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.