New Zealand doctors writing in the British Lancet medical journal say unpublished data from trials of the breast cancer drug Herceptin show that it may be up to a third less effective than has been claimed.
The doctors, from the Government's Pharmaceutical Management Agency (Pharmac), said in the journal that they may have identified a new case of publication bias - in which drug companies publish only trial results that are favourable, the Guardian newspaper reported.
The missing data is important and related to a trial involving nearly 1000 women, wrote Dr Scott Metcalfe and his colleagues at Pharmac.
In the trial, the North Central Cancer Treatment Group (NCCTG), one group of women with early breast cancer were given Herceptin at the same time as chemotherapy drugs called taxanes. Another group were given Herceptin after they had finished chemotherapy.
The results from those given Herceptin and taxanes concurrently were good - they cut the number of breast cancers that recurred by a third - and have been published, said the New Zealand researchers.
But results from the women who were given taxanes followed by Herceptin - the method used in Britain - have not been published in a peer-reviewed journal.
Experts were aware of both results, as they featured in an oral presentation at a cancer meeting in 2005 and on a slide presentation on a website.
What they showed, said the Pharmac doctors, was that only a comparatively small number of cancers were prevented from coming back and, statistically, there was a reasonable chance that Herceptin did not protect the women at all.
"The selective release of data from the NCCTG study has far-reaching implications for women with 'Herceptin-susceptible' early breast cancer," the doctors said.
"Without these data . . . (Herceptin) seems more effective than it probably is," wrote Dr Metcalfe. With the missing data the trial "shows a treatment effect one-third less than initially estimated".
Another big Herceptin trial, called Hera, looked at the use of the drug following chemotherapy instead of at the same time and had very positive results.
In New Zealand, Pharmac advises doctors to give it at the same time as a taxane.
Roche, the maker of Herceptin, said the NCCTG trial was sponsored by the National Cancer Institute in the US. There was a pre-agreed trial protocol and an independent monitoring committee oversaw the results.
It was up to the investigators to publish the data, said Roche, adding: "Four large trials involving more than 13,000 patients have consistently shown that Herceptin extends survival when administered either sequentially or concurrently with adjuvant chemotherapy."
Pharmac this month began a court-ordered consultation on whether it should begin funding 12-month courses of the breast cancer drug Herceptin.
In July 2006 Pharmac decided to back the nine-week course of the drug, used to combat the aggressive HER2 positive form of breast cancer, at an estimated cost to District Health Boards of about $6 million a year.
To fund the 12-month course would cost about $25 million.
Eight breast cancer patients, promoting themselves as the "Herceptin Heroines", took Pharmac to court to challenge the decision.
The High Court in Wellington earlier this year found insufficient consultation had been done on the possible benefits of the longer course and instructed Pharmac to start over.
Pharmac chief executive Matthew Brougham said the agency stood by its original decision but was aware Herceptin was a relatively new drug and new information had emerged since then.
"To help people make meaningful responses, and to be transparent about our current thinking, it is important we make it clear in consultation what our proposal is - the proposal is for funding to be declined," Mr Brougham said.
Pharmac said that any relevant data presented at a major oncology conference in the United States at the end of May will also be considered, and that its board will make a decision on the new consultation as early as June.