Many active pharmaceutical ingredients (APIs) can be crystallized in
different crystal forms through a phenomenon known as polymorphism.
Crystallization processes and crystal structures are often complex and
unpredictable. APIs can exist in different physical forms (polymorphs,
solvates, salts, co-crystals or amorphous).
Although
morphology and particle size distribution are important solid-state
characteristics, the uncontrolled occurrence of multiple physical forms
of an API can have significant effects on the performance of the
material during processing, manufacturing, storage, and administration.
Such
differences may have a potentially significant impact on the stability
(physical & chemical) and bioavailability/bioequivalence of
solid/liquid oral dosage form and also relevant for intellectual
property considerations because solids with superior properties can be
patent protected. It is estimated that 30-50 per cent of pharma
compounds exhibit polymorphism. A new polymorph can appear suddenly in a
manufactured drug, with devastating results and it is taken very
seriously by the pharma industry.
Paracetamol,
is the most widely used antipyretic and analgesic in the world. Though
the drug seems to be simple, it has been shown to exist in two
polymorphic forms. One is monoclinic Form-I, which is marketed whereas
Form-II is orthorhombic. Similarly, Famotidine which is an excellent
histamine H2 receptor antagonist is also found to exist in two different
polymorphic forms, metastable polymorph B and stable polymorph A.
Enalapril maleate is an ACE inhibitor known to exist in two polymorphic
modifications (I and II) with Form II being the more thermodynamically
stable form.
Terazosin
hydrochloride is a selective b1 adrenoceptor receptor antagonist that
is known to exist in three anhydrous forms (I, II, III), as a
monohydrate and as a dihydrate. Torsemide is a diuretic known to exist
in two non solvated modifications (forms I and II) and in a
nonstoichiometric channel inclusion compound consisting of water and
alcohol (Form A). Carbamazepine is an anticonvulsant that exists in
three non-solvated forms (a, ß, g) and in a dihydrate form, with the
dihydrate exhibiting the lowest solubility. Piroxicam, a nonsteroidal,
anti-inflammatory drug widely prescribed all over the world exists in
three forms I, II and III. Norfloxacin, is the widely used synthetic
broad spectrum antibacterial fluoroquinolone for the treatment of
prostate and urinary tract infections. This drug exists in two anhydrous
polymorphs (A and B), an amorphous form and several hydrated forms. Of
the two anhydrous polymorphs, Form B is the most stable at room
temperature. But the commercial sample of norfloxacin used is the Form
A, which is metastable at room temperature.
Regulatory
authorities worldwide, however, increasingly require pharmaceutical
companies to demonstrate absolute control over their production
processes. Withdrawal of HIV-protease inhibitor ritonavir because of
unexpected appearance of less soluble polymorph made company lose an
estimated $250 million in sales, as well as hundreds of millions of
dollars trying to recover the original polymorph, while the product was
off the market. Such an event clearly indicates its potential clinical
impact on patients and logistical, regulatory, and financial
consequences for manufacturers. It is therefore necessary to invest in
time and resources required to secure a robust understanding of the
extent of physical form diversity for a new API and the factors that
control it.
Many complex legal issues arising from polymorphism
and most of these issues are concerned with patent cases. The most
famous case is the Zantac patent case, which is concerned with the
solid-state form of Glaxo's major drug, ranitidine hydrochloride, for
the treatment of peptic ulcers. This is a polymorphic drug capable of
adopting two crystal structures. A process resulting in the
crystallization of Form I was patented in 1978; two years later a more
stable crystalline Form II appeared which was also patented and which
subsequently, became the active ingredient for Zantac formulations. When
the patent expired in 1995, other generic companies also came in this
field and subsequent legal battles resulted.
Novartis patent
case, which was dealt in Madras High Court, was about a life saving
cancer drug 'Gleevac' containing imatinib mesylate. Gleevac offers a
cure to the life threatening form of the cancer 'chronicmyeloid
leukaemia'. Novartis invented imatinib in 1992 and patented it in 1993
in US and other countries. The company applied for a patent in India in
1998 for ß-crystalline form of imatinib mesylate, which led to a legal
scrutiny of Patents Act 1970.
Crucially, predicting the
occurrence and likely extent of polymorphism in a specific molecule,
particularly with conformational flexibility of a typical pharmaceutical
molecule, is a significant challenge to theory. Whilst considerable
advances have been made in the area of crystal structure prediction in
recent years, experimental polymorph screens are an essential component
of preclinical drug development. This provides information for the
control of solid form throughout the subsequent scale up, formulation,
manufacture, storage, and usage of pharmaceuticals.
Screening strategies
Ritonavir
served as a wake-up call for the pharmaceutical industry, which became
more interested in exploring the solid state of drug candidates at an
earlier stage. Now polymorph screening is a common procedure, using
robots to carry out hundreds or thousands of crystallisation experiments
using different solvents or solvent mixtures and different cooling and
evaporation rates. The crystallization process is not well understood
but involves nuclei forming within a supersaturated solution of the
compound. The nuclei act as templates for the formation of further
crystals within the solution.
Without nucleation, the
supersaturated solution may not crystallise for a long time, which is
why chemists sometimes try to form nuclei by scratching the side of the
vessel containing the solution with a glass rod. Tiny crystals of the
desired polymorph - if they exist - can act as nuclei or seeds for
crystallising that polymorph. But seeding doesn't always work as planned
- if seeds of an undesired polymorph are present, even in a very small
amount, they will tend to seed that polymorph, instead of the wanted
one. Impurities can also act as seeds.
Fundamental objective in
an experimental polymorph screening is to recrystallise the target API
under a wide range of conditions as can be achieved within the
constraints of available compound, time and resources. Crystallisation
is a highly complex process, there are several process variables that
can affect the outcome. Supersaturation as the driving force of
crystallisation is the key thermodynamic variable that affects the
kinetics of crystal nucleation and growth. To increase the probability
of discovering all the relevant forms, the multiparameter space that
contributes to solid form diversity should be covered as broadly as
possible. This is usually achieved by designing a rational set of the
process variables (table1).
In early stage of development,
high-throughput (HT) screening with fully automated robotic systems
capable of performing thousands of crystallizations per week with only
few grams of API consumption can be prove to be very useful. HT
screening is most commonly carried out in 96-well plate systems in which
the particular solid, dissolved in a suitable solvent, is initially
dispensed using automated liquid handling systems. Different levels of
supersaturation can be achieved by, for example, heating/cooling,
evaporation, and by varying the nominal concentration of API. Similarly
to bench-scale crystallisations, slurry experiments can also be
implemented in HT systems. Once the crystallization occurs, solid state
formed under each set of test conditions is evaluated and results are
used to identify the experimental factors controlling polymorphic
outcomes and also to assess the completeness of the experimental screen.
However the selection of best available crystalline candidate must be
based on certain physical and chemical parameters as outlined in table
2.
Variation and alternatives to solution crystallization
approach includes contact line crystallization, crystallization under
high pressure, crystallization in constrained environments such as
nanometer scale pores in glass bead, on self assembled monolayer, and in
glass capillaries, mechanically induced changes, slurries,
heteroseeding, and templating, polymer hetero nuclei and microarrays, in
situ thermal transformations and recrystallisation from amorphous
solids, growth from vapour phase via reverse sublimation.
For
the best overall results, one should design strategies to best fit the
development process of drugs from screening to controlled scale-up of
the crystallization process and complemented with the full range of
physico-chemical evaluation.
Evaluation and characterization techniques
Thorough
solid state characterization of a new drug substance is recognized as
an essential part of preformulation research. It is the goal of
preformulation scientists to investigate and characterize polymorphs of a
pharmaceutical solid and select a form with the best combination of
desired properties to proceed to the formulation stage. In addition,
regulatory agencies require that understanding and controlling the
physical forms of a polymorphic drug substance be demonstrated,
especially if drug bioavailability is affected by its polymorphism.
Techniques commonly used to study solid-state properties are listed in
Table 3.
The method of choice for a specific case depends on the
key parameters one needs to determine and how deeply they have to be
investigated. Usually it is advisable to use two or more complementary
methods to obtain a reliable knowledge of the forms. Single-crystal and
powder x-ray diffraction are the gold standards for determining how
molecules are packed in the crystal; advances in this technology means
that even smaller crystals can be used. Differential scanning
calorimetry (DSC) allows the detailed study of the crystal's behaviour
as it is heated and cooled, to determine the stability range of
different polymorphs and to identify any changes from one polymorph to
another. 'DSC is a good way of finding new polymorphic forms,' comments
Chris Frampton, chief scientific officer of SAFC-Pharmorphix, UK.
Infrared,
Raman, and nuclear magnetic resonance spectroscopy (NMR) are used
because the different hydrogen bonding patterns in polymorphs give rise
to different spectra; added to these are solid state NMR and terahertz
(far infrared) spectroscopy, emerging technologies which will further
refine the characterization of polymorphs.
The development of a
new pharmaceutical product requires a deep understanding of solid-state
phenomena. In order for a pharmaceutical product to succeed, the
possible existence and performance of various solid forms need to be
thoroughly investigated and the IP strategies carefully considered
before commercial launch. Therefore, solid form screening has become an
essential part of pharmaceutical development and product lifecycle
management.
The tools available for solid form screening have
evolved radically during the past decade - we can now explore the solid
forms computationally, perform thousands of experimental
crystallizations with miniaturised high-throughput screening
technologies, and identify new solid phases fast. We should, however,
pay special attention on the evaluation part of the screening.
Understanding of basic thermodynamics together with robust design of
experiments and powerful data analysis are the keys to successful solid
form screening.
Mrunali R. Patel is faculty, Indukaka Ipcowala
College of Pharmacy. Rashmin B. Patel, Jolly R Parikh and Bharat G.
Patel are faculty A. R. College Of Pharmacy & G. H. Patel Institute
of Pharmacy, Vallabh Vidyanagar , Gujarat.