UPDATE 1-FDA unclear if benefits of GSK multiple myeloma drug outweigh risks

(Recasts, adds quotes, analyst comment)

LONDON, July 10 (Reuters) - The U.S. Food and Drug Administration said on Friday it was unclear whether the benefits of GlaxoSmithKline's experimental treatment for a common form of blood cancer outweigh the risks ahead of a review of a side-effect which affects the eyes.

Approval for belantamab mafodotin as a treatment for multiple myeloma, a rival to Johnson & Johnson and Genmab's Darzalex treatment, is seen as important for GSK's growing oncology portfolio.

In submissions ahead of a meeting of the Oncologic Drugs Advisory Committee (ODAC) on July 14, the FDA said it had concerns about how belantamab mafodotin can cause deposits to gather on the cornea, known as keratopathy.

The FDA highlighted the frequency of the side effect, its severity, the lack of clear mitigation strategies and incomplete data on whether it is reversible.

GSK is confident in the benefit/risk profile of the drug, and proposes it should be managed through modifying or interrupting the course of treatment.

"There is uncertainty whether the proposed dose modification strategy is sufficient to mitigate the risks," a briefing document published on the FDA's website said.

"It is not clear whether the benefit outweighs the risks of ocular toxicity."

Broker Jefferies said that while it was usual for the document to flag contentious issues, "our confidence is dented by FDA's remarks".

In its mid-stage study DREAMM-2, nearly 100 patients were given the belantamab mafodotin at the dosage for which GSK has submitted for approval.

Of those patients, 27% experienced medically severe keratopathy. One patient discontinued treatment due to the side effect but no patients experienced life threatening symptoms.

The trial was testing the drug, also known as GSK2857916, in patients who had received four to seven prior other treatments, including Darzalex.

GSK has other trials planned aimed at showing its benefits in earlier stages of treatment. (Reporting by Alistair Smout; Additional reporting by Ankur Banerjee; Editing by Alexander Smith)