Johnson & Johnson’s (J&J) prostate cancer radiotherapy JNJ-6420 outlook was hampered by safety issues that resulted in four deaths in a Phase I trial, despite the drug demonstrating promising efficacy in treating prostate cancer.

The safety and efficacy results were announced as part of an abstract submitted to the American Society of Clinical Oncology (ASCO) Annual Meeting ahead of a data presentation at the event, which is scheduled to take place from 31 May to 4 June.

JNJ-6420 is an anti-hK2 antibody-based targeted radiotherapy that works by delivering an actinium-based radioligand. This payload emits high-energy short-range alpha particles that target cancer cells.

In the Phase I trial (NCT04644770), out of 57 patients who had intravenously received JNJ-6420 at 150 μCi or higher, 35 had Grade 3 or higher treatment-emergent adverse events (TEAEs), with 21 experiencing serious treatment-emergent adverse events (TEAE). The abstract highlighted thrombocytopenia and interstitial lung disease (ILD) as TEAEs of note due to repeated dosing.

Nine patients left the trial due to treatment-related adverse events (TRAE), and four TRAEs resulted in death.  

The radiotherapy’s safety and preliminary clinical were evaluated in the Phase I study, along with determining doses for progression into Phase II studies. According to, metastatic castration-resistant prostate cancer patients who have previously undergone treatment with at least one prior androgen receptor targeted therapy were enrolled in the trial.

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Whilst the drugmaker deals with the apparent high toxicity, the abstract reported “prolonged clinical, biochemical, and radiographic responses” in patients receiving the drug at 150 μCi and higher. Out of the 57 patients, three had a confirmed objective response rate, including one complete response and two partial responses.

Data demonstrated a PSA50 response rate of 45.6%, a measure of prostatic-specific antigen. This biomarker is important in prostate cancer as a PSA response is associated with prolonged overall survival.

Radiotherapy dollars

J&J is part of a growing number of pharma companies pursuing radiopharmaceutical candidates. Bristol Myers Squibb announced its arrival on the scene with a $3.6bn acquisition of RayzeBio – the latter’s assets also use actinium-based payloads. Eli Lilly followed its $1.4bn Point Biopharma deal last year with a $1.1bn partnership with Aktis Oncology just last week. AstraZeneca entered the arena in March 2024 with a $2bn takeover deal for Fusion Pharmaceuticals.

Novartis is spearheading the field with its lutetium-based prostate cancer treatment Pluvicto. The drug generated sales of $980m in 2023 and is predicted to surpass the $4.3bn mark by 2030, according to GlobalData’s Pharma Intelligence Centre.

GlobalData is the parent company of Clinical Trials Arena.  

The big pharma doubled down on this space earlier this month, spending a potential $1.7bn to acquire Mariana Oncology.