How Does PSMA RLT Kill Cells? Alpha vs Beta Radiobiology "Presentation" - Ana Ponce Kiess
April 19, 2025
At the 2025 UCSF-UCLA PSMA Conference, Ana Ponce Kiess compares alpha and beta particle radiobiology in PSMA radioligand therapy. She explains how ionizing radiation kills cells through DNA damage, highlighting that alpha particles create dense ionization causing complex clustered damage with prominent double-strand breaks, while beta particles produce sparse ionization with more single-strand breaks. Using cell studies, she demonstrates alpha radiation generates more chromosomal aberrations and unrepairable damage with slower repair kinetics, explaining why dose fractionation has minimal effect with alpha therapy.
Biography:
Ana Ponce Kiess, MD, PhD, Assistant Professor, Radiation Oncology and Molecular Radiation Sciences, Johns Hopkins University School of Medicine, Baltimore, MD
Biography:
Ana Ponce Kiess, MD, PhD, Assistant Professor, Radiation Oncology and Molecular Radiation Sciences, Johns Hopkins University School of Medicine, Baltimore, MD
Read the Full Video Transcript
Ana Ponce Kiess: All right. How does PSMA-RLT kill cells? Alpha versus beta radiobiology in eight minutes. So I'm going to limit the scope of this by just briefly touching on how ionizing radiation generally kills cells and then go through a comparison of alphas to low LET radiation and the steps from ionization to cell death, with the caveat that I am not an expert radiobiologist.
So ionizing radiation generally kills cells through a combination of direct and indirect DNA damage, with the indirect DNA damage mediated by free radicals that require the presence of oxygen. And depending on the type and extent of DNA damage, it can be lethal, sublethal, or potentially lethal, and then repair failure leads also into cell death.
Beta particles have less dense ionization density—so more sparse—and they cause more indirect DNA damage and have a combination of base damage, single-strand breaks, and double-strand breaks. And in general, low LET radiation, like X-rays, has about 1,000 single-strand breaks for every 20 to 30 double-strand breaks. Alpha particle decay—alpha particles—result in very dense ionization and much more complex clustered DNA damage, with usually double-strand breaks prominent among those clusters.
So this is another nice picture showing the influence of ionization density and LET tracks on DNA damage sites. And again, showing more potential for direct DNA damage with alpha particles and, therefore, less dependence on oxygen. This is a nice study comparing actinium-PSMA-I&T and lutetium-PSMA-I&T in PC3-PIP prostate cancer cell culture. Twenty-four hours after incubation with actinium-PSMA-I&T, there were more DNA double-strand breaks shown in red and clusters than after incubation with 1,000 times higher lutetium-PSMA-I&T.
The types of DNA damage again can be lethal, potentially lethal, or sublethal damage. This is from the classic Hall and Giaccia Radiobiology book. Examples of lethal DNA damage after ionizing radiation include these three examples. So on the left, if you have two different pre-replication chromosomes and you have a double-strand break in each chromosome, they can have illegitimate union. And then when they replicate, form a dicentric chromosome.
In the middle, if you have DNA double-strand breaks in both arms of a post-replication chromosome, you can form a ring that then replicates into an overlapping ring. And then if you have a post-replication chromosome that has a break in each chromatid arm, it can form a sister union and then a dicentric chromatid. All of these are lethal chromosomal aberrations that result in cell death.
For sublethal or potentially lethal DNA damage, there's DNA damage repair afterward. But for lethal damage, there's less capacity for repair. So this shows an experiment of alpha particle beam versus gamma ray beam in cell culture of lung cancer cells, showing increased chromosome fragments with alpha particle radiation. There are also other studies that show increased chromosomal aberrations, like we showed on the prior chart—the lethal aberrations after alphas—and more unrepairable DNA damage.
DNA damage repair of sublethal or potentially lethal damage can be through non-homologous end joining or through homologous recombination. Alpha radiation has been shown to be slower to repair double-strand DNA breaks. So this is a study in HeLa cell culture showing γ-H2AX double-strand break foci decreasing rapidly in the X-ray-treated cells. So in the first few hours, they repair a lot of the double-strand breaks versus with carbanions, and even more so with alpha particles—a slower and less complete repair of double-strand DNA breaks. And this leads to less resistance mechanisms for alphas and influences the cell survival curve.
There are different modalities of cell death after ionizing radiation, including apoptosis, autophagy, mitotic catastrophe, and necrosis. Mitotic catastrophe is known to be prominent in all forms of ionizing radiation-induced cell death. So there's formation of multinucleate cells and micronuclei, and the cell death occurs in mitosis.
I found out, in preparing these slides, that the International Committee on Cell Death does not actually consider mitotic catastrophe, though, to be a form of cell death itself, but more of an intermediate step leading to one of these other forms of cell death. But that can be really hard to categorize in cells that have undergone heavy radiation because they don't have all the characteristics of apoptosis or necrosis. So how each of these cell death pathways changes as a function of LET, apparently, is not well understood.
One thing that is well understood in radiobiology of alphas versus betas is the cell survival curves for uniform radiation and cell culture. So this is showing the log-linear scale of surviving fraction versus absorbed dose—for alphas it's monoexponential or linear on that scale, and for betas or X-rays, the classic linear-quadratic shape is seen—mostly due to the repair of DNA.
And this has been shown in many experiments of X-rays versus alpha beam radiation in different cell types. And importantly, it has implications for the effect of fractionation. So on the right, you see that the cell survival curve for one versus two fractions of alpha beam radiation is basically on the same line—no difference—versus you get sparing if you split X-rays into two doses.
And then this slide shows that concept on a four-dose regimen. So you get more sparing due to DNA repair by fractionating low LET radiation. But I want to say, with the caveat that this does not account for repopulation of bone marrow cells from unexposed cells because it assumes uniform radiation of all the cells, which doesn't happen in vivo.
So there's a lot of research more recently that has shown other mechanisms of cell death besides just DNA damage response in both alpha and beta RLT. So this is a nice review from Dr. Pouget more recently showing that the contributions of membrane damage, mitochondrial damage, calcium dysregulation all contribute to cell death after alpha emitter radiotherapy. And also that bystander effects—mediated by cytokines and gap junctions—and also systemic immune effects may influence cell death.
And then one more caveat—again, the importance of micro-biodistribution and microdosimetry limit a lot of what we see in these cell culture experiments, because alpha radiobiology is highly, highly dependent—because of its short range—on the cell geometry and the proximity to the nucleus. So the distribution of the alpha particles at the organ level, the tissue level, the cellular, and the subcellular level all will affect the dose response. And in vivo RLT radiobiology will be very different than in vitro.
So in summary, compared to beta particles, alpha particles have more dense ionization, more potential for direct DNA damage, more clustered DNA lesions with double-strand breaks, less dependence on oxygen, reduced capacity to repair DNA, and have a monoexponential survival curve after uniform radiation. Thank you.
Ana Ponce Kiess: All right. How does PSMA-RLT kill cells? Alpha versus beta radiobiology in eight minutes. So I'm going to limit the scope of this by just briefly touching on how ionizing radiation generally kills cells and then go through a comparison of alphas to low LET radiation and the steps from ionization to cell death, with the caveat that I am not an expert radiobiologist.
So ionizing radiation generally kills cells through a combination of direct and indirect DNA damage, with the indirect DNA damage mediated by free radicals that require the presence of oxygen. And depending on the type and extent of DNA damage, it can be lethal, sublethal, or potentially lethal, and then repair failure leads also into cell death.
Beta particles have less dense ionization density—so more sparse—and they cause more indirect DNA damage and have a combination of base damage, single-strand breaks, and double-strand breaks. And in general, low LET radiation, like X-rays, has about 1,000 single-strand breaks for every 20 to 30 double-strand breaks. Alpha particle decay—alpha particles—result in very dense ionization and much more complex clustered DNA damage, with usually double-strand breaks prominent among those clusters.
So this is another nice picture showing the influence of ionization density and LET tracks on DNA damage sites. And again, showing more potential for direct DNA damage with alpha particles and, therefore, less dependence on oxygen. This is a nice study comparing actinium-PSMA-I&T and lutetium-PSMA-I&T in PC3-PIP prostate cancer cell culture. Twenty-four hours after incubation with actinium-PSMA-I&T, there were more DNA double-strand breaks shown in red and clusters than after incubation with 1,000 times higher lutetium-PSMA-I&T.
The types of DNA damage again can be lethal, potentially lethal, or sublethal damage. This is from the classic Hall and Giaccia Radiobiology book. Examples of lethal DNA damage after ionizing radiation include these three examples. So on the left, if you have two different pre-replication chromosomes and you have a double-strand break in each chromosome, they can have illegitimate union. And then when they replicate, form a dicentric chromosome.
In the middle, if you have DNA double-strand breaks in both arms of a post-replication chromosome, you can form a ring that then replicates into an overlapping ring. And then if you have a post-replication chromosome that has a break in each chromatid arm, it can form a sister union and then a dicentric chromatid. All of these are lethal chromosomal aberrations that result in cell death.
For sublethal or potentially lethal DNA damage, there's DNA damage repair afterward. But for lethal damage, there's less capacity for repair. So this shows an experiment of alpha particle beam versus gamma ray beam in cell culture of lung cancer cells, showing increased chromosome fragments with alpha particle radiation. There are also other studies that show increased chromosomal aberrations, like we showed on the prior chart—the lethal aberrations after alphas—and more unrepairable DNA damage.
DNA damage repair of sublethal or potentially lethal damage can be through non-homologous end joining or through homologous recombination. Alpha radiation has been shown to be slower to repair double-strand DNA breaks. So this is a study in HeLa cell culture showing γ-H2AX double-strand break foci decreasing rapidly in the X-ray-treated cells. So in the first few hours, they repair a lot of the double-strand breaks versus with carbanions, and even more so with alpha particles—a slower and less complete repair of double-strand DNA breaks. And this leads to less resistance mechanisms for alphas and influences the cell survival curve.
There are different modalities of cell death after ionizing radiation, including apoptosis, autophagy, mitotic catastrophe, and necrosis. Mitotic catastrophe is known to be prominent in all forms of ionizing radiation-induced cell death. So there's formation of multinucleate cells and micronuclei, and the cell death occurs in mitosis.
I found out, in preparing these slides, that the International Committee on Cell Death does not actually consider mitotic catastrophe, though, to be a form of cell death itself, but more of an intermediate step leading to one of these other forms of cell death. But that can be really hard to categorize in cells that have undergone heavy radiation because they don't have all the characteristics of apoptosis or necrosis. So how each of these cell death pathways changes as a function of LET, apparently, is not well understood.
One thing that is well understood in radiobiology of alphas versus betas is the cell survival curves for uniform radiation and cell culture. So this is showing the log-linear scale of surviving fraction versus absorbed dose—for alphas it's monoexponential or linear on that scale, and for betas or X-rays, the classic linear-quadratic shape is seen—mostly due to the repair of DNA.
And this has been shown in many experiments of X-rays versus alpha beam radiation in different cell types. And importantly, it has implications for the effect of fractionation. So on the right, you see that the cell survival curve for one versus two fractions of alpha beam radiation is basically on the same line—no difference—versus you get sparing if you split X-rays into two doses.
And then this slide shows that concept on a four-dose regimen. So you get more sparing due to DNA repair by fractionating low LET radiation. But I want to say, with the caveat that this does not account for repopulation of bone marrow cells from unexposed cells because it assumes uniform radiation of all the cells, which doesn't happen in vivo.
So there's a lot of research more recently that has shown other mechanisms of cell death besides just DNA damage response in both alpha and beta RLT. So this is a nice review from Dr. Pouget more recently showing that the contributions of membrane damage, mitochondrial damage, calcium dysregulation all contribute to cell death after alpha emitter radiotherapy. And also that bystander effects—mediated by cytokines and gap junctions—and also systemic immune effects may influence cell death.
And then one more caveat—again, the importance of micro-biodistribution and microdosimetry limit a lot of what we see in these cell culture experiments, because alpha radiobiology is highly, highly dependent—because of its short range—on the cell geometry and the proximity to the nucleus. So the distribution of the alpha particles at the organ level, the tissue level, the cellular, and the subcellular level all will affect the dose response. And in vivo RLT radiobiology will be very different than in vitro.
So in summary, compared to beta particles, alpha particles have more dense ionization, more potential for direct DNA damage, more clustered DNA lesions with double-strand breaks, less dependence on oxygen, reduced capacity to repair DNA, and have a monoexponential survival curve after uniform radiation. Thank you.