Phytopharm, the Cambridge drugs company, suffered a setback yesterday after US pharmaceuticals group Pfizer withdrew from a partnership deal to develop a new anti-obesity drug from a rare cactus.

The news sent Phytopharm's shares down by almost a third, even though Pfizer said its decision to pull out of the £13m licence deal was down to internal restructuring and the closure of its Nutraceutical group, and was not due to a failure of clinical trials.

The medicine, known as P57, was developed from the hoodia, a cactus-like plant eaten by Kalahari bushmen to stave off hunger during hunting trips.

Pfizer said clinical data encouraged further study of P57 as a therapy for obesity, but the company had decided that its development might be best achieved by another organisation. Phytopharm was free to license P57 to other parties.

Dr Richard Dixey, chief executive of Phytopharm, said despite the shares' fall - they closed down 85p at 175p - the announcement would have little impact on his company.

"It is cash neutral, and in the long run could be to our benefit. Pfizer have stressed the drug is worthy of further development and said we are free to re-license.

"With the global market for anti-obesity drugs estimated at $8bn a year, there is a lot of interest. I estimate we will have re-licensed within six months and, as we will be further forward in our research, it will be with a substantially better deal than the one we had with Pfizer."