Syros Pharmaceuticals has stopped development of its intravenous CDK7 inhibitor SY-1365 after early clinical data came up short. The lackluster results led Syros to switch its attention to an oral CDK7 inhibitor, SY-5609, that is due to enter the clinic early next year.

        In August, Syros sketched out a series of upcoming milestones in the development of SY-1365 and talked up opportunities for accelerated development of the asset. Syros expected to pass the first milestone in the fourth quarter with the release of initial safety and efficacy data on the candidate in heavily pretreated ovarian cancer patients. 

        Syros is yet to share data on the ovarian cancer patients, one of two populations targeted in the initial expansion of a phase 1 trial, but did provide a qualitative assessment of why it canned the drug. 

        “Initial clinical activity and tolerability data from the expansion of the phase 1 trial of SY-1365 did not support an optimal profile for patients, particularly in light of an increasing focus on oral targeted agents in cancer,” Syros wrote in a statement.

        SY-5609 is an oral molecule that Syros claims is a more selective and potent inhibitor of CDK7. In preclinical models, those advantages have translated into greater anti-tumor activity, according to Syros. 

        However, while SY-5609 may be a better drug, the decision to scrap SY-1365 still pushes back Syros’ plans. SY-1365 was part of a two-pronged R&D push that Syros CEO Nancy Simonian saw as providing “fast-to-market strategies in three distinct AML and ovarian cancer patient populations” when she spoke to investors in August.

        SY-5609 is further back in development. Syros plans to start a clinical trial of the asset in certain types of solid tumors in the first quarter of next year. The clinical timeline puts Syros well behind Carrick Therapeutics, which started a phase 1 trial of its oral CDK7 inhibitor CT7001 in 2017.

        Shares in Syros fell 18% in premarket trading.