GlaxoSmithKline has already dipped deep into its coffers for an in-licensing spree this year, and has now added to its tally with a $1 billion deal to get its hands on a candidate skin cancer drug developed by Synta Pharmaceuticals.

The terms of the deal sees GSK pay $80 million upfront for the compound, STA-4783, which is just entering Phase III development as a treatment for malignant melanoma. The deal also stacks a lot of the potential payments at the front end – it includes almost a half billion dollars in development milestones – so champagne corks may well be popping at Massachusetts-based Synta for some time to come.

So what is GSK buying into? STA-4783 is a first-in-class compound that Synta and GSK believe has real potential in improving survival in malignant melanoma patients – something that has been hard to achieve with current treatments mainly because relapse rates are so high.

Synta’s CEO Safi Bahcall said GSK was one of a number of companies bidding for STA-4783, and the size of the deal was a reflection of the strong clinical data for the compound as well as its ‘very substantial commercial opportunity’.

The drug is described as an oxidative stress inducer and is thought to work by rendering melanoma cells more susceptible to destruction by chemotherapeutic agents. A combination of STA-4783 and paclitaxel has already been shown to double progression-free survival to 3.7 months compared with 1.8 months for patients treated with paclitaxel alone in a Phase IIb study.

Bahcall also said additional data showing STA-4783’s potential alongside other widely-used, first-line anticancer drugs would be presented at an American Association for Cancer Research (AACR) meeting later this month in San Francisco.

Melanoma rising faster than any other cancer

According to the World Health Organization, the number of malignant melanoma cases worldwide is increasing faster than that of any other cancer. Meanwhile, STA-4783 could also have potential in other tumour types characterised by high levels of oxidative stress, such as breast, prostate, ovarian and pancreatic cancers, as well as leukaemias.

GSK’s latest deal comes hard on the heels of a $2.5 billion agreement with Anacor for boron-based anti-infectives and anti-inflammatory compounds. In fact, in the last 12 months GSK has been among the top Big Pharma spenders on in-licensing, fulfilling a vow made by CEO Jean-Pierre Garnier to grow organically and by collaboration, rather than going down the mega-merger route.

It has inked a $1.5 billion deal with Targacept for central nervous system drugs targeting neuronal nicotinic receptors, offered nearly $700 million for rights to a restless legs syndrome drug developed from XenoPort, boosted its cancer and drug delivery portfolios with a $55 million purchase of Praecis Pharmaceuticals, and paid an undisclosed amount for an antidepressant developed by Fabre Kramer.

Synta’s shares actually dropped a little over 9% after the announcement, although analysts said this was likely a result of profit-taking as news of a deal on the drug had been anticipated.