The EMA has kicked off its review of Sanofi and Regeneron’s immuno-oncology drug cemiplimab, which is aiming to become the sixth drug in the PD-1/PD-L1 inhibitor class to reach the market.

Sanofi has filed cemiplimab in the first instance as a treatment for advanced cutaneous squamous cell carcinoma (CSCC), which it says is the “deadliest form of non-melanoma skin cancer”. As of yet, none of the other PD-1/PD-L1-targeting drugs are approved for CSCC.

The marketing application for cemiplimab comes from a single-arm, open-label trial involving 82 patients which showed an overall response rate with the PD-1 inhibitor of 46% after at least six months’ follow-up, which Sanofi and Regeneron said was among the highest observed to date with a PD-1 targeting agent in solid tumours. It is in the first stage in a rolling license application for the drug, which is already underway in the US.

Cemiplimab is one of nine drugs expected to be filed by Sanofi before the middle of next year, according to the company’s December 2017 R&D day. The company is hoping to build a market for the drug in CSCC and follow up with additional indications, including first- and second-line non-small cell lung cancer (NSCLC), which will pit the drug against its cancer immunotherapy rivals.

Sanofi and Regeneron first partnered on cemiplimab in 2015, and earlier this year agreed to boost their investment in the drug to a least $1.65bn - around $1bn more than originally intended - to help it break into one of the most hotly-contested pharma markets, currently led by Bristol-Myers Squibb and Merck & Co/MSD.

In addition to chasing companies in the category with already-marketed drugs - which also include Roche, AstraZeneca and Merck KGaA/Pfizer - Novartis’ PDR001 and INCSHR-1210 from Incyte/Jiangsu Hengrui are in phase III testing with several more candidates in phase II.

Regeneron CEO Leonard Schleifer said last month the riling application for cemiplimab in the US should be completed by the end of the first quarter and the company is already “preparing for launch”, with approval in the US possible before the end of the year.

The two partners also see the drug as a foundation for a series of combination therapies for cancer with other checkpoint inhibitors and bispecific antibody drugs, but maintain that CSCC is a significant and important opportunity on its own.