Actinium-225

Ac-225 does not decay once; it opens a four-alpha chain, releasing on the order of 27 MeV of charged-particle energy per parent decay. Alpha particles are high-LET: they deposit that energy in dense, short ionization tracks, and the peroxide yield per unit energy (G) rises with ionization density. Decay for decay, Ac-225 is the most radiolytically intense isotope in the clinical roster.

What sets it apart is intensity, not bulk activity. Targeted-alpha doses are administered in single-digit megabecquerels, not the gigabecquerels of a beta therapy, roughly a thousandfold less activity. Yet the roughly 27 MeV each parent decay deposits across its four-alpha chain is so radiolytically productive that the modeled peroxide still climbs into the hundreds of micromolar, within range of the gigabecquerel beta therapeutics rather than far below them. The roughly 10-day half-life then holds that burden across a long clock. Ac-225's story is intensity per decay carried over a long clock, not large administered activity.

The high-LET peroxide behavior is drawn from the radiation-chemistry literature (Pastina & LaVerne), which documents how hydrogen-peroxide yield increases as ionization density rises: the reason an alpha emitter is modeled with a higher yield than a beta of the same energy.

DuraRad lines the vial wall with a sol-gel coating that holds two immobilized enzymes: manganese superoxide dismutase, which converts superoxide into hydrogen peroxide and oxygen, and catalase, which breaks that hydrogen peroxide down into water and oxygen. Radiolysis of the water generates reactive oxygen throughout the dose, at random positions and in every direction; the coating acts as a distributed sink at the wall, consuming the superoxide and peroxide that diffuse to it and lowering their steady-state concentration in the vial. Because the enzymes are fixed in the coating rather than dissolved in the dose, they work without becoming an additive in a product already dispensed at trace mass. Clearing the superoxide and peroxide the alpha tracks produce keeps their concentration low and lowers the oxidative burden on the radiolabeled molecule.

Ac-225's oxidative burden is unusual: extraordinarily intense per decay, because a four-alpha chain lays down dense, high-LET ionization tracks with a high peroxide yield, from an administered activity of only single-digit megabecquerels, a thousandfold below a beta therapy. The near-10-day half-life then sustains that intense production across a long clock, so the modeled peroxide accumulates into the hundreds of micromolar before the source fades. Enzymatic clearance runs on a timescale of minutes, far faster than Ac-225 generates peroxide, so the wall sink keeps pace easily and the model holds peroxide well below the uncoated level across the window. The modeled proportional suppression is large: the value here is lowering the intense per-decay reactive oxygen the dense alpha tracks produce and reducing the load on the radiolabeled product. The Q4 2026 radioactive validation at Washington University in St. Louis is designed to confirm this modeled behavior.