The genetic disease is due to abnormal haemoglobin, which causes the red blood cells to become sticky and look like a ‘sickle’. Sickle cells can aggregate together and block vessels. Image credit: Shutterstock/SciePro.

The US Food and Drug Administration (FDA) has given the green light to Fulcrum Therapeutics to continue a study investigating its candidate FTX-6058 for treatment of sickle cell disease, after the agency lifted a clinical hold.

Shares in Fulcrum opened 34.6% higher on 22 August compared to the pre-announcement market close. The company’s market cap is $365m.

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In February 2023, the FDA halted clinical development of Fulcrum’s polycomb repressive complex 2 (PRC2) inhibitor FTX-6058 due to evidence of haematological malignancies in submitted preclinical data. It also pointed towards clinical and non-clinical data from other PRC2 inhibitors that showed similar effects.

After working closely with the FDA to resolve the hold, Fulcrum has been greenlit to resume the Phase Ib trial (NCT05169580) investigating the candidate in adults with sickle cell disease. In a programme update presentation, the company outlined the remainder of the trial is planned to resume at 12mg followed by a 20mg dose, each administered for 12 weeks. It also made several modifications to key inclusion criteria – the FDA had previously requested Fulcrum define the population whose benefit from continued treatment would outweigh the risks, in a Clinical Hold Letter sent to the company.

A small-molecule inhibitor of embryonic ectoderm development (EED), FTX-6058 downregulates foetal globin repressors. B-cell lymphoma/leukaemia 11A (BCL11A) is one such gene impacted, causing an increase in foetal haemoglobin (HbF). Promoting high levels of HbF, which the drug has already demonstrated in interim analyses, is one of the most promising approaches for treating sickle cell disease. The candidate received orphan drug designation from the FDA in October 2022.

“Based on the initial data from the Phase Ib trial, which showed increasing levels of HbF with each dose escalation, we believe in the potential of FTX-6058 to not only shift the current standard of care but importantly, offer these patients a differentiated oral option. We look forward to building on these results with plans to resume enrolment for patients with SCD,” said Alex C. Sapir, Fulcrum’s president and CEO in a statement.

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FTX-6058 isn’t the only sickle cell disease treatment on the FDA’s radar. bluebird bio’s lovo-cell gene therapy has a Prescription Drug User Fee Act (PDUFA) goal date of 20 December 2023. Vertex and CRISPR Therapeutics also submitted a BLA to the FDA in April 2023 for its collaborative therapy exa-cel (exagamglogene autotemcel).