literature review article - max flint

Gel networks in pharmaceuticals Max Flint

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Gel networks and their influence on the crystallisation

of pharmaceutical ingredients

Max Flint

Department of Chemistry, University of Sheffield, Sheffield S3 7HF, UK.

[email protected]

Abstract. Gels have been utilised for crystal growth for decades now due to their myriad

properties which make them an ideal medium for crystallisation. The need for gel networks

in the discipline of crystallography has been apparent for many years in light of the need

for large, high quality samples often only accessible via the use of gels. There has been a

recent surge of research interest in the field of Low Molecular Weight Gelators (LMWGs),

molecules which reversibly form a supramolecular network and can be used to grow

crystals of hitherto inaccessible forms. Particular topics explored include the effect of using

gel networks to affect crystal habit, polymorphism and optical isomerism, with focus on how

these properties are relevant when considering the manipulation of organic crystal

structures such as Active Pharmaceutical Ingredients (APIs). This article will begin with a

brief assessment of crystal growth in gels and go on to consider how these gels can be

implemented, to great effect, in the pharmaceutical industry.

Introduction

Growing crystals in gels has been practised for around 120 years,1 beginning with

Liesegang in 1896 and his formation of the famous ‘Liesegang rings’.2 What followed was a

great deal of attention from notable scientists such as Ostwald and Rayleigh, and then by

geologists who saw the research as a way of explaining certain crystal formations found in

rock formations. Later research (throughout the early-mid 20th century) served to further

illustrate the usefulness of gel media for the growth of high quality crystal structures. We

can see examples of this kind of growth in many forms in nature and perhaps most

beautiful is the formation of the mollusc shell, whereby the crystallisation of calcium

carbonate occurs in an organic medium comprising of glycoproteins and polysaccharides

(Figure 1).3 It is clear from many instances, both natural and anthropomorphic, that the

control of crystal growth to favour a particular kind (morphology, polymorph, habit etc.)

over others is immensely important and the current academic interest in the field is

reflective of this.

Figure 1. A Giant Clam. The formation

of a mollusc shell can be thought of as

the crystallisation of CaCO3 in an

organic gel network.

A gel can be described as a semi-solid system in which a liquid is responsible for the

majority of the weight, while the remaining weight (~1%) is accounted for by a 3D network

of fibres formed by a certain type of molecule known as a gelator.4,5 A hydrogel is a gel in

which the liquid phase is water, while an organogel describes one in which the liquid phase

is an organic solvent, such as toluene. Their solid-like rheology, occurring due to the liquid

being immobilised in the fibrous network (Figure 2) by surface tension, prevents convection

and reduces the number of nucleation sites. This greatly slows down diffusion and,

therefore, the rate at which the crystals grow.6 These features of the gel allow the crystal to

grow uniformly in all directions, effectively serving to produce much higher quality crystals,

with far fewer defects than if the process were undertaken in a non-gel aqueous medium.


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Figure 2. Left: A basic scheme of the structure of a gel. Right: In the case of LMWGs (here ‘LMOG’ describes a

‘Low Molecular Weight Organogelator’), as will be explored in this article, small molecules such as (bis)urea self-

assemble reversibly to form supramolecular networks.

Polymeric gelators e.g. gelatin and silica, are used in many different aqueous and non-

aqueous environments. LMWGs are an alternative class of gelators that have only recently

been explored and they are able to self-assemble away from equilibrium, also in a diverse

variety of solvents. These supramolecular gels can be distinguished from polymeric gels in

that they form Self-Assembled Fibrillar Networks (SAFINs) via various non-covalent

interactions.7 This makes their formation thermally reversible and they can be easily

transformed back into a fluid. The reasons why these types of gelators have caught so much

attention in recent years are numerous and range (in the context of pharmaceuticals) from

the improved synthesis of certain ingredients to the ways in which drugs are delivered.

Seeing as the reason why many treatments have, in the past, been unsuccessful due to

their inability to effectively deal with issues such as tumour targeting and problems

associated with intravenous chemotherapy treatments,8 further research into the

mechanism of crystal growth seems necessary.

LMWGs are remarkable in that they are formed through self-assembly via intermolecular

forces such as hydrogen bonding, aromatic stacking and the hydrophobic effect. What

makes them so useful in terms of pharmaceuticals, both for their delivery and their

synthesis, is that they can be tailored to undergo the sol-gel transition as a response to a

wide range of stimuli, such as pH triggers, temperature change or a trigger in the form of

anion-tuning. In most cases, the gel simply acts as an inert medium in which the crystal is

free to form. However, recent developments have demonstrated that their potential reaches

further: they can in fact influence the ways in which the crystals form, including

enantiomorphism, polymorphism and habit.9 The ability to control these features has long

been sought after, as many organic compounds are found to exist in multiple solid forms on

a frequent basis.10 Thus far, work on LMWGs has been dominated by biomineralisation

studies on the formation of CaCO3 crystals. However, this article will focus primarily on the

plethora of applications to improve the availability of high-quality pharmaceutical

ingredients.

Discussion

Crystal Growth in Gels – Habit Modification

Gels are often described as an ideal medium in which to grow crystals. This is attributed to

the suppression of convection currents in the bulk liquid, rendering diffusion the controlling

factor in crystal growth. The dramatic reduction in the number of nucleation sites is

brought about by the increased viscosity of the gel. This reduces the number of random

collisions between molecules and therefore the number of nucleation sites.11 Put simply, if

the crystals are only able to grow slowly and from very few sites, then large, microscopically

ordered crystals with few defects can be obtained. The crystals grown often display novel

morphologies, which is a particularly exciting phenomenon for the field of pharmaceuticals

due to the enormous potential for drugs with new properties.

Crystal habit is the characteristic external shape of a crystal. One early study into the

manipulation of crystal habits dates back to 1979 following the earlier work of Henisch.

Barium molybdate crystals were grown under various conditions and in-depth

investigations were carried out into the morphology and sizes of their samples. Although

the temperature at which they were grown had no effect on the crystals, they did manage to

generate crystals of different habits by modifying both pH and concentration.12 A more

recent example of influence over crystal habit using gels is the use of hydrogel media to

modify the habit of Aspargine monohydrate crystals. Aspargine, first isolated by Pierre

Jean Robiquet in 1806,13 is an amino acid, one of the fundamental building blocks of

proteins and is essential for the development of the human brain.14 Swift and co-workers


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used a range of hydrogels to synthesise Aspargine monohydrate (Asn.H2O) crystals and the

morphologies of these crystals were compared against those grown in alternative aqueous

media. They were able to ascertain that the use of gels as media for crystal growth is a

viable pathway to obtaining new morphologies for crystals.15 This research serves as an

important indicator that gels have the potential to be used to great effect in the

pharmaceutical industry.

Table 1. Summary of different media used by Swift and

co-workers11 to obtain novel morphologies for Asn.H2O

crystals with the Miller indices indicated. Since this

study, much more work has gone into understanding the

mechanism of crystal growth in gel media in the hope

that the properties of the gel can be tailored to favour a

desired product.

Using Gels to Aid Polymorphism Screening

Polymorphism describes the phenomenon of crystallisation into two or more structurally

distinct compounds which share identical chemical composition. The importance of

polymorphism in the pharmaceutical industry was recognised first in 1969 by McCrone and

Haleblian.16 They recognised that if polymorphism in pharmaceutical ingredients goes

unmonitored and uncontrolled then dangerous variations in drug availability to the patient

will be prevalent. This issue is seen as so important that the common practice now

(enforced in the US by the Food and Drug Administration) is to identify and analyse the

known polymorphic forms of an active pharmaceutical ingredient (API) at all stages of drug

development - a long and arduous process known as polymorphism screening.10 It is

necessary because polymorphic forms of APIs are considered to be different chemical

entities by drug regulators due to their different physio-chemical properties in solution.

This is an area where gel networks are starting to be used. For example, interesting

research has been undertaken by Steed and co-workers looking into growing organic

crystals (in this case, pharmaceuticals) using anion-tuned gel phase materials (Figure 3)

i.e. using LMWGs to generate a responsive gel material, one in which triggered gelation can

occur. They have concluded that bis(urea) organogels can be used to access different

polymorphs of pharmaceutical ingredients, such as carbamazepine. This efficient way of

accessing different polymorphs of a crystal can provide a powerful tool for the process of

pharmaceutical polymorph screening.17

Figure 3. A single crystal of carbamazepine is recovered via acetate-anion-dissolution of a gel. Note the large size

of the crystal, suitable for single crystal X-ray crystallography, and the ease with which it was recovered.

One of the first examples of successful growth of an API in gels was of (±)-modafinil in

tetramethoxysilane (TMOS) gels by Coquerel and co-workers,18 with the focus prior to this

being on inorganic crystals. Up until 2006, organic crystals of the metastable racemic

polymorphic form III of (±)-modafinil (Figure 4) had not been grown of a sufficient size to be

studied properly by X-ray diffraction. Crystallisation in TMOS gels was found to be an

effective technique for the growth of this form, where crystallisation in solution had

previously failed. The presence of, and accessibility to, different polymorphs of the same

API is significant in the context of pharmaceuticals. Papers like those published by

Coquerel et al show that gels could be used not only to develop previously inaccessible

polymorphs of APIs with possibly useful properties, but also to exclude those which are

inactive, or even harmful.


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Figure 4. (±)-modafinil in TMOS gels. The number of crystals then decreases with the degree of supersaturation

involved (from left to right the water/methanol ratio decreases). Crystals grown in the centre tube were mainly of

form III, a form which has proved elusive in other aqueous media.

However, as recently as 2015 it has been acknowledged that there is still a need for novel,

modern polymorph screening technologies due to the regulatory necessity of being aware of

all possible forms of a new drug substance. Steed and co-workers expanded upon their work

undertaken in 2010 using LMWGs to a more pharmaceutically relevant approach by

tailoring the structure of the gelator itself to mimic that of the drug substance, in this case

the anti-cancer drug cisplatin.19 Cisplatin displays only limited polymorphism due to its

simple structure and labile chloro ligands and thus presented an interesting challenge.

However, it was determined that the structural similarities between the cisplatin-mimetic

gelator and cisplatin together with the relatively ordered assembly of the gel enhanced the

influence of the ‘C3’ gels (Figure 5) on cisplatin crystallization. This work represents a new

and advanced pharmaceutical crystallisation strategy for the discovery of novel

polymorphs.

Figure 5. A scheme showing the formation of cisplatin-mimetic gelators and the structure of Cisplatin. The C3 gel

was found to be the most versatile and effective gelator, and in the presence of C3 consistently high-quality

crystals formed.

Using Gels to Influence Chirality in APIs

The importance of chirality in the pharmaceutical industry is something of immense

significance. The most well-publicised and far-reaching consequence of this phenomenon

came to light in the late 1950s in West Germany when a company called Chemie

Grünenthal developed and sold Thalidomide under the trade name ‘Contergan’. One

enantiomer of the drug, (R)-thalidomide, was an effective cure for morning sickness in

pregnant women while the other, (S)-thalidomide, had horrific effects on the foetus.

Although purifying and administering only (R)-thalidomide is futile20 (Figure 6),

acknowledging this case study is a useful exercise in demonstrating the need for

enantiomerically pure APIs. Gels can be used not only for growing only one enantiomer of

an API (chiral gel networks) but also for the separation of enantiomers in capillary

electrophoresis, a method for separation and analysis of macromolecules and their

fragments using a polymeric gel as a medium, based on their size and charge.21

Figure 6. The two enantiomers of thalidomide. The acidic hydrogen indicated above means that the compound will

rapidly racemise at physiological pH, so even if an enantiomerically pure drug were administered, it would still be

potentially very harmful to a foetus. The case study nevertheless serves to highlight the importance of chirality in

the pharmaceutical industry.

The advantages of providing a drug in its pure enantiomeric form are summarised

succinctly by Professor Gohel of L. M. College of Pharmacy,22 the most important one being


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the reduced likelihood of side-effects. There is a need to identify and isolate the

enantiomers of racemates which are useful, not harmful and the pharmacodynamic effects

of which are known.23 Supramolecular gel networks clearly represent a potential solution

where this is concerned.

A study undertook in 2000 involved using proteins as chiral selectors to separate

enantiomers of drugs. A silica gel was immobilised by a 3D protein network and

enantioseparation occurs by each enantiomer interacting differently with an immobilized or

adsorbed protein selector.24 More recently, studies have shown that gels made from chiral

building blocks can be used to amplify both chirality and crystal growth in crystals. That is

to say, enantiomeric excesses are able to convert racemic mixtures into enantiomerically

enriched mixtures. Petrova and Swift were able to use agarose gel (a naturally occurring

chiral polysaccharide) as an effective medium for the growth of large enantiomeric excesses

of either d- or l-NaClO3, where using pure aqueous solutions yielded a racemate. The

growth of d-NaClO3 crystals was favoured when the aqueous gel was imparted with 48%

weight NaClO3 at 279K, with enantiomeric excess reaching as high as 22% (Figure 7).25

Figure 7. Enantiomeric excess gained from crystallisation in gel rather than aqueous media.

Sánchez et al used organogelators 1 and 2 (Figure 8) to investigate the enantiomeric and

polymorphic outcome of the crystallisation of various APIs (Aspirin, Caffeine,

Carbamazepine and Indomethacin) in gel media using toluene as the solvent. The crystals

can be easily recovered from the gel by washing and shaking the samples in toluene. This is

advantageous as it is a simple procedure which does nothing to alter the crystalline

product. Non-covalent interactions (i.e. those utilised by LMWGs) play a significant part in

the amplification of chirality in biological systems,26 and this realisation was explored and

extended to investigate the manipulation of the aforementioned APIs.

Figure 8. Structure of organogelators 1 and 2 used by

Suarez et al to investigate the influence of chirality in

gel network structure on the growth of some APIs.

Conclusions – Looking Forward

The ability to grow organic, pharmaceutical crystals in gel media has been demonstrated

along with the ability to easily recover these crystals without altering or harming them. It

has also been shown that gel networks, specifically those formulated using LMWGs, can be

an extremely useful and versatile tool in helping to solve the issue of limited access to

crystal forms of APIs which have previously been difficult to grow and recover. The

potential of these media for influencing the properties of APIs represents a significant

milestone on the road to easier and cheaper polymorphism screening and habit

modification. Overall, this new tool in the field of crystallisation is something which could

revolutionise the discipline, as it could shed light on the growth and nucleation processes,

both of which are relatively poorly understood, especially when considering crystal habit

modification. There is now a diverse and substantial library of different supramolecular

gels which will allow us to choose the correct gelator to suit the crystallisation conditions.

This class of gelator opens up huge opportunities for fine-tuning the interactions we

observe between the gel and the solute, allowing us to hope for much greater control over

the crystallisation of APIs (and other products of interest) in the not-too-distant future.

As our understanding of this new frontier in soft materials science grows, so will the scope

for applying gel networks to challenges other than those presented by API growth. For

example, we have seen that gel-phase crystallisations can offer high-quality single crystals


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and this can be of particular use in macromolecular crystallography, where the crystal

quality of small samples is very important. The developments in the field of metallogels

such as the cisplatin-mimetic gel mentioned earlier (Figure 5) are of immense significance

for future studies. As recently as January 2016, a paper has reported the discovery of “A

novel low molecular weight supergelator showing an excellent gas adsorption, dye

adsorption, self-sustaining and chemosensing properties in the gel state”.27 LMWGs clearly

have enormous potential in industry and medicine, and represent a new and exciting

branch of chemistry, the versatility of which will surely be instrumental in allaying a wide

range of future and current issues.

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19 A. Dawn, K. S. Andrew, D. S. Yufit, Y. Hong, J. P. Reddy, C. D. Jones, J. A. Aguilar, and J. W. Steed,

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2016, 6, 14009.

literature review article - max flint

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