Several times in the past year Imutran has announced its hope of starting pig-to-human organ transplants within12 months. Fortunately, the date for human experiments is being constantly put back, because we are nowhere near ready for them, scientifically or ethically.

Imutran's latest announcement was of the results of removing kidneys from seven Cynomolgus monkeys and replacing them with genetically engineered pig kidneys. One monkey died at six days of ureteric obstruction, two died at eight days of acute vascular rejection, and the remaining four were killed at between 13 and 35 days because of progressive severe anaemia.

To announce that these results are extremely encouraging and presage "extended periods of survival", even if it reassured shareholders, was surely over-optimistic. There has so far been no publication in a peer-reviewed journal of the results of placing pig organs into monkeys.

The ethical dilemmas have yet to be discussed widely, although they were considered by the Nuffield Council on Bioethics and the US Institute of Medicine. Both were concerned about the possible transmission of infectious diseases by xenografts, and recommended that all recipients should consent to long-term follow-up, and that the first trials on humans should only involve adults capable of giving their consent freely.

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Neither suggested how successful animal studies needed to be before starting human trials. Yet that success, or lack of it, is an important factor in the risk/benefit assessment of human trials that research ethics committees must conduct, and potential recipients must be aware of if they are to give an informed consent. I believe that at least 50 per cent of pig to monkey xenografts, in a cohort of at least ten monkeys, should after two years be functioning well, before any ethics committee approve human trials.

However, commercial pressure is bound to look for earlier human trials. Salomon Brothers, the investment bank, has shown just how much is at stake for Sandoz, which owns Imutran and a patent, valid until 2012, on cyclosporin - the most effective drug to prevent organ rejection. The report suggests that Sandoz will sell each pig organ for about $12,000, and that sales of organs and cyclosporin could increase Sandoz's annual income by $7-8 billion by 2010.

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Richard Nicholson is editor of the Bulletin of Medical Ethics.