Ultrasound mediated drug delivery: A 21st century phoenix?

Therapeutic ultrasound is now rapidly rising, phoenix-like, from the ashes, or rather from a long period of lack of interest. The timing of this is not altogether clear, but most probably it is due to the rapid growth in interest and sophistication of imaging and molecular biology techniques. It is not widely recognised that the application of ultrasound energy to the human body was initially with therapeutic intent. Szent-Györgi, publishing in Nature in 1933 [1], reported that ultrasound had no effect on Ehrlich's carcinoma, but the first publication on the successful application of therapy ultrasound in human tumours was from Horvath (1944) who reported the treatment of skin metastases [2]. It was not until 1950 that John Wild [3] first reported the use of sonar pulses to look at structures within the body, thus opening up the field of medical diagnostic ultrasound, with Howry and Donaldson publishing their pioneering work later that decade [4–7].

Ultrasonic energy is attractive for therapeutic usage. An ultrasound wave incident on tissue is scattered by the tissue structures, thus allowing its use as an imaging modality. Between 60% and 80% of the incident energy (depending on the tissue type) remains within the tissue and is absorbed, leading to tissue heating, which may be harnessed for therapeutic benefit. In addition, the rarefaction portion of the ultrasonic pressure wave is capable of drawing gas out of solution, giving rise to the phenomenon of acoustic cavitation. Controlled cavitation, whether using native or introduced bubbles, has been shown to produce effects on the extracellular membrane that facilitate drug and gene delivery. By careful choice of ultrasound frequency and acoustic pressure amplitude, the relative contributions of thermal and mechanical interaction mechanisms can be varied. This can be used to good therapeutic effect. The millimetric wavelengths of ultrasound in tissue allow its tight focusing deep within the body, a property that has been used to good effect in high intensity focused ultrasound (HIFU), and is now being employed in ultrasound mediated drug delivery. The non-ionising nature of ultrasound means that there are no tolerance considerations, and repeated treatments are possible. The dual imaging/therapy nature of medical ultrasound presents the possibility of designing ‘seek and destroy’ ultrasound techniques in which the target is identified using B-mode or Doppler ultrasound, and then exposed to a high energy therapy field.

Table 1 lists published applications of therapeutic ultrasound, and the year of their first publication (taken from a Scopus search using the terms ‘ultrasound’ and ‘therapy’). It can be seen that a wide variety of sites and conditions have been exposed, with only those in bold still being in use today, albeit rarely using the original exposure conditions.

Table I.  Applications of therapeutic ultrasound listed under their first year of reporting (excluding HIFU). Applications in bold are still under active consideration.

1947	Sarcoma, neuritis
1950	Arthrosis, arthritis, ischialgia, neuralgia, thrombosis & embolism
1951	Skin disease, wound healing, rheumatism, asthma, painful joints, ophthalmic disorders, phantom pain – amputation
1952	Pleural effusion, frostbite, bursitis, dentistry, prostatic hypertrophy, drug delivery, peripheral vascular disease, scleroderma, haemorrhoids
1953	Neuroblastoma
1954	Ménière's disease
1955	Carcinoma
1956	Ulcers, uncontrollable hiccup, urological stones, elephantiasis
1957	Tinnitis, haematoma, drug delivery across blood–brain barrier
1958	Verrucas
1959	Fractures
1960	Pressure sores, phonophoresis, pain relief
1961	Keloid scars
1962	Dupuytren's contracture
1963	Lumbago, silicosis
1967	Carpal tunnel syndrome, whiplash
1968	Chilblains
1969	Trismus, frozen shoulder
1971	Tonsilitis
1972	Pancreatitis
1974	Allergic rhinitis
1996	Gene transfection

Tissue ablation using high intensity focused beams is becoming clinically widespread [8–16]. The most recent clinical applications hark back to the original intent of therapeutic ultrasound research in the areas of neurology and ophthalmology [17], [18]. HIFU employs intensities of up to 35  kW cm⁻², with associated pressure amplitudes of ∼10 MPa. Despite there being little evidence for the efficacy of physiotherapy ultrasound, it continues to be in common clinical use, especially for pain relief and sports injuries [19–22].

In the field of oncology in particular, there is interest in the development of localised therapies. These have the advantage of reducing unwanted side effects, and may allow increased drug uptake. Ultrasound echoes the so-called ‘magic bullet’ approach of Paul Ehrlich [23]. He postulated that if it were possible to develop a compound that could selectively target a disease-causing organism, then it should be possible to attach a toxin to this compound, and thus only attack the organism of interest. For the ultrasound ‘magic bullet’ the vehicle carrying the drug must survive in the circulation until it reaches its target, be able to release its payload when exposed to ultrasound, and, preferably, play a part in promoting the uptake of the therapeutic agent locally.

The area of therapy ultrasound research that is showing the most growth is that which explores the potential of ultrasound to enhance drug delivery by thermal or mechanical means. There is now considerable interest in combining ultrasound exposure with microbubbles that act as the vehicle for the localised delivery of drug, genes, or other agents, and this is the major topic of this special issue. This form of therapeutic ultrasound uses intensities up to ∼10 Wcm⁻².

The simplest technique of this nature is to introduce ultrasound contrast agents (microbubbles) and the agent of interest simultaneously. This is done when blood clots are exposed to ultrasound in the presence of microbubbles and tissue plasminogen activator (tPa) or urokinase. This has been shown to accelerate clot dissolution, and is the basis for sonothrombolysis for the treatment of stroke [24–26]. In this application, it is most probably the action of the microbubble facilitated cavitation on the clot surface that increases drug access to the clot interior.

Ultrasonically driven microbubbles have been shown to produce transient pores on the cell surface through which molecules may pass. This has exciting potential applications in the brain where it has been shown that the blood–brain barrier may be opened temporarily [18], [27], [28].

Ultrasound mediated enhancement of the delivery of drug or gene therapies has been studied in a number of in vitro and in vivo experimental models [29]. Each model has associated advantages and disadvantages. In vitro systems allow isolation of different mechanisms of action and good ultrasound dosimetry, but have little clinical relevance. It is difficult to quantify ultrasound exposures in vivo and to identify the individual elements responsible for the effects seen.

There has been considerable interest in gene therapy for a number of conditions. Transfection using naked DNA on its own has low efficiency, and so new vectors are being sought. Both viral and non-viral vectors have been considered. The first report that ultrasound may increase transfection efficiency came from Kim et al. [30]. This important research field is the subject of three papers in this issue [31–33]. While it has been suggested that the presence of microbubbles is a pre-requisite for ultrasound mediated improvement in transfection [34], Liu et al. (2012) report such an increase in their absence [33].

Modern technology allows the modification of the microbubble shell to therapeutic advantage. Owen et al. [35] describe the use of magnetic microbubbles capable of carrying a drug payload that can be drawn to the target volume using an external magnetic field, and be disrupted using a focused ultrasound beam. An alternative approach is to attach specific ligands to the bubble surface [36].

Ultrasound-mediated enhancement of drug delivery in the presence of microbubbles relies predominantly on harnessing acoustic cavitation events to produce transient effects in the cell membrane. Ultrasound is also capable of producing thermal effects. It is therefore a useful approach to use thermally sensitive carriers for drug delivery. A number of different temperature-sensitive drug delivery systems have been studied. Rapoport here describes the use of polymeric micelles the vehicles for drug delivery [37]. Thermally sensitive liposomes also show considerable promise. Doxorubicin loaded liposomes that are destroyed at temperatures around 40°C are being studied extensively, and are now entering clinical trial [28], [39]. Yudina & Moonen [41] have exposed thermally sensitive liposomes in the presence of microbubbles, in order to capitalise on both the thermal and cavitational mechanisms induced by ultrasound.

The papers in this special issue have been chosen to give a representative flavour of the current state of the art in ultrasound mediated drug and gene delivery. The coverage is inevitably not completely inclusive, but gives a snapshot of activity at this time. Therapeutic ultrasound techniques are still under development, and look extremely promising, but in order to determine their clinical utility widespread large-scale clinical trials are urgently needed.