The Phase 1a/1b trial evaluating Five Prime Therapeutics‘ checkpoint inhibitor FPA150 in patients with advanced solid cancers has begun dosing patients ahead of schedule, the company announced.

FPA150 is an antibody that targets the B7-H4 protein in malignant cells. This protein is found in numerous solid tumors and belongs to the family of checkpoint molecules, such as PD-L1, which prevent immune cells from attacking cancer cells.

But FPA150 works in two ways. It blocks the signals from B7-H4, helping immune cells detect and eliminate tumor cells, and directly kills B7-H4-positive cells.

The study (NCT03514121) is testing FPA150 in patients with advanced solid tumors, including those with breast, ovarian, endometrial, and urinary system cancers. Patients must have high B7-H4 levels in their tumor cells to be eligible.

“B7-H4 is generating excitement in investigators because it’s expressed in tumor types that are currently not well served by immunotherapy, including breast and gynecologic cancers,” Helen Collins, MD, senior vice president and chief medical officer of Five Prime, said in a press release. “We’re hopeful that a targeted therapy like FPA150 will provide clinical benefit in these patients who have limited treatment options.”

The Phase 1 trial is being conducted in two parts. In Phase 1a, researchers are working with 10 patients with any solid tumor to determine the maximum tolerated dose and recommended dose for further testing. FPA150’s safety is also being tested.

In Phase 1b — the dose-expansion portion — researchers will examine the established dose in select patient groups, including those with breast, ovarian, and endometrial cancers with high B7-H4 levels. Researchers will then assess the number of patients achieving a partial or complete response to treatment.

Investigators are also examining how FPA150 behaves in the body, including how it is absorbed, metabolized and secreted.

“We’re excited to be ahead of schedule in opening up our dose exploration basket cohort for FPA150, our first-in-class B7-H4 antibody, in patients whose tumors overexpress B7-H4,” Collins said.

The study is still recruiting and is expected to enroll approximately 268 participants across eight clinical centers in the United States. For information, visit the study webpage.