RNAi experts hope for a knock-out

For Dr John McKinley, CEO of Brisbane RNA-interference therapeutics developer Benitec (ASX:BLT), the power and promise of RNAi technology is captured in one word: druggability.

McKinley says it is now theoretically possible to inhibit each of the 30,000-odd genes in the human genome with RNAi molecules that will act with matchless precision, avoiding many of the unwanted side-effects associated with conventional drugs.

"We are certainly going to be able to inhibit previously undruggable targets, with far greater potency than comparable therapeutics based on antisense oligonucleotides, or ribozymes," he says.

The very nature of the technology -- the fact that it produces a specific and predictable effect on a target gene -- means the path to the clinic for RNAi therapeutics is likely to be considerably faster and cheaper than for conventional, small-molecule drugs, according to McKinley. Results from in vitro cell-culture experiments and in vivo trials in animal models are very promising, and there is a near-universal consensus among research leaders that RNAi therapeutics would revolutionise human medicine.

"Having said that, we should be cautious when making predictions, because the same things were being claimed for antisense technology 10 years ago," McKinley observes.

He says it has taken just two years for the pharmaceutical industry to adopt RNAi as its preferred gene-knockout tool for functional genomics research. Whether researchers employed the short interfering (si) or DNA-directed (dd) RNAi approach, both were regarded as superior to any previous technology -- ribozymes, antisense nucleotides, and transposon tagging -- for identifying gene targets for new drugs or therapeutics.

A measure of the pace at which the technology was advancing, was that the first RNAi therapeutic was likely to enter human clinical trials within a matter of weeks. US company Acuity has developed an experimental treatment for age-related macular degeneration, that uses a siRNA to inhibit the gene coding for vascular endothelial growth factor (VEGF).

"I believe the US Food and Drug Administration has just approved Acuity's Investigational New Drug (IND) application. If that's the case, we're already at the dawn of therapeutic proof of principle," McKinley says. "Three companies are developing siRNA treatments for macular degeneration, even though reports of the proposed trials say treatment will involve injections directly into the eye."

McKinley says Acuity's molecule acts as an angiogenesis inhibitor, blocking production of VEGF, which drives the growth of small blood vessels that, in aged people, can overgrow the light-sensing retina at the rear of the eye, resulting in loss of vision and, eventually, blindness. He says the same RNAi molecule's anti-angiogenesis activity also makes it a promising anti-cancer therapy.

Benitec will have its own experimental RNAi therapeutic for HIV-AIDS in clinical trials by the end of next year, McKinley says. The AIDS virus infects helper-inducer T-lymphocytes by binding its GP120 coat protein to the CD4 receptor on the host cell's surface, but also requires the CCR5 co-receptor to be present. A mutation found in Europeans and their descendants, which reaches its highest frequency in Scandinavia and Iceland, deletes the CCR5 co-receptor. Individuals who are homozygous for the mutation are immune to AIDS.

Benitec's RNAi therapeutic will mimic the protected, homozygous state, and McKinley says it will be delivered as a ddRNAi construct in genetically modified stem cells extracted from the infected individual's own bone marrow.

The construct will simultaneously target several genes essential to the virus' replication, minimising the chances of the virus exploiting its notorious capacity for mutation to evade the new therapy.

McKinley says Benitec also plans have its hepatitis C virus (HCV) in human clinical trials by early 2006. "We'll also be announcing a cancer target soon, which we hope to take it into clinical trials in 2005," he says. "We're one of the leaders in bringing RNAi therapeutics into trials, thanks to our associations with the City of Hope Cancer Centre and Stanford University. We're going as fast as we can to get our therapies into phase I trials, so that we can demonstrate proof of principle.

"When this happens, it will become easier to get IND approval of other RNAi therapeutics, and things will begin to happen even more rapidly."

McKinley admits that, in these early days of RNAi technology, it is not yet clear what level of expression must be achieved with ddRNAi or siRNA constructs to knock down the expression of target genes in tissues or organs. "We're getting very good correlations between expression and gene knockdown from our experiments, but for a cancer therapeutic, you can't miss a single messenger RNA molecule -- it has to be 100 per cent."

McKinley says that while Benitec's research is focused on DNA-directed RNAi (ddRNAi) therapeutics, it is also experimenting with siRNA constructs, and has several patents assigned and under filing. The company doesn't regard either technology as inherently superior -- Benitec is aiming to develop products covering both fields.

"We're looking at ddRNAi as something that will persist in the patient's system and continually knock down whatever we're trying to 'kill', whether it's a virus or a cancer or a metabolic disorder," he says.

He predicts ddRNAi will dominate in the field of anti-viral therapeutics, where Benitec's commercial activity is currently focused. "We have a major interest in viral pathogens like HIV and HCV, and we have a range of potential delivery mechanisms."

Delivering the goods

"There's still a lot to be done in the area of delivery -- there are issues involved in getting small molecules into cells or organs, in a way that doesn't induce an interferon response," McKinley says. "With siRNA you probably need higher doses. DdRNAi offers an easier route, because we have 15 years of gene-therapy research to rely on. We're effectively the payload for gene-therapy vectors -- it's our natural position, and we see the resulting therapeutics as the core of our future business.

"Viral gene-therapy vectors like lentiviruses, and adeno-associated viruses (AAVs) will have a big role to play in Benitec's future. We're looking at a lentivirus to deliver our HIV therapy, and an AAV for our hepatitis C therapy. The big question is: will RNAi therapeutics be accepted by the regulatory authorities, the medical profession and the community? We have an answer to the first question -- the FDA has already approved over 100 gene therapy trials.

"However, we're looking at other non-viral means of delivery such as packaging RNAi constructs in liposomes. There are lots of ideas for non-viral delivery systems - if we can maximise their use, there should be fewer problems with doctors and patients accepting the technology."