Patient demands for new therapy are understandable but Pharmac should be left well alone to get best deals on drugs.

Two petitions are circulating for the funding of the new immunotherapy drug against melanoma, Keytruda. Both were initiated by women suffering from this form of cancer and the petitions will be hard for the Government to refuse.

Indeed, it is a wonder the Cabinet has not already caved in to the pressure since Pharmac assessed it as a "low priority" for its budget. Judith Collins, before her return to the Cabinet was announced this week, had hinted the Government would provide additional funds for it, but so far Health Minister Jonathan Coleman has said little. As a doctor he probably respects Pharmac's decision.

So does the Green Party's health spokesman, Kevin Hague, a former district health board official who has been critical of Labour's hasty promise to fund the drug if elected. What is the point of taxpayers maintaining a professional medical assessment panel if politicians prefer to make these decisions? We need to trust Pharmac unless there is something wrong with its system of evaluating new medicines. Our health reporter Martin Johnston has provided a useful outline of its system in a feature today.

Contrary to widespread belief, it appears Pharmac does not have a formula such as the "quality adjusted life year" (QALY) used in Britain. That system enables the benefit of a drug to be measured by the extra years of life it offers, adjusted for the level of health the patient has for those additional years, and the cost of the drug can be divided by the QALY to see how it compares to all other medicines that may be provided from public funds. Britain has decided to fund Keytruda, as has Australia and others.

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Pharmac's system is not simply a cost-benefit evaluation, says its chief executive, Steffan Crausaz. He explains that it uses nine criteria such as "the health needs of all eligible people in New Zealand" and "such other criteria as Pharmac thinks fit". That probably includes discretion to reject a new drug if the price demanded by the manufacturer seems out of all proportion to the costs of its discovery, development and clinical trials. While those costs can be high, and it is reasonable for manufacturers to use their successful drugs to also cover the costs of their unsuccessful research, price gouging is too easy for some of their products.

It is certainly possible for Keytruda. Clinicians say it is the only treatment that can be effective against melanoma that has advanced to internal organs. Chemotherapy and other routine treatments are not. Most people diagnosed with the cancer would pay any price for Keytruda if they had the money and since they do not have it, it is perfectly understandable they might want the taxpayer to pay any price for it. The asking price ranges from $122,000 to $163,000 a year, depending on the patient's weight.

Pharmac seems to be good at bargaining these prices down and its announced assessment of Keytruda may give it more leverage. But our report today shows this drug is not a proven cure; it has so far reduced the tumours of 45 per cent of patients by more than 30 per cent, and another 35 per cent have had their tumours shrink by less than 30 per cent. Perhaps it is early days. The hope is exciting. But politicians and the public should trust Pharmac to decide.