Among the most clinically meaningful numbers buried in Aktis Oncology‘s first-in-human dosimetry readout for AKY-2519: a predicted salivary gland absorbed dose of just 4.2 Gy RBE at the proposed therapeutic schedule of 8 MBq × 4 doses of actinium-225. That figure matters because salivary toxicity has been a persistent, dose-limiting liability for PSMA-targeted radiopharmaceuticals like lutetium-177 PSMA-617, and it directly constrains how aggressively clinicians can treat. If AKY-2519 holds that profile through therapeutic dosing, the B7-H3 target becomes more than an alternative — it becomes a structural improvement in the tolerability architecture of prostate cancer radiopharmaceuticals.
The dataset itself comes from 16 patients with metastatic castration-resistant prostate cancer evaluated at the University of Pretoria and Steve Biko Academic Hospital. Aktis used two imaging surrogates — gallium-68 for PET/CT biodistribution and low-dose, non-therapeutic lutetium-177 for SPECT/CT dosimetry — rather than administering therapeutic actinium-225 directly, which is the correct design for a first-in-human characterization. Tumor uptake persisted for at least six days post-administration, a retention profile that is essential for actinium-225 given its 10-day half-life. The predicted kidney dose of 16 Gy RBE and liver dose of roughly 10 Gy RBE sit within ranges that have been clinically managed in other alpha-emitter programs, though those organs will require close surveillance as therapeutic cohorts expand. Bone marrow absorbed dose was low at 1.3 Gy RBE, which reduces concern about hematologic suppression.
B7-H3 is expressed not just in mCRPC but across lung, colorectal, and a range of other solid tumors — and critically, expression in normal tissues is limited. That biology gives Aktis a justification for the parallel Phase 1b it plans to launch in pan-tumor B7-H3-positive patients in the second half of 2026, alongside the ongoing mCRPC-dedicated trial enrolling both PLUVICTO-naïve and PLUVICTO-experienced patients. The latter group is especially important: capturing patients who have progressed on lutetium-177 PSMA-617 tests whether B7-H3 targeting can deliver meaningful activity in a PSMA-refractory setting, which would address a genuine gap rather than simply compete for the same first-line patients.
The single number to watch in the 2027 preliminary efficacy readout is PSA response rate in the PLUVICTO-experienced cohort — that subgroup will determine whether AKY-2519 earns a distinct clinical niche or gets absorbed into an increasingly crowded radiopharmaceutical field fighting over the same treatment-naive patients.
Jon Napitupulu is Director of Media Relations at The Clinical Trial Vanguard. Jon, a computer data scientist, focuses on the latest clinical trial industry news and trends.
