Angewandte
`..
`Reviews
`
`D.-K. Bucˇar et al.
`
`Polymorphism
`Disappearing Polymorphs Revisited
`Dejan-Kresˇimir Bucˇar,* Robert W. Lancaster,* and Joel Bernstein*
`
`International Edition: DOI: 10.1002/anie.201410356
`German Edition:
`DOI: 10.1002/ange.201410356
`
`Keywords:
`crystallization · drug formulation ·
`nucleation · polymorphism ·
`solid-state chemistry
`
`Angewandte
`Chemie
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`6972
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`www.angewandte.org
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` 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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`Angew. Chem. Int. Ed. 2015, 54, 6972 – 6993
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`Polymorphism
`
`Nearly twenty years ago, Dunitz and Bernstein described a selection
`of intriguing cases of polymorphs that disappear. The inability to
`obtain a crystal form that has previously been prepared is indeed
`a frustrating and potentially serious problem for solid-state scientists.
`This Review discusses recent occurrences and examples of disap-
`pearing polymorphs (as well as the emergence of elusive crystal forms)
`to demonstrate the enduring relevance of this troublesome, but always
`captivating, phenomenon in solid-state research. A number of these
`instances have been central issues in patent litigations. This Review,
`therefore, also highlights the complex relationship between crystal
`chemistry and the law.
`
`Angewandte
`Chemie
`
`From the Contents
`
`1. Introduction
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`2. Disappearing Polymorphs—The
`Concept and Misconceptions
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`6974
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`3. Recent Instances of
`Disappearing Polymorphs and
`Elusive Crystal Forms
`
`4. Recovering Disappeared
`Polymorphs
`
`5. Outlook
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`1. Introduction
`
`There is a continual and increasing demand for crystalline
`molecular materials with specific, fit-for-purpose physico-
`chemical properties.[1–6] Interest in polymorphism, crystalli-
`zation, and (in industry) in robust process development has
`surged over the last two decades,[7, 8] as evidenced by the
`immense growth in knowledge concerning the design, prep-
`aration, and characterization of crystalline materials.[9] This
`expanding interest and demand for promising materials drives
`investigations of the solid form (i.e. polymorphs, solvates,
`hydrates, and amorphous materials) landscapes[8, 10] of poten-
`tially relevant compounds, with the goal of identifying the
`optimally performing solid among them.
`A broad range of crystallization techniques is generally
`employed to search for the most stable crystal form in
`hundreds or (in some cases) thousands of experimental
`attempts.[11] New crystal forms can, however, emerge unex-
`pectedly long after the carefully designed and executed
`screening experiments are completed. Such a sudden emer-
`gence of a new crystal form can be unsettling and problem-
`atic, especially in the late stages of a product development or
`even following launch, because the newly emerged form can
`exhibit different (possibly undesired) properties. Equally
`disruptive is the emergence of a thermodynamically more-
`stable crystal form, in accord with Ostwalds Rule of Stages,[12]
`concurrent with the disappearance of the less-stable known
`forms that signal a loss of control of the production process.
`While it may create roadblocks in the development process or
`even the marketed product of the solid form of a compound of
`interest, the consequences of the appearance of a new form
`are not necessarily negative. The serendipitous appearance of
`a new form may provide a substance with improved charac-
`teristics.
`Unfortunately, our current understanding of the mecha-
`nisms and processes involved in the nucleation and growth of
`crystals is still
`insufficient
`for precise control over the
`formation or disappearance of a polymorph (or any other
`crystal form).[13, 14] Nearly twenty years ago, Dunitz and
`Bernstein presented an overview of the disappearing poly-
`morph phenomenon[15] that has captivated and intrigued
`solid-state scientists since. In their review, Dunitz and
`Bernstein voiced their belief that crystal
`forms do not
`
`disappear permanently; on the contrary, once a solid form
`has been obtained, in principle it can always be reproduced if
`the right experimental conditions are met.[15–18] In the same
`spirit as the earlier survey, this Review aims to discuss
`selected recent occurrences of disappearing polymorphs and
`of elusive crystal forms that have not only triggered the
`curiosity of researchers, but have also affected the business of
`pharmaceutical and health care companies. These examples
`illustrate how apparently stable polymorphs can suddenly
`disappear, and how elusive crystal forms can be prepared
`given the availability of conditions specifically designed to
`promote their formation. The uncontrolled loss of a crystal
`form can have serious consequences, and there is thus an
`urgent need to develop methods that provide absolute control
`over crystal nucleation and growth,[13, 14] which is still an art,
`rather than a routine procedure.[19]
`In addition to citing examples of disappearing polymorphs
`from the literature and our own laboratories, the 1995 review
`dealt with a number of issues that are still the subjects of
`debate. There have also been a number of patent litigations in
`which the same issues have arisen and have been interpreted
`
`[*] Dr. D.-K. Bucˇar, Dr. R. W. Lancaster
`Department of Chemistry, University College London
`20 Gordon Street, London WC1H 0AJ (United Kingdom)
`E-mail: d.bucar@ucl.ac.uk
`r.lancaster@ucl.ac.uk
`Prof. Dr. J. Bernstein
`Faculty of Natural Sciences, New York University Abu Dhabi
`P.O. Box 129188, Abu Dhabi (United Arab Emirates)
`and
`New York University Shanghai, Pudong New Area, Shanghai 200122
`(China)
`and
`Department of Chemistry, Ben-Gurion University of the Negev
`Beer Sheva, 84120 (Israel)
`E-mail: joel.bernstein@nyu.edu
`yoel@bgu.ac.il
` 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co.
`KGaA. This is an open access article under the terms of the Creative
`Commons Attribution License, which permits use, distribution and
`reproduction in any medium, provided the original work is properly
`cited.
`
`Angew. Chem. Int. Ed. 2015, 54, 6972 – 6993
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` 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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`6973
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`.
`Angewandte
`Reviews
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`variously by the courts. We will deal initially with those
`aspects of the subject and follow with the descriptions of
`a number of recent cases of disappearing polymorphs (and
`other crystal forms), as well as further details on some of
`those previously cited.
`
`2. Disappearing Polymorphs—The Concept and
`Misconceptions
`
`One of us (J.B.) recently recounted the genesis of the 1995
`review,[20] which was based on earlier cases in the laboratories
`of both Bernstein and Dunitz as well as additional examples
`we had encountered in the course of our involvement in the
`ranitidine hydrochloride litigations.
`In the twenty-year
`interim we have experienced numerous additional examples
`in which the phenomena described therein were either
`misinterpreted or misunderstood. Hence, we review some of
`those here.
`
`2.1. The Concept
`
`As we described in the section of the 1995 review headed
`“Vanishing Polymorphs”, a disappearing polymorph refers to
`a crystal form that has been prepared at least once and whose
`existence has been established experimentally by some
`observation or measurement. Subsequent attempts to prepare
`the same crystal
`form by the same procedure lead to
`a different crystal form, alone or together with the old one.
`If a mixture appears in the first instance, then very often in
`subsequent preparations the new form dominates and the old
`form is no longer obtained.
`The phase rule limits to one the number of stable crystal
`forms that may exist under a specific set of conditions. The
`old—“disappeared”—form is generally less stable than the
`new one under those specific conditions. In thermodynamic
`terms, it is metastable, although that does not necessarily
`imply that it would spontaneously convert into a more stable
`form; it only means that it is at a higher energy minimum than
`the most stable state. To invoke a familiar example: diamond
`is metastable with respect to graphite; nevertheless, as is
`widely advertised, “diamonds are forever”.
`
`D.-K. Bucˇar et al.
`
`The fact that a crystal form once existed, but is now
`difficult to prepare by the same method that was previously
`used, does not mean that it is impossible to prepare again. It
`has not been relegated to the “crystal form cemetery”.[21]
`Every crystallization is a competition between kinetic and
`thermodynamic factors. As noted in the last sentence of the
`1995 review, “it is always possible to obtain [the old form]
`again; it is only a matter of finding the right experimental
`conditions”—thermodynamic and kinetic.
`Recovering a crystal form that has disappeared may
`require considerable time and effort and invoke some
`inventive and creative chemistry. The examples given below
`will demonstrate the kinds of strategies that have been
`employed to recover crystal forms that have disappeared.
`
`2.2. Seeds and Seeding
`
`The 1995 review also contains a section headed “Seeding”.
`Intentional seeding is a well-known technique for inducing
`crystallization and is widely used, especially in the pharma-
`ceutical
`industry. Unintentional seeding arises from the
`presence of small amounts—indeed, in principle one particle
`is sufficient—of the solid material that is present even as
`a contaminant. As we noted earlier, “Unintentional seeding is
`often invoked as an explanation of phenomena which are
`otherwise difficult to interpret. We shall argue in favor of this
`explanation, although there is no consensus about the size and
`range of activity of such seeds, which have never actually been
`directly observed.”[15]
`The situation this statement describes has led to consid-
`erable controversy, particularly in the framework of patent
`litigations involving crystal forms. That controversy very
`much represents the clash between the cultures of science and
`the law, and in light of that controversy it seems appropriate,
`indeed compelling, to put the phenomenon of unintentional
`seeding into a proper scientific perspective in this Review.
`Virtually every chemist has at some time attempted to
`crystallize a compound. Crystallization is perhaps the classic
`method of purification, and the technique is one of the first
`mentioned in purification methods in any undergraduate
`organic chemistry laboratory textbook. Practicing chemists
`soon learn, often simply by experience, that it is frequently
`very difficult to crystallize a newly synthesized substance,
`
`Kresˇo Bucˇar obtained a BSc in chemistry
`with Dr. Ernest Mesˇtrovic´ at the University
`of Zagreb, and a PhD in chemistry from the
`University of Iowa with Prof. Leonard R.
`MacGillivray. He then started his independ-
`ent research career as a Royal Society
`Newton International Fellow at the Univer-
`sity of Cambridge with Prof. William Jones.
`While in Cambridge, he was also a Bye-
`Fellow at Sidney Sussex College. He recently
`joined the Department of Chemistry at Uni-
`versity College London as UCL Excellence
`Fellow. His research interests mainly concern
`molecular cocrystals and their applications.
`
`Robert Lancaster joined Glaxo in 1969,
`studied part-time to obtain a Grad RIC
`degree. He gained exposure to the phenom-
`enon of polymorphism in the mid-70s and
`completed his PhD at the University of East
`Anglia in 1986. He worked closely with
`process and pharmaceutical development
`scientists, specifically looking at issues sur-
`rounding all aspects of crystallization and
`polymorphism. He had some exposure to
`legal aspects of two high profile Glaxo
`(GSK) drugs. After retiring from GSK, he
`joined Prof. Sally Price’s group at University
`College London on a part-time basis.
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` 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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`Angew. Chem. Int. Ed. 2015, 54, 6972 – 6993
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`Polymorphism
`
`while subsequent crystallizations are considerably more
`facile. The situation was documented over half a century
`ago by Wiberg in his classic text “Laboratory Technique in
`Organic Chemistry” in the section entitled “Inducing Crys-
`tallization”: “When a compound is prepared for the first time
`in a laboratory, it is often observed that it is relatively difficult
`to effect crystallization. However, once the compound has been
`obtained in the crystalline state, it is usually easy to effect
`crystallization, and it has been suggested that after initial
`crystallization crystal nuclei are present in the laboratory and
`induce crystallization”.[22] In the current context those nuclei
`are unintentional seeds.
`Many laymen are initially skeptical about a phenomenon
`caused by particles that cannot be seen, although very few
`would accept an invitation for a casual—and unprotected—
`visit to the pneumonia ward at their local hospital. The
`approximate limit of visual detection for the naked eye is
`6 g. We pointed out
`a crystal that weighs approximately 10
`earlier that a speck of that size contains approximately 1016
`molecules and while there are various estimates of the size of
`a critical nucleus that could act as a seed even the largest—
`a few million molecules[23]—would mean that an invisible
`particle could contain up to 1010 of such unintentional seeds.
`Where do these microscopic particles come from? As
`noted elsewhere, depending on our location, the air contains
`a vast number of submicroscopic particles. For a normal urban
`environment there are approximately 106 airborne particles of
`0.5 micrometer diameter or larger per cubic foot, the number
`being reduced by an order of magnitude in an uninhabited
`rural environment. A sitting individual generates roughly one
`million dust particles ( 0.3 micrometer diameter) per minute
`(a visible particle is usually  10 micrometers).[24] Clean
`rooms for various purposes (e.g. surgery, biological or
`pharmaceutical preparations,
`semiconductor
`fabrication)
`employ very sophisticated technology to remove these
`particles and to prevent subsequent contamination. There-
`fore, the possible presence of seeds of a newly formed
`polymorph in a laboratory, a manufacturing facility, or any
`location having been exposed to that form cannot be casually
`dismissed; indeed its presence would be hard to avoid. In his
`comprehensive monograph on crystallization, Mullin notes
`that, “Atmospheric dust frequently contains particles of the
`crystalline product itself, especially in industrial plants or in
`laboratories where quantities of
`the material have been
`
`Joel Bernstein studied chemistry at Cornell
`University (BA 1962) and Yale University
`(PhD 1967). He was then a postdoctoral
`fellow with Ken Trueblood at UCLA and
`then with Gerhardt Schmidt at the Weiz-
`mann Institute. He is professor emeritus at
`Ben-Gurion University of the Negev in
`Israel, which he joined in 1971 and from
`which he retired in 2010 as the Barry and
`Carol Kaye Professor of Applied Science. He
`is currently Global Distinguished Professor of
`Chemistry at New York University having
`taught at Abu Dhabi and Shanghai. His
`research interests concern chemistry of the organic solid state, particularly
`polymorphism.
`
`Angewandte
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`handled…Once a certain crystalline form has been prepared
`in a laboratory or plant, the working atmosphere inevitably
`becomes contaminated with seeds of the particular material.”[23]
`So much for the atmosphere. What about the crystallizing
`medium, usually a solution? The normal determination that
`dissolution has been completed is made by visual inspection.
`If the solution is clear to the human eye all the solute is
`assumed to be in solution. Mullin has also pointed out that
`“aqueous solutions as normally prepared in the laboratory
`may contain > 106 solid particles per cm3…”.[23] These can be
`impurities or particles of the solute that have not undergone
`complete dissolution, and can serve as seeds for the subse-
`quent crystallization.
`The presence and influence of microscopic seeds and their
`influence on crystallization is thus well established. Never-
`theless, it is difficult for many who lack practical laboratory
`experience to accept
`their existence. In the history of
`chemistry there have been many instances of inductive
`reasoning in understanding chemical phenomena. The exis-
`tence of atoms was proposed and accepted for nearly two
`hundred years before an atom was actually “seen”. Yet no
`chemist doubts the existence of atoms or the ability to make
`and break bonds between them.
`The presence and influence of seeds may be invoked to
`explain the disappearance of one crystal form at the expense
`of a new form. In such a case, the unintentional seeding by the
`new form may be quite aggressive, preventing the crystal-
`lization of the old form. However, there is no intrinsic reason
`why every system is influenced by such aggressive uninten-
`tional seeding. There are many known examples of multi-
`crystalline materials in which the various forms can be
`prepared and maintained in the known presence of other
`forms. As for polymorphism in general, every system is
`unique and must be individually studied and characterized to
`understand how to prepare and characterize each form.
`
`2.3. “Universal Seeding”
`
`The publicity surrounding some cases of aggressive
`unintentional seeding led to discussions, particularly in legal
`circles, of the alleged phenomenon of universal seeding—that
`is, in some cases of disappearing polymorphs, when the old
`form could not be made by the old process somehow, there
`was an implication that the entire universe must be seeded. To
`put the matter to rest it is important to quote a footnote from
`the 1995 review: “The claim for universal seeding, taken
`literally, is obviously absurd. After all, the universe is estimated
`to contain about a millimole of stars, so one seed per star (per
`solar system)—not much—would need about 100 kg of the
`compound in question (Mr 100)”.
`A number of cases of aggressive seeding have attained
`considerable notoriety, and these will be described below. In
`instances where various locations at considerable distance
`have become “infected” with a new form within a relatively
`short time, it has been possible to trace the source of the
`seeding in successively affected locations.
`
`Angew. Chem. Int. Ed. 2015, 54, 6972 – 6993
`
` 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
`
`www.angewandte.org
`
`6975
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`..
`Angewandte
`Reviews
`
`D.-K. Bucˇar et al.
`
`3. Recent Instances of Disappearing Polymorphs
`and Elusive Crystal Forms
`
`This section describes several of the most (in)famous
`recent cases of disappearing polymorphs and other crystal
`forms. In addition, in relation to the sudden and unexpected
`disappearance of a well-known crystal form, we consider it
`particularly relevant to describe cases where elusive crystal
`forms, believed to be non-existent, were prepared.
`
`3.1. Ranitidine Hydrochloride
`
`In the early 1970s, James Black at (then) Smith, Kline &
`French identified the histamine type 2 (H2) receptor and from
`the preparation of a series of H2-receptor antagonists
`developed the first antiulcer drug, cimetidine (Tagamet),
`for which he won the 1988 Nobel Prize in Medicine. H2-
`receptor antagonists are among the miracle drugs of the 20th
`century. Prior to their introduction (and the subsequent entry
`of proton pumps) there were millions of sufferers of peptic
`ulcers worldwide with a significant number of fatalities; since
`their introduction, the surgical procedure for removing peptic
`ulcers has essentially been eliminated from the modern
`medical school curriculum.
`The dramatic success of cimetidine led to industry-wide
`efforts to develop additional H2-receptor antagonists. In 1977,
`Allen & Hanbury (then a part of Glaxo Group Research, now
`GSK) developed ranitidine and its hydrochloride (Figure 1 a),
`for which a US patent was issued in 1978.[25] The preparation
`of the hydrochloride following the multistep synthesis of
`ranitidine base is given in “Example 32” of the patent
`(Figure 1 b).
`Subsequent development of the drug over nearly four
`years involved batch scale-ups to a multi-kilogram scale in the
`companys pilot plant by employing essentially the chemistry
`described in Example 32.[10] The batch prepared on April 15,
`1980 failed the quality control IR analysis, which exhibited
`1, and suggested
`a hitherto unobserved sharp peak at 1045 cm
`the formation of a new crystal form designated Form 2. The
`subsequent four batches exhibited increasing amounts of
`Form 2 and the same process no longer produced the (now
`designated) Form 1. Considerable efforts to revert to the
`production of Form 1 by essentially the same process were
`unsuccessful. Thus, this is clearly a case of a disappearing
`polymorph. Serendipitously, Form 2 had considerably
`improved filtering and drying characteristics which,
`in
`addition to the novelty of the new polymorph, formed the
`basis for a patent application, granted in 1985.[10] The crystal
`structures of both forms have been subsequently determined;
`both forms crystallize in the monoclinic P21/n, space group,
`wherein the nitroethenediamine moiety of the ranitidine
`cations displays different conformations and degrees of
`disorder (Figure 1 c).[26–28] This is thus also an example of
`conformational polymorphism.[29]
`Glaxo launched ranitidine hydrochloride in 1984 as
`Zantac and by 1992 it was the worlds best-selling drug at
`US$ 3.44 billion per year, when sales for the next largest drug
`(Bayers Adalat Procardia) were about half that amount.[30]
`
`Figure 1. a) Molecular structure of ranitidine hydrochloride. b) Exam-
`ple 32 from patent US 4128658A (“Aminoalkyl furan derivatives”), the
`apparently straightforward procedure for the preparation of Form 1 of
`ranitidine hydrochloride. c) Overlay of the ranitidine cation from
`Form 1 (blue) and Form 2 (red). Form 2 features a disordered nitro-
`ethenediamine moiety.
`
`In accord with the terms of the 1984 Hatch–Waxman Act in
`the US, by 1990 a number of generic drug companies were
`planning to enter the market with Form 1 in anticipation of
`the 1995 expiration of the Form 1 patent. Attempts to make
`Form 1 were based on carrying out Example 32. As transpired
`in the course of the subsequent litigations, essentially all of
`those attempts started with commercial Form 2; hence, at the
`very least Form 2—thus seeds of Form 2—were present in the
`environment in which attempts to follow Example 32 were
`being carried out.
`Following numerous attempts to prepare Form 1 accord-
`ing to Example 32, which led almost exclusively to Form 2,
`the Canadian generic firm Novopharm claimed that Glaxo
`had never made Form 1 and sought approval from the Food
`and Drug Administration (FDA) to market Form 2. Glaxo
`aimed to prevent Novopharm (and others) from entering the
`market with Form 2 by suing them for the infringement
`(actually, virtual
`infringement under the Hatch–Waxman
`Act) of their Form 2 patent. Novopharm admitted infringe-
`ment of Form 2, but argued that the Form 2 patent was
`inherently anticipated in the Form 1 patent, since their
`attempts to prepare Form 1 according to Example 32 led to
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` 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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`Angew. Chem. Int. Ed. 2015, 54, 6972 – 6993
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`Polymorphism
`
`Form 2. Novopharm contended that the experimental proce-
`dures underpinning the Form 1 patent were flawed, to which
`Glaxo responded that the oppositions experiments were
`contaminated with seed crystals and hence not a faithful
`reproduction of Example 32 [Clearly, there were no seeds of
`Form 2 anywhere prior to April 15, 1980].
`The legal concept of inherency in the United States
`implies a consistent result of a process, that is, it must be
`invariable or inevitable that one obtains the later claimed
`result to establish inherency. Thus, to support its case against
`inherent anticipation, in principle Glaxo had to demonstrate
`that Example 32 did not inevitably or invariably yield Form 2
`but could in fact yield Form 1.
`To do so, in the course of the August 1993 trial, the
`notebooks of David Collin, who been the first to prepare
`ranitidine hydrochloride were examined, cross-examined, and
`compared to the wording in Example 32. Collins notebooks
`contain three slightly different examples. As is common for
`a laboratory notebook, the texts are not word-for-word
`identical nor is any one identical
`to the language in
`Example 32, and there was much discussion over the differ-
`ences and what they would mean to a practicing chemist (one
`“skilled in the art” in patent lexicon).
`In addition, one of Glaxos witnesses, Sir Jack Baldwin of
`the University of Oxford, in 1993 had two of his senior
`postdoctoral fellows complete the entire synthesis of raniti-
`dine base according to the Form 1 patent followed by the
`reproduction of Example 32 using that prepared base. They
`also obtained Form 1 three times.
`Those six instances of the preparation of Form 1 according
`to Example 32 were sufficient to overcome the inherency
`argument. The Form 2 patent was found to be valid and
`Novopharm (and others) were restricted from marketing
`Form 2 prior to its anticipated expiration in 2002.
`Legal footnotes. A number of litigation cases ensued. It
`was surprising that Glaxo could no longer make Form 1 in the
`original pilot plant, but even so, at the time, the concept of
`disappearing polymorphs and the role of unintentional
`seeding were treated with skepticism by those who had no
`personal experience of
`the phenomenon. For instance,
`counsel for Novopharm included the following in his opening
`statement to the court:[31]
`“Theres also testimony in this case which is under
`a protective order from a third party pharmaceutical company
`that did the same thing. They reproduced example 32 and got
`Form 2. So we have six different locations or incidents where
`example 32 had been reproduced to yield Form 2, not Form 1.
`Whats Glaxos response to this? Seed crystals, theyre in the
`air. You cant see them. You cant smell them. You cant taste
`them. You also cant detect them but theyre there, and these
`seed crystals fall out of the sky, and theyre very intelligent
`because they know when youre running one of these example
`32 experiments. They fall out of the sky and they fall in your
`reaction beaker and it causes not Form 1 to be produced, but
`Form 2, and thats why when we run this experiment today we
`get the Form 2 product and not the Form 1.”
`“Well, I submit that if one believes in Santa Claus we might
`believe in these seed crystals, but if were beyond that, were not
`going to believe in these seed crystals, and even if you do, the
`
`Angewandte
`Chemie
`
`techniques that were used in these reproductions would,
`without a doubt, exclude these seed crystals because these
`seed crystals to survive in the method that has been used for
`these reproductions have to defy standard chemical princi-
`ples”.
`Some excerpts from the cross-examination of one of
`Novopharms witnesses regarding seeding:
`Question (cross-examining attorney): “I think the issue of
`seeding is one that I would have expected to come from
`a crystallographer. Have you made a study of the subject of
`seeding?”
`Answer: “Ive found in my experiments that I cant see any
`seeding effects.”
`Question: “You found that you cant see. My question was,
`have you done a study of the science of seeding which takes in
`account the myriad of works of those who can see. Have you
`made such a study?”
`Answer: “Ive done a literature search to see if a theoretical
`phenomena (sic) like the hypothetical
`theory of universal
`seeding could be found in all of the chemical abstract literature,
`and the only references I found to something called universal
`seeding had to do with entries like, universal prevention of
`fungus on weed seeds by using certain different fungicides.
`Thats the only type of reference I could come up with when I
`scanned the chemical literature.”
`Question: “…Now, did you go to any meeting of profes-
`sional crystallographers or did you consult crystallographers to
`see whether there was a body of knowledge that you hadnt
`found?”
`…Answer: “I reported my negative findings to the
`[attorneys].”
`
`3.2. Ritonavir
`
`Perhaps the most notorious recent example of a disap-
`pearing polymorph is that of ritonavir (Figure 2 a), an
`antiviral compound marketed by Abbott Laboratories in
`1996 as Norvir in the form of semisolid gel capsules for the
`treatment of the acquired immune deficiency syndrome
`(AIDS). The capsules were based on the only known crystal
`form, Form I, discovered during the development process. In
`1998, however, a new and significantly less-soluble polymorph
`of ritonavir unexpectedly precipitated out in the semisolid gel
`capsules, thereby leading to failed dissolution tests for the
`capsule.[32, 33] Subsequent studies showed that the new form,
`referred to as Form II, exhibited a significantly lower
`solubility in hydroalcoholic solutions than the marketed
`Form I.[33] In addition, it was found that Form II rendered
`Form I unobtainable in any laboratory to which Form II had
`been introduced. There was even speculation that
`the
`conversion of Form I into Form II in the laboratory was
`facilitated simply by the presence of an individual who had
`previous exposure to Form II (or the contaminants that were
`subsequently shown to enable the formation of Form II). As
`a result of these events, ritonavir had to be temporarily
`withdrawn from the market.[34]
`Crystallographic analyses showed that the crystal struc-
`ture of Form I is characterized by ritonavir stacks resembling
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`Angew. Chem. Int. Ed. 2015, 54, 6972 – 6993
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` 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
`
`www.angewandte.org
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`6977
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`Angewandte
`Reviews
`
`Figure 2. a) Molecular structure of ritonavir. b) Crystal structures of its
`Form I (left) and Form II (right). c) Formation of the presumed
`heteronuclear seed of ritonavir Form II through a base-catalyzed
`reaction.
`
`a b-sheet structure (Figure 2 b).[32] The structure is sustained
`by N-H(amide)···O(amide) and O-H(hydroxy)···N(thiazole)
`hydrogen bonds (with first-order graph set N1 = C(4)C11)[35]).
`The crystal structure of Form II, on the other hand,
`is
`comprised of heavily hydrogen-bonded one-dimensional
`ritonavir stacks (Figure 2 b).[36] Each molecule in the stack is
`hydrogen bonded to two other molecules through a total of
`eight N-H(amide)···O(amide), N-H(amide)···O(hydroxy),
`(N1 =
`and O-H(hydroxy)···O(amide)
`hydrogen
`bonds
`C(6)C(9)C(11)C(12)).[32, 35] The higher calculated crystal den-
`sity of Form I suggested it is also the more stable crystal
`form.[37] In addition, a survey of the Cambridge Structural
`Database (CSD) indicated that Form I ritonavir exhibits
`a statistically more favorable conformation of the carbamate
`moiety.[32] The analysis was in agreement with an NMR study
`in solution that revealed the existence of two conformers in
`solution in a ratio of roughly 99:1. The conformers could not
`be unambiguously resolved as Forms I and II, but it was noted
`that the observed 99:1 relationship of the two conformers is
`consistent with the initial discovery of a single polymorph,
`that is, Form I.[32] The higher stability of Form II was, in the
`end, attributed to the formation of a hydrogen-bond pattern
`wherein, unlike in Form I, “all of the strong hydrogen bond
`donors and acceptors have been satisfied”.[33] This argument is
`consistent with the observation that Form II has a higher
`1) than
`melting point and heat of fusion (ca. 125 88C, 87.8 J g
`1).[33]
`Form I (ca. 122 8C, 78.2 J g
`
`D.-K. Bucˇar et al.
`
`A recent logistic regression hydrogen-bonding propensity
`study involving Forms I and II (using the CSD as data source)
`reported that the kinetically more favored Form I displays
`a statistically doubtful hydrogen-bond pattern. Specifically, it
`was found that Form I entails statistically improbable hy-
`droxy-thiazoyl and ureido-ureido interactions—despite the
`hydrogen-bond donors and acceptors availability for the
`realization of high-propensity hydrogen bonds.[38]
`The origin of Form II was initially unclear, as it was
`established that ritonavir solutions would crystallize as
`Form II only if seeded with Form II—even at amounts as
`low as 1 ppm. Heterogeneous nucleation was identified as
`a possible cause of the formation of Form II. Specifically, it
`was found that ritonavir degrades in a base-catalyzed reaction
`to form a carbamate-bearing product (Figure 2 c) structurally
`related to the conformation of ritonavir in Form II.[32] It was
`also found that the degradation product forms very rapidly
`and that, consistent with its greater stability, it exhibits a lower
`