ASCO24: J&J Reports Four Patient Deaths in Early-Stage Prostate Cancer Trial

J&J Switzerland_iStock, yuelan

Pictured: J&J's building in Switzerland/iStock, yuelan

Four patients died in Johnson & Johnson’s Phase I study for its investigational targeted radiopharmaceutical candidate JNJ-6420 in metastatic castration-resistant prostate cancer, according to a Thursday readout released ahead of next week’s American Society of Clinical Oncology annual meeting.

Thursday’s data come from 57 patients who had received at least one dose of JNJ-6420, of whom 31 stayed on treatment for 24 weeks or longer. At the time of the interim analysis, three patients had confirmed objective response, including one patient who showed complete response.

Almost half of the patients who had received at least a 150-μCi dose of the candidate saw a 50% decline in prostate-specific antigen levels, while 14% reached this benchmark. At six months, disease control rate was 28.1%.

According to the abstract, the results suggest that one to two doses of 150-μCi JNJ-6420 can elicit “profound and durable biochemical and radiographic responses” in treated patients.

However, in terms of safety, J&J’s investigational radiotherapy appears to be fairly toxic. More than 60% of patients developed treatment-emergent adverse events (TEAE) that were at least grade 3 in severity, while almost 37% had a serious TEAE. Common adverse events included anemia, low platelet count and abnormal reductions in white blood cells.

Nine patients dropped out of the study due to severe side effects that were deemed related to the study drug.

JNJ-6420 is a potentially first-in-class targeted radiopharmaceutical candidate that works by using an anti-hK2 antibody to deliver an actinium-based radioligand. The payload can emit high-energy alpha particles across short ranges, allowing JNJ-6420 to specifically target prostate cancer cells.

With Thursday’s readout, J&J joins the growing number of companies that have started pumping dollars in the emerging targeted radiopharma field. Among them is Bristol Myers Squibb’s acquisition of RayzeBio for $3.6 billion in December 2023. Like JNJ-6420, RayzeBio’s candidates use actinium-based payloads. Its lead program, RYZ101, is being developed for gastroenteropancreatic neuroendocrine tumors and small-cell lung cancer.

In March 2024, AstraZeneca also joined the radiopharma race with its potential $2.4 billion acquisition of Fusion Pharmaceuticals, which is also working on an actinium-based candidate. Its lead program is FPI-2265, which will face off against JNJ-6420 in metastatic castration-resistant prostate cancer.

Earlier this month, Novartis also bet $1 billion in radiopharma by buying Mariana Oncology and was followed by Eli Lilly, which on Tuesday entered a $1.1 billion partnership with Aktis Oncology.

Tristan Manalac is an independent science writer based in Metro Manila, Philippines. Reach out to him on LinkedIn or email him at or

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