UPDATE 2-GSK starts late-stage trials of melanoma drugs

* Phase III to focus on patients with BRAF V600 mutation

* MEK, BRAF inhibitors promising in early Phase I/II tests

* GSK also testing drugs in combinations, in other cancers

(Adds details of other drug candidates, GSK share price)

By Kate Kelland

LONDON, Jan 24 (Reuters) - British drugmaker GlaxoSmithKline (GSK.L) said on Monday it was starting two global late stage trials of drugs in advanced or metastatic melanoma in patients whose tumours have a particular gene mutation.

Melanoma is the deadliest form of skin cancer. Once it returns and spreads after initial treatment there are virtually no effective alternative treatments available.

The studies will assess the efficacy and safety of two experimental drugs -- known as BRAF and MEK inhibitors and called GSK2118436 and GSK1120212 -- to see whether they can stop or slow the progression of skin cancer in patients whose tumours contain a BRAF V600 mutation, GSK said in a statement.

The BRAF V600 mutation occurs in around 50 to 60 percent of melanoma patients.

"By focusing our research on patients with the V600 mutation, we are striving to understand how our ... inhibitors can best be used to treat patients with metastatic melanoma," Paolo Paoletti, president of GSK Oncology, said.

If GSK's drugs prove successful in melanoma, they are likely to have to compete with other keenly-watched experimental drugs from Roche (ROG.VX), Plexxikon and Bristol-Myers Squibb (BMY.N).

Roche said last week that data on a drug it is developing with private-held firm Plexxikon, called RG7204 or PLX4032, showed it helped patients live longer and extended the period in which their disease does not get worse. [ID:nLDE70I04W]

Oncologists and analysts are also closely watching Bristol's Phase III drug ipilimumab. [ID:nN05163435].

Melanoma is a type of cancer which develops when melanocytes -- pigment cells in the skin -- become malignant and start to grow and divide at an abnormally quick pace, spreading into the surrounding surface layers of skin.

Metastatic melanoma occurs when melanoma spreads to other parts of the body.

According to the World Health Organisation, 132,000 melanoma skin cancers occur globally each year and approximately 19,000 women and 22,000 men die annually from malignant melanoma.

Data published last May from earlier stage trials showed the two GSK drugs were promising in the treatment of this most deadly type of skin cancer. [ID:nLDE64J1W2]

Paoletti said GSK was also studying the two drugs -- alone and in combination with other medicines -- in other difficult-to-treat forms of cancer including pancreatic cancer, refractory or relapsed leukaemias and other solid tumours.

GSK shares were trading 1.6 percent higher by 1520 GMT. (Editing by David Hulmes and Alexander Smith)