Early results bode well for Psivida's biosilicon

The first four patients with inoperable liver cancer involved in a Phase IIa clinical trial of an in situ radiotherapy treatment developed by Perth nano-biotech company Psivida (ASX:PSD) have experienced regressions of up to 60 per cent in their tumours.

Although the small trial at Singapore General Hospital involves only eight patients, and seeks only to establish that pSivida's patented BrachySil therapy is safe and well-tolerated, CEO Gavin Rezos said today the company knew the treatment would be safe, and the evidence for a significant therapeutic benefit had "exceeded all expectations",

Oncologists at the hospital used a fine-gauge needle to deliver a single dose of BrachySil, consisting of 20-micron particles of porous silicon doped with the phosphorus radioisotope 32P.

The procedure requires only a local anaesthetic, and patients can go home the next day. It places a localised 'charge' of the short-lived radioisotope directly into the tumour. 32P has a half life of only 14 days, and decays by emitting beta particles, which have an effective cell-killing range of only 8mm.

Rezos said the pronounced regression of the patients' tumours was "very good news", given that it involved only a single, very localised injection of Brachysil that could irradiate only a small volume of the tumours.

He said pSivida is developing a multi-injector that will enable surgeons to place three or four doses of BrachySil strategically around a tumour from a single entry point in the skin, to irradiate most of the tumour's volume.

The Singapore Hospital surgeons conducted the trial in collaboration with Psivida's Singaporean subsidiary, Psioncology.

"The [Singapore] result is very important," Rezos said. "It means we can expand BrachySil therapy to other tumours, including tumours of the breast, bladder - or virtually any solid tumour," he said.

"It's very cheap to manufacture - we don't need to make it ourselves. We can source the P-doped silicon from almost anywhere in the world, because it's widely used in the semiconductor industry."

The company's press release said the current global market for brachytherapy - the use of radionuclides to irradiate tumours from within - is estimated at around US$1 billion.

Current commercial brachytherapy products like Sirtex's SIR-Spheres, which are delivered by injection into the hepatic artery, are restricted to this application because of the risk of leakage of the radionuclide to other areas of the body. In BrachySil therapy, the radionuclide remains at the site of the injection, because it is an integral component of the crystal structure of BioSilicon.

Rezos said BrachySil can also be used for localised chemotherapy, by charging its porous microstructure with cancer-killing drugs.

Rezos said Psivida will now proceed with a study to determine the optimum dosage of BrachySil; it expects to begin marketing BrachySil worldwide in 2007.