Graham Ruecroft PhD, of Oxford-based Prosonix, explains the benefits that ultrasonics can bring to drug formulators in terms of improved yields and reaction times
The Titanic sank in 1912 and one of the many consequences of this terrible disaster was a competition, prompted in the same year, to discover and develop non-visual methods for detection of icebergs. This led to the echo sounding technique developed by Langevin that became known as SONAR (SOund Navigation And Ranging).
Continuing this nautical theme: Lord Rayleigh, in 1917, discovered that severe erosion of ships" propellers at a fast spinning rate was a result of localised high pressure bubble collapse.
Two interesting facts, you may say, but importantly these events paved the way over the ensuing decades for the emergence of power ultrasound as a tool in many areas of science and processing. In fact, the most marked event in the history of commercial ultrasonics came in 1918, when Langevin discovered that the piezoelectric effect1 was spectacularly enhanced when the quartz was sandwiched between two steel shims (the Langevin triplet). Today, magneto-restrictive2 and piezoelectric transducers are the underpins of modern ultrasonic processing equipment.
Many of us are familiar with the use of ultrasound in diverse fields such as medical imaging and diagnostics, biological cell-disruption, non-destructive testing of materials, thermoplastic welding, food processing and SONAR, to name but a few. Large-scale equipment is now available for the application of high intensity power ultrasound to wastewater treatment (in particular anaerobic digestion) and to a range of activities in the food processing sector, (such as emulsification, microbial fermentation, cell disintegration and homogenising).
The application of power ultrasound (20 - 100kHz; but it can be as high as 2MHz) to crystallisation and chemical processing is an intensification technology that has undergone serious development over the past 15 years or so, and has a rosy future ahead. After all, it has been known for 80 years that ultrasound can have interesting effects on chemical and biochemical systems as well as influencing events in crystallisation, but it took the chemical and pharmaceutical communities another 40-50 years to see the benefits it might bring to their industry.
Maybe the absence of scale-up equipment led to that intransigence. However, recent advances in equipment have made its implementation at industrial scale feasible. Here at Prosonix, we have seen a lot of interest in the application of ultrasound to crystallisation in the pharmaceutical and fine chemicals sectors of industry and have used our experience in crystallisation and ultrasonic engineering to design and build ultrasonic processing equipment to allow production of pharmaceutical ingredients with a desirable morphology and particle size distribution.
Our equipment allows distribution of acoustic energy into a liquid very effectively by using a number of low-power transducers (now 21 in a 5L flow-cell) bonded to the outside of a cylindrical duct.3 This avoids the problems of using high-powered probe-based equipment where metal particles can be shed into the crystallising liquor. Typical equipment for batch production of pharmaceuticals is shown in Figure 1 but perhaps the ultimate in sonocrystallisation is the rapidly developing technology that involves atomisation and ultrasound in the production of micro- and nano-crystalline particles for drug delivery (SAX).
By applying power ultrasound to a liquid medium, highly excited acoustic bubbles can be formed similar to those observed by Rayleigh and with the same potent properties when sufficient acoustic power is used (low power leads to more stable cavitation). This phenomenon is commonly referred to as cavitation. As with all sound waves, there are expansion and contraction pressure variances in the sine wave (see Figure 2). When power ultrasound is applied to a liquid medium the expanding pressure wave can literally tear the solution apart where there are structural defects (foreign matter, particles, dust, dissolved gas) under the negative pressure exerted to create cavities. Conversely on the compression phase these cavities or bubbles collapse violently to generate regions of extreme excitation, temperature (5000K) and pressure (2000 atm) and, as you might expect, release of shockwaves. Think of them as transient high-energy microreactors.
Many areas of process and synthetic chemistry can benefit from the use of power ultrasound leading to improved yield and rate: metal activation in heterogeneous chemistry and catalysis and improved liquid-liquid or solid-liquid mixing in heterogeneous systems, while useful and in fact the main area of interest, are cases of false sonochemistry.4 True sonochemistry (and sonobiochemistry) can be represented, for example, by homogeneous and heterogeneous electron transfer/radical processes.
Sonocrystallisation utilises power ultrasound and resultant acoustic cavitation to assist in nucleation of metastable solutions and subsequent crystal growth. The reasons why such local and transient energy concentrations assist with nucleation are difficult to explain but local dramatic temperature and pressure changes, shockwaves and rapid local cooling rates may all contribute to nucleation in the regions of the supersaturated solution or perhaps we can simply overcome the energy barriers associated with nucleation.
Cavitation appears to be particularly effective as a means of inducing nucleation in a controlled and reproducible way and this provides a well-defined start point for the crystallisation process. This allows focus on controlling the crystal growth via the residence time in the crystalliser. We have used this combined approach to influence crystal size distribution, assist in morphological control, eliminate impurities in the crystal and improve solid-liquid separation behaviour.
Sononucleation can also eliminate the requirement to add seed crystals, which can be particularly advantageous in contained pharmaceutical crystallisation processes. Reduced acoustic power levels can lead to streaming effects rather than stable or violent transient cavitation, which can be useful to help crystal growth.
In many respects, the ease or difficulty of carrying out a crystallisation process can be linked to an understanding of the metastable zone (MZ)5, as shown in Figure 3. Typically the application of high-intensity 20 kHz ultrasound can lead to narrowing of the metastable zone width (MZW) in much the same way as you would see by using seed crystals or when scratching the side of a glass flask containing a supersaturated solution a fairly typical "trick" in crystallisation but one that has been cautiously attributed to cavitation.
This MZW narrowing can range from a few degrees to 20 or more when crystallizing sugars. By narrowing the MZW it is possible to "tailor" a crystal size distribution between the extreme cases of a short burst of ultrasound to nucleate at lower levels of supersaturation and allow growth to large crystals, as well as the production of small crystals via continuous (or perhaps a longer single burst) insonation throughout the duration of the process. The optimum needs to be determined by experimental investigation.
It is possible that ultrasound may also induce secondary nucleation by mechanically disrupting crystals or loosely bound agglomerates that have already formed. The overall technique lends itself extremely well to almost any crystallisation process of valuable pharmaceuticals and proteins.
One particular small molecule new pharmaceutical we have worked extensively on has been shown to exhibit troublesome behaviour in terms of crystal habit. When high (labile) levels of supersaturation are reached in a standard cooling crystallisation, high nucleation rates, along with poorly controlled crystallisation, leads to the proliferation of a distinct needle habit (see figure 4), leading to poorly stirred slurries and variable product bulk density.
By using ultrasound at much lower levels of supersaturation highly desirable rhombic/plate type habit can be easily produced and managed.
Polymorphism is common among organic materials and given that small-molecule drugs can be flexible and exhibit significant internal freedom, it is no surprise that such entities can exist as two or more crystalline phases with different packing in the crystal lattice. Isolation of the "wrong" polymorph brings substantial problems in pharmaceutical applications, but through careful application of ultrasound to a crystallising system at the right level of supersaturation, the ground state polymorph (the most thermodynamically favoured and least soluble) or one near the ground state can be isolated.
Using sonocrystallisation to produce the desired particle size and distribution can be extremely valuable in avoiding secondary processing techniques such as micronisation. Why manufacture large crystals and then obliterate them with such a sledgehammer tool when you could produce smaller particles ready for tableting or better still microcrystalline inhalable therapeutics by using sonocrystallisation?
Drug delivery, globally, is worth some US$100bn. As part of this, drug delivery by inhalation is a significant market and one that is growing rapidly. A large proportion of new drug candidates today are water insoluble (but oil soluble) and are poorly bioavailable leading to abandoned development efforts. However, there is hope for such new chemical entities by formulating them into crystalline nanosuspensions: sub-micron colloidal dispersions of pure particles of drug substance stabilised by surfactants. Oral nanosuspensions have been specifically used to increase the rate, absorption and bioavailability of drug compounds, sometimes with a 15-fold increase in the highest plasma drug concentration.
Inhalation is the choice delivery method for a number of drugs used in therapeutic areas such as asthma and other respiratory disorders, lung tumours, diabetes and COPD to name but a few. For superior delivery the optimum particle size can be from 500 nm to 6 mm, although it is difficult to obtain 1-5 mm particle by conventional batch type crystallisation.
Typically, mesoscopic particles for drug inhalation are manufactured by pharmaceutical technologies that can at best be described as primitive. These include micronisation whereby large and perhaps regular crystals are broken by aggressive milling. The energetic process leads to irregular shaped 1 - 5 mm particles that can also undergo morphological change, perhaps leading to unwanted surface polymorph transformation and amorphicity. Stability and shelf life can also be severely affected, and the particles can be very highly charged, which undermines flow-rates and makes it harder to make aerosolised and dry powder inhalers.
We need to learn to engineer organic micro and nanocrystals and be able to control their properties with desired and consistent micro and macro structures. Importantly, we need to fully characterise their performance enhancing attributes (PEA) such as overall particle shape, morphology, aerodynamic properties and resistance to agglomeration, mindful that the therapeutic efficacy of suspension-based pressurised inhalers and dry powder inhalation formulations depend upon the physical properties of the particle being inhaled.
A number of processes utilise the dispersion properties of supercritical carbon dioxide for preparation of microcrystalline particles. Perhaps the best known is SEDS (Solution Enhanced Dispersion by Supercritical Fluids), which was developed by Bradford Particle Design (BPD now part of Nektar Therapeutics) in the 1990s. The technique allows simultaneous dispersion, solvent extraction and particle formation in a single step. In conjunction with Rob Price at the University of Bath we are developing an ambient temperature and pressure process using a combination of Solution Atomisation and sonoXtallisation (SAX) to deliver particles with optimum size and morphology that are suitable for formulation by nebulisation.
SAX (see figure 5) is used to reproducibly prepare spherical crystalline particles using a process involving: (i) formation of a drug substance solution; (ii) generation of an aerosol; (iii) collection of the aerosol droplets in a vessel containing non-solvent, and (iv) application of ultrasound to induce crystallisation.
The use of this technology allows us to produce regular spherical particles in a reproducible fashion and with superior aerodynamic properties (see figure 6). Changing the aerosol droplet volume, size, velocity, solvent, and nucleation conditions allow us to vary particle size and morphology indeed, careful control of parameters suggest we may see a "dial-a-particle" SAX technology in the near future.
Interestingly, the particles have a unique nanotopology that improves the absorption characteristics by virtue of the increased particle surface area for a given size and volume. Another big advantage is the suspension stability: we have found that a suspension of SAX particles resists settling compared with suspensions of particles obtained by other techniques including micronisation (Figure 7). This is incredibly important in manufacture and use of drug inhalers with respect to repeatable dose to the patient and reduced shelf life of the drug inhaler, leading to premature disposal when still filled with a drug substance.
Many groups in industry and academia have shown that the controlled delivery of power ultrasound assists in nucleation and crystallisation of small drug-like molecules.3 Perhaps the real strengths lie in the ability to: (i) generate seeds from mother liquor circumventing the need for external seeds in GMP operation; (ii) form the desired morphology, and (iii) produce microcrystalline particles.
We can look forward to seeing increased productivity, from pharmaceuticals to bulk inorganics, by preparing crystals with improved purity and physical properties. Sonocrystallisation can, and we believe will, become a core technology in the pharmaceutical industry and we can expect to see many more industrial applications in the near future.
With SAX we will see the emergence of a superior technology and secondary unit operation to supersede micronisation for the manufacture of microcrystalline drug substances, and indeed complex APIs and proteins. A rosy future awaits power ultrasound in particle science.