Health Minister Jonathan Coleman is keeping decisions on drugs "at arm's length" with Pharmac and has conceded National's interference in 2008 over Herceptin was a mistake.

The makers of Keytruda have been forced to amend trial data given to doctors, after overstating the benefits of the melanoma drug.

The ground-breaking new cancer drug is at the centre of a major PR campaign, by both the drug company and terminal cancer patients, to gain public funding through Pharmac.

Made by Pharmaceutical company Merck Sharp and Dohme (MSD), Keytruda is the brand name for Pembrolizumab - a biologic drug for terminal melanoma patients, that has produced promising results in early clinical trials.



Keytruda branding was understood to be a major feature of a recent New Zealand Society of Oncologists conference, where slides showing some of the latest clinical trial data were presented.

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In a letter to conference attendees, Merck's oncology clinical director Colin Yeoman said the headline on one of the slides was misleading.



It was able to be interpreted that 100 per cent of patients that had received the drug as a first-line treatment had had a postive response.



"MSD was concerned that using the heading from the presentation... 'Duration of Response in First Line' could be misinterpreted as the response in all first line patients," said Yeoman in the letter.



"Our intent was therefore to clearly communicate that only the responding patients were represented in the [right-hand chart].



"Despite our best intent, having reviewed this and in the absence of published updated duration data for this patient cohort; the information on this slide does not support the 30 months claim. It does however, support that in responding first-line patients, the responses were durable."



That unsupported claim was that "most patients that responded to first-line treatment were still responding at 30 months".

According to the internationally-recognised standard, which defines the level of tumour shrinkage required to constitute a response to a drug, 45 per cent of clinical trial patients had responded to Keytruda.

Of those, 13.5 per cent had "no identifiable tumour mass left", according to Merck's own data.

The drug has entered the political arena, with the Labour Party vowing to override Pharmac's decision-making process and ordering the national drug purchaser to fund the drug.

Leader Andrew Little has said he did not believe the move would undermine Pharmac's negotiations with Merck, who would be guaranteed of a sale under a Labour Government.

But Health Minister Jonathan Coleman has conceded National got it wrong, when it made the same pledge for breast cancer drug Herceptin in 2008.

When National came into Government that year, it made the funding available for the Ministry of Health to fund it rather than Pharmac, though it helped with negotiations.

Pharmac's view then, was that a nine-week course of Herceptin was justified for those with HER2 breast cancer but it rejected pressure for a 52-week course.

A number of studies since have indicated nine weeks of treatment was as effective, with fewer risks of cardiac problems, while other studies have found similar outcomes for "overall survival" and "disease free survival" among those treated with Herceptin for 9-week and those given 52-week courses.