Drug Delivery & Formulation

Pharmaceutical Analysis using Terahertz Terahertz pulsed spectroscopy can be used to characterise and quantify various solid-state forms of active pharmaceutical substances – notably polymorphs – both in bulk form and as finished tablets.

By Paul Smith at TeraView

Paul Smith is a Pharmaceutical Business Development Consultant for TeraView. He managed the introduction of TeraView’s first pharmaceutical product, the TPIspectra1000, in 2003; this was awarded the New Product Gold Award at PITTCON 2004 and also the 2004 R&D 100 award. Mr Smith has been responsible for formulating TeraView’s Technology Access Partnership (TAP) strategy programme which has successfully signed up a number of pharmaceutical companies. This collaborative partnership gives companies early access to TeraView’s unique Far-IR/THz technology, for example in polymorph screening and non-invasive 3D chemical mapping of solid dosage forms. He also recently initiated and implemented TeraView’s Process Analytical Technology (PAT) Pharmaceutical Business Strategy. He obtained a BSc in Genetics from the University of Nottingham (UK) and an MSc in Computer Science from Birkbeck College, London University; more recently, he obtained an MBA from the University of Warwick (UK). It has long been known that many pharmaceutical solids expected to have a dramatic impact on the way that can exist in more than one crystalline form (1); these pharmaceutical compounds are analysed – as well as other different forms or polymorphs have the same chemical aspects of drug discovery and formulation. formula but different crystalline structures. This can lead to different physicochemical properties and may TERAHERTZ LIGHT also impact on the pharmaceutical properties of a Terahertz (THz=1012Hz) radiation represents the last compound. For example, different polymorphs may unexplored frontier of the radio wave and light have different rates of dissolution or bioavailability, spectrum. The so-called ‘’ encompasses and the stability of a drug may also be affected frequencies invisible to the naked eye in the range from (2). Polymorphism is very common among drug 100GHz (1011Hz) up to roughly 30THz (3x1012Hz). compounds and has been observed in 67% of steroids, The former limit lies just above the region, 40% of sulphonamides and 63% of barbiturates. whereas the latter limit is located adjacent to frequencies; the gap is a fusion between and The formation of different polymorphs can be controlled infrared light (Figure 1). Conventional microwave during crystallisation by parameters such as choice of sources do not work fast enough (at high enough solvent, cooling rate and the degree of super-saturation frequencies) to efficiently produce radiation in the gap, of the solution. Once in the desired crystalline form, whereas laser diode sources have been limited by thermal however, the polymorphic state may change over time or effects. However, recently, advances in ultra-fast pulsed as a result of pharmaceutical processing operations. The laser technology have led to the generation and detection investigation of crystallographic polymorphism of drugs is thus of paramount importance to the pharmaceutical Figure 1: Terahertz – where radio waves and light meet industry. Failure to identify the existence of polymorphs could lead to loss of patent protection (for example, Visible/infrared Radio the GlaxoSmithKline antidepressant, PaxilTM/SeroxalTM, lasers Mobile waves phones paroxetine), or a company could be penalised by the regulatory authorities if a different polymorph were found during the manufacture of a product.

Polymorphs can be investigated by a variety of techniques including X-ray diffraction, NMR spectroscopy and Raman spectroscopy. This article will focus on a new Terahertz ‘gap’ (1012Hz) technology, Terahertz pulsed spectroscopy (TPS), which is

Innovations in Pharmaceutical Technology 73 Figure 2: Schematic of Terahertz pulsed imaging (TPI) information on the chemical constituents or structural nature of the material. This is done simply by using a Pulsed red laser Generation Imaging mathematical (Fourier) transform of the Terahertz pulse in Beam X Splitter OAP Sample the time domain to produce a spectrum. The bandwidth or Ti:Sapphire Y range of frequencies covered by the Terahertz pulse can be Solid-state adjusted and optimised depending on the type of ultra-fast pump laser pulsed laser employed, and the used in generating and detecting the radiation. Detection

Balanced Photodiodes TeraView has applied its cutting-edge technology to the WP OAP Variable Semiconductor Optical Delay imaging and chemical analysis of a variety of objects. TPI Computer is non-ionising and less hazardous to use than X-Rays, Control and the power levels used are generally lower on average than the background encountered in of broad bandwidth Terahertz light for the first time. everyday life. A wide variety of common materials (living This dramatic advance was made possible by applying tissue, plastics, clothing, cardboard and semiconductors) new concepts in semiconductor physics to these are semi-transparent at Terahertz frequencies, and this – commercially available laser systems. together with the ability to supply compositional and other diagnostic data – opens up a vast number of Terahertz Pulsed Imaging (TPI) potential commercial applications. These include Terahertz waves have a frequency of over a trillion cycles per medical imaging and diagnosis, security screening, non- second, and can be generated by illuminating a suitably destructive testing and – of most relevance to the engineered semiconductor crystal with ultra-fast pulses of pharmaceutical industry – pharmaceutical analysis. visible light (Figure 2). This bombardment generates Terahertz pulses which are very fast – lasting only of the POLYMORPH RECOGNITION order of 100 femto-seconds (1 femto = 10-15) – and with large bandwidth (>10THz). When used to visualise internal The unique spectral imaging characteristics of combining structures, a TPI system works much like a radar system, Terahertz pulsed imaging (TPI) and Terahertz pulsed although TPI can use transmitted as well as reflected pulses. spectroscopy (TPS) have been used to investigate various A Terahertz pulse illuminates the target, and portions of the pharmaceutical applications – notably the characterisation pulse are reflected from the internal layers. The delay of the of polymorphic forms of drugs. Factors such as solubility, pulse as it passes through the object gives a precise measure stability and bioavailability can vary by up to five times for of the distance to the various surfaces inside the object. different polymorphs, and it is important for manufacturers So, by scanning the beam across the target (Figure 2), a to understand whether their drugs can exist as different complete three-dimensional picture can be built up of the polymorphs and, if so, whether there is interconversion internal structure. The coherent detection of these Terahertz between them. Take the case of the anti-ulcer drug, pulses is then achieved by illuminating a second crystal with ranitidine (Figure 3); the molecule can exist in two active the visible beam (Figure 2). forms, known as forms 1 and 2. The original product, GSK’s Zantac, contains form 2 of the drug, whereas the The electric field of the Terahertz field is measured as a Apotex product Apo-Ranitidine contains form 1. function of time with femto-second resolution yielding depth information, and this same waveform can then be In many cases, polymorphs can be recognised visually by mathematically transformed to produce an absorption their crystalline habit or their different colouration; spectrum for the sample. Because this spectral however, polymorphism can also be demonstrated using information is available for each pixel, TPI is a a variety of experimental techniques, ranging from spectroscopic imaging technique that can determine simple measurements to more sophisticated methods of chemical composition as well as structural features. analysis, including:

Terahertz Pulsed Spectroscopy (TPS) X-ray diffraction (3) Terahertz pulsed spectroscopy (TPS) can be used to obtain Infrared (IR) (4) the spectrum of an imaged object or a material, yielding Raman spectroscopy (5)

74 Innovations in Pharmaceutical Technology Nuclear magnetic resonance (NMR) or differences in the crystalline structure of the material, spectroscopy (6) giving rise to different crystal oscillation (or phonon Terahertz pulsed spectroscopy (TPS) (7, 8) vibrations). This ability to distinguish between solid-state forms has resulted in TPS being extended to investigate Single Crystal X-Ray Diffraction the active ingredient in finished tablets. Single crystal X-ray diffraction is still the benchmark method for determining the existence of a polymorph, as One clear advantage of the technique for analysing solid it gives the crystalline structure directly. However, it is dosage forms is that – in the majority of cases – little or severely limited when applied to drug formulations, no sample preparation is required, and spectra containing particularly when crystalline excipients can interfere with information about the active ingredient can be obtained active diffraction peaks. from an intact tablet or capsule. Although dosage forms frequently contain many different ingredients, the active Mid-Infrared or Raman Spectroscopy ingredient is usually sufficiently different chemically from The individual atomic bonds within a molecule generate commonly used excipients such that peaks belonging to a discrete vibrational mode which can be characterised by the active can be readily identified in a mixture of either mid-infrared or Raman spectroscopy. Different polymorphs in the finished tablet. crystalline structures can modify these vibrations and can thus be probed using these techniques. However, Terahertz data is complementary to Raman spectroscopy; as differences in peak positions or peak intensities are small the selection rules are different for the two techniques, TPS and sometimes hard to measure; this is because provides new data which adds to that obtained by Raman intramolecular vibrations are probed, rather than the spectroscopy. Terahertz spectroscopy provides information intermolecular (or phonon modes) that characterise a on both high-frequency (just below IR) and low-frequency crystalline structure. Raman spectroscopy can create an vibrational modes; the latter are difficult to assess with extra, serious problem in that the laser irradiation can Raman as a result of proximity to the visible excitation line. induce a phase change, or initiate photochemical Unlike Raman spectrosopy, excitation via powerful laser reactions. In samples being interrogated, this can be radiation is not required with Terahertz spectroscopy; thus, avoided by careful choice of experimental conditions. phase changes or photochemical reactions in polymorphs do not occur as they sometimes do with Raman. The Near-Infrared (NIR) Spectroscopy technique has a fast data acquisition rate; currently raw data With near-infrared (NIR) spectroscopy, the situation waveforms can be obtained in less than 100 microseconds. becomes more complex due to the fact that the spectra This opens up opportunities for incorporation of the obtained will consist of many combination and overtone bands of the fundamental vibrations observed Figure 3: Terahertz spectra (fingerprints) of the different crystallographic forms (polymorphs) of ranitidine hydrochloride: crystal form 1, apotex’s in the mid-infrared region, making analysis difficult. apo-ranitidine; and crystal form 2, GlaxoSmithKline’s ZantacTM Differences in the electronic environment for different crystalline states also allow the use of solid-state NMR; The crystalline form affects drug solubility and other aspects of drug performance. although the technique is inherently insensitive (9), it Form I Form II has been applied to finished drug products. 4 5 4 61cm-1 3 Optical or Electron Microscopy 2 3 1 0 Optical or electron microscopy can provide secondary or 53cm-1 supporting evidence for the existence of a polymorph, but 43cm-1 2 only spectroscopic techniques (including single crystal X- 39cm-1 32cm-1 ray diffraction) can determine the crystalline morphology. Absorption

1 Terahertz Pulsed Spectroscopy Terahertz pulsed spectroscopy can be used to characterise 0 and quantify various solid-state forms of bulk active pharmaceutical ingredients (APIs) including polymorphs. 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Differences in spectra arise because of differing Frequency (THz) Frequency (THz) intermolecular interactions or molecular conformations,

Innovations in Pharmaceutical Technology 75 technology into a high-throughput polymorph screening is currently being investigated. Additionally, TPI is very system. As most plastic and polyethylene packaging is sensitive to DNA hybridisation and other interactions, and transparent in the Terahertz range, direct inspection of could be used for research in this field. packaged materials for QA/QC purposes or extended shelf- life applications should be possible. FUTURE DEVELOPMENTS

Polymorphic Impurities Over the past year, TPS has been shown to be a The US Food and Drug Administration (FDA) Guidelines technique that can provide information about the indicate that, if technically possible and in cases where drug physical state of a pharmaceutical active ingredient product performance testing doesn’t provide adequate during the processing and storage of a tablet indication of a change in the ratio of polymorphs, the formulation. Moreover, there is scope for further polymorph form should be monitored during stability development of the technique for these and other testing of a drug product. TPS shows promise as a applications. Advanced data processing methods, such as technique which could be used to address this challenge. chemometrics, will facilitate a greater understanding of solid dose formulations. It has been shown that it is possible to detect polymorphic impurities in tablets using a model system. Tablets In conclusion, it has been shown that TPS is capable of containing a small amount of polymorph 2 as an impurity discriminating between polymorphs in finished in polymorph 1 were prepared and analysed; it was found pharmaceutical products and that it can be used to that it was possible to detect the polymorphic impurity at determine the level of an active pharmaceutical substance a 10% level (relative to the other polymorph). The actual within a tablet. Several large pharmaceutical companies limit of detection for polymorphic impurities is dependent are now exploring the potential of the technique, and it on the method of analysis (bulk or microscopic sampling), may also have possible application in the FDA process the intensity of the Terahertz absorption signal from the analytical technology (PAT) initiative. active ingredient, the concentration of both the drug and the polymorphic impurity, and the magnitude of the The author can be contacted at spectral differences between the two polymorphs. [email protected]

Changes During Processing References Pharmaceutical companies characterise the bulk drug form of an API, but changes in solid-state form may 1. Haleblian JK (1975). J Pharm Scien, 64(8), occur during processing. Operations that involve energy 1269-1288. input, solvents or pressure are most likely to effect a 2. Byrn S, Pfeiffer R, Ganey M, et al. (1995). transformation. As TPS is a high-throughput technique Pharmaceut Res, 12 (7), 945-954. which has been shown to be sensitive to the crystalline 3. Suryanarayanan R and Herman CS, (1991). I J structure of a drug, it is envisaged that it will have Pharmaceutics, 77, 287-295. application in pharmaceutical manufacturing processes. 4. Ryan JA, Compton SV, Brooks MA and Compton DAC (1991). J Pharm Biomed Analy, 9, 303-310. Changes During Stability/Storage 5. Taylor LS and Langkilde FW, (2000). J Pharm Sci, During stability testing programmes, tablets are often 89, 1342-1353. exposed to challenging temperature and humidity 6. Saindon PJ, Cauchon NS, Sutton PA, et al. (1993). regimes. It is necessary to monitor the drug substance in Pharmaceutical Research, 10, 197-203. the formulation during stability testing, particularly if 7. Taday PF, Bradley IV, Arnone DD, and Pepper there are indications of a change in drug product M (2003). J Pharm Sci, 92, 831-838. performance from either dissolution testing or chemical 8. Strachan CJ, Rades T, Newnham DA, et al. (2004). stability measurements. TPS is expected to have Using terahertz pulsed spectroscopy to study application in this area. crystallinity of pharmaceutical materials. Chemical Physics Letters, 390, 20-24. Other Pharmaceutical Applications 9. Bugay DE (2001). Characterization of the solid-state: The use of TPI in proteomics and drug discovery to spectroscopic techniques. Advn Drug Del Rev, determine protein 3D structure, folding and characterisation 48, 43-65.

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