitamin A exposed to air
`at room temperature for several years or heated at
`1008C for 5 h gave at least five ketones on TLC
`plates treated with 2,4-dinitrophenylhydrazine [14].
`We have found that vitamin A is a gummy yellow
`solid after 5 months of exposure to room light,
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`119
`
`cated because it produces a variety of products. Xu
`[18] has
`carried out
`a preliminary study of
`methionine oxidations by monitoring the disappear-
`ance of
`the peptide as well as monitoring the
`formation of the sulfoxide and sulfone products, as
`shown in Fig. 2.
`Three model peptides (Fig. 3) were investigated:
`DL-Ala-DL-Met (a zwitterionic dipeptide), N-formyl-
`Met-Leu-Phe methyl ester (a neutral tripeptide), and
`Met-enkaphalin acetate salt (a weakly acidic penta-
`peptide). These peptides were chosen for investiga-
`tion because both crystalline and amorphous material
`could be produced. To complete the study in a
`reasonable time, the oxidation of these peptides was
`accelerated by exposure of
`the peptides to UV
`radiation (254 nm) using a 15 W tube. Fig. 4 shows
`the results for
`the study of
`the degradation of
`crystalline and amorphous DL-Ala-DL-Met. Similar
`
`Fig. 2. Methionine oxidation reactions [18].
`
`Fig. 3. Model peptides DL-Ala-DL-Met (a zwitterionic dipeptide),
`N-formyl-Met-Leu-Phe methyl ester (a neutral
`tripeptide), and
`Met-enkaphalin acetate salt (a weakly acidic pentapeptide).
`
`Fig. 1. Autooxidation scheme of b-carotene [11].
`
`temperature, and air. Elemental analysis indicated
`that each molecule of vitamin A took up six oxygen
`atoms, and mass spectral studies indicated extensive
`degradation and the presence of many products.
`Diluents with antioxidant properties stabilize vita-
`min A palmitate in vitamin preparations
`[15].
`Aluminum salts of fatty acids such as stearic acid
`stabilize vitamin A, as does combination with gelatin
`and dextrin, which probably contain reducing sugars
`[16].
`An interesting correlation of the melting points of
`the vitamin A (retinoic acid) esters with their zero
`order rate of solid state decomposition has been
`observed [13]. As the melting point increased, the
`rate of decomposition decreased. The rates of de-
`composition of these esters in solution were virtually
`identical. These results were interpreted in terms of
`crystal lattice energy. It was argued that the higher
`melting esters had more crystal-lattice energy and
`thus were more stable to the solid–gas oxidation
`reaction. Thus, the higher the melting point the more
`efficient
`the packing and, conceivably,
`the less
`permeable the crystal is to reacting gas. A better
`measure of lattice energies in a series of compounds
`is based on the heat of sublimation [17].
`
`4.1.2. Peptides
`There is substantial interest in understanding more
`about
`the solid-state oxidation of methionine in
`peptides and proteins. One impediment to studying
`this oxidation reaction is that the process is compli-
`
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`
`Fig. 4. UV induced degradation (left) and oxidation (right) of crystalline and amorphous DL-Ala-DL-Met at ambient conditions [18].
`
`studies of N-formyl-Met-Leu-Phe methyl ester, and
`Met-enkaphalin acetate salt were also carried out but
`are not shown. These studies show that amorphous
`material oxidizes and degrades faster than crystalline
`material in each of these cases.
`To rule out particle size as an explanation for the
`enhanced reactivity of the amorphous material, crys-
`talline DL-Ala-DL-Met was partitioned into different
`size fractions and each fraction was oxidized. Fig. 5
`shows the results of this study. It is clear from this
`
`figure that particle size does not affect the reactivity
`of the crystalline material. Regardless of the particle
`size the crystalline material is much less reactive
`compared to the amorphous material.
`This preliminary study shows that amorphous
`peptides are much more reactive towards oxygen
`than their crystalline counterparts and that the differ-
`ence in reactivity is probably not due to particle size.
`It
`is likely that
`the amorphous material
`is more
`reactive because it has more mobility.
`
`Fig. 5. The effect of particle size on the UV-induced Met oxidation of DL-Ala-DL-Met at ambient conditions [18].
`
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`121
`
`4.1.3. Steroids
`Hydrocortisone modifications have been exten-
`sively studied to investigate the stability of steroids.
`Hydrocortisone 21-tert-butylacetate (HTBA) yielded
`40% of
`the 11-keto product, cortisone 21-tert-
`butylacetate (CTBA), upon standing at room tem-
`perature for 1–2 years (Fig. 6) [19,20]. In contrast,
`other esters of this steroid, including the 21-ethyl
`ester, were completely resistant to air oxidation, even
`after 15 years at room temperature. The oxidation of
`hydrocortisone 21-tert butylacetate is accelerated by
`heat and greatly accelerated by free radical initiators
`and ultraviolet light.
`In our laboratory, HTBA has been obtained in five
`crystalline forms [21]. Forms I, II, and III were
`obtained from absolute ethanol solution either at
`
`Fig. 6. Oxidation of hydrocortisone 21-tert-butylacetate.
`
`Table 1
`Crystal forms of hydrocortisone 21-tert-butylacetate
`
`room temperature or in the refrigerator. X-ray pow-
`der diffraction studies indicated that
`they were
`different forms and elemental analysis showed that
`they contained varying amounts of ethanol within the
`crystals.
`The results of these studies are shown in Table 1.
`During crystallization from ethanol a mixture of
`Forms I, II, and III often appeared, but a pure single
`form could be obtained under certain conditions. A
`new form designated Form IV was produced when
`Forms I, II, and III were heated at 1208C. Forms I
`and II underwent desolvation and phase transforma-
`tion to Form IV, while Form III appeared to trans-
`form via a melt recrystallization process to Form IV.
`However, this observation needs to be substantiated
`by another method of analysis. Another form (Form
`V) was isolated from pyridine. All crystal forms,
`except Forms I and V, were inert to irradiation with
`ultraviolet light.
`Form I was oxidized from HTBA to cortisone
`21-tert-butylacetate (CTBA) under irradiation with
`ultraviolet light in air. A known weight and given
`size of crystals were put in vials and irradiated at
`308C. The extent of formation of CTBA was de-
`termined by integrating the C methyl NMR signal,
`18
`and the content of ethanol was measured by gas
`
`Form
`
`Crystal class
`Space group
`˚a (A)
`˚b (A)
`˚
`c (A)
`a
`b
`g
`Solvent of
`crystallization
`
`EtOH content
`a
`mp(8C)
`UV oxidation
`
`I
`
`hexagonal
`P6
`1
`17.485
`17.485
`15.376
`908
`908
`1208
`ethanol
`propanol
`n-amyl alcohol
`acetonitrile
`d
`0.9
`170–180
`reaction
`
`II
`
`monoclinic
`P2
`1
`12.440
`7.710
`14.724
`908
`88.78
`908
`ethanol
`
`1.0
`110–120
`no reaction
`
`b
`
`III
`
`triclinic
`¯
`P1
`23.0
`12.5
`29.0
`748
`1478
`748
`ethanol-
`tert-butanol
`
`–
`123–126
`no reaction
`
`c
`
`IV
`
`V
`
`unstable
`
`heat forms
`I, II, or III
`
`–
`234–238
`no reaction
`
`pyridine
`
`–
`
`reaction
`
`a The exact melting temperature may vary from one crystal to another.
`b Opaque at this temperature range, with final melting at 234–2388C.
`c After melting, the melt resolidified as the temperature was rising and finally remelted at 234–2388C.
`d When crystallized from ethanol, Form I contains ethanol; when crystallized from the other solvents, no solvent of crystallization is
`present [21].
`
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`
`Table 2
`Percent desolvation and oxidation of crystalline hydrocortisone
`a
`21-tert-butylacetate 0.9 ethanolate upon exposure to UV light
`
`Days
`
`% EtOH lost
`
`% Cortisone formed
`
`1
`2
`3
`6
`10
`14
`21
`
`43.3
`75.6
`83.3
`88.9
`93.3
`95.6
`96.7
`
`20.0
`38.9
`50.0
`52.9
`56.3
`66.7
`71.4
`
`a
`
`From Ref. [21].
`
`chromatography. The percent desolvation and oxida-
`tion of HTBA are shown in Table 2. Although the
`loss of ethanol was faster than the oxidation, this
`desolvation–oxidation behavior is different from that
`of dihydrophenylalanine hydrate [22],
`in which
`nearly complete loss of water precedes the oxidation.
`In addition, the crystal opaqueness phenomenon of
`desolvation has not been found in crystal Form I.
`During desolvation, a significant change in appear-
`ance of Form I (or other similar crystals studied) can
`be observed only under polarized light. After a week,
`a circular pattern of decolorization moved from the
`ends toward the center, but the crystal remained clear
`under transmitted light even after 42 days. Crystallo-
`graphic studies show that desolvated Form I is still a
`single crystal with a reasonably good diffraction
`pattern.
`The crystal structure of Form I has been de-
`termined. The structure of this form is consistent
`
`with structures of other hydrocortisone derivatives
`[23]. It is proposed that the reactivity of HTBA Form
`I toward oxygen is due to the crystal packing, which
`allows penetration of oxygen down the axis of the
`helix of the crystal. It is further proposed that ethanol
`of crystallization is normally along this axis, and that
`the exit of the ethanol from the crystal further aids
`oxygen penetration. However,
`further crystallo-
`graphic studies are required to confirm these hypoth-
`eses.
`Our laboratory carried out a study of an additional
`series of hydrocortisone esters [4]. Table 3 summa-
`rizes the results of
`these studies and lists the
`crystallographic data, whether solvent of crystalliza-
`tion was located in the crystal and the type of crystal.
`Three types of crystal forms were defined as:
`
`1. Type A crystal forms are nonstoichiometric sol-
`vates that are reactive in the presence of oxygen.
`2. Type B crystal forms are stoichiometric solvates
`that are unreactive in the presence of oxygen.
`3. Type C crystal forms do not contain solvent and
`are also unreactive in the presence of oxygen.
`
`The structure of both crystal forms of hydrocortisone
`21-butanoate provide good examples of these esters.
`Crystals of Form I hydrocortisone 21-butanoate were
`crystallized from toluene and was found to belong to
`space group P2 2 2 [4]. Form II of hydrocortisone
`1 1 1
`21-butanoate was crystallized from 2-propanol and
`found to be hexagonal, belonging to the space group
`
`Table 3
`Summary of cell parameters and crystal data for hydrocortisone esters studied
`
`Ester
`
`Type
`
`a
`
`Solvent
`
`Space
`group
`
`a
`
`b
`
`c
`
`a
`
`b
`
`g
`
`propanoate Form I
`butanoate Form I
`butanoate Form II
`pentanoate Form I
`hexanoate Form I
`cyclopentylacetate
`Form I
`butanoate, 9a-fluoro
`Form I
`pentanoate, 9a-fluoro
`Form I
`
`C
`C
`A
`A
`A
`A
`
`A
`
`A
`
`IPA
`PhMe
`IPA
`EtOH
`EtOH
`EtOH
`
`EtOH
`
`EtOH
`
`1
`
`P2 2 2
`1 1 1
`P2 2 2
`1 1 1
`P6
`P6
`1
`P6
`P6
`
`1
`
`1
`
`16.807
`17.011
`17.704
`17.991
`
`16.810
`17.004
`17.736
`17.980
`
`15.117
`15.194
`15.035
`15.165
`
`P6
`
`1
`
`P6
`
`1
`
`16.832
`
`16.827
`
`15.135
`
`17.188
`
`17.193
`
`15.141
`
`z
`
`4
`4
`6
`6
`6
`6
`
`6
`
`6
`
`Final R
`
`Solvent
`located
`
`0.079
`0.084
`0.089
`0.075
`0.088
`0.098
`
`0.057
`
`0.069
`
`No
`No
`No
`Yes
`Yes
`Yes
`
`Yes
`
`Yes
`
`90
`90
`90
`90
`90
`90
`
`90
`
`90
`
`90
`90
`90
`90
`90
`90
`
`90
`
`90
`
`90
`90
`120
`120
`120
`120
`
`120
`
`120
`
`a Type A — reactive, nonstoichiometric solvate; type B — unreactive, stoichiometric solvate; type C — unreactive, unsolvated crystal
`form. All of these compounds give unique XRPD patterns [4].
`
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`
`

`
`S.R. Byrn et al. / Advanced Drug Delivery Reviews 48(2001)115–136
`
`123
`
`P6 . Hydrocortisone 21-butanoate also crystallized
`1
`in this hexagonal space group from methanol, etha-
`nol, 1-propanol, acetone, ethyl acetate, tert-butylace-
`tate, and DMF. Crystals formed from all of these
`solvents show identical X-ray powder diffraction
`patterns. These forms contain the steroid molecules
`in a helix with a tunnel down the center.
`Both forms of the 21-butanoate ester were exposed
`to UV light at 254 nm for 15 days and analyzed
`using both NMR and HPLC for oxidation to the
`corresponding cortisone 21-ester. Analysis showed
`that Form II (hexagonal form) oxidized, while Form
`I (orthorhombic form) did not oxidize. Under the
`same conditions, Form I (orthorhombic form) of
`hydrocortisone 21-propanoate (isostructural to Form
`I of the 21-butanoate ester) also did not oxidize.
`Moreover, all of the other hexagonal crystal forms
`listed in Table 3 oxidized under these conditions.
`Note that we prefer to classify these forms as Type A
`even though these forms are stoichiometric solvates
`containing a 1:2 ratio of solvent:hydrocortisone ester.
`This is because it is difficult to make the distinction
`between stoichiometric and nonstoichiometric sol-
`vates in hexagonal steroids.
`the maximum
`Initial experiments showed that
`amount of oxidation of powdered samples of Form II
`was about 30% regardless of their time of exposure
`to UV light. This is consistent with the reactivity of
`hydrocortisone tert-butylacetate. It is hypothesized
`that
`this happens because oxidation is limited to
`molecules near the surface of the solid. Form II
`(hexagonal) crystals were exposed to UV light and
`washed with chloroform. Both the washings and the
`
`washed crystals remaining were analyzed using
`HPLC. Oxidation product was found in the washings
`but not in the residual crystals, indicating that the
`inner parts of these crystals were not oxidized. This
`result further supports the hypothesis that oxidation
`is limited to molecules near the surface.
`This surface oxidation phenomenon seems to be
`quite odd for the hexagonal crystal form, especially
`with the presence of a tunnel through the center of
`the unit cell and the close proximity of the 11-b-OH
`to the inner surface of the tunnel. One hypothesis
`that may explain this result is that the tunnel is too
`small to allow oxygen to penetrate the crystal. To
`test this hypothesis, the cross sectional area of this
`tunnel was measured. However, simply measuring
`this area by measuring the tunnel area on a projec-
`tion perpendicular to the c-axis is not appropriate
`because of the helical nature of the crystal packing.
`In doing so, only a small area would be measured
`because of the overlapping of atoms from various
`molecules in the zigzag tunnel. In order to obtain a
`more accurate approximation to the true cross sec-
`tional area, the c-axis of the tunnel is cut into 3
`˚
`layers (Fig. 7). Each layer is 5.04 A deep (3 of the
`unit cell c dimension) along the c-axis and consists
`of different parts of each molecule.
`Hydrogen atoms lining the tunnel were used in
`calculated positions. The area was determined by
`delineating the boundaries of the tunnel and de-
`termining the area by the cut-and-weigh method. The
`cross-sectional area of each layer was determined to
`2˚
`be 23.9 A . This area is obviously large enough for
`molecular oxygen to pass in and out of the tunnel.
`
`Fig. 7. Cross sectional areas of the three layers of the tunnel down the c-axis of hydrocortisone 21-butanoate Form I [24].
`
`Mylan Ex 1033, Page 9
`
`

`
`124
`
`S.R. Byrn et al. / Advanced Drug Delivery Reviews 48(2001)115–136
`
`Therefore, a narrow tunnel is not the cause for the
`limited oxidation in this hexagonal crystal form.
`An alternative hypothesis is that the presence of
`solvent of crystallization in the tunnel of the hexa-
`gonal crystal form prevents internal oxidation. Crys-
`tallographic studies