JNJ-6143, which came from Johnson & Johnson’s $3 billion takeover of Halda Therapeutics in 2025, showed a 75% overall response rate in the Phase 1 readout.
An investigational prostate cancer drug that Johnson & Johnson got from the $3 billion acquisition of Halda Therapeutics late last year has elicited a response in a small group of patients during an early-stage study—results that analysts at Leerink Partners dubbed “immaculate.”
Of 16 treated patients who had measurable disease through imaging, 12 had complete or partial responses, according to a press announcement on Sunday. This corresponded to a confirmed overall response rate (ORR) of 75% for patients receiving the asset, a daily oral drug that locks a cancer-specific protein with a protein crucial for cell survival to trigger cell death.
J&J presented the results, from the Phase 1 portion of a Phase 1/2 trial, on Saturday at the 2026 meeting of the European Society for Medical Oncology in Madrid. The study is evaluating escalating doses of JNJ-6143 ranging from 12.5 mg to 150 mg daily, with an overall enrollment target of 190 patients with metastatic castration-resistant prostate cancer (mCRPC) whose disease progressed after treatment with androgen receptor pathway inhibitors. Patients enrolled so far received a median of four prior therapies, including chemotherapy and prostate-specific membrane antigen–targeted radioligand therapy.
“JNJ-6143 could be a game changer for the treatment of prostate cancer and for JNJ’s oncology franchise projects,” Leerink said in an investor note on Sunday evening. “Early Phase 1 activity supports [JNJ-6143’s] potentially best-in-class profile among” therapies for mCRPC given after androgen receptor pathway inhibitors.
JNJ-6143 uses Halda’s regulated induced proximity targeting chimera (RIPTAC) platform. Molecules produced via this approach lock together a cancer-specific protein and a protein critical for a cell’s survival with the goal of inducing cell death.
The trial’s primary objective was to assess safety. J&J has settled on 50 mg as the preliminary optimal dose for JNJ-6143 to advance its development, including into a separate Phase 2 study for metastatic prostate cancer. At this level, the most common side effects included nausea, and elevated cholesterol and triglyceride levels. Of the 38 patients on this dose, four interrupted their dose while one reduced their dose and another discontinued treatment due to adverse events.
Preliminary efficacy data at the 50-mg dose showed that 77% of patients had at least a 50% decrease in prostate-specific antigen (PSA), a biomarker used to screen for prostate cancer. A PSA reduction of at least 90% was reported in nearly half of patients at this dose level.
“JNJ-6143’s highly encouraging early-stage PSA response look better than most therapies in development in the same setting,” Leerink wrote, pointing to Pfizer’s Phase 3 asset mevrometostat as one potential competitor in the advanced prostate cancer space.
With these data, JNJ-6143 emerged “as a standout at ESMO” thanks to the 75% ORR and “an exceptionally encouraging safety profile for an oral monotherapy in heavily pretreated mCRPC,” Leerink analysts wrote in a second Sunday investor note.
