SpringWorks plans $115M IPO to take ex-Pfizer drugs to market

SpringWorks Therapeutics has filed to raise $115 million in an IPO. The money will take SpringWorks through to regulatory filings for two former Pfizer drugs against rare tumors.

Pfizer licensed the drugs, gamma secretase inhibitor nirogacestat and MEK inhibitor mirdametinib, to SpringWorks in 2017. SpringWorks, which was conceived by Pfizer, raised $103 million from backers including Bain Capital in conjunction with the four-asset licensing deal before adding another $125 million to its coffers in April through a series B round.

SpringWorks still had $185 million of the money left as of the end of June. But with a phase 3 trial of nirogacestat in desmoid tumors underway and a phase 2b of mirdametinib neurofibromatosis type 1-associated plexiform neurofibromas (NF1-PN) set to start in the third quarter, SpringWorks wants more cash.

If SpringWorks hits its IPO fundraising target, it expects to have enough money to see it through to 2022, by when it should have completed the phase 2b and 3 trials and, if the data are positive, be on the way to getting nirogacestat and mirdametinib approved and on the market.

Nirogacestat moved into phase 3 on the strength of data linking the drug to multi-year responses in patients with desmoid tumors, a rare type of soft tissue tumor. There are no FDA approved treatments for the indication and surgical resection suffers from high rates of disease recurrence.  

Mirdametinib achieved an objective response rate of 42% in phase 2 but SpringWorks thinks it may be able to improve on that figure. The phase 2 protocol specified that patients had to experience a 15% or greater reduction in the volume of their target tumor within around eight months of starting treatment to continue in the trial. 

However, some NF1-PN patients treated with other MEK inhibitors spent 12 months on treatment before experiencing an objective response, leading SpringWorks to speculate that the earlier phase 2 trial of mirdametinib was too short. The phase 2b will keep patients on therapy for up to 24 months.

The longer dosing period may have implications for the safety of mirdametinib. Earlier in development, adverse events led to the termination of one phase 2 trial and the cessation of enrollment in another. The adverse events happened at far higher doses than those SpringWorks will test in the phase 2b, but, as the company notes in its Securities and Exchange Commission filing, it is treating patients for longer this time around.