Blueprint Medicines Corp. said its investigational drug BLU-667 shrank tumors in certain thyroid cancer patients in an early stage trial.

Updated data from the ongoing phase 1 study, called Arrow, showed that 90% of patients saw radiographic tumor reductions after treatment with BLU-667.

The data cut-off date was Sept. 14.

The trial enrolled patients with both papillary thyroid cancer, or PTC, and medullary thyroid cancer, or MTC. PTC is the most common form of the disease; MTC is rare but more aggressive.

Overall, 49% of MTC patients responded to treatment, with one having no detectable cancer and 16 seeing at least a 50% reduction in tumor size. All PTC patients responded to treatment, with two seeing at least 50% tumor shrinkage.

BLU-667 also showed durable activity, with certain patients staying on therapy for more than 15 months. The drug had greater efficacy at higher doses.

The company said BLU-667 worked consistently regardless of patients having been previously treated or having receptor tyrosine kinase-altered tumors.

Additionally, BLU-667 was well-tolerated across the 69 patients enrolled in the study, with mostly mild adverse events. However, Blueprint Medicines said two patients had to discontinue therapy due to severe adverse events.

"We believe these results begin to reveal the potential of BLU-667 to transform the care of patients with RET-altered thyroid cancer, and we look forward to seeing the data continue to mature as additional patients are treated at the recommended phase 2 dose for longer durations," Blueprint Medicines Chief Medical Officer Andy Boral said in a news release.

Cambridge, Mass.-based Blueprint Medicines presented the updated data at the American Thyroid Association's annual meeting. The company had reported positive data from the Arrow trial in April.