Nuclear bombs to cure cancer

Delft Outlook, 2010.1

he first clinical trial of radiation therapy using minute radioactive microspheres has recently been launched in Utrecht. Once again, TU Delft researchers are thinking one step ahead.

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he future scenario goes something like this. Imagine that your doctor suspects you have cancer, yet is uncertain about the tumour’s exact location and whether or not it has metastasised. An injection of microspheres and specific protein fragments could quickly clarify the situation. Owing to the nature of these microspheres, they are clearly visible by magnetic resonance imaging (MRI), while the protein fragments ensure that the microspheres only bind to cancerous cells. In this way, any metastases become visible almost immediately after the injection. More importantly, treatment can start immediately, because these same microspheres (which are made of the metal holmium) can be made radioactive by placing them for several hours in a neutron beam emitted by a nuclear reactor. A swarm of radioactive nanoparticles is then injected into the bloodstream, where, like guided missiles, the particles search for tumour cells. Once the particles locate their targets, it is only a matter of time before the holmium microsphere unleashes its deadly beta radiation in the surrounding tissue. This is, in effect, nuclear warfare combined with precision bombing. Two such injections will cause tumours to shrink. Is this pure science fiction? Yes, of course it is. Yet Utrecht University’s Faculty of Veterinary Medicine has recently demonstrated technology so amazing that it verges on science fiction. In his laboratory at the university, Professor Jolle Kirpensteijn treats tumours in cats and dogs with a syringe full of radioactive holmium microspheres, developed at the Radiology & Nuclear Medicine department of the University Medical Center Utrecht (UMCU). The microspheres are made radioactive at the Reactor Institute Delft (RID). The holmium microspheres are injected directly into the tumour. Writing about a cat named Lucky (what’s in a name?), Prof. Kirpensteijn states: “We have achieved a CR (complete remission), something which had previously been considered impossible. There is currently no clinical evidence of tumours, and a swab that was made several weeks ago revealed no trace of any remaining tumour cells.” As befits a good scientist, he qualifies his findings as follows: “N = 1 [just the one patient, ed.], so perhaps it is a good idea to publish an article in the Journal of Veterinary Medicine, in order to attract more patients with oral tumours?” In other words, people whose pets have been diagnosed with an oral tumour would be well advised to take them to Utrecht.
Radioactive holmium microspheres were first used to treat a human patient in a clinical trial late last year. Researchers from Delft and Utrecht regard this clinical trial as a first step towards a promising form of internal radiation therapy. “It’s a very exciting time for us,” says medical biologist Dr Frank Nijsen (UMCU). “We’ve been working on this project for the past fifteen years, and we’ve worked out all the details.”
The current clinical trial is focused on the treatment of liver cancer, and there is a good reason for this: 80 percent of patients with liver tumours cannot be treated using available methods. This new and experimental radiotherapy involves bombarding liver tumours with radioactive holmium microspheres. At just 30 micrometers in diameter, 30 of these microspheres placed side-by-side would span a distance of less than 1 millimetre.

Three-in-one
“Holmium is a really great element,” says Professor Bert Wolterbeek (Applied Sciences). He walks over to a poster of the periodic table on the wall of his office and points a well-aimed finger at a black square marked ‘Ho’. “The non-irradiated material is 100 percent holmium-165,” he explains, “but if you bombard that with neutrons, a large proportion of the material is converted into the radioactive isotope holmium-166.” This form of holmium disintegrates when subjected to emissions of electrons (beta radiation) and gamma rays, and after just one day the radioactivity is halved.
Another advantage of holmium is that it ‘lights up’ beautifully on MRI images. Prof. Wolterbeek is very enthusiastic about it: “It’s a case of ‘three-inone’: you can see the particles flowing through the bloodstream in the MRI; the gamma radiation pinpoints sites of intense activity; and the beta radiation ills any tumour tissue in the immediate vicinity.” At 30 micrometers in diameter, the holmium microspheres are exactly the right size to become trapped in the liver, where they deliver their radiation from a blockage, or embolism. After being injected into the main artery carrying blood to the liver, the microspheres flow down ever narrower arteries, before eventually getting stuck and delivering a dose of deadly radiation to the tumour. Tumours are very good at rapidly creating a large network of blood vessels. With holmium therapy, however, this ability becomes their Achilles’ heel. The extensive blood supply carries most of the radioactivity deep into the tumour. The diameter is critical. If the microspheres are too large, they will not penetrate into the tumour. If they are too small, they will pass right through the tumour and pose a hazard to other tissues. In addition to holmium, the microspheres also contain poly(lactic acid); however, this does make them vulnerable to radiation. Prof. Wolterbeek says that the reactor institute has extended the exposure time to six hours, while reducing the intensity of the neutron beam and the gamma radiation, in an attempt to minimise any potential damage to the poly(lactic acid). The current work is part of a Phase I clinical trial, in which the safety of the treatment itself must be demonstrated. In this context, it is expected that twenty patients will be treated in Utrecht. During the next stage, which the researchers say could easily take two to three years, the efficacy of holmium therapy must be demonstrated in a group of around eighty patients. In the ‘Chemistry Building’ on the Julianalaan in Delft, a group headed by Dr Kristina Djanashvili is attempting to transform Nijsen and Wolterbeek’s ‘holmium bomb’ into a guided missile. Although this research is still at an early stage, the preliminary work has been completed. The plan is to create radioactive nanoparticles which, after being injected into the body, attach themselves to cancer cells and irradiate the tumour from within. In a room opening off one of the long corridors, Dr Djanashvili, Dr Joop Peters (recently retired, but of whom everyone says ‘we simply can’t manage without him’), and PhD student, Florian Maier, are seated around a table. On a sheet of paper, Djanashvili draws a picture that vaguely resembles a ‘Smartie’ (a type of sweet), but is actually a nanosphere encased in a couple of layers. This is what the group is working on: nano-sized holmium spheres that are resistant to radiation and capable of finding their way independently through the body. The drawing is of a spherule about 100 nanometres in diameter. Thirty of these would not be enough to span a distance of 1 millimetre; for that you would need 10,000 of them. These objects are so small that they can pass unhindered through even the narrowest of capillaries. The holmium particle itself is only 70 nanometres in diameter – the optimum size for an MRI signal. The particle is encased in a layer of silicon about 15 nanometres thick. While this may sound fairly straightforward, preparing holmium particles of exactly the right size from a solution is a complex process, according to Maier. The group has moreover opted to encase these particles in silicon, as this is more resistant to radiation than the poly(lactic acid) used for the microspheres. On the outside of the Smartie, perpendicular to its surface, Dr Djanashvili then draws some sticks that represent polyethylene glycol molecules and act as a ‘cloak of invisibility’, hiding the particles from the immune system. This enables the nanospheres to drift along in the bloodstream, virtually unnoticed, until they reach their destination. Other molecules on the surface of the nanosphere must ensure that it binds to a tumour cell. “Tumour cells have specific receptors on their cell membrane,” says Dr Djanashvili. “We use peptides [protein fragments, ed.] that bind to these specific receptors on tumour cells.” The researchers hope that this binding process will allow the radioactive nanospheres in the bloodstream to independently attach themselves to tumours, thereby targeting their deadly radiation to best effect. “We want to keep the chemistry as simple as possible,” Dr Peters adds. “Once the holmium nanospheres have been irradiated, we ‘bolt on’ the appropriate components and then the solution is ready for injection.”

Hurdles
There are however still quite a few hurdles to overcome before the researchers arrive at that stage. Dr Frank Nijsen (UMCU) still has his doubts about the specificity with which these microspheres bind to tumour cells: “Ever since monoclonal antibodies were first developed, about thirty years ago now, researchers have dreamt of designing proteins or antibodies capable of discriminating between normal cells and tumour cells.” Dr Nijsen agrees that the science is indeed improving, but tumour cells and body cells are very much alike, so the distinction is never 100 percent. He estimates that only five to ten percent of the injected antibody-linked, radioactive material actually reaches its target, while 60 to 80 percent will be removed by the liver, and the remainder will go on to damage healthy tissue. Dr Nijsen points out that antibody therapies often proceed no further than Phase I clinical trials, as they are either too toxic or induce an immune response in the body.
This is not the only approach to internal radiation therapy, however, and work is already progressing in other related areas. One example cited by Dr Djanashvili involved the use of holmium particles of a suitable diameter, yet without a peptide layer for specific binding. These particles are normally retained within blood vessels; however, the blood vessels in tumours are often very leaky, which allows the particles to penetrate into the surrounding tissue where they can do their work.  Dr Nijsen, in turn, is extremely interested in the work being carried out by his veterinary colleague, Prof. Kirpensteijn, and he is eager to repeat the success of the professor’s single-shot therapy in humans. If microspheres could be developed with just the right diameter and level of radioactivity, then internal radiotherapy in humans should also be feasible. Encouraged by that early success, researchers are currently attempting to find the most effective form for what might eventually be known as ‘Holmium Therapy’.